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NUTRITION IN PLANTS (SYLLABUS SECTION 4.1 : 2011-2013)

STRUCTURE AND ROLES OF LEAF AND CHLOROPLAST 1. The figure is an electronmicrograph of a chloroplast.

(a) (i) Name X to Z. [3 marks]

X – stroma; Y – thylakoid; Z – starch grain

(ii) On the above figure indicate with a label line marked with the letter W where photophosphorylation takes place. [1 mark]

(b) Describe how chloroplasts are specialized for photosynthesis. [3 marks]

Chloroplasts provide a large surface area where photosynthetic pigments cane be accommodated by having many thylakoid membranes. The light-dependant and light-independent reactions occur in two separate regions within the chloroplast, the thylakoids and stroma, respectively.

2. The rate of photosynthesis at different wavelengths of light can be measured and plotted on a graph. This is called an action spectrum and is shown on the figure below.

Describe the effects of different wavelengths of light on the rate of photosynthesis. [3 marks] The rate of photosynthesis is high at both ends of the visible spectrum, coinciding with the blue-violet (450nm) and red (650nm) wavelengths. The lowest rate is around 550nm, coinciding with the green wavelength.

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PHOTOSYNTHESIS 3. The diagram below is a summary of the light-dependent reaction of photosynthesis.

(a) Name compound X. ATP [1 mark] (b) Name the gas Y and describe how it is produced. [3 marks]

Oxygen. It is produced by photolysis. Water splits in the presence of light on the reaction centre of PSII.

(c) State where in a chloroplast the light-dependent reaction takes place. [1 mark] Thylakoid membranes.

(d) Name one pigment which is responsible for the capture of light energy during the light-dependent reaction of photosynthesis. [1 mark] Chlorophyll a/b; β-carotene

4. The diagram summarises the light-dependent reaction in photosynthesis.

(a) Where, in the chloroplast, does the light-dependent reaction take place? Thylakoid membranes

[1 mark] (b) During this reaction water molecules are broken down to yield oxygen, electrons and hydrogen

ions (protons). (i) What is the name given to the process in which the water molecules are broken down? [1 mark]

Photolysis.

(ii) What happens to the electrons produced in this process? [1 mark] Taken up by PSII to replace those lost.

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(iii) What happens to the hydrogen ions? [1 mark] Hydrogen ions and electrons are taken up by NADP to form NADPH+H+. 5. In the chloroplasts of green plants, pigment molecules are organized into photosynthetic units

called photosystems.

(a) (i) Name two pigment molecules found in chloroplasts. [2 marks]

Chlorophyll a/b; β-carotene

(ii) State one role of pigment molecules in a photosystem. [1 mark] Trap light energy and convert it into chemical energy.

(iii) State the location of photosystems within the chloroplast. Thylakoid membranes [1 mark]

(b) The production of ATP in chloroplasts is known as photophosphorylation. There are two types of

photophosphorylation, cyclic and non-cyclic. (i) Complete the following table comparing the two types of photophosphorylation.

Cyclic Non-cyclic

Photosystem(s) involved

PSI PSI, PSII

End product(s)

ATP Oxygen, ATP, NADPH+H+

[4 marks] (ii) Explain the role of ATP in the light independent stage of photosynthesis. [3 marks]

ATP is used for the purposes in the Calvin cycle. It is first used to provide energy during the conversion of 3-phosphoglycerate into glyceraldehyde 3-phosphate. Then it is used in the regeneration of RuBP. ATP provides the phosphate group and energy to change ribulose monophosphate to ribulose bisphosphate.

(c) Some chloroplasts have been shown to possess photosystem I only. Explain why these chloroplasts are unable to form sugars. [3 marks]

Since only PSII provides NADPH+H+ that provides the reducing power during the Calvin cycle needed to convert 3-phosphoglycerate into glyceraldehyde 3-phosphate.

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6. The light dependent stage of photosynthesis takes place on thylakoid membranes in chloroplasts. These membranes surround the thylakoid space (lumen) and are arranged into stacks known as grana. The diagram summarizes the processes that take place at the thylakoid membrane.

(a) State the general name of the pigment complexes shown as E and F on the diagram. [1 mark]

Antenna complex/ light-harvesting complex

(b) Name the pigment represented by P680 and P700. Chlorophyll a [1 mark] (c) Name the type of molecule represented by G. Protein [1 mark] (d) State, using the information in the diagram above, why the pH of the thylakoid space (lumen) is

lower than that of the stroma. [1 mark] As H+ are pumped into the lumen.

(e) Explain the function of this pH gradient. [3 marks] A gradient of hydrogen ions is required for ATP to be generated by chemiosmosis.

Hydrogen ions move down their concentration gradient back into the stroma through a channel protein. As these pass through, the enzyme ATP synthase joins ADP and Pi, liberating ATP. One ATP molecule is formed for every three hydrogen ions that pass through.

(f) Herbicides (weed killers), such as diquat and paraquat, act on the chloroplast thylakoids. They

interfere with electron transport by accepting electrons and prevent the light dependent stage of photosynthesis from taking place. Explain how this causes plants to die. [5 marks] The light-dependant stage of photosynthesis generates the energized carriers ATP and NADPH+H+ which are later used to make sugars in the light-independent stage. These sugars can then be converted into all the other types of organic molecules the plant requires. If these carriers are absent, then no sugars can be formed and the plant dies due to lack of organic materials.

(g) Some weed species are not killed when herbicides are applied. Suggest why. [2 marks] They are resistant to the herbicide.

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7. The figure below gives an outline of the Calvin cycle.

(a) Indicate on the figure, by using the letter P, where the ATP from photophosphorylation is utilized in

the Calvin cycle. [2 marks] (b) Name the enzyme involved in the fixation of carbon dioxide. [1 mark] Rubisco / Ribulose bisphosphate carboxylase (c) Name substance Q. Glyceraldehyde 3-phosphate [1 mark] (d) State two macromolecules, other than carbohydrates, that can be formed from substance Q. Proteins; nucleic acids; lipids [2 marks] 8. The diagram below shows the main stages of the light-independent reaction in photosynthesis. (a) Write in the boxes in the diagram the number of carbon atoms in each of the molecules as

appropriate. [1 mark]

(b) What is the role of ATP in the conversion of: (i) glycerate 3-phosphate to triose phosphate; [1 mark] Provides energy to change the carboxyl group into the aldehyde one. (ii) ribulose phosphate to ribulose bisphosphate? [1 mark] Provides the phosphate group and the energy to add this group to ribulose phosphate. (c) Explain why `light-independent reaction' is a better name than `dark reaction' [1 mark] Since the reaction occurs in the light and not in darkness. However, it does not need light

to proceed.

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FACTORS AFFECTING PHOTOSYNTHESIS 9. The graph below shows the rate of photosynthesis of a plant at different light intensities. All other

external factors were kept constant.

(a) (i) What external factor is limiting photosynthesis between A and B? Light intensity [1 mark] (ii) Give a reason for your answer. [1 mark] Rate increases when light intensity increases. (b) Explain how temperature limits the rate of photosynthesis. [1 mark] As temperature affects the rate at which enzymes required for photosynthesis, like rubisco,

function. (c) Explain how one feature of the leaf might affect the rate of photosynthesis. [3 marks] Surface area – the greater it is, the higher will the rate be. The more light energy is

absorbed, the more chemical energy in the plant. Amount of chlorophyll present – same reason.

10. The graph shows the relation between the rate of photosynthesis and carbon dioxide concentration at different light intensities and temperatures in a species of flowering plant.

(a) Explain why measuring the rate of uptake of carbon dioxide by the plant would not give a true

figure for the rate of photosynthesis. [1 mark] As the plant uses the carbon dioxide it produces itself by respiration. (b) Giving a reason for your answer in each case, what is likely to be limiting the rate of

photosynthesis in this species of plant:

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(i) at 0.01% carbon dioxide, a temperature of 20°C and a light intensity of 6.2 kilolux; [2 marks] Carbon dioxide is the limiting factor, since on increasing this, the rate increased. (ii) outside in full sunlight at a temperature of 4°C. [2 marks] Temperature is the limiting factor. Photosynthesis involves enzymes to proceed. (c) Under what conditions would an increase in temperature from 12°C to 20°C increase the rate of

photosynthesis? [1 mark] An increase in light intensity. 11. An experiment was carried out to investigate the effect of light intensity on the rate of

photosynthesis of an aquatic plant, using the apparatus shown in the diagram below.

(a) State TW0 environmental conditions, other than light intensity, which would need to be

controlled. For each condition, describe how control could be achieved. [4 marks] Temperature – placing a beaker full of water in front of the pondweed. Concentration of HCO3

- – using a fresh supply of the HCO3- solution for each experiment

(b) The plant was allowed to carry out photosynthesis for 10 minutes. Describe how you would use

the apparatus to determine the volume of oxygen produced by the plant during this 10-minute period. [4 marks] The bubbles collected are made to move over the capillary tubing by using the syringe. The length of the bubble (l) is measured and the volume is calculated from πr2l.

(c) Using this apparatus, the volume of oxygen produced after I0 minute periods was determined at

different light intensities. The results of this investigation are shown in the graph below.

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Comment on the results of this investigation. [5 lines, 3 marks] The volume of oxygen, i.e. rate of photosynthesis, increases at a fast rate when the light intensity is increased up to 2000 lux, but slows down above this value. This indicates that some other factor, not light, starts becoming a limiting factor. Carbon dioxide concentration or temperature may be limiting.

12. The graph below shows how the wavelength of light affects the rate of photosynthesis.

(a) (i) What name is given to the relationship between wavelength of light and the rate of

photosynthesis, as shown by this graph? Action spectrum [1 mark]

(ii) From the graph, state the optimum wavelength of light for photosynthesis. 450 nm [1 mark]

(iii) Explain the effect on the rate of photosynthesis of varying the wavelength of light from 550 to 700 nm. [2 marks]

The rate of photosynthesis decreases when the light intensity is increased from 550 nm to 575 nm, increases up to 625 nm. Then it decreases slowly up to 675 nm but drops sharply above this value until rate falls to zero at 700 nm.

(b) (i) Name the stage of photosynthesis that produces oxygen. Light-dependant reactions[1 mark]

(ii) State where in a chloroplast this stage would occur. Thylakoid membranes [1 mark]

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C3 AND C4 PLANTS 13. The graphs show the effects of light intensity and temperature on the rate of photosynthesis in

two species of plant, A and B.

(a) (i) Over what range of light intensities is light a limiting factor for photosynthesis in species B? 0-20 arbitrary units. [1 mark] (ii) Species A normally grows in tropical areas. Give two pieces of evidence from the graphs

which support this. [2 marks] Higher rate of photosynthesis at higher light intensities and temperature. (b) Species A has a specialised method of photosynthesis which helps it to concentrate carbon

dioxide in some types of cells. The diagram shows the interconversion of some organic compounds, which contain either three carbon atoms (C3) or four carbon atoms (C4). It also shows the transfer of these compounds between two cells in a leaf of species A.

(i) When carbon dioxide enters cell X, it combines with the C3 compound to produce a C4

compound. Explain how this causes a large amount of carbon dioxide to enter cell X. [2 marks] PEP carboxylase has a high affinity for carbon dioxide and is not inhibited by oxygen.

(ii) Use information from the diagram to explain how a high concentration of carbon dioxide is built up in cell Y. [2 marks] C4 compound enters cell Y and breaks down to release carbon dioxide.

(iii) Why is it useful for plants growing in tropical areas to be able to build up a high concentration of carbon dioxide in their photosynthesising tissues? [2 marks]

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These plants close their stomata during the day to reduce evaporation. However, by doing so, oxygen builds up inside the leaf. This situation leads to rubisco fixing oxygen instead of carbon dioxide. Thus to avoid this happening, tropical plants have a mechanism which enables carbon dioxide concentration to build up inside their photosynthesizing tissues.

GENERAL QUESTIONS 14. The diagram below is a simplified representation of some steps in the light dependent phase of

photosynthesis.

(a) In which part of the chloroplast do the light-dependent reactions of photosynthesis occur?

Thylakoid membranes [2 marks] (b) What do P680 and P700 represent? [2 marks] Reaction centres of photosystem II and I respectively. They are special molecules of

chlorophyll a, able to lose electrons when they absorb light energy. (c) Which of the two photosystems is stimulated by higher photon energies? P680 [1 mark]

(d) What is the name given to the process in which ADP is converted to ATP? [1 mark] Photophosphorlyation. (e) How is Photosystem II brought back to the ground state after releasing electrons? [3 marks]

The electrons PSII loses are replaced when water splits. During photolysis, light splits water into oxygen, hydrogen ions and electrons.

(f) Why are electrons from Photosystem II passed on to Photosystem I? [3marks] To generate ATP by chemiosmosis and to supply PSI with electrons to replace those being lost from it when light shines.

(g) What happens to the products of the light-dependent phase during the light-independent phase of photosynthesis? [3marks] ATP is used in two reactions during the light-independent phase while all the NADPH+H+ is used at one stage. ATP provides the energy needed to reduce 3-phosphoglcyerate into glyceraldehde-3 phosphate and NADPH+H+ provides the reducing power . ATP also provides the phosphate group needed to change RuP into RuBP.

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15.(a) Which of the following statements about photophosphorylation and the Calvin-Benson cycle is NOT true? (i) The products of photophosphorylation are ATP, NADPH + H+

and O2

(ii) The products of the Calvin-Benson cycle are CO2 and sugar (iii) Photophosphorylation requires chlorophyll a (iv) ATP and NADPH + H+

produced in photophosphorylation are used in the Calvin-Benson cycle

(v) Photophosphorylation is activated by light 15 (b) Select the correct path that an electron could take during the complete process of

photosynthesis. (i) C02 ribulose bisphosphate G3P glucose (ii) H2O photosystem I photosystem II NADPH + H+ G3P (iii) photosystem II H2O photosystem I NADPH + H+ glucose (iv) CO2 photosystem I NADPH + H+ ribulose bisphosphate G3P (v) H20 photosystem II photosystem I NADPH + H+ G3P

15 (c). Choose all of the following materials that are not cycled between the light reactions and the

Calvin-Benson cycle. (i) Water (ii) NADP+

(iii) ATP (iv) Oxygen (v) NADPH + H+

15(d). The following graph shows changes in the concentration of substances A, B, and C during a

“lollipop” experiment in which the CO2 supply was eliminated at time 30. Explanation:

Based on these data, which of the following shows the correct pathway for the substances involved? (i) A + CO2 B C (ii) C + CO2 B A (iii) A + CO2 C B (iv) B + CO2 C A (v) A + B C + CO2

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15 (e) Which of the following statements about chemiosmotic ATP production in chloroplasts and mitochondria is not true? (i) Both depend on membrane proteins that are ATP synthases. (ii) In both, ATP is generated by passive diffusion of H+. (iii) Both depend on a proton gradient across a membrane. (iv) Both depend on the impermeability of cell membranes to charged particles. (v) The electron carriers are the same in both.

15 (f) Which one of the following reactions is least likely to occur within the cells of a C4 plant?

(i) The Calvin-Benson cycle (ii) Photorespiration (iii) CO2 + RuBP 2 3PG (iv) Photophosphorylation (v) CO2 + PEP oxaloacetate

16 (a) Which one of the following categories does not differ between a C3 and a C4 plant?

(i) Initial C02 acceptor (ii) Extent of photorespiration (iii) Enzyme catalysing reaction that fixes CO2

(iv) Presence of Calvin-Benson cycle (v) Leaf anatomy

16 (b) Which of the following features is not characteristic of some CAM plants?

(i) Stomata open only at night (ii) Trap CO2 with PEP carboxylase (iii) PEP produced photosynthetically during day {NOTE: PEP is made during glycolysis] (iv) Calvin-Benson cycle during day only (v) CO2 captured by rubisco from four-carbon acids

16(c ). Which of the following processes would not be inhibited if a plant cell were placed in the dark?

(i) Cellular respiration (ii) Cyclic photophosphorylation (iii) Non-cyclic photophosphorylation (iv) Photorespiration (v) The Calvin-Benson cycle

16(d) Which one of the following statements about NAD+

and NADPH is not true? (i) Both compounds are oxidation-reduction agents. (ii) NAD+

participates in respiratory reactions; NADPH participates in photosynthetic reactions.

(iii) NADPH is restricted to photosynthetic autotrophs; NAD+ is restricted to

heterotrophs. (iv) Both compounds exist in two different molecular forms. (v) Both compounds function as electron carriers.

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17. (a) Indicate whether the following statements about photosynthesis are true or false:

Statement True/False ATP is produced in cyclic photophosphorylation and used up in the Calvin-Benson cycle True

Excited chlorophyll is an oxidising agent False In non-cyclic photophosphorylation CO2 reacts with RuBP False Chlorophyll pigments absorb light near both ends of the spectrum True

In the Calvin-Benson cycle ATP and NADP + H+ are formed as 3PG is reduced False

Chlorophyll can accept energy from accessory pigments such as carotenoids True

(3) (b) Choose two of the statements you marked FALSE and in each case give reasons for your

answer. [6 marks] Excited chlorophyll is a reducing agent as it donates electrons. In the Calvin-Benson cycle CO2 reacts with RuBP. In the Calvin-Benson cycle ATP and NADP + H+ are used as 3PG is reduced.

(c) Explain briefly what you understand by each of the following: [2 marks each]

“non-cyclic photophosphorylation”; The light-requiring part of photosynthesis where electrons move from PS II to PS I and end on NADP to form NADP + H+ and ATP. Oxygen is produced as a waste product. “oxidising agent”; An oxidising agent can be defined as either a chemical compound that readily transfers oxygen atoms, or a substance that gains electrons in a redox chemical reaction. “NADP”; a coenzyme that binds loosely to the enzyme. Acts as a hydrogen carrier. Becomes reduced during the light dependent reaction. “RuBP”; Is a compound that fixes carbon dioxide from the air in the presence of rubisco during the Calvin cycle, producing glycerate phosphate.

18. The graph in Fig 3 below shows the degree of CO2 uptake by an alfalfa plant over a 48-hr period.

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(a) What is the time of maximum photosynthetic activity? Noon [1 mark] (b) Why is the level of CO2 fixation negative at certain times? [3 lines, 3 marks]

As the rate of respiration is higher than that of photosynthesis. Food is consumed rather than formed.

(c) Alfalfa is grown as animal fodder and the mass of plant tissue produced is therefore of great importance.

Which of the following climatic conditions would you consider to be beneficial to an alfalfa crop grown without irrigation? In each case give reasons far your answer. (i) Long periods of overcast sky No. Lack of light. (ii) Low atmospheric CO2 levels No. Carbon dioxide is limiting. (iii) Prolonged windy conditions No. Stomata close. (iv) Clear skies throughout the growing period. Yes. Maximum light. [1 mark each] 19. The photograph is an electronmicrograph of parts of two cells from a maize leaf.

(a) Explain why it is not possible to see the detail shown in this photograph when using a light

microscope at the same magnification. Lower resolution [2 marks] (b) This section has been stained with a heavy metal. Explain why the feature labelled P appears

uniformly black in colour. Material absorbs electrons. [3 marks]

(c) (i) Identify features X and Y. X – cell wall, Y – plasmodesma [2 marks] (ii) Suggest the function of feature Y. Communication between cells. [1 mark] (d) (i) Construct a suitable table showing three structural differences between the chloroplast labelled

A and that labelled B. [3 marks]

A B Few grana More grana Smaller grana Larger grana More starch grains Fewer starch grains

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(ii) It is thought that the process of photophosphorylation is of much greater importance in chloroplast B than it is in chloroplast A. Explain the evidence in the photograph that supports this. [2 marks] Photophosphorylation occurs on the thylakoid membranes, of which there are more in chloroplast B. A preparation of chloroplasts can be made from grinding suitable tissue in a pestle and mortar and separating the chloroplasts by centrifugation. It is very difficult to produce a pure suspension of undamaged chloroplasts.

(e) Suggest and explain one precaution that could be taken to: (i) avoid contamination with nuclei; [2 marks]

Centrifuge many times.

(ii) prevent damage to the partially permeable outer membranes of the chloroplasts when they are released from the cells. [2 marks]

Use low speeds. (f) Uptake of phosphate labelled with radioactive 32P and of carbon dioxide labelled with radioactive

14C were measured in isolated chloroplast preparations. The investigation was carried out in the light and in the dark. The results are shown in the table.

(i) Name the substance to which the phosphate is added and the product formed during photophosphorylation. [2 marks] The substance is ADP and the product is ATP.

(ii) Describe what happened to the carbon dioxide when the preparation was illuminated. [2 marks] Carbon dioxide was fixed. It was trapped by RuBP in the presence of rubisco. (iii) Use the results of the phosphate-uptake investigation to explain why there is more carbon

dioxide taken up in the light than in the dark. [3 marks] In the light, carbon dioxide is fixed by RuBP. At the same time a lot of phosphate-labelled substance was taken up to generate ATP needed for the Calvin cycle.

20. (a) Briefly describe how ATP is produced as a result of light striking chlorophyll molecules. [2 marks]

Electrons containing a lot of energy are lost from the reaction centre of PSII. These are trapped by an electron acceptor which forms part of the electron transport chain. The electron moves along the electron transport chain, losing energy in the process. This energy loss is used to pump hydrogen ions into the thylakoid interior. ATP is generated by ATP synthase when hydrogen ions move down their concentration gradient through the channel, part of the enzyme complex.

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(b) Although photosynthesis generates ATP, plants also generate ATP by respiration. Explain why this is necessary. [2 marks]

All the ATP generated by photosynthesis is used in the formation of organic molecules. ATP for other cell processes is generated by respiration.

The diagram below shows the main steps in the light-independent stage of photosynthesis. (i) For what is ATP used in step X? To supply the phosphate group and the energy to join this

to ribulose phosphate. [1 mark] (ii) Name compound Y. 3-phosphoglycerate / glycerate phosphate

21. The blue dye DCPIP can be converted to colourless reduced DCPIP by gaining electrons. This is

summarised below. A suspension of chloroplasts was made by grinding fresh leaves in buffer solution and centrifuging the mixture. Tubes were then prepared and treated in the following way.

(a) (i) In tube A, from where do the electrons come that reduce the DCPIP? [1 mark]

Excited photosystems.

(ii) What normally happens to these electrons in a photosynthesising leaf? [1 mark] Are accepted by NADP+ at the end of noncyclic photophosphorlyation..

(b) The chloroplast suspension may be contaminated with mitochondria. Explain the evidence from this investigation that the presence of mitochondria was not responsible for reduction of the DCPIP. [2 marks]

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As in the dark there was no colour change.