NEET-Pattern Practice Questions

How This Chapter Is Asked in NEET

Photosynthesis in Higher Plants is one of the two or three highest yielding chapters in the whole paper. Nothing here is derived and almost nothing is calculated - the marks go to the student who learnt the exact molecule, the exact number and the exact compartment.

Every question in this chapter is one of four shapes:

1. Name the molecule, the enzyme, the pigment or the scientist. "The primary acceptor of $\mathrm{CO_2}$ in a $\mathrm{C_4}$ plant" - PEP. "The most abundant enzyme in the world" - RuBisCO.
2. Give the number or the wavelength. "ATP and NADPH per $\mathrm{CO_2}$ fixed" - 3 and 2. "The reaction centre of PS I" - $700\ \mathrm{nm}$.
3. Say which cell, which membrane side or which compartment. "Where the Calvin cycle runs in a $\mathrm{C_4}$ plant" - the bundle sheath cells. "Where protons accumulate" - the lumen of the thylakoid.
4. Compare $\mathrm{C_3}$ with $\mathrm{C_4}$. Anatomy, acceptor, enzyme, first product, photorespiration, temperature optimum - a whole family of items from one table.

The traps repeat, and in this chapter they are always the same ones:

• Priestley against Ingenhousz. Priestley showed that plants restore to the air whatever breathing animals and burning candles remove, using a bell jar with a candle, a mouse and a mint plant, in 1770; he discovered oxygen in 1774. Ingenhousz showed that sunlight is essential, by placing a similar setup once in the dark and once in the sunlight, and with an aquatic plant showed that only the green part of the plants could release oxygen. Air restored is Priestley; sunlight is Ingenhousz.
• Engelmann gave the ACTION spectrum, not the absorption spectrum. A prism, the green alga Cladophora, a suspension of aerobic bacteria, the bacteria accumulating mainly in the region of blue and red light - that is the first action spectrum of photosynthesis. An absorption spectrum belongs to a pigment; an action spectrum belongs to the process.
• The oxygen comes from water, not from $\mathrm{CO_2}$. That is van Niel's inference, drawn from purple and green sulphur bacteria that use $\mathrm{H_2S}$ and give sulphur or sulphate instead of $\mathrm{O_2}$, and it was later proved by using radioisotopic techniques. It is also why the corrected equation carries twelve waters:

$$\mathrm{6CO_2 + 12H_2O} \xrightarrow{\text{light}} \mathrm{C_6H_{12}O_6 + 6H_2O + 6O_2}$$

• Chlorophyll a is the chief pigment and the reaction centre. The accessory pigments - chlorophyll b, xanthophylls and carotenoids - only absorb light and transfer the energy to chlorophyll a. They do not form a reaction centre, and a plant with chlorophyll b but no chlorophyll a cannot photosynthesise.
• $\mathrm{P_{700}}$ belongs to PS I and $\mathrm{P_{680}}$ to PS II, and the photosystems are named in the sequence of their discovery, and not in the sequence in which they function during the light reaction. PS I was found first and so took the number one, but PS II acts first.
• The Z scheme starts at PS II. Trace it as PS II, uphill, down the cytochromes, PS I, uphill again, down to $\mathrm{NADP^+}$, and remember that the Z shape appears only when the carriers are placed in a sequence on a redox potential scale - not in space and not in time.
• Water splitting is associated with PS II, and it happens on the INNER side of the thylakoid membrane, so the protons and the $\mathrm{O_2}$ collect in the LUMEN:

$$\mathrm{2H_2O \rightarrow 4H^+ + O_2 + 4e^-}$$

• Cyclic photophosphorylation makes ATP ONLY, never $\mathrm{NADPH + H^+}$, and it happens where PS II and the NADP reductase enzyme are absent - in the stroma lamellae. It also runs when only light of wavelengths beyond $680\ \mathrm{nm}$ is available, because $\mathrm{P_{700}}$ can still be excited but $\mathrm{P_{680}}$ cannot.
• The proton gradient builds in the LUMEN in photosynthesis, but in the INTERMEMBRANE SPACE in respiration. The stroma is where protons are taken from, not where they gather.
• $\mathrm{CF_0}$ is embedded in the membrane and forms a transmembrane channel for facilitated diffusion of protons; $\mathrm{CF_1}$ protrudes on the outer surface of the thylakoid membrane, on the side that faces the stroma, and undergoes the conformational change that synthesises ATP. The two are swapped in options constantly.
• $\mathrm{RuBP}$ is a 5-carbon ketose sugar giving a 3-carbon first product, and two molecules of 3-PGA at that. The 2-carbon acceptor is the historical wrong guess - scientists spent many years hunting for one before the 5-carbon $\mathrm{RuBP}$ turned up, and the paper still offers it as an option.
• 3 ATP and 2 NADPH per $\mathrm{CO_2}$, and 18 ATP and 12 NADPH per glucose over 6 turns. The favourite wrong option is 12 ATP and 12 NADPH, written by a student who forgot the one extra ATP spent in regeneration.
• The Calvin cycle happens in ALL photosynthetic plants. A $\mathrm{C_4}$ plant is not an alternative to it - it is an extra pump placed in front of it.
• PEP is the $\mathrm{C_4}$ acceptor and it sits in the MESOPHYLL, which LACKS RuBisCO; the bundle sheath has RuBisCO but LACKS PEPcase. Learn the two cells with the two enzymes as one line, because swapping them is the standard trap. Note also that the initial carboxylation reaction occurs in the mesophyll in both plant types.
• Photorespiration makes neither sugar nor ATP, and it consumes ATP, releasing $\mathrm{CO_2}$ that the plant had already fixed. Its biological function is not known yet, so any option that gives it a purpose is wrong.
• Light saturation occurs at 10 per cent of full sunlight, so except for plants in shade or in dense forests, light is rarely a limiting factor in nature. The distractor set offers 50 per cent or 100 per cent.
• $\mathrm{CO_2}$ is the major limiting factor, with $\mathrm{C_4}$ plants saturating at about $360\ \mu\mathrm{L\,L^{-1}}$ and $\mathrm{C_3}$ saturation seen only beyond $450\ \mu\mathrm{L\,L^{-1}}$ - so it is the $\mathrm{C_3}$ plants that present levels limit. The higher number belongs to $\mathrm{C_3}$, which feels backwards and is exactly why it is asked.

One habit pays more than any other here. When two options both look plausible, ask which compartment, which cell or which photosystem the statement actually belongs to. The wrong option in this chapter is almost never invented - it is a true fact moved one step sideways: water splitting given to PS I, NADPH given to cyclic flow, RuBisCO given to the $\mathrm{C_4}$ mesophyll, the proton gradient given to the stroma.

Before You Start

Give yourself 50 minutes for 50 questions and look nothing up. Mark every question where you were choosing between two options rather than certain - those marked questions, not only the wrong ones, are your revision list.

The facts most likely to decide your score, by topic:

Early experiments and the equations. Photosynthesis needs chlorophyll, light and $\mathrm{CO_2}$, shown by the variegated or partly covered leaf and by the $\mathrm{KOH}$ experiment, in which $\mathrm{KOH}$-soaked cotton absorbs $\mathrm{CO_2}$ and the enclosed half tests negative for starch. Priestley, 1770 - a candle goes out and a mouse suffocates in a bell jar, but a mint plant keeps both going; plants restore to the air whatever breathing animals and burning candles remove; he discovered oxygen in 1774. Ingenhousz - sunlight is essential; bubbles form around the green parts of an aquatic plant in bright sunlight but not in the dark; only the green part releases oxygen. Sachs, about 1854 - glucose is produced when plants grow and is usually stored as starch, and the green substance is located in special bodies later called chloroplasts. Engelmann - a prism, the green alga Cladophora and aerobic bacteria; the bacteria gathered in the blue and red regions; the first action spectrum of photosynthesis, resembling roughly the absorption spectra of chlorophyll a and b. van Niel - photosynthesis is a light-dependent reaction in which hydrogen from a suitable oxidisable compound reduces carbon dioxide to carbohydrates; in green plants the donor is $\mathrm{H_2O}$, in purple and green sulphur bacteria it is $\mathrm{H_2S}$ and the product is sulphur or sulphate, not $\mathrm{O_2}$; hence the $\mathrm{O_2}$ comes from water, later proved by radioisotopic techniques. Empirical equation $\mathrm{CO_2 + H_2O \rightarrow [CH_2O] + O_2}$, with $\mathrm{[CH_2O]}$ a carbohydrate; corrected equation $\mathrm{6CO_2 + 12H_2O \rightarrow C_6H_{12}O_6 + 6H_2O + 6O_2}$. Photosynthesis is a multistep process, not a single reaction.

Chloroplast and pigments. Photosynthesis occurs in green leaves and in other green parts. Mesophyll cells have a large number of chloroplasts, aligned along the walls so that they get the optimum quantity of incident light. The chloroplast membrane system is the grana, the stroma lamellae and the matrix stroma. The membrane system traps light energy and synthesises ATP and NADPH; the stroma runs the enzymatic reactions that synthesise sugar, which in turn forms starch. Light reactions or photochemical reactions are directly light driven; dark reactions or carbon reactions are not directly light driven but depend on ATP and NADPH - they do not occur in darkness and are not light-independent. Four pigments - chlorophyll a bright or blue green, chlorophyll b yellow green, xanthophylls yellow, carotenoids yellow to yellow-orange. A pigment is a substance that has an ability to absorb light at specific wavelengths. Chlorophyll a absorbs maximally in the blue and red regions, and those regions show the highest rate of photosynthesis, so chlorophyll a is the chief pigment associated with photosynthesis. There is no complete one-to-one overlap between the absorption spectrum of chlorophyll a and the action spectrum of photosynthesis. The accessory pigments absorb light and transfer the energy to chlorophyll a; they enable a wider range of wavelengths to be utilised and protect chlorophyll a from photo-oxidation.

Photosystems. Light reactions include light absorption, water splitting, oxygen release, and the formation of high-energy chemical intermediates, ATP and NADPH. The pigments are organised into two discrete photochemical light harvesting complexes (LHC) within PS I and PS II, named in the sequence of their discovery and not in the sequence in which they function. PS II functions first, PS I second. The LHC are made up of hundreds of pigment molecules bound to proteins. All the pigments except one molecule of chlorophyll a form the antennae, which help make photosynthesis more efficient by absorbing different wavelengths of light; the single chlorophyll a molecule forms the reaction centre. PS I - absorption peak at $700\ \mathrm{nm}$, hence $\mathrm{P_{700}}$. PS II - absorption maxima at $680\ \mathrm{nm}$, hence $\mathrm{P_{680}}$.

Z scheme and water splitting. In PS II the reaction centre chlorophyll a absorbs $680\ \mathrm{nm}$ red light, causing electrons to become excited and jump into an orbit farther from the atomic nucleus. An electron acceptor picks them up and passes them to an electron transport system consisting of cytochromes, and this movement is downhill on a redox potential scale. The electrons are not used up in the chain but are passed on to the pigments of PS I. Simultaneously electrons in the reaction centre of PS I are excited by $700\ \mathrm{nm}$ light and transferred to another acceptor molecule that has a greater redox potential, then downhill to $\mathrm{NADP^+}$, reducing it to $\mathrm{NADPH + H^+}$. The whole scheme is the Z scheme, and the shape appears when all the carriers are placed in a sequence on a redox potential scale. The electrons removed from PS II are replaced by electrons from the splitting of water. Splitting of water is associated with PS II; water is split into $\mathrm{2H^+}$, $\mathrm{[O]}$ and electrons, giving $\mathrm{2H_2O \rightarrow 4H^+ + O_2 + 4e^-}$. The water splitting complex is on the inner side of the thylakoid membrane, so the protons and the $\mathrm{O_2}$ are released into the lumen.

Cyclic and non-cyclic photophosphorylation. Phosphorylation is the process through which ATP is synthesised by cells, in mitochondria and chloroplasts; photo-phosphorylation is the synthesis of ATP from ADP and inorganic phosphate in the presence of light. Non-cyclic - both photosystems work in series, first PS II and then PS I, connected through an electron transport chain as in the Z scheme; water is the donor; oxygen is released; both ATP and $\mathrm{NADPH + H^+}$ are made; the site is the grana lamellae. Cyclic - only PS I is functional, the electron is circulated within the photosystem and cycled back to the PS I complex; only ATP is made and no $\mathrm{NADPH + H^+}$; no water splitting and no oxygen; the likely site is the stroma lamellae, which lack PS II as well as the NADP reductase enzyme; it also occurs when only light of wavelengths beyond $680\ \mathrm{nm}$ is available for excitation.

Chemiosmosis. ATP synthesis is linked to development of a proton gradient across the thylakoid membrane. Proton accumulation is towards the inside of the membrane, in the lumen, whereas in respiration protons accumulate in the intermembrane space of the mitochondria. Three causes - (a) splitting of water on the inner side of the membrane puts protons into the lumen; (b) the primary acceptor of electrons, located towards the outer side of the membrane, transfers its electron to an H carrier and not to an electron carrier, so that molecule removes a proton from the stroma and releases it into the lumen; (c) the NADP reductase enzyme on the stroma side takes protons from the stroma for the reduction of $\mathrm{NADP^+}$ to $\mathrm{NADPH + H^+}$. Protons in the stroma decrease while protons accumulate in the lumen, creating a proton gradient and a measurable decrease in pH in the lumen. It is the breakdown of the gradient that leads to the synthesis of ATP, as protons move back to the stroma through the transmembrane channel of the $\mathrm{CF_0}$ of the ATP synthase, which carries out facilitated diffusion. $\mathrm{CF_1}$ protrudes on the outer surface of the thylakoid membrane on the side that faces the stroma and undergoes the conformational change that synthesises ATP. Chemiosmosis requires a membrane, a proton pump, a proton gradient and ATP synthase.

Biosynthetic phase and the first products. The products of the light reaction are ATP, NADPH and $\mathrm{O_2}$; the $\mathrm{O_2}$ diffuses out of the chloroplast. The biosynthetic phase does not directly depend on the presence of light but depends on ATP and NADPH besides $\mathrm{CO_2}$ and $\mathrm{H_2O}$. Immediately after light becomes unavailable the biosynthetic process continues for some time and then stops; if light is made available again, the synthesis starts again. Melvin Calvin, just after world war II, used radioactive $\mathrm{{}^{14}C}$ in algal photosynthesis studies and found the first $\mathrm{CO_2}$ fixation product to be a 3-carbon organic acid, 3-phosphoglyceric acid or PGA - hence the Calvin cycle. Another group of plants gives a 4-carbon organic acid, oxaloacetic acid or OAA, as the first stable product. The primary acceptor is a 5-carbon ketose sugar, ribulose bisphosphate; scientists believed it would be a 2-carbon compound and spent many years looking for one.

The Calvin cycle. The pathway operates in a cyclic manner - the $\mathrm{RuBP}$ is regenerated - and occurs in all photosynthetic plants, whatever other pathway they have. Three stages in order - carboxylation, reduction, regeneration. Carboxylation is the fixation of $\mathrm{CO_2}$ into a stable organic intermediate and is the most crucial step; $\mathrm{CO_2}$ is used for the carboxylation of $\mathrm{RuBP}$ by $\mathrm{RuBP}$ carboxylase, giving two molecules of 3-PGA; since the enzyme also has an oxygenation activity it is more correctly called $\mathrm{RuBP}$ carboxylase-oxygenase, RuBisCO. Reduction is a series of reactions leading to the formation of glucose and uses 2 ATP for phosphorylation and 2 NADPH for reduction per $\mathrm{CO_2}$ fixed. Regeneration of $\mathrm{RuBP}$ requires one ATP for phosphorylation and no NADPH. Hence 3 ATP and 2 NADPH per $\mathrm{CO_2}$, and 6 turns for one glucose - six $\mathrm{CO_2}$, 18 ATP and 12 NADPH in; one glucose, 18 ADP and 12 NADP out. It is probably to meet this difference in the number of ATP and NADPH used in the dark reaction that the cyclic phosphorylation takes place.

The $\mathrm{C_4}$ pathway. Plants adapted to dry tropical regions have the $\mathrm{C_4}$ pathway, and although their first fixation product is the $\mathrm{C_4}$ acid OAA, they use the Calvin cycle as the main biosynthetic pathway. They are special in five ways - a special type of leaf anatomy, tolerance of higher temperatures, a response to high light intensities, the lack of photorespiration and greater productivity of biomass. The particularly large cells around the vascular bundles are the bundle sheath cells, and such leaves have Kranz anatomy; Kranz means wreath. Bundle sheath cells may form several layers and have a large number of chloroplasts, thick walls impervious to gaseous exchange and no intercellular spaces. The Hatch and Slack pathway is cyclic - the primary acceptor is the 3-carbon phosphoenol pyruvate (PEP) in the mesophyll, the enzyme is PEP carboxylase or PEPcase, and the mesophyll lacks RuBisCO; OAA forms in the mesophyll, then malic or aspartic acid, which move to the bundle sheath; there the $\mathrm{C_4}$ acids are broken down to release $\mathrm{CO_2}$ and a 3-carbon molecule, which returns to the mesophyll and is converted to PEP again. The bundle sheath cells are rich in RuBisCO but lack PEPcase, and the Calvin pathway runs in all the mesophyll cells of $\mathrm{C_3}$ plants but only in the bundle sheath cells of $\mathrm{C_4}$ plants. Examples - maize and sorghum.

Photorespiration. RuBisCO is the most abundant enzyme in the world, and its active site can bind to both $\mathrm{CO_2}$ and $\mathrm{O_2}$. It has a much greater affinity for $\mathrm{CO_2}$ when the $\mathrm{CO_2}$ to $\mathrm{O_2}$ ratio is nearly equal, the binding is competitive, and the relative concentration of the two gases determines which will bind. In $\mathrm{C_3}$ plants some $\mathrm{O_2}$ binds to RuBisCO and $\mathrm{CO_2}$ fixation is decreased; $\mathrm{RuBP}$ then gives one molecule of phosphoglycerate and one molecule of phosphoglycolate, a 2-carbon compound - the pathway called photorespiration. There is neither synthesis of sugars nor of ATP; it results in the release of $\mathrm{CO_2}$ with the utilisation of ATP; there is no synthesis of ATP or NADPH; and the biological function of photorespiration is not known yet. In $\mathrm{C_4}$ plants photorespiration does not occur, because the $\mathrm{C_4}$ acid broken down in the bundle sheath increases the intracellular concentration of $\mathrm{CO_2}$ at the enzyme site, ensuring RuBisCO functions as a carboxylase and minimising the oxygenase activity - the enzyme itself is the same.

Factors affecting photosynthesis. Plant or internal factors - the number, size, age and orientation of leaves, mesophyll cells and chloroplasts, internal $\mathrm{CO_2}$ concentration and the amount of chlorophyll. External factors - the availability of sunlight, temperature, $\mathrm{CO_2}$ concentration and water. Blackman's (1905) Law of Limiting Factors - if a chemical process is affected by more than one factor, then its rate will be determined by the factor which is nearest to its minimal value. Light has three aspects - quality, intensity and duration of exposure; there is a linear relationship between incident light and $\mathrm{CO_2}$ fixation rates at low light intensities, and at higher intensities the rate does not increase further as other factors become limiting; light saturation occurs at 10 per cent of full sunlight, so light is rarely a limiting factor in nature except for plants in shade or in dense forests; increase in incident light beyond a point causes the breakdown of chlorophyll and a decrease in photosynthesis. $\mathrm{CO_2}$ is the major limiting factor; the atmospheric concentration is between 0.03 and 0.04 per cent; an increase up to 0.05 per cent raises fixation rates, and beyond this the levels can become damaging over longer periods. At low light neither group responds to high $\mathrm{CO_2}$; at high light both $\mathrm{C_3}$ and $\mathrm{C_4}$ plants show an increase. $\mathrm{C_4}$ plants saturate at about $360\ \mu\mathrm{L\,L^{-1}}$, while $\mathrm{C_3}$ saturation is seen only beyond $450\ \mu\mathrm{L\,L^{-1}}$, so current $\mathrm{CO_2}$ availability is limiting to the $\mathrm{C_3}$ plants - which is why greenhouse crops such as tomatoes and bell pepper are grown in a carbon dioxide enriched atmosphere. The dark reactions, being enzymatic, are temperature controlled, while the light reactions are temperature sensitive to a much lesser extent; the $\mathrm{C_3}$ temperature optimum is $20^\circ$ to $25^\circ\mathrm{C}$ and the $\mathrm{C_4}$ optimum is $30^\circ$ to $40^\circ\mathrm{C}$; tropical plants have a higher temperature optimum than plants adapted to temperate climates. Water acts more through its effect on the plant than directly - water stress closes the stomata, reducing $\mathrm{CO_2}$ availability, and makes leaves wilt, reducing leaf surface area and metabolic activity.