Transport across cell membranes

Question 1

What is the cell-surface membrane otherwise known as?

Answer 1

plasma membrane

Question 2

Describe the structure of a phospholipid bilayer and why it structured is this way

Answer 2

They are made up of phospholipids:

• Hydrophilic head: point to the outside of the cell-surface membrane as it is attracted to water on both sides
• Hydrophobic tails: point to the centre of the cell membrane which is repelled by the water on both sides

Question 3

What is the function of phospholipids in the membrane?

Answer 3

• Allows lipid-soluble substances to enter and leave the cell
• Prevents water-soluble substances entering + leaving the cell
• Makes the membrane flexible and self-sealing

Question 4

What is the function of a membrane?

Answer 4

• A partially permeable barrier regulating entry/exit of particles
• Has binding sites
• Confers antigenic properties to the cell
• Site of certain reactions
• Compartmentalizes different cell reactions in organelles

Question 5

Describe the fluid mosaic model

Answer 5

Mosaic - the phospholipid bilayer has proteins of different shapes + sizes embedded in it
Fluid - phospholipids can move freely through the bilayer (membrane can constantly change shape + is flexible

Question 6

What are extrinsic proteins made up of and where are they found?

Answer 6

• Protein that is found on the surface of membrane (don’t extend completely across bilayer)
• May have carbohydrates bound to them e.g glycoproteins

Question 7

Describe the function of extrinsic proteins in the membrane

Answer 7

• Can give mechanical support to membrane with glycolipids (helping them give mechanical support)
• Act as enzymes or receptors for molecules e.g hormones

Question 8

What are intrinsic proteins made up of and where are they found?

Answer 8

• Made up of protein only

- Span the membrane from one side to the other

Question 9

Name two types of intrinsic proteins

Answer 9

Carrier proteins and protein channels

Question 10

Describe the function of carrier proteins in the membrane

Answer 10

• Bind to ions/molecules
• Change shape in order to move these molecules across the membrane
• They are specific to proteins + use kinetic energy from the molecule only

Question 11

Give two examples of carrier proteins

Answer 11

Glucose and amino acids

Question 12

Describe the function of protein channels in the membrane

Answer 12

Form water-filled tubes + allow water-soluble ions to diffuse across membrane
Channels are selective - will remain closed unless specific ion is present

Question 13

Describe three of the functions of proteins in the membrane

Answer 13

• 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 14

Where is cholesterol found?

Answer 14

Within phospholipid bilayer

Question 15

Describe three of the functions of cholesterol in the membrane

Answer 15

• Makes membrane less fluid at high temp.
• Reduces lateral movement of other molecules (including phospholipids)
• Add strength to membranes
• Pulls together the fatty acid tails + limit their movement + that of other molecules but doesn’t make membrane too rigid as a whole
• Hydrophobic: important in preventing loss/leakage of water + dissolved ions from cell

Question 16

What is a glycolipid made up of and where is it found?

Answer 16

• Made up of carbohydrate covalently bonded to a lipid
  Lipid portion is embedded into bilayer + carbohydrate portion extends outside of cell
  (Extended section acts as a receptor for specific chemicals)

Question 17

Describe the function of glycolipids in the membrane

Answer 17

• Acts as recognition + binding sites
• Helps maintain stability of membrane
• Helps cells to attach to one another + so forms tissues

Question 18

What is a glycoprotein made up of and where is it found?

Answer 18

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

Question 19

Describe the functions of glycoproteins in the membrane

Answer 19

• Act as recognition sites
• Act as cell-surface receptors (specifically for hormones + neurotransmitters)
• Allow cells to recognise one another e.g lymphocytes recognise an organism’s own cells
• Help cells to attach to one another (form tissues)

Question 20

Why do most molecules not freely diffuse across the cell-surface membrane?

Answer 20

Many are:

• Not soluble in lipids and therefore cannot 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 they are small enough to pass through, they are repelled
• Electrically charged (or polar) and so they have difficulty passing through the non-polar hydrophobic tails in the phospholipid bilayer