Unit 2 - Topic 8 - Osmosis and Plant Transport

Question 1

Definition of Osmosis

Answer 1

Osmosis is the diffusion of water from a dilute solution to a more concentrated solution, down the concentration gradient, across a selectively permeable membrane.

Question 2

How are cell membranes ‘selectively permeable’?

Answer 2

Cell membranes are selectively permeable as they allow certain small molecules to pass through and enter, but prevent the movement and entry of larger molecules.

Question 3

Plant cells have a cell wall. Describe its role in controlling what enters and exits the cell.

Answer 3

Plant cells have a cellulose cell wall which is totally permeable and has no role in controlling what enters and exits the cell. It will allow both small and large molecules to pass through.

Question 4

Name an experiment which can be used to observe Osmosis. Describe the apparatus and the results.

Answer 4

The Visking Tubing experiment can be used to demonstrate Osmosis. Concentrated glucose solution is held in Visking tubing, which is then placed in a beaker of pure water. The tubing will appear flaccid at first. Water will move from the dilute solution (pure water) into the tubing to the more concentrated solution (glucose solution). The tubing will as a result increase in size and become turgid. The smaller molecules (water molecules) will be able to pass through, whereas the larger molecules (glucose molecules) will not.

Question 5

Give another name for Visking tubing.

Answer 5

Dialysis tubing

Question 6

Describe how cell Lysis occurs in animal cells.

Answer 6

Animal cells do not have a cell wall. Due to the absence of this structure, nothing can stop the cell membrane form expanding too far, causing damage. In a dilute solution, when water moves into the cell in excess, osmosis can cause animal cells to swell up and burst.

Question 7

Describe how crenation occurs in animal cells

Answer 7

In concentrated solutions, water loss causes the cells to shrink. When this happens to red blood cells, it is called crenation.

Question 8

How does the structure of a plant cell aid in the entry of water?

Answer 8

Normally, a plant cell contains a more concentrated solution than its surroundings. This will cause water to move into the cell. The vacuole will expand, pushing the cell membrane against the cell wall. This cause the turgor (pressure) necessary for support. The cell wall prevents the membrane from expanding too far and causing damage. The cell wall therefore limits the water intake of the cell.

Question 9

What happens to the appearance of a plant cell in the event of water loss?

Answer 9

If a plant cell is surrounded by a more concentrated solution, the cell will lose water via osmosis. Water will move out of the cell sap (vacuole), through the cell membrane and out of the cell. The cell will lose turgor (pressure) and the membrane will pull away from the cell wall as the vacuole shrinks. This is known as plasmolysis.

Question 10

In many experiments, changes in mass of plants is often used to observe osmosis, such as potato cylinders for example. When measuring changes after being placed in a solution, it is more accurate to weigh the sample rather than to measure its length or width. Why?

Answer 10

When measuring length in centimetres, we go to the nearest millimetre (one decimal place), whereas when measuring the mass, we go to two decimal places, improving our accuracy.

When measuring samples, there will also be human error which will hinder the accuracy of our results.

Also, when measuring the mass, we get an idea of the overall change of the sample, not just one factor.

Question 11

Isotonic solution definition

Answer 11

Surrounding solution is in equilibrium with cell contents, i.e same concentration.

Question 12

Hypertonic solution definition

Answer 12

Surrounding solution has a lower concentration of H2O than the cell.

Question 13

Hypotonic solution definition

Answer 13

Surrounding solution has a higher concentration of H2O than the cell.

Question 14

Plants use water for…

Answer 14

Support, (turgor)

Transpiration (The movement of water up through a plant, evaporation from leaf cells followed by diffusion out of the stomata)

Transport, as water moves up through a plant, in the xylem, it carries minerals

Water is a raw material for photosynthesis

Question 15

In an experiment where different initial volumes of potato cylinders were used, why is it important that % change in mass is used, and not change in mass?

Answer 15

The initial mass of each potato is different, therefore the percentage change in mass must be measured and calculated to get accurate results.

Question 16

Formula for change in mass

Answer 16

Final mass - initial mass = Change in mass

Question 17

Formula for percentage change in mass

Answer 17

Difference/Original x100

Question 18

Transpiration definition

Answer 18

Transpiration is the movement of water into and out of the plant. The water enters the plant in the roots, travels through the xylem, is evaporated in the spongy Mesophyll cells, diffuses through the air spaces and out through the pores (stomata).

Question 19

Explain how the cuticle and guard cells help reduce the loss of water from the leaf.

Answer 19

The waxy cuticle traps water from leaving the cell and prevents evaporation due to overheating. The guard cells will also close the stomata (small pores) and prevent water from evaporating out of the pores.

Question 20

What is the transpiration stream?

Answer 20

The transpiration stream is the continuous movement of water through the plant from root to leaf.

Question 21

Plants often need to reduce water loss by transpiration, they do this by two methods…

Answer 21

The guard cells close the stomata on the underside of the leaf

The waxy cuticle on the upper side of the leaf reduces evaporation

Question 22

Name some of the factors which affect the rate of transpiration.

Answer 22

Temperature - Higher temperatures increases the rate of evaporation, which will in turn increase the rate of transpiration.

Wind speed - Transpiration is increased on windy days as the wind carries water vapour away from the leaf surface, thus increasing the moisture gradient between the leaf and the surrounding atmosphere, thus increasing the rate of transpiration.

Humidity - On humid days there is more moisture in the air and the moisture gradient between the plant and atmosphere is reduced thus decreasing the rate of transpiration.

The surface area of leaves and the number of leaves will both increase the number of pores (stomata) available for evaporation to take place, thus affecting the rate of transpiration.

Question 23

List the ideal weather conditions for transpiration to occur.

Answer 23

Dry (humidity)
Warm temperature
High wind speed
Low cloud cover, increased light intensity

Question 24

How can the rate of water loss be measured?

Answer 24

The rate of water loss can be measured or compared in different conditions by a potometer

Question 25

What are the disadvantages concerned with the bubble potometer experiment?

Answer 25

The apparatus measures the water uptake by a cut shoot. It does not accurately measure the exact amount of transpiration as some of the water entering the plant is used and does not evaporate/leave the plant.

Question 26

What are the advantages concerned with the bubble potometer experiment?

Answer 26

It is an excellent method of comparing transpiration in different conditions.

Question 27

Describe the assembly of the bubble potometer apparatus.

Answer 27

Assemble the apparatus underwater in a water trough.

Cut the plant shoot underwater to avoid the formation of air bubbles.

The resevoir or syringe present in the apparatus is to reset the single air bubble back to its starting position, thus resetting the apparatus for repeats. This resets the bubble back to zero on the scaled tubing/metre rule.

Air leaks will hinder the uptake of water and the accuracy of our results, so the apparatus must be sealed with Vaseline at the junctions, particularly between the shoot and the neck of the potometer.

Question 28

How is the uptake of water measured?

Answer 28

The distance the air bubble moves

Question 29

How is the uptake of water in a given time measured?

Answer 29

The distance the air bubble moves in a given period of time.

Question 30

How can the potometer be used to investigate and measure how environmental factors which affect transpiration affect the rate of water uptake?

Answer 30

Certain modifications can be made to the apparatus.

Wind speed - The addition of a fan

Temperature - The addition of a heater

Humidity - The addition of a clear polythene bag, which will still allow light to enter, around the shoot

Question 31

Describe the apparatus and assembly required to carry out a weight potometer experiment to investigate the rate of water uptake in terms of overall mass.

Answer 31

A plant shoot is placed in a beaker of water, which is then placed on a top pan balance.

A layer of oil is placed on the water surface to prevent evaporation not associated with transpiration.

This means that only water loss through transpiration in the leaf is measured.

If humid conditions are desirable, a clear polythene bag can be placed around the plant shoot.

This experiment can also be carried out in a plant pot, where the layer of oil is substituted for a layer of Aluminium foil over the soil surface to prevent evaporation not associated with transpiration from occurring.

The plant pot is then placed on a top pan balance.

And like the other experiments, if humid conditions are desirable a clear polythene bag may be placed around the plant shoot.

Question 32

What is the formula for water uptake mass?

Answer 32

Initial mass of plant pot and contents - final mass of plant pot and contents

= Water uptake mass

Question 33

Describe how a bubble could be introduced in the water column in the bubble potometer apparatus.

Answer 33

The bubble can be introduced by lifting the delivery tubing out of the trough of water and then re-submerging it. This will create a single bubble within the apparatus.

Question 34

Why does leaf surface area affect the rate of transpiration?

Answer 34

A large leaf surface area will contain more stomata for water to evaporate out of and will have a larger surface area for evaporation to occur.