The membrane has a bi-phospholipid layer, as there are two pairs of head and tails.

How are they made:
Phospholipids spontaneously create a membrane in an aqueous environment.
The head is always towards the water.

The Endomembrane system:
The ER, Mitochondria, Golgi Body, Lysosomes, etc. (Just NOT peroxisomes)
The ER takes the most amount of space.

Membranes are made up of THREE molecules:

  1. Two phospholipids
  2. Channel Proteins
  3. Cholesterol for structure

Two kinds of membranes:
If the tails are short and small they might just form a Micelle (small, single-layered cell).

Compartmentalization:
The fact that certain things like the lysosomes are enclosed in their own acidic environment makes it possible for them to work optimally, without damaging the other organelles.

Peripheral proteins:
Loosely bound on the surface and DON’T go through.
They are Hydrophilic, with some charges and polar side groups so they can interact with the membrane head.
Some have hydrophobic cores.

Integral Proteins:
They go into the membrane (some go through Transmembrane proteins/channel proteins).
Typically 20 – 25 hydrophobic amino acids in an alpha helix are enough for a membrane-spanning protein.
Can be hydrophilic AND hydrophobic and penetrate the interior of the bilayer.

The Fluid Mosaic Model:
The membrane IS NOT static, but always moving, and structures in it are held in by mostly Hydrophobic interactions (not as strong as covalent bonds).
The things in the membrane can semi-freely move around in the membrane.

Cholesterol:
A steroid that is randomly distributed, to limit the fluidity of the bilayer. (It regulates and expands the range of temperatures the membrane works in):

  • In Cold environments it makes the membrane MORE fluid
  • In Warm environments it TIGHTENS the membrane (making is tougher)

Membrane fluidity:
The more Saturated the lipids are the more dense and rigid the membrane is.
This is because the unsaturated fats are bend, and can’t tighten together as easily.
Some fish actually make themselves more saturated when the temperature in the environment decreases.

Carbohydrates in the membrane:
VERY important as they work as an ID-Card, so cells can recognize each other, and immune cells can see if the cell is harmless or dangerous.

Glycoproteins:
One or more carbohydrates have attached to a peripheral membrane protein (Blue color).
(2 – 60 monosaccharides).

Glycolipids:
One or more carbohydrates have attached to the lipid in the membrane (Green color).
(2 – 60 monosaccharides).