Introduction to Biomembranes

Biomembrane Transport

Educational level: this is a tertiary (university) resource.

Subject classification: this is a science resource.

Type classification: this is a lesson resource.

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

Example of a Lipid Bilayer

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)