Membrane Structure and Function

Slide 1 ______Chapter 7

Membrane Structure and ______Function ______

PowerPoint® Lecture Presentations for Biology Eighth Edition Neil Campbell and Jane Reece ______

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Slide 2 Overview: The Membrane Barrier ______

• The plasma membrane is the boundary that separates the living cell from its surroundings ______• All eukaryotic membranes exhibit selective permeability, allowing some substances to cross them more easily than others ______Membrane Channel Protein ______

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Slide 3 Concept 7.1: Cellular membranes are fluid mosaics ______of lipids and proteins

• Phospholipids are the most abundant lipid in membranes • 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 ______

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______Slide 4 Fig. 7-5 ______The Fluidity of Membranes Lateral movement Flip-flop Most of the (~107 times per second) (~ once per month) lipids, and (a) Movement of phospholipids some proteins, ______Fluid Viscous drift laterally

Unsaturated hydrocarbon Saturated hydro- ______tails with kinks carbon tails Rarely does a (b) Membrane fluidity molecule flip-flop transversely across the membrane ______

Cholesterol (c) Cholesterol within the animal cell membrane ______

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Slide 5 Fig. 7-6 ______Demonstration of Protein Drift in a Membrane

RESULTS ______Membrane proteins

Mixed proteins ______after 1 hour Mouse cell Human cell Hybrid cell ______

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Fig. 7-5b Slide 6 •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 ______

Fluid Viscous ______

Unsaturated hydrocarbon Saturated hydro- ______tails with kinks carbon tails

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______Slide 7 Fig. 7-5c ______•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

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Slide 8 Fig. 7-7 ______Fibers of extracellular matrix (ECM)

______Glyco- Carbohydrate protein Glycolipid EXTRACELLULAR SIDE OF MEMBRANE ______

Transmembrane protein Cholesterol

Peripheral Proteins provide most proteins Integral membrane-specific protein ______CYTOPLASMIC SIDE functions OF MEMBRANE

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Slide 9 Fig. 7-8 ______

N-terminus EXTRACELLULAR SIDE

What types ______of side chains are on outside of ______these helices?

C-terminus CYTOPLASMIC α Helix SIDE ______

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______Slide 10 Fig. 7-9ac ______

Signaling molecule

Enzymes Receptor ______

______ATP Signal transduction (a) Transport (b) Enzymatic activity (c) Signal transduction ______

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Slide 11 Fig. 7-9df ______

Glyco- ______protein

(d) Cell-cell recognition (e) Intercellular joining (f) Attachment to the cytoskeleton ______and extracellular Carbohydrates often play important roles matrix (ECM) on the plasma membrane. Covalently bonded to lipids (forming glycolipids) or more commonly to proteins (forming glycoproteins) ______

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Fig. 7-10 ER Slide 12 1 ______Membranes Transmembrane have glycoproteins Secretory distinct protein inside and The Glycolipid asymmetrical outside distribution of ______Golgi 2 faces apparatus proteins, lipids, and Vesicle carbohydrates is determined when the ______membrane is 3 built by the ER Plasma membrane: and Golgi Cytoplasmic face 4 apparatus Extracellular face Transmembrane Secreted glycoprotein ______protein

Membrane glycolipid

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______Slide 13 Selective Permeability ______

• A cell must exchange materials with its surroundings and compartmentalize its interior. ______• This process is controlled by the membranes. • Membranes are selectively permeable, regulating the cell’s molecular traffic ______

• What does “Selective Permeability” mean? ______

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Slide 14 The Permeability of the Lipid Bilayer ______

Hydrophobic (nonpolar) molecules, such as hydrocarbons, can dissolve in the lipid bilayer and pass through the membrane rapidly ______

estradiol testosterone ______

Charged or strongly polar molecules, such as ions, ______sugars and proteins, do not cross the membrane easily glucose proteins Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings ______

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Slide 15 ______Some small molecules, even if polar, can pass through the lipid bilayer - very slowly...... ______-oxygen -carbon dioxide -urea ______-water ______H2O

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______Slide 16 Diffusion: True or False ______

1. Diffusion requires that the membrane be permeable to the substance that is diffusing 2. Diffusion is spontaneous and passive – requiring no Energy ______3. A substance moves against its concentration gradient during diffusion 4. The concentration of one substance can affect the diffusion of another 5. Diffusion occurs because populations of molecules move randomly, but individual molecules can be directional (moving in a particular direction across a membrane) ______6. Diffusion results from thermal motion (heat) 7. Osmosis is the diffusion of a solute across a selectively-permeable membrane. ______8. At dynamic equilibrium, as many molecules cross in one direction across the membrane as in the other direction

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Fig. 7-11 Slide 17 Membrane (cross section) ______

one

solute WATER ______

Net diffusion Net diffusion Equilibrium

Net diffusion ______two solutes

Net diffusion Net diffusion Equilibrium ______

Net diffusion Net diffusion Equilibrium Net diffusion ______

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Fig. 7-12 Lower Higher Same concentration Slide 18 concentration concentration of sugar ______of solute (sugar) of sugar

______H2O

Selectively permeable membrane ______

Water bound to sugar creates less “free” water OSMOSIS: the special case of water (solvent) ______diffusion. Osmosis

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______Slide 19 Water Balance of Cells ______

• Tonicity is the ability of a solution to cause a cell to gain or lose water (e.g., potato in lab) – Considers both solute concentration, and ______– Membrane permeability – Depends on concentration of non-penetrating solutes • 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

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Fig. 7-13

Slide 20 Hypotonic solution Isotonic solution Hypertonic solution ______

H2O H2O H2O H2O

(a) Animal cell ______

Lysed Normal Shriveled

H O H O H2O H2O 2 2 ______

(b) Plant cell ______Turgid (normal) Flaccid Plasmolyzed

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Slide 21 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 ______

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______Slide 22 ______

• Hypertonic or hypotonic environments can create osmotic problems ______• 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 ______

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Fig. 7-14 Slide 23 Filling vacuole 50 µm ______

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(a) A contractile vacuole fills with fluid that enters from a system of canals radiating throughout the cytoplasm. Contracting vacuole ______

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(b) When full, the vacuole and canals contract, expelling fluid from the cell. ______

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Slide 24 Transmembrane ‘Transport Proteins’ can help ______hydrophilic substances cross the membrane

1. Facilitated Diffusion 1. Aqueous channels for charged or polar molecules to diffuse through ______-or - 2. Passive carrier mechanisms that move molecules from higher to lower concentration – usually in either direction ______2. Active Transport 1. ATP-dependent carrier that can move charged or polar molecules against their concentration gradient ______3. All Transport Proteins are very specific for their molecules

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______Fig. 7-17 Slide 25 Passive transport Active transport ______

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AT P ______Diffusion Facilitated diffusion

Both passive and active carrier proteins undergo a change in shape ______that opens the solute-binding site on the pother side of the membrane

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Slide 26 ______

• Channel proteins include – Aquaporins, for facilitated diffusion of water ______– Ion channels that open or close in response to a stimulus (gated channels) ______What kind of “gating” have you heard about?

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Slide 27 ______

• Active transport allows cells to maintain concentrations of things different from what would occur naturally ______– We have calcium pumps, hydrogen ion pumps and sodium-potassium pumps, among others – We also have glucose transporters and amino acid transporters – even large protein transporters ______

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______Slide 28 ______

• What other way have we already learned that big molecules, like proteins, lipids and ______carbohydrates, can be moved in or out or around within a cell? ______

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Slide 29 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 ______

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Slide 30 Fig. 7-16-1 ______

EXTRACELLULAR FLUID [Na+] high Example: [K+] low ______Active Na+ - natural Transport Na+ gradients? ______[Na+] low Na+ CYTOPLASM [K+] high 1 Cytoplasmic Na+ binds to the sodium-potassium pump. ______

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______Slide 31 Fig. 7-16-2 ______

Na+ ______

Na+ Na+ ______ATP P ADP 2 Na+ binding stimulates phosphorylation by ATP. ______

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Slide 32 Fig. 7-16-3 ______

Na+ Na+

Na+ ______

______P

3 Phosphorylation causes the protein to change its ______shape. Na+ is expelled to the outside.

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Slide 33 Fig. 7-16-4 ______

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______P P 4 K+ binds on the extracellular side and ______triggers release of the phosphate group.

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______Slide 34 Fig. 7-16-5 ______

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5 Loss of the phosphate restores the protein’s original ______shape.

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Slide 35 Fig. 7-16-6 ______

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K+ is released, and the cycle repeats. ______Why do we want Na+ outside of the cell and K+ inside?

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Slide 36 Neurons and muscle cells in animals and phloem ______cells in plants rely on electrical signaling.

• Electricity is the energy created by the movement of charged particles – it’s named for the example of electrons ______• When a cell uses electricity it does it by allowing ions that it has concentrated by active transport to rush from one side of the membrane to the other through channel proteins • The opening and closing of the channels determines when ______the electrical current is flowing • Voltage is a measure of how many ions are on the move • Membrane potential is a measure of how many ions have ______been actively concentrated across a membrane

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______Slide 37 ______

• Concentrated ions diffuse faster than uncharged molecules ______• 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) ______

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Slide 38 ______

• 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 • Mitochondria and chloroplasts use a proton pump to help make ATP ______

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Slide 39 Fig. 7-18 ______– EXTRACELLULAR + FLUID AT P – + H+ ______

H+ Proton pump H+

– + H+ ______H+ – + CYTOPLASM H+ – + ______Where else have you heard of a proton pump? ______

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______Slide 40 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 ______

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Fig. 7-19 Slide 41 – + ______ATP H+ H+ – + Proton pump H+ H+ ______– + H+ H+ – + H+ Diffusion of H+ ______Sucrose-H+ cotransporter H+ – Sucrose + ______– + Sucrose

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Slide 42 ______

• 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 ______

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Animation: Receptor-Mediated Endocytosis

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______Fig. 7-20c RECEPTOR-MEDIATED ENDOCYTOSIS Slide 43 Coat protein ______Receptor Coated vesicle ______Coated pit Ligand

A coated pit Coat and a coated ______protein vesicle formed during receptor- mediated endocytosis (TEMs) ______Plasma membrane 0.25 µm

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Slide 44 Fig. 7-UN3 ______What will happen?

______Environment: “Cell” 0.01 M sucrose 0.03 M sucrose 0.01 M glucose 0.02 M glucose 0.01 M fructose ______

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Slide 45 You should now be able to: ______

1. Define the following terms: amphipathic molecules, aquaporins, diffusion ______2. Explain how membrane fluidity is influenced by temperature and membrane composition 3. 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 ______

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______Slide 46 ______

4. Explain how transport proteins facilitate diffusion ______5. Explain how an electrogenic pump creates voltage across a membrane, and name two electrogenic pumps ______6. Explain how large molecules are transported across a cell membrane ______

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