Introduction to Biomembranes
Biomembrane Transport
| Educational level: this is a tertiary (university) resource.
|
| Subject classification: this is a science resource .
|
| Resource type: this resource is a lesson.
|
A biomembrane is a structure presenting all cells being it the plasma membrane or internal membranes (e.g. endoplasmic reticulum, Golgi apparatus).
All Membranes consist of three substances
- Lipids
- Proteins
- Carbohydrates (Glyco-Proteins, -lipids)
The ratio between lipid and proteins depends on the function of the cell, some examples of different cell membrane compositions are shown in the table below.
| Membrane |
% By weight |
| |
Protein |
Lipid |
Carbohydrate |
| Myelin |
18 |
79 |
3 |
| Human erythtocyte |
49 |
43 |
8 |
| Mouse liver |
44 |
52 |
4 |
| Ameba |
54 |
42 |
4 |
| Chloroplast spinach lamellae |
70 |
30 |
0 |
| Halobacterium purple membrane |
75 |
25 |
0 |
| Mitochondrial inner membrane |
76 |
24 |
0 |
Membranes as Lipid Bilayers
Membranes are made of a lipid by layer of phospholipids which are made up of a hydrophilic "polar" heads(water) and hydrophobic fatty acid tails (oil). Phospholipids will spontaneously form lipid bilayers in aqueous solution; tails aggregate and exclude water from a core, the structure is stabalised by tail-tail/ head-head interactions. The polar head region's charge is variable and fatty acid (R) chains vary in length from C12 to C22 as well as in their degree of unsaturation (number of double bonds).Each C=C bond produces a rigid kink in the the phospholipid tail. In 1972 Singer and Nicholson proposed the "Fluid Mosaic" model suggesting that the membrane itself is a two dimensional sea of mobile lipid in which proteins diffuse or 'float'.
Mechanisms by which substances cross the membrane
- Diffusion
- Osmosis (net water flow)
- Transport (Passive or Active)
