NEET Biology: Plant Physiology · Chapter 1

Photosynthesis in Higher Plants

What to remember

Photosynthesis is a light-driven process in which green plants make carbohydrate from CO₂ and water. The oxygen released comes from water, not from CO₂.
The light reaction (in the thylakoid membranes) makes ATP and NADPH. The biosynthetic phase (Calvin cycle, in the stroma) uses them to reduce CO₂ to sugar. Making one glucose needs 18 ATP and 12 NADPH.
C3 plants fix CO₂ first as 3-PGA (3-carbon) using RuBisCO. C4 plants fix CO₂ first as OAA (4-carbon) using PEP carboxylase in mesophyll cells, then run the Calvin cycle in bundle sheath cells. C4 plants have no visible photorespiration.
Photorespiration is the oxygenase action of RuBisCO. It gives off CO₂, makes no ATP and no NADPH, and wastes fixed carbon. The overall rate of photosynthesis is set by the slowest factor (Blackman's law of limiting factors).

Early experiments and the nature of photosynthesis

Photosynthesis is the process by which green plants, algae and some bacteria use light energy to make food. It is the basis of almost all life on Earth. It is also the source of the oxygen in the air. The overall reaction for green plants is:

6CO₂ + 12H₂O → C₆H₁₂O₆ + 6H₂O + 6O₂ (in light, with chlorophyll)

The table below lists the experiments that built our understanding.

Scientist	Work	Conclusion
Joseph Priestley (1770s)	Kept a mint plant with a burning candle or a mouse in a closed bell jar	Plants restore to the air what burning candles and breathing animals remove
Jan Ingenhousz	Repeated the work with an aquatic plant in light and in dark	Sunlight is essential, and only the green parts of the plant release oxygen
Julius von Sachs	Tested leaves for starch	Green parts make glucose, which is stored as starch
T. W. Engelmann	Used a prism to split light and placed Cladophora (a filamentous green alga) with aerobic bacteria	Bacteria collected in the blue and red regions, where more oxygen was released. This gave the first action spectrum
Cornelius van Niel	Studied purple and green sulphur bacteria	Photosynthesis is a light-dependent reduction of CO₂ by a hydrogen donor. In bacteria the donor is H₂S. In green plants the donor is H₂O, so the oxygen comes from water
Robin Hill	Used isolated chloroplasts with an artificial electron acceptor	Isolated chloroplasts in light evolve oxygen without CO₂ being fixed. This shows that water is split in the light reaction

Van Niel's general equation is 2H₂A + CO₂ → 2A + CH₂O + H₂O. For bacteria, A is sulphur. For green plants, A is oxygen.

Later work with the heavy isotope of oxygen (¹⁸O) confirmed that the O₂ released comes from water.

Site of photosynthesis and the pigments

Photosynthesis takes place in the green parts of the plant, mainly the leaves, and inside the chloroplasts of mesophyll cells.

Structure of the chloroplast	Description	Function
Outer and inner membrane	Double membrane envelope	Encloses the stroma
Stroma	Colourless matrix with enzymes, DNA and ribosomes	Site of the biosynthetic phase (Calvin cycle)
Thylakoids	Flattened membrane sacs with an inner space called the lumen	Site of the light reaction
Grana (singular granum)	Stacks of thylakoids	Contain the pigments and the photosystems
Stroma lamellae	Flat membrane tubes joining one granum to another	Connect the grana and carry out cyclic flow

Pigments absorb light. A chromatography of leaf extract shows four main pigments.

Pigment	Colour in chromatography	Role
Chlorophyll a	Bright or blue green	Main pigment; the reaction centre is always chlorophyll a
Chlorophyll b	Yellow green	Accessory pigment
Xanthophylls	Yellow	Accessory pigment
Carotenoids	Yellow to yellow-orange	Accessory pigment; protect chlorophyll from photo-oxidation

The absorption spectrum is a graph of the amount of light a pigment absorbs at each wavelength. Chlorophyll a absorbs mainly in the blue region (about 430 nm) and the red region (about 660 nm). The action spectrum is a graph of the rate of photosynthesis at each wavelength. The action spectrum of photosynthesis closely follows the absorption spectra of chlorophyll a together with the accessory pigments. Photosynthesis is highest in the blue and red regions.

The accessory pigments absorb light and pass the energy to chlorophyll a. Together they make the photosynthetic apparatus work over a wider range of wavelengths.

Light reaction

The light reaction (photochemical phase) includes light absorption, water splitting, oxygen release, and the formation of ATP and NADPH.

The pigments are organised into two photosystems, PS I and PS II. Each photosystem has a light harvesting complex (LHC), also called the antenna. The LHC has many molecules of chlorophyll and accessory pigments. They pass energy to a single chlorophyll a molecule, which is the reaction centre.

Feature	PS I	PS II
Reaction centre	P700 (absorption peak at 700 nm)	P680 (absorption peak at 680 nm)
Position on the thylakoid	Non-appressed regions, that is, stroma lamellae and the margins of the grana	Appressed regions of the grana
Water splitting	Not associated	Associated, on the lumen side

Non-cyclic photophosphorylation (Z scheme). Both photosystems work in series. The path is:

PS II (P680) → electron acceptor → plastoquinone → cytochrome b6f complex → plastocyanin → PS I (P700) → electron acceptor → ferredoxin → NADP reductase → NADPH

When light hits PS II, the electrons are excited and leave the reaction centre. They move down the electron transport chain to PS I. In PS I they are excited again and move to NADP⁺, which is reduced to NADPH. The graph of this flow, with energy on one axis, looks like the letter Z. Hence the name Z scheme.

Splitting of water. The electrons lost by PS II are replaced by electrons from water.

2H₂O → 4H⁺ + O₂ + 4e⁻

The water splitting complex is on the lumen side of the thylakoid membrane. It is associated with PS II. Manganese is a cofactor of this reaction. Oxygen is a by-product. The protons stay inside the lumen.

Cyclic photophosphorylation. Only PS I works. Electrons leave P700, pass to an acceptor, go through the cytochrome b6f complex and come back to P700. There is no water splitting, no O₂ release and no NADPH. Only ATP is made. This happens in the stroma lamellae, where PS II and NADP reductase are absent.

Chemiosmotic hypothesis. ATP is made because of a proton gradient across the thylakoid membrane. The lumen becomes rich in H⁺ because:

1. Water is split on the lumen side and releases H⁺.
2. As electrons pass along the chain, plastoquinone moves protons from the stroma into the lumen.
3. NADP reductase is on the stroma side. It removes H⁺ from the stroma when NADP⁺ is reduced to NADPH. This lowers the H⁺ concentration in the stroma.

So the lumen has a high concentration of protons and the stroma has a low one. This gradient stores energy. The protons then move back to the stroma through ATP synthase. ATP synthase has two parts: CF₀ is embedded in the membrane and forms a channel for protons. CF₁ sticks out on the stroma side and makes ATP from ADP and Pi.

ATP and NADPH are called the assimilatory power. They are made on the stroma side and used in the stroma.

Biosynthetic phase: the Calvin cycle and the C4 pathway

The biosynthetic phase does not need light directly. It uses the ATP and NADPH from the light reaction. It takes place in the stroma. It stops soon if the light reaction stops.

Calvin cycle (C3 pathway). It has three stages.

Stage	What happens	Enzyme or need
Carboxylation	CO₂ is added to RuBP (a 5-carbon sugar, ribulose bisphosphate). The product splits into two molecules of 3-PGA, the first stable product	RuBisCO
Reduction	3-PGA is reduced to glyceraldehyde 3-phosphate (a triose phosphate)	ATP and NADPH
Regeneration	RuBP is formed again from triose phosphates so that the cycle continues	ATP

For every CO₂ fixed, the cycle needs 3 ATP and 2 NADPH. Six turns fix six CO₂ and make one glucose. So one glucose needs 18 ATP and 12 NADPH. RuBisCO is the most abundant enzyme on Earth. It is a carboxylase and also an oxygenase.

Plants that use the Calvin cycle as the only fixation path are called C3 plants. Examples are wheat, rice and potato.

C4 pathway. It was worked out by M. D. Hatch and C. R. Slack. It is found in plants of tropical and dry regions, such as maize, sorghum and sugarcane. These plants have Kranz anatomy (Kranz means wreath). Large bundle sheath cells form rings around the vascular bundles. Bundle sheath cells have many chloroplasts, thick walls and no intercellular spaces. The mesophyll cells lie outside them.

The steps are:

1. In mesophyll cells, the primary CO₂ acceptor is PEP (phosphoenolpyruvate, a 3-carbon compound). The enzyme is PEP carboxylase (PEPcase). The first stable product is OAA (oxaloacetic acid, 4-carbon).
2. OAA is changed to malic acid (or aspartic acid). It moves to the bundle sheath cells.
3. In the bundle sheath, malic acid is broken down. It releases CO₂ and pyruvate (3-carbon).
4. The CO₂ enters the Calvin cycle in the bundle sheath cells. Only here is RuBisCO active.
5. Pyruvate goes back to the mesophyll and is changed to PEP, using ATP.

The C4 pathway acts as a CO₂ pump. It keeps the CO₂ concentration high around RuBisCO. So RuBisCO works as a carboxylase and not as an oxygenase.

Photorespiration and the C3 and C4 comparison

Photorespiration happens when RuBisCO binds O₂ instead of CO₂. This is more likely when the O₂ concentration is high and the CO₂ concentration is low. RuBP then combines with O₂. The product is one molecule of 3-PGA and one molecule of a 2-carbon compound (phosphoglycolate). The 2-carbon compound is changed in a series of steps called the C2 cycle. The steps take place in three organelles: chloroplast, peroxisome and mitochondrion. CO₂ is released. There is no synthesis of sugar, ATP or NADPH. In fact, energy is lost. So photorespiration is a wasteful process.

Feature	C3 plants	C4 plants
Examples	Wheat, rice, potato	Maize, sorghum, sugarcane
Kranz anatomy	Absent	Present
Chloroplasts	One type, in mesophyll	Two types: mesophyll and bundle sheath
First CO₂ acceptor	RuBP	PEP
First stable product	3-PGA (3-carbon)	OAA (4-carbon)
Main carboxylating enzyme	RuBisCO	PEP carboxylase in mesophyll, then RuBisCO in bundle sheath
Calvin cycle in	Mesophyll cells	Bundle sheath cells
Photorespiration	Present	Absent (not seen)
Optimum temperature	About 20 to 25 °C	About 30 to 45 °C
Response to high CO₂ and light	Rate rises when CO₂ is raised	Saturates at lower CO₂
Productivity	Lower	Higher

Factors affecting photosynthesis

The rate of photosynthesis depends on both the plant and the environment. The plant factors include the number, size, age and orientation of leaves, the mesophyll cells and the chloroplasts, and the amount of chlorophyll. The external factors are light, CO₂ concentration, temperature and water.

Blackman's law of limiting factors. If a process depends on several factors, its rate is limited by the factor that is nearest to its minimum value. If one factor is in short supply, adding more of the others does not raise the rate.

Factor	Effect
Light	At low intensity, the rate rises in proportion to light. Saturation is reached at about 10 percent of full sunlight. So in open sunlight, light is rarely a limiting factor. Very strong light breaks down pigments and lowers the rate. Quality of light (wavelength) also matters
CO₂	The main limiting factor in nature, because the air has only about 0.03 to 0.04 percent. Raising it to about 0.05 percent raises the rate in C3 plants. C4 plants saturate at about 360 µL per litre. C3 plants respond up to about 450 µL per litre
Temperature	The dark (enzymatic) reactions are more sensitive to temperature than the light reaction. C4 plants have a higher optimum than C3 plants. Tropical plants have a higher optimum than temperate plants
Water	Water stress closes the stomata, so less CO₂ enters. Leaves also wilt, which reduces the surface area and slows metabolism

NEET traps

1. Priestley showed that plants restore air. Ingenhousz showed that light and green parts are needed. Sachs showed starch formation. Do not swap them.
2. Engelmann used Cladophora and aerobic bacteria. Van Niel used purple and green sulphur bacteria.
3. The oxygen released comes from water, not from CO₂. This is shown by van Niel's idea and isotope work.
4. P680 belongs to PS II. P700 belongs to PS I. The number is the wavelength of peak absorption.
5. Water splitting is linked to PS II. Cyclic flow uses PS I only. Cyclic flow gives only ATP.
6. CF₀ is the membrane channel. CF₁ sticks out into the stroma and makes ATP.
7. In the thylakoid, protons build up in the lumen, not in the stroma.
8. The first stable product in C3 plants is 3-PGA (3-carbon). In C4 plants it is OAA (4-carbon).
9. In C4 plants, PEP carboxylase works in mesophyll cells. The Calvin cycle works in bundle sheath cells.
10. Photorespiration needs RuBisCO acting as an oxygenase. It does not make ATP or NADPH. It is not the same as mitochondrial respiration.

One-liners

1. Priestley used a mint plant, a candle and a mouse in a closed jar.
2. Ingenhousz showed that only the green parts in sunlight release oxygen.
3. Sachs showed that glucose is stored as starch.
4. Engelmann gave the first action spectrum, using Cladophora.
5. Hill showed that isolated chloroplasts evolve oxygen in light.
6. The light reaction takes place in the grana thylakoids. The Calvin cycle takes place in the stroma.
7. Chlorophyll a appears bright or blue green in chromatography, and chlorophyll b appears yellow green.
8. The Z scheme has PS II first and PS I second.
9. Water splitting gives 4H⁺, O₂ and 4 electrons for two water molecules.
10. ATP synthase is driven by H⁺ moving from the lumen to the stroma.
11. RuBisCO is the carboxylating enzyme of the Calvin cycle and also has oxygenase activity.
12. Six turns of the Calvin cycle give one glucose and need 18 ATP and 12 NADPH.
13. Hatch and Slack worked out the C4 pathway.
14. Kranz anatomy means a ring of bundle sheath cells with many chloroplasts around a vascular bundle.
15. Light saturates at about 10 percent of full sunlight, and CO₂ is the usual limiting factor.

Practice questions

Which scientist showed that plants restore to the air what burning candles and breathing animals remove?
1. Joseph Priestley
2. Jan Ingenhousz
3. T. W. Engelmann
4. Julius von Sachs
Answer
A. Joseph Priestley
Priestley used a mint plant with a burning candle or a mouse in a closed jar.
Sachs showed that the green parts of a plant make glucose, which is stored as
1. protein
2. cellulose
3. starch
4. fat
Answer
C. starch
Sachs tested leaves for starch, the stored form of glucose.
Engelmann split light with a prism and used which organism with aerobic bacteria to find the action spectrum?
1. Volvox
2. Nostoc
3. Chlamydomonas
4. Cladophora
Answer
D. Cladophora
Engelmann used the filamentous green alga Cladophora; bacteria gathered in the blue and red regions.
The work of van Niel on purple and green sulphur bacteria led to the idea that the oxygen released in green plant photosynthesis comes from
1. chlorophyll
2. water
3. carbon dioxide
4. glucose
Answer
B. water
In bacteria the hydrogen donor is H₂S; in green plants it is H₂O, so O₂ comes from water.
Which pigment forms the reaction centre in both photosystems?
1. Chlorophyll b
2. Chlorophyll a
3. Xanthophyll
4. Carotenoid
Answer
B. Chlorophyll a
The reaction centre is always a chlorophyll a molecule.
The reaction centre of photosystem II has its absorption peak at
1. 680 nm
2. 700 nm
3. 640 nm
4. 740 nm
Answer
A. 680 nm
PS II has P680 and PS I has P700.
Cyclic photophosphorylation produces
1. ATP and O₂
2. ATP only
3. ATP and NADPH
4. NADPH and O₂
Answer
B. ATP only
In cyclic flow electrons return to PS I, so there is no water splitting, no O₂ and no NADPH.
Which part of ATP synthase projects on the stroma side of the thylakoid and makes ATP?
1. CF₀
2. Plastocyanin
3. Ferredoxin
4. CF₁
Answer
D. CF₁
CF₀ is the membrane channel for protons; CF₁ sticks out into the stroma and makes ATP.
The first stable product of the Calvin cycle is
1. oxaloacetic acid
2. phosphoenolpyruvate
3. 3-phosphoglyceric acid
4. ribulose bisphosphate
Answer
C. 3-phosphoglyceric acid
RuBP plus CO₂ gives two molecules of 3-PGA, the first stable product.
The number of ATP and NADPH needed in the Calvin cycle to make one glucose molecule is
1. 12 ATP and 18 NADPH
2. 36 ATP and 24 NADPH
3. 18 ATP and 12 NADPH
4. 6 ATP and 6 NADPH
Answer
C. 18 ATP and 12 NADPH
Each CO₂ needs 3 ATP and 2 NADPH; six CO₂ give 18 ATP and 12 NADPH.
In C4 plants, the primary acceptor of CO₂ in mesophyll cells is
1. 3-phosphoglyceric acid
2. oxaloacetic acid
3. ribulose bisphosphate
4. phosphoenolpyruvate
Answer
D. phosphoenolpyruvate
PEP (3-carbon) accepts CO₂ and forms OAA through PEP carboxylase.
In C4 plants, the Calvin cycle takes place in the
1. bundle sheath cells
2. epidermal cells
3. root cortex cells
4. mesophyll cells
Answer
A. bundle sheath cells
Malic acid releases CO₂ in the bundle sheath, where RuBisCO is active.
Which pair of plants shows C4 photosynthesis?
1. Maize and sorghum
2. Potato and pea
3. Pea and wheat
4. Wheat and rice
Answer
A. Maize and sorghum
Maize, sorghum and sugarcane are C4 plants; wheat, rice and potato are C3.
Photorespiration involves which set of cell organelles?
1. Chloroplast, Golgi body and mitochondrion
2. Chloroplast, peroxisome and mitochondrion
3. Chloroplast, vacuole and nucleus
4. Peroxisome, endoplasmic reticulum and lysosome
Answer
B. Chloroplast, peroxisome and mitochondrion
The C2 cycle runs across chloroplast, peroxisome and mitochondrion.
Blackman's law of limiting factors states that the rate of a process is limited by the factor
1. that is present in the largest amount
2. that acts first in the process
3. that is the most easily measured
4. that is nearest to its minimum value
Answer
D. that is nearest to its minimum value
The slowest or least available factor sets the rate.
In the Z scheme, electrons from the splitting of water first replace the electrons lost by
1. ATP synthase
2. photosystem I
3. photosystem II
4. NADP reductase
Answer
C. photosystem II
Water splitting is linked to PS II, whose lost electrons are replaced by those from water.
Given below are two statements: Statement I: Xanthophylls and carotenoids are accessory pigments. Statement II: Carotenoids protect chlorophyll from photo-oxidation. Choose the correct answer.
1. Both Statement I and Statement II are correct
2. Both Statement I and Statement II are incorrect
3. Statement I is correct but Statement II is incorrect
4. Statement I is incorrect but Statement II is correct
Answer
A. Both Statement I and Statement II are correct
Both are accessory pigments and carotenoids have a protective role.
Given below are two statements: Statement I: The action spectrum of photosynthesis follows the absorption spectrum of chlorophyll a alone. Statement II: Photosynthesis is highest in the blue and red regions of light. Choose the correct answer.
1. Both Statement I and Statement II are correct
2. Both Statement I and Statement II are incorrect
3. Statement I is correct but Statement II is incorrect
4. Statement I is incorrect but Statement II is correct
Answer
D. Statement I is incorrect but Statement II is correct
The action spectrum follows chlorophyll a together with the accessory pigments, so Statement I is wrong.
Given below are two statements: Statement I: Non-cyclic photophosphorylation involves both photosystems. Statement II: Cyclic photophosphorylation releases oxygen. Choose the correct answer.
1. Both Statement I and Statement II are correct
2. Both Statement I and Statement II are incorrect
3. Statement I is correct but Statement II is incorrect
4. Statement I is incorrect but Statement II is correct
Answer
C. Statement I is correct but Statement II is incorrect
Cyclic flow uses PS I only and has no water splitting, so no O₂ is released.
Given below are two statements: Statement I: ATP and NADPH are made during photorespiration. Statement II: Photorespiration is initiated when RuBisCO binds O₂ instead of CO₂. Choose the correct answer.
1. Both Statement I and Statement II are correct
2. Both Statement I and Statement II are incorrect
3. Statement I is correct but Statement II is incorrect
4. Statement I is incorrect but Statement II is correct
Answer
D. Statement I is incorrect but Statement II is correct
Photorespiration makes neither ATP nor NADPH; Statement II is true.
Given below are two statements: Statement I: In C4 plants the first fixation of CO₂ is done by PEP carboxylase in mesophyll cells. Statement II: C4 plants do not show visible photorespiration. Choose the correct answer.
1. Both Statement I and Statement II are correct
2. Both Statement I and Statement II are incorrect
3. Statement I is correct but Statement II is incorrect
4. Statement I is incorrect but Statement II is correct
Answer
A. Both Statement I and Statement II are correct
The CO₂ pump keeps CO₂ high around RuBisCO in the bundle sheath.
Given below are two statements: Statement I: During ATP synthesis in chloroplasts, protons move from the lumen to the stroma through CF₀. Statement II: Protons accumulate in the stroma during the light reaction. Choose the correct answer.
1. Both Statement I and Statement II are correct
2. Both Statement I and Statement II are incorrect
3. Statement I is correct but Statement II is incorrect
4. Statement I is incorrect but Statement II is correct
Answer
C. Statement I is correct but Statement II is incorrect
Protons build up in the lumen, not the stroma, so Statement II is wrong.
Which of the following statements are correct? 1. Water splitting is associated with photosystem II. 2. The water splitting complex is on the lumen side of the thylakoid membrane. 3. Electrons from water pass directly to NADP⁺.
1. 1 and 2 only
2. 2 and 3 only
3. 1 and 3 only
4. 1, 2 and 3
Answer
A. 1 and 2 only
Statements 1 and 2 are true. Electrons from water go to PS II first, so 3 is wrong.
Which of the following statements about the Calvin cycle are correct? 1. Carboxylation of RuBP is catalysed by RuBisCO. 2. Six turns of the cycle are needed to make one glucose. 3. The first stable product is a 4-carbon acid.
1. 2 and 3 only
2. 1, 2 and 3
3. 1 and 2 only
4. 1 and 3 only
Answer
C. 1 and 2 only
The first stable product is the 3-carbon 3-PGA, so 3 is wrong.
Which of the following statements are correct? 1. C4 plants have a higher temperature optimum than C3 plants. 2. C4 plants saturate at a lower CO₂ concentration than C3 plants. 3. C3 plants show Kranz anatomy.
1. 1, 2 and 3
2. 1 and 3 only
3. 2 and 3 only
4. 1 and 2 only
Answer
D. 1 and 2 only
Kranz anatomy is found in C4 plants, so 3 is wrong.
Given below are two statements: Statement I: At low light intensity the rate of photosynthesis rises with light. Statement II: In open sunlight light is usually the main limiting factor for most plants. Choose the correct answer.
1. Both Statement I and Statement II are correct
2. Both Statement I and Statement II are incorrect
3. Statement I is correct but Statement II is incorrect
4. Statement I is incorrect but Statement II is correct
Answer
C. Statement I is correct but Statement II is incorrect
Saturation is reached at about 10 percent of full sunlight, so light is rarely limiting in the open.
Assertion (A): The oxygen released in photosynthesis comes from water. Reason (R): Water is split by photosystem II in the light reaction.
1. Both A and R are true and R is the correct explanation of A
2. Both A and R are true but R is not the correct explanation of A
3. A is true but R is false
4. A is false but R is true
Answer
A. Both A and R are true and R is the correct explanation of A
Water splitting releases O₂, H⁺ and electrons.
Assertion (A): C4 plants show high photosynthetic rates at high temperature. Reason (R): In C4 plants, CO₂ is concentrated in the bundle sheath cells, where RuBisCO acts as a carboxylase.
1. Both A and R are true and R is the correct explanation of A
2. Both A and R are true but R is not the correct explanation of A
3. A is true but R is false
4. A is false but R is true
Answer
A. Both A and R are true and R is the correct explanation of A
High CO₂ around RuBisCO prevents oxygenase action and photorespiration.
Assertion (A): Chlorophyll a is the primary pigment of photosynthesis. Reason (R): Carotenoids are accessory pigments that pass energy to chlorophyll a.
1. Both A and R are true and R is the correct explanation of A
2. Both A and R are true but R is not the correct explanation of A
3. A is true but R is false
4. A is false but R is true
Answer
B. Both A and R are true but R is not the correct explanation of A
Both are true, but the accessory role of carotenoids does not explain why chlorophyll a is primary.
Assertion (A): The stroma of the chloroplast is the site of the Calvin cycle. Reason (R): The enzymes of the Calvin cycle are located in the thylakoid membranes.
1. Both A and R are true and R is the correct explanation of A
2. Both A and R are true but R is not the correct explanation of A
3. A is true but R is false
4. A is false but R is true
Answer
C. A is true but R is false
The Calvin cycle enzymes are in the stroma.
Assertion (A): Photorespiration is a useful process that produces ATP. Reason (R): RuBisCO shows oxygenase activity when the O₂ concentration is high.
1. Both A and R are true and R is the correct explanation of A
2. Both A and R are true but R is not the correct explanation of A
3. A is true but R is false
4. A is false but R is true
Answer
D. A is false but R is true
Photorespiration wastes energy; the oxygenase statement is true.
Assertion (A): The light reaction makes ATP and NADPH. Reason (R): The biosynthetic phase takes place in the stroma.
1. Both A and R are true and R is the correct explanation of A
2. Both A and R are true but R is not the correct explanation of A
3. A is true but R is false
4. A is false but R is true
Answer
B. Both A and R are true but R is not the correct explanation of A
Both are true but R does not explain A.
Match List I with List II. List I: a. Priestley b. Sachs c. Engelmann d. van Niel List II: i. Purple and green sulphur bacteria ii. Starch tested in green parts of leaves iii. Aerobic bacteria and a prism of light iv. Mint plant and a burning candle
1. a-iii, b-ii, c-iv, d-i
2. a-iv, b-ii, c-iii, d-i
3. a-ii, b-iv, c-iii, d-i
4. a-iv, b-ii, c-i, d-iii
Answer
B. a-iv, b-ii, c-iii, d-i
Priestley: mint plant and candle; Sachs: starch; Engelmann: prism and bacteria; van Niel: sulphur bacteria.
Match List I with List II. List I: a. Chlorophyll a b. Chlorophyll b c. Xanthophylls d. Carotenoids List II: i. Yellow to yellow-orange ii. Yellow iii. Bright or blue green iv. Yellow green
1. a-iii, b-iv, c-i, d-ii
2. a-iii, b-iv, c-ii, d-i
3. a-ii, b-iv, c-iii, d-i
4. a-iv, b-iii, c-ii, d-i
Answer
B. a-iii, b-iv, c-ii, d-i
Colours in chromatography: chlorophyll a bright or blue green, b yellow green, xanthophylls yellow, carotenoids yellow to yellow-orange.
Match List I with List II. List I: a. Plastoquinone b. Plastocyanin c. Ferredoxin d. ATP synthase List II: i. Carries electrons from cytochrome b6f to photosystem I ii. Passes electrons on towards NADP reductase iii. Moves protons into the lumen as it carries electrons iv. Makes ATP from ADP and Pi
1. a-i, b-iii, c-ii, d-iv
2. a-iii, b-ii, c-i, d-iv
3. a-iii, b-i, c-iv, d-ii
4. a-iii, b-i, c-ii, d-iv
Answer
D. a-iii, b-i, c-ii, d-iv
Plastoquinone carries electrons and moves protons; plastocyanin goes to PS I; ferredoxin passes electrons to NADP reductase.
Match List I with List II. List I: a. Carboxylation b. Reduction c. Regeneration d. Primary fixation in C4 plants List II: i. Formation of OAA from PEP ii. Uses ATP to form RuBP iii. Addition of CO₂ to RuBP iv. Uses ATP and NADPH to form triose phosphate
1. a-iii, b-ii, c-iv, d-i
2. a-iii, b-iv, c-ii, d-i
3. a-iii, b-iv, c-i, d-ii
4. a-i, b-iv, c-ii, d-iii
Answer
B. a-iii, b-iv, c-ii, d-i
Carboxylation adds CO₂ to RuBP; reduction uses ATP and NADPH; regeneration uses ATP; C4 fixation forms OAA.
Match List I with List II. List I: a. Hatch and Slack b. Blackman c. Hill d. Ingenhousz List II: i. Isolated chloroplasts evolve oxygen in light ii. Sunlight and green parts are needed to release oxygen iii. C4 pathway iv. Law of limiting factors
1. a-i, b-iv, c-iii, d-ii
2. a-iii, b-iv, c-i, d-ii
3. a-iv, b-iii, c-i, d-ii
4. a-iii, b-iv, c-ii, d-i
Answer
B. a-iii, b-iv, c-i, d-ii
Hatch and Slack: C4 pathway; Blackman: limiting factors; Hill: chloroplasts evolve O₂; Ingenhousz: light and green parts.
Match List I with List II. List I: a. P700 b. P680 c. CF₀ d. Calvin cycle List II: i. Reaction centre of PS II ii. Reaction centre of PS I iii. Membrane channel of ATP synthase iv. Takes place in the stroma
1. a-ii, b-iii, c-i, d-iv
2. a-ii, b-i, c-iv, d-iii
3. a-i, b-ii, c-iii, d-iv
4. a-ii, b-i, c-iii, d-iv
Answer
D. a-ii, b-i, c-iii, d-iv
P700 is the PS I centre, P680 the PS II centre, CF₀ the channel, and the Calvin cycle runs in the stroma.
A greenhouse grower raises CO₂ from about 0.03 percent to about 0.05 percent for a C3 crop, with light and temperature at their best. What is the expected result?
1. The rate stays the same because light is limiting
2. Photorespiration stops completely
3. The rate of photosynthesis rises
4. The rate of photosynthesis falls
Answer
C. The rate of photosynthesis rises
CO₂ is the main limiting factor, and C3 plants respond to increases in CO₂.
Isolated chloroplasts are given water and an artificial electron acceptor in light, with no CO₂. What is released?
1. Oxygen
2. Carbon dioxide
3. Glucose
4. Starch
Answer
A. Oxygen
This is the Hill reaction: water is split and O₂ is evolved without CO₂ fixation.
Thylakoid membranes are made leaky to protons. Which process is mainly lost?
1. Splitting of water at the oxygen evolving complex
2. Absorption of light by chlorophyll
3. Synthesis of ATP by ATP synthase
4. Carboxylation of RuBP by RuBisCO
Answer
C. Synthesis of ATP by ATP synthase
ATP synthesis depends on the proton gradient across the membrane.
Light intensity is raised from about 10 percent of full sunlight to full sunlight and the rate of photosynthesis does not rise. What is the likely reason?
1. CO₂ has become more abundant
2. Water has become more abundant
3. RuBisCO has stopped working
4. Light saturation has been reached
Answer
D. Light saturation has been reached
Saturation is reached at about 10 percent of full sunlight.
On a hot, dry day the stomata of a C3 leaf close. Internal CO₂ falls and O₂ rises. Which process increases?
1. Photorespiration
2. Fixation of CO₂ by PEP carboxylase
3. Cyclic photophosphorylation only
4. Splitting of water by PS I
Answer
A. Photorespiration
Low CO₂ and high O₂ favour the oxygenase action of RuBisCO.
How many NADPH molecules are needed by the Calvin cycle to make two molecules of glucose?
1. 12
2. 24
3. 36
4. 18
Answer
B. 24
One glucose needs 12 NADPH, so two need 24.
When light split by a prism falls on Cladophora with aerobic bacteria, the bacteria gather mainly in the blue and red regions. What does this show?
1. Photosynthesis is highest in green light
2. Oxygen is absorbed by the alga in red light
3. Bacteria make food in blue light
4. Photosynthesis, and so oxygen release, is highest in blue and red light
Answer
D. Photosynthesis, and so oxygen release, is highest in blue and red light
Bacteria collect where most oxygen is released, giving the action spectrum with peaks in blue and red.

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