Chapter 7 Membrane Structure and Function 7 1

Chapter 7 Membrane Structure and Function { 7. 1 Cellular membranes are fluid mosaics of lipids and proteins

Overview: Life at the Edge The plasma membrane is a selectively permeable boundary, allowing some substances to cross it more easily than others Phospholipids have 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 Main components: Carbohydrates Integral proteins Phospholipids Peripheral proteins Copyright © 2008 Pearson Education, Inc. , publishing as Pearson Benjamin Cummings

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

Fig. 7 -3 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 Phospholipid bilayer Hydrophobic regions of protein Hydrophilic regions of protein

Fig. 7 -4 The fluid mosaic model is supported by freeze-fracture studies, (a specialized preparation technique that splits a membrane along the middle of the phospholipid bilayer) TECHNIQUE RESULTS Extracellular layer Knife Plasma membrane Proteins Inside of extracellular layer Cytoplasmic layer Inside of cytoplasmic layer

Fig. 7 -5 • Phospholipids in the plasma membrane can move within the bilayer • Most of the lipids, and some proteins, drift laterally • Rarely does a molecule flipflop transversely across the membrane Lateral movement (~107 times per second) Flip-flop (~ once per month) (a) Movement of phospholipids Fluid Unsaturated hydrocarbon tails with kinks Viscous Saturated hydrocarbon tails (b) Membrane fluidity Cholesterol (c) Cholesterol within the animal cell membrane

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

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

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-bound to the surface of the membrane Integral proteins-completely embedded in membrane Transmembrane proteins: span membrane Copyright © 2008 Pearson Education, Inc. , publishing as Pearson Benjamin Cummings

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

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Fig. 7 -8 The hydrophobic regions of an integral protein consist of one or more stretches of nonpolar amino acids, often coiled into alpha helices N-terminus C-terminus Helix EXTRACELLULAR SIDE CYTOPLASMIC SIDE

Fig. 7 -9 Signaling molecule Enzymes Six major functions of membrane proteins ATP (a) Transport Receptor Signal transduction (b) Enzymatic activity (c) Signal transduction (e) Intercellular joining (f) Attachment to the cytoskeleton and extracellular matrix (ECM) Glycoprotein (d) Cell-cell recognition

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) Cell surface carbohydrates vary from species to species Why blood transfusions must be type-specific Copyright © 2008 Pearson Education, Inc. , publishing as Pearson Benjamin Cummings

Chapter 7 Membrane Structure and Function { Concept 7. 2: Membrane structure results in selective permeability

Membranes are selectively permeable 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 Hydrophobic (nonpolar) molecules, such as hydrocarbons, pass through the membrane rapidly Polar molecules, (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 Channel proteins are transport proteins that act like tunnels aquaporins facilitate the passage of water 3 billion water molecules/aquaporin/second!!!! 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

Chapter 7 Membrane Structure and Function { 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 At dynamic equilibrium, as many molecules cross one way as cross in the other direction Substances diffuse down their concentration gradient molecules may exhibit a net movement in one direction Does not require energy/work 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

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

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

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Fig. 7 -12 Lower concentration of solute (sugar) Higher concentration of sugar H 2 O Selectively permeable membrane 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 Osmosis Same concentration of sugar

Water Balance of Cells Without Walls 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

Fig. 7 -13 Hypotonic solution H 2 O Isotonic solution H 2 O Hypertonic solution H 2 O (a) Animal cell Lysed H 2 O Normal H 2 O Shriveled H 2 O (b) Plant cell Turgid (normal) Flaccid Plasmolyzed

Osmoregulation, the control of water balance, is a necessary adaptation for life in such environments 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 (such as ions and hydrophilic substantces) across the plasma membrane Channel proteins provide corridors and include: Aquaporins, for facilitated diffusion of water Ion channels that open or close in response to a stimulus (gated channels) Facilitated diffusion is still passive because the solute moves down its concentration gradient Copyright © 2008 Pearson Education, Inc. , publishing as Pearson Benjamin Cummings

Fig. 7 -15 EXTRACELLULAR FLUID Channel protein Solute CYTOPLASM (a) A channel protein Carrier protein (b) A carrier protein Solute

The Need for Energy in Active Transport Active transport moves substances against their concentration gradient (from the side where they are less concentrated to the side where they are more concentrated). requires energy, (ATP) Active transport is performed by specific proteins embedded in the membranes Active transport allows cells to maintain concentration gradients that differ from their surroundings Copyright © 2008 Pearson Education, Inc. , publishing as Pearson Benjamin Cummings

Fig. 7 -16 -1 The sodium-potassium pump is one type of active transport system EXTRACELLULAR FLUID [Na+] high [K+] low Na+ CYTOPLASM Na+ [Na+] low [K+] high 1 Cytoplasmic Na+ binds to the sodium-potassium pump.

Fig. 7 -16 -2 The sodium-potassium pump is one type of active transport system Na+ Na+ P ADP ATP 2 Na+ binding stimulates phosphorylation by ATP.

Fig. 7 -16 -3 The sodium-potassium pump is one type of active transport system Na+ Na+ P 3 Phosphorylation causes the protein to change its shape. Na+ is expelled to the outside.

Fig. 7 -16 -4 The sodium-potassium pump is one type of active transport system K+ K+ P 4 K+ binds on the extracellular side and triggers release of the phosphate group. P

Fig. 7 -16 -5 The sodium-potassium pump is one type of active transport system K+ K+ 5 Loss of the phosphate restores the protein’s original shape.

Fig. 7 -16 -6 The sodium-potassium pump is one type of active transport system K+ K+ 6 K+ is released, and the cycle repeats.

Fig. 7 -16 -7 EXTRACELLULAR FLUID [Na+] high [K+] low Na+ Na+ CYTOPLASM Na+ [Na+] low [K+] high P ADP 2 1 ATP P 3 K+ K+ K+ + K K+ P K+ 6 5 4 P

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Fig. 7 -17 Passive transport Active transport ATP Diffusion Facilitated diffusion

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 The inside of the cell is negatively (-) charged A positively charged ion (like Na+) is attracted to the negative charges inside the cell 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) An electrogenic pump is a transport protein that generates voltage across a membrane In animal cells: Na-K pump In plants/fungi/bacteria cells: proton pump Copyright © 2008 Pearson Education, Inc. , publishing as Pearson Benjamin Cummings

Figure 7. 18 An electrogenic pump – ATP EXTRACELLULAR FLUID + – + H+ H+ Proton pump H+ – CYTOPLASM + – – H+ H+ + + H+

Fig. 7 -19 – + ATP – H+ + Proton pump H+ H+ – H+ H+ + – + Sucrose-H+ cotransporter H+ H+ Diffusion of H+ H+ Sucrose – Cotransport occurs when – active transport of a solute indirectly drives transport of another solute + + Sucrose

Chapter 7 Membrane Structure and Function { Concept 7. 5: Bulk transport across the plasma membrane occurs by exocytosis and endocytosis

Exocytosis & Endocytosis Large molecules, such as polysaccharides and proteins, cross the membrane in bulk via vesicles Bulk transport requires energy In exocytosis, transport vesicles migrate to the membrane, fuse with it, and release their contents, (used by secretory cells) In endocytosis, the cell takes in macromolecules by forming vesicles from the plasma membrane (3 types) Phagocytosis (“cellular eating”) Pinocytosis (“cellular drinking”) Receptor-mediated endocytosis Copyright © 2008 Pearson Education, Inc. , publishing as Pearson Benjamin Cummings

Fig. 7 -20 In phagocytosis a cell engulfs a particle in a vacuole then fuses with a lysosome to digest the particle PHAGOCYTOSIS 1 µm EXTRACELLULAR CYTOPLASM FLUID Pseudopodium of amoeba “Food”or other particle Bacterium Food vacuole An amoeba engulfing a bacterium via phagocytosis (TEM) PINOCYTOSIS 0. 5 µm Plasma membrane Pinocytosis vesicles forming (arrows) in a cell lining a small blood vessel (TEM) Vesicle RECEPTOR-MEDIATED ENDOCYTOSIS Coat protein Receptor In receptor-mediated endocytosis, binding of ligands (molecule that binds to receptor) to receptors triggers vesicle formation Coated vesicle Coated pit Ligand A coated pit and a coated vesicle formed during receptormediated endocytosis (TEMs) Coat protein Plasma membrane 0. 25 µm In pinocytosis, molecules are taken up when extracellular fluid is “gulped” into tiny vesicles

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