transport across membranes

Question 1

explain why a cell membrane may be described as a fluid-mosaic?

Answer 1

the position of the molecules within the membrane is fluid – they are able to move around within the membrane
membrane is made up from a variety of different molecules arranged into a mosaic.

Question 2

explain the arrangement of phospholipids in a cell-surface membrane

Answer 2

bilayer OR Water is present inside and outside a cell
hydrophobic (fatty acid) tails point away/are repelled from water OR hydrophilic (phosphate) heads point to/are in/are attracted to water

Question 3

many different substances enter and leave a cell by crossing its cell surface membrane

describe how substances can cross a cell surface membrane

Answer 3

1 (simple / facilitated) diffusion from high to low concentration / down concentration gradient;
2 small / non-polar / lipid-soluble molecules pass via phospholipids / bilayer
OR
large / polar / water-soluble molecules go through proteins
3 water moves by osmosis / from high water potential to low water potential / from less to more negative water potential
4 active transport is movement from low to high concentration / against concentration gradient
5 active transport / facilitated diffusion involves proteins / carriers
6 active transport requires energy / ATP
7 eef. to Na+ / glucose co-transport

Question 4

the movement of substances across cell membranes is affected by membrane structure

describe how

Answer 4

phospholipid (bilayer) allows movement/diffusion of non-polar/lipid-soluble substances
phospholipid (bilayer) prevents movement/diffusion of polar/ charged/lipid-insoluble substances OR (Membrane) proteins allow polar/charged substances to cross the membrane/bilayer
carrier proteins allow active transport;
channel/carrier proteins allow facilitated diffusion/co-transport
shape/charge of channel / carrier determines which substances move
number of channels/carriers determines how much movement
membrane surface area determines how much diffusion/movement
cholesterol affects fluidity/rigidity/permeability;

Question 5

What is Fick’s Law?

Answer 5

Rate of Diffusion = (surface area × concentration gradient) / diffusion distance

Question 6

What are 4 factors affecting the rate of diffusion?

Answer 6

1. Temperature
2. Surface Area
3. Concentration Gradient
4. Diffusion Distance / Pathway

Question 7

How does increasing temperature affect the rate of diffusion? (1)

Answer 7

increased kinetic energy so faster rate of diffusion of molecules

Question 8

How does increasing surface area affect the rate of diffusion? (1)

Answer 8

larger surface area provides for space for molecules so faster rate of diffusion

Question 9

How does increasing concentration gradient affect the rate of diffusion? (1)

Answer 9

as concentration difference increases, rate of diffusion increases

Question 10

How does increasing diffusion distance affect the rate of diffusion? (1)

Answer 10

longer diffusion distance, slower rate of diffusion

Question 11

What happens if the tertiary structure of the carrier protein changes? (1)

Answer 11

Facilitated diffusion / active transport can’t occur (because the binding site has changed shape and is no longer complementary to the molecule, therefore there will not fit/bind)

Question 12

Describe the relationship between size and surface area to volume ratio of organisms (1)

Answer 12

As the size of an organism increases, it’s surface area to volume ratio decreases

Question 13

Give two similarities in the movement of substances by diffusion and by osmosis. (2)

Answer 13

1. (Movement) down a gradient / from high concentration to low concentration
2. Passive / not active processes
   OR
3. Don’t use energy from respiration / ATP