CELL MEMBRANE CELL MEMBRANE - is a 2-dimensional fluid mosaic of lipids and proteins and also is in constant motion. - It’s a thin, elastic structure, ~7.5 nanometers thick and envelops the cell. - Composed mainly of proteins and lipids. - Basic structure is a lipid bilayer - a thin film of ~ 2 molecules thick and continuous over the entire cell surface.

2. CHOLESTEROL 3. GLYCOLIPID. CELL MEMBRANE Lipid bilayer is made of 3 types of molecules ; 1. PHOSPHOLIPIDS 2. CHOLESTEROL 3. GLYCOLIPID.

Cholesterol

iii) GLYCOLIPIDs -are lipids with attached carbohydrate groups ~ 5% of membrane lipids - Non-polar parts - are the fatty acid “tails” Polar parts - are the attached carbohydrate groups Only appear on membrane layer that faces extra-cellular fluid (one reason for the asymmetry of the two sides).

Asymetrical Distribution of Lipids Extracellular Intracellular Modified from Figure 11-17, Page 355 from: Essential Cell Biology by Alberts et al. 1997, Garland Publishing Inc. New York, NY

Cholesterol can fill gaps between phospholipids Saturated FA’s = increase in fluidity Modified from Figure 11-16 and Panel 2-4 from: Essential Cell Biology: An introduction to the Molecular Biology of the Cell by Alberts, Bray, Johnson, Lewis, Raff, Roberts and Walter 1997, Garland Publishing Inc. New York, NY

MEMBRANE PROTEINS 1. PERIPHERAL PROTEINS Are membrane proteins that are located on the periphery of membranes and they are either on the cell surface or on the inside of cell They associate with membrane lipids or integral proteins at inner or outer surface of the membrane. They can be stripped away from membrane by methods that do not disrupt membrane integrity FUNCTION 1. As enzymes on cell surfaces 2. As regulatory portions of ion channels and transmembrane receptors 3. Roles in cell signaling – by some reversible attachment of proteins on cell surface

Membrane Proteins 14

The proteins in the plasma membrane may provide a variety of major cell functions. Fig. 8.9 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings 15

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2. SELECTIVE PERMEABILITY PROPERTIES OF CELL MEMBRANES 2 main properties are; 1. FLUIDITY 2. SELECTIVE PERMEABILITY

PROPERTIES OF CELL MEMBRANES 1. FLUIDITY – (2-dimensional fluid in constant motion) SIGNIFICANCE : Allows for fusion of membranes (e.g. fusion of vesicles with organelles) Allows for diffusion of new lipids and new proteins laterally, so they are equally distribution Allows for diffusion of proteins and other molecules laterally across the membrane in signaling/reactions Allows for proper separation of membranes during cell division

MEMBRANE FLUIDlTY is determined by; PROPERTIES OF CELL MEMBRANES - FLUIDITY MEMBRANE FLUIDlTY is determined by; 1. LIPID COMPOSITION 2. TEMPERATURE

Phospholipids

Cholesterol 22

Cholesterol

PROPERTIES OF CELL MEMBRANES - FLUIDITY 2. TEMPERATURE Organisms regulate lipid composition (thus membrane fluidity) in response to temperature. Cold/ low temperatures – membranes “gel” and are not fluid. Hot/ high temperature – membranes are too fluid and become “leaky” allowing ions to cross. Hibernating animals incorporate more unsaturated fatty acids (fatty acids with double bonds) to prepare for drop in their body temperature .

PROPERTIES OF CELL MEMBRANES - SELECTIVE PERMEABILITY SELECTIVE PERMEABILITY OF MEMBRANE – they are selective for the movement of molecules across the membrane. MOVEMENT OF MOLECULE across membrane is limited by; i) SIZE - small molecules cross membrane and large molecules do not E.g. Water, O2, CO2, ethanol (46 MW) and glycerol (92 MW) can cross the membrane - Glucose (180 MW) can NOT cross membrane ii) POLARITY - Hydrophobic molecules can “dissolve” in the lipid bilayer, not polar molecules. E.g. Ethanol is more hydrophobic than glycerol so crosses membrane faster iii) IONIC CHARGE-– membranes are highly impermeable to ions But ions and large molecules do pass across biological membranes – through proteins that pass through membranes (integral proteins – channel/transport protein)

15.1 A pure phospholipid bilayer acts as a selectively permeable barrier Figure 15-1

Membrane Transport Simple Diffusion - 2 types of molecules 1. Small, nonpolar Oxygen 2. Small, polar, noncharged H2O (some - more later) Ethanol

Water Channels- The aquaporins Selectivity filter generated by hydrophobic residues that line the channel allowing only one molecule of water to pass at a time HO H H2O H2O H2O Aquaporins

Membrane Transport

Ion Channels -small aqueous holes Properties selective fast passive gated - open or closed

Ion Channels -small aqueous holes Properties selective fast passive gated - open or closed

CHANNELS The pore in some channels can be opened or closed. Opening/ closing of channels are controlled/gated by a specific stimulus. Example of a specific stimulus: i) Voltage; - VOLATGE - GATED CHANNEL ii) Ligand; - LIGAND -GATED CHANNELS iii) Specific stress; - STRESS – ACTIVATED CHANNELS

VOLTAGE - GATED CHANNEL Example; Na+ voltage gated channels opens when the membrane potential depolarizes (i.e. becomes more positive). It has activation and inactivation gates.

LIGAND- GATED CHANNEL Binding of a chemical (ligand) to a specific site on the receptor causes a change in membrane potential and causes it to allow a specific ion to pass through the channel in the membrane.

STRESS - ACTIVATED GATED ION CHANNEL The channels open/close when a physical stress is applied to the channel protein E.g. Auditory hair cells converts a physical stress to an electrical signal.

Selectivity Filter K+ O H - O O

Selectivity Filter K+ O H - O O

Selectivity Filter K+ O H - O O

Selectivity Filter O H - O H - K+ O O

Selectivity Filter O H - O H - O O K+

Selectivity Filter O H - O H - O O K+ O H - O H -

Selectivity Filter O H - O H - O O K+ O H - O H -

Selectivity Filter O H O H Na+ O O

Selectivity Filter Na+ O H O O

Selectivity Filter Na+ O H O O

زندگي ، خواب گراني است به ارزاني عمر

3 Types of Gating

Active Transport REQUIRES ENERGY! Used to move molecules against a gradient REQUIRES ENERGY! Two types . Pumps . Coupled Transport (co-transport)

Na+/K+ ATPase

Primary Active Transport: Pumps Products Figure 5-24: Mechanism of the Na+ - K+ -ATPase (75%)

Secondary Active Transport: Uses Kinetic Energy of [ion] Cotransports [Ion ] restored using ATP Figure 5-25: Sodium-glucose symporter

Endocytosis Exocytosis

(extracellular fluid) pinocytosis (extracellular fluid) 1 3 3 2 vesicle containing extracellular fluid (cytoplasm) cell (b) Figure: 03-07 Title: Two types of endocytosis. Caption: (a) To capture drops of liquid, a dimple in the plasma membrane deepens and eventually pinches off as a fluid-filled vesicle, which contains a random sampling of the extracellular fluid. (b) Pseudopodia encircle an extracellular particle. The ends of the pseudopodia fuse, forming a large vesicle that contains the engulfed particle. phagocytosis food particle pseudopod 1 2 3 particle enclosed in vesicle

Endocytosis and Exocytosis: VacuoleTransport Figure 5-28: Receptor-mediated endocytosis and exocytosis

Transepithelial and Transcytosis Cross two membranes Apical Basolatera Absorption Secretion

Transepithelial and Transcytosis Figure 5-30: Transepithelial transport of glucose

Transepithelial and Transcytosis Figure 5-31: Transcytosis across the capillary endothelium