cell biology-potato practical

What is diffusion?

Spreading out of particles of a gas or substances in solution causing a...

● Net movement from an area of higher concentration to an area of lower conc. (down a conc. gradient)

Give examples of substances transported in / out of cells by diffusion

Oxygen and carbon dioxide in gas exchange

● Urea (a waste product) out of cells into blood plasma for excretion (removal) in the kidney

Describe 3 factors that increase rate of diffusion

1. Increasing concentration gradient (difference in concentrations)

2. Increasing temperature (so increasing kinetic energy)

3. Increasing surface area of membrane

How is surface area to volume ratio (SA:V) calculated?

Divide surface area (size length x side width x number of sides) by volume (length x width x depth)

How are single-celled organisms able to transport enough molecules in and out to meet their needs?

• Have a (relatively) large surface area to volume ratio (and short diffusion distance)

Explain why multicellular organisms require exchange surfaces and

transport systems?

Surface area to volume ratio is too small

● Diffusion distance too long (some cells too far from surface)

● So diffusion rate per unit volume too slow

Describe how the effectiveness of an exchange surface is increased

1. Increased surface area of cell membrane

2. Thin wall / membrane to provide a short diffusion path

3. (In animals) efficient blood supply

4. (In animals, for gaseous exchange) being ventilated

Explain how the small intestine is adapted for exchanging materials

1. Internal surface covered in many (villi) → ↑

surface area to maximise diffusion

2. Wall of villi are thin / one cell thick → short diffusion distance

3. Good blood supply to villi → moves food molecules away to

maintain a steep conc. gradient

4. Very long → increasing time for absorption

5. Many mitochondria in cells → ↑ respiration to transfer energy

for active transport

Explain how the lungs are adapted for gas exchange

1. Large number of alveoli (tiny air sacs)→ increase surface area to maximise diffusion

2. Walls of alveoli / capillary one cell thick → short diffusion distance

3. Alveoli have a good blood supply → remove O2 quickly to maintain a steep conc. gradient

4. Ventilation → brings in O2 to maintain a steep concentration gradient

Explain how gills in fish are adapted for gas exchange

Many villi → ↑ surface area to maximise diffusion

2. Very thin → short diffusion distance

3. Good blood supply → remove O2 quickly to maintain a steep conc. gradient

Explain how plant roots are adapted for exchanging materials

1. Many root hairs, root hair cells have long projections → ↑ surface area to maximise diffusion

2. Hairs are one cell thick / thin → short diffusion distance

3. Many mitochondria in cells → ↑ respiration to transfer energy for active transport

Explain how plant leaves are adapted for exchanging materials

1. Flattened shape, many stomata (pores) and internal air spaces → ↑ surface area

2. Thin → short diffusion distance

What is osmosis?

Diffusion of water

● From a dilute solution (high water conc.) to a more concentrated solution (low water conc.)

● Through a partially permeable membrane

Required practical 2-Investigate the effect of a range of concentrations of salt or sugar solutions on the mass of plant tissue.

1. Use a scalpel / cork borer to cut up a potato (remove peel) into 6 identical size cylinders

2. Blot dry then measure initial mass of each

3. Place each in a different concentration of sugar solution for 24 hours

4. Blot potato dry with paper towel then measure final mass of each

5. Percentage change in mass = (change in mass / starting mass) x 100

6. Repeat and calculate a mean for each concentration

7. On a graph, plot % change in mass (y axis) against concentration of sugar solution (x axis

What are the independent, dependent and control variables?

• Independent - concentration of sugar solution

● Dependent - change in mass of plant tissue

● Control - volume of solution, temperature of solution, time in

solution, starting size / length / shape of potato, variety of potato

What is active transport?

Movement of substances from a dilute to a more concentrated solution (against a conc. gradient)

● Requires energy from respiration

Give examples of active transport

• Absorption of mineral ions from soil into plant root hair cells → for healthy growth

● Absorption of sugar molecules from gut to blood → for cellular respiration to release energy