Life at the Edge The plasma membrane is the boundary that separates the living cell from its nonliving surroundings The plasma membrane exhibits selective.

Life at the Edge The plasma membrane is the boundary that separates the living cell from its nonliving surroundings The plasma membrane exhibits selective permeability, allowing some substances to cross it more easily than others

Cellular membranes are fluid mosaics of lipids and proteins
Phospholipids are the most abundant lipid in the plasma membrane Phospholipids are amphipathic molecules, containing hydrophobic and hydrophilic regions The fluid mosaic model states that a membrane is a fluid structure with a “mosaic” of various proteins embedded in it

WATER Hydrophilic head Hydrophobic tail WATER

Hydrophilic region of protein Phospholipid bilayer Hydrophobic region of protein

The Fluidity of Membranes
Phospholipids in the plasma membrane can move within the bilayer Most of the lipids, and some proteins, drift laterally Rarely does a molecule flip-flop transversely across the membrane

Lateral movement (~107 times per second) Flip-flop (~ once per month) Movement of phospholipids

As temperatures cool, membranes switch from a fluid state to a solid state
The temperature at which a membrane solidifies depends on the types of lipids Membranes rich in unsaturated fatty acids are more fluid than those rich in saturated fatty acids Membranes must be fluid to work properly; they are usually about as fluid as salad oil

Unsaturated tails prevent packing. Saturated tails pack together.
Fluid Viscous Unsaturated tails prevent packing. Saturated tails pack together. (a) Unsaturated versus saturated hydrocarbon tails (b) Cholesterol reduces membrane fluidity at moderate temperatures, but at low temperatures hinders solidification. Figure 5.5 Factors that affect membrane fluidity Cholesterol 9

Fluid Viscous Unsaturated hydrocarbon tails with kinks Saturated hydrocarbon tails Membrane fluidity

The steroid cholesterol has different effects on membrane fluidity at different temperatures
At warm temperatures (such as 37°C), cholesterol restrains movement of phospholipids At cool temperatures, it maintains fluidity by preventing tight packing

Cholesterol Cholesterol within the animal cell membrane

Some proteins in the plasma membrane can drift within the bilayer
Proteins are much larger than lipids and move more slowly

Membrane proteins Mouse cell Mixed proteins after 1 hour Human cell Hybrid cell

Membrane Proteins and Their Functions
A membrane is a collage of different proteins embedded in the fluid matrix of the lipid bilayer Proteins determine most of the membrane’s specific functions Peripheral proteins are not embedded Integral proteins penetrate the hydrophobic core and often span the membrane

Fibers of extracellular matrix (ECM) Glycoprotein Carbohydrate Glycolipid EXTRACELLULAR SIDE OF MEMBRANE Cholesterol Microfilaments of cytoskeleton Peripheral proteins Integral protein CYTOPLASMIC SIDE OF MEMBRANE

Integral proteins that span the membrane are called transmembrane proteins
The hydrophobic regions of an integral protein consist of one or more stretches of nonpolar amino acids, often coiled into alpha helices

EXTRACELLULAR SIDE N-terminus C-terminus CYTOPLASMIC SIDE a Helix

Six major functions of membrane proteins:
Transport Enzymatic activity Signal transduction Cell-cell recognition Intercellular joining Attachment to the cytoskeleton and extracellular matrix (ECM)

Transport Enzymatic activity Signal transduction Signal Enzymes
Receptor ATP Transport Enzymatic activity Signal transduction

Cell-cell recognition Intercellular joining Attachment to the
Glyco- protein Cell-cell recognition Intercellular joining Attachment to the cytoskeleton and extracellular matrix (ECM)

The Role of Membrane Carbohydrates in Cell-Cell Recognition
Cells recognize each other by binding to surface molecules, often carbohydrates, on the plasma membrane Membrane carbohydrates may be covalently bonded to lipids (forming glycolipids) or more commonly to proteins (forming glycoproteins) Carbohydrates on the external side of the plasma membrane vary among species, individuals, and even cell types in an individual

Synthesis and Sidedness of Membranes
Membranes have distinct inside and outside faces The asymmetrical distribution of proteins, lipids and associated carbohydrates in the plasma membrane is determined when the membrane is built by the ER and Golgi apparatus

ER Transmembrane glycoproteins Secretory protein Glycolipid Golgi apparatus Vesicle Plasma membrane: Cytoplasmic face Extracellular face Transmembrane glycoprotein Secreted protein Plasma membrane:

Membrane structure results in selective permeability
A cell must exchange materials with its surroundings, a process controlled by the plasma membrane Plasma membranes are selectively permeable, regulating the cell’s molecular traffic

The Permeability of the Lipid Bilayer
Hydrophobic (nonpolar) molecules, such as hydrocarbons, can dissolve in the lipid bilayer and pass through the membrane rapidly Polar molecules, such as sugars, do not cross the membrane easily

Transport Proteins Transport proteins allow passage of hydrophilic substances across the membrane Some transport proteins, called channel proteins, have a hydrophilic channel that certain molecules or ions can use as a tunnel Channel proteins called aquaporins facilitate the passage of water

Other transport proteins, called carrier proteins, bind to molecules and change shape to shuttle them across the membrane A transport protein is specific for the substance it moves

Passive transport is diffusion of a substance across a membrane with no energy investment
Diffusion is the tendency for molecules to spread out evenly into the available space Although each molecule moves randomly, diffusion of a population of molecules may exhibit a net movement in one direction At dynamic equilibrium, as many molecules cross one way as cross in the other direction

Diffusion of one solute
Molecules of dye Membrane (cross section) WATER Net diffusion Net diffusion Equilibrium Diffusion of one solute

Substances diffuse down their concentration gradient, the difference in concentration of a substance from one area to another No work must be done to move substances down the concentration gradient The diffusion of a substance across a biological membrane is passive transport because it requires no energy from the cell to make it happen

Diffusion of two solutes
Net diffusion Net diffusion Equilibrium Net diffusion Net diffusion Equilibrium Diffusion of two solutes

Effects of Osmosis on Water Balance
Osmosis is the diffusion of water across a selectively permeable membrane The direction of osmosis is determined only by a difference in total solute concentration Water diffuses across a membrane from the region of lower solute concentration to the region of higher solute concentration

Lower concentration of solute (sugar) Higher concentration of sugar Same concentration of sugar H2O Selectively permeable membrane: sugar molecules cannot pass through pores, but water molecules can Osmosis

Water Balance of Cells Without Walls
Tonicity is the ability of a solution to cause a cell to gain or lose water Isotonic solution: solute concentration is the same as that inside the cell; no net water movement across the plasma membrane Hypertonic solution: solute concentration is greater than that inside the cell; cell loses water Hypotonic solution: solute concentration is less than that inside the cell; cell gains water

Animals and other organisms without rigid cell walls have osmotic problems in either a hypertonic or hypotonic environment To maintain their internal environment, such organisms must have adaptations for osmoregulation, the control of water balance The protist Paramecium, which is hypertonic to its pond water environment, has a contractile vacuole that acts as a pump

50 µm Filling vacuole 50 µm Contracting vacuole

Water Balance of Cells with Walls
Cell walls help maintain water balance A plant cell in a hypotonic solution swells until the wall opposes uptake; the cell is now turgid (firm) If a plant cell and its surroundings are isotonic, there is no net movement of water into the cell; the cell becomes flaccid (limp), and the plant may wilt In a hypertonic environment, plant cells lose water; eventually, the membrane pulls away from the wall, a usually lethal effect called plasmolysis

Hypotonic solution Isotonic solution Hypertonic solution Animal cell H2O H2O H2O H2O Lysed Normal Shriveled Plant cell H2O H2O H2O H2O Turgid (normal) Flaccid Plasmolyzed

Facilitated Diffusion: Passive Transport Aided by Proteins
In facilitated diffusion, transport proteins speed movement of molecules across the plasma membrane Channel proteins provide corridors that allow a specific molecule or ion to cross the membrane Carrier proteins undergo a subtle change in shape that translocates the solute-binding site across the membrane

EXTRACELLULAR FLUID Channel protein Solute CYTOPLASM

Carrier protein Solute

Active transport uses energy to move solutes against their gradients
Facilitated diffusion is still passive because the solute moves down its concentration gradient Some transport proteins, however, can move solutes against their concentration gradients

The Need for Energy in Active Transport
Active transport moves substances against their concentration gradient Active transport requires energy, usually in the form of ATP Active transport is performed by specific proteins embedded in the membranes The sodium-potassium pump is one type of active transport system

Cytoplasmic Na+ bonds to the sodium-potassium pump
EXTRACELLULAR FLUID [Na+] high [K+] low Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ [Na+] low [K+] high ATP P Na+ P CYTOPLASM ADP Cytoplasmic Na+ bonds to the sodium-potassium pump Na+ binding stimulates phosphorylation by ATP. Phosphorylation causes the protein to change its conformation, expelling Na+ to the outside. K+ K+ K+ K+ K+ P P K+ Extracellular K+ binds to the protein, triggering release of the phosphate group. Loss of the phosphate restores the protein’s original conformation. K+ is released and Na+ sites are receptive again; the cycle repeats.

Passive transport Active transport ATP Diffusion Facilitated diffusion

Maintenance of Membrane Potential by Ion Pumps
Membrane potential is the voltage difference across a membrane Two combined forces, collectively called the electrochemical gradient, drive the diffusion of ions across a membrane: A chemical force (the ion’s concentration gradient) An electrical force (the effect of the membrane potential on the ion’s movement)

An electrogenic pump is a transport protein that generates the voltage across a membrane
The main electrogenic pump of plants, fungi, and bacteria is a proton pump

– EXTRACELLULAR FLUID + ATP – + H+ H+ Proton pump H+ – + H+ H+ – + CYTOPLASM H+ – +

Cotransport: Coupled Transport by a Membrane Protein
Cotransport occurs when active transport of a solute indirectly drives transport of another solute Plants commonly use the gradient of hydrogen ions generated by proton pumps to drive active transport of nutrients into the cell

Sucrose-H+ cotransporter
– + ATP H+ H+ – + Proton pump H+ H+ – + H+ – + H+ Diffusion of H+ Sucrose-H+ cotransporter H+ – + – + Sucrose

Bulk transport across the plasma membrane occurs by exocytosis and endocytosis
Small molecules and water enter or leave the cell through the lipid bilayer or by transport proteins Large molecules, such as polysaccharides and proteins, cross the membrane via vesicles

Exocytosis In exocytosis, transport vesicles migrate to the membrane, fuse with it, and release their contents Many secretory cells use exocytosis to export their products

Endocytosis In endocytosis, the cell takes in macromolecules by forming vesicles from the plasma membrane Endocytosis is a reversal of exocytosis, involving different proteins

Three types of endocytosis:
Phagocytosis (“cellular eating”): Cell engulfs particle in a vacuole Pinocytosis (“cellular drinking”): Cell creates vesicle around fluid Receptor-mediated endocytosis: Binding of ligands to receptors triggers vesicle formation

An amoeba engulfing a bacterium via phagocytosis (TEM)
EXTRACELLULAR FLUID Pseudopodium of amoeba Solutes Pseudopodium Bacterium 1 m Food vacuole “Food” or other particle An amoeba engulfing a bacterium via phagocytosis (TEM) Figure 5.18a Exploring endocytosis in animal cells (part 1: phagocytosis) Food vacuole CYTOPLASM 56

0.25 m Pinocytosis Plasma membrane Coat protein
Pinocytotic vesicles forming (TEMs) Coated pit Figure 5.18b Exploring endocytosis in animal cells (part 2: pinocytosis) Coated vesicle 57

0.25 m Receptor-Mediated Endocytosis Coat Plasma protein membrane
Figure 5.18c Exploring endocytosis in animal cells (part 3: receptor-mediated endocytosis) Top: A coated pit Bottom: A coated vesicle forming during receptor- mediated endocytosis (TEMs) 58

Animations and Videos How Diffusion Works Diffusion Osmosis Bozeman - Osmosis Demo Bozeman - Diffusion Demo Plasmolysis Hemolysis and Crenation Contractile Vacuole

Animations and Videos Bozeman - Water Potential How Facilitated Diffusion Works Sodium-Potassium Pump Exchange Bozeman - Transport Across Cell Membrane Cotransport Proton Pump Amoeboid Movement Second Messengers (cAMP and Ca+2 Pathways)

Animations and Videos Chemical Synapse – 1 Chemical Synapse – 2 Voltage-Gated Channels and the Action Potential Clathrin-Coated Pits and Vesicles Receptors Linked to a Protein Channel Passive Transport Active Transport by Group Translocation

Animations and Videos Secondary Active Transport Organization of the Golgi Antiport Uniport...Carrier Protein Gated and Non-gated Channels Symport Cellulose Synthesis during Elongation Signal Transduction Pathway

Animations and Videos Signaling by Secreted Molecules Signal Transduction 2nd Messenger Signal Amplification – 1 Signal Amplification – 2 Cotranslational Targeting of Secretory Proteins to the ER Mechanism of Tyrosine Kinase

Animations and Videos Chapter Quiz Questions – 1 Chapter Quiz Questions - 2

Which of the following best describes the structure of a biological membrane?
two layers of phospholipids with proteins embedded between the two layers a mixture of covalently linked phospholipids and proteins that determines which solutes can cross the membrane and which cannot two layers of phospholipids with proteins either spanning the layers or on the surface of the layers a fluid structure in which phospholipids and proteins move freely between sides of the membrane two layers of phospholipids (with opposite orientations of the phospholipids in each layer) with each layer covered on the outside with proteins Answer: C

Which of the following best describes the structure of a biological membrane?
two layers of phospholipids with proteins embedded between the two layers a mixture of covalently linked phospholipids and proteins that determines which solutes can cross the membrane and which cannot two layers of phospholipids with proteins either spanning the layers or on the surface of the layers a fluid structure in which phospholipids and proteins move freely between sides of the membrane two layers of phospholipids (with opposite orientations of the phospholipids in each layer) with each layer covered on the outside with proteins

Which of the following statements about osmosis is correct?
If a cell is placed in an isotonic solution, more water will enter the cell than leaves the cell. Osmotic movement of water into a cell would likely occur if the cell accumulates water from its environment. The presence of aquaporins (proteins that form water channels in the membrane) should speed up the process of osmosis. If a solution outside the cell is hypertonic compared to the cytoplasm, water will move into the cell by osmosis. Osmosis is the diffusion of water from a region of lower water concentration to a region of higher water concentration. Answer: C 67

Which of the following statements about osmosis is correct?
If a cell is placed in an isotonic solution, more water will enter the cell than leaves the cell. Osmotic movement of water into a cell would likely occur if the cell accumulates water from its environment. The presence of aquaporins (proteins that form water channels in the membrane) should speed up the process of osmosis. If a solution outside the cell is hypertonic compared to the cytoplasm, water will move into the cell by osmosis. Osmosis is the diffusion of water from a region of lower water concentration to a region of higher water concentration. 68

a charged amino acid like lysine a polar amino acid like serine
Which of the following amino acids would most likely be present in the outer side of a transmembrane domain of an integral membrane protein? a charged amino acid like lysine a polar amino acid like serine a special amino acid like glycine or proline a hydrophobic amino acid like valine any of the above, with no preference Answer: D Transmembrane domains primarily consist of  helices of hydrophobic amino acids. 69

a charged amino acid like lysine a polar amino acid like serine
Which of the following amino acids would most likely be present in the outer side of a transmembrane domain of an integral membrane protein? a charged amino acid like lysine a polar amino acid like serine a special amino acid like glycine or proline a hydrophobic amino acid like valine any of the above, with no preference 70

Which of the following molecules will diffuse most quickly across a lipid bilayer membrane? [Hint: Which one has a nonpolar covalent bond that leads to no partial charge on the atoms?] H2O O2 H2PO4 glucose Na Answer: B 71

Which of the following molecules will diffuse most quickly across a lipid bilayer membrane? [Hint: Which one has a nonpolar covalent bond that leads to no partial charge on the atoms?] H2O O2 H2PO4 glucose Na Answer: B 72

receptor-mediated endocytosis phagocytosis facilitated diffusion
Cells (e.g., bacteria) are taken up by other cells (e.g., an immune cell) by which of the following? pinocytosis exocytosis receptor-mediated endocytosis phagocytosis facilitated diffusion Answer: D 73

receptor-mediated endocytosis phagocytosis facilitated diffusion
Cells (e.g., bacteria) are taken up by other cells (e.g., an immune cell) by which of the following? pinocytosis exocytosis receptor-mediated endocytosis phagocytosis facilitated diffusion 74

Consider the amino acids along the  helices of a membrane spanning protein (see figure). Those facing the fatty acid chains would be expected to be _____, while those facing the inner pore would often be ______. acidic; basic hydrophilic; hydrophobic aromatic; acidic hydrophobic; hydrophilic cysteines; glycines Answer: D

Consider the amino acids along the  helices of a membrane spanning protein (see figure). Those facing the fatty acid chains would be expected to be _____, while those facing the inner pore would often be ______. acidic; basic hydrophilic; hydrophobic aromatic; acidic hydrophobic; hydrophilic cysteines; glycines

Based on the current model of the membrane, which statement is incorrect?
Glycoproteins tend to have oligosaccharides on their outward facing side. Transmembrane proteins often bind with cytosolic proteins, but not with extracellular molecules. The combinations of phospholipids in the two faces of the membrane often differ. Phospholipids tend to move faster laterally along the membrane than do the proteins. Some transmembrane proteins function as active transport systems. Answer: B

Based on the current model of the membrane, which statement is incorrect?
Glycoproteins tend to have oligosaccharides on their outward facing side. Transmembrane proteins often bind with cytosolic proteins, but not with extracellular molecules. The combinations of phospholipids in the two faces of the membrane often differ. Phospholipids tend to move faster laterally along the membrane than do the proteins. Some transmembrane proteins function as active transport systems.

Assume that each of the following items experiences a similar magnitude of energy difference driving their diffusion across a pure lipid bilayer. If ranked in order from fastest to slowest, which of the following items would likely be second in terms of how much of it crosses the bilayer in a given time? molecular oxygen sucrose insulin glucose water Answer: E

As shown on the next slide, two solutions with similar solute concentration are separated by a membrane that allows only water to pass, and one solution is of greater volume than the other. Which choice best describes what will happen next? The energy in the pressure gradient will drive water to the left side, which will cause the solute concentration on the right side to increase. Water will pass to the right side and increase its volume. Any water that passes to the left side will create a tension on the right side, pulling the water back to the right side. The water will flow to the left side, down its pressure gradient, until the two sides achieve equal height. Since there is no concentration gradient present, there will be no net movement of water. Answer: A

Two solutions with similar solute concentration are separated by a membrane that allows only water to pass, and one solution is of greater volume than the other. Which choice best describes what will happen next? The energy in the pressure gradient will drive water to the left side, which will cause the solute concentration on the right side to increase. Water will pass to the right side and increase its volume. Any water that passes to the left side will create a tension on the right side, pulling the water back to the right side. The water will flow to the left side, down its pressure gradient, until the two sides achieve equal height. Since there is no concentration gradient present, there will be no net movement of water.

One strategy used by many animal species to avoid the need for their body cells to have either a stiff external cell wall or active contractile vacuoles is to have membranes that are impermeable to water and keep it from entering their cells. use aquaporins to actively pump water out through a hydrophilic pore. keep the concentration of solutes in their cytosol the same as that found in pond water. maintain a high internal pressure to constantly push extra water out of cells. carefully regulate the solute concentration of the extracellular solution to which their cells are exposed. Answer: E

One strategy used by many animal species to avoid the need for their body cells to have either a stiff external cell wall or active contractile vacuoles is to have membranes that are impermeable to water and keep it from entering their cells. use aquaporins to actively pump water out through a hydrophilic pore. keep the concentration of solutes in their cytosol the same as that found in pond water. maintain a high internal pressure to constantly push extra water out of cells. carefully regulate the solute concentration of the extracellular solution to which their cells are exposed.

An artificial liposome, whose membrane contains no proteins, is loaded with a 0.03 M sucrose solution and put into pure water (see figure on next slide). Which choice best describes what will quickly happen next? Since there are no membrane proteins, nothing will cross the membrane. Sucrose will diffuse out of the liposome. Water will diffuse down its concentration gradient into the liposome, causing it to burst. Water will enter the liposome, diluting the sucrose concentration there down to 0 M. The pressure of the surrounding solution will push water out of the liposome, causing it to shrink over time. Answer: C

An artificial liposome, whose membrane contains no proteins, is loaded with a 0.03 M sucrose solution and put into pure water. Which choice best describes what will quickly happen next? Since there are no membrane proteins, nothing will cross the membrane. Sucrose will diffuse out of the liposome. Water will diffuse down its concentration gradient into the liposome, causing it to burst. Water will enter the liposome, diluting the sucrose concentration there down to 0 M. The pressure of the surrounding solution will push water out of the liposome, causing it to shrink over time. Answer: C

A correct distinction between facilitated diffusion and active transport of a substance across a biological membrane is that active transport requires conformational changes in the transport protein associated with the transport process, and facilitated diffusion does not. active transport requires an integral membrane protein to carry out the transport, and facilitated diffusion does not. facilitated diffusion requires a protein lined pore in the membrane, and active transport does not. facilitated diffusion depends on an existing energy gradient acting on the transported substance, while active transport makes such a gradient. facilitated diffusion requires cellular energy (often from ATP hydrolysis), but active transport does not. Answer: D

A correct distinction between facilitated diffusion and active transport of a substance across a biological membrane is that active transport requires conformational changes in the transport protein associated with the transport process, and facilitated diffusion does not. active transport requires an integral membrane protein to carry out the transport, and facilitated diffusion does not. facilitated diffusion requires a protein lined pore in the membrane, and active transport does not. facilitated diffusion depends on an existing energy gradient acting on the transported substance, while active transport makes such a gradient. facilitated diffusion requires cellular energy (often from ATP hydrolysis), but active transport does not.

Consider various transport systems in a hypothetical cell (see figure)
Consider various transport systems in a hypothetical cell (see figure). Which one of these systems would both be a passive system and not alter the membrane potential through its operation? A B C D E Answer: D

Consider various transport systems in a hypothetical cell (see figure)
Consider various transport systems in a hypothetical cell (see figure). Which one of these systems would both be a passive system and not alter the membrane potential through its operation? A B C D E

In the hypothetical cell with the membrane transport systems and conditions shown (see figure), which represents a sodium ion channel, and energy gradients in what form influence the direction of movement of sodium ions through this channel? A; concentration gradient B; concentration and electrical gradients C; concentration gradient D; electrical gradient E; concentration and electrical gradients Answer: B

In the hypothetical cell with the membrane transport systems and conditions shown (see figure), which represents a sodium ion channel, and energy gradients in what form influence the direction of movement of sodium ions through this channel? A; concentration gradient B; concentration and electrical gradients C; concentration gradient D; electrical gradient E; concentration and electrical gradients

Receptor-mediated endocytosis produces vesicles that
typically deliver the items they take up to the nucleus of the cell. carry macromolecules and cells for delivery to the lysosomal compartment. assist in the removal of certain items from the cytosol of the cell. when formed cause there to be more total surface area available in the plasma membrane. have receptors which can indicate at their cytosolic side if ligands are bound or not on their extracellular/ lumen side.

Receptor-mediated endocytosis produces vesicles that
typically deliver the items they take up to the nucleus of the cell. carry macromolecules and cells for delivery to the lysosomal compartment. assist in the removal of certain items from the cytosol of the cell. when formed cause there to be more total surface area available in the plasma membrane. have receptors which can indicate at their cytosolic side if ligands are bound or not on their extracellular/ lumen side. Answer: E

Life at the Edge The plasma membrane is the boundary that separates the living cell from its nonliving surroundings The plasma membrane exhibits selective.

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Life at the Edge The plasma membrane is the boundary that separates the living cell from its nonliving surroundings The plasma membrane exhibits selective.

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Presentation on theme: "Life at the Edge The plasma membrane is the boundary that separates the living cell from its nonliving surroundings The plasma membrane exhibits selective."— Presentation transcript:

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