4.1 Structure of the cell-surface membrane

Question 1

What are the 5 main components in a plasma membrane?

Answer 1

• phospholipids
• proteins
• cholesterol
• glycolipids
• glycoproteins

Question 2

Why are phospholipids important in plasma membranes?

Answer 2

• hydrophilic heads of both phospholipid layer point to outside of cell, attract water on both sides
• hydrophobic tails both point inside of cell and repel water

Question 3

What is the function of the phospholipids in a membrane? (3)

Answer 3

• allow lipid-soluble substances to enter and exit cell
• prevent water-soluble substances entering and exiting cell
• make membrane flexible and self-sealing

Question 4

How are proteins embedded in the phospholipid belayer? (2 main ways)

Answer 4

• some proteins occur in the surface of the bilayer and provide mechanical support as cell receptors for molecules such as hormones
• some proteins span from one side of the bilayer to the other and act as channels for either water-soluble ions or molecules (e.g. glucose and amino acids) to diffuse across the membrane

Question 5

What are the functions of proteins in the membrane? (4)

Answer 5

• provide structural support
• act as channels for transportation
• allow active transport across membrane (carrier proteins)
• cell-surface receptors

Question 6

What is the function of cholesterol in the membrane? (2)

Answer 6

• control the fluidity of the membrane and provide strength
• prevent leakage of water and dissolved ions from the cell

Question 7

What are the functions of glycolipids in the membrane? (3)

Answer 7

• act as recognition sites
• help maintain stability of the membrane
• help cells attach to one another and form tissues

Question 8

What are the role of glycoproteins in the membrane? (3)

Answer 8

• act as recognition sites
• help cells attach to one another and form tissues
• allow cells to recognise one another, e.g. lymphocytes can recognise organisms own cells.

Question 9

Why is the arrangement of a phospholipid bilayer membrane known as the FLUID-MOSAIC MODEL?

Answer 9

FLUID - flexible structure that constantly changes shape
MOSAIC - proteins embedded vary in shape, size and pattern like stones or tiles of a mosaic