Membrane Structure and Function Chapter 7 Membrane Structure and Function

You should be able to: Define the following terms: amphipathic molecules, aquaporins, diffusion Explain how membrane fluidity is influenced by temperature and membrane composition Distinguish between the following pairs or sets of terms: peripheral and integral membrane proteins; channel and carrier proteins; osmosis, facilitated diffusion, and active transport; hypertonic, hypotonic, and isotonic solutions Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Explain how transport proteins facilitate diffusion Explain how an electrogenic pump creates voltage across a membrane, and name two electrogenic pumps Explain how large molecules are transported across a cell membrane Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Overview: Life at the Edge The plasma membrane is the boundary that separates the living cell from its surroundings The plasma membrane exhibits selective permeability, allowing some substances to cross it more easily than others Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Fig. 7-1 Figure 7.1 How do cell membrane proteins help regulate chemical traffic?

Concept 7.1: 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 For the Cell Biology Video Structure of the Cell Membrane, go to Animation and Video Files. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Membrane Models: Scientific Inquiry Membranes have been chemically analyzed and found to be made of proteins and lipids Scientists studying the plasma membrane reasoned that it must be a phospholipid bilayer Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

WATER Hydrophilic head Hydrophobic tail WATER Fig. 7-2 Figure 7.2 Phospholipid bilayer (cross section) Hydrophobic tail WATER

In 1935, Hugh Davson and James Danielli proposed a sandwich model in which the phospholipid bilayer lies between two layers of globular proteins Later studies found problems with this model, particularly the placement of membrane proteins, which have hydrophilic and hydrophobic regions In 1972, J. Singer and G. Nicolson proposed that the membrane is a mosaic of proteins dispersed within the bilayer, with only the hydrophilic regions exposed to water Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Phospholipid bilayer Hydrophobic regions of protein Hydrophilic Fig. 7-3 Phospholipid bilayer Figure 7.3 The fluid mosaic model for membranes Hydrophobic regions of protein Hydrophilic regions 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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

(a) Movement of phospholipids Fig. 7-5 Lateral movement (~107 times per second) Flip-flop (~ once per month) (a) Movement of phospholipids Fluid Viscous Unsaturated hydrocarbon tails with kinks Saturated hydro- carbon tails (b) Membrane fluidity Figure 7.5 The fluidity of membranes Cholesterol (c) Cholesterol within the animal cell membrane

(a) Movement of phospholipids Fig. 7-5a Lateral movement (107 times per second) Flip-flop ( once per month) Figure 7.5a The fluidity of membranes (a) 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 that those rich in saturated fatty acids Membranes must be fluid to work properly; they are usually about as fluid as salad oil Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Unsaturated hydrocarbon tails with kinks Saturated hydro- carbon tails Fig. 7-5b Fluid Viscous Unsaturated hydrocarbon tails with kinks Saturated hydro- carbon tails Figure 7.5b The fluidity of membranes (b) 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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

(c) Cholesterol within the animal cell membrane Fig. 7-5c Cholesterol Figure 7.5c The fluidity of membranes (c) Cholesterol within the animal cell membrane

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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Fig. 7-7 Fibers of extracellular matrix (ECM) Glyco- Carbohydrate protein Carbohydrate Glycolipid EXTRACELLULAR SIDE OF MEMBRANE Figure 7.7 The detailed structure of an animal cell’s plasma membrane, in a cutaway view Cholesterol Microfilaments of cytoskeleton Peripheral proteins Integral protein CYTOPLASMIC SIDE OF MEMBRANE

Peripheral proteins are bound to the surface of the membrane Integral proteins penetrate the hydrophobic core 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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

EXTRACELLULAR N-terminus SIDE C-terminus CYTOPLASMIC SIDE  Helix Fig. 7-8 EXTRACELLULAR SIDE N-terminus Figure 7.8 The structure of a transmembrane protein C-terminus CYTOPLASMIC SIDE  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) Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

(b) Enzymatic activity (c) Signal transduction Fig. 7-9ac Signaling molecule Enzymes Receptor Figure 7.9a–c Some functions of membrane proteins ATP Signal transduction (a) Transport (b) Enzymatic activity (c) Signal transduction

(d) Cell-cell recognition (e) Intercellular joining (f) Attachment to Fig. 7-9df Glyco- protein Figure 7.9d–f Some functions of membrane proteins (d) Cell-cell recognition (e) Intercellular joining (f) 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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

ER 1 Transmembrane glycoproteins Secretory protein Glycolipid Golgi 2 Fig. 7-10 ER 1 Transmembrane glycoproteins Secretory protein Glycolipid Golgi apparatus 2 Vesicle Figure 7.10 Synthesis of membrane components and their orientation on the resulting membrane 3 Plasma membrane: Cytoplasmic face 4 Extracellular face Transmembrane glycoprotein Secreted protein Membrane glycolipid

Concept 7.2: 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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

A transport protein is specific for the substance it moves 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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Concept 7.3: 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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Membrane (cross section) Fig. 7-11 Molecules of dye Membrane (cross section) WATER Net diffusion Net diffusion Equilibrium (a) Diffusion of one solute Figure 7.11 The diffusion of solutes across a membrane Net diffusion Net diffusion Equilibrium Net diffusion Net diffusion Equilibrium (b) Diffusion of two solutes

Membrane (cross section) Fig. 7-11a Molecules of dye Membrane (cross section) WATER Figure 7.11a The diffusion of solutes across a membrane Net diffusion Net diffusion Equilibrium (a) 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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

(b) Diffusion of two solutes Fig. 7-11b Net diffusion Net diffusion Equilibrium Figure 7.11b The diffusion of solutes across a membrane Net diffusion Net diffusion Equilibrium (b) Diffusion of two solutes

Effects of Osmosis on Water Balance Osmosis is the diffusion of water across a selectively permeable membrane Water diffuses across a membrane from the region of lower solute concentration to the region of higher solute concentration Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Higher concentration Lower Same concentration concentration of sugar Fig. 7-12 Lower concentration of solute (sugar) Higher concentration of sugar Same concentration of sugar H2O Selectively permeable membrane Figure 7.12 Osmosis 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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

cell cell Hypotonic solution Isotonic solution Hypertonic solution H2O Fig. 7-13 Hypotonic solution Isotonic solution Hypertonic solution H2O H2O H2O H2O (a) Animal cell Lysed Normal Shriveled H2O H2O H2O H2O Figure 7.13 The water balance of living cells (b) Plant cell Turgid (normal) Flaccid Plasmolyzed

Hypertonic or hypotonic environments create osmotic problems for organisms Osmoregulation, the control of water balance, is a necessary adaptation for life in such environments The protist Paramecium, which is hypertonic to its pond water environment, has a contractile vacuole that acts as a pump Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

(a) A contractile vacuole fills with fluid that enters from Fig. 7-14 50 µm Filling vacuole (a) A contractile vacuole fills with fluid that enters from a system of canals radiating throughout the cytoplasm. Contracting vacuole Figure 7.14 The contractile vacuole of Paramecium: an evolutionary adaptation for osmoregulation (b) When full, the vacuole and canals contract, expelling fluid from the cell.

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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

In a hypertonic environment, plant cells lose water; eventually, the membrane pulls away from the wall, a usually lethal effect called plasmolysis Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Facilitated Diffusion: Passive Transport Aided by Proteins In facilitated diffusion, transport proteins speed the passive movement of molecules across the plasma membrane Channel proteins provide corridors that allow a specific molecule or ion to cross the membrane Channel proteins include Aquaporins, for facilitated diffusion of water Ion channels that open or close in response to a stimulus (gated channels) For the Cell Biology Video Water Movement through an Aquaporin, go to Animation and Video Files. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Channel protein Solute (a) A channel protein Solute Carrier protein Fig. 7-15 EXTRACELLULAR FLUID Channel protein Solute CYTOPLASM (a) A channel protein Figure 7.15 Two types of transport proteins that carry out facilitated diffusion Solute Carrier protein (b) A carrier protein

Carrier proteins undergo a subtle change in shape that translocates the solute-binding site across the membrane Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Some diseases are caused by malfunctions in specific transport systems, for example the kidney disease cystinuria Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Concept 7.4: 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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

The sodium-potassium pump is one type of active transport system Active transport allows cells to maintain concentration gradients that differ from their surroundings The sodium-potassium pump is one type of active transport system For the Cell Biology Video Na+/K+ATPase Cycle, go to Animation and Video Files. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Cytoplasmic Na+ binds to Fig. 7-16-1 EXTRACELLULAR FLUID [Na+] high [K+] low Na+ Na+ [Na+] low Na+ Figure 7.16, 1 The sodium-potassium pump: a specific case of active transport CYTOPLASM [K+] high Cytoplasmic Na+ binds to the sodium-potassium pump. 1

Na+ binding stimulates Fig. 7-16-2 Na+ Na+ Na+ ATP P Figure 7.16, 2 The sodium-potassium pump: a specific case of active transport ADP Na+ binding stimulates phosphorylation by ATP. 2

Phosphorylation causes the protein to change its Fig. 7-16-3 Na+ Na+ Na+ Figure 7.16, 3 The sodium-potassium pump: a specific case of active transport P Phosphorylation causes the protein to change its shape. Na+ is expelled to the outside. 3

extracellular side and triggers release of the phosphate group. Fig. 7-16-4 K+ K+ P Figure 7.16, 4 The sodium-potassium pump: a specific case of active transport P K+ binds on the extracellular side and triggers release of the phosphate group. 4

restores the protein’s original shape. Fig. 7-16-5 K+ K+ Figure 7.16, 5 The sodium-potassium pump: a specific case of active transport Loss of the phosphate restores the protein’s original shape. 5

K+ is released, and the cycle repeats. 6 K+ K+ Fig. 7-16-6 Figure 7.16, 6 The sodium-potassium pump: a specific case of active transport K+ K+ is released, and the cycle repeats. 6

1 2 3 6 5 4 EXTRACELLULAR FLUID [Na+] high Na+ [K+] low Na+ Na+ Na+ Fig. 7-16-7 EXTRACELLULAR FLUID [Na+] high Na+ [K+] low Na+ Na+ Na+ Na+ Na+ Na+ Na+ [Na+] low ATP P Na+ P CYTOPLASM [K+] high ADP 1 2 3 K+ Figure 7.16, 1–6 The sodium-potassium pump: a specific case of active transport K+ K+ K+ K+ P K+ P 6 5 4

Facilitated diffusion Fig. 7-17 Passive transport Active transport ATP Diffusion Facilitated diffusion Figure 7.17 Review: passive and active transport

How Ion Pumps Maintain Membrane Potential Membrane potential is the voltage difference across a membrane Voltage is created by differences in the distribution of positive and negative ions Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

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) Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

An electrogenic pump is a transport protein that generates voltage across a membrane The sodium-potassium pump is the major electrogenic pump of animal cells The main electrogenic pump of plants, fungi, and bacteria is a proton pump Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

– EXTRACELLULAR FLUID + ATP – + H+ H+ Proton pump H+ – + H+ H+ H+ – + Fig. 7-18 – EXTRACELLULAR FLUID + ATP – + H+ H+ Proton pump H+ – + H+ H+ H+ Figure 7.18 An electrogenic pump – + 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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

– + H+ ATP H+ – + H+ H+ – + H+ H+ – + H+ H+ – + – + Diffusion of H+ Fig. 7-19 – + H+ ATP H+ – + Proton pump H+ H+ – + H+ H+ – + H+ Diffusion of H+ Sucrose-H+ cotransporter Figure 7.19 Cotransport: active transport driven by a concentration gradient H+ – Sucrose + – + Sucrose

Concept 7.5: 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 in bulk via vesicles Bulk transport requires energy Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Endocytosis In endocytosis, the cell takes in macromolecules by forming vesicles from the plasma membrane Endocytosis is a reversal of exocytosis, involving different proteins There are three types of endocytosis: Phagocytosis (“cellular eating”) Pinocytosis (“cellular drinking”) Receptor-mediated endocytosis Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

In phagocytosis a cell engulfs a particle in a vacuole The vacuole fuses with a lysosome to digest the particle For the Cell Biology Video Phagocytosis in Action, go to Animation and Video Files. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

In pinocytosis, molecules are taken up when extracellular fluid is “gulped” into tiny vesicles Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

PINOCYTOSIS Plasma membrane Vesicle 0.5 µm Fig. 7-20b Pinocytosis vesicles forming (arrows) in a cell lining a small blood vessel (TEM) Vesicle Figure 7.20 Endocytosis in animal cells—pinocytosis

In receptor-mediated endocytosis, binding of ligands to receptors triggers vesicle formation A ligand is any molecule that binds specifically to a receptor site of another molecule Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Coat protein Receptor Coated vesicle Coated pit Ligand A coated pit Fig. 7-20c RECEPTOR-MEDIATED ENDOCYTOSIS Coat protein Receptor Coated vesicle Coated pit Ligand A coated pit and a coated vesicle formed during receptor- mediated endocytosis (TEMs) Coat protein Figure 7.20 Endocytosis in animal cells—receptor-mediated endocytosis Plasma membrane 0.25 µm

Facilitated diffusion Fig. 7-UN1 Passive transport: Facilitated diffusion Channel protein Carrier protein

Fig. 7-UN2 Active transport: ATP

Environment: 0.01 M sucrose “Cell” 0.01 M glucose 0.01 M fructose Fig. 7-UN3 Environment: 0.01 M sucrose 0.01 M glucose 0.01 M fructose “Cell” 0.03 M sucrose 0.02 M glucose

An animal cell membrane will be more fluid at room temperature if it contains a) more cholesterol. b) longer chain fatty acids. c) more cis-unsaturated and polyunsaturated fatty acids. d) more trans-unsaturated fatty acids. e) any of the above Answer: c This question relates to Concept 7.1.

Osmosis If a marine algal cell is suddenly transferred from seawater to freshwater, the algal cell will initially a) lose water and decrease in volume. b) stay the same: neither absorb nor lose water. c) absorb water and increase in volume. Answer: c This question relates to Concept 7.3.

Transport Kinetics: Passive Diffusion Which of the curves below illustrates uptake of nitrous oxide into the cell, if nitrous oxide diffuses passively across the membrane? A B C Answer: c This question relates to Concept 7.3. The X-axis is the difference in concentration, expressed as the concentration outside the cell minus the concentration inside the cell. A negative value means that the concentration is higher inside the cell than outside the cell.

Transport Kinetics: Passive Diffusion Which of the curves below illustrates uptake of lactose into the cell via a passive transporter? A B C Answer: b This question relates to Concept 7.3. The X-axis is the difference in concentration, expressed as the concentration outside the cell minus the concentration inside the cell. A negative value means that the concentration is higher inside the cell than outside the cell. Facilitated diffusion shows saturation kinetics, as the rate becomes limited by the number of transporter proteins in the membrane at saturating concentrations of solute.

Transport Kinetics: Passive Diffusion Which of the curves below illustrates uptake of phosphate into the cell by an active transport system? A B C Answer: a This question relates to Concept 7.4. The X-axis is the difference in concentration, expressed as the concentration outside the cell minus the concentration inside the cell. A negative value means that the concentration is higher inside the cell than outside the cell. Active transport will work against a concentration gradient (negative values of the X-axis in the figure above) and show saturation kinetics.