4.1 Structure of the cell-surface membrane

Question 1

What are the 5 structures of a cell membrane

Answer 1

1. Phospholipids
2. Glycolipids
3. Glycoproteins
4. Cholesterol
5. Proteins

Question 2

Why are phospholipids important components of membranes

Answer 2

1. The hydrophillic heads of both phospholipid layers point outside of the membrane attracted by water on both sides
2. The hydrophobic tails of both phospholipid layers point into the centre of the cell membrane

Question 3

What functions do phospholipids have within the membrane

Answer 3

1. Allow lipid-soluble substances to enter and exit the cell
2. Prevent water soluble substances from entering and leaving the cell
3. Make the membrane flexible and self-sealing

Question 4

What are the two different types of ways proteins are embedded in cell membranes

Answer 4

1. Some occur in the surface of the bilayer and never extend across it. These give mechanical support and are known as extrinsic proteins
2. Others completely span the bilayer. Some are protein channels and others are carrier proteins. These are known as intrinsic proteins

Question 5

What are the functions of proteins within the cell membrane

Answer 5

1. Provide structural support
2. Act as channels transporting water-soluble substances across the membrane
3. Allow active transport across the membrane through carrier proteins
4. Form cell-surface receptors for identifying cells
5. Help cells adhere together
6. Act as receptors, e.g. for hormones.

Question 6

What is the function of cholesterol within the membrane

Answer 6

1. Reduce lateral movement of other molecules including phospholipids
2. Make the membrane less fluid at high temperatures
3. Prevent water and ion leakage from the cell

Question 7

What is a glycoprotein

Answer 7

A carbohydrate covalently bonded with an extrinsic protein

Question 8

What is the function of glycoproteins

Answer 8

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 9

What is a glycolipid

Answer 9

Carbohydrate chains covalently bonded to lipids

Question 10

What is the function of glycolipids

Answer 10

1. Act as recognition sites
2. Help cells to attach to one another and so form tissues
3. Allow cells to recognise one another, e.g. lymphocytes can recognise an organisms own cells

Question 11

Name the 5 functions cell membranes have within cells

Answer 11

1. Control the entry and exit of materials in organelles like mitochondria and chloroplasts
2. Separate organelles from cytoplasm so that specific metabolic reactions can take place within them
3. Provide an internal transport system. e.g. ER
4. isolate enzymes that may be harmful to the cell e.g. Lysozymes
5. Provide surfaces on which reactions can occur. e.g. RER

Question 12

What are the 4 possible reasons as to why most molecules do not freely diffuse across cell membranes

Answer 12

1. Not soluble in lipids and cannot pass through the phospholipid layer
2. Too large to pass through the membrane channels
3. Of the same charge as the protein channels and so, even if they’re the right size to pass through, they are repelled
4. Electrically charged and therefore have difficulty passing through the non-polar hydrophobic tails in the bilayer

Question 13

Why are membranes known as ‘fluid’

Answer 13

Because each individual phospholipid molecules can move relative to one another. Fluid because it constantly changes shape

Question 14

Why are membranes known as ‘mosaic’

Answer 14

Because the proteins embedded in the bilayer vary in shape, size and pattern.