Unit 1.1 Topic 7

Biological Membranes - fluid boundaries.

Surrounding all cells, separating the cell contents from the outside world, are membranes. In eukaryotic cells, membranes are also found around many organelles. As well as separating the cell components from each other, membranes carry out a number of different functions. The basis structure of all membranes is the same.

The roles of membranes.

The major roles of membranes include:

• separating cell contents from the outside environment.
• separating cell components from cytoplasm.
• cell recognition and signalling.
• holding the components of some metabolic pathways in place.
• regulating the transport of materials into or out of cells.

The nature of phosopholipids.

The basic structure of a phospholipid consist of a phosphate head, which is hydrophilic - water loving, while the two fatty acid tails are hydrophobic- water hating. These properties come from the way charges are distributed across the molecule.

Molecules with charges that are evenly distributed around the molecule do not evenly dissolve or mix with water, and in fact repel water molecules. You can see this if try to mix oil and water. The oil quickly separates from the water, forming a layer on the surface, which can easily be skimmed off. Molecules with more unevenly distributed charges can interact with water molecules quite easily.

If phospholipids molecules are mixed with water, they form a layer at the water surface. The phosphates head sticks into the water, while the fatty acid tails stick up out of the water.

Layers and bilayers.

If phospholipids molecules are completely surrounded by water, a bilayer can form. Phosphate heads on each side of the bilayer stick into the water, while the hydrophobic…