Cell-surface-membrane

Question 1

Why are phospholipids important components of the cell-surface membrane?

Answer 1

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

Question 2

What moves through the membrane via the phospholipid portion?

Answer 2

Lipid-soluble material

Question 3

What are the functions of the phospholipids?

Answer 3

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

Question 4

How are proteins connected to the cell-surface membrane?

Answer 4

They are interspersed throughout the cell-surface membrane

Question 5

What is one way proteins are embedded into the phospholipid bilayer?

Answer 5

• Some proteins occur in the surface of the bilayer and never extend completely across it
• They act to give mechanical support to the membrane
• Or they are in conjunction with glycolipids, as cell receptors for molecules such as hormones

Question 6

What is another way proteins are embedded into the phospholipid bilayer?

Answer 6

• Other proteins completely span the phospholipid bilayer from one side to the other
• Some are protein channels, which form water filled tubes to allow water soluble ions to diffuse across the membrane
• Others are carrier proteins that bind to ions or molecules(glucose and amino acids) then change shape in order to move these molecules across the membrane

Question 7

What are the functions of the proteins in the membranes?

Answer 7

• 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 e.g. for hormones

Question 8

Where do cholesterol occur?

Answer 8

In the phospholipid bilayer

Question 9

What do cholesterol add to the cell-surface membrane?

Answer 9

They add strength

Question 10

What are cholesterol in terms to water and what does this mean?

Answer 10

They are very hydrophobic and so play an important role in preventing loss of water and dissolved ions from the cell

Question 11

How do cholesterol limit the movement of the membrane?

Answer 11

They pull together the fatty acid tails of the phospholipid molecules, limiting their movement and that of other molecules but without making the membrane as a whole too rigid

Question 12

What are the functions of the cholesterol in the membrane?

Answer 12

• Reduce 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

Question 13

What are glycolipids?

Answer 13

Made up of a carbohydrate covalently bonded with a lipid

Question 14

What does the carbohydrate portion of the glycolipid do within the membrane?

Answer 14

It extends from the phospholipid bilayer into the watery environment outside the cell where it acts as a cell-surface receptor for specific chemicals

Question 15

What are the functions of the glycolipids in the membrane?

Answer 15

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

Question 16

What are glycoproteins in cell membranes?

Answer 16

Carbohydrate chains are attached to many extrinsic proteins on the outer surface of the cell membrane

Question 17

What do glycoproteins act as in cell membranes?

Answer 17

They also act as cell-surface receptors, more specifically for hormones and neurotransmitters

Question 18

What are the functions of the glycoproteins in the membrane?

Answer 18

• Act as recognition sites
• Help cells to attach to one another and so form tissues
• Allow cells to recognise one another, e.g. lymphocytes can recognise an organism’s own cells

Question 19

What does the cell-surface membrane control?

Answer 19

The movement of substances into and out of the cell

Question 20

In general, why do most molecules not freely diffuse across the cell-surface membrane?

Answer 20

• Not soluble in lipids so can’t pass through the phospholipid bilayer
• Too large to pass through the channels in the membrane
• Of the same charge as the charge on the protein channels and so even if small enough, are repelled
• Electrically charges (polar) and so have difficulty passing through the non-polar hydrophobic tails in the phospholipid bilayer

Question 21

What is the fluid-mosaic model?

Answer 21

The way in which all the various molecules are combined into the structure of the cell-surface membrane

Question 22

Why is the cell-surface membrane known as fluid?

Answer 22

• Because the individual phospholipid molecules can move relative to one another
• This gives the membrane a flexible structure that is constantly changing shape