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Chapter 3: The Living Parts

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1 Chapter 3: The Living Parts

2 Cell Theory The cell is the basic living unit Organismal functions depend on individual and collective cell functions Biochemical activities of cells are dictated by their specific subcellular structures Continuity of life has a cellular basis

3 Cell Diversity Over 200 different types of human cells
Types differ in size, shape, subcellular components, and functions

4 Generalized Cell All cells have some common structures and functions
Human cells have three basic parts: Plasma membrane—flexible outer boundary Cytoplasm—intracellular fluid containing organelles Nucleus—control center

5 Plasma Membrane Also called cell membrane
Layer of lipids and proteins in a constantly changing fluid mosaic Plays a dynamic role in cellular activity Separates intracellular fluid (ICF) from extracellular fluid (ECF) Interstitial fluid (IF) = ECF that surrounds cells 75% phospholipids (lipid bilayer) Phosphate heads: polar and hydrophilic Fatty acid tails: nonpolar and hydrophobic (Review Fig. 2.16b) 5% glycolipids Lipids with polar sugar groups on outer membrane surface 20% cholesterol Increases membrane stability and fluidity

6 Membrane Proteins Integral proteins
Firmly inserted into the membrane (most are transmembrane) Functions: Transport proteins (channels and carriers), enzymes, or receptors Transport Receptors for signal transduction Attachment to cytoskeleton and extracellular matrix Enzymatic activity Intercellular joining Cell-cell recognition

7 Membrane Proteins Peripheral proteins
Loosely attached to integral proteins Include filaments on intracellular surface and glycoproteins on extracellular surface Functions: Enzymes, motor proteins, cell-to-cell links, provide support on intracellular surface, and form part of glycocalyx

8 Membrane Junctions Three types: Tight junction
Prevent fluids and most molecules from moving between cells Desmosome “Rivets” or “spot-welds” that anchor cells together Gap junction Transmembrane proteins form pores that allow small molecules to pass from cell to cell For spread of ions between cardiac or smooth muscle cells

9 Types of Membrane Transport
Passive processes No cellular energy (ATP) required Substance moves down its concentration gradient Active processes Energy (ATP) required Occurs only in living cell membranes Plasma membranes are selectively permeable Some molecules easily pass through the membrane; others do not

10 Passive Processes What determines whether or not a substance can passively permeate a membrane? Lipid solubility of substance Channels of appropriate size Carrier proteins

11 Carrier-mediated facilitated diffusion
Passive Processes Simple diffusion Carrier-mediated facilitated diffusion Channel-mediated facilitated diffusion Osmosis Diffusion Nonpolar lipid-soluble (hydrophobic) substances diffuse directly through the phospholipid bilayer Facilitated Diffusion Certain lipophobic molecules (e.g., glucose, amino acids, and ions) use carrier proteins or channel proteins Exhibit specificity (selectivity) Are saturable; rate is determined by number of carriers or channels Can be regulated in terms of activity and quantity Carrier Proteins Transmembrane integral proteins transport specific polar molecules (e.g., sugars and amino acids) Binding of substrate causes shape change in carrier Aqueous channels formed by transmembrane proteins selectively transport ions or water Two types: Leakage channels Always open Gated channels Controlled by chemical or electrical signals

12 Tonicity: The ability of a solution to cause a cell to shrink or swell
Isotonic: A solution with the same solute concentration as that of the cytosol Hypertonic: A solution having greater solute concentration than that of the cytosol Hypotonic: A solution having lesser solute concentration than that of the cytosol When osmosis occurs, water enters or leaves a cell Change in cell volume disrupts cell function

13 Figure 3.9 (a) Isotonic solutions (b) Hypertonic solutions
(c) Hypotonic solutions Cells retain their normal size and shape in isotonic solutions (same solute/water concentration as inside cells; water moves in and out). Cells lose water by osmosis and shrink in a hypertonic solution (contains a higher concentration of solutes than are present inside the cells). Cells take on water by osmosis until they become bloated and burst (lyse) in a hypotonic solution (contains a lower concentration of solutes than are present in cells). Figure 3.9

14 Membrane Transport: Active Processes
Two types of active processes: Active transport Vesicular transport Three types Both use ATP to move solutes across a living plasma membrane Active transport Requires carrier proteins (solute pumps) Moves solutes against a concentration gradient Types of active transport: Primary active transport Energy from ATP causes shape change in transport protein so that solutes (ions) are “pumped” across the membrane Secondary active transport Depends on an ion gradient created by primary active transport Energy stored in ionic gradients is used indirectly to drive transport of other solutes Sodium-potassium pump (Na+-K+ ATPase) Located in all plasma membranes Involved in primary and secondary active transport of nutrients and ions Maintains electrochemical gradients essential for functions of muscle and nerve tissues Cotransport—always transports more than one substance at a time Symport system: Two substances transported in same direction Antiport system: Two substances transported in opposite directions Substances such as sugars, amino acids, and ions are cotransported Vesicular Transport Transport of large particles, macromolecules, and fluids across plasma membranes Requires cellular energy (e.g., ATP) Functions: Exocytosis—transport out of cell Examples: Hormone secretion Neurotransmitter release Mucus secretion Ejection of wastes Endocytosis—transport into cell Transcytosis—transport into, across, and then out of cell Substance (vesicular) trafficking—transport from one area or organelle in cell to another

15 Membrane Potential Separation of oppositely charged particles (ions) across a membrane creates a membrane potential (potential energy measured as voltage) Resting membrane potential (RMP): Voltage measured in resting state in all cells Ranges from –50 to –100 mV in different cells Results from diffusion and active transport of ions (mainly K+) Extremely important in the transmission of nerve impulses

16 Generation and Maintenance of RMP
The Na+ -K+ pump continuously ejects Na+ from cell and carries K+ back in K+ continually leaks out of cell through K+ leakage channels Membrane interior becomes negative (relative to exterior) because of large anions trapped inside cell

17 Generation and Maintenance of RMP
Electrochemical gradient begins to attract K+ back into cell RMP is established at the point where the electrical gradient balances the K+ concentration gradient A steady state is maintained because the rate of active transport is equal to and depends on the rate of Na+ diffusion into cell

18 1 2 3 K+ diffuse down their steep Extracellular fluid
concentration gradient (out of the cell) via leakage channels. Loss of K+ results in a negative charge on the inner plasma membrane face. 1 Extracellular fluid K+ also move into the cell because they are attracted to the negative charge established on the inner plasma membrane face. 2 A negative membrane potential (–90 mV) is established when the movement of K+ out of the cell equals K+ movement into the cell. At this point, the concentration gradient promoting K+ exit exactly opposes the electrical gradient for K+ entry. 3 Potassium leakage channels Protein anion (unable to follow K+ through the membrane) Cytoplasm Figure 3.15

19 Cell-Environment Interactions
Involves glycoproteins and proteins of glycocalyx Cell adhesion molecules (CAMs) Membrane receptors

20 Roles of Cell Adhesion Molecules
Anchor cells to extracellular matrix or to each other Assist in movement of cells past one another CAMs of blood vessel lining attract white blood cells to injured or infected areas Stimulate synthesis or degradation of adhesive membrane junctions Transmit intracellular signals to direct cell migration, proliferation, and specialization

21 Roles of Membrane Receptors
Contact signaling—touching and recognition of cells; e.g., in normal development and immunity Chemical signaling—interaction between receptors and ligands (neurotransmitters, hormones and paracrines) to alter activity of cell proteins (e.g., enzymes or chemically gated ion channels) G protein–linked receptors—binding activates a G protein, affecting an ion channel or enzyme or causing the release of an internal second messenger

22 Second Messenger System

23 Cytoplasm Located between plasma membrane and nucleus Cytosol
Water with solutes (protein, salts, sugars, etc.) Cytoplasmic organelles Metabolic machinery of cell Inclusions Granules of glycogen or pigments, lipid droplets, vacuoles, and crystals Inclusions may be present or may not

24 Cytoplasmic Organelles
Membranous Mitochondria Peroxisomes Lysosomes Endoplasmic reticulum Golgi apparatus Nonmembranous Cytoskeleton Centrioles Ribosomes

25 Mitochondria Double-membrane structure with shelflike cristae
Provide most of cell’s ATP via aerobic cellular respiration Contain their own DNA and RNA

26 Ribosomes Granules containing protein and rRNA
Site of protein synthesis Free ribosomes synthesize soluble proteins Membrane-bound ribosomes (on rough ER) synthesize proteins to be incorporated into membranes or exported from the cell

27 Endoplasmic Reticulum (ER)
Interconnected tubes and parallel membranes enclosing cisternae Continuous with nuclear membrane Two varieties: Rough ER Smooth ER Rough ER External surface studded with ribosomes Manufactures all secreted proteins Synthesizes membrane integral proteins and phospholipids Smooth ER Tubules arranged in a looping network Enzyme (integral protein) functions: In the liver—lipid and cholesterol metabolism, breakdown of glycogen, and, along with kidneys, detoxification of drugs, pesticides, and carcinogens Synthesis of steroid-based hormones In intestinal cells—absorption, synthesis, and transport of fats In skeletal and cardiac muscle—storage and release of calcium

28 Golgi Apparatus Stacked and flattened membranous sacs
Modifies, concentrates, and packages proteins and lipids Secretory vesicles leave trans face of Golgi stack and move to designated parts of cell

29 Lysosomes Spherical membranous bags containing digestive enzymes (acid hydrolases) Digest ingested bacteria, viruses, and toxins Degrade nonfunctional organelles Break down and release glycogen Break down bone to release Ca2+ Destroy cells in injured or nonuseful tissue (autolysis)

30 Endomembrane System Overall function
Produce, store, and export biological molecules Degrade potentially harmful substances

31 Peroxisomes Membranous sacs containing powerful oxidases and catalases Detoxify harmful or toxic substances Neutralize dangerous free radicals (highly reactive chemicals with unpaired electrons)

32 Cytoskeleton Elaborate series of rods throughout cytosol Microtubules
Determine overall shape of cell and distribution of organelles Microfilaments Involved in cell motility, change in shape, endocytosis and exocytosis Intermediate filaments Resist pulling forces on the cell and attach to desmosomes

33 Centrosome “Cell center” near nucleus
Generates microtubules; organizes mitotic spindle Contains centrioles: Small tube formed by microtubules

34 Cellular Extensions Cilia and flagella
Whiplike, motile extensions on surfaces of certain cells Contain microtubules and motor molecules Cilia move substances across cell surfaces Longer flagella propel whole cells (tail of sperm)

35 Cellular Extensions Microvilli
Fingerlike extensions of plasma membrane Increase surface area for absorption Core of actin filaments for stiffening

36 Nucleus Cellular control center Most cells are uninucleate
Red blood cells are anucleate Skeletal muscle cells, bone destruction cells, and some liver cells are multinucleate Genetic library with blueprints for nearly all cellular proteins Responds to signals and dictates kinds and amounts of proteins to be synthesized

37 Nuclear Envelope Double-membrane barrier containing pores Outer layer is continuous with rough ER and bears ribosomes Inner lining (nuclear lamina) maintains shape of nucleus Pore complex regulates transport of large molecules into and out of nucleus

38 Nucleoli Dark-staining spherical bodies within nucleus Involved in rRNA synthesis and ribosome subunit assembly

39 Chromatin Threadlike strands of DNA (30%), histone proteins (60%), and RNA (10%) Arranged in fundamental units called nucleosomes Condense into barlike bodies called chromosomes when the cell starts to divide

40 Defines changes from formation of the cell until it reproduces
Cell Cycle Defines changes from formation of the cell until it reproduces Includes: Interphase Cell division (mitotic phase)

41 Period from cell formation to cell division
Interphase Period from cell formation to cell division Nuclear material called chromatin Four subphases: G1 (gap 1)—vigorous growth and metabolism G0—gap phase in cells that permanently cease dividing S (synthetic)—DNA replication G2 (gap 2)—preparation for division

42 DNA Replication DNA helices begin unwinding from the nucleosomes Helicase untwists the double helix and exposes complementary chains The Y-shaped site of replication is the replication fork Each nucleotide strand serves as a template for building a new complementary strand

43 DNA polymerase only works in one direction
DNA Replication DNA polymerase only works in one direction Continuous leading strand is synthesized Discontinuous lagging strand is synthesized in segments DNA ligase splices together short segments of discontinuous strand

44 DNA Replication End result: two DNA molecules formed from the original
This process is called semiconservative replication

45 Mitotic (M) phase of the cell cycle
Cell Division Mitotic (M) phase of the cell cycle Essential for body growth and tissue repair Does not occur in most mature cells of nervous tissue, skeletal muscle, and cardiac muscle

46 Includes two distinct events:
Cell Division Includes two distinct events: Mitosis—four stages of nuclear division: Prophase Metaphase Anaphase Telophase Cytokinesis—division of cytoplasm by cleavage furrow

47 Cytokinesis Begins during late anaphase Ring of actin microfilaments contracts to form a cleavage furrow Two daughter cells are pinched apart, each containing a nucleus identical to the original

48 Telophase and Cytokinesis
Nuclear envelope forming Nucleolus forming Contractile ring at cleavage furrow Telophase and Cytokinesis Telophase Figure 3.33

49 Control of Cell Division
“Go” signals: Critical volume of cell when area of membrane is inadequate for exchange Chemicals (e.g., growth factors, hormones, cyclins, and cyclin-dependent kinases (Cdks))

50 Control of Cell Division
“Stop” signals: Contact inhibition Growth-inhibiting factors produced by repressor genes

51 Protein Synthesis DNA is the master blueprint for protein synthesis Gene: Segment of DNA with blueprint for one polypeptide Triplets of nucleotide bases form genetic library Each triplet specifies coding for an amino acid

52 Roles of the Three Main Types of RNA
Messenger RNA (mRNA) Carries instructions for building a polypeptide, from gene in DNA to ribosomes in cytoplasm Ribosomal RNA (rRNA) A structural component of ribosomes that, along with tRNA, helps translate message from mRNA Transfer RNAs (tRNAs) Bind to amino acids and pair with bases of codons of mRNA at ribosome to begin process of protein synthesis

53 Transcription Transfers DNA gene base sequence to a complementary base sequence of an mRNA Steps DNA unwound and RNA polymerase attaches to begin copy Two strands of mRNA are generated and sent from nucleus to ribosome for translation

54 Translation Steps mRNA passes through ribosome where is will be met by tRNA. Anticodon on tRNA matches codon on mRNA for transfer of amino acid. Amino acid strings generate a polypeptide which ultimately generate a protein.

55 Each three-base sequence on DNA is represented by a codon
Genetic Code Each three-base sequence on DNA is represented by a codon Codon—complementary three-base sequence on mRNA


57 Other Roles of DNA Intron (“junk”) regions of DNA code for other types of RNA: Antisense RNA Prevents protein-coding RNA from being translated MicroRNA Small RNAs that interfere with mRNAs made by certain exons Riboswitches Folded RNAs that act as switches regulating protein synthesis in response to environmental conditions

58 Extracellular Materials
Body fluids (interstitial fluid, blood plasma, and cerebrospinal fluid) Cellular secretions (intestinal and gastric fluids, saliva, mucus, and serous fluids) Extracellular matrix (abundant jellylike mesh containing proteins and polysaccharides in contact with cells)

59 Theories of Cell Aging Wear and tear theory: Little chemical insults and free radicals have cumulative effects Immune system disorders: Autoimmune responses and progressive weakening of the immune response Genetic theory: Cessation of mitosis and cell aging are programmed into genes. Telomeres (strings of nucleotides on the ends of chromosomes) may determine the number of times a cell can divide.

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