1. Membrane Structure & Function
BIOLOGY 11 ADVANCED
Membrane Structure & Function

2. Life At The Edge
The plasma membrane is the boundary that separates the cell from it’s surroundings. Remarkably enough, it’s only 8nm thick, and yet controls all the traffic into and out of the cell.
The plasma membrane is selectively permeable – which means that is allows some substances to cross through more easily than others.
The most abundant lipids in most membranes are phospholipids – amphipathic molecules that have both a hydrophilic and hydrophobic region.

3. The Fluid Mosaic Model
The membrane is a fluid structure with a “mosaic” of various proteins embedded in or attached to a double layer (bilayer) of phospholipids.
Phospholipids move laterally in a membrane, but flip-flopping across the membrane is quite rare. Lateral movement occurs around 107 times per second, which means phospholipids can travel about 2μm in one second – the length of a typical bacteria cell.
The steroid cholesterol is wedged between phospholipid molecules in the plasma membrane of cells, and has different effects on membrane fluidity at different temperatures.
Membranes must be fluid to work properly!
There are two major populations of membrane proteins.

4. Integral Proteins
Integral Proteins – Penetrate the hydrophobic core of the lipid bilayer. Many are trans membrane proteins, which completely span the membrane. The hydrophobic regions of an integral protein consists of one or more stretches of nonpolar amino acids, usually coiled into α helixes. The hydrophilic parts of the molecule are exposed to the aqueous solutions on either side of the membrane.

5. Peripheral Proteins
Peripheral Proteins – Are not embedded in the lipid bilayer at all; they are appendages loosely bound to the surface of the membrane, often to the exposed parts of integral proteins.

6. Glycoproteins
Glycoproteins – Carbohydrates that are bonded to protein. Note: glyco refers to the presence of carbohydrate.

7. Six Major Functions Transport
A protein that spans the membrane may provide a hydrophilic channel across the membrane that is selective for a particular solute. Other transport proteins shuttle a substance from one side to the other by changing shape. Some of these proteins hydrolyze ATP as an energy source to actively pump substances across the membrane.

8. Six Major Functions 2. Enzymatic Activity
A protein built into the membrane may be an enzyme with its active site exposed to substances in the adjacent solution. In some cases, several enzymes in a membrane are organized as a team that carries out sequential steps of a metabolic pathway.

9. Six Major Functions 3. Signal Transduction
A membrane protein may have a binding site with a specific shape that fits the shape of a chemical messenger, such as a hormone. The external messenger (signal) may cause a conformational change in the protein (receptor) that relays the message to the inside of the cell.
An example of this would be turning glucose into pyruvate.

10. Six Major Functions 4. Cell-cell recognition
Some glycoproteins serve as identification tags that are specifically recognized by other cells.

11. Six Major Functions 5. Intercellular Joining
Membrane proteins of adjacent cells may hook together in various kinds of junctions, such as gap junctions or tight junctions. .

12. Six Major Functions
6. Attachment to the cytoskeleton and extracellular matrix (ECM)
Microfilaments or other elements of the cytoskeleton may be bonded to membrane proteins, a function that helps maintain shape and stabilizes the location of certain membrane proteins. Proteins that adhere to the ECM can coordinate extracellular and intracellular changes.