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Membrane Structure and Function

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1 Membrane Structure and Function
7 Membrane Structure and Function Lecture Presentation by Nicole Tunbridge and Kathleen Fitzpatrick

2 Life at the Edge Plasma Membrane
Boundary separates living cell from surroundings Selective permeability Allows some substances to pass through more easily than others

3 Concept 7.1: Cellular membranes are fluid mosaics of lipids and proteins
Fluid Mosaic Model: Cell membrane has a fluid structure with a “mosaic” of various proteins embedded in it Proteins not randomly distributed in the membrane Phospholipid Bilayer: Stable boundary between two aqueous compartments Phospholipids Most abundant lipid in membrane Amphipathic molecules, with hydrophobic and hydrophilic regions

4 WATER WATER Hydrophilic head Hydrophobic tail Figure 7.2
Figure 7.2 Phospholipid bilayer (cross section) Hydrophobic tail

5 Fibers of extra- cellular matrix (ECM)
Figure 7.3 Fibers of extra- cellular matrix (ECM) Glyco- protein Glycolipid Carbohydrate EXTRACELLULAR SIDE OF MEMBRANE Cholesterol Figure 7.3 Updated model of an animal cell’s plasma membrane (cutaway view) Microfilaments of cytoskeleton Peripheral proteins Integral protein CYTOPLASMIC SIDE OF MEMBRANE

6 The Fluidity of Plasma Membranes
Phospholipids can move within the bilayer Most lipids and some proteins drift laterally Rarely, a lipid may flip-flop transversely Membranes must be fluid to work properly Membranes are usually about as fluid as salad oil

7 Mixed proteins after 1 hour
Figure 7.4-3 Membrane proteins Mixed proteins after 1 hour Mouse cell Human cell Figure Inquiry: Do membrane proteins move? (step 3) Hybrid cell

8 Temperature and Fluidity
Temperatures   Membranes fluid, solid Temperature at which a membrane solidifies depends on types of lipids EX: Unsaturated fatty acids: more fluid than saturated Steroid: Cholesterol Different effects 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

9 (a) Unsaturated versus saturated hydrocarbon tails
Figure 7.5 (a) Unsaturated versus saturated hydrocarbon tails Fluid Viscous Unsaturated tails prevent packing. Saturated tails pack together. (b) Cholesterol within the animal cell membrane Cholesterol reduces membrane fluidity at moderate temperatures, but at low temperatures hinders solidification. Figure 7.5 Factors that affect membrane fluidity Cholesterol

10 Evolution of Differences in Membrane Lipid Composition
Variations in Lipid Composition Among Species Possible adaptations to specific environmental conditions EX: Ability to change the lipid composition Evolutionary response to temperature changes in organisms’ habitats

11 Membrane Proteins and Their Functions
Membrane is a collage of different proteins Proteins are often grouped together and embedded in the fluid matrix of the lipid bilayer Proteins determine most of the membrane’s specific functions Peripheral Proteins: Bound to the surface Integral Proteins: Penetrate the hydrophobic core Transmembrane Proteins: Integral proteins that span the membrane Hydrophobic Regions: Consist of one or more stretches of nonpolar amino acids, often coiled into alpha helices

12 N-terminus EXTRACELLULAR SIDE α helix CYTOPLASMIC SIDE C-terminus
Figure 7.6 N-terminus EXTRACELLULAR SIDE Figure 7.6 The structure of a transmembrane protein α helix CYTOPLASMIC SIDE C-terminus

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

14 Signaling molecule Enzymes ATP Receptor Signal transduction
Figure 7.7 Signaling molecule Receptor Enzymes ATP Signal transduction (a) Transport (b) Enzymatic activity (c) Signal transduction Figure 7.7 Some functions of membrane proteins Glyco- protein (d) Cell-cell recognition (e) Intercellular joining (f) Attachment to the cytoskeleton and extracellular matrix (ECM)

15 Surface Proteins and Receptors
Surface protein CD4 and “co-receptor” CCR5 Needed in order for HIV to bind to the immune cell HIV cannot enter the cells of resistant individuals that lack CCR5 Thoughts on possible targeted treatment ideas?

16 Receptor (CD4) but no CCR5 Co-receptor (CCR5) Plasma membrane
Figure 7.8 HIV Receptor (CD4) Receptor (CD4) but no CCR5 Figure 7.8 The genetic basis for HIV resistance Co-receptor (CCR5) Plasma membrane (a) (b)

17 Role of Membrane Carbohydrates in Cell-Cell Recognition
Mr. T-Cell Cell-Cell Recognition Bind to molecules on the extracellular surface of the membrane Often carbohydrates Carbohydrates on the external side of the plasma membrane vary among species, individuals, and even cell types in an individual Glycolipids: Carbohydrates covalently bonded to lipids Glycoproteins: Carbohydrates covalently bonded to proteins

18 I pity the fool who doesn’t learn about cell signaling!

19 Synthesis and Sidedness of Membranes
Membranes have distinct inside and outside “faces” Plasma membrane has an asymmetrical distribution of proteins, lipids, and associated carbohydrates Determined when the membrane is built by the ER and Golgi apparatus

20 Transmembrane glycoproteins
Figure 7.9 Transmembrane glycoproteins Secretory protein Golgi apparatus Vesicle Attached carbohydrate Glycolipid ER lumen Plasma membrane: Cytoplasmic face Extracellular face Figure 7.9 Synthesis of membrane components and their orientation in the membrane Transmembrane glycoprotein Secreted protein Membrane glycolipid

21 Concept 7.2: Membrane structure results in selective permeability
Cells exchange materials with surroundings Controlled by the plasma membrane Plasma Membrane Selectively permeable Regulate cell’s molecular “traffic” The Permeability of Lipid Bilayer Hydrophobic/Nonpolar Molecules (hydrocarbons) Can dissolve in the lipid bilayer, pass through rapidly Hydrophilic/Polar Molecules (ions) Do not cross easily

22 Transport Proteins Allow passage of hydrophilic/polar substances across the membrane Specific for the substance it moves Channel Proteins Have a hydrophilic channel that certain ions can use Aquaporins: Facilitate passage of water Carrier Proteins Bind to molecules, change shape, & act as a shuttle

23 Concept 7.3: Passive transport is diffusion of a substance across a membrane with no energy investment Passive Transport The movement of a substance across a biological membrane requiring no energy to be expended by the cell Diffusion Osmosis

24 Diffusion Across the Membrane
Tendency for molecules to spread out evenly into the available space Molecules move randomly, but the diffusion may be directional Dynamic Equilibrium: Equal #’s of molecules cross in both directions Molecules diffuse down their concentration gradient, from high  low. Region along which the density of a chemical substance increases or decreases No work needed to move substances down the concentration gradient

25 Membrane (cross section)
Figure 7.10 Molecules of dye Membrane (cross section) WATER Net diffusion Net diffusion Equilibrium (a) Diffusion of one solute Figure 7.10 The diffusion of solutes across a synthetic membrane Net diffusion Net diffusion Equilibrium Net diffusion Net diffusion Equilibrium (b) Diffusion of two solutes

26 Effects of Osmosis on Water Balance
The diffusion of water molecules across a selectively permeable membrane Moves from the side of the membrane with a lower solute concentration  the region of higher solute concentration Seeks equal solute concentration on both sides Molecules never stop moving

27 Lower concentration of solute (sugar) Higher concentration of solute
Figure 7.11 Lower concentration of solute (sugar) Higher concentration of solute More similar concentrations of solute Sugar molecule H2O Selectively permeable membrane Figure 7.11 Osmosis Osmosis

28 Water Balance of Cells Without Cell Walls
Tonicity: Surrounding solutions cause cells 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

29 Hypotonic Isotonic Hypertonic H2O H2O H2O H2O (a) Animal cell Lysed
Figure 7.12 Hypotonic Isotonic Hypertonic H2O H2O H2O H2O (a) Animal cell Lysed Normal Shriveled Cell wall Plasma membrane H2O Plasma membrane H2O H2O H2O Figure 7.12 The water balance of living cells (b) Plant cell Turgid (normal) Flaccid Plasmolyzed

30 Osmoregulation Control of solute concentrations and water balance
Necessary adaptation for life in aquatic environments Hypertonic or hypotonic environments create osmotic problems for organisms Example: Paramecium, (protist), is hypertonic to its pond water environment Has a contractile vacuole that acts as a pump

31 50 μm Contractile vacuole Figure 7.13
Figure 7.13 The contractile vacuole of Paramecium caudatum

32 Water Balance of Cells with Cell Walls
Cell walls help maintain water balance A plant cell in a hypotonic solution swells until the wall opposes uptake, making the cell turgid (firm) Isotonic Environment: Plant cell has no net movement of water into the cell  cell becomes flaccid (limp) Hypertonic Environment: Plant cells lose water Membrane pulls away from the cell wall Causes plant to wilt Plasmolysis: Lethal effect of wilting

33 Facilitated Diffusion: Passive Transport Aided by Proteins
Transport proteins speed the movement of molecules across the plasma membrane Channel Proteins: Provide corridors for a specific molecule or ion to cross the membrane Carrier Proteins: Undergo a subtle change in shape and translocates the solute-binding site across the membrane Aquaporins facilitate the diffusion of water Ion channels facilitate the diffusion of ions Gated Channels: Ion channels open or close in response to a stimulus

34 EXTRACELLULAR FLUID (a) A channel protein CYTOPLASM
Figure 7.14 EXTRACELLULAR FLUID (a) A channel protein Channel protein Solute CYTOPLASM Figure 7.14 Two types of transport proteins that carry out facilitated diffusion Carrier protein Solute (b) A carrier protein

35 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, and the transport requires no energy Some transport proteins, however, can move solutes against their concentration gradients

36 Active Transport Moves substances against their concentration gradients REQUIRES ENERGY – usually ATP Performed by specific proteins embedded in the membranes Allows cells to maintain concentration gradients that differ from their surroundings Example: Sodium-Potassium Pump

37 Figure 7.15 EXTRACELLULAR FLUID [Na+] high [K+] low Na+ Na+ Na+ Na+ Na+ [Na+] low [K+] high P ATP Na+ CYTOPLASM ADP 2 K+ 1 K+ 6 Na+ Na+ Na+ K+ Figure 7.15 The sodium-potassium pump: a specific case of active transport K+ P K+ K+ 3 P P 5 i 4

38 Facilitated diffusion ATP
Figure 7.16 Passive transport Active transport Figure 7.16 Review: passive and active transport Diffusion Facilitated diffusion ATP

39 Ion Pumps Maintain Membrane Potential
Membrane Potential: Voltage difference across a membrane Voltage created by differences in the distribution of + and - ions across a membrane Electrochemical Gradient: Combined chemical and electrical forces drive the diffusion of ions across a membrane Chemical Force: Ion’s concentration gradient Electrical Force: Effect of the membrane potential on the ion’s movement

40 Electrogenic Pumps Transport protein that generates voltage across a membrane Example: Sodium-Potassium Pump (animal cells) Proton Pump: The main electrogenic pump of plants, fungi, and bacteria Help store energy that can be used for cellular work

41 ATP − + EXTRACELLULAR FLUID − + H+ H+ H+ Proton pump H+ − + H+ − + H+
Figure 7.17 ATP + EXTRACELLULAR FLUID + H+ H+ H+ Proton pump H+ + H+ + H+ CYTOPLASM Figure 7.17 A proton pump

42 Cotransport Coupled transport by a membrane protein
Occurs when active transport of a solute indirectly drives transport of other substances Commonly Used in Plant Cells Proton pumps generate a gradient of H+ ions Use the gradient to drive active transport of nutrients into the cell

43 Sucrose-H+ cotransporter
Figure 7.18 + Sucrose Sucrose Sucrose-H+ cotransporter Diffusion of H+ H+ + H+ H+ + H+ H+ Figure 7.18 Cotransport: active transport driven by a concentration gradient H+ H+ Proton pump H+ ATP + H+

44 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 Via transport proteins Large Molecules: Cross the membrane in bulk, via vesicles Requires energy EX: Polysaccharides, proteins

45 Exocytosis Many secretory cells use exocytosis to export their products Transport vesicles migrate to the membrane Fuse with membrane Release their contents outside the cell

46 Endocytosis Reversal of exocytosis, involving different proteins
Plasma membrane folds inward to form vesicles. Membrane surrounds the macromolecules that the cell takes into it as the vesicles form. Types of Endocytosis Phagocytosis (“cellular eating”) Pinocytosis (“cellular drinking”) Receptor-mediated endocytosis

47 Receptor-Mediated Endocytosis
Figure 7.19 Receptor-Mediated Endocytosis Phagocytosis Pinocytosis EXTRACELLULAR FLUID Solutes Pseudopodium Receptor Plasma membrane Coat protein “Food” or other particle Coated pit Figure 7.19 Exploring endocytosis in animal cells Coated vesicle Food vacuole CYTOPLASM

48 Types of Endocytosis Phagocytosis Pinocytosis
Cell engulfs a particle in a vacuole Vacuole fuses with a lysosome to digest the particle Pinocytosis Molecules dissolved in droplets are taken up when extracellular fluid is “gulped” into tiny vesicles Receptor-mediated Endocytosis Binding of ligands to receptors triggers vesicle formation Ligand: Any molecule that binds specifically to a receptor site of another molecule

49 Pseudopodium of amoeba
Figure 7.19a Phagocytosis EXTRACELLULAR FLUID Solutes Pseudopodium of amoeba Pseudopodium Bacterium 1 μm Food vacuole An amoeba engulfing a bacterium via phago- cytosis (TEM) “Food” or other particle Figure 7.19a Exploring endocytosis in animal cells (part 1: phagocytosis) Food vacuole CYTOPLASM

50 Pinocytotic vesicles forming (TEMs)
Figure 7.19b Pinocytosis Plasma membrane 0.25 μm Coat protein Pinocytotic vesicles forming (TEMs) Coated pit Figure 7.19b Exploring endocytosis in animal cells (part 2: pinocytosis) Coated vesicle

51 Receptor-Mediated Endocytosis
Figure 7.19c Receptor-Mediated Endocytosis Plasma membrane Coat protein Receptor 0.25 μm Figure 7.19c Exploring endocytosis in animal cells (part 3: receptor-mediated endocytosis) Top: A coated pit. Bottom: A coated vesicle forming during receptor- mediated endocytosis (TEMs)


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