Cell Membrane + Transport

Question 1

What is the function of the cell membrane?

Answer 1

• control which substances enter and leave the cell
• they’re partially permeable so they only let certain molecules through
• substances can move across by diffusion, osmosis and active transport
• recognising chemicals circulating through the tissue fluid

Question 2

Whys is the membrane semi permeable?

Answer 2

the phospholipid bi-layer as he hydrophilic heads are attracted to water and hydrophobic tails are repelled by water
- it is the polar nature of the phospholipids that give it its semi permeable nature

Question 3

What is meant by semi permeable membrane?

Answer 3

• small non polar or lipid soluble molecules can pass freely through
• large polar or water soluble molecules are prevented from passing directly through the phospholipids

Question 4

What is the fluid mosaic model?

Answer 4

Fluid = membrane is constantly changing shape and the phospholipids constantly changing position
Mosaic = many different types of proteins placed at many different random positions in the membrane

Question 5

How does the structure of phospholipids relate to its function?

Answer 5

phospholipid molecules form a barrier to dissolved substances
- head = hydrophilic (attracts water )
- tail = hydrophobic (repels water)
- they arrange themselves into a bi-layer and the centre is hydrophobic so the membrane doesn’t allow water soluble substances to diffuse through it

Question 6

How is cholesterol important in the cell membrane?

Answer 6

gives the membrane stability
- its keeps membrane stable at normal body temp - without it cells would burst open
- its a type of lipid that fits between the phospholipids and binds to the hydrophobic tails of the phospholipids causing them to pack more closely together
- this restricts the movement of the phospholipids, making the membrane less fluid and more rigid
- this helps maintain the shape of animal cells which is important for cells that aren’t supported by other cells e.g. red blood cells which float freely in the blood

Question 7

What do intrinsic channel proteins do?

Answer 7

form pores in the membrane and allow water soluble molecules and ions to diffuse through with a concentration gradient

Question 8

What do intrinsic carrier proteins do?

Answer 8

move water soluble molecules and ions against a concentration gradient by active transport
- changes shape to pass molecules through

Question 9

What do extrinsic proteins do?

Answer 9

act as receptor molecules for other chemicals

Question 10

What are the differences and similarities between intrinsic and extrinsic proteins?

Answer 10

both act as structural support for the membrane
i = completely span the membrane from the inside to the outside
e = do not span the membrane but only partly embedded in the phospholipids either in the inner or outer layer

Question 11

What may be attached to some proteins on the outer surface?

Answer 11

glycoprotein or glycolipid - all of these together form a glycocalyx which is for recognition sites for chemicals and may also act as antigens

Question 12

What happens to the membrane at below 0℃?

Answer 12

Membrane is rigid as phospholipids don’t move very much as they don’t have a lot of energy. The channel and carrier proteins denature increasing the permeability of membrane

Question 13

What happens to the membrane at 0-45 ℃?

Answer 13

Phospholipids can move and aren’t packed tightly so it is partially permeable

Question 14

What happens to the membrane at above 45 ℃?

Answer 14

Bi layer starts to melt so becomes more permeable. Channel and carrier proteins denature so cant control what enters or leaves

Question 15

What is the difference between active and passive transport?

Answer 15

a = - requires energy in the form of ATP
- bulk transport - endo + exocytosis, uses vesicles
- active transport uses carrier proteins
p = - only requires kinetic energy of particles
- diffusion, facilitated diffusion and osmosis

Question 16

What is Fick’s Law?

Answer 16

• Increasing the SA across which the particles diffuse or increasing the size of the conc gradient will increase the rate of diffusion
• increasing the distance will decrease the rate

Question 17

What is simple diffusion?

Answer 17

the net movement of particles from an area of higher conc to an area of lower conc.
- this continues until particles are evenly distributed throughout the liquid/gas (equilibrium is reached)
-particles diffuse down a conc gradient

Question 18

What factors affect the rate of diffusion?

Answer 18

• temp
• conc gradient
• SA
• distance / thickness
• size of molecule (small diffuse fastest)

Question 19

What is active transport?

Answer 19

the movement of molecules and ions from an area of lower conc to an area of higher conc (against a conc gradient)

Question 20

What is osmosis?

Answer 20

the net movement of water molecules from an area of higher/negative water potential to an area of lower/more negative water potential across a partially permeable membrane

Question 21

What does active transport require?

Answer 21

energy in the form of ATP - hydrolysis of ATP into ADP and Pi which causes the transport protein to change shape and the Pi molecule is released from the protein after the molecule is transported to the protein reverts back to its original shape.

Question 22

How long does osmosis occur?

Answer 22

continues until dynamic equilibrium is reached and the two sides are isotonic

Question 23

What does isotonic mean?

Answer 23

• the water potentials are equal
• cells in isotonic solution won’t gain or lose water so there is no net movement

Question 24

What does hypotonic mean?

Answer 24

• water potential of solution is more positive than the cells (less negative)
• animal cells will burst (osmotic lysis)as there is no cell wall
• plant cells become turgid as they have a strong cell wall

Question 25

What does hypertonic mean?

Answer 25

• water potential of solution is more negative than the cell
• animal and plant cell will shrink/shrivel due to large volume of water leaving the cell by osmosis - cell plasmalysed (cell membrane shrinks)