Membrane Structure and Function
1. Membrane Models
2. Fluid mosaic of lipids, proteins, and carbohydrates
3. Selective permeability
4. Passive transport. Osmosis.
5. Active Transport
6. Exocytosis and endocytosis
The cell membrane
- The plasma membrane is the boundary that separates the living cell from its nonliving
- Membranes are of crucial importance to life, because a cell must spate itself from the
outside environment for two major reasons
o It must keep its molecules of life (DNA, RNA. Proteins) from dissipating away.
o It must keep out foreign molecules that damage or destroy the cells components
1. Membrane Models
- Two generations of membrane models
o A) The Davidson-Danielli model. Sandwiched phospholipid bilayer between two protein
Widely accepted until 1970.
o B) The fluid mosaic model disperses the proteins and immerses them in the
phospholipid bilayer, which is in a fluid state. Singer and Nicolson proposed
- Proteins are individually embedded in the phospholipid bilayer, rather than forming a solid
coat spread upon the surface.
- Hydrophilic portions of both proteins and phospholipids are maximally exposed to water
resulting in a stable membrane structure
- Hydrophobic portions of proteins and phospholipids are in the nonaqueous environment
inside the bilayer.
- Membrane is a mosaic of proteins inserted in a fluid bilayer of phospholipids.
The cell membrane
- The cell membrane functions as a semi-permeable barrier, allowing very few molecules
across it while fencing the majority of organically produced chemicals inside the cell.
- The most common molecule in the model is the phospholipid which is polar
(hydrophilic)head and two (hydrophobic) tails
- (a) water can be coated with a single layer of phospholipid molecules
- The hydrophilic heads of phospholipids are immersed in water, and the hydrophobic tails
are excluded from water.
- (b) a bilayer of phospholipids forms a stable boundary between two aqueous
- This arrangement exposes the hydrophilic parts of the molecules to water and shields the
hydrophobic parts from water.
2. The fluid mosaic of lipids, proteins and carbohydrates
- A membrane is held together primarily by the hydrophobic interactions, which are much
weaker than covalent bonds.
Movement of phospholipids
- Most of the lipids and some of the proteins can
drift in the plane of the membrane, but not from
one layer to another.
- Phospholipids move quickly along the
membranes plane averaging 2 MicroM per second. Proteins drift more slowly.
Evidence for the drifting of membrane proteins
- When researchers fuse a human cell with a mouse cell, it takes
less than an hour for the membrane proteins of the two
species to completely mix in the membrane of the hybrid cell.Membrane fluidity
- Tails with kinks are keeping molecules from packing together, enhancing membrane fluidity.
Cholesterol within the membrane
- Cholesterol reduces membrane fluidity by reducing phospholipid
movement at the moderate temperatures and also hinders
solidification at low temperature: it make the membrane less fluid
at warm temps and more fluid at lower temp.
Sidedness of the plasma membrane
- The membrane has distinct cytoplasmic and extracellular sides.
- The bifacial quality determined when the membrane is first synthesized and modifies by the
ER and Golgi
- The side facing the inside of the ER, Golgi and vesicles is topologically equivalent to the
extracellular surface of the plasma membrane,
- The other side always faces the cytosol, from the time the
membrane is made by the ER to the time it is added to the plasma
membrane by fusion of a vesicle.
- The small green trees represent the