Chapter 7 Membrane Structure and Function Concept 7

Chapter 7 Membrane Structure and Function

Concept 7. 1: Cellular membranes are fluid mosaics of lipids and proteins Phospholipids Most abundant lipid in plasma membrane Amphipathic : hydrophobic & hydrophilic regions

Fluid mosaic model of membrane structure Membrane is a fluid structure with a “mosaic” of various proteins embedded in it

Davson-Danielli sandwich model (1935) Stated that membrane was made up of phospholipid bilayer sandwiched between two protein layers Issues? Looked like rows of beads in electron micrographs Proteins not soluble

In 1972, Singer & Nicolson Proposed that membrane proteins are dispersed & individually inserted into phospholipid bilayer Hydrophobic region of protein Phospholipid bilayer Figure 7. 3 Hydrophobic region of protein

Fluidity of Membranes Phospholipids in plasma membrane Can move within bilayer Lateral movement (~107 times per second) (a) Movement of phospholipids Figure 7. 5 A Flip-flop (~ once per month)

Proteins in plasma membrane Can drift within bilayer EXPERIMENT Researchers labeled the plasma mambrane proteins of a mouse cell and a human cell with two different markers and fused the cells. Using a microscope, they observed the markers on the hybrid cell. RESULTS Membrane proteins + Mouse cell Human cell Hybrid cell Figure 7. 6 Mixed proteins after 1 hour CONCLUSION The mixing of the mouse and human membrane proteins indicates that at least some membrane proteins move sideways within the plane of the plasma membrane.

Type of hydrocarbon tails in phospholipids Affects fluidity of plasma membrane Fluid Unsaturated hydrocarbon tails with kinks (b) Membrane fluidity Figure 7. 5 B Viscous Saturated hydro. Carbon tails

Cholesterol Different effects at different temps Low temps breaks up tight packing (prevents hardening) High temps binds lipids together (prevents runniness) Cholesterol Figure 7. 5 (c) Cholesterol within the animal cell membrane

Membrane Proteins and Their Functions Collage of different proteins embedded in fluid matrix of lipid bilayer Fibers of extracellular matrix (ECM) Glycoprotein Carbohydrate Glycolipid EXTRACELLULAR SIDE OF MEMBRANE Microfilaments of cytoskeleton Figure 7. 7 Cholesterol Peripheral protein Integral CYTOPLASMIC SIDE protein OF MEMBRANE

Integral proteins Penetrate hydrophobic core of lipid bilayer Often transmembrane proteins, completely spanning the membrane N-terminus C-terminus a Helix CYTOPLASMIC SIDE

Peripheral proteins Appendages loosely bound to surface of membrane

6 major functions of membrane proteins (a) Transport. (left) A protein that spans membrane may provide a hydrophilic channel across the membrane that is selective for a particular solute. (right) Other transport proteins shuttle a substance from one side to the other by changing shape. Some of these proteins hydrolyze ATP as an energy source to actively pump substances across the membrane. ATP (b) Enzymatic activity. protein built into membrane Enzymes may be an enzyme with its active site exposed to substances in the adjacent solution. In some cases, several enzymes in a membrane are organized as a team that carries out sequential steps of a metabolic pathway. (c) Signal transduction. protein may have a binding site with a specific shape that fits the shape of a chemical messenger, such as a hormone. The external messenger (signal) may cause a conformational change in the protein (receptor) that relays the message to the inside of the cell. Figure 7. 9 Signal Receptor

(d) Cell-cell recognition. Some glyco-proteins serve as identification tags that are specifically recognized by other cells. Glycoprotein (e) (f) Intercellular joining. Membrane proteins of adjacent cells may hook together in various kinds of junctions, such as gap junctions or tight junctions (see Figure 6. 31). Attachment to cytoskeleton & extracellular matrix (ECM). Microfilaments or other elements of the cytoskeleton may be bonded to membrane proteins, a function that helps maintain cell shape and stabilizes the location of certain membrane proteins. Proteins that adhere to the ECM can coordinate extracellular and intracellular changes (see Figure 6. 29). Figure 7. 9

Role of Membrane Carbohydrates in Cell-Cell Recognition Cell-cell recognition Cell’s ability to distinguish one type of cell from another Importance ? immunity, sorting cells during development

7. 2: Membrane structure results in selective permeability Cell must exchange materials with surroundings - controlled by plasma membrane

Hydrophobic (nonpolar) molecules hydrocarbons, CO 2, O 2 Can pass through membrane rapidly (lipid soluble) Hydrophilic (polar molecules) sugars, H 2 O Do not cross membrane rapidly

Transport Proteins Allow passage of hydrophilic substances across membrane

7. 3: Passive transport is diffusion of a substance across a membrane with no energy investment Diffusion Tendency for molecules to spread out evenly into available space

Substances diffuse down concentration gradient, high low concentration

Lower concentration of solute (sugar) Osmosis Higher concentration of sugar Same concentration of sugar Movement of water across a semipermeable membrane Affected by concentration gradient of dissolved substances Selectively permeable membrane: sugar molecules cannot pass through pores, but water molecules can Figure 7. 12 Water molecules cluster around sugar molecules More free water molecules (higher concentration) Fewer free water molecules (lower concentration) Osmosis Water moves from an area of higher free water concentration to an area of lower free water concentration

Water Balance of Cells Without Walls Tonicity Ability of solution to cause cell to gain or lose water Great impact on cells w/o walls

If solution is isotonic Concentration of solutes is same as inside the cell No net movement of water

If solution is hypertonic Concentration of solutes is greater than it is inside cell Cell will lose water Don’t drink _____ water!

If solution is hypotonic Concentration of solutes is less than it is inside cell Cell will gain water Can you drink too much water?

Water balance in cells w/o walls Organisms w/o rigid cell walls must have special adaptations for osmoregulation Hypotonic solution (a) Animal cell. An animal cell fares best in an isotonic environment unless it has special adaptations to offset the osmotic uptake or loss of water. H 2 O Lysed Figure 7. 13 Isotonic solution Hypertonic solution H 2 O Normal H 2 O Shriveled

Water Balance of Cells with Walls Cell walls - help maintain water balance Plants, prokaryotes, fungi, some protists (b) Plant cells are turgid (firm) and generally healthiest in a hypotonic environment, where the uptake of water is eventually balanced by the elastic wall pushing back on the cell. H 2 O Turgid (normal) Figure 7. 13 H 2 O Flaccid H 2 O Plasmolyzed

Terms for cells with walls turgid hypotonic environment Very firm, healthy state in most plants flaccid isotonic or hypertonic environment

10. You observe plant cells under a microscope that have just been placed in an unknown solution. First the cells plasmolyze; after a few minutes, the plasmolysis reverses and the cells appear normal. What would you conclude about the unknown solute? a) It is hypertonic to the plant cells, and its solute can not cross the pant cell membranes. b) It is hypotonic to the plant cells, and its solute can not cross the pant cell membranes. c) It is isotonic to the plant cells, but its solute can cross the plant cell membranes. d) It is hypertonic to the plant cells, but its solute can cross the plant cell membranes. e) It is hypotonic to the plant cells, but its solute can cross the plant cell membranes.

An artificial cell consisting of an aqueous solution enclosed in a selectively permeable membrane has just been immersed in a beaker containing a different solution. The membrane is permeable to water and to the simple sugars glucose and fructose but completely impermeable to the disaccharide sucrose.

1. Which solute(s) will exhibit a net diffusion into the cell? a) sucrose b) glucose c) fructose

2. Which solute(s) will exhibit a net diffusion out of the cell? a) sucrose b) glucose c) fructose

3. Which solution is hypertonic to the other? a) the cell contents b) the environment

4. In which direction will there be a net osmotic movement of water? a) out of the cell b) into the cell c) neither

Facilitated Diffusion: Passive Transport Aided by Proteins Transport proteins speed movement of molecules across plasma membrane 2 types 1. Channel proteins 2. Carrier proteins

Channel proteins Provide corridors that allow a specific molecule or ion to cross EXTRACELLULAR FLUID Channel protein Solute CYTOPLASM (a) A channel protein (purple) has a channel through which water molecules or a specific solute can pass. Figure 7. 15

Carrier proteins Undergo change in shape that translocates solute-binding site across membrane Carrier protein Solute (b) A carrier protein alternates between two conformations, moving a solute across the membrane as the shape of the protein changes. The protein can transport the solute in either direction, with the net Figure 7. 15 movement being down the concentration gradient of the solute.

7. 4: Active transport uses energy to move solutes against their gradients Moves substances against their concentration gradient Requires energy, usually in form of ATP

sodium-potassium pump (animals) One type of active transport (nerves, muscles) 1 Cytoplasmic Na + binds to the sodium-potassium pump. EXTRACELLULAR [Na+] high FLUID [K+] low Na+ 2 Na+ binding stimulates phosphorylation by ATP. Na+ Na+ [Na+] low Na+ [K+] high CYTOPLASM ATP P ADP Na+ Na+ 3 K+ is released and Na + sites are receptive again; the cycle repeats. 4 Phosphorylation causes the K+ protein to change its conformation, expelling Na + to the outside. P K+ K+ K+ 5 Loss of the phosphate restores the protein’s original conformation. Figure 7. 16 K+ K+ 6 Extracellular K + binds to the P Pi protein, triggering release of the Phosphate group.

Review: Passive and active transport compared Passive transport. Substances diffuse spontaneously down their concentration gradients, crossing a membrane with no expenditure of energy by the cell. The rate of diffusion can be greatly increased by transport proteins in the membrane. Active transport. Some transport proteins act as pumps, moving substances across a membrane against their concentration gradients. Energy for this work is usually supplied by ATP Diffusion. Hydrophobic molecules and (at a slow rate) very small uncharged polar molecules can diffuse through the lipid bilayer. Figure 7. 17 Facilitated diffusion. Many hydrophilic substances diffuse through membranes with the assistance of transport proteins, either channel or carrier proteins.

Maintenance of Membrane Potential by Ion Pumps Membrane potential Voltage difference across membrane Inside is negative compared to outside Electrochemical gradient Caused by concentration electrical gradient of ions across a membrane

Electrogenic pump Transport protein that generates voltage across a membrane – ATP EXTRACELLULAR FLUID + – + H+ H+ Proton pump H+ – + H+ H+ + – CYTOPLASM Figure 7. 18 – + + H+

Cotransport: Coupled Transport by a Membrane Protein Active transport driven by a concentration gradient, ex. H 2 O pumped uphill does work on way down – + H+ ATP H+ + – H+ Proton pump H+ – + Sucrose-H+ cotransporter H+ Diffusion of H+ H+ – – Figure 7. 19 + + Sucrose

Based on the model of sucrose uptake in this figure, which of the following experimental treatments would increase the rate of sucrose transport into the cell? a) b) c) d) e) decreasing extracellular sucrose concentration decreasing extracellular p. H decreasing cytoplasmic p. H adding an inhibitor that blocks the regeneration of ATP adding a substance that makes the membrane more permeable to hydrogen ions

7. 5: Bulk transport across plasma membrane occurs by exocytosis & endocytosis Large proteins Cross membrane by different mechanisms

Exocytosis Transport vesicles migrate to plasma membrane, fuse with it, and release their contents (hormones, neurotransmitters)

Endocytosis Cell takes in macromolecules by forming new vesicles from the plasma membrane

3 types of endocytosis In phagocytosis, a cell engulfs a particle by Wrapping pseudopodia around it and packaging it within a membraneenclosed sac large enough to be classified as a vacuole. The particle is digested after the vacuole fuses with a lysosome containing hydrolytic enzymes. PHAGOCYTOSIS EXTRACELLULAR CYTOPLASM FLUID Pseudopodium 1 µm Pseudopodium of amoeba “Food” or other particle Bacterium Food vacuole An amoeba engulfing a bacterium via phagocytosis (TEM). In pinocytosis, the cell “gulps” droplets of extracellular fluid into tiny vesicles. It is not the fluid itself that is needed by the cell, but the molecules dissolved in the droplet. Because any and all included solutes are taken into the cell, pinocytosis is nonspecific in the substances it transports. Figure 7. 20 PINOCYTOSIS 0. 5 µm Plasma membrane Pinocytosis vesicles forming (arrows) in a cell lining a small blood vessel (TEM). Vesicle

Receptor-mediated endocytosis enables the cell to acquire bulk quantities of specific substances, even though those substances may not be very concentrated in the extracellular fluid. Embedded in the membrane are proteins with specific receptor sites exposed to the extracellular fluid. The receptor proteins are usually already clustered in regions of the membrane called coated pits, which are lined on their cytoplasmic side by a fuzzy layer of coat proteins. Extracellular substances (ligands) bind to these receptors. When binding occurs, the coated pit forms a vesicle containing the ligand molecules. Notice that there are relatively more bound molecules (purple) inside the vesicle, other molecules (green) are also present. After this ingested material is liberated from the vesicle, the receptors are recycled to the plasma membrane by the same vesicle. RECEPTOR-MEDIATED ENDOCYTOSIS Coat protein Receptor Coated vesicle Ligand Coated pit A coated pit and a coated vesicle formed during receptormediated endocytosis (TEMs). Coat protein Plasma membrane 0. 25 µm