CELLS: Cell Membrane Structure

Question 1

Why might some particles not diffuse across?

Describe the permeability of cell-surface membranes:

Answer 1

Partially permeable - some particles may not diffuse across because:

• they are not soluble in lipids and cannot pass through phospholipid layer.
• they are too large to pass through channels in membrane.
• they are of the same charge as charge on protein channels and so are repelled.
• electrically charged/polar and so find it difficult to pass through non-polar hydrophobic tails of bilayer.

Question 2

How can substances move across the cell-surface membrane?

Answer 2

Diffusion, osmosis or active transport.

Question 3

What model was proposed in 1972?

Why?

Answer 3

The fluid mosiac model was suggested to describe the arrangement of molecules in the membrane.

Question 4

Why is the model for cell membranes known as the ‘fluid mosaic’ model?

Answer 4

Fluid - because individual phospholipid molecules can move relative to one another, giving the membrane a flexible structure that is constantly changing in shape.

Mosaic - because the proteins embedded in the phospholipid bilayer vary in shape, size and pattern in the same way as the tiles in mosiacs do.

Question 5

Label this diagram:

Answer 5

Question 6

5 to list

Describe the function of membranes within a cell:

Answer 6

• Control exit and entry of materials in discrete organelles, like mitochondria and chloroplasts.
• Separate organelles from cytoplasm so that specific metabolic reactions can take place within them.
• Provide an internal transport system, like the ER.
• Isolate enzymes that might damage the cell, like lysosomes.
• Provide surfaces on which reactions can occur.

Question 7

List the different components of cell membranes:

Answer 7

• Phospholipids
• Cholesterol
• Glycoproteins
• Glycolipids
• Channel protein (intrinsic proteins)
• Extrinsic proteins

Question 8

What do phospholipids form?

Answer 8

A bilayer

Question 9

Why are phospholipids important parts of the cell membrane?

Answer 9

• Hydrophilic heads of both phospholipid layers point to the outside of the cell surface membrane attacted by water on both sides.
• Hydrophobic tails of both phospholipid layers point into the centre of the cell membrane, repelled by the water on both sides.

Question 10

What are the functions of phospholipids in the cell-surface membrane?

Answer 10

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

Question 11

What is the role of extrinsic (surface) proteins in the phospholipid bilayer?

Answer 11

• Give mechanical support to the membrane
• In conjunction with glycolipids, act as cell receptors for molecules like hormones.

Question 12

Channel proteins

Answer 12

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

Question 13

Carrier proteins.

Answer 13

Proteins on the cell-surface membrane that bind to molecules or ions and change shape in order to move these molecules across the membrane.

Question 14

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

Answer 14

Channel proteins

Question 15

Proteins on the cell-surface membrane that bind to molecules or ions and change shape in order to move these molecules across the membrane.

Answer 15

Carrier proteins.

Question 16

list 5

What are the functions of proteins in the cell membrane?

Answer 16

• 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.

Question 17

What type of molecule is cholesterol?

Answer 17

It is a type of lipid.

It is present is all cell membranes (except bacterial cell membranes).

Question 18

What do cholesterol molecules fit between?

Answer 18

Phospholipids.

Question 19

Why do cholesterol molecules make the membrane less fluid and more rigid?

Answer 19

Molecules bind to the hydrophobic tails of the phospholipids, causing them to pack more closely togther.

It also resticts their movement.

Question 20

What is the function of cholesterol?

Answer 20

• 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 21

What are glycoproteins made of?

Answer 21

A carbohydrate covalently bonded with a protein.

Question 22

Where does the carbohydrate portion of a glycolipid extend in to?

Answer 22

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

Question 23

What are the functions of glyolipids in the membrane?

Answer 23

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

Question 24

Glycoproteins

Answer 24

Carbohydrate chains attached to proteins on the outer surface of the cell membrane, which act as cell-surface receptors.

Question 25

What is the function of glycoproteins in the cell surface membrane?

Answer 25

• Act as recognition sites
• Help cells attach to one another and so form tissues.
• Allows cells to recognise one another, eg. so lymphocytes can recognise the body’s own cells.

Question 26

Why are beetroots used to investigate the permeabiliry of the cell membrane?

Answer 26

Beetroot cells contain a coloured pigment that leaks out - the higher the permeability of the membrane, the more pigment leaks out of the cell.

Question 27

How could you investigate how temperature affects beetroot membrane permeability?

Answer 27

• Use scalpel to carefully cut 5 equal pieces of beetroot.
• Rinse pieces to remove pigment released during cutting.
• Add each to piece to a different test tube containing 5cm³ water.
• Place in water bath at different temperatures for same length of time.
• Remove pieces of beetroot from tubes, leaving just coloured liquid.
• Colorimeter to measure light absorbed - higher absorbance, the more pigment has been released, so teh higher the permeability of the membrane.

Question 28

How do you calibrate a colorimeter?

Answer 28

Give 5 minutes to stabilise.

Take a measurement through pure water to calibrate.

Question 29

What happens to the permeability of membranes at temperatures below 0°C?

Answer 29

• The phospholipids don’t have much energy. so they can’t move very much.
• They’re packed closely together and so membrane is rigid.
• Channel proteins and carrier proteins in membrane deform, increasing permeability of membrane.
• Ice crystals may form and pierce membrane making it highly permeable.

Question 30

What happens to the permeability of the membrane at 0-45°C?

Answer 30

• Phospholipids can move and aren’t packed closely together - membrane is partially permeable.
• As temperature increases, phospholipids move more because they ahve more energy - this increases permeability.

Question 31

Temperatures above 45°C?

Answer 31

• Phospholipid bilyaer starts to melt and membrane becomes more permeable.
• Water inside cell starts to expand, putting pressure on membrane.
• Channel proteins and carrier proteins deform so can’t control what enters and eits - increases permeability of membrane.

Question 32

Carbohydrate chains attached to proteins on the outer surface of the cell membrane, which act as cell-surface receptors.

Answer 32

Glycoproteins