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CELLS: THE LIVING UNITS

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1 CELLS: THE LIVING UNITS
OVERVIEW OF THE CELLULAR BASIS OF LIFE

2 Cell Theory Cells are the basic structural and functional units of life The activity of an organism depends on both the individual and the collective activities of its cells The biochemical activities of a cell are dictated by their organelles The continuity of life has a cellular basis

3 Cells vary greatly in their size, shape, and function

4 CELL DIVERSITY

5 CELL DIVERSITY

6 CELL DIVERSITY

7 Characteristics of Cells
All cells are composed primarily of carbon, hydrogen, nitrogen, and oxygen

8 Characteristics of Cells
All cells have the same basic parts and some common functions A generalized human cell contains the plasma membrane, the cytoplasm, and the nucleus

9 CELL STRUCTURE

10 Plasma Membrane: Structure
Plasma membrane (cell membrane) defines the extent of the cell, separating two of the body’s major fluid compartments: Intracellular fluid within cells Extracellular fluid outside cells

11 The Fluid Mosaic Model Plasma membrane is composed of a double layer of phospholipids embedded with small amounts of cholesterol and proteins dispersed in it The phospolipid bilayer is composed of two layers of phospholipids lying tail to tail: Polar head is charged and hydrophilic (hydro=water, philic=loving) Exposed to water inside (intracellular) and outside (extracellular) the cell Attracted to water Nonpolar tail is made of two fatty acid chains and is hydrophobic (phobia=hating) Avoid water Line up in the center of the membrane

12 FLUID MOSIAC MEMBRANE

13 The Fluid Mosaic Model All biological membranes share a common structure: They are composed of two parallel sheets of phospholipid molecules lying tail to tail, with their polar heads exposed to water inside and outside This self-orienting property of phospholipids encourages biological membranes to self-assemble into closed, generally spherical, structures and to reseal themselves quickly when torn

14 The Fluid Mosaic Model The inward-facing and outward-facing surfaces of the plasma membrane differ in the kinds and amounts of lipids they contain: The majority of membrane phospholipids are unsaturated (like phosphatidyl choline), a condition which kinks their tails (increasing the space between them) and increases fluidity Glycolipids, phospholipids with attached sugar groups, are found only in the outer membrane (5% of membrane) Sugar group makes that end of the glycolipid molecule polar, whereas the fatty acid tails are nonpolar Cholesterol (20% of membrane) stabilizes the lipid membrane by wedging its platelike hydrocarbon rings between the phospholipid tails and restraining movement of the phospholipids Lipid rafts (20%), dynamic assemblies of saturated phospholipids (which pack together tightly) associated with unique lipids called sphinolipids and lots of cholesterol are also found only in the outer membrane More stable and orderly and less fluid than the rest of the membrane Include or exclude specific proteins to various extents Assumed to function in cell signaling

15 The Fluid Mosaic Model Two distinct populations of membrane proteins:
Integral Peripheral

16 FLUID MOSIAC MEMBRANE

17 Functions of Membrane Proteins
Proteins make up about 50% of the plasma membrane by mass and are responsible for most of the specialized membrane functions Transport Enzymatic activity Receptors for signal transduction Intercellular joining Cell-cell recognition Attachment to the cytoskeleton and extracellular matrix (ECM)

18 FLUID MOSIAC MEMBRANE

19 MEMBRANE PROTEIN

20 Functions of Membrane Proteins
Transport: (a): A protein that spans the membrane may provide a hydrophilic channel across the membrane that is selective for a particular solute (b): Some transport proteins hydrolyze ATP as an energy source to actively pump substances across the membrane

21 MEMBRANE PROTEIN

22 Functions of Membrane Proteins
Enzymatic activity: A protein built into the membrane may be an enzyme with its active site exposed to substances in the adjacent solution In some cases, several enzymes in a membrane act as a team that catalyzes sequential steps of a metabolic pathway as indicated (right to left) here

23 MEMBRANE PROTEIN

24 Functions of Membrane Proteins
Receptors for signal transduction: A membrane protein exposed to the outside of the cell may have a binding site with a specific shape that fits the shape of a chemical messenger, such as a hormone The external signal may cause a conformational change in the protein that initiates a chain of chemical reactions in the cell

25 MEMBRANE PROTEIN

26 Functions of Membrane Proteins
Intercellular joining: Membrane proteins of adjacent cells may be hooked together in various kinds of intercellular junctions Some membrane proteins (CAMs) of this group provide temporary binding sites that guide cell migration and offer cell-to-cell interactions

27 MEMBRANE PROTEIN

28 Functions of Membrane Proteins
Cell-Cell recognition: Some glycoproteins (proteins bonded to short chains of sugars) serve as identification tags that are specifically recognized by other cells

29 MEMBRANE PROTEIN

30 Functions of Membrane Proteins
Attachment to the cytoskeleton and extracellular matrix (ECM): Elements of the cytoskeleton (cell’s internal supports) and the extracellular matrix (ECM) may be anchored to membrane proteins, which help maintain cell shape and fix the location of certain membrane proteins Others play a role in cell movement or bond adjacent cells together

31 MEMBRANE PROTEIN

32 The Fluid Mosaic Model There are two distinct types of membranes proteins: Integral Peripheral

33 FLUID MOSIAC MEMBRANE

34 Integral Membrane Proteins
Firmly inserted into the plasma membrane Some protrude from one membrane face only, BUT most are transmembrane proteins that span the entire width of the membrane and protrude on BOTH sides All have BOTH hydrophobic and hydrophilic regions: This allows them to interact BOTH with the nonpolar lipid tails buried in the membrane and with water inside and outside the cell

35 FLUID MOSIAC MEMBRANE

36 Integral Membrane Proteins
Mainly involved in transport: Some cluster together to form channels, or pores, through which small, water-soluble molecules or ions can move, thus bypassing the lipid part of the membrane Some act as carriers that bind to a substance and then move it through the membrane Some are receptors for hormones or other chemical messengers and relay messages to the cell interior (process called signal transduction)

37 FLUID MOSIAC MEMBRANE

38 Peripheral Membrane Proteins
Are not embedded in the lipid of the plasma membrane, but attach rather loosely to integral proteins or membrane phospholipids and are easily removed without disrupting the membranes Include a network of filaments that helps support the membrane from its cytoplasmic side Peripheral proteins may function as enzymes or in mechanical functions of the cell, such as changing cell shape during cell division and muscle cell contraction, or linking cells together

39 FLUID MOSIAC MEMBRANE

40 Peripheral Membrane Proteins
Many of the proteins that abut the extracellular space are: Glycoproteins that have branching sugar groups The term glycocalyx (sugar coated) is used to describe the fuzzy, sticky carbohydrate-rich area at the cell surface The glycocalyx is enriched BOTH by glycolipids and by glycoproteins secreted by the cells that cling to its surface

41 FLUID MOSIAC MEMBRANE

42 Glycocalyx Because every cell type has a different pattern of sugars in its glycocalyx, the glycocalyx provides highly specific biological markers by which approaching cells recognize each other; Example: A sperm recognizes an ovum (egg cell) by the ovum’s unique glycocalyx Cells of the immune system identify a bacterium by binding to certain membrane glycoproteins in the bacterial glycocalyx Definite changes in the glycocalyx occur in a cell that is becoming cancerous A cancer cell’s glycocalyx may change almost continuously, allowing it to keep ahead of immune system recognition mechanisms and avoid destruction

43 Fluid Mosaic Model The plasma membrane is a dynamic fluid structure that is in constant flux Its consistency is like that of olive oil The lipid molecules of the bilayer move freely from side to side, parallel to the membrane surface, but their polar-nonpolar interactions prevent them from flip-flopping or moving from one leaflet (half of the bilayer) to the other Some of the proteins float freely Others, particularly the peripheral proteins, are restricted in their environments because they are “tethered” to intercellular structures that make up the cytoskeleton

44 Specialization of the Plasma Membrane Microvilli
Microvilli are fingerlike extensions of the plasma membrane that increase the surface area of the cell Most often found on the surface of absorptive cells such as intestinal and kidney tubule cells Have a core of actin filaments A contractile protein, BUT in microvilli it appears to function as a mechanical stiffener

45 CELL JUNCTIONS

46 Specialization of the Plasma Membrane Membrane Junctions
Three factors act to bind cells together: 1. Glycoproteins in the glycocalyx act as an adhesive 2. Wavy contours of the membranes of adjacent cells fit together in a tongue-and-groove fashion 3. Special membrane junctions are formed

47 Specialization of the Plasma Membrane Tight Junctions
Type of membrane junction in which integral proteins on adjacent cells fuse together to form an impermeable junction that encircles the cell Prevent molecules from passing through the extracellular space between adjacent cells Example: Tight junctions between epithelial cells lining the digestive tract keep digestive enzymes and microorganisms in the intestine from seeping into the bloodstream

48 CELL JUNCTIONS

49 Specialization of the Plasma Membrane Desmosomes
Desmosomes: binding bodies Are mechanical couplings that are scattered like rivets along the sides of adjoining cells that prevent their separation Abundant in tissues subjected to great mechanical stress: Skin Heart muscles

50 CELL JUNCTIONS

51 Specialization of the Plasma Membrane Desmosomes
On the cytoplasmic face (outside) of each plasma membrane is a buttonlike thickening called a plaque Adjacent cells are held together by thin linker protein filaments (cadherins) that extend from the plaques and interdigitate like the teeth of a zipper in the intercellular space Thicker protein filaments (intermediate filaments), which form part of the cytoskeleton) extend from the cytoplasmic side (inside) across the width of the cell to anchor to the plaque on the cell’s opposite side NOT ONLY bind neighboring cells together, they also contribute to a continuous internal network of strong “guy-wires” Distributes tension throughout a cellular sheet and reduces the chance of tearing when a tissue is stressed

52 CELL JUNCTIONS

53 Specialization of the Plasma Membrane Gap Junctions
Gap junctions or nexus (bond) Are a communication junction between cells that allows substances (chemicals) to pass between adjacent cells Adjacent membranes are very close, and the cells are connected by hollow cylinders called connexons composed of transmembrane proteins The many different types of connexon proteins vary the selectivity of the gap junction channels Ions, simple sugars, and other small molecules pass through these water-filled channels from one cell to the next Present in electrically excitable tissues: Heart Smooth muscle

54 CELL JUNCTIONS

55 Membrane Transport Our cells are bathed in an extracellular fluid called interstitial fluid that is derived from the blood: It is like a rich, nutritious soup It contains thousands of ingredients, including amino acids, sugars, fatty acids, vitamins, regulatory substances such as hormones and neurotransmitters, salts, and waste products To remain healthy, each cell must extract from this mix the exact amounts of the substances it needs at specific times

56 Membrane Transport The plasma membrane is a selectively (differentially) permeable barrier, regulating how substances pass into and out of the cell Allows some substances to pass while excluding others: Allows nutrients to enter the cell, but keeps many undesirable substances out Keeps valuable cell proteins and other substances in the cell, but allows wastes to exit

57 Membrane Transport Substances move through the plasma membrane in essentially two ways: Passively: Substances cross the membrane without any energy input from the cell Actively: Cell provides the metabolic energy (ATP) needed to move substances across the membrane

58 Passive Processes Passive processes do not use energy and move substances down a concentration gradient (high to low) Two main types of passive transport in cells are: Diffusion: An important means of passive membrane transport for every cell of the body Filtration: Generally occurs only across capillary walls

59 Passive Process Diffusion
Diffusion is a process in which substances scatter evenly throughout the environment from an area of higher concentration to an area of lower concentration Molecules move randomly, collide and ricochet off one another, changing direction with each collision Overall effect of this erratic movement is that molecules or ions move away from areas where they are in higher concentration to areas where their concentration is lower, so we say that molecule diffuse along, or down, their concentration gradient until equilibrium The greater the difference in concentration between the two areas, the faster the net diffusion of the particles

60 DIFFUSION

61 Passive Process Diffusion
Driving force is the kinetic energy of the molecules themselves: The speed is influenced by: Molecular size: Smaller the faster Temperature: Warmer, the faster Example: Peeling an onion, releases volatile substances that diffuse through the air, dissolving in the fluid film covering your eyes forming irritating sulfuric acid

62 Passive Process Diffusion
Because of its hydrophobic core, the plasma membrane is a physical barrier to free diffusion However, a molecule will diffuse through the membrane if the molecule is: 1. Lipid soluble (Simple Diffusion) 2. Small enough to pass through membrane channels (Simple Diffusion) 3. Assisted by a carrier molecule (Facilitated Diffusion)

63 Passive Process Simple Diffusion
Nonpolar and lipid-soluble substances diffuse directly through the lipid bilayer Examples: oxygen, carbon dioxide, fat-soluble vitamins, and alcohol

64 DIFFUSION

65 Passive Process Facilitated Diffusion
In facilitated diffusion substances are moved through, even though they are unable to pass through the lipid bilayer of the plasma membrane, by either: Binding to protein carriers in the membrane Moving through channels Examples: glucose and other sugars, amino acids, and ions

66 Facilitated Diffusion Carriers
Is a transmembrane integral protein (sometimes called a permease) that shows specificity for molecules of a certain polar substance or class of substances that are too large to pass through membrane channels Examples: sugars and amino acids Mechanism: Carrier-Mediated Facilitated Diffusion Changes in shape of the carrier allow it to first envelop and then release the transported substance, shielding it en route from the nonpolar regions of the membrane Essentially, the binding site is moved from one face of the membrane to the other by changes in the conformation of the carrier protein

67 DIFFUSION

68 Facilitated Diffusion Channels
Transmembrane proteins that serve to transport substances, usually ions or water, through aqueous channels from one side of the membrane to the other Types of Channels are: Open Channels: Are always open (leakage channels) and simply allow ion or water fluxes according to concentration gradients Gated and Controlled Channels: Gated: Binding or association sites exist within the channel and the channel is selective due to pore size and the charges of the amino acids lining the channel Controlled: open or close by various chemical or electrical signals

69 DIFFUSION

70 Passive Process Osmosis
Osmos=pushing Is the diffusion of a solvent, such as water, through a selectively permeable membrane Even though water is highly polar, it passes via osmosis through the lipid bilayer This is surprising because you’d expect it to be repelled by the hydrophobic lipid tails Hypothetical: Random movements of the membrane lipids open small gaps between their wiggling tails, allowing water to slip and slide its way through the membrane by moving from gap to gap Water also moves freely and reversibly through water-specific channels constructed by transmembrane proteins called aquaporins (AQP) Abundant in Red Blood Cells and Kidney Tubules

71 Passive Process Osmosis
Occurs whenever the water concentration differs on the two sides of a membrane If the solute concentration on the two sides of the membrane differs, water concentration differs as well (as solute concentration increases, water concentration decreases) The extent to which water’s concentration is decreased by solutes depends on the NUMBER, NOT THE TYPE, of solute particles, because one molecule or one ion of solute (theoretically) displaces one water molecule Osmolarity: total concentration of all solute particles in a solution

72 Osmolarity When equal volumes of aqueous solutions of different osmolarity are separated by a membrane that is permeable to all molecules in the system, net diffusion of both solute and water occurs, each moving down its own concentration gradient Eventually, equilibrium is reached when the water concentration on the left equals that on the right, and the solute concentration on both sides is the same

73 OSMOSIS

74 Osmolarity If we consider the same system, but make the membrane impermeable to solute molecules, we see quite a different result: Water quickly diffuses from the left to the right compartment and continues to do so until its concentration is the same on the two sides of the membrane Notice that in this case equilibrium results from the movement of WATER ALONE (the solutes are prevented from moving) Also the movement of water leads to dramatic changes in the volumes of the two compartments

75 OSMOSIS

76 Osmolarity In the this U-tube example, the tube can receive and compensate additional fluid BUT in a cell this may not be feasible As water diffuses into the cell, the point is finally reached where the hydrostatic pressure (the back pressure exerted by water against the membrane) within the cell is equal to its osmotic pressure—the cell’s tendency to resist further (net) water entry The higher the amount of nondiffusable (or nonpenetrating) solutes in a cell, the higher the osmotic pressure and the greater the hydrostatic pressure that must be developed to resist further net water entry

77 Osmolarity Many molecules, particularly intracellular proteins and selected ions, are prevented from diffusing through the plasma membrane Consequently, any change in their concentration alters the water concentration on the two sides of the membrane and results in a net loss or gain of water by the cell The ability of a solution to change the shape or tone of cells by altering their internal water volume is called tonicity

78 Tonicity (a):Solutions with the same concentration of nonpenetrating solutes as those found in cells (0.9% saline or 5% glucose) are isotonic (same tonicity) Cells exposed to such solutions retain their normal shape, and exhibit no net loss or gain of water The body’s extracellular fluids and most intravenous solutions (solutions infused into the body via a vein) are isotonic

79 TONICITIES

80 Tonicity (b):Solutions with a higher concentration of nonpenetrating solutes than is seen in the cell (Example: strong saline solution) are hypertonic Cells immersed in hypertonic solutions lose water and shrink, or crenate

81 TONICITIES

82 Tonicity (c): Solutions that are more dilute (contain a lower concentration of nonpenetrating solutes) than cells are called hypotonic Cells placed in a hypotonic solution plump up rapidly as water rushes into them Distilled water represents the most extreme example of hypotonicity Because it contains NO solutes, water continues to enter cells until they finally burst or lyse

83 TONICITIES

84 Osmolarity vs Tonicity
Osmolarity and tonicity are not the same thing A solution’s osmolarity is based solely on its total solute concentration A solution’s tonicity is based on how the solution affects cell volume, which depends on: 1. Solute concentration 2. Solute permeability of the plasma membrane

85 Osmolarity Expressed as osmoles per liter (osmol/L) where 1 osmol is equal to 1 mole of nonionizing molecules Determined by multiplying molarity (moles per liter, or M) by the number of particles resulting from ionization: Example: Since NaCl ionizes to Na+ + Cl-, a 1-M solution of NaCl is a 2-Osm solution Substances that do not ionize (e.g., glucose), molarity and osmolarity are the same

86 Osmolarity A 0.3-osmol/L solution of NaCl is isotonic because sodium ions are usually prevented from diffusing through the plasma membrane But if the cell is immersed in a 0.3-osmol/L solution of a penetrating solute, the solute will enter the cell and water will follow Cell will swell and burst, just as if it had been placed in pure water

87 Osmotic and Hydrostatic Pressure
Osmosis is extremely important in determining distribution of water in the various fluid-containing compartments of the body (in cells, in blood, etc.) Osmosis continues until osmotic and hydrostatic pressures acting at the membrane are equal Example: Water is forced out of capillary blood by the hydrostatic pressure of the blood against the capillary wall, but the presence in blood of solutes that are too large to cross the capillary membrane draws water back into the bloodstream As a result, very little net loss of plasma fluid occurs

88 HOMEOSTATIC IMBALANCE
Hypertonic solutions are sometimes infused intravenously into the bloodstream of edematous patients (those swollen because water is retained in their tissues) to draw excess water out of the extracellular space and move it into the bloodstream so that it can be eliminated by the kidneys Hypotonic solutions may be used (with care) to rehydrate the tissues of extremely dehydrated patients In less extreme cases of dehydration, drinking hypotonic fluids (colas, apple juice, and sports drinks) usually does the trick

89 Passive Process Filtration
Filtration is a pressure-driven process that forces water and solutes through a membrane or capillary wall by fluid, or hydrostatic pressure Is a passive transport process that involves a pressure gradient: Pushes solute-containing fluid (filtrate) from a higher-pressure area to a lower-pressure area Hydrostatic pressure: Exerted by blood forces fluid out of the capillaries containing solutes that are vital to the tissues Provides the fluid ultimately excreted by the kidneys as urine Not selective: only blood cells and protein molecules too large to pass through membrane pores or the paracellular path (between cells) are held

90 Active Processes Use energy (ATP) to move substances across a membrane
Substances moved actively across the plasma membrane are usually unable to pass in the necessary direction by any passive transport processes Substances may be too large to pass through the channels, incapable of dissolving in the lipid bilayer, or unable to move down its concentration gradient There are two major mechanisms of active membrane transport: Active transport Vesicular transport

91 Active Transport Similar to facilitated diffusion in that both require carrier proteins that combine specifically and reversibly with the transported substances HOWEVER, facilitated diffusion always honors concentration gradients because its driving force is kinetic energy IN CONTRAST, the active transporters or solute pumps move solutes, most importantly ions (such as Na+, K+, and Ca2+), uphill against a concentration gradient To do this work, cells must expend the energy of ATP Very selective involving chemicals that cannot pass by diffusion Classified according to their energy source

92 Na+K PUMPS

93 Primary Active Transport
Energy to do work comes directly from hydrolysis of ATP Results in the phosphorylation of the transport protein, a step that causes the protein to change its conformation in such a manner that it “pumps” the bound solute across the membrane

94 Na+K PUMPS

95 Primary Active Transport
Sodium-potassium pump: Carrier is an enzyme called Na+-K+ ATPase The concentration of K+ inside the cell is times higher than that outside The concentration of Na+ outside the cell is times higher than that inside

96 Na+K PUMPS

97 Primary Active Transport
Because Na+ and K+ leak slowly but continuously through channels in the plasma membrane along their concentration gradient (and cross rapidly in stimulated muscles and nerve cells), the Na+-K+ pump operates more or less continuously as an antiport to simultaneously drive Na+ out of the cell against a steep concentration gradient and pump K+ back in

98 Na+K PUMPS

99 Primary Active Transport
Calcium pumps actively segregate ionic calcium from the intracellular fluid into specific organelles or eject it from the cell

100 Secondary Active Transport
Transport is driven indirectly by energy stored in ionic gradients created by operation of primary active transport pumps Are all coupled systems (move more than one substance at a time) If the two transported substances are moved in the same direction, the system is a symport system (sym=same) If the transported substances cross the membrane in opposite directions, the system is an antiport system (anti=opposite, against)

101 SECONDARY ACTIVE TRANSPORT

102 Secondary Active Transport
A single ATP-powered pump, such as the Na+-K+ pump, can indirectly drive the secondary active transport of several other solutes Moving sodium (normally higher outside) across the plasma membrane against its concentration gradient (from inside to outside), the pump stores energy in the ion gradient (potential) Just as water pumped uphill can do work as it flows down (turning a turbine or water wheel)

103 SECONDARY ACTIVE TRANSPORT

104 Secondary Active Transport
A substance pumped across a membrane can do work as it leaks back, downhill along its concentration gradient Thus, as sodium moves back into the cell with the help of a carrier protein (facilitated diffusion), other substances are dragged along or cotransported by a common carrier protein Some sugars, amino acids, and many ions are cotransported in this way into cells lining the small intestine Though the cotransported substances both move passively, Na+ has to be pumped back into the lumen of the intestine to maintain its diffusion gradient

105 SECONDARY ACTIVE TRANSPORT

106 Vesicular Transport Energized by either:
ATP (adenosine triphosphate) GTP (guanosine triphosphate) Means by which large particles, macromolecules, and fluids are transported across the plasma membrane, or within the cell Exocytosis is a process used to move substances from inside the cell to the extracellular environment Endocytosis is a process used to move substances from the extracellular environment into the cell Transcytosis is a process that moves substances into, across, and then out of the cell Vesicular trafficking is a process that moves substances from one area in the cell to another (or organelle) Vesicle moves from one organelle to another (ER to Golgi Apparatus)

107 Exocytosis Typically stimulated by a cell-surface signal such as:
Binding of a hormone to a membrane receptor, accounts for hormone secretion Neurotransmitter release Mucus secretion In some cases, ejection of wastes

108 Exocytosis The substance to be removed from the cell is first enclosed in a membranous sac called a vesicle Vesicle migrates to the plasma membrane, fuses with it, and then ruptures, spilling the sac contents out of the cell Special transmembrane proteins on the vesicle called v-SNAREs (v for vesicles) recognize certain plasma membrane proteins, called t-SNAREs (t for target), and bind Lipid layers corkscrew without mixing

109 EXOCYTOSIS

110 Endocytosis, Transcytosis, Vesicular Trafficking
Involve the use of an assortment of protein-coated vesicles of three types and, with some exceptions, all are mediated by membrane receptors

111 Clathrin-coated Vesicles
Provide the main route for endocytosis and transcytosis of bulk solids, most macromolecules, and fluids Substance to be taken into the cell by endocytosis is progressively enclosed by an infolding of the plasma membrane called a coated pit Bristlelike clathrin protein coating on the cytoplasmic face of the vesicle Acts to deform the membrane to produce the vesicle and in cargo selection

112 ENDOCYTOSIS

113 Clathrin-coated Vesicles
Once inside, the vesicle loses its fuzzy coat and then typically fuses with a processing and sorting vesicle called an endosome 1. Contents of the endosome may be recycled back to the plasma membrane (many receptors and membrane components)

114 ENDOCYTOSIS

115 Clathrin-coated Vesicles
2. Combined with a lysosome, a specialized cell structure containing digestive enzymes, where the ingested substance is degraded or released (iron or cholesterol)

116 ENDOCYTOSIS

117 Clathrin-coated Vesicles
3. Transported completely across the cell and released by exocytosis on the opposite side (transcytosis) Common in the endothelial cells lining blood vessels because it provides a quick means to get substances from the blood to the interstitial fluid

118 ENDOCYTOSIS

119 Clathrin-coated Vesicles
Three types of endocytosis using clathrin-coated vesicles are: Phagocytosis Pinocytosis Receptor-mediated endocytosis

120 Phagocytosis Type of endocytosis in which some relatively large or solid material, such as a clump of bacteria or cell debris, is engulfed by the cell Cytoplasmic extensions called pseudopods form and flow around the particle and engulf it Vesicle formed is called a phagosome In most cases, the phagosome then fuses with a lysosome and its contents are digested In humans, certain WBC and macrophages

121 ENDOCYTOSIS

122 Pinocytosis Also called fluid-phase endocytosis (cell drinking)
Bit of infolding plasma membrane surrounds a very small volume of extracellular fluid containing dissolved molecules This droplet enters the cell in an endosome Unlike phagocytosis, pinocytosis is a routine activity of most cells, affording them a nonselective way of sampling the extracellular fluid Important in cells that absorb nutrients, such as cells that line the intestines

123 Receptor-mediated endocytosis
Very selective Main mechanism for the specific endocytosis and transcytosis of most macromolecules by body cells Receptors for this process are plasma membrane proteins that bind only with certain substances Both the receptors and attached molecules are internalized in a clathrin-coated pit Substances taken up include: Enzymes Insulin Some hormones Low-density lipoproteins (such as cholesterol attached to a transport protein) Iron UNFORTUNATELY: flu viruses and diphtheria toxin use this route to enter and attack our cells

124 ENDOCYTOSIS

125 Non-clathrin-coated Vesicles
Non-clathrin-coated vesicles, or caveolae, are inpocketings of the cell membrane that capture specific molecules in vesicles lined with caveolin, not clathrin Smaller than clathrin-coated vesicles Thinner and composed of a different protein (caveolin) Capture specific molecules (folate, tetanus toxin) Precise role in the cell is still being worked out

126 Coated Vesicles Clathrin (left) / Caveolin (right)

127 Generating and Maintaining a Resting Membrane Potential
A membrane potential is a voltage across the cell membrane that occurs due to a separation of oppositely charged particles (ions) The resting membrane potential is a condition in which the inside of the cell membrane is negatively charged compared to the outside, and ranges in voltage from -5 to -100 millivolts The minus sign before the voltage indicates that the inside of the cell is negative compared to its outside This voltage (or charge separation) exists ONLY at the membrane The resting membrane potential is determined mainly by the concentration gradient of potassium (K+), and by the differential permeability of the plasma membrane to K+ and other ions

128 Generating and Maintaining a Resting Membrane Potential
K+ and protein anions (negative) predominate inside body cells Extracellular fluid contains relatively more Na+, which is largely balanced by Cl- The unstimulated plasma membrane is somewhat permeable to K+ because of leakage channels, but impermeable to the protein anions Potassium therefore diffuses out of the cell along its concentration gradient but the protein anions (-) are unable to follow, so loss of positive charges makes the membrane interior more negative

129 MEMBRANE POTENTIAL

130 Generating and Maintaining a Resting Membrane Potential
As more and more K+ leaves the cell, the negativity of the inner membrane face becomes great enough to attract K+ back toward and even into the cell At the point where potassium concentration gradient is balanced by the membrane potential (-70 mV), one K+ enters the cell as one leaves, and the resting potential is established

131 Generating and Maintaining a Resting Membrane Potential
Other ions do contribute to the resting membrane potential, but only minimally Sodium is strongly attracted to the cell interior by its concentration gradient, BUT because the membrane is nearly impermeable to sodium, K+ outflow is not balanced by Na+ inflow and Cl- entry is resisted by the negative charge of the interior, even though the membrane is permeable to Cl-

132 Generating and Maintaining a Resting Membrane Potential
In a cell at rest, very few ions cross its plasma membrane However, Na+ and K+ are not at equilibrium and there is some net movement of K+ out of the cell and of Na+ into the cell because of its strong pull into the cell by both its concentration gradient and the interior negative charge If ONLY passive forces were at work, these ion concentrations would eventually become equal inside and outside the cell

133 Generating and Maintaining a Resting Membrane Potential
Instead, the cell exhibits a steady state in which diffusion causes ionic imbalances that polarizes the membrane, and active transport processes maintain that membrane potential The rate of active transport is equal to, and depends on, the rate of Na+ diffusion into the cell If more Na+ enters, more is pumped out (like a leaky boat, the more water comes in, the faster you have to pump out) The Na+-K+ pump couples sodium and potassium transport and, on average, each turn of the pump ejects 3Na+ out of the cell and carries 2K+ back in

134 Generating and Maintaining a Resting Membrane Potential
The membrane is always times more permeable to K+ ATP-dependent Na+-K+ pump maintains both the membrane potential and the osmotic balance Were Na+ NOT continuously removed from cells, in time so much would accumulate intracellularly that the osmotic gradient would draw water into the cells, causing them to burst

135 MEMBRANE POTENTIAL

136 MEMBRANE POTENTIAL

137 Cell-Environmental Interactions
Cells can interact directly with other cells, respond to extracellular chemicals, and interact with molecules that direct migration Whether cells interact directly or indirectly, the glycocalyx (externally facing glycoproteins on a cell’s plasma membrane) is always involved: Two large families: Cell adhesion molecules Plasma membrane receptors

138 Roles of Cell Adhesion Molecules (CAMs)
Are glycoproteins that play roles in embryonic development, wound repair, and immunity These sticky glycoproteins (cadherins and integrins) act as: 1. Molecular Velcro cells use to anchor themselves to molecules in the extracellular space and to each other 2. Arms that migrating cells use to haul themselves past one another 3. SOS signals sticking out from the blood vessel lining that rally protective white blood cells to a nearby infected or injured area 4. Mechanical sensors that respond to local tension at the cell surface by stimulating synthesis or degradation of adhesive membrane junctions

139 Roles of Membrane Receptors
Huge and diverse group of integral proteins and glycoproteins that serve as binding sites Some membrane receptors function in: Contact signaling Electrical signaling Chemical signaling

140 Contact Signaling The actual coming together and touching of cells, is the means by which cells recognize one another Important for normal development and immunity Some bacteria and other infectious agents use contact signaling to identify “preferred” target tissues or organs

141 Electrical Signaling Certain plasma membrane receptors are channel proteins that respond to changes in membrane potential by opening or closing the gates associated with an ion channel Common in excitable tissues like neural and muscle tissues

142 Chemical Signaling Most plasma membranes
Signaling chemicals that bind specifically to plasma membrane receptors are called ligands: Neurotransmitters (nervous system) Hormones (endocrine system) Paracines (chemicals that act locally and are rapidly destroyed) Different cells respond in different ways to the same ligand Example: Acetylcholine: stimulates skeletal cells to contract BUT inhibits cardiac (heart) cells Thus, a target cell’s response depends on the internal machinery that the receptor is linked to, not the specific ligand that binds to it Effects: Alter the shape of the receptor Initiate enzymatic reactions in the cell after combing with the receptor Example: G proteins Open or close ion gates or channels resulting in the excitability of the cell Nitric oxide NO: Consisting of one atom of oxygen and one atom of nitrogen Has one unpaired electron that makes it a highly reactive free radical that reacts rapidly with other key molecules An environmental pollutant First known gas to act as a biological messenger Important in the neural, cardiovascular, and immune system

143 G PROTEIN RECEPTORS

144 CYTOPLASM The cytoplasm is the cellular material between the cell (plasma) membrane and the nucleus, and is the site of most cellular activity There are three major elements of the cytoplasm: Cytosol: viscous, semitransparent fluid in which the other cytoplasmic elements are suspended Colloid and true solution properties Largely water with dissolved solutes (salts, sugars, etc.) Cytoplasmic organelles: metabolic machinery of the cell Engineered to carry out a specific function for the cell Cytoplasmic inclusions: chemical substances that may or may not be present, depending on the cell type Examples: stored nutrients: Glycogen: liver and muscle cells Lipid: fat cells Melanin (pigment): skin and hair cells

145 Cytoplasmic Organelles
Little organs Specialized cellular compartments, each performing its own job to maintain the life of the cell Membranous organelles: Bounded by a membrane similar in composition to the plasma membranre (minus the glycocalyx) This membrane enables them to maintain an internal environment different from that of the surrounding cytosol Examples: Mitochondria Peroxisomes Lysosomes Endoplasmic reticulum Golgi apparatus Nonmembranous organelles: Cytoskeleton Centrioles Ribosomes

146 Mitochondria Sausage-shaped membranous organelle
In living cells they squirm, elongate, and change shape almost continuously Power plants of the cell, providing most of its ATP supply Enclosed by two membranes, each with the general structure of the plasma membrane Outer membrane is smooth and featureless Inner membrane folds inward, forming shelflike cristae that protrude into the matrix, the gel-like substance within the mitochondrion Intermediate products of food fuels are broken down to water and carbon dioxide by teams of enzymes, some dissolved in the mitochondrial matrix and others forming part of the crista membrane

147 MITOCHONDRION

148 Mitochondria Site of aerobic respiration (requires oxygen)
Contain their own DNA and RNA and are able to reproduce themselves Capable of fission Contain approximately 37 genes that direct the synthesis of some proteins required for mitochondrial functions Believed that mitochondria arose from bacteria that invaded the ancestors of plant and animal cells

149 Ribosomes (a):Small staining granules consisting of protein and ribosomal RNA Each ribosome has two globular subunits that fit together Site of protein synthesis

150 Ribosome

151 Ribosomes Some float freely in the cytoplasm:
Make soluble proteins that function in the cytosol Some are attached to membranes, forming a complex called the rough endoplasmic reticulum: Synthesize proteins destined either for incorporation into cell membranes or for export from the cell Ribosomes can switch back-and-forth between the two types

152 Endoplasmic reticulum
Is an extensive system of interconnected tubes and parallel membranes enclosing fluid-filled cavities, called cisternae, that coils and twist throughout the cytosol Continuous with the nuclear membrane Two varieties: Rough ER Smooth ER

153 Rough Endoplasmic Reticulum
External surface is studded with ribosomes that: Manufacture all proteins that are secreted from cells Abundant in secretory cells, antibody-producing plasma cells, and liver cells, which produce most blood proteins Membrane factory: integral proteins and phospholipids that form part of all cellular membranes are manufactured

154 Rough Endoplasmic Reticulum Attachment of Ribosomes
1. Presence of a short signal sequence on a newly forming protein causes the mRNA-ribosome complex to be directed to the rough ER by a signal-recognition particle (SRP), which binds to a receptor site that includes a pore and an enzyme to clip the signal sequence

155 ER PROTEIN SYNTHESIS

156 Rough Endoplasmic Reticulum Attachment of Ribosomes
2. Once attached to the ER receptor site, the signal recognition site (SRP) is released and the growing polypeptide snakes through the ER membrane into the cristerna

157 ER PROTEIN SYNTHESIS

158 Rough Endoplasmic Reticulum Attachment of Ribosomes
3. The signal sequence initially remains attached to the receptor but shortly it is clipped off by an enzyme As protein synthesis continues, sugar groups (Y) may be added to the protein

159 ER PROTEIN SYNTHESIS

160 Rough Endoplasmic Reticulum Attachment of Ribosomes
4. In this example, the completed protein is released from the ribosome and folds into its 3-D conformation, a process aided by molecular chaperones Transmembrane proteins are only partially translocated and remain embedded in the membrane

161 ER PROTEIN SYNTHESIS

162 Rough Endoplasmic Reticulum Attachment of Ribosomes
5. The protein is enclosed within a coatomer—coated transport vesicle that pinches off the ER The transport vesicles make their way to the Golgi apparatus, where further processing of the proteins occurs

163 ER PROTEIN SYNTHESIS

164 Smooth ER Continuation of rough ER, consisting of a looping network of tubules Its enzymes (all integral proteins forming part of its membranes) play no role in protein synthesis Instead, they catalyze reactions involved in several processes: 1. Lipid metabolism, cholesterol synthesis, and synthesis of the lipid components of lipoproteins (in liver cells) 2. Synthesis of steroid-based hormones such as sex hormones (testosterone-synthesizing cells of the testes are full of smooth ER)

165 Smooth ER 3. Absorption, synthesis, and transport of fats (in intestinal cells) 4. Detoxification of drugs, certain pesticides, and carcinogens (in liver and kidneys) 5. Breakdown of stored glycogen to form free glucose (in liver cells especially) 6. Important role in calcium ion storage and release during muscle contraction in skeletal and cardiac muscle

166 ENDOPLASMIC RETICULUM

167 Golgi Apparatus Is a series of stacked, flattened, membranous sacs, shaped like hollow dinner plates, associated with swarms of tiny groups of membranous vesicles The main function of the Golgi apparatus is to modify, concentrate, and package the proteins and lipids made at the rough ER The transport vesicles that bud off from the rough ER move to and fuse with the membranes at its convex cis face (receiving side), of the Golgi apparatus

168 GOLGI APPARATUS

169 Golgi Apparatus Inside the apparatus, the proteins are modified:
Some sugar groups are trimmed while others are added, and in some cases, phosphate groups are added Various proteins are tagged for delivery to a specific address, sorted, and packaged in at least three types of vesicles that bud from the concave trans face (shipping side) of the Golgi stack 1.Vesicles containing proteins destined for export pinch off from the trans face as secretory vesicles, or granules, which migrate to the plasma membrane and discharge their contents from the cell by exocytosis (Pathway 1) Specialized secretory cells, such as the enzyme-producing cells of the pancreas, have a very prominent Golgi apparatus 2.The Golgi apparatus creates vesicles containing lipids and transmembrane proteins for incorporation into the cell membrane or other membranous organelles (Pathway 2) 3.Packages digestive enzymes into the membranous lysosomes that remain in the cell (Pathway 3)

170 GOLGI ROLE

171 Lysosomes Spherical membranous organelles that contain digestive enzymes Abundant in phagocytes, the cells that dispose of invading bacteria and cell debris Digest almost all kinds of biological molecules functioning best in acidic environments Thus called acid hydrolases The lysosomal membrane is adapted to serve lysosomal functions in two important ways: 1.Contains H+ (proton) pumps, ATPases that gather hydrogen ions from the surrounding cytosol to maintain the organelle’s acidic pH 2.It retains the dangerous acid hydrolases while permitting the final products of digestion to escape so that they can be used by the cell or excreted Hence, lysosomes provide sites where digestion can proceed safely within a cell

172 LYSOSOMES

173 Lysosomes Function as a cell’s demolition crew:
Digesting particles taken in by endocytosis, particularly ingested bacteria, viruses, and toxins Degrading worn-out or nonfunctional organelles Performing metabolic functions, such as glycogen breakdown and release Breaking down nonuseful tissues, such as the webs between fingers and toes of a developing fetus and the uterine lining during menstruation Breaking down bone to release calcium ions into the blood

174 HOMEOSTASIS IMBALANCE
Tay-Sachs Disease: Mostly in Jews from Central Europe Congenital (at birth) Listlessness, motor weakness progressing to mental retardation, seizures, and ultimately death within 18 months Glycogen and certain lipids in the brain are degraded by lysosomes at a relatively constant rate BUT in this disease the lysosomes lack an enzyme needed to break down a glycolipid abundant in nerve cell membranes As a result, the nerve cell lysosomes swell with undigested lipids, which interfere with nervous system functioning

175 Endomembrane System System of organelles that work together mainly:
1. To produce, store, and export biological molecules 2. To degrade potentially harmful substances Includes the ER, Golgi apparatus, secretory vesicles, lysosomes, and nuclear membrane The plasma membrane, though not actually an endo membrane, is also functionally part of this system

176 ENDOMEMBRANES

177 Peroxisomes Membranous sacs containing a variety of powerful enzymes, such as: Oxidases: Use molecular oxygen (O2) to detoxify harmful substances, including alcohol and formaldehyde Most important function is to neutralize dangerous free radicals, highly reactive chemicals with unpaired electrons that can scramble the structure of biological molecules Oxidase converts free radicals to hydrogen peroxide, which is also reactive and dangerous but is quickly converted to water by catalase enzymes Catalases: Converts hydrogen peroxide to water Free radicals and hydrogen peroxide are normal by-products of cellular metabolism, but they have devastating effects on cells if allowed to accumulate Numerous in liver and kidney cells, which are very active in detoxification Look like small lysosomes but they are self-replicating organelles formed by a simple pinching in half of preexisting peroxisomes Unlike lysosomes, they do not arise by budding from the Golgi apparatus

178 Cytoskeleton Series of rods running through the cytosol, supporting cellular structures and aiding in cell movement There are three types of rods in the cytoskeleton:not covered by membranes Microtubules Microfilaments Intermediate filaments

179 Microtubules Largest diameter
Hollow tubes made of spherical protein subunits called tubulins Most radiate from a small region of cytoplasm near the nucleus called the centrosome Constantly growing from the centrosome, disassembling, and then reassembling

180 CYTOSKELETON

181 CENTRIOLES

182 Microtubules Stiff but bendable
Determine the overall shape of the cell, as well as the distribution of cellular organelles: Mitochondria, lysosomes, and secretory granules attach to the microtubules like ornaments hanging from the limb of a Christmas Tree These organelles are continually pulled along the microtubules and repositioned by motor proteins (kinesins, dyneins, and others)

183 CYTOSKELETON

184 Microfilaments Thinnest elements of the cytoskeleton
Strands of the protein actin (ray) Each cell has its own unique arrangements (NO TWO CELLS ARE ALIKE) Nearly all cells have a fairly dense cross-linked network of microfilaments attached to the cytoplasmic side of their plasma membrane that strengthens the cell surface

185 CYTOSKELETON

186 Microfilaments Most are involved in cell motility or changes in cell shape You can say that cells move when they get their act(in) together (b): Muscle cells: actin interact with myosin (protein) Form cleavage furrow that pinches a cell in two during cell division Responsible for the membrane changes in endocytosis and exocytosis Movement of cilia

187 CYTOSKELETON

188 Intermediate Filaments
Tough, insoluble protein fibers that have a diameter between those of microfilaments and microtubules Constructed like woven ropes Most stable and permanent of the cytoskeletal elements High tensile strength Act as internal guy wires to resist pulling forces exerted on the cell, and they attach to desmosomes Protein composition varies in different cell types resulting in numerous names: Examples: Nerve cells: neurofilaments Epithelial cells: keratin filaments

189 CYTOSKELETON

190 Centrosome and Centrioles
The centrosome is a region near the nucleus in which a group of microtubules is anchored The centrosome functions as a microtubule organizing center: Other than the granular-looking matrix it contains paired centrioles Small, barrel-shaped organelles oriented at right angles to each other Consists of a pinwheel array of nine triplets of microtubules, arranged to form a hollow tube Form the bases of cilia and flagella

191 CENTRIOLES

192 Centrosome and Centrioles
The centrosome matrix is best known for its generation of microtubules and its role of organizing the mitotic spindle during cell division

193 MITOSIS

194 Cellular Extensions Cilia and Flagella
Cilia are whiplike, motile cellular extensions that occur, typically in large numbers, on the exposed surfaces of some cells Ciliary action is important in moving substances in one direction across cell surfaces Example: Ciliated cells that line the respiratory tract propel mucus laden with dust particles and bacteria upward away from the lungs When a cell is about to form cilia, the centrioles multiply and line up beneath the plasma membrane at the free cell surface Microtubules then begin to sprout from each centriole, forming the ciliary projections by exerting pressure on the plasma membrane When the projections formed by centrioles are substantially longer, they are called flagella Notice that cilia propel other substances across a cell’s surface, whereas a flagellum propels the cell itself

195 Cellular Extensions Cilia and Flagella
Centrioles forming the bases of cilia and flagella are commonly referred to as basal bodies The pattern of microtubules in the cilium or flagellum itself (9 doublets, or pairs, of microtubules encircling one central pair) differs slightly from that of a centriole (9 microtubule triplets)

196 CENTRIOLES

197 Cilia/Flagella

198 Cilia and Flagella Microtubules are definitely involved in the coordinated cilia activity Extending from the microtubule doublets are arms composed of the motor protein dynein Cilia move when the dynein side arms grip adjacent doublets and start to crawl along their length The collective bending action of all the doublets causes the cilium to bend

199 Cilia/Flagella

200 Cilia and Flagella As a cilium moves, it alternates rhythmically between a propulsive power stroke, when it is nearly straight and moves in an arc, and a recovery stroke, when it bends and returns to its initial position With these two strokes, the cilium produces a pushing motion in a single direction The activity of cilia in a particular region is coordinated so that the bending of one cilium is quickly followed by the bending of the next and then the next, creating a current at the cell surface that brings to mind the traveling waves that pass across a field of grass on a windy day

201 CILIA

202 THE NUCLEUS The nucleus is the control center of the cell and contains the cellular DNA Most cells have only one nucleus, but very large cells may be multinucleate Presence of more than one nucleus usually signifies that a larger-than-usual cytoplasmic mass must be regulated All body cells except mature red blood cells (anucleate) have nuclei The nucleus is larger than the cytoplasmic organelles It has three regions: Nuclear envelope (membrane) Nucleoli Chromatin

203 NUCLEUS

204 Nuclear Envelope Is a double-membrane barrier (separated by a fluid-filled space) surrounding the nucleus Outer membrane is continuous with the rough ER of the cytoplasm and is studded with ribosomes on its external face Inner membrane is lined by a network of protein filaments ( the nuclear lamina) that maintains the shape of the nucleus At various points, nuclear pores penetrate areas where the membranes of the nuclear envelope fuse A complex of proteins, called a pore complex, lines each nuclear pore and regulates passage of large particles into and out of the nucleus Like other cell membranes, the nuclear envelope is selectively permeable, but here passage of substances is much freer than elsewhere Protein molecules imported from the cytoplasm and RNA molecules exported from the nucleus pass easily through the relatively large pores The nuclear envelope encloses the fluid and solutes of the nucleus, the nucleoplasm

205 NUCLEUS

206 Nucleoli Dark-staining spherical bodies within the nucleus
NOT membrane bound There are typically one or two nucleoli per nucleus, but there may be more Site of the assembly of ribosomal subunits: Therefore, large in actively growing cells that are making large amounts of tissue proteins

207 NUCLEUS

208 Nucleoli Associated with nucleolar organizer regions, which contain the DNA that issues genetic instructions for synthesizing ribosomal RNA (rRNA) As molecules of rRNA are synthesized, they are combined with proteins to form the two kinds of ribosomal subunits (the proteins are manufactured on ribosomes in the cytoplasm and imported into the nucleus) Most of these subunits leave the nucleus through the nuclear pores and enter the cytoplasm, where they join to form functional ribosomes

209 NUCLEUS

210 Chromatin (a): Appears as a fine, unevenly stained network, but special techniques reveal it as a system of bumpy threads weaving their way through the nucleoplasm Is roughly half DNA, the genetic material of the cell, and half globular histone proteins: Nucleosomes are the fundamental unit of chromatin, consisting of discus-shaped cores or clusters of eight histone proteins connected like beads on a string by a DNA molecule DNA winds around each nucleosome and continues on to the next cluster via linker DNA segments

211 CHROMOSOMES

212 Chromatin Histones provide physical means for packing the very long DNA molecules in a compact, orderly way, they also play an important role in gene regulation: In a nondividing cell, addition of phosphate or methyl groups to histone exposes different DNA segments, or genes, so that they can dictate the specifications for protein synthesis When a cell is preparing to divide, chromatin condenses into dense, rodlike chromosomes Chromosome compactness avoids entanglement and breakage of the delicate chromatin strands during the movements that occur during cell division

213 CHROMOSOMES

214 CELL GROWTH and REPRODUCTION
The Cell Life Cycle A series of changes a cell goes through from the time it is formed to the time it reproduces, encompasses two major periods: Interphase: in which the cell grows and carries on its usual activities Cell Division (mitotic phase): during which it divides into two cells

215 CELL CYCLE

216 Interphase Period from cell formation to cell division during which it is metabolically very active and growing (metabolic phase or growth phase) while preparing for the next cell division

217 CELL CYCLE

218 Interphase Subphases Interphase is the period from cell formation to cell division, and has three subphases: In all three subphases, the cell grows by producing proteins and organelles, however, chromatin is reproduced only during the S phase G1 (gap 1): cell is synthesizing proteins and actively growing: Metabolically very active Time variable depending on the cell type Minutes to years Cells that permanently cease dividing are said to be in the Go phase During this phase virtual no activity is related to cell division However, as this phase ends, the centrioles start to replicate in preparation for cell division

219 CELL CYCLE

220 Interphase Subphases S (synthetic) phase: DNA is replicated
Ensuring that the two future cells being created will receive identical copies of the genetic material New histones are made and assembled into chromatin

221 CELL CYCLE

222 Interphase Subphases G2 (gap 2): Final phase of Interphase Very brief
Enzyme and other proteins needed for division are synthesized and distributed throughout the cell By the end of G2, centriole replication (begun in G1) is complete The cell is now ready to divide Throughout S and G2, the cell continues to grow and carries on with business as usual

223 CELL CYCLE

224 DNA DNA: deoxyribonucleic acid Double stranded Deoxyribose sugar
Nitrogenous bases: A=adenine T=thymine C=cytosine G=guanine

225 DNA Replication Before a cell can divide, its DNA must be replicated exactly, so that identical copies of the cell’s genes can be passed on to each of its offspring Replication begins simultaneously on several chromatin threads and continues until all the DNA has been replicated Takes place when: 1.DNA helices begin unwinding from the nucleosomes 2. (b): Helicase enzyme untwists the double helix, the hydrogen bonds between its base pairs are broken and the DNA molecule separates into two complementary nucleotide chains, exposing the nitrogenous bases The site of separation is called the replication bubble The Y-shaped region at each end of the replication bubble is called the replication fork 3.Gradually the DNA molecule separates

226 DNA REPLICATION

227 DNA Replication 4.The DNA helix uncoils, and each nucleotide strand of the DNA acts as a template for the construction of a complementary nucleotide strand from free DNA precursors dissolved in the nucleoplasm Nucleotide base pairing is always complementary: Adenine (A) bonds to Thymine (T) Guanine (G) bonds to Cytosine (C) Hence, the order of the nucleotides on the template strand determines the order on the strand being built Example: A TACTGC sequence on a template strand would bond to new nucleotides with the order ATGACG

228 DNA REPLICATION

229 DNA Replication 5. (a): At sites where DNA synthesis is to occur, the needed machinery gradually accumulates until several different proteins (mostly enzymes) are present in a large complex called a replisome The actual initiation of DNA synthesis requires formation of short (about 10 bases long) RNA primers by primase enzymes which are part of the replisome These primers are eventually replaced by DNA nucleotides

230 DNA REPLICATION

231 DNA Replication 6. Once the primer is in place, DNA polymerase III comes into the picture Continuing from the primer, it positions complementary nucleotides along the template strand and then covalently links them together DNA polymerase works only in one direction Consequently, one strand, the leading strand, is synthesized continuously following the movement of the replication fork The other strand, called the lagging strand, is constructed in segments in the opposite direction and requires that a primer initiate replication of each segment

232 DNA REPLICATION

233 DNA Replication 7.(b): The short segments of DNA are then spliced together by DNA ligase The end result is that two DNA molecules are formed from the original DNA helix and are identical to it Each new molecule consists of one old and new nucleotide strand This mechanism of DNA replication is referred to as semiconservative replication Replication also involves the generation of two new telomeres (tel=end; mer=piece), snugly fitting nucleoprotein caps that prevent degradation of the ends of the chromatin strands

234 DNA REPLICATION

235 DNA Replication 8. As soon as replication ends, histones (synthesized in the cytoplasm and imported into the nucleus) associate with the DNA, completing the formation of two new chromatin strands The chromatin strands, united by a buttonlike centromere (believed to be a stretch of repetitive DNA), condense to form chromatids The chromatids remain attached, held together by the centromere and a protein complex called cohesin, until the cell has entered the anaphase stage of mitotic cell division They are then distributed to the daughter cells ensuring that each has identical genetic information

236 CHROMOSOMES

237 Cell Division Process necessary for growth and tissue repair
In most body cells, cell division, which is called the M (mitotic) phase of the cell life cycle, involves two distinct events: Mitosis: division of the nucleus Cytokinesis: division of the cytoplasm

238 Mitosis Series of events that parcel out the replicated DNA of the mother cell to two daughter cells Four phases: actually a continuous process, with one phase merging smoothly into the next: Prophase Metaphase Anaphase Telophase

239 CELL CYCLE

240 MITOSIS

241 MITOSIS

242 Cytokinesis Division of the cytoplasm, begins during late anaphase and is completed after mitosis ends The plasma membrane over the center of the cell (the spindle equator) is drawn inward to form a cleavage furrow by the activity of a contractile ring made of actin filaments The furrow deepens until the cytoplasmic mass is pinched into two parts, so that at the end of cytokinesis there are two daughter cells Each is smaller and has less cytoplasm than the mother cell, but is genetically identical to it The daughter cells then enter the interphase portion of the life cycle until it is their turn to divide

243 Control of Cell Division
The signals that prod cells to divide are poorly understood, but we know that the ratio of cell surface area to cell volume is important The amount of nutrients a growing cell requires is directly related to its volume Volume increases with the cube of cell radius, whereas surface area increases with the square of the radius Example: A 64-fold (43) increase in cell volume is accompanied by only a 16-fold (42) increase in surface area Consequently, the surface area of the plasma membrane becomes inadequate for nutrient and waste exchange when a cell reaches a certain critical size Cell division solves this problem because the smaller daughter cells have a favorable surface-area-to-volume ratio These surface-volume relationships help explain why most cells are microscopic in size

244 Interphase Period of a cell’s life when it is carrying out its normal metabolic activities and growing Toward the end of this phase: Microtubule arrays (asters) are seen extending from the centrosomes Centrioles begin replicating (G1 through G2 phases) DNA is replicated (S phase) Final preparations for mitosis are completed (G2 phase) Centriole pair finishes replicating into two pairs

245 MITOSIS

246 Early Prophase Chromatin threads coil and condense, forming barlike chromosomes Since DNA replication has occurred during interphase, each chromosome is actually made up of two identical chromatin threads Chromatids of each chromosome are held together by a small, buttonlike body called a centromere and a protein complex called cohesin After the chromatids separate, each is considered a new chromosome

247 MITOSIS

248 Early Prophase As the chromosomes appear, the nucleoli disappear
Cytoskeletal microtubules disassemble Centriole pairs separate from one another Act as focal points for growth of a new assembly of microtubules called the mitotic spindle As these tubules lengthen, they push the centrioles farther and farther apart, propelling them toward opposite ends (poles) of the cell

249 MITOSIS

250 Late Prophase While the centrioles are still moving away from each other, the nuclear envelope fragments, allowing the spindle to occupy the center of the cell and to interact with the chromosomes Meanwhile, some of the growing spindle microtubules attach to special protein-DNA complexes, called kinetochores, at each chromosome’s centromere Such microtubules are called kinetochore microtubules

251 MITOSIS

252 Late Prophase The remaining spindle microtubules, which do not attach to any chromosomes, are called polar microtubules The tips of the polar microtubules are linked near the center an push against each other forcing the poles apart The kinetochore microtubules pull on each chromosome from both poles resulting in a tug-of-war that ultimately draws the chromosomes to the middle of the cell

253 MITOSIS

254 Metaphase Chromosomes cluster at the middle of the cell, with their centromeres precisely aligned at the exact center (equator) of the spindle An enzyme called separase cleaves cohesin, triggering separation of the chromatids at the metaphase-anaphase transition

255 MITOSIS

256 Anaphase Begins abruptly as the centromeres of the chromosomes split, and each chromatid now becomes a chromosome in its own right The kinetochore fibers, moved along by motor proteins in the kinetochores, rapidly disassemble at their kinetochore ends by removing tubulin subunits, and gradually pull each chromosome toward the pole it faces

257 MITOSIS

258 Anaphase Polar microtubules slide past each other and lengthen (process presumed to be driven by kinesin motor molecules), and push the two poles of the cell apart, causing the cell to elongate This process of moving and separating the chromosomes is helped by the fact that the chromosomes are short, compact bodies Diffuse threads of extended chromatin would tangle, trail, and break, which would damage the genetic material and result in its imprecise division and movement to the daughter cells

259 MITOSIS

260 Telophase and Cytokinesis
Telophase begins as soon as chromosomal movement stops The identical sets of chromosomes at the opposite poles of the cell uncoil and resume their threadlike extended-chromatin form The new nuclear envelope, derived from the rough ER, re-forms around each chromatin mass Nucleoli reappear within the nuclei, and the spindle breaks down and disappears Mitosis is now ended; The cell, for just a brief period, is binucleate (has two nuclei) and each new nucleus is identical to the original mother nucleus

261 MITOSIS

262 Telophase and Cytokinesis
As mitosis draws to a close, cytokinesis completes the division of the cell into two daughter cells Cytokinesis occurs as a contractile ring of peripheral microfilaments forms at the cleavage furrow and squeezes the cells apart Cytokinesis actually begins during late anaphase and continues through and beyond telophase

263 MITOSIS

264 Control of Cell Division
Two other mechanisms that influence when cells divide are: Chemical signals released by other cells: Growth factors, hormones, etc. Availability of space Normal cells stop proliferating when they begin touching, a phenomenon called contact inhibition Cancer cells lack many of the normal controls and therefore divide wildly, which makes them dangerous to their host

265 Control of Cell Division
The stepwise processes of the cell cycle are timed by rhythmic fluctuations in the activity of protein kinases (enzymes that catalyzes the transfer of phosphate from ATP to an acceptor) called cyclin-dependent kinases (Cdks), because they are active only when bound to a cyclin, a protein whose concentration varies cyclically

266 Control of Cell Division
A Cdk-cyclin (cyclin-dependent kinase) complex called MPF (M-phase promoting factor) acts at the G2 phase to trigger mitosis (a): the graph shows how MPF activity fluctuates with the level of cyclin in the cell The cyclin level rises throughout interphase (G1, S, and G2 phases), then falls abruptly during mitosis (M phase) The peaks of MPF activity and cyclin concentration correspond Cdk is present at a constant level (NOT SHOWN)

267 CELL CYCLE

268 Control of Cell Division
(b)(1): By the G2 checkpoint (black bar), enough cyclin is available to produce many molecules of MPF (M-phase promoting factor) (2): MPF promotes mitosis by phosphorylating various proteins, including other enzymes

269 CELL CYCLE

270 Control of Cell Division
3. One effect of MPF is the initiation of a sequence of events leading to the breakdown of its own cyclin 4. The Cdk (cyclin-dependent kinase) component of MPF (M-phase promoting factor) is recycled Its kinase activity will be restored by association with new cyclin that accumulates during interphase

271 CELL CYCLE

272 Cell Division There are three main events of cell division
Mitosis is the process of nuclear division in which cells contain all genes Meiosis is the process of nuclear division found only in egg and sperm cells in which the cells have half the genes found in other body cells Cytokinesis is the process of dividing the cytoplasm Control of cell division depends on surface-volume relationship, chemical signaling, and contact inhibition

273 Protein Synthesis In addition to directing its own replication, DNA serves as the master blueprint for protein synthesis: Although cells also make lipids and carbohydrates, DNA does not dictate their structure DNA specifies ONLY the structure of protein molecules, including the enzymes that catalyze the synthesis of all classes of biological molecules that act as structural or functional molecules Proteins are composed of polypeptide chains made up of amino acids Each gene is a segment of DNA that carries instructions for one polypeptide chain

274 Protein Synthesis The four nucleotide bases (A, G, T, and C) are the letters of the genetic dictionary, and the information of DNA is found in the sequence of these bases Each sequence of three bases, called a triplet, can be thought of as a word that specifies a particular amino acid Example: AAA codes for amino acid phenylalanine CCT codes for glycine The sequence of triplets in each gene forms a sentence that tells exactly how a particular polypeptide is to be made; it specifies the number, kinds, and order of amino acids needed to build a particular polypeptide Most genes of higher organisms contain exons (coding region) that specify amino acid informational sequences separated by noncoding segments called introns (noncoding space between the exons of the DNA of a gene)

275 Role of RNA DNA NEVER leaves the nucleus: RNA: ribonucleic acid
Protein synthesis takes place at the ribosomes DNA requires not only a decoder, but also a messenger The decoding and messenger functions are carried out by RNA RNA: ribonucleic acid Single stranded Ribose sugar Base uracil (U) instead of thymine (T)

276 Role of RNA RNA exists in three forms that decode and carry out the instructions of DNA in protein synthesis: Transfer RNA (tRNA): Small, roughly cloverleaf-shaped molecules Transfer amino acids from the cytoplasm to the ribosomes Ribosomal RNA (rRNA): Part of the ribosome Messenger RNA (mRNA): Coded copy of DNA that carries genetic code from nucleus to the ribosome All three types of RNA are constructed on the DNA in the nucleus, then released from the DNA to migrate to the cytoplasm while the DNA recoils to its original form

277 Role of RNA Polypeptide synthesis involves two major steps:
Transcription: DNA information is encoded in mRNA Translation: the information carried by mRNA is decoded at the ribosomes and used to assemble polypeptides with the tRNA which transports the amino acids to the ribosome RNA code to Protein code

278 DNA to PROTEIN

279 Transcription Genetic code is rewritten (transformed) from one format (DNA) to another (mRNA) Once mRNA is made, it detaches and leaves the nucleus via nuclear pore Only DNA and mRNA are involved in the transcription process

280 Making the mRNA Complement
(a):Gene-activating chemical called a transcription factor stimulates loosening of the histones at the site-to-be of gene transcription and then binds to the promoter Promoter is a special DNA sequence adjacent to the startpoint of the structural gene that specifies where mRNA synthesis starts and which DNA strand is going to serve as the template strand Once gene activation has occurred, transcription is ready to begin

281 TRANSCRIPTION

282 Making the mRNA Complement
To make the mRNA complement, the transcription factor mediates binding of RNA polymerase, an enzyme that directs the synthesis of mRNA Once bound, the RNA polymerase unwinds base pairs of the DNA helix at a time Using incoming ribonucleoside triphosphates as subtrates, it aligns them with complementary DNA bases on the template strand and joins the RNA nucleotides together: Example: If a particular DNA triplet is AGC, the mRNA sequence synthesized at that site will be UCG When the polymerase codes a special sequence called a termination signal, transcription ends and the newly formed messenger pulls off the DNA template

283 TRANSCRIPTION

284 Making the mRNA Complement
For each triplet, or three-base sequence, on DNA, the corresponding three-base sequence on mRNA is called a codon: Since there are four kinds of RNA (or DNA) nucleotides, there are 43, or 64, possible codons Three of these 64 codons are stop messages that call for termination of a polypeptide All the rest code for amino acids Since there are only about 20 amino acids, some are specified by more one codon

285 GENETIC CODE

286 Translation Translation is the process of converting the language of nucleic acids (base nucleotides sequence) to the language of proteins (amino acids sequence) Occurs in the cytoplasm and involves all three varieties of RNA When it reaches the cytoplasm, the mRNA molecule carrying instructions for a particular protein binds to a small ribosomal subunit by base pairing to rRNA Then tRNA transfers amino acids, dissolved in the cytoplasm, to the ribosome There are approximately 20 different types of tRNA, each capable of binding with a specific amino acid The attachment process is controlled by a synthetase enzyme and is activated by ATP Once its amino acid is loaded, the tRNA migrates to the ribosome, where it maneuvers the amino acid into the proper position, as specified by the mRNA codons

287 Translation

288 Translation Shaped like a handheld drill, tRNA is well suited to its dual function The amino acid is bound to one end of tRNA, at a region called the stem At the other end, the head, is its anticodon, a three-base sequence complementary to the mRNA codon calling for the amino acid carried by that particular tRNA Anticodons form hydrogen bonds with complementary codons, meaning that a tRNA is the link between the language of nucleic acids and the language of proteins Thus, if the mRNA codon is UUU, which specifies phenylalanine, the tRNAs carrying phenylalanine will have the anticodon AAA, which can bind to the correct codons

289 Translation

290 Translation The ribosome is more than just a passive attachment site for mRNA and tRNA Besides its binding site for mRNA, it has three binding sites for tRNA: An “A” (aminoacyl) site for incomong tRNA A “P” (peptidyl) site for the tRNA holding the growing polypeptide chain An “E” (exit) site for outgoing tRNA Like a vise, the ribosome holds the tRNA and mRNA close together to coordinate the coupling of codons and anticodons, and positions the next (incoming) amino acid for addition to the growing polypeptide chain

291 Translation

292 Translation When the initiator tRNA binds to the mRNA, it occupies the P site on the ribosome and the A and E sites are vacant Now the ribosome slides the mRNA strand along, bringing the next codon into position to be read by an aminoacyl-tRNA coming into the A site (1) It is at this point that the ribosome does a little proofreading to make sure of the codon-anticodon match That accomplished, (2) an enzyme in the large ribosomal particle bonds the amino acid of the initiator tRNA to that of the tRNA at the A site (3): The ribosome then translocates the tRNA that is now carrying two amino acids to the P site, and ratchets the initiator tRNA to the E site, (4) from which it moves away from the ribosome Musical chair situation continues: peptidyl-tRNAs transferring their polypeptide cargo to the amino-acyl-tRNAs, and then the P site to E site, and A site to P site movements of the tRNAs

293 Translation

294 Translation As mRNA is progressively read, its initial portion passes through the ribosome and may become attached successively to several other ribosomes, all reading the same message simultaneously and sequentially Such a multiple ribosome-mRNA complex is called a polyribosome, and it provides an efficient system for producing many copies of the same protein The mRNA strand continues to be read sequentially until its last codon, the stop codon enters the ribosomal groove The completed polypeptide chain is then released from the ribosome, and the ribosome separates into its two subunits

295 POLYRIBOSOME

296 INFORMATION: DNA to RNA

297 DNA FINGERPRINT

298 EXTRACELLULAR MATERIALS
Substances contributing to body mass that are found outside the cells There are three classes of extracellular material: Body fluids consist mainly of interstitial fluid, blood plasma, and cerebrospinal fluid, and are important to transport and solute dissolution Cellular secretions include substances aiding in digestion (intestinal and gastric fluids) or functioning as lubrication (saliva, mucus, and serous fluids) Extracellular matrix is a jellylike substance consisting of proteins and polysaccharides Helps to hold body cells together

299 DEVELOPMENTAL ASPECTS OF CELLS
Embryonic and Fetal Development of Cells Embryonic cells are exposed to different chemical signals that cause them to follow different pathways in development Chemical signals influence development by switching genes on and off Cell differentiation is the process of cells developing specific and distinctive features Apoptosis is the programmed cell death of stressed, surplus developing cells

300 DEVELOPMENTAL ASPECTS OF CELLS
Development of Cells Through Adolescence Most organ systems are well formed and functional before birth The body continues to form new cells throughout childhood and adolescence During young adulthood, cell numbers remain relatively constant, but local changes in the rate of cell division are common

301 DEVELOPMENTAL ASPECTS OF CELLS
Effect of Aging on Cells The wear and tear theory considers the cumulative effect of slight chemical damage and the production of free radicals Cell aging may also be a result of autoimmune responses and progressive weakening of the immune response The genetic theory of cell aging suggests that cessation of mitosis and cell aging are genetically programmed


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