OCR AS F211 Cell membranes 1.1.2

• a) outline the roles of membranes within cells and at the surface of cells

The plasma membrane:

• separates cell contents from the outside environment
• regulates the passage of substances into and out of cells
• is involved in cell recognition, cell siganlling and cell adhesion

Organelle membranes:

• divide the cell into compartments with specialised functions
• isolate potentially harmful enzymes in lysosomes
• provide surfaces for holding chlorophyll
• provide surfaces for holding enzymes and coenzymes for making ATP and attachment sites for ribosomes

b) state that the plasma (cell surface) membranes are partially permeable barriers

c) describe, with the aid of diagrams, the fluid mosaic model of membrane structure

membranes are made up of a bilayer of phospholipids. The membrane is not rigid, the phospholipids can move around making the membrane 'fluid'. The membrane contains a sprinkling or 'mosaic' of proteins.

d) describe the roles of the components of the cell membrane:

• phospholipids consist of a phosphate ion (head) and two fatty acid chains (tails). The head is hydrophilic, the tails are hydrophobic. Phospholipids naturally form a bilayer which ensures that the fatty acid chains are kept away from water inside the cell (cytoplasm)
• cholesterol is a lipid molecule found in some eukaryotic cell membranes. It gives the membrane mechanical stability and reduces permeability to water, ions and hydrophilic molecules
• glycoproteins are carbohydrate branches attached to proteins. They are important for cell signalling (some are hormone receptors); cell recognition (they help the immune system recognise cells as 'self') and cell adhesion (they help bind cells together in tissues
• gylcolipids are carbohydrate branches attached to phospholipids. They are important for cell signalling and cell recognition
• proteins some span the whole membrane such as channel proteins and carrier proteins. Channel proteins contain water-lined channels (pores) to allow ions and some large water-soluble molecules across the membrane. Carrier proteins carry large water-soluble molecules across the membrane either passively (e.g. glucose) or actively (e.g. magnesium ion). Sme proteins occur on the surface of the phospholipid bilayer while others extend into the phospholipid layer. Some are enzymes (involved in metabolic reactions) and some are important for cell signalling (they are hormone receptor sites)

e) outline the effect of changing temperature on membrane structure and permeability

the phospholipid bilyaer is usually:

• permeable to small molecules e.g. ethanol, water
• permeable to large lipid-soluble molecules e.g. steroid hormones
• impermeable to ions and large water-soluble molecules e.g. glucose

membrane permeability increases with temperature:

up to 60 degrees celcius, this is due to increased kinetic energy of molecules (increased diffusion rate) and increased movement of…