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OCR Biology A2

F214 Communication, Excretion and Homeostasis Unit 4.3: Photosynthesis

JWh

8/8/2014

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3.1 Define the terms autotroph and heterotroph;

Photosynthesis is a process whereby light energy is converted to chemical energy which can be used to synthesis large, organic molecules from smaller, inorganic substrates.

Plants and other organisms which are photosynthetic (for example some bacteria) are known as autotrophs, they make organic compounds from small inorganic compounds e.g. CO2.

More specifically, those autotrophs which undergo photosynthesis are known as photoautotrophs.

Animals, and many other organisms, cannot synthesise their own food, but instead digest organic molecules. These are known as heterotrophs.

A simplified equation for photosynthesis is: Carbon Dioxide + Water → Glucose + Oxygen

6CO2 + 6H2O → C6H12O6 + 6O2

Photosynthesis is considered to be the most fundamental biochemical process, as all organisms require the products of photosynthesis to undergo aerobic respiration.

Autotrophs are found at the start of all food chains.

Q1. The diagram shows some of the early stages in the development of life on Earth. ozone layer develops and protects organisms from harmful UV rays

autotrophs evolved X released

more efficient respiration possible(i) What gas is represented by X?........................................................................................................................ (1 mark)

State that light energy is used during photosynthesis to produce complex organic molecules;

Energy cannot be created or destroyed simply changed from one form to another.

First Law of Thermodynamics

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Absorption and Action Spectrums

The action spectrum shows the rate of photosynthesis at different wavelengths.

The absorption spectrum shows how strongly the pigments absorb at different wavelengths.

The absorption spectrum and action spectrum show that the wavelengths that are most strongly absorbed (red and blue) are the ones that cause photosynthesis to proceed at the fastest rate.

Green is reflected, causing leaves to look green. The shorter the wavelength, the more energy it contains. During photosynthesis the light energy is converted into chemical energy. The absorbed light excites electrons in the pigment molecules and the energy

can be passed on to be used by the plant

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Q2.          The percentage of light absorbed by an aquatic plant was measured when it was exposed to different wavelengths. The rate of photosynthesis was also measured at each wavelength of light. The results are shown in the graph.

(a)     Describe and explain the relationship between light absorption and the rate of photosynthesis for the wavelengths of light between 410 nm and 500 nm.

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Explain how respiration in plants and animals depends upon the products of photosynthesis;

The glucose made may be used to make other substances or it may be used in respiration. The oxygen may diffuse out through the stomata or it may be used in respiration.

State that in plants photosynthesis is a two-stage process taking place in chloroplasts;

Photosynthesis is a process which occurs in two stages, both entirely inside the chloroplast.

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A chloroplast is an organelle within photosynthetic cells.

1. The granal membranes provide a large surface area for the attachment of the photosynthetic pigments (chlorophylls and carotenoids), electron carriers and enzymes for the light-dependent reactions

2. A network of proteins in the grana hold the pigments in a very precise manner that forms the photosystems allowing for maximum absorption of light

3. The granal membranes have many ATP synthase enzymes attached to them which, help to manufacture ATP molecules

4. The fluid of the stroma holds all of the enzymes needed to carry out the light-independent reactions

5. The stroma fluid surrounds the grana, and so the products of the light-dependent reactions can directly and readily pass into the stroma for the light-independent reactions

6. Chloroplasts contain both DNA and ribosomes so they can quickly and easily manufacture photosynthetic proteins

Chloroplasts vary, but most are disc-shaped and approximately 2-10μm long. Each has a double membrane (called an envelope), consisting of the inner

membrane and outer membrane.

Explain, with the aid of diagrams and electron micrographs, how the structure of chloroplasts enables them to carry out their functions;

Chloroplasts contain stacks of flattened membrane compartments. Each stack is called a granum (plural: grana) and each compartment is a

thylakoid. Small, thin membranal extensions connect different grana, called inter-

granal lamellae. The fluid surrounding the grana is called stroma. Starch grains can also be found in the stroma matrix, as well as DNA and

ribosomes, which can be used to make proteins.

The two stages of photosynthesis are the light-dependent reactions (the first or ‘light’ stage) and the light-independent reactions (the second or ‘dark’ stage).

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The first stage takes place in the grana, and the second stage in the stroma fluid of the chloroplast.

Q3.     In which part of a chloroplast does the light-independent reaction occur?

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Chloroplasts structures 1. Outer membrane

The outer membrane is permeable to many small ions. 2. Inner membrane

Has transport proteins, so controls the entry and exit of substances. 3. Stroma

Fluid filled matrix which contains all the enzymes for the light independent stage

Contains starch grains and oil droplets Contains DNA and prokaryote type ribosomes

4. Thylakoid Flattened membrane bound sacs Holds photosytems Absorb light Carryout ATP synthesis

5. Grana A stack of thylakoids together

6. Inter-granal lamellae Joins one granum to another.

Define the term photosynthetic pigment; Explain the importance of photosynthetic pigments in photosynthesis; State that the light-dependent stage takes place in thylakoid membranes and that the light-independent stage takes place in the stroma;

Photosynthetic pigments and photosystems are embedded in the thylakoid membrane among grana.

They are coloured compounds which absorb light of a short range of wavelengths and reflect light of other wavelengths.

These are called photosynthetic pigments. A photosynthetic pigment can absorb some light energy of specific wavelengths.

Various photosynthetic pigments are arranged into small structures called photosystems in the granal membrane.

The pigment on the inner membrane to the organelle is called chlorophyll a, which is the primary pigment.

Xanthophyll and Carotene pigments are called the secondary pigments or accessory pigments.

As light hits the secondary pigments which absorb the light, they get excited and a pair of electrons are passed through the pigments and through to the primary pigment

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Outline how light energy is converted to chemical energy (ATP and reduced NADP) in the light-dependent stage:

Light-dependent reactions The ‘light’ stage of photosynthesis: photolysis of water and

photophosphorylation The first stage, involved in photosynthesis is called the light-dependent

stage. The light-dependent reactions occur in the thylakoid membranes inside

chloroplasts. It is the photosystems embedded in these membranes which are of

importance. For one set of light-dependent reactions to happen, two photosystems are

used: photosystem I (PSI, P680), and photosystem II (PSII, P700). The light-dependent stage is focused on converting light energy (trapped

by pigments in the photosystems) into chemical energy.

Photons (light particles) strike photosystems I and II Energy is transferred from the photons to electrons in the primary pigment

reaction centre (Chlorophyll a P700) which become excited Electrons leave the chlorophyll a and pass through into the stroma of the

chloroplast. Energy is transferred from the photons to electrons in the primary pigment

reaction centre (Chlorophyll a P680) which become excited Electrons leave the chlorophyll a and pass onto an electron carrier protein. The electron carrier protein in turn becomes excited as it receives the

electrons. As it becomes excited it pumps protons (H+) across the thylakoid membrane into the thylakoid lumen from the stroma.

Photolysis of water in the thylakoid lumen. Enzymes in photosystem II split water into protons (H+), electrons (e-) and oxygen.

2H2O → 4H+ + 4e- + O2

The electrons replace those lost from photosystem II chlorophyll a in the primary pigment reaction centre.

The protons (H+) from photolysis of water along with those pumped into the thylakoid lumen accumulate.

This creates a proton gradient across the thylakoid membrane. The thylakoid lumen is acidic as a result of this build up.

Protons cannot pass out of the thylakoid lumen as the inner membrane is impermeable to them.

There are places where they can pass out of the lumen into the stroma – through an enzyme called ATP synthetase.

The protons flow down their concentration gradient by a process known as chemiosmosis.

This flow of protons through ATP synthetase generates a proton motive force.

This proton motive force is used to turn the head unit of the ATP synthetase enzyme which physically combines a phosphate to ADP creating ATP.

This is known as photophosphorylation. The protons now in the stroma combine with the electrons lost from

photosystem I to make hydrogen. This happens on a NADP coenzyme reducing it to NADPH.

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Both ATP and NADPH are in the stroma and are used here in the light independent stage of photosynthesis.

Q4. Explain

(i)      the role of chlorophyll in photolysis;

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(ii)     how the energy of light is converted into chemical energy in the light-dependent reactions.

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Q5. .          The diagram shows the light-dependent reactions of photosynthesis.

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

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(b)     Name the substances in boxes A, B and C.

A ................................................................

B ..................…........ + ....................…......

C ................................................................. (3)

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(i) In the presence of light, a molecule of water is split using an enzyme found within photosystem II, with a peak absorption of light wavelength 680nm (written P680), into an atom of oxygen, as well as two electrons and two protons. The oxygen is a by-product which is not used in this reaction

(ii) The two hydrogen ions (protons) remain inside the thylakoid space, but the electrons are accepted by photosystem II when light is present, as the photons of light excite the electrons, so that they are then accepted by an electron acceptor and passed along various electron carriers (or cytochromes)

(iii) The movement of electrons along the thylakoid membrane between different cytochromes releases energy, which pumps protons across the membrane from the stroma into the thylakoid inner space

(iv) This creates a concentration gradient of protons over the membrane as they begin to accumulate, and so protons begin to diffuse back through to the stroma through channel proteins, which are associated with the enzyme ATP synthase. As proteins flow through the channel, the enzyme is activated, as the transport of the proton drives the rotation of the enzyme so that one ADP molecule and one phosphate group form a molecule of ATP – this is photophosphorylation (production of ATP using light energy)

(v) There are two types of photophosphorylation which take place during the light-dependent reactions. One type involves only photosystem I (which has a peak absorption of 700, so is also called P700), and the other both the photosystems.

(vi) Non-cyclic photophosphorylation : The entire light-dependent stage is the process of non-cyclic photophosphorylation. This involves the flow of electrons from the photolysed water (the original electron donor) through the photosystems and cytochromes to the reduction of NADP. Non-cyclic photophosphorylation involves the steps 1-4 as seen above. In the diagram above of the Z-pathway, non-cyclic photophosphorylation can be seen producing ATP on the diagonal stroke of the Z-shape travelling towards photosystem I.

(vii) The electrons lost by photosystem I are replaced by the electrons from oxidised photosystem II, and the electrons donated from photolysed water replace those then lost by PSII. This flow of electrons is what makes the Z-pathway.

(viii) Cyclic photophosphorylation also occurs, but only at photosystem I. Electrons are lost from PSI as photons of light excite them, and passed around electron carriers and then back to the chlorophyll of PSI where they first came from, in a cycle. This movement of electrons does generate small amounts of ATP, but does not involve photolysis of water or reduce any NADP. The ATP produced may be used for other functions where energy is required.

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Outline how the products of the light-dependent stage are used in the light-independent stage (Calvin cycle) to produce triose phosphate (TP);

1. Ribulose bisphosphate (RUBP) is a 5 carbon molecule found in the stroma of chloroplasts. 2. RUBP binds to the active site of an enzyme RUBISCO (ribulose bisphosphate carboxylase) along with carbon dioxide (1 carbon molecule)3. RUBP (5C) and carbon dioxide (1C) are fixed to produce two molecules of GP (Glycerate-3-phosphate) each with 3 carbons (6C in total). If carbon dioxide is in short supply then RUBP is not converted to GP. This keeps RUBP levels high and GP levels low. 4. GP is stable and will remain as GP unless ATP and NADPH are produced from the light dependent reaction. If NADPH and ATP have been produced then the two molecules of GP are converted to two molecules of TP (triose phosphate), both of which are also 3 carbon molecules. 5. NADPH will become oxidised to NADP and ATP will be hydrolysed to ADP releasing energy. Both NADP and ADP are now available to be recycled in the light dependent stage of photosynthesis. 7. TP is stable and will remain as TP unless ATP is produced from the light

dependent reaction. If ATP has been produced then most of the two molecules of TP are recycled back into the 5 carbon molecule of RUBP.

8. ADP is now available to be recycled in the light dependent stage of photosynthesis.

9. One carbon of TP per cycle is not recycled back to RUBP but in fact will make 1/6 of a glucose molecule. The cycle happens 6 times to produce a 6 carbon hexose sugar = Glucose.

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Q6. The diagram shows a summary of the light-independent reaction of photosynthesis.

(a)     (i)      Complete the boxes to show the number of carbon atoms in the molecules.

(2)(ii)     In which part of a chloroplast does the light-independent reaction

occur?.............................................................................................................

(1)(iii)     Which process is the source of the ATP used in the conversion of

glycerate3-phosphate (GP) to triose phosphate?.............................................................................................................

(1)(iv)    What proportion of triose phosphate molecules is converted to

ribulose bisphosphate (RuBP)?.............................................................................................................

(1)(b)     Lowering the temperature has very little effect on the light-dependent

reaction, but it slows down the light-independent reaction. Explain why the light-independent reaction slows down at low temperatures...................................................................................................................................................................................................................................................................................................................................................................

(2)(Total 7 marks)

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Describe the effect on the rate of photosynthesis, and on levels of GP, RuBP and TP, of changing carbon dioxide concentration, light intensity and temperature.

Light intensity:In bright light RuBP and TP are in excess quantities and GP is in a low relative concentration as it is being used to form TPand TP is then used in the regeneration of RuBP. In dim light the light dependant stage stops and thus the products from this stage are not then passed onto the light in-dependant stage so this also ceases. So GP becomes in excess as it is no longer being used to form TP and TP/RuBP levels decrease as they are no longer being formed in dim light.

Carbon Dioxide concentration:An increase in carbon dioxide concentration may lead to an increase in the carbon dioxide fixation. This then means more molecules of GP can be converted into TP so these levels are in excess. RuBP is being carboxylated to form molecules of GP and therefore has low levels of relative concentration. If the concentration of Carbon dioxide falls below 0.01% the carbon dioxide acceptor will begin to accumulate and therefore will not be producing GP so TP is subsequently not formed either.

Temperature:Temperature has a strange effect on rubisco. At high temperatures it stops catalysing the combination of CO2 with RuBP, and instead combines oxygen with RuBP. This is called photorespiration (but has nothing to do with respiration!)This is wasteful to the plant as it reduces the rate of photosynthesis when temperature and light intensity levels are high.

Q7.In an investigation of the light-independent reaction, the amounts of glycerate

3-phosphate (GP) and ribulose bisphosphate (RuBP) in photosynthesising cells were measured under different environmental conditions.Figure 1 shows the effect of reducing the carbon dioxide concentration on the amounts of glycerate 3-phosphate and ribulose bisphosphate in photosynthesising cells.

Figure 1

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(i)      Explain why there is twice the amount of glycerate 3-phosphate as ribulose bisphosphate when the carbon dioxide concentration is high...........................................................................................................................................................................................................................

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(ii)     Explain the rise in the amount of ribulose bisphosphate after the carbon dioxide concentration is reduced...........................................................................................................................................................................................................................

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(c)     Figure 2 shows the results of an experiment in which photosynthesising cells were kept in the light and then in darkness.

Figure 2

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(i)      In the experiment the cells were supplied with radioactively labelled 14CO2. Explain why the carbon dioxide used was radioactively labelled...........................................................................................................................................................................................................................

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(ii)     Explain how lack of light caused the amount of radioactively labelled glycerate 3-phosphate to rise.....................................................................................................................................................................................................................................................................................................................................................................................................................................................

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RuBisCO The enzyme RuBisCO (short for ribulose biphosphate carboxylase-oxygenase) is the most abundant enzyme on earth, as it makes approximately 50% of leaf protein. It is of upmost importance to life. Although you can see that the Calvin cycle uses RuBisCO to combine a molecule of RuBP and carbon dioxide, as the name of RuBisCO suggests, oxygen can also fit into the enzyme complex. This results in a reaction called photorespiration. Photorespiration is a process whereby oxygen combines with RuBP in the place of carbon dioxide. This lowers the efficiency of photosynthesis in plants, as this undoes a lot of the work of the process so far and also leads to the formation of the toxic hydrogen peroxide. When photorespiration begins to occur, i.e. when there is a high concentration of oxygen in the atmosphere in relation to the concentration of carbon dioxide, the reactions catalysed by RuBisCO produce two products: phosphoglycerate (PGA) and phosphoglycolate (PPG). PGA re-enters the Calvin cycle, being converted back into RuBP, and so is not too problematic, but this does slow down rate of photosynthesis somewhat. PPG, however, is much more difficult to get rid of, as it has to leave the chloroplast and enter mitochondria (among other organelles), undergoing a long series of reactions before the end products can re-enter the Calvin cycle. This obviously lowers photosynthetic efficiency. So why does RuBisCO have oxygenase functions? Due to the controversial functions of photorespiration, there is no exact answer at the moment, although many scientists have speculated. Certain theories have suggested that, since the combination of RuBP and oxygen leads to the formation of hydrogen peroxide (H2O2), it is to aid homeostatic mechanisms as this substance makes a key contribution to cellular redox homeostasis. It has also been suggested that RuBisCO allows oxygen to combine with RuBP to stop free ATP and NADP from mixing with oxygen and forming radicals (compounds with unpaired electrons and incomplete outer energy levels) which can damage the cell’s metabolic function.

Discuss limiting factors in photosynthesis with reference to carbon dioxide concentration, light intensity and temperature;

Limiting Factor A variable that limits the rate of a particular process (in this case

photosynthesis). If the factor is increased then the process will take place at a higher

rate. Where the process is affected by a number of different factors the

limiting factor is the one whose magnitude limits the rate of the process.

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Limiting factors for photosynthesis will be Light intensity

Light intensity limits photosynthesis in the light dependent stage Low light intensity means less photons strike the photosystems Less excitation of electrons and so less NADP reduced to NADPH and

less ATP produced.

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Carbon dioxide concentration in the air Carbon dioxide concentration limits photosynthesis in the light

independent stage Low carbon dioxide concentration means less action of the enzyme

RUBISCO Less RUBP is fixed to carbon dioxide producing less GP and hence less

TP. Less glucose is produced.

Temperature temperature limits photosynthesis in the light independent stage Low temperatures means less action of the enzyme RUBISCO and ATP

synthetase High temperatures would lead to denaturation of RUBISCO and ATP

synthetase Less RUBP is fixed to carbon dioxide producing less GP and hence less

TP. Less glucose is produced. Less ATP produced.

Water availability Water limits photosynthesis in the light independent stage Low water levels no photolysis of water and hence a shortage of

electrons. Fewer electrons means less NADP reduced to NADPH and less ATP

produced.

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Less water leads to stomatal closure and so less gas exchange (i.e. less carbon dioxide available)

Less RUBP is fixed to carbon dioxide producing less GP and hence less TP.

Less glucose is produced. Q8.

.          Mountains are harsh environments. The higher up the mountain, the lower the temperature becomes. The diagram shows a forest growing on the side of a mountain.The upper boundary of the forest is called the tree line. Trees do not grow above the tree line.

(a)     (i)      The position of the tree line is determined by abiotic factors.What is meant by an abiotic factor?

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(ii)     Other than temperature, suggest one abiotic factor that is likely to affect the position of the tree line on the mountain.

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Describe how to investigate experimentally the factors that affect the rate of photosynthesis.

To measure photosynthesis the uptake of substrates or the appearance of products

1. We could measure Volume of oxygen produced Amount of carbon dioxide used Increase in dry mass of plant

It is usually measured using the volume of oxygen produced per minute by an aquatic plant

Although this has some limitations Some of the oxygen that is produced will be used by the plant in

respiration There could be some dissolved nitrogen in the gas collected Some oxygen could be trapped in the leaves The apparatus used is a Photpsynthometer (audus microburette) It is set up so it is air tight with no air bubbles

Q9.. A scientist investigated the uptake of radioactively labelled carbon dioxide in chloroplasts. She used three tubes, each containing different components of chloroplasts. She measured the uptake of carbon dioxide in each of these tubes.Her results are shown in the table.

Tube Contents of Tube Uptake of radioactivelylabelled CO2 /

counts per minut

A Stroma and grana 96,000B Stroma, ATP, Reduced NADP 97,000C Stroma 4,000

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(a) Name the substance which combines with carbon dioxide in a chloroplast.............................................................................................................................................(1 mark)(b) Explain why the results in tube B are similar to those in tube A.....................................................................................................................................................................................................................................................................................................................................................................................................................................(1 mark)(c) Use the information in the table to predict the uptake of radioactively labelled carbon dioxide if tube A was placed in the dark. Explain your answer...........................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................

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Q1. Oxygen 1

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Q2.             The more light absorbed, the greater the rate of photosynthesis;Light provides the energy for light dependent reactions / exciting electrons in chlorophyll; 2 max

Q3. Stroma 1

Q4. (i)      Absorbs light/energy; Loses electrons/becomes positively charged/is oxidised; Accepts electrons from water/from OH– ; Causes more water to dissociate/pulls equilibrium to the right; max 3

(ii)     Electrons raised to higher energy level/electrons excited; Use of electron carriers/cytochromes/acceptors; For production of ACT

Q5. (a)     Grana/thylakoids/ lamellae;1

(b)     A = oxygen/O2

B = ADP and phosphate/Pi / C = reduced NADP; 3

Q6. (a)     (i)      RuBP – 5; GP – 3; TP – 3; Glucose – 6;(all correct = 2 marks; 3 or 2 correct = 1 mark)

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(ii)     stroma;1

(iii)     light-dependent reaction / (photo)phosphorylation;(accept photolysis)

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(iv)    5 out of 6 / 83% / equivalent;1

(b)     enzymes involved / not a photochemical reaction;slow rate of enzyme/chemical reaction at low temperature /less kinetic energy / fewer collisions;

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Q7. (a); (i)      RuBP combines with carbon dioxide to produce 2 x GP;1

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(ii)     less used to combine with carbon dioxide /less used to form glycerate 3-phosphate;

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(b)     (i)      used in photosynthesis allows detection of products;1

(ii)     ATP and reduced NADP not formed;GP is not being used to form RuBP / is being formed from RuBP;

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Q8.  (a)      (i)     Non-living / physical;

(ii)     Accept an abiotic factor that may limit photosynthesis;

E.g.      Water

             Light

             Carbon dioxide

             

Q9. (a) Ribulose bisphosphate 1

(b) ATP and reduced NADP are produced in grana/thylakoids/ present in A/both tubes;

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(c)1. 4 000;

2. Light-dependent reaction does not occur /ATP and reduced NADP are not produced;

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