Summary and Exam Tips

The Chapter in One Read

Green plants make their own food by photosynthesis. During this process carbon dioxide from the atmosphere is taken in by leaves through stomata and used for making carbohydrates, principally glucose and starch. Photosynthesis takes place only in the green parts of the plants, mainly the leaves, and within the leaves the mesophyll cells have a large number of chloroplasts that are responsible for $\mathrm{CO_2}$ fixation. That is the whole chapter in outline, and everything else is detail hung on it.

The early experiments settled what is needed and where the oxygen comes from. Priestley showed that plants restore to the air whatever breathing animals and burning candles remove; Ingenhousz showed that sunlight is essential and that only the green part releases oxygen; Sachs showed that glucose is produced when plants grow and is usually stored as starch; Engelmann gave the first action spectrum of photosynthesis; and van Niel showed that the $\mathrm{O_2}$ evolved by a green plant comes from $\mathrm{H_2O}$, not from carbon dioxide, which is why the correct equation carries twelve waters:

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

Within the chloroplasts, the membranes are sites for the light reaction, while the chemosynthetic pathway occurs in the stroma. Photosynthesis has two stages - the light reaction and the carbon fixing reactions. In the light reaction the light energy is absorbed by the pigments present in the antenna, and funnelled to special chlorophyll a molecules called reaction centre chlorophylls. There are two photosystems, PS I and PS II. PS I has a $700\ \mathrm{nm}$ absorbing chlorophyll a $\mathrm{P_{700}}$ molecule at its reaction centre, while PS II has a $\mathrm{P_{680}}$ reaction centre that absorbs red light at $680\ \mathrm{nm}$ - and the two are named in the sequence of their discovery, not in the sequence in which they function. After absorbing light, electrons are excited and transferred through PS II and PS I, and are finally passed to $\mathrm{NADP^+}$, forming $\mathrm{NADPH + H^+}$. During this process a proton gradient is created across the membrane of the thylakoid. The breakdown of the proton gradient, due to movement through the $\mathrm{CF_0}$ part of the ATP synthase enzyme, releases enough energy for synthesis of ATP. Splitting of water molecules is associated with PS II, resulting in the release of $\mathrm{O_2}$ and protons and the transfer of electrons to PS II. Because the water splitting complex sits on the inner side of the thylakoid membrane, the protons collect in the lumen - the opposite address from respiration, where protons accumulate in the intermembrane space of the mitochondria. When only PS I is functional the electron is cycled back to the PS I complex, and that cyclic flow makes ATP only, not $\mathrm{NADPH + H^+}$.

In the carbon fixation cycle, $\mathrm{CO_2}$ is added by the enzyme RuBisCO to a 5-carbon compound $\mathrm{RuBP}$, that is converted to 2 molecules of 3-carbon PGA. This is then converted to sugar by the Calvin cycle, and the $\mathrm{RuBP}$ is regenerated. During this process ATP and NADPH synthesised in the light reaction are utilised - 3 ATP and 2 NADPH for every $\mathrm{CO_2}$ fixed, and so 18 ATP and 12 NADPH over the 6 turns that make one glucose. This phase does not directly depend on the presence of light, but it depends on the products of the light reaction, which is why it continues for some time after light becomes unavailable and then stops. RuBisCO also catalyses a wasteful oxygenation reaction in $\mathrm{C_3}$ plants - photorespiration - in which $\mathrm{RuBP}$ gives one phosphoglycerate and one phosphoglycolate, no sugar and no ATP are made, ATP is used up, and the biological function is not known yet.

Some tropical plants show a special type of photosynthesis called the $\mathrm{C_4}$ pathway. In these plants the first product of $\mathrm{CO_2}$ fixation that takes place in the mesophyll is a 4-carbon compound, oxaloacetic acid, made by PEPcase acting on the 3-carbon acceptor PEP in mesophyll cells that lack RuBisCO. In the bundle sheath cells the Calvin pathway is carried out for the synthesis of carbohydrates - and because the $\mathrm{C_4}$ acid is broken down there to release $\mathrm{CO_2}$, the concentration of $\mathrm{CO_2}$ at the enzyme site stays high, RuBisCO acts as a carboxylase and photorespiration does not occur.

Finally, the rate of photosynthesis is very important in determining the yield of plants, and it is set by whichever factor is nearest to its minimal value - Blackman's Law of Limiting Factors, 1905. Light saturates at 10 per cent of full sunlight, so light is rarely limiting in nature, while $\mathrm{CO_2}$ is the major limiting factor, and it is the $\mathrm{C_3}$ plants, not the $\mathrm{C_4}$ plants, that present atmospheric levels hold back.

Master Quick Recap

The early experiments

• Chlorophyll, light and $\mathrm{CO_2}$ are required for photosynthesis. A variegated leaf or a partly covered leaf shows starch only in the green parts, in the presence of light.
• $\mathrm{KOH}$-soaked cotton absorbs $\mathrm{CO_2}$, so the enclosed half of the leaf tests negative for starch - $\mathrm{CO_2}$ is required.
• Priestley, 1770 - a candle burning in a bell jar goes out and a mouse suffocates, but with a mint plant in the jar the mouse stayed alive and the candle continued to burn. Plants restore to the air whatever breathing animals and burning candles remove. He discovered oxygen in 1774.
• Ingenhousz - the same setup once in the dark and once in the sunlight, showing sunlight is essential; with an aquatic plant, small bubbles formed around the green parts in bright sunlight but not in the dark, and the bubbles were oxygen, so only the green part of the plants could release oxygen.
• Julius von 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.
• T. W. Engelmann - a prism to split light, the green alga Cladophora, a suspension of aerobic bacteria to detect the sites of $\mathrm{O_2}$ evolution; the bacteria accumulated mainly in the region of blue and red light, giving the first action spectrum of photosynthesis, which resembles roughly the absorption spectra of chlorophyll a and b.
• Empirical equation: $\mathrm{CO_2 + H_2O \rightarrow [CH_2O] + O_2}$, in which $\mathrm{[CH_2O]}$ represents a carbohydrate.
• Cornelius van Niel - photosynthesis is essentially a light-dependent reaction in which hydrogen from a suitable oxidisable compound reduces carbon dioxide to carbohydrates, $\mathrm{2H_2A + CO_2 \rightarrow 2A + CH_2O + H_2O}$.
• In green plants $\mathrm{H_2O}$ is the hydrogen donor and is oxidised to $\mathrm{O_2}$; in purple and green sulphur bacteria $\mathrm{H_2S}$ is the donor and the product is sulphur or sulphate, not $\mathrm{O_2}$.
• The $\mathrm{O_2}$ evolved comes from $\mathrm{H_2O}$, not from carbon dioxide, later proved by using radioisotopic techniques, hence $\mathrm{6CO_2 + 12H_2O \rightarrow C_6H_{12}O_6 + 6H_2O + 6O_2}$.
• Photosynthesis is not a single reaction but a multistep process.

The chloroplast and the pigments

• Photosynthesis takes place in green leaves and also in other green parts of the plant.
• Mesophyll cells have a large number of chloroplasts, aligned along the walls so that they get the optimum quantity of the incident light.
• The chloroplast membrane system consists of the grana, the stroma lamellae and the matrix stroma.
• Division of labour: the membrane system traps the light energy and synthesises ATP and NADPH; in the stroma, enzymatic reactions 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 - this does not mean that they occur in darkness or that they are not light-dependent.
• 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 shows maximum absorption in the blue and the red regions, and those wavelengths also show a higher 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 - chlorophyll b, xanthophylls and carotenoids - absorb light and transfer the energy to chlorophyll a.
• They enable a wider range of wavelengths of incoming light to be utilised for photosynthesis, and they protect chlorophyll a from photo-oxidation.

The photosystems

• Light reactions include light absorption, water splitting, oxygen release, and the formation of high-energy chemical intermediates, ATP and NADPH. Several protein complexes are involved.
• The pigments are organised into two discrete photochemical light harvesting complexes (LHC) within PS I and PS II.
• They are named in the sequence of their discovery, and not in the sequence in which they function during the light reaction - 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 - the reaction centre chlorophyll a has an absorption peak at $700\ \mathrm{nm}$, hence $\mathrm{P_{700}}$. PS II - absorption maxima at $680\ \mathrm{nm}$, hence $\mathrm{P_{680}}$.

Electron transport, the Z scheme and water splitting

• In PS II the reaction centre chlorophyll a absorbs $680\ \mathrm{nm}$ wavelength of 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 of electrons is downhill in terms of a redox potential scale.
• The electrons are not used up as they pass through the chain, but are passed on to the pigments of PS I.
• Simultaneously, electrons in the reaction centre of PS I are excited by red light of $700\ \mathrm{nm}$ and transferred to another acceptor molecule that has a greater redox potential, then downhill again to $\mathrm{NADP^+}$, and the addition of these electrons reduces $\mathrm{NADP^+}$ to $\mathrm{NADPH + H^+}$.
• The whole scheme of transfer of electrons is called the Z scheme, due to its characteristic shape, and the shape is formed when all the carriers are placed in a sequence on a redox potential scale.
• The electrons moved from PS II must be replaced, and this is achieved by electrons available due to splitting of water.
• The splitting of water is associated with PS II; water is split into $\mathrm{2H^+}$, $\mathrm{[O]}$ and electrons, giving $\mathrm{2H_2O \rightarrow 4H^+ + O_2 + 4e^-}$, and this creates oxygen, one of the net products of photosynthesis.
• The water splitting complex is associated with PS II, which is physically located on the inner side of the membrane of the thylakoid, 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.
• When the two photosystems work in a series, first PS II and then PS I, the process is non-cyclic photo-phosphorylation, and the two photosystems are connected through an electron transport chain as in the Z scheme.
• Both ATP and $\mathrm{NADPH + H^+}$ are synthesised by non-cyclic electron flow, and water splitting and $\mathrm{O_2}$ release belong here.
• When only PS I is functional, the electron is circulated within the photosystem and the phosphorylation occurs due to cyclic flow of electrons.
• A possible location is the stroma lamellae, because the membrane or lamellae of the grana have both PS I and PS II, while the stroma lamellae membranes lack PS II as well as the NADP reductase enzyme.
• The excited electron does not pass on to $\mathrm{NADP^+}$ but is cycled back to the PS I complex through the electron transport chain, so the cyclic flow results only in the synthesis of ATP, but not of $\mathrm{NADPH + H^+}$.
• Cyclic photophosphorylation also occurs when only light of wavelengths beyond $680\ \mathrm{nm}$ is available for excitation.

Chemiosmosis

• The chemiosmotic hypothesis explains how ATP is synthesised in the chloroplast: ATP synthesis is linked to development of a proton gradient across the membranes of the thylakoid.
• Here the proton accumulation is towards the inside of the membrane, that is in the lumen; in respiration, protons accumulate in the intermembrane space of the mitochondria.
• Cause (a): splitting of the water molecule takes place on the inner side of the membrane, so the protons produced accumulate within the lumen of the thylakoids.
• Cause (b): the primary acceptor of electrons, located towards the outer side of the membrane, transfers its electron not to an electron carrier but to an H carrier, which removes a proton from the stroma while transporting an electron and releases it into the lumen when it passes the electron on.
• Cause (c): the NADP reductase enzyme is located on the stroma side of the membrane, and the protons needed for the reduction of $\mathrm{NADP^+}$ to $\mathrm{NADPH + H^+}$ are also removed from the stroma.
• Protons in the stroma decrease in number while protons accumulate in the lumen, creating a proton gradient across the thylakoid membrane as well as a measurable decrease in pH in the lumen.
• It is the breakdown of this gradient that leads to the synthesis of ATP, as protons move across the membrane to the stroma through the transmembrane channel of the $\mathrm{CF_0}$ of the ATP synthase, by facilitated diffusion.
• $\mathrm{CF_1}$ protrudes on the outer surface of the thylakoid membrane on the side that faces the stroma, and the breakdown of the gradient causes a conformational change in $\mathrm{CF_1}$ which makes the enzyme synthesise several molecules of ATP.
• Chemiosmosis requires a membrane, a proton pump, a proton gradient and ATP synthase.
• The ATP and NADPH are used immediately in the biosynthetic reaction taking place in the stroma, responsible for fixing $\mathrm{CO_2}$ and synthesis of sugars.

The 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 is dependent on the products of the light reaction - 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.
• Calling the biosynthetic phase the dark reaction is arguably a misnomer - it neither requires darkness nor is it independent of light.
• Melvin Calvin, just after world war II, used radioactive $\mathrm{{}^{14}C}$ in algal photosynthesis studies and found that the first $\mathrm{CO_2}$ fixation product was a 3-carbon organic acid, 3-phosphoglyceric acid or PGA; he also worked out the complete biosynthetic pathway, hence the Calvin cycle.
• Another group of plants gives a 4-carbon organic acid, oxaloacetic acid or OAA, as the first stable product of $\mathrm{CO_2}$ fixation - hence two main types of $\mathrm{CO_2}$ assimilation, the $\mathrm{C_3}$ and the $\mathrm{C_4}$ pathway.
• The primary acceptor of $\mathrm{CO_2}$ is a 5-carbon ketose sugar, ribulose bisphosphate. Scientists believed the acceptor would be a 2-carbon compound and spent many years trying to identify one.

The Calvin cycle

• The pathway operates in a cyclic manner - the $\mathrm{RuBP}$ is regenerated.
• The Calvin pathway occurs in all photosynthetic plants; it does not matter whether they have $\mathrm{C_3}$ or $\mathrm{C_4}$ or any other pathway.
• 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 of the Calvin cycle.
• $\mathrm{CO_2}$ is utilised for the carboxylation of $\mathrm{RuBP}$, catalysed by $\mathrm{RuBP}$ carboxylase, resulting in the formation of two molecules of 3-PGA, and since the enzyme also has an oxygenation activity it is more correctly called $\mathrm{RuBP}$ carboxylase-oxygenase, RuBisCO.
• Reduction is a series of reactions that lead to the formation of glucose, using 2 ATP for phosphorylation and 2 NADPH for reduction, per $\mathrm{CO_2}$ fixed.
• Regeneration of $\mathrm{RuBP}$ is crucial if the cycle is to continue uninterrupted, and requires one ATP for phosphorylation - and no NADPH.
• Hence 3 ATP and 2 NADPH are required for every $\mathrm{CO_2}$ molecule entering the Calvin cycle.
• Six molecules of $\mathrm{CO_2}$ and 6 turns of the cycle give one molecule of glucose - in: six $\mathrm{CO_2}$, 18 ATP and 12 NADPH; out: one glucose, 18 ADP and 12 NADP.
• 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 $\mathrm{C_3}$ pathway, or the Calvin cycle, as the main biosynthetic pathway.
• $\mathrm{C_4}$ plants 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 are said to have Kranz anatomy. Kranz means wreath, a reflection of the arrangement of cells.
• 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 a cyclic process. The primary $\mathrm{CO_2}$ acceptor is the 3-carbon phosphoenol pyruvate (PEP), present in the mesophyll cells, and the enzyme is PEP carboxylase, or PEPcase.
• The mesophyll cells lack the RuBisCO enzyme.
• OAA is formed in the mesophyll cells, then other 4-carbon compounds like malic acid or aspartic acid form in the mesophyll itself and are transported to the bundle sheath cells.
• In the bundle sheath cells these $\mathrm{C_4}$ acids are broken down to release $\mathrm{CO_2}$ and a 3-carbon molecule, and the 3-carbon molecule is transported back to the mesophyll, where it is converted to PEP again, completing the cycle.
• The $\mathrm{CO_2}$ released in the bundle sheath cells enters the Calvin pathway, a pathway common to all plants.
• The bundle sheath cells are rich in RuBisCO but lack PEPcase.
• The Calvin pathway occurs in all the mesophyll cells of $\mathrm{C_3}$ plants; in $\mathrm{C_4}$ plants it does not take place in the mesophyll cells but only in the bundle sheath cells. Examples of $\mathrm{C_4}$ plants - 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 $\mathrm{O_2}$ - hence the name carboxylase-oxygenase.
• RuBisCO 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 $\mathrm{O_2}$ and $\mathrm{CO_2}$ determines which of the two will bind to the enzyme.
• In $\mathrm{C_3}$ plants some $\mathrm{O_2}$ does bind to RuBisCO, and hence $\mathrm{CO_2}$ fixation is decreased. $\mathrm{RuBP}$, instead of being converted to 2 molecules of PGA, binds with $\mathrm{O_2}$ to form one molecule of phosphoglycerate and one molecule of phosphoglycolate, which is a 2-carbon compound - this pathway is called photorespiration.
• In the photorespiratory pathway there is neither synthesis of sugars, nor of ATP. Rather, it results in the release of $\mathrm{CO_2}$ with the utilisation of ATP. There is no synthesis of ATP or NADPH.
• The biological function of photorespiration is not known yet.
• In $\mathrm{C_4}$ plants photorespiration does not occur, because they have a mechanism that increases the concentration of $\mathrm{CO_2}$ at the enzyme site - the $\mathrm{C_4}$ acid from the mesophyll is broken down in the bundle sheath cells to release $\mathrm{CO_2}$, increasing the intracellular concentration of $\mathrm{CO_2}$ and ensuring that RuBisCO functions as a carboxylase, minimising the oxygenase activity.
• Productivity and yields are better in $\mathrm{C_4}$ plants, and they show tolerance to higher temperatures.

Factors affecting photosynthesis

• The rate of photosynthesis is very important in determining the yield of plants, including crop plants.
• 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; they depend on the genetic predisposition and the growth of the plant.
• External factors - the availability of sunlight, temperature, $\mathrm{CO_2}$ concentration and water.
• Usually one factor is the major cause, or the one that limits the rate, and at any point the rate will be determined by the factor available at sub-optimal levels.
• 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: it is the factor which directly affects the process if its quantity is changed.
• Light has three aspects - light quality, light intensity and the duration of exposure to light.
• There is a linear relationship between incident light and $\mathrm{CO_2}$ fixation rates at low light intensities, and at higher light intensities the rate gradually does not show further increase, as other factors become limiting.
• Light saturation occurs at 10 per cent of the full sunlight, so except for plants in shade or in dense forests, light is rarely a limiting factor in nature.
• 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 for photosynthesis. The atmospheric concentration is between 0.03 and 0.04 per cent, and an increase up to 0.05 per cent can raise $\mathrm{CO_2}$ fixation rates, beyond which the levels can become damaging over longer periods.
• At low light conditions neither group responds to high $\mathrm{CO_2}$ conditions; at high light intensities both $\mathrm{C_3}$ and $\mathrm{C_4}$ plants show an increase in the rates of photosynthesis.
• $\mathrm{C_4}$ plants saturate at about $360\ \mu\mathrm{L\,L^{-1}}$, while $\mathrm{C_3}$ plants show saturation only beyond $450\ \mu\mathrm{L\,L^{-1}}$ - so current availability of $\mathrm{CO_2}$ levels is limiting to the $\mathrm{C_3}$ plants.
• Greenhouse crops such as tomatoes and bell pepper are grown in a carbon dioxide enriched atmosphere, which leads to higher yields.
• The dark reactions, being enzymatic, are temperature controlled; the light reactions are also temperature sensitive but are affected to a much lesser extent.
• $\mathrm{C_4}$ plants respond to higher temperatures and show a higher rate of photosynthesis, while $\mathrm{C_3}$ plants have a much lower temperature optimum, and tropical plants have a higher temperature optimum than plants adapted to temperate climates.
• The effect of water is more through its effect on the plant than directly on photosynthesis - water stress causes the stomata to close, reducing $\mathrm{CO_2}$ availability, and also makes leaves wilt, reducing the surface area of the leaves and their metabolic activity.

The Whole Pathway, End to End

Read the table across, one row at a time. Light energy enters at the top left and leaves as glucose at the bottom right, and every arrow in between is one row. The first four rows are the light reaction in the thylakoid membrane; the last three are the Calvin cycle in the stroma.

Two totals to carry away from the table. Per $\mathrm{CO_2}$ fixed, 3 ATP and 2 NADPH. Per glucose, over 6 turns, 18 ATP and 12 NADPH - and the mismatch between the ATP and the NADPH demand is the reason given for cyclic photophosphorylation.

The Eight Comparisons Worth Memorising

1. Light reaction against dark reaction - the light reaction, or photochemical phase, happens in the membrane system of grana and stroma lamellae, is directly light driven, and includes light absorption, water splitting, oxygen release and the formation of ATP and NADPH; the dark reaction, or carbon reaction, or biosynthetic phase, happens in the stroma, is not directly light driven but depends on ATP and NADPH, and fixes $\mathrm{CO_2}$ and synthesises sugars. The name is misleading - it does not mean they occur in darkness or that they are not light-dependent, and the proof is that the process continues for some time after light becomes unavailable and then stops.
2. Absorption spectrum against action spectrum - an absorption spectrum plots how much light a pigment absorbs at each wavelength and belongs to a molecule; an action spectrum plots the rate of photosynthesis at each wavelength and belongs to the process. Engelmann described the first action spectrum, using a prism, Cladophora and aerobic bacteria. The two do not overlap completely, which is the evidence that pigments other than chlorophyll a contribute.
3. Chlorophyll a against the accessory pigments - chlorophyll a is bright or blue green, absorbs maximally in the blue and the red, is the chief pigment associated with photosynthesis, and is the only pigment that forms a reaction centre; the accessory pigments - chlorophyll b (yellow green), xanthophylls (yellow) and carotenoids (yellow to yellow-orange) - absorb light and transfer the energy to chlorophyll a, enable a wider range of wavelengths to be utilised, and protect chlorophyll a from photo-oxidation. A plant with chlorophyll b but no chlorophyll a cannot photosynthesise.
4. PS I against PS II - PS I was discovered first and carries the number one, has $\mathrm{P_{700}}$, absorbing at $700\ \mathrm{nm}$, functions second in the light reaction, is present in both grana and stroma lamellae, and passes electrons on to $\mathrm{NADP^+}$; PS II was discovered second, has $\mathrm{P_{680}}$, absorbing red light at $680\ \mathrm{nm}$, functions first, is present only in the grana lamellae, and is the photosystem associated with the splitting of water and the release of oxygen. The numbers are the order of discovery, not the order of function.
5. Cyclic against non-cyclic photophosphorylation - cyclic uses only PS I, circulates the electron back to the PS I complex, needs no external electron donor, has no water splitting and no oxygen release, gives ATP only, and is likely sited in the stroma lamellae, which lack PS II as well as the NADP reductase enzyme; it also runs when only light beyond $680\ \mathrm{nm}$ is available. Non-cyclic uses both photosystems in series, first PS II and then PS I, connected through an electron transport chain as in the Z scheme, takes water as the donor, releases $\mathrm{O_2}$, gives both ATP and $\mathrm{NADPH + H^+}$, and is sited in the grana lamellae, which have both PS I and PS II.
6. $\mathrm{C_3}$ against $\mathrm{C_4}$ pathway - the $\mathrm{C_3}$ plant has $\mathrm{RuBP}$, a 5-carbon acceptor, the enzyme RuBisCO, and PGA, a 3-carbon first product; the $\mathrm{C_4}$ plant has PEP, a 3-carbon acceptor, the enzyme PEPcase, and OAA, a 4-carbon first product, and then runs the Hatch and Slack pathway - malic or aspartic acid to the bundle sheath, $\mathrm{CO_2}$ released there, the 3-carbon molecule back to the mesophyll to make PEP again. Both have RuBisCO and both run the Calvin cycle; only the $\mathrm{C_4}$ plant has PEPcase. The name comes from the first product, never from the acceptor.
7. $\mathrm{C_3}$ against $\mathrm{C_4}$ leaf anatomy - a $\mathrm{C_3}$ leaf has no Kranz anatomy, one $\mathrm{CO_2}$-fixing cell type, the mesophyll, RuBisCO in the mesophyll, and bundle sheath cells that are not specialised; a $\mathrm{C_4}$ leaf has Kranz anatomy, two $\mathrm{CO_2}$-fixing cell types, the mesophyll and the bundle sheath, RuBisCO in the bundle sheath and PEPcase in the mesophyll, and particularly large bundle sheath cells, often in several layers, with a large number of chloroplasts, thick walls impervious to gaseous exchange and no intercellular spaces. The initial carboxylation reaction occurs in the mesophyll in both - that is the row students lose.
8. Photorespiration against respiration - photorespiration happens only in the light, only in $\mathrm{C_3}$ plants, begins when $\mathrm{O_2}$ binds the active site of RuBisCO instead of $\mathrm{CO_2}$, gives one phosphoglycerate and one phosphoglycolate, releases $\mathrm{CO_2}$ with the utilisation of ATP, produces neither sugars nor ATP nor NADPH, and has a biological function that is not known yet; respiration happens in the light and in the dark, in all cells, oxidises substrate to release energy, and produces ATP. Photorespiration wastes carbon the plant had already fixed; respiration pays for the cell's work.

Writing the Chapter-End Exercises Well

Class 11 has no board paper, but the nine chapter-end exercises are the best test of this chapter that exists. Between them they cover the two plant types, the accessory pigments, the light graph and the three comparisons - which is very nearly the whole chapter. Marks are lost here cheaply, by naming a molecule and never saying which cell it sits in.

Laying out a $\mathrm{C_3}$-versus-$\mathrm{C_4}$ comparison. One exercise asks you to compare the $\mathrm{C_3}$ and $\mathrm{C_4}$ pathways, another asks for the anatomy of the leaf in $\mathrm{C_3}$ and $\mathrm{C_4}$ plants, and two more - whether you can tell a $\mathrm{C_3}$ plant from a $\mathrm{C_4}$ plant by looking at it externally, and by which internal structure you can tell - are answered from the same material. Write these as a two-column table, not as a paragraph. Prose loses marks because the examiner cannot see the pairs. Put the rows in this order: cell in which the Calvin cycle runs - mesophyll against bundle sheath; cell in which the initial carboxylation occurs - mesophyll in both; primary $\mathrm{CO_2}$ acceptor - $\mathrm{RuBP}$, 5 carbons, against PEP, 3 carbons; enzyme of the first fixation - RuBisCO against PEPcase; first fixation product - PGA, 3 carbons, against OAA, 4 carbons; presence of RuBisCO - in both; presence of PEPcase - only in $\mathrm{C_4}$; Kranz anatomy - absent against present; photorespiration - high in $\mathrm{C_3}$ at high light and at low $\mathrm{CO_2}$, negligible in $\mathrm{C_4}$; $\mathrm{CO_2}$ fixation rate at high light - low against high; temperature optimum - $20^\circ$ to $25^\circ\mathrm{C}$ against $30^\circ$ to $40^\circ\mathrm{C}$; examples - wheat and rice against maize and sorghum. For the external question, say plainly that you cannot tell from outside - the difference is anatomical and biochemical, not in the shape or colour of the plant - and then give the internal answer: a vertical section of the leaf, looking for the bundle sheath around the vascular bundles, that is Kranz anatomy.

A full cyclic-versus-non-cyclic answer. The comparison exercise also asks for cyclic against non-cyclic photophosphorylation, and a complete answer needs six rows, not two: which photosystems are involved; whether they work in series; the path of the electron - circulated back to PS I against travelling from PS II to $\mathrm{NADP^+}$; the external electron donor - none against water; whether water is split and oxygen released; and the products - ATP only against ATP and $\mathrm{NADPH + H^+}$. Add the site as a seventh row if you have space - stroma lamellae against grana lamellae - and give the reason with it: the stroma lamellae lack PS II as well as the NADP reductase enzyme. Most students write only the products row and stop; that is half a mark out of the several on offer.

Reading the light-intensity graph question. One exercise gives you the curve of the effect of light on the rate of photosynthesis and asks at which point light is a limiting factor, what could be limiting in the region near the origin, and what the flat part of the curve represents. Read the curve in three parts and answer in the same three parts. On the rising, linear portion, light is the limiting factor - there is a linear relationship between incident light and $\mathrm{CO_2}$ fixation rates at low light intensities, so more light gives more fixation. In that low region, light is the factor nearest to its minimal value, though $\mathrm{CO_2}$ concentration and temperature are also present as possible limits. The plateau is light saturation - the rate does not show further increase as other factors become limiting, and the limiting factor there is no longer light but $\mathrm{CO_2}$ concentration or temperature. Finish with the number: light saturation occurs at 10 per cent of the full sunlight, and light is rarely a limiting factor in nature except for plants in shade or in dense forests.

The habit all nine exercises reward. Name the molecule, then the enzyme, then the location, in that order. "How is $\mathrm{CO_2}$ fixed in a $\mathrm{C_4}$ plant" is answered as PEP accepts it - the enzyme is PEPcase - and this happens in the mesophyll cells, which lack RuBisCO. "Why does RuBisCO carry out more carboxylation in $\mathrm{C_4}$ plants" is answered as the $\mathrm{C_4}$ acid is broken down to release $\mathrm{CO_2}$ - this raises the intracellular $\mathrm{CO_2}$ concentration at the enzyme site - and in the bundle sheath cells, where RuBisCO sits, so it acts as a carboxylase and the oxygenase activity is minimised. "Why are so few cells of a $\mathrm{C_4}$ plant so productive" is answered as only the bundle sheath cells run the Calvin cycle - but they are fed a concentrated supply of $\mathrm{CO_2}$ by the mesophyll - so there is no photorespiration to waste what is fixed, and productivity and yields are better. Most marks lost in this chapter are lost by giving only the first of the three.

NEET Strategy

What the paper asks. Photosynthesis is one of the highest yielding chapters in the paper, and almost all of it is recall of a molecule, a number, a place or a comparison. Nothing has to be derived. That makes it a chapter where speed comes from certainty, and the minutes you save here are minutes you keep for the calculation-heavy chapters elsewhere.

The marking is +4 and -1. On a pure-recall question, a coin flip between two options loses marks on average. If both the molecule and the compartment have genuinely gone, skip it and bank the time.

Budget. Aim for 25 to 30 seconds per question in this chapter. If a question is still open at forty seconds, staring at it will not fix it - mark it and move.

The four shapes, in the order they are easiest to attempt.

1. Give the number or the wavelength. These are the fastest if the ladder is in your head - $700\ \mathrm{nm}$ and $680\ \mathrm{nm}$; 3 ATP and 2 NADPH; 18 ATP and 12 NADPH; 10 per cent of full sunlight; $360$ and $450\ \mu\mathrm{L\,L^{-1}}$; 0.03 to 0.04 per cent. Answer in five seconds or move on.
2. Name the molecule, enzyme, pigment or scientist. The clue is given, so there is nothing to work out - just recall. Most abundant enzyme in the world - RuBisCO. First action spectrum - Engelmann. Primary acceptor in a $\mathrm{C_4}$ plant - PEP.
3. Say which cell, which membrane side or which compartment. Read until one clue fixes the place and stop reading. "Protons accumulate" is the lumen; "the Calvin cycle in a $\mathrm{C_4}$ plant" is the bundle sheath; "protrudes on the side facing the stroma" is $\mathrm{CF_1}$.
4. Compare $\mathrm{C_3}$ with $\mathrm{C_4}$, or spot the incorrect statement. Leave these for last. A "which is NOT" question forces you to check all four options and costs two to three times what the others cost.

Read the direction word before you read the options. NOT, INCORRECT and respectively are the three words that turn a fact you know into a mark you lose. Underline them as you read the stem. In a respectively question, match the first pair first - it usually eliminates two options in one step.

One last habit that fits this chapter in particular. When two options both look right, ask which compartment, which cell or which photosystem the statement actually belongs to. The wrong option here 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, the Calvin cycle taken away from $\mathrm{C_4}$ plants.

The Mistakes That Cost the Most Marks

1. Swapping Priestley and Ingenhousz. Priestley showed that plants restore to the air whatever breathing animals and burning candles remove, in 1770, and discovered oxygen in 1774. Ingenhousz showed that sunlight is essential, and with an aquatic plant that only the green part of the plants could release oxygen. Air restored is Priestley; sunlight is Ingenhousz.
2. Calling Engelmann's result an absorption spectrum. He described the first action spectrum of photosynthesis, using a prism, the green alga Cladophora and a suspension of aerobic bacteria. An absorption spectrum belongs to a pigment; an action spectrum belongs to the process, and the two do not overlap completely.
3. Saying the oxygen comes from carbon dioxide. The $\mathrm{O_2}$ evolved by the green plant comes from $\mathrm{H_2O}$, which is exactly why the correct equation carries twelve molecules of water, not six: $\mathrm{6CO_2 + 12H_2O \rightarrow C_6H_{12}O_6 + 6H_2O + 6O_2}$.
4. Giving an accessory pigment a reaction centre. The single chlorophyll a molecule forms the reaction centre; all the other pigments form the antennae and only absorb light and transfer the energy to chlorophyll a. A plant with chlorophyll b but no chlorophyll a cannot photosynthesise.
5. Pairing $\mathrm{P_{700}}$ with PS II, or letting PS I act first. $\mathrm{P_{700}}$ 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. The Z scheme starts at PS II.
6. Putting water splitting at PS I, or the protons in the stroma. Splitting of water is associated with PS II, and because the complex is on the inner side of the thylakoid membrane, the protons and the $\mathrm{O_2}$ are released into the lumen. In photosynthesis protons collect in the lumen; in respiration, in the intermembrane space of the mitochondria.
7. Letting cyclic photophosphorylation make NADPH. Cyclic flow results only in the synthesis of ATP, but not of $\mathrm{NADPH + H^+}$, because it happens where PS II as well as the NADP reductase enzyme are absent - in the stroma lamellae. It also runs when only light beyond $680\ \mathrm{nm}$ is available.
8. Swapping $\mathrm{CF_0}$ and $\mathrm{CF_1}$. $\mathrm{CF_0}$ is embedded in the thylakoid membrane and forms the 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.
9. Getting the Calvin arithmetic wrong, or the acceptor. 3 ATP and 2 NADPH per $\mathrm{CO_2}$, and 18 ATP and 12 NADPH per glucose over 6 turns - the wrong answer 12 ATP and 12 NADPH forgets the extra ATP spent in regeneration. And the acceptor is the 5-carbon ketose sugar $\mathrm{RuBP}$, giving two molecules of the 3-carbon PGA; the 2-carbon acceptor is the historical wrong guess.
10. Taking the Calvin cycle away from $\mathrm{C_4}$ plants, or putting RuBisCO in their mesophyll. The Calvin pathway occurs in all photosynthetic plants, and in a $\mathrm{C_4}$ plant it runs only in the bundle sheath cells. The mesophyll has PEPcase and lacks RuBisCO; the bundle sheath has RuBisCO and lacks PEPcase. While you are there, remember that photorespiration makes neither sugar nor ATP, uses up ATP, and its biological function is not known yet, and that $\mathrm{C_4}$ plants escape it by concentration at the enzyme site, not by having a different RuBisCO.

A Short Revision Plan

First pass - one hour and fifteen minutes. Read sections 1 to 12 straight through without stopping to memorise. You are building the map: the early experiments and the equation, then the chloroplast and its pigments, then the two photosystems, the Z scheme and water splitting, then the two kinds of photophosphorylation, then chemiosmosis, then the biosynthetic phase, the Calvin cycle, the $\mathrm{C_4}$ pathway, photorespiration, the comparison and the limiting factors. Do not stop to learn numbers on this pass - just notice that energy moves from light, to electrons, to ATP and NADPH, to sugar, and that every step has an address inside the chloroplast.

Second pass - one hour and thirty minutes. Work the Solved Examples in each section, writing the answers out rather than reading them, then check your wording against the answer given and mark only the words you missed. The marking-scheme keywords are in bold in every answer for exactly this purpose. Pay particular attention to the items on where the oxygen comes from, the products of cyclic flow, the three causes of the proton gradient, and the ATP and NADPH arithmetic.

Third pass - forty-five minutes. Learn the seven-row pathway table in this section and the eight comparisons above. Then say the chain out loud in one run: light absorbed by the antennae, funnelled to the reaction centre chlorophyll a; $\mathrm{P_{680}}$ in PS II excited at $680\ \mathrm{nm}$; the electron to the acceptor, down the cytochromes to PS I; water split on the inner side of the membrane giving four protons into the lumen, oxygen and four electrons to replace those lost; $\mathrm{P_{700}}$ in PS I excited at $700\ \mathrm{nm}$; the electron to an acceptor of greater redox potential, then down to $\mathrm{NADP^+}$, giving $\mathrm{NADPH + H^+}$ by NADP reductase on the stroma side; protons gathering in the lumen from water splitting, from the H carrier and from the stroma side reduction; the gradient collapsing through $\mathrm{CF_0}$ and turning $\mathrm{CF_1}$ to make ATP; then in the stroma, $\mathrm{CO_2}$ onto $\mathrm{RuBP}$ by RuBisCO giving two 3-PGA, reduction on 2 ATP and 2 NADPH, regeneration on 1 ATP, six turns, one glucose. Then the numbers: $700$ and $680$ nanometres; 2 waters giving 4 protons, 1 oxygen and 4 electrons; 3 ATP and 2 NADPH per $\mathrm{CO_2}$; 18 ATP and 12 NADPH per glucose; 10 per cent of full sunlight; 0.03 to 0.04 per cent $\mathrm{CO_2}$ rising usefully to 0.05 per cent; $360$ and $450\ \mu\mathrm{L\,L^{-1}}$; $20^\circ$ to $25^\circ\mathrm{C}$ and $30^\circ$ to $40^\circ\mathrm{C}$; 1770, 1774, about 1854 and 1905.

Fourth pass - one hour and ten minutes. Sit the 50 NEET-Pattern Practice Questions under time, then go back only to the rows of the tables your wrong answers came from. Redo the same questions three days later; the second attempt is what tells you whether the compartments and the numbers stuck.

The night before. Read the seven-row pathway table once, then the eight comparisons, then the ten mistakes. Nothing else, and no new material. If you have time for only one thing, read down the Where column of the pathway table and say aloud which cell, which membrane and which side each stage happens on. This chapter rewards a clean recent pass far more than a long one.