CELL MEMBRANECELL 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.

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

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

Extracellular

Intracellular

Cholesterol can fill gaps between phospholipids

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

Saturated FA’s = increase in fluidity

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

•The proteins in the plasma membrane may provide a variety of major cell functions.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Fig. 8.9

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

PROPERTIES OF CELL MEMBRANES - FLUIDITY

MEMBRANE FLUIDlTY is determined by;

1 .LIPID COMPOSITION

2 .TEMPERATURE

Phospholipids

Cholesterol

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

2. 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

H2O H2O

HO H HO

H2OAquaporins

• Selectivity filter generated by hydrophobic residues that line the channel allowing only one molecule of water to pass at a time

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 CHANNELii) Ligand; - LIGAND -GATED CHANNELSiii) 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.

O

K+O

H

H

-

OH

H

-

O

Selectivity Filter

O

K+O

H

H

-

OH

H

-

O

Selectivity Filter

OK+

OH

H

-

OH

H

-

O

Selectivity Filter

O K+OH

H

-

OH

H

-

O

Selectivity Filter

O

K+

OH

H

-

OH

H

-

O

Selectivity Filter

O

K+

OH

H

-

OH

H

-

O

Selectivity Filter

OH

H

-

OH

H

-

O

K+

OH

H

-

OH

H

-

O

Selectivity Filter

OH

H

-

OH

H

-

O

Na+

O

Selectivity Filter

OH

H OH

H

O O

Selectivity Filter

Na+OH

H OH

H

O O

Selectivity Filter

Na+OH

H OH

H

عمر ارزاني به است گراني خواب ، زندگي

3 Types of Gating

Active Transport

• Used to move molecules against a gradient

–REQUIRES ENERGY!

• Two types

1 . Pumps

2 . Coupled Transport (co-transport)

Na+/K+ ATPase

Primary Active Transport: Pumps Products

Figure 5-24: Mechanism of the Na+ - K+ -ATPase (75%)

• Cotransports

• [Ion ] restored– using ATP

Secondary Active Transport: Uses Kinetic Energy of [ion]

Figure 5-25: Sodium-glucose symporter

EndocytosisExocytosis

(extracellular fluid)

(cytoplasm)

food particle

particle enclosed in vesicle

phagocytosis

vesicle containing extracellular fluid

cell

pseudopod

pinocytosis

(a)

(b)

133

2

1 32

Endocytosis and Exocytosis: VacuoleTransport

Figure 5-28: Receptor-mediated endocytosis and exocytosis

• Cross two membranes – Apical – Basolatera

• Absorption• Secretion

Transepithelial and Transcytosis

Transepithelial and Transcytosis

Figure 5-30: Transepithelial transport of glucose

Transepithelial and Transcytosis

Figure 5-31: Transcytosis across the capillary endothelium