Cell Membranes

Question 1

Phospholipids

Answer 1

The hydrophilic 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 into the centre of the cell membrane, repelled by the water on both sides.
The functions of phospholipids in the membrane are to:
• allow lipid-soluble substances to enter and leave the cell
• prevent water-soluble substances entering and leaving the cell
• make the membrane flexible and self-sealing.

Question 2

Proteins

Answer 2

Some proteins occur in the surface of the bilayer They act either to give mechanical support to the membrane or,, as cell receptors for molecules such as hormones.
• Other proteins completely span the phospholipid bilayer from one side to the other. Some are protein channels, which form water-filled tubes to allow water-soluble ions to diffuse across the membrane. Others are carrier proteins that bind to ions or molecules like glucose and amino acids, then change shape in order to move molecules across the membrane

Question 3

Functions of the protein, in the membrane

Answer 3

• provide structural support
• act as channels transporting water-soluble substances across the membrane
• allow active transport across the membrane through carrier proteins
• form cell-surface receptors for identifying cells
• help cells adhere together
• act as receptors, for example for hormones.

Question 4

Cholesterol plus functions

Answer 4

Cholesterol occur within the phospholipid bilayer They add strength to the membranes.
. Cholesterol molecules are very hydrophobic and preventing loss of water and dissolved ions from the cell. They also pull together the fatty acid tails of the phospholipid molecules, limiting their movement and that of other molecules but without making the membrane as a whole too rigid.
The functions of cholesterol in the membrane are to:
• reduce lateral movement of other molecules including phospholipids
• make the membrane less fluid at high temperatures
• prevent leakage of water and dissolved ions from the cell.

Question 5

Glycolipids

Answer 5

Glycolipids are made up of a carbohydrate covalently bonded with a lipid.
The carbohydrate portion extends from the phospholipid bilayer into the watery environment outside the cell where it acts as a cell-surface receptor for specific chemicals
The functions of glycolipids in the membrane are to:
• act as recognition sites
• help maintain the stability of the membrane
• help cells to attach to one another and so form tissues.

Question 6

Glycoproteins

Answer 6

Carbohydrate chains are attached to many extrinsic proteins on the outer surface of the cell membrane. These glycoproteins also act as cell-surface receptors, more specifically for hormones and neurotransmitters.
The functions of glycoproteins in the membrane are to:
• act as recognition sites
• help cells to attach to one another and so form tissues
• allows cells to recognise one another,

Question 7

Permeability

Answer 7

Most molecules do not freely diffuse due to
-not soluble in lipids and therefore cannot pass through the phospholipid layer
• too large to pass through the channels in the membrane
• of the same charge as the charge on the protein channels and so, even if they are small enough to pass through, they are repelled
• electrically charged (in other words are polar) and therefore have difficulty passing through the non-polar hydrophobic tails in the phospholipid bilayer.

Question 8

Fluid mosaic model

Answer 8

fluid because the individual phospholipid molecules can move relative to one another. This gives the membrane a flexible structure that is constantly changing in shape

mosaic because the proteins that are embedded in the phospholipid bilayer vary in shape, size and pattern in the same way as the stones or tiles of a mosaic.