Chapter 7 Chapter 7 MembranesMembranes

Functions of membranesFunctions of membranes1. Boundaries and serve as permeability barriers.2. Sites of specific proteins and therefore of specific

functions. 3. Membrane proteins regulated the transport of solutes. 4. Membrane proteins detect and transmit electrical and

chemical signals. 5.   Membrane proteins medicate cell adhesion and cell-to-

cell communication.

Figure 7-1

The Prominence of membrane around and within Eukaryotic cells

Models of Membranes structure

1. Lipid nature of membrane: It is the pioneering work of Chlarles Overton in the 1890s.

2. Lipid monolayer: It is the working of Irving Langmair in 1900s.

3. Lipid bilayer: 1935s Hugh Davson & James Danielli suggested that proteins are present in membranes and its consist of lipids bilayer that are coated on both sides with thin sheet of proteins .

Models of Membranes structure 1. Unite membranes: In 1950s J.David

Robertson suggested that all cellular membranes share a common underlying structure.

2. Fluid mosaic model :Its consummation in 1970s was proposed by S.Jonathon singer & Groth Nicolson.

3. Membrane protein structure: Unwin & Richard Henderson used electron microscopy to determine the three dimensional structure of unfixed crystallized bacteriorhodopsion and its position in the membrane.

Timeline for development of the fluid mosaic model

Figure 7-5

The fluid mosaic model of membrane

structure

Membrane LipidsMembrane Lipids The main classes of membrane lipids:1. Glycolipids2. Sterols3. Phospholipids

Figure 7-6

The main classes of membrane lipids

GlycolipidsGlycolipidsGlycolipids are formed by the addition of carbohydrate groups

to lipids. The most common glycolipids are:1. Cerebrosides: Also known as neutral glycolipids because

of the uncharged sugars as head groups.2. Gangliosides*Both of the above types of glycolipids are found mostly in

brain cells and nerve cell membranes.

SterolsSterolsThe major sterols in animal membranes are

cholesterol. It maintains and stabilizes membranes of our bodies. Plants have different types of sterols similar in function but different in structure, even fungi have a unique sterol known as ergosterol.

*Sterols aren’t fount in the inner membrane of mitochondria or chloroplasts.

PhospholipidsPhospholipids Phospholipids are the most abundant lipids in membranes. There are 2 types of phospholipids:A-Phosphoglycerides: glycerol-basedB-Sphingolipids: sphingosine-based The most common phophoglyerides are: 1-Phosphatydylcholine2-Phosphatidlyethanolamine3-Phophatidylamine*The portions of any of these lipids vary according to the type of membrane and its function.

Fatty AcidsFatty Acids Fatty acids are components of all membrane lipids except sterols The asymmetry of membranes depends partly on the degree of

unsaturation in the fatty acids and the different types of lipids there are.

Unsaturated fatty acids fail to tightly pack because of the kinks made from double bonds.

kink

Membrane FluidityMembrane FluidityMembrane fluidity is caused by movement of lipids from

one monolayer to the other.The type of movement's maybe any of the following:1. Transverse Diffusion (flip-flop) 2. Lateral Diffusion (movement in the plane of

the membrane)3. Or Rotation of phospholipids about their long

axisOne of the techniques used to overview these

movements includes Florescence recovery after photobleaching also known as FRAP.

Membrane ProteinsMembrane Proteins A technique called freeze fracturing was

used to support the fluid mosaic model. Membranes contain:1. Integral2. Peripheral3. Lipid-Anchored proteins4. Fatty acid-anchored membrane proteins5. Isoprenylated membrane proteins6. GPI-anchored membrane proteins

Figure 7-16a

Membrane ProteinsMembrane Proteins

Peripheral Membrane ProteinsPeripheral Membrane Proteins Peripheral membrane proteins are bound to membrane

surfaces through weak electrostatic forces and hydrogen bonding with the hydrophilic portions of the lipid bilayer, perhaps with the polar head groups of membrane lipids.

Spectrin, Ankyrin, and Band 4.1 are main peripheral proteins found in erythrocyte membranes. These proteins are found on the inner surface of the plasma membrane and forms a skeletal network that maintains the erythrocyte shape.

Integral Membrane ProteinsIntegral Membrane Proteins Integral membrane proteins have hydrophobic regions

embedded within the membrane which makes it difficult to remove them. Because of their affinity for the lipid bilayer they are difficult to extract and study.

Types of integral membrane proteins:1. Integral monotopic proteins: Those that protrude only

from one side2. Transmembrane proteins: Those that have hydrophilic

regions protruding from both sides of the membrane, they can be:

A- Single pass: Cross the membrane once (Example: Glycophorin)

B- Multipass: Cross the membrane several times (Example: Band 3)

Lipid-Anchored Membrane Proteins:Lipid-Anchored Membrane Proteins: Lipid-anchored proteins are located on either side of the surfaces of the lipid bilayers but are covalently bonded to lipid molecules embedded within the bilayer.

Fatty Acid-Anchored Membrane Proteins:Fatty Acid-Anchored Membrane Proteins: Its Its synthesized in the cytosol and then is attached to a saturated fatty acid usually myristic acid.

Isoprenylated Membrane Proteins:Isoprenylated Membrane Proteins: Its synthesized in the cytosol as soluble cytosolic proteins then is modified.

GPI-Anchored Membrane Proteins:GPI-Anchored Membrane Proteins: GPI (glycosylphophatidylinositol) is a glycolipid found in the outer layer of the plasma membrane. These types of membrane proteins are made in the ER.

Isolation of ProteinsIsolation of Proteins SDS- Sodium Dodecyle Sulphate: Allows

integral membrane proteins to be isolated in addition to being fractionated and analyzed by electrophoresis.

Membrane protiens have many functions which include:

1. Enzymatic2. Electron transporters3. Transport proteins that transport nutrients 4. Transport ATPase

Glycosylated ProteinsGlycosylated Proteins The addition of carbohydrate groups to the protein is

called glycosylation. If the linkage of the carbohydrate is to a nitrogen atom of an amino group then its called N-Linked Glycosylation. If the carbohydrate group is linked to an oxygen atom of a hydroxyl group its known as O-Linked Glycosylation.

Glycoproteins and glycolipids that protrude from the outer surface of plasma membranes form a surface coat called glycocalyx.