Structure of the cell-surface membrane

Phosoholipid bilayer

• hydrophilic heads point out - attracted by water on both sides

• hydrophobic tails point in - repelled by water on both sides

Function of phospholipids in bilayer

• allow lipid-soluble substances to enter and leave cell

• prevent water-soluble substances entering and leaving cell

• make membrane flexible and self-sealing

Proteins

Proteins are interspersed throughout cell surface membrane

• protein channels - allow water-soluble ions diffuse across membrane

• carrier proteins - bind to ions/molecules like glucuse/amino acids then change shape to move these across the membrane

Functions of proteins in membrane

• structural support

• act as channels transporting water-soluble substances

• allow active transport across membrane through carrier proteins

• form cell-surface receptors for identifying cells

• help cells adhere together

• act as receptors e.g. hormones

Cholesterol functions

• add strength to membrane

• hydrophobic and so prevent loss of water and dissolved ions from cell

• reduce lateral movement of other molecules e.g. phospholipids (pull together fatty acid tails)

• make membrane less fluid at high temperatures

Glycolipids + functions in membrane

Made of a carb covalently bonded with a lipid

• act as recognition sites

• help maintain stability of membrane

• help cells attach to one another and so forms tissues

Glycoproteins + functions in membrane

Carb chains attach to many extrinsic proteins on outer surface of cell membrane

• act as recognition sites

• help cells to attach to one another and so form tissues

• allows cells to recognise one another e.g. lymphocytes

Why can’t most molecules freely diffuse across cell-surface membrane?

• not soluble in lipids and so cannot pass through phospholipid bilayer

• too large to pass through channels in membrane

• of the same charge as charge of protein channels and so are repelled

• polar so have difficulty passing through non-polar hydrophobic tails in bilayer

Fluid mosaic model

• fluid - phospholipid molecules can move relative to one another = flexible structure

• mosaic - proteins embedded in bilayer vary in shape, size and pattern