Movement of Substances Into and Out of Cells

Question 1

Define diffusion (2)

Answer 1

• The net movement of particles from an area of high concentration to an area of low concentration
• Passive process (no energy is required)

Question 2

Explain the factors that affect the rate of diffusion (4)

Answer 2

• Concentration gradient: greater = faster, more particles are randomly moving down than against it
• Temperature: greater = faster, greater kinetic energy which results in more frequent collisions
• Surface area to volume ratio: greater = faster, more space for the particles to move through
• Distance: shorter = faster, less distance for particles to pass through

Question 3

Describe diffusion in living organisms (2)

Answer 3

• Diffusion used when they cross the cell membrane, which is partially permeable
• Allows smaller molecules (e.g. oxygen, amino acids, glucose) to diffuse across but larger molecules (e.g. starch, proteins) cannot

Question 4

Describe diffusion in unicellular organisms (2)

Answer 4

• Can transport molecules directly into body from the surrounding air (have a relatively large surface area to volume ratio)
• Have low metabolic demands, so diffusion across surface is sufficient

Question 5

Describe diffusion in multicellular organisms (3)

Answer 5

• Surface area to volume ratio is small so they are unable to rely on diffusion alone
• Therefore cell surfaces and organ systems have a number of adaptations
• For example: alveoli in lungs, villi in small intestines, root hair cells in plants

Question 6

Define osmosis (2)

Answer 6

• The net movement of water from an area of low solute concentration to high solute concentration through a semi permeable membrane
• Passive process (not energy is required)

Question 7

What does it mean when a solution is isotonic to the cell?

Answer 7

The concentration of sugar in an external solution is same as the internal, so there is no movement

Question 8

What does it mean when a solution is hypertonic to the cell?

Answer 8

The concentration of sugar in an external solution is higher than the internal, so water moves out

Question 9

What does it mean when a solution is hypotonic to the cell?

Answer 9

The concentration of sugar in an external solution is lower than the internal, so water moves in

Question 10

Describe osmosis in animal cells (2)

Answer 10

• Concentrated solution: cell loses water, become shrivelled
• Dilute solution: cell swells and may burst

Question 11

Why are turgid plant cells essential? (2)

Answer 11

• Prevents wilting
• Internal pressure is developed, this provides structural support for leaves and stems

Question 12

Define active transport (2)

Answer 12

• The movement of particles from an area of low concentration to an area of high concentration
• Active process, requires energy from respiration

Question 13

Give 2 examples of active transport (1 in animals, 1 in plants)

Answer 13

• Absorption of the products of digestion into the bloodstream from the small intestine
• Absorption of mineral ions from the soil into the root hair cells of plants

Question 14

Describe an experiment to investigate diffusion in non-living systems (7)

Answer 14

• 1cm3 cube of agar (sodium hydroxide and phenolphthalein indicator)
• Put in hydrochloric acid
• Remove and wash with water (to stop further reaction)
• Cut cube in half and measure distance the acid has caused the agar to become colourless from outside inwards
• Repeat and calculate mean
• Repeat with different concentrations of hydrochloric acid
• You can repeat this experiment to see how other factors affect rate of diffusion (e.g. temperature, s.a. to volume ratio)

Question 15

Describe an experiment to investigate osmosis in potatoes (5)

Answer 15

• Place different sucrose solutions in different boiling tubes (include 0% for a control)
• Dry potato strips on a paper towel and measure the masses
• Put each potato strip into each sucrose solution for 20 minutes
• Record change in mass
• Repeat tests at each solution several times with potato strips of similar mass and s.a. to volume ratio

Question 16

How does a plant cell become turgid? (2)

Answer 16

• Placed in a dilute solution
• It absorbs water by osmosis

Question 17

How does a plant cell become flaccid and what happens? (4)

Answer 17

• Placed in a concentrated solution, - It loses water by osmosis
• The cell membrane no longer pushes against the wall
• Decreases in volume

Question 18

How does a plant cell become plasmolysed and what does it lead to? (4)

Answer 18

• An excessive loss of water by osmosis
• Leads to a gap between the cell membrane and cell wall
• Cell contents shrink, causing wilting
• This is a protective action which reduce’s the plant’s s.a. to volume ratio, reducing water loss