Topic 4 - Transport Across Cell Membranes

Question 1

Describe the structure of the phospholipid bilayer

Answer 1

• The hydrophilic heads of both phospholipids layers point to the outside of the cell-surface membrane attracted by water on sides.
• The hydrophobic tails of both phospholipid layers point into the centre of the cell membrane, repelled by the water on both sides.

Question 2

What are the functions of the phospholipid bilayer

Answer 2

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

Question 3

What are the functions of proteins 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

What are the functions of cholesterol in the cell membrane

Answer 4

• They add strength to the membranes
• Are very hydrophobic so prevent water loss and dissolved ion loss from the cell
• Reduce lateral movement of other molecules including phospholipids
• Make the membrane less fluid at high temperatures

Question 5

What are the functions of glycolipids in the cell membrane

Answer 5

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