Membranes

A membrane is:

Fluid - made up of phospholipids which are liquid and move easily within each layer

Mosaic - from the surface the membrane looks like a mosaic picture made of tiles. The proteins and phospholipids form the pattern.

The membrane is a thin layer that is made up of serveral types of molecules:

• Phospholipids - specialised lipids. They form a bilayer. Act as a barrier for water-soluble molecules between the cytoplasm and the outside. Their hydrocarbon tails are hydrophobic and point inwards. Held together by weak bonds. The phospholipid heads are hydrophilic and face outwards towards both the cytoplasm and the outside of the cell, both of which are water-based.
• Proteins - large proteins extend across the membrane, smaller ones lie on the surface.
• Carbohydrate chains - attached to some proteins (forming glycoproteins) or to some phospholipids (forming glycolipids).
• Cholesterol molecules - found only in eukaryotic cells.

They:

• determine what is allowed through
• act as a barrier to many water-soluble molecules
• prevent LARGE protein molecules, such as cytoplasm and enzymes, leaving cells
• are permeable to selected molecules
• allow larger molecules to pass through pores or via carriers, e.g glucose
• or to enter or leave by endocytosis and exocytosis
• act as receptors or as recognition sites
• increase the surface area by folding into microvilli (hair-like projections on the surface to increase the area)