Cell membrane by dr.qamar

PLASMA MEMBRANESBY

DR. QAMAR SULTAN

Objectives What are cell membrane? Composition of cell membrane. The structural models of plasma membrane. Function of lipids, proteins and carbohydrates in the plasma

membrane. Functions of plasma membrane in general.

PLASMA MEMBRANES

Plasma membrane. Endo-membrane system.

Nuclear membrane. Endoplasmic reticulum. Vesicles. Golgi apparatus. Chloroplast in plants.

COMPOSITION OF BIOMEMBRANES The cell membrane (also called

the plasma membrane), which envelops the cell, is a thin, pliable,elastic structure only 7.5 to 10 nanometers thick.

It is composed almost entirely of proteins and lipids.

The approximate composition is:

proteins, 55 per cent; phospholipids, 25 per cent;cholesterol, 13 per cent; other lipids, 4 per cent; carbohydrates, 3 per cent.

STRUCTURAL MODELS OF PLASMA MEMBRANE Sandwich model. Unit membrane model. Fluid Mosaic Membrane model.

PLASMA MEMBRANE- FLUID MOSAIC MODEL

Fluid lipid bilayer embedded with proteins.

Phospholipids Polar end: hydrophilic. Non-polar end:

hydrophobic. Small amount of

carbohydrates. Cholesterol :

between phospholipids. Contributes to fluidity and

stability of cell membrane.

PHOSPHOLIPIDS: The “Backbone” of the Membrane

PHOSPHOLIPIDS: The “Backbone” of the Membrane

Cartoon of a phospholipid molecule

This cartoon depicts the basic Amphipathic structure common to all phospholipids

Fatty acids

Glycerol pluspolar side Group.

PHOSPHOLIPIDSPHOSPHOLIPIDS Cartoon of a phospholipid molecule

.

Membrane lipids

1. Phospholipids.

2. Glycolipids.

3. Cholesterol.

Phospholipids are amphipathic molecules

Cell membrane is essentially a bilayer of phospholipids.

Phospholipids:

hydrophobic疏水的

hydrophilic亲

Membrane Proteins INTEGRAL PROTEINS

These are the proteins floating within the lipid sea. Have both hydrophilic and hydrophobic areas. Penetrate completely through or partially into the phospholipid bilayer. Hydrophobic areas are embedded inside the bilayer to avoid contact with water.

TRANSMEMBRANE PROTEINS

These are a type of integral proteins. And these are proteins embedded in lipid layer or passing through and through the membrane. Examples: G protein coupled receptors, ion channels, pumps, transporters.

PERIPHERAL PROTEIN

Are attached to inner and outer surface of membrane. Hydrophilic (water-loving). In contact with hydrophilic heads of phospholipids. G- proteins.

Channels and carriers are needed to get ions across the bilayer

Channels and carriers are needed to get ions across the bilayer

FUNCTIONS FOR MEMBRANE PROTEINS

Form water-filled pathways, or channels across bilayer for transport.

Carrier molecules for transport. Membrane-bound enzymes. Receptor sites. Cell adhesion molecules (CAMs)

which perform the function of intercellular joining.

Recognition molecules. enzymes

Carbohydrates

GLYCOCALYX:

The loose carbohydrate coat on the outer surface ( hydrophilic in nature) of cell. It is made of

Glycoproteins = Carbohydrate + protein Glycolipids = Carobhydrate + lipid. IMPORTANT FUNCTIONS OF GLYCOCALYX:

(1) Many of them have a negative electrical charge, which gives most cells an overall negative surface charge that repels other negative objects e.g. bacteria.

(2) The gly-cocalyx of some cells attaches to the glycocalyx of other cells, thus attaching cells to one another.

(3) Many of the carbohydrates act as receptor substances for binding hormones, such as insulin; when bound, this combination activates attached internal proteins that, in turn, activate a cascade of intracellular enzymes.

(4) Some carbohydrate moieties enter into immune reactions.

BIO-MEMBRANE FUNCTIONS

1. It subdivides cell into compartments (organelles). And also separates the

ICF from ECF. Selective permeability of plasma membrane is the basis of

differential concentration of electrolytes in ECF and ICF.

2. Regulation of cell contents: It allows for different conditions between

inside and outside of cell.

3. It provides protection to the cell

4. It acts as material exchange gateway (transport function).5. It allow selective receptivity and signal transduction by providing

transmembrane receptors that bind signaling molecules6. It allow cell recognition.7. It provide anchoring sites for cytoskeleton components. This allows the cell

to maintain its shape. 8. It provides physical and functional connections with other cells or the

extracellular matrix so that the cells group together to form tissues.