4. Transport Across Cell Membranes

Question 1

What are plasma membranes?

Answer 1

All membranes around and within all cells

Question 2

What do phospholipids form?

Answer 2

A bilayer

Question 3

Why are phospholipids important components of cell surface membranes?

Answer 3

• The hydrophobic heads point to the outside of the cell, attracted to water on both sides
• The hydrophobic tails point into the centre of the cell membrane, repelled by the water

Question 4

What material moves through the membrane?

Answer 4

Lipid soluble material

Question 5

What are the functions of phospholipids in the membrane?

Answer 5

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

Question 6

How are proteins embedded in the phospholipid belayer?

Answer 6

• Some act to give mechanical support
• Some are protein channels or carrier proteins

Question 7

What are the functions of proteins in the membrane?

Answer 7

• Provide structural support
• Act as channels transporting water soluble substances
• Allow active transport across the membrane through carrier proteins
• Form cell surface receptors for identifying cells
• Help cells adhere together
• Act as receptors

Question 8

Where are cholesterol molecules?

Answer 8

Within the phospholipid belayer

Question 9

What are the functions of cholesterol in the membrane?

Answer 9

• Recuce lateral movement
• Make the membranes less fluid at high temps
• Prevent leakage of water and dissolved ions from the cell

Question 10

What are glycolipids made of?

Answer 10

Carbohydrate covalently bonded with a lipid

Question 11

What are the functions of glycolipids in the membrane?

Answer 11

• Act as recognition sites
• Help maintain stability of the membrane
• Help cells to attach to one another and so form tissues

Question 12

What are the functions of glycoproteins?

Answer 12

• Act as recognition sites
• Help cells attach to one another
• Allows cells to recognise one another

Question 13

What does the cell surface membrane control?

Answer 13

The movement of substances in and out of the cell

Question 14

Why do most molecules not freely diffuse across the membrane?

Answer 14

• Not soluble in lipids
• Too large
• Of the same charge as the charge on the protein so are repelled
• Electrically charged

Question 15

What is the fluid mosaic model?

Answer 15

The model that shows the various molecules all combined in the structure of the cell-surface membrane