A2 02 Photosynthesis

A2 Biology Exam DoctorPHOTOSYNTHESIS

Question2

Question3

Question1


QUESTION 1

The graph below shows the effect of light intensity on photosynthesis.

(a) What is the limiting factor in region X?

[1](b) Comment on the shape of the graph at Y.

[3]

Temperature.




[3]

The rate of photosyntheis is slowing down. There is almost too much light and another factor like water or CO2 is now slowing it down.

(a) light intensity; 1(b) rate of increase is slowing; light intensity is not limiting the rate; carbon dioxide concentation or temperature is now limiting; 3 TOTAL 4

MARK SCHEME


(a) What is the limiting factor in region X? [1]

(b) Comment on the shape of the graph at Y. [3]

No, over that steep part of the graph it is light intensity that is holding back the rate. You can tell this because as the light intensity increases, so does the rate.

No it isn’t! Over the whole of Y, as light intensity increases, so does the rate of photosynthesis. However, it would be true to say that the rate of increase is slowing down – the curve is getting less steep. So, what is happening is that as the light intensity increases, it is having progressively less stimulating effect. This is because another factor, possibly light or CO2

concentration is now limiting the overall rate.

We simply do not know this MP 3

Total 1 out of 4

Temperature.




[3]

The rate of photosyntheis is slowing down. There is almost too much light and another factor like water or CO2 is now slowing it down.


QUESTION 2

(b) Using information from the graph and your own knowledge, suggest a mechanism for stomatal opening.

The graph shows results of investigations into the mechanism of stomatal opening.

(a) Describe the relationship between stomatal aperture width and starch concentration (i) between: 8.00 am and noon.

[1]

(ii) between: 4.00 pm and 8.00 pm. [1]

[4]




[1]

(ii) between: 4.00 pm and 8.00 pm. [1]

[4]

Starch concentration goes down as width increases.

Starch concentration increases as width decreases.

Starch is being broken down into glucose in the morning. The sugars are respired and used to make ATP which enables the guard cells to open the stomata. At 4.00 pm the starch runs out, ATP production stops and the guard cells close.



(a) Describe the relationship between stomatal aperture width and starch concentration between:

(i) 8.00 am and noon.

(ii) 4.00 pm and 8.00 pm.

(a)(i) stomatal width increases as concentration of starch decreases; 1 (ii) stomatal width decreases as concentration of starch increases; 1

(b)(i) light stimulates conversion of starch to sugars; therefore as light intensity increases, starch decreases; sugars reduce water potential of guard cells; water enters osmotically; as guard cells swell, stomatal pore opens; ref uneven thickening of guard cell walls; max 4

TOTAL 6

MARK SCHEME




[1](ii) between: 4.00 pm and 8.00 pm.

[1]

[4]

Starch concentration goes down as width increases.Starch concentration increases as width decreases.

Starch is being broken down into glucose in the morning. The sugars are respired and used to make ATP which enables the guard cells to open the stomata. At 4.00 pm the starch runs out, ATP production stops and the guard cells close.

Good concise description

The student hasn’t looked at the graph closely enough, nor considered why the examiner asked points (i) and (ii). The examiner was trying to tell the student that light may be involved in stomatal opening. Total 3 out of 6

The examiner has awarded the mark as a combination of MP 1+2


QUESTION 3

(ii) In which part of the chloroplast does the light independent reactions occur?

The diagram below shows the ultrastructure of a chloroplast

A

B

C

(a)(i) Identify structures A, B and C.

(b) Distinguish between oxidative phosphorylation and photophosphorylation.

[3]

A

BC

[1]

[4]



A

B

C



[3]

A

BC

[1]

[4]

the matrixa granum

a ribosome

The light independent reaction occurs in the matrix..

Oxidative phosphorylation generates ATP from the oxidation of reduced coenzymes. Photophosphorylation generates ATP from light energy and produces reduced NAD.

MARK SCHEME(a) (i) A = stroma; B = granum/stack of thylakoids; C = ribosome;

3 (ii) stroma;

1

(b) oxidative phosphorylation uses energy gained from oxidation; of reduced coenzymes/NADH; to produce ATP; photophosphorylation uses light energy; to produce ATP; and reduced NADPH;

4 TOTAL 8



A

B

C





A

B

C



[3]

A

BC

[1]

[4]

the matrixa granum

a ribosome

The light independent reaction occurs in the matrix.

Oxidative phosphorylation generates ATP from the oxidation of reduced coenzymes. Photophosphorylation generates ATP from light energy and produces reduced NAD.

A is the stroma. The matrix is found in the mitochondrion.

But don’t waste time repeating the question.

Accept matrix here – it was penalised above.

MP2 MP3

MP2

MP4

Photosynthesis uses NADP/NADPH, respiration uses NAD/NADH.

Total 7 out of 8

A2 02 Photosynthesis

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