Topic 2B - Cell membranes

Question 1

What does the fluid mosaic model (1972) show us?

Answer 1

The arrangement of molecules in the membrane

Question 2

How do phospholipids molecules relate to the Fluid mosaic model?

Answer 2

In the model, phospholipid molecules form a continuous, double layer.
- This bilayer is fluid because the phospholipids are constantly moving

Question 3

How do cholesterol molecules relate to the Fluid mosaic model?

Answer 3

Cholesterol molecules are present within the bilayer

Question 4

How do proteins relate to the Fluid mosaic model?

Answer 4

1. Proteins are scattered through the bilayer, like tiles in a mosaic (including channel and carrier proteins which allow large molecules/ions to pass through membrane)
2. Receptor proteins on the cell-surface membrane allow the cell to respond to chemicals like hormones .
3. Some proteins can move sideways through the bilayer, whilst some are fixed
4. Some proteins have a polysaccharide chain attached - This is called glycoproteins

Question 5

How do lipids relate to the Fluid Mosaic model?

Answer 5

Some lipids also have a polysaccharide chain attached - called glycolipids

Question 6

How do phospholipids act as a barrier to dissolved substances?

Answer 6

The centre of the bilayer is hydrophobic so the membrane doesnt allow water-soluble substances, like ions, through it

Question 7

What is cholesterol and how does it give the membrane stability?

Answer 7

1. Cholesterol is a type of lipid that’s present in all cell membranes (except bacterial cell membranes)
2. Cholesterol molecules fit between the phospholipids. They bind to their hydrophobic tails, causing them to pack more closely together, making the membrane less fluid and more rigid
3. Cholesterol helps maintain the shape of animal cells . This is especially important for cells that aren’t supported by other cells, whch float free in the blood.

Question 8

What is diffusion?

Answer 8

Diffusion is the net movement of particles (molecules or ions) from an area of higher concentation to an aea of lower concentration

Question 9

Define the concentration gradient

Answer 9

The conc gradient is the path from an area of higher concentration to an area of lower concentration.
- Particles diffuse down a concentration gradient

Question 10

What type of process in diffusion?

Answer 10

It is a passive process - no energy is needed for it to happen

Question 11

On what condition can particles diffuse across cell membranes and give an example?

Answer 11

They must be to be able to move freely through membrane

• e.g. Oxygen and Carbon Dioxide can move freely through the membrane as they’re small, so they can pass through spaces between the phospholipids.
• They’re also non-polar, which makes them soluble in lipids, so they can dissolve in the hydrophobic bilayer

Question 12

What’s simple diffusion? (specifically)

Answer 12

It is a type of diffusion where molecules diffuse directly through a cell membrane

Question 13

What molecules would diffuse very slowly though the phospholipid bilayer and why?

Answer 13

1. Larger molecules (amino acids/glucose) as they’re big

2. Charged particles (ions and polar molecules) as they’re water soluble and the centre of the bilayer is hydrophobic

Question 14

How can the diffusion of the molecules mentioned earlier be sped up?

Answer 14

Through facilitated diffusion:

• large or charged particles diffuse through carrier and channel proteins in the membrane instead
• faciliated diffusion moves particles down a conc gradient just like simple diffusion
• It’s a passive process so it doesn’t use energy

Question 15

How do carrier proteins move large molecules through the membrane? (DIAGRAM)

Answer 15

Different carrier proteins facilitate the diffusion of different molecules
1) First, a large molecule attaches to a carrier protein in the membrane
2) Then, the protein changes shape
3 This releases the molecule on the opposite side of the membrane

Question 16

What is the ‘binding site’ of a carrier protein?

Answer 16

It is the site where large molecules will attach onto in the carrier protein

Question 17

How do channel proteins move large molecules through the membrane? (DIAGRAM)

Answer 17

Different channel proteins facilitate the diffusion of different charged particles
- Channel proteins form pores in the membrane for charged particles to diffuse through (down the conc gradient)

Question 18

What does the rate of simple diffusion depend on?

Answer 18

1. The conc gradient - The higher it is, the faster the rate of diffusion. As diffusion takes place, the difference in conc between 2 sides of the membrane decreases until it reaches an equilibrium (conc is equal on both sides). This means that diffusion slows down over time
2. The thickness of the exchange surface - The thinner the exchange surface (like the shorter the distance the particles have to travel), the faster the rate of diffusion
3. The surface area - the larger the surface area of the cell surface membrane, the faster the rate of diffusion

Question 19

How do microvilli increase the surface area for faster diffusion?

Answer 19

• Microvilli give the cell a larger surface area (in human cells, microvilli can increase the surface area by 600 times)
• A larger surface area means that more particles can be exchanged in the same amount of time, increasing the rate of diffusion

Question 20

What does the rate of facilitated diffusion depend on?

Answer 20

1. The conc gradient - the higher the conc gradient, the faster the rate of facilitated diffusion (up to a point). As equilibrium is reached the rate of facilitated diffusion will level off
2. The number of channel or carrier proteins - once all the proteins in a membrane are in use, facilitated diffusion can’t happen any faster, even if you increase the concentration gradient. So the greater the number of channel/carrier proteins in the cell membrane, the faster the rate of facilitated diffusion

Question 21

How would you work out the rate of diffusion on a straight line graph and a curved graph?

Answer 21

Straight line graph - find gradient of line

Curved line graph - draw tangent at point and find gradient of tangent

Question 22

Define osmosis.

Answer 22

Osmosis is the diffusion of water molecules across partially permeable membrane, from an area of higher water potential (i.e higher concentration of water molecules) to an area of lower water potential (lower concentration of water molecules)

Question 23

Define water potential.

Answer 23

It is the potential (likelihood) of water molecules to diffuse out of or into a solution

Question 24

What has the highest water potential?

Answer 24

Pure water has the highest water potential. All solutions have a lower water potential than pure water

Question 25

Define isotonic

Answer 25

When two solutions have the same water potential

Question 26

What does the rate of osmosis depend on?

Answer 26

1. The water potential gradient - the higher the water potential gradient, the faster the rate of osmosis. As osmosis takes place, the difference in water potential on either side of the membrane decreases, so the rate of osmosis levels off over time
2. The thickness of the exchange surface - the thinner the exchange surface the faster the rate of osmosis
3. The surface area of the exchange surface - the larger the surface area, the faster the rate of osmosis

Question 27

What does active transport use to move molecules and ions across membranes?

Answer 27

They use energy to move molecules and ions across membranes

Question 28

Which direction does active transport work in? (in terms of the concentration gradient)

Answer 28

It works against the concentration gradient

Question 29

What protein is involved in active transport?

Answer 29

Carrier proteins

Question 30

What are the 3 main differences between facilitated diffusion and active transport? (DIAGRAM)

Answer 30

1. Active transport moves solutes from low to high conc but in facilitated diffusion, its the other way round
2. Active transport needs energy - facilitated diffusion
3. Active transport does not use channel proteins whilst facilitated diffusion does

Question 31

How is ATP involved in active transport?

Answer 31

• ATP is a common energy source of energy in the cell, produces by respiration
• ATP undergoes a hydrolysis reaction, splitting into ADP and Pi. This releases energy so that the solutes can be transported

Question 32

What are co-transporters? (DIAGRAM)

Answer 32

They are a type of carrier protein

1. They bind to 2 molecules at once
2. The conc gradient of one of the molecules is used to move the other molecule against its own conc gradient

Question 33

What factors affect the rate of active transport?

Answer 33

1. The speed of individual carrier proteins - the faster they work, the faster the rate
2. The number of carrier proteins present - the more, the faster the rate
3. The rate of respiration in the cell and availability of ATP - if respiration is inhibited, active transport can’t take place.

Question 34

Briefly state how glucose is absorbed into the bloodstream

Answer 34

It is absorbed from the lumen of the ileum (final part of small intestine)
- It is absorbed by co-transport as the conc of glucose (in the small intestine) is too low for it to diffuse into the blood

Question 35

Describe how glucose enters the ileum epithelium

Answer 35

1. Sodium ions are actively transported out of the ileum epithelial cells, into the blood, by a NA-K pump. This creates a conc gradient so there’s a higher conc of sodium ions in the lumen of the ileum than in the cell
2. This causes sodium ions to diffuse fom the lumen of the ileum into the epithelial cells, down their conc gradient. They do this via the Na-glucose co-transporter proteins
3. The co-transporter carries glucose into the cell with the sodium. As a result, the conc of glucose in the cell increases
4. Glucose diffuses out of the cell, into the blood, down its conc gradient through a protein channel, by facilitated diffusion

Question 36

State what is involved in the co-transport of sodium ions and glucose (in general)

Answer 36

• Sodium ions move into the cell down their concentration gradient
• This moves glucose into the cell too against its concentration gradient