The Fluid Mosaic Model and Passive Transport

The fluid mosaic model

A cell surface membrane is a phospholipid bilayer about $7$ nm thick. Each phospholipid has a hydrophilic phosphate head that faces the water on either side and two hydrophobic fatty acid tails that face inwards, so the bilayer forms spontaneously in water. The layer is fluid because individual phospholipids are free to move sideways, and it is a mosaic because proteins of many kinds are scattered through it. Intrinsic proteins span the whole bilayer and act as channels, carriers, pumps or receptors; extrinsic proteins sit on one surface only. Cholesterol packs between the phospholipid tails and regulates fluidity: it stops the membrane becoming too fluid when warm and stops the tails packing solid when cold. Short carbohydrate chains attached to proteins and lipids form glycoproteins and glycolipids, which act in cell recognition and as receptor sites.

What can cross, and how

Small non-polar molecules such as oxygen, carbon dioxide and steroid hormones dissolve in the hydrophobic core and cross by simple diffusion, moving down a concentration gradient without any protein and without ATP. Water crosses partly by simple diffusion and mainly through protein channels called aquaporins. Ions and larger polar molecules such as glucose cannot pass the hydrophobic core, so they use facilitated diffusion through channel or carrier proteins — still down a gradient and still without ATP, but limited by the number of proteins available.

Fick's law tells you what to change

The rate of diffusion is proportional to $\dfrac{\text{surface area} \times \text{difference in concentration}}{\text{distance}}$. Every gas exchange adaptation in the syllabus is an application of this one relationship: alveoli and villi give a huge surface area, a wall one cell thick gives a short distance, and a rich blood supply or ventilation keeps the concentration difference steep.