Introduction to Biomembranes

Biomembrane Transport

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Subject classification: this is a science resource.

Type classification: this is a lesson resource.

A biomembrane is a structure present in all cells, being either the external plasma membrane or internal membranes (e.g. endoplasmic reticulum, Golgi apparatus). Membranes exist in order to allow the cell to control and change its internal environment, and to protect important organelles from environmental damage or attack from pathogens. All Membranes consist of at least three substances:

Lipids
Proteins
Carbohydrates (Glyco-Proteins, -lipids, sterols for structure)

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 stabilised 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
- For hydrophobic molecules, occurs through the phospholipid bilayer down a concentration gradient (i.e. from a region of high concentration of the substance to a region of low concentration)
- For hydrophillic molecule diffusion, see Transport
Osmosis (net water flow)
- As water is hydrophillic, it cannot pass through the hydrophobic cell membrane, and so requires special hydrophillic proteins called aquaporins in order to assist their transport.
Transport (Passive or Active)
- Transport via proteins. This can be passive (i.e. down a concentration gradient, no energy input) or active (against a concentration gradient, requiring energy input)