Plants

Question 1

A student investigated the distribution of stomata on leaves from two species of plant. She removed small pieces from the lower surface of the leaves of each plant species. She mounted these pieces on separate microscope slides. She then counted the number of stomata in several parts of the epidermis on each piece of leaf tissue using an optical microscope.

(a) Suggest appropriate units the student should use to compare the distribution of stomata on leaves.

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(1)

(b) The pieces of leaf tissue examined were very thin.

Explain why this was important.

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(2)

(c) Give two reasons why it was important that the student counted the number of stomata in several parts of each piece of leaf tissue

Answer 1

Stomata per mm2 or cm2

OR

Number per mm2 or cm2;

Accept: mm−2 or cm−2.

Reject: per μm2 or μm−2.

Reject: the use of a solidus / as being equivalent to per.

Ignore: ‘amount’.

1

(b) 1. Single/few layer(s) of cells;

Accept: more/too many/overlapping.

‛Single layer’ without reference to cells/tissue should not be credited.

2. So light can pass through;

2

(c) 1. Distribution may not be uniform

OR

So it is a representative sample;

Question 2

One of the two plant species used by the student in this investigation was a xerophyte.

Other than the distribution of stomata, suggest and explain two xerophytic features the leaves of this plant might have.

1. _________________________________________________________________

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2. _________________________________________________________________

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(2)

(e) The student then compared the rate of transpiration (evaporation of water) from the two species of plant. She did this by measuring the rate of water uptake by each plant species.

Suggest two reasons why the rate of water uptake by a plant might not be the same as the rate of transpiration.

Answer 2

Hairs so ‘trap’ water vapour and water potential gradient decreased;

2. Stomata in pits/grooves so ‘trap’ water vapour and water potential gradient decreased;
3. Thick (cuticle/waxy) layer so increases diffusion distance;
4. Waxy layer/cuticle so reduces evaporation/transpiration.
5. Rolled/folded/curled leaves so ‘trap’ water vapour and water potential gradient decreased;
6. Spines/needles so reduces surface area to volume ratio;

1, 2 and 5. Accept: humid/moist air as ‛water vapour’ but not water/moisture on its own.

1, 2 and 5. Accept: diffusion gradient as equivalent to water potential gradient.

1, 2 and 5. Accept: less exposed to air as an alternative to water potential gradient.

6. Accept: spines/needles so ‘reduce area’.

2 max

Question 3

e) The student then compared the rate of transpiration (evaporation of water) from the two species of plant. She did this by measuring the rate of water uptake by each plant species.

Suggest two reasons why the rate of water uptake by a plant might not be the same as the rate of transpiration.

Answer 3

e) 1. Water used for support/turgidity;

2. Water used in photosynthesis;
3. Water used in hydrolysis;
4. Water produced during respiration;

Question 4

The stomata close when the light is turned off.

Explain the advantage of this to the plant.

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(2)

(c) (i) Treatment 2 shows that even when the lower surface of the leaf is sealed there is still some uptake of carbon dioxide.

Suggest how this uptake of carbon dioxide continues.

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(1)

(ii) In both Treatment 1 and Treatment 2, the uptake of carbon dioxide falls to zero when the light is turned off.

Explain why.

Answer 4

(Mean rate of) carbon dioxide uptake was constant and fell after the light turned off;

Ignore absence of arbitrary units in both marking points.

Both ideas needed for mark.

Accept ‘stayed at 4.5’ as equivalent to ‘was constant’.

2. Uptake fell from 4.5 to 0 / uptake started to fall at 60 minutes and reached lowest at 80 minutes / uptake fell over period of 20 minutes;

One correct use of figures required.

Accept fell to nothing / no uptake for 0.

2

(ii) 1. (Because) water is lost through stomata;

2. (Closure) prevents / reduces water loss;
3. Maintain water content of cells.

This marking point rewards an understanding of reducing water loss e.g. reduce wilting, maintain turgor, and is not related to photosynthesis.

2 max

(c) (i) (Carbon dioxide uptake) through the upper surface of the leaf / through cuticle.

1

(ii) 1. No use of carbon dioxide in photosynthesis (in the dark);

2. No diffusion gradient (maintained) for carbon dioxide into leaf / there is now a diffusion gradient for carbon dioxide out of leaf (due to respiration)

Question 5

A student who saw these results concluded that as the carbon dioxide concentration of the air had increased the number of stomata per mm2 in leaves had decreased.
Do the results support this conclusion?

Answer 5

(Overall data show) negative correlation;

Do not allow description of correlation because in question stem

2. Little change in number of stomata in last 10 000 years;
3. Small sample size;
4. Only one species studied;
5. Other factors / named factor may have affected number of stomata;
6. Evidence does not support the conclusion between 30 000 and 25 000 years ago / between 5000 years ago and present day;

Accept reference to either one of these age ranges

7. Appropriate reference to standard deviations (in comparing means);