Cells: Cell Surface Membrane

Question 1

Cell surface membrane definition

Answer 1

The plasma membrane that surrounds cells and forms the boundary between the cytoplasm and the environment. The main components of the plasma membrane. It is comprised of 40% lipids (i.e phospholipids, cholesterol), 50% proteins and 10% carbohydrate groups (i.e glycoproteins, glycolipids)

Question 2

Functions of the phospholipid bilayer

Answer 2

1. To allow lipid soluble molecules to enter and leave the cell
2. Prevent water soluble molecules entering and leaving the cell
3. Make the membrane flexible

Question 3

Most molecules do not move freely across the membrane because they are…

Answer 3

1. Not lipid soluble
2. Too large
3. Of the same charge as the charge on the protein channels
4. Polar

Question 4

Membranes are ‘selectively permeable’: Phospholipid bilayers are impermeable to:

Answer 4

1. Large molecules
2. Charged molecules and ions

These molecules require transport proteins: different molecules require different proteins, therefore, a cell can ‘select’ which molecules can enter by controlling the proteins in its membrane

Question 5

Glycolipid definition

Answer 5

Glycolipids are made of a carbohydrate covalently bonded to 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

Question 6

Functions of glycolipids in the membrane

Answer 6

1. Act as recognition sites
2. Help maintain the stability of the membrane
3. Help cells to attach to one another and so form tissues

Question 7

Glycoprotein definition

Answer 7

Carbohydrate chains are attached to many extrinsic proteins on the outer surface of the membrane. These glycoproteins act as cell surface receptors usually for hormones and neurotransmitters

Question 8

Glycoproteins functions

Answer 8

1. Act as recognition sites
2. Help cells to attach to one another and so form tissues
3. Allows cells to recognise one another e.g lymphocytes can recognise ‘self’

Question 9

Phospholipid bilayer aspects

Answer 9

• One layer has its hydrophilic heads pointing inwards - interacting with the water inside the cells
• The other had its hydrophilic heads pointing outwards - interacting with the water that surrounds the cell
• The hydrophobic tails of both layers point to the centre of the membrane

Question 10

Functions of the phospholipid bilayer

Answer 10

1. To allow lipid soluble molecules to enter and leave the cell
2. Prevent water soluble molecules entering and leaving the cell
3. Make the membrane flexible

Question 11

Fluid mosaic model

Answer 11

A model suggesting an explanation for the arrangement of molecules within the membrane.
- Phospholipid molecules form a continuous bilayer - this is fluid because the phospholipids can move relative to one another: this means it’s flexible
- Mosaic because the proteins can vary in size and shape like the stones in a mosaic

Question 12

Proteins in the bilayer

Answer 12

They are randomly embedded in the bilayer (mosaic), and are classified in two ways:

Extrinsic and Intrinsic

Question 13

Extrinsic Proteins

Answer 13

Found either on the surface of the bilayer or partially embedded into it. They can give mechanical support, or if combined with glycolipids form cell receptors for e.g hormones. This allows communication between cells and also recognition, as White Blood Cell’s recognise the type of glycoprotein as being self and will not produce an immune response

Question 14

Intrinsic proteins

Answer 14

They span the bilayer acting as carrier proteins, protein channels or co-transporters

Question 15

Functions of proteins

Answer 15

• To provide support
• To act as carriers for water soluble substances
• Allow active transport across the membrane by forming ion channels for sodium and potassium
• Form recognition sites
• To help cells adhere (stick) together
• Act as receptors i.e for hormones

Question 16

Cholesterol functions

Answer 16

• Cholesterol molecules fit in between the phospholipids to reduce the membranes fluidity by pulling together the fatty acid tails of the phospholipid. The more cholesterol molecules there are the less fluid the membrane, this helps to prevent the membrane from breaking up
• Reduces the 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