Structure of the cell-surface membrane

Role of membranes:

• Cell communication
• Partially permeable
• Site of chemical reactions

Fucntions of the cell-surface membrane:

• Separate cell from environment
• Control the entry and exit of molecules as partially permeable
• Receptor for hormones and neuro transmitters
• Allows different conditions to be established inside and outside a cell

Functions of membrans within a cell:

• Compartmentalise organelles so that specific metabolic reactions can take place within them
• Control the entry and exit of materials is discrete organelles such as mitochondria and chloroplasts
• Provide an internal transport system e.g. endoplasmic reticulum
• Isolate enzymes that might damage the cell e.g. lysosomes
• Provide surfaces on which reactions can occur e.g protein systhesis using ribosomes on RER

Molecules that form its structure -

Phospholipids:

• The hydrophillic heads of both phospholipid layers point to the outside of the cell-surface membrane attracted by water on both sides
• The hydrophobic tails of both phospholipid layers point to the centre of the cell membrane, repelled by water on both sides

Function of phospholipids in the membrane -

• Allow lipid-soluble substances to enter and leave the cell
• Prevent water-soluble subtances entering and leaving the cell
• Make the membrane flexible and self-sealing

Proteins:

Extrinsic proteins (peripheral) -

• Are on the surafce of the bilayer and never extend completely across it
• Act either to give mechanical support to the membrane
• Or in conjunction with glycolipids as cell receptors for molecules such as hormones

Intrinsic proteins (intergral) -

• Completely span the phospholipid bilayer from one side to another
• There are two types

Channel proteins -

• Form water-filled tubes to allow water-soluble ions to diffuse across the membrane

Carrier proteins -

• Bind to ions or molecules like glucose and amino acids
• Then change shape in order to move molecules…