Cell membranes

Question 1

What is the cell-surface membrane

Answer 1

The name given specifically to the plasma membrane that surrounds cells and forms the boundary between the cell cytoplasm and the environment.

Question 2

Why are phospholipids important components of cell surface membranes

Answer 2

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

Question 3

What is the function of phospholipids in the cell membrane

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

What role do proteins play in diffusion across the cell surface membrane

Answer 4

They allow molecules that are not lipid soluble or too large to diffuse across the phospholipid layer to diffuse into the cell.

Question 5

What are the two ways in which proteins are embedded in the phospholipid bilayer

Answer 5

• some proteins are found in the surface of the bilayer and never extend fully across it
• some proteins completely span the phospholipid bilayer from one side to the other.

Question 6

What is the role of the proteins in the cell surface membrane that are found in the surface of the phospholipid bilayer but do not extend fully across it

Answer 6

They act to either give mechanical support to the membrane,or, in conjunction with glycolipids, as cell receptors for molecules such as hormones

Question 7

How do channel proteins help molecules to diffuse across the cell-surface membrane

Answer 7

They form water filled tubes that allow water-soluble ions to diffuse across the membrane.

Question 8

How do carrier proteins help to move molecules across the cell surface membrane

Answer 8

They bind to ions or molecules like glucose and amino acids, then change shapes in order to move these molecules across the membrane.

Question 9

List the role of proteins in the cell- surface membrane

Answer 9

1) provide structural support
2) Act as channels supporting water-soluble substances across the membrane
3) Allow active transport across the membrane through carrier proteins
4) form cell-surface receptors for identifying cells
5) help cells adhere together
6) act as receptors for example for hormones

Question 10

How does cholesterol molecules being hydrophobic relate to their function in the cell surface membrane

Answer 10

It helps them prevent the loss of water and dissolved ions from the cell. They also pull together the fatty acid tails of the phospholipid molecules which limits their movement and that of other molecules but without making the membrane too rigid

Question 11

Where are cholesterol molecules found in a cell membrane

Answer 11

They can be found within the phospholipid bilayer

Question 12

List the functions of cholesterol molecules in the cell membrane

Answer 12

1) add strength
2) reduce lateral movement of other molecules including phospholipids
3) make the membrane less fluid at high temperatures
4) prevent leakage of water and dissolved ions from the cell

Question 13

What are glycolipids

Answer 13

Molecules made up of a carbohydrate covalently bonded with a lipid.

Question 14

Describe how glycolipids are found in the cell surface membrane

Answer 14

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

Question 15

What is an example of how glycolipids are used in the body

Answer 15

The human ABO blood system operates as a result of glycolipids on the cell surface membrane.

Question 16

List the roles of glycolipids in the cell-surface membrane

Answer 16

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

Question 17

Describe how glycoproteins are found in the cell surface membrane

Answer 17

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

Question 18

List the roles of glycoproteins in the cell surface membrane

Answer 18

• Act as cell surface receptors, more specifically for hormones and neurotransmitters
• Act as recognition sites
• help cells to attach to one another and so form tissues
• allows cells to recognise one another, for example, lymphocytes can recognise an organisms own cells.

Question 19

Explain why most molecules do not freely diffuse across the cell surface membrane.

Answer 19

• most molecules are not lipid soluble and so cannot pass through the phospholipid layer
• many molecules are too large to pass through the channels in the membrane
• many molecules have the same charge as the charge on the protein channels and so even if they are small enough to pass through, they are repelled
• many molecules are polar and so have difficulty passing through the non polar hydrophobic tails in the phospholipid bilayer

Question 20

List the functions of the membranes found within cells

Answer 20

• Control the entry of material in and out of discrete organelles such as chloroplasts and mitochondria.
• Separate material from the cytoplasm so that specific metabolic reactions can take place within them.
• Provide an internal transport system (eg. Endoplasmic reticulum)
• Isolate enzymes that might damage the cell (eg. Lysosomes)
• Provide surfaces on which reactions can occur (eg. Ribosomes on RER)

Question 21

What is the name of the model that describes how molecules are arranged in a cell surface membrane

Answer 21

The fluid-mosaic model

Question 22

Explain why ‘fluid’ is used to describe the cell surface membrane in the fluid mosaic model

Answer 22

The fluid part describes the fact that the individual phospholipid molecules can move relative to each other which gives the membrane a flexible structure that is constantly changing in shape.

Question 23

Explain why ‘mosaic’ is used to describe the structure of a cell membrane in the fluid-mosaic model

Answer 23

The mosaic refers to the fact that the proteins that are embedded in the phospholipid bilayer vary in size, shape and pattern like mosaic tiles.