JESS'S A LEVELS: Biology: Chapter 4: Cell surface membrane: Fluid mosaic structure, it's components and function

Biology: Chapter 4: Cell surface membrane: Fluid mosaic structure, it's components and function

Phospholipid

Phospholipids are lipid molecules that consist of a polar head (hydrophilic) and non-polar tail (hydrophobic) because one of the fatty acid groups is replaced with a phosphate group, making the head polar.
This means in water or solutions, the water-loving phospholipid heads will be in the liquid whereas the non-polar hydrophobic tails will avoid water or liquid, and are either on the surface of the water (monolayer) or point towards each other, forming a layer.
Micelle: Ball-like structure formed by phospholipids, where the polar heads are on the exterior, shielding the hydrophobic tails which point in towards each other.
Bi-layer: Two layered structure - shown in the diagram below. Basic structure of membranes.

The phospholipid bilayer forms a membrane-bound compartment where chemicals can be isolated from the external environment, and exchange between the cell/organelles and the outside environment (eg. respiration and excretion) can be controlled.

Diagram of liposomes (vesicles), micelle and the bilayer sheet

Cell surface (plasma) membrane.

The phospholipid bi-layer is visible using the electron microscope on a very high magnification (x100,000 -One hundred thousand)
Around 7 nm thick

Fluid: Because phospholipids and proteins can move around by diffusion - Phospholipids mainly move sideways within their own layer (monolayer) , while some proteins can move within the bi-layer. The fluidity of the bilayer is similar to that of olive oil.
Mosaic: Pattern of scattered proteins when the membrane is viewed from above.
Model: Because we cannot magnify a cell enough to see the cell membrane, so we just have a model of what we predict the membrane looks like.

Double layer (bi-layer) of phospholipids
Individual phospholipid molecules move around their own mono-layers by diffusion.
Phospholipid tails point inward, forming non-polar hydrophobic interior
Phospholipid heads face outwards, into the aqueous (water-containing) medium that surrounds the membranes.
Some phospholipid tails are saturated and some are unsaturated (eg. cholesterol).
The more unsaturated they are, the more fluid the membrane is; this is because unsaturated fatty acid tails are bent, and therefore fit together more loosely.
Tail length also affects fluidity: the longer the tail, the more fluid the membrane is.
When temperature decreases, membranes become less fluid because there is less kinetic energy.
Some organisms, such as bacteria and yeasts, who cannot regulate their own body temperature, respond by increasing the proportion of unsaturated fatty acids in their membranes to maintain fluidity.

Unsaturated tails in the phospholipid bi-layer

Fluid mosaic model