CELL MEMBRANE STRUCTURE AND FUNCTION
Life at the edge
The plasma membrane controls traffic into and out of the cell it
The plasma membrane is selectively permeable, thus it allows some
substances to cross it more easily than others.
Cellular membranes are fluid mosaics of lipids and proteins
A phospholipid has a hydrophilic and a hydrophobic region.
Most of the proteins within membranes have both hydrophobic and
The fluid mosaic model, the membrane is a fluid structure with a ‘mosaic’
of various proteins embedded in or attached to a double layer of
The fluidity of membranes
A membrane is held together by hydrophobic interactions, which are
much weaker than covalent bonds.
A membrane remains fluid as temperature decreases until the
phospholipids settle into a closely packed arrangement and the
The membrane remains fluid to a lower temperature if it is rich in
phospholipids with unsaturated hydrocarbon tails.
The twists in the tails where double bonds are located, unsaturated
hydrocarbon tails cannot pack together as closely as saturated
hydrocarbon tails, and this makes the membrane more fluid.
The steroid cholesterol, which is between the phospholipid molecules in
the plasma membranes of animal cells, has different effects on membrane
fluidity at different temperatures.
At a higher temperature, the cholesterol makes the membrane less fluid
by restraining phospholipid movement.
Cholesterol is a temperature buffer that resists changes in membrane
fluidity that can be caused by changes in temperature.
Membrane proteins and their functions
A membrane is a collage of different proteins embedded in the fluid
matrix of the lipid bilayer.
Phospholipids from the main fabric of the membrane, but proteins
determine most of the membrane’s functions.
There are two membrane proteins:
- Integral proteins; penetrate the hydrophobic core of the lipid bilayer.
- Peripheral proteins are loosely bound to the surface of the membrane,
often to exposed parts of integral proteins.
On the cytoplasmic side of the plasma membrane, some membrane
proteins are held in place by attachment to the cytoskeleton. On the extracellular side, certain membrane proteins are attached to
fibers of the extracellular matrix.
The role of membrane carbohydrates in cell-cell recognition
Cell-cell recognition, a cell’s ability to distinguish one type of
neighbouring cell from another.
Membrane carbohydrates are covalently bonded to lipids, forming
molecules called glycolipids. But some are covalently bonded to proteins
Synthesis and sidedness of membranes
1. Synthesis of membrane proteins and lipids in the ER. Carbohydrates are
added to the proteins, making them glycoproteins. The carbohydrate
portions are then modified.
2. Inside the golgi apparatus, the glycoproteins undergo further
carbohydrate modification, and lipids obtain carbohydrates, becoming
3. The transmembrane proteins, membrane glycolipids, and secretory
proteins are transported in vesicles to the plasma membrane.
4. The vesicles fuse with the membrane, releasing secretory proteins from
the cell. Vesicle fusion positions the carbohydrates of membrane
glycoproteins and glycolipids on the outside of the plasma membrane.
The permeability of the lipid bilayer
Nonpolar molecules, carbon dioxide and oxygen, are hydrophobic and can
therefore dissolve in the lipid bilayer of the membrane and cross it easily,
without the aid of membrane proteins.
Polar molecules, glucose and other sugars, pass slowly through the lipid
bilayer, and even water, and extremely polar molecule does not cross
A charged atom or molecule and its surrounding shell of water find the
hydrophobic layer of the membrane even more difficult to cross.
Proteins built into the membrane play key roles in regulating transport.
Cell membranes are permeable to specific ions and a variety of polar
These hydrophilic substances can avoid contact with the lipid bilayer by
passing through transport proteins.
Some transport proteins, called channel proteins, function by having a
hydrophilic channel that certain molecules or atomic ions use as a stunnel
through the membrane.
Carrier proteins hold onto their passengers and change shape in way that
shuttles them across the membrane.
A transport protein is specific for the substance it moves, allowing only a
certain substance/s to cross the membrane. Passive transport is dif