Q1: Cell membrane structure

Question 1

Cell surface membrane

Answer 1

The plasma membrane that surrounds cells and forms the boundary between the cell cytoplasm and the environment.

Question 2

Functions of the cell surface membrane

2

Answer 2

1. allows different conditions to be established inside and outside a cell
2. controls of the movement of substances in and out of the cell

Question 3

Structure of phospholipids in the cell surface membrane

Answer 3

hydrophilic heads of both layers point to the outside,

hydrophobic tails of both layers point into the centre of the cell membrane

Question 4

Functions of phospholipids in the cell surface membrane

3

Answer 4

1. allow lipid soluble substances to enter and leave the cell
2. prevent water soluble substances from entering and leaving the cell
3. make the membrane flexible and self-sealing

Question 5

Proteins arrangement in the cell surface membrane

Answer 5

interspersed throughout

Question 6

Two ways proteins are embedded in the phospholipid bilayer

Answer 6

1. Some occur in the surface: mechanical support or receptors
2. Others completely span the bilayer: protein channels and carrier proteins

Question 7

Cell surface membrane is made up of a

Answer 7

phospholipid bilayer

Question 8

Protein channels

Answer 8

Form water-filled tubes to allow water-soluble ions to diffuse across the membrane.

Question 9

Carrier proteins

Answer 9

Bind to ions or molecules then changes shape in order to move them across the membrane.

Question 10

Functions of proteins in the membrane

5

Answer 10

• provide structural support
• protein channels
• carrier proteins
• receptors
• help cells adhere together

Question 11

Characteristic of cholesterol molecules

Answer 11

very hydrophobic

Question 12

How does cholesterol decrease fluidity of the cell surface membrane at high temperatures?

Answer 12

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

Question 13

Functions of cholesterol in the membrane

3

Answer 13

• reduce lateral movement of other molecules
• decrease fluidity at high temperatures
• prevent leakage of water and dissolved ions from the cell

Question 14

Glycolipids

Answer 14

carbohydrate covalently bonded with a lipid

Question 15

How do glycolipids act as receptors for specific chemicals?

Answer 15

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

Question 16

Functions of glycolipids in the membrane

3

Answer 16

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

Question 17

Glycoproteins

Answer 17

Carbohydrate chains attached to many extrinsic proteins on the outer surface of the cell membrane, act as cell surface receptors

Question 18

Functions of glycoproteins in the membrane

3

Answer 18

• act as recognition sites
• help cells attach to one another and so form tissues
• allows cells to recognise one another

Question 19

Reasons why a molecule can’t freely diffuse across the cell surface membrane

Answer 19

• not soluble in lipids
• too large to pass through the channels
• same charge as on the protein channels, repelled
• electrically charged, polar

Question 20

Fluid-mosaic model

Answer 20

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

Question 21

Why is the cell-surface membrane fluid?

Answer 21

individual phospholipids molecules can move relative to one another, so the membrane is constantly changing shape.

Question 22

Why is the cell-surface membrane a mosaic?

Answer 22

The proteins that are embedded in the bilayer are different shapes, sizes and patterns.

Question 23

Intrinsic proteins

Answer 23

Span both bilayers of the plasma membrane,

Channels work carrier proteins.

Question 24

Extrinsic proteins

Answer 24

Proteins found in the surface of the plasma membrane.