Photosynthesis in Higher Plants | CBSE Class 11 Biology Notes

This note covers NCERT Class 11 Biology Chapter 11, Photosynthesis in Higher Plants: the early experiments, the chloroplast and its pigments, the light reaction and the Z scheme, chemiosmosis, the Calvin cycle, the C₄ pathway, photorespiration, and the factors that affect the rate of photosynthesis. It follows the current NCERT text section by section. Every name, date, wavelength and number is given as NCERT states it, because this chapter is examined through exact details: which photosystem, which enzyme, which cell, how many ATP.

What is photosynthesis, and why does it matter?

Green plants synthesise the food they need through photosynthesis, and are therefore called autotrophs. All other organisms that depend on green plants for food are heterotrophs.

Definition: Photosynthesis is a physico-chemical process by which green plants use light energy to drive the synthesis of organic compounds.

NCERT gives two reasons why photosynthesis is important:

• it is the primary source of all food on earth, and
• it is responsible for the release of oxygen into the atmosphere by green plants.

Simple experiments from earlier classes show that chlorophyll, light and CO₂ are required for photosynthesis.

Which early experiments built our understanding of photosynthesis?

The scientists and their contributions are asked as one-mark and matching questions. Learn the organism or material each one used.

Four bell jars. In (a) a mouse and a burning candle are in a closed bell jar; an arrow leads to (b), where the candle has gone out and the mouse lies still. In (c) a mint plant has been placed in the bell jar with the candle and the mouse; an arrow leads to (d), where the candle is still burning and the mouse is alive.

See Fig. 11.1 in your NCERT textbook

In Engelmann's experiment, what were the bacteria for?

The aerobic bacteria were used to detect the sites of O₂ evolution. They gathered where the alga released most oxygen, which was in blue and red light. The action spectrum he described roughly resembles the absorption spectra of chlorophyll a and b.

How did the equation of photosynthesis change?

By the middle of the nineteenth century it was known that plants use light energy to make carbohydrates from CO₂ and water. The empirical equation for oxygen-evolving organisms was then understood as:

CO₂ + H₂O → [CH₂O] + O₂ (in the presence of light)

where [CH₂O] represents a carbohydrate, for example glucose, a six-carbon sugar.

Van Niel, a microbiologist, expressed photosynthesis in a general form:

2H₂A + CO₂ → 2A + CH₂O + H₂O (in the presence of light)

In green plants H₂O is the hydrogen donor and is oxidised to O₂. Some organisms do not release O₂ during photosynthesis. In purple and green sulphur bacteria H₂S is the hydrogen donor, and the oxidation product is sulphur or sulphate, depending on the organism, and not O₂. From this van Niel inferred that the O₂ evolved by the green plant comes from H₂O and not from carbon dioxide. This was later proved using radioisotopic techniques. The correct overall equation is therefore:

6CO₂ + 12H₂O → C₆H₁₂O₆ + 6H₂O + 6O₂ (in the presence of light)

where C₆H₁₂O₆ represents glucose. This is not a single reaction. It is a description of a multistep process.

Note: NCERT asks why twelve molecules of water are shown as substrate. All the O₂ released comes from water. Six molecules of O₂ contain twelve oxygen atoms, so twelve molecules of H₂O are needed to supply them. Six molecules of water appear again on the product side.

Where does photosynthesis take place?

Photosynthesis takes place in the green leaves of plants, and also in other green parts. The mesophyll cells in the leaves have a large number of chloroplasts. The chloroplasts usually align themselves along the walls of the mesophyll cells so that they get the optimum quantity of incident light.

Within the chloroplast there is a membranous system consisting of grana, the stroma lamellae, and the matrix, the stroma. There is a clear division of labour.

A diagrammatic representation of an electron micrograph of a section of chloroplast. The labels are: outer membrane, inner membrane, stromal lamella, grana, stroma, ribosomes, starch granule and lipid droplet. In a drawing, show the grana as stacks joined by stromal lamellae, lying in the stroma.

See Fig. 11.2 in your NCERT textbook

Note: "Dark reaction" is a name given by convention. NCERT warns that it should not be taken to mean that these reactions occur in darkness or that they are not light-dependent. They depend on ATP and NADPH from the light reaction.

How many pigments are involved in photosynthesis?

A chromatographic separation of leaf pigments (paper chromatography) shows that the colour of leaves is due to four pigments, not one.

Pigments are substances that have an ability to absorb light at specific wavelengths. Chlorophyll a shows maximum absorption in the blue and the red regions of the spectrum, and these are also the wavelengths at which the rate of photosynthesis is higher. NCERT concludes that chlorophyll a is the chief pigment associated with photosynthesis.

All three graphs have the wavelength of light on the x-axis, from 400 to 700 nm. Graph (a) plots the absorbance of light by chloroplast pigments: chlorophyll a, chlorophyll b and the carotenoids each have their own curve; chlorophyll a and chlorophyll b each show a peak towards the blue end and a second peak towards the red end, while the carotenoids absorb only towards the blue end. Graph (b) is the action spectrum: the rate of photosynthesis, measured by O₂ release, is high in the blue region, dips in the middle of the spectrum and rises to a second peak in the red region before falling steeply near 700 nm. Graph (c) superimposes the action spectrum on the absorption spectrum of chlorophyll a: the peaks fall in the same regions, but the action spectrum stays well above the chlorophyll a curve in between, so the overlap is not one-to-one.

See Fig. 11.3 a, b and c in your NCERT textbook

Most photosynthesis takes place in the blue and red regions of the spectrum, but some takes place at the other wavelengths of the visible spectrum too. This is because of the accessory pigments. Chlorophyll is the major pigment responsible for trapping light, but other thylakoid pigments, chlorophyll b, xanthophylls and carotenoids, also absorb light and transfer the energy to chlorophyll a. The accessory pigments do two things:

• they enable a wider range of wavelengths of incoming light to be utilised for photosynthesis, and
• they protect chlorophyll a from photo-oxidation.

What is the light reaction, and what are photosystems?

The light reactions, or the photochemical phase, include four events: light absorption, water splitting, oxygen release, and the formation of the 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 Photosystem I (PS I) and Photosystem II (PS II). The LHC are made up of hundreds of pigment molecules bound to proteins.

• Each photosystem has all the pigments, except one molecule of chlorophyll a, forming a light harvesting system also called the antennae. These pigments make photosynthesis more efficient by absorbing different wavelengths of light.
• The single chlorophyll a molecule forms the reaction centre. The reaction centre is different in the two photosystems.

A cluster of pigment molecules receives a photon. The energy is passed from pigment molecule to pigment molecule to the reaction centre, and from the reaction centre an electron goes to the primary acceptor. The labels are: photon, pigment molecules, reaction centre and primary acceptor.

See Fig. 11.4 in your NCERT textbook

Note: The photosystems are named in the sequence of their discovery, and not in the sequence in which they function during the light reaction. In the light reaction PS II acts first, then PS I.

How do electrons travel in the Z scheme?

The transfer of electrons from water to NADP⁺ happens in an ordered sequence.

1. In photosystem II, the reaction centre chlorophyll a absorbs red light of 680 nm wavelength. Its electrons become excited and jump into an orbit farther from the atomic nucleus.
2. These electrons are picked up by an electron acceptor, which passes them to an electron transport system consisting of cytochromes.
3. The electrons move downhill along the electron transport chain, in terms of an oxidation-reduction or redox potential scale. They are not used up. They are passed on to the pigments of photosystem I.
4. Simultaneously, electrons in the reaction centre of PS I are excited when they receive red light of wavelength 700 nm, and are transferred to another acceptor molecule that has a greater redox potential.
5. These electrons then move downhill again, this time to a molecule of energy-rich NADP⁺. The addition of these electrons reduces NADP⁺ to NADPH + H⁺.

Definition: The Z scheme is the whole scheme of transfer of electrons, starting from PS II, uphill to the acceptor, down the electron transport chain to PS I, excitation of electrons, transfer to another acceptor, and finally downhill to NADP⁺, reducing it to NADPH + H⁺. It gets its name from its characteristic shape, which is formed when all the carriers are placed in a sequence on a redox potential scale.

The LHC of photosystem II is drawn low on the left and the LHC of photosystem I a little higher on the right; light falls on both. From the LHC of PS II an arrow rises to an e⁻ acceptor. From there an arrow slopes down through the electron transport system to the LHC of PS I, and ADP + iP is converted to ATP along this slope. From the LHC of PS I a second arrow rises to another e⁻ acceptor, and from it a short arrow leads to NADP⁺, which becomes NADPH. At the bottom, H₂O gives 2e⁻ + 2H⁺ + [O], and these electrons feed PS II. The two rises joined by the downward slope make the Z shape.

See Fig. 11.5 in your NCERT textbook

How does PS II keep supplying electrons?

The electrons that were moved from PS II must be replaced. This is achieved by electrons made available by the splitting of water. The splitting of water is associated with PS II. Water is split into 2H⁺, [O] and electrons. This creates oxygen, one of the net products of photosynthesis.

2H₂O → 4H⁺ + O₂ + 4e⁻

The electrons needed to replace those removed from photosystem I are provided by photosystem II.

The water splitting complex is associated with PS II, which is physically located on the inner side of the membrane of the thylakoid. The protons and the O₂ formed are therefore released on the inner side, into the lumen.

What is the difference between cyclic and non-cyclic photophosphorylation?

The process through which ATP is synthesised by cells, in mitochondria and chloroplasts, is named phosphorylation.

Definition: Photophosphorylation is the synthesis of ATP from ADP and inorganic phosphate in the presence of light.

A single photosystem, Photosystem I, with chlorophyll P700. Light excites an electron, which rises to the e⁻ acceptor, passes down the electron transport system and returns to P700, completing a closed loop. ADP + iP is converted to ATP as the electron passes down the electron transport system. NADP⁺ and water do not appear in the figure.

See Fig. 11.6 in your NCERT textbook

How is ATP made in the chloroplast? The chemiosmotic hypothesis

The chemiosmotic hypothesis explains how ATP is synthesised in the chloroplast. As in respiration, ATP synthesis in photosynthesis is linked to the development of a proton gradient across a membrane. Here the membranes are those of the thylakoid. There is one difference: in photosynthesis the protons accumulate towards the inside of the membrane, in the lumen. In respiration, protons accumulate in the intermembrane space of the mitochondria when electrons move through the electron transport system.

Three steps that build the proton gradient

1. The splitting of the water molecule takes place on the inner side of the membrane, so the protons produced by the splitting of water accumulate within the lumen of the thylakoids.
2. As electrons move through the photosystems, protons are transported across the membrane. 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. This molecule removes a proton from the stroma while transporting an electron. When it passes its electron to the electron carrier on the inner side of the membrane, the proton is released into the lumen.
3. The NADP reductase enzyme is located on the stroma side of the membrane. Along with the electrons that come from the acceptor of electrons of PS I, protons are necessary for the reduction of NADP⁺ to NADPH + H⁺. These protons are also removed from the stroma.

So within the chloroplast, protons in the stroma decrease in number, while protons accumulate in the lumen. This creates a proton gradient across the thylakoid membrane, and a measurable decrease in pH in the lumen.

How the gradient makes ATP

It is the breakdown of this gradient that leads to the synthesis of ATP. The gradient is broken down by the movement of protons across the membrane to the stroma, through the transmembrane channel of the CF₀ of the ATP synthase. The ATP synthase enzyme has two parts.

Definition: Chemiosmosis requires four things: a membrane, a proton pump, a proton gradient and ATP synthase.

A thylakoid is drawn as a closed membrane. The stroma outside is marked low H⁺ and the lumen inside is marked high H⁺. In the membrane, from left to right, sit PS II (P680), plastoquinone, the cytochrome b6f complex, plastocyanin (PC) and PS I (P700); light falls on both photosystems. Water is oxidised at PS II on the lumen side, giving O₂ and H⁺. H⁺ is also picked up from the stroma by plastoquinone, the H carrier, and released into the lumen near the cytochrome complex. On the stroma side of PS I, Fd and FNR reduce NADP⁺ + H⁺ to NADPH. At the bottom the ATP synthase has CF₀ in the membrane and CF₁ projecting into the stroma; H⁺ flows through it from the lumen to the stroma, and ADP + Pi is converted to ATP. An arrow shows the electrochemical potential gradient running from high in the lumen to low in the stroma.

See Fig. 11.7 in your NCERT textbook

The ATP, along with the NADPH produced by the movement of electrons, is used immediately in the biosynthetic reaction taking place in the stroma, which is responsible for fixing CO₂ and the synthesis of sugars.

Where are the ATP and NADPH used?

The products of the light reaction are ATP, NADPH and O₂. Of these, O₂ diffuses out of the chloroplast. ATP and NADPH are used to drive the processes leading to the synthesis of food, more accurately sugars. This is the biosynthetic phase of photosynthesis.

The biosynthetic phase does not directly depend on the presence of light. It depends on the products of the light reaction, ATP and NADPH, besides CO₂ and H₂O. The evidence is simple: immediately after light becomes unavailable, the biosynthetic process continues for some time and then stops. If light is then made available, the synthesis starts again. This is why calling it the "dark reaction" is a misnomer.

How was the first product of CO₂ fixation found?

Just after World War II, Melvin Calvin used radioactive ¹⁴C in algal photosynthesis studies. This led to the discovery that the first CO₂ fixation product was a 3-carbon organic acid, 3-phosphoglyceric acid (PGA). He also contributed to working out the complete biosynthetic pathway, which is called the Calvin cycle after him.

Experiments over a wide range of plants then showed another group of plants in which the first stable product of CO₂ fixation was an organic acid with 4 carbon atoms, oxaloacetic acid (OAA). CO₂ assimilation during photosynthesis is therefore of two main types.

What is the primary acceptor of CO₂?

Scientists expected that, since the first product was a C₃ acid, the primary acceptor would be a 2-carbon compound. They spent many years trying to identify one. The studies very unexpectedly showed that the acceptor molecule was a 5-carbon ketose sugar, ribulose bisphosphate (RuBP).

What are the three stages of the Calvin cycle?

Calvin and his co-workers worked out the whole pathway and showed that it operates in a cyclic manner: the RuBP is regenerated. The Calvin pathway occurs in all photosynthetic plants. It does not matter whether they have C₃ or C₄ (or any other) pathways.

1. Carboxylation. Carboxylation is the fixation of CO₂ into a stable organic intermediate. It is the most crucial step of the Calvin cycle. CO₂ is utilised for the carboxylation of RuBP, catalysed by the enzyme RuBP carboxylase, and two molecules of 3-PGA are formed. Since this enzyme also has an oxygenation activity, it is more correct to call it RuBP carboxylase-oxygenase, or RuBisCO.
2. Reduction. A series of reactions that lead to the formation of glucose. The steps involve the utilisation of 2 molecules of ATP for phosphorylation and 2 of NADPH for reduction per CO₂ molecule fixed.
3. Regeneration. Regeneration of the CO₂ acceptor molecule RuBP is crucial if the cycle is to continue uninterrupted. The regeneration steps require one ATP for phosphorylation to form RuBP.

A circle with three numbered stages. At the top right, CO₂ + H₂O from the atmosphere joins ribulose-1,5-bisphosphate at stage 1, carboxylation, giving 3-phosphoglycerate. At stage 2, reduction, ATP + NADPH enter and ADP + Pi + NADP⁺ leave, giving triose phosphate. From triose phosphate an arrow leads out of the cycle to sucrose and starch. At stage 3, regeneration, ATP enters and ADP leaves, and ribulose-1,5-bisphosphate is formed again.

See Fig. 11.8 in your NCERT textbook

Worked example: ATP and NADPH for one molecule of glucose

1. For one CO₂ fixed, reduction uses 2 ATP and 2 NADPH.
2. For the same CO₂, regeneration uses 1 ATP.
3. So each CO₂ entering the Calvin cycle needs 2 + 1 = 3 ATP and 2 NADPH.
4. The fixation of six molecules of CO₂ and 6 turns of the cycle are required to form one molecule of glucose.
5. ATP needed = 6 × 3 = 18. NADPH needed = 6 × 2 = 12.

Note: The Calvin cycle uses more ATP than NADPH: 3 ATP for every 2 NADPH. NCERT says it is probably to meet this difference that cyclic phosphorylation takes place, because cyclic flow makes ATP without making NADPH.

What is the C₄ pathway, and what is Kranz anatomy?

Plants adapted to dry tropical regions have the C₄ pathway. Although these plants have the C₄ oxaloacetic acid as the first CO₂ fixation product, they use the C₃ pathway, the Calvin cycle, as the main biosynthetic pathway. NCERT lists five ways in which C₄ plants are special:

• they have a special type of leaf anatomy,
• they tolerate higher temperatures,
• they show a response to high light intensities,
• they lack a process called photorespiration, and
• they have greater productivity of biomass.

Definition: Kranz anatomy is the leaf anatomy of C₄ plants, in which particularly large bundle sheath cells surround the vascular bundles. "Kranz" means "wreath" and reflects the arrangement of the cells.

The bundle sheath cells may form several layers around the vascular bundles. They are characterised by:

• a large number of chloroplasts,
• thick walls impervious to gaseous exchange, and
• no intercellular spaces.

NCERT's examples of C₄ plants are maize and sorghum. The presence of the bundle sheath in a vertical section of a leaf helps to identify a C₄ plant.

The Hatch and Slack pathway, step by step

The C₄ pathway is named the Hatch and Slack pathway. Like the Calvin cycle, it is a cyclic process.

1. The primary CO₂ acceptor is a 3-carbon molecule, phosphoenol pyruvate (PEP), present in the mesophyll cells. The enzyme responsible for this fixation is PEP carboxylase (PEPcase). The mesophyll cells lack the RuBisCO enzyme.
2. The C₄ acid OAA is formed in the mesophyll cells.
3. OAA then forms other 4-carbon compounds, such as malic acid or aspartic acid, in the mesophyll cells themselves. These are transported to the bundle sheath cells.
4. In the bundle sheath cells these C₄ acids are broken down to release CO₂ and a 3-carbon molecule.
5. The 3-carbon molecule is transported back to the mesophyll, where it is converted to PEP again. This completes the cycle.
6. The CO₂ released in the bundle sheath cells enters the C₃ or Calvin pathway. The bundle sheath cells are rich in RuBisCO but lack PEPcase.

Two cells are drawn one above the other, a mesophyll cell above and a bundle sheath cell below, joined by plasmodesmata. In the mesophyll cell, atmospheric CO₂ enters as HCO₃⁻ and meets phosphoenolpyruvate at the box marked Fixation, giving a C₄ acid. The C₄ acid passes down through the box marked Transport into the bundle sheath cell. There, at the box marked Decarboxylation, it gives CO₂, which goes to fixation by the Calvin cycle, and a C₃ acid. The C₃ acid is transported back up to the mesophyll cell, where Regeneration turns it into phosphoenolpyruvate again. The plasma membrane and cell wall are also labelled.

See Fig. 11.9 in your NCERT textbook

The Calvin pathway is thus common to C₃ and C₄ plants. In C₃ plants it occurs in all the mesophyll cells. In C₄ plants it does not take place in the mesophyll cells, and occurs only in the bundle sheath cells.

What is photorespiration, and why do C₄ plants not show it?

To understand photorespiration, start with the first step of the Calvin pathway, in which RuBP combines with CO₂ to form 2 molecules of 3PGA, catalysed by RuBisCO.

RuBP + CO₂ → 2 × 3PGA (catalysed by RuBisCO)

RuBisCO is the most abundant enzyme in the world. Its active site can bind to both CO₂ and O₂, hence the name carboxylase-oxygenase. RuBisCO has a much greater affinity for CO₂ when the CO₂ : O₂ ratio is nearly equal. The binding is competitive: it is the relative concentration of O₂ and CO₂ that determines which of the two will bind to the enzyme.

Definition: Photorespiration is the pathway in C₃ plants in which RuBP, instead of being converted to 2 molecules of PGA, binds with O₂ to form one molecule of phosphoglycerate and one of phosphoglycolate (a 2-carbon compound).

What NCERT says about the photorespiratory pathway:

• In C₃ plants some O₂ does bind to RuBisCO, and hence CO₂ fixation is decreased.
• There is neither synthesis of sugars nor of ATP.
• It results in the release of CO₂ with the utilisation of ATP.
• There is no synthesis of ATP or NADPH.
• The biological function of photorespiration is not known yet.

In C₄ plants photorespiration does not occur, because they have a mechanism that increases the concentration of CO₂ at the enzyme site. The C₄ acid from the mesophyll is broken down in the bundle sheath cells to release CO₂, which increases the intracellular concentration of CO₂. This ensures that RuBisCO functions as a carboxylase, minimising the oxygenase activity. This is why productivity and yields are better in C₄ plants. In addition, these plants show tolerance to higher temperatures.

How do C₃ and C₄ plants compare?

NCERT Table 11.1 is left blank for the student to fill in. The rows below are filled from the chapter text.

Note: Do not write that C₄ plants lack RuBisCO or lack the Calvin cycle. Both are present, but only in the bundle sheath cells. What the mesophyll cells of a C₄ plant lack is RuBisCO, and what the bundle sheath cells lack is PEPcase.

Which factors affect the rate of photosynthesis?

The rate of photosynthesis is very important in determining the yield of plants, including crop plants. Photosynthesis is under the influence of several factors, both internal and external.

All these factors affect the rate simultaneously, but usually one factor is the major cause, or is the one that limits the rate. At any point the rate is determined by the factor available at sub-optimal levels.

Definition: 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.

NCERT's example: despite the presence of a green leaf and optimal light and CO₂ conditions, the plant may not photosynthesise if the temperature is very low. Given the optimal temperature, the leaf will start photosynthesising.

Light

Distinguish between light quality, light intensity and the duration of exposure to light.

• At low light intensities there is a linear relationship between incident light and CO₂ fixation rates.
• 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. Hence, 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.

The x-axis is light intensity and the y-axis is the rate of photosynthesis. The curve rises from the origin as a straight line (region A), bends over (point B) and then runs flat (region C). A dashed vertical line from B meets the x-axis at D, and a dashed horizontal line at the height of the flat part meets the y-axis at E. In region A the rate rises with light, so light is the limiting factor. In region C more light gives no more photosynthesis, so some other factor is limiting. D is the light intensity at which the rate stops rising, and E is the maximum rate reached.

See Fig. 11.10 in your NCERT textbook

Carbon dioxide concentration

Carbon dioxide is the major limiting factor for photosynthesis.

• The concentration of CO₂ is very low in the atmosphere, between 0.03 and 0.04 per cent.
• An increase in concentration up to 0.05 per cent can cause an increase in CO₂ fixation rates. Beyond this the levels can become damaging over longer periods.
• At low light conditions neither C₃ nor C₄ plants respond to high CO₂ conditions. At high light intensities both show an increase in the rate of photosynthesis.
• C₄ plants show saturation at about 360 µl L⁻¹. C₃ plants respond to increased CO₂ concentration, and saturation is seen only beyond 450 µl L⁻¹. Thus the current availability of CO₂ is limiting to C₃ plants.
• Because C₃ plants respond to higher CO₂ with higher rates of photosynthesis and higher productivity, some greenhouse crops such as tomatoes and bell pepper are grown in a carbon dioxide enriched atmosphere, which leads to higher yields.

Temperature

• The dark reactions, being enzymatic, are temperature controlled. The light reactions are also temperature sensitive, but are affected to a much lesser extent.
• C₄ plants respond to higher temperatures and show a higher rate of photosynthesis, while C₃ plants have a much lower temperature optimum.
• The temperature optimum also depends on the habitat to which the plant is adapted. Tropical plants have a higher temperature optimum than plants adapted to temperate climates.

Water

Water is one of the reactants in the light reaction, but its effect as a factor is more through its effect on the plant than directly on photosynthesis.

• Water stress causes the stomata to close, which reduces the CO₂ availability.
• Water stress also makes leaves wilt, which reduces the surface area of the leaves and their metabolic activity.

How do you answer the NCERT exercise questions?

Can you tell whether a plant is C₃ or C₄ by looking at it externally? Which internal structure tells you?

Not reliably from the outside. C₄ plants are adapted to dry tropical regions, but the chapter gives no external feature that separates the two groups. The internal structure does: in a vertical section of the leaf, C₄ plants show Kranz anatomy, with large bundle sheath cells around the vascular bundles that have many chloroplasts, thick walls and no intercellular spaces.

Very few cells in a C₄ plant carry out the Calvin pathway, yet C₄ plants are highly productive. Why?

The C₄ acid brought from the mesophyll is broken down in the bundle sheath cells, which raises the CO₂ concentration at the site of RuBisCO. RuBisCO therefore works as a carboxylase and photorespiration does not occur, so no fixed carbon is lost. C₄ plants also tolerate higher temperatures and respond to high light intensities.

Why does RuBisCO carry out more carboxylation in C₄ plants?

Whether RuBisCO binds CO₂ or O₂ depends on their relative concentration. In C₄ plants RuBisCO is found in the bundle sheath cells, where the breakdown of the C₄ acid keeps the intracellular concentration of CO₂ high. This minimises the oxygenase activity.

Would a plant with chlorophyll b but no chlorophyll a carry out photosynthesis? Why do plants have accessory pigments?

No. Chlorophyll a is the chief pigment, and a chlorophyll a molecule forms the reaction centre of each photosystem. Chlorophyll b and the other accessory pigments absorb light and transfer the energy to chlorophyll a. They allow a wider range of wavelengths to be used, and protect chlorophyll a from photo-oxidation.

Figure 11.10: where is light limiting, and what do C and D represent?

Light is the limiting factor in region A, where the rate rises in a straight line with light intensity. C is the region of light saturation, where the rate does not increase further because other factors have become limiting. D is the light intensity at which the rate reaches its maximum.

Compare the anatomy of the leaf in C₃ and C₄ plants.

In C₄ leaves the vascular bundles are surrounded by particularly large bundle sheath cells, which may form several layers, with a large number of chloroplasts, thick walls impervious to gaseous exchange and no intercellular spaces (Kranz anatomy). C₃ leaves do not have this arrangement, and their Calvin cycle runs in the mesophyll cells.

Glossary

• Autotroph — An organism, such as a green plant, that synthesises the food it needs through photosynthesis.
• Action spectrum — A graph of the rate of photosynthesis at different wavelengths of light; first described by Engelmann using Cladophora.
• Accessory pigments — Chlorophyll b, xanthophylls and carotenoids, which absorb light and transfer the energy to chlorophyll a.
• Light harvesting complex (LHC) — Hundreds of pigment molecules bound to proteins, organised within PS I and PS II to absorb light.
• Reaction centre — The single chlorophyll a molecule of a photosystem, P700 in PS I and P680 in PS II.
• Z scheme — The path of electrons from PS II through the electron transport chain to PS I and on to NADP⁺, named for its shape.
• Photophosphorylation — The synthesis of ATP from ADP and inorganic phosphate in the presence of light.
• Chemiosmosis — ATP synthesis driven by the breakdown of a proton gradient across a membrane, through ATP synthase.
• RuBP — Ribulose bisphosphate, the 5-carbon ketose sugar that is the primary acceptor of CO₂ in the Calvin cycle.
• RuBisCO — RuBP carboxylase-oxygenase, the enzyme that fixes CO₂ to RuBP and can also bind O₂; the most abundant enzyme in the world.
• PGA — 3-phosphoglyceric acid, the 3-carbon acid that is the first product of CO₂ fixation in the C₃ pathway.
• PEP — Phosphoenol pyruvate, the 3-carbon primary CO₂ acceptor of the C₄ pathway, present in the mesophyll cells.
• Kranz anatomy — The leaf anatomy of C₄ plants, with large bundle sheath cells arranged like a wreath around the vascular bundles.
• Photorespiration — The pathway in C₃ plants in which RuBisCO binds O₂, forming phosphoglycerate and phosphoglycolate, with no sugar or ATP made.
• Law of Limiting Factors — Blackman's (1905) law that the rate of a process is determined by the factor nearest to its minimal value.

Common errors and misconceptions

• Misconception: The oxygen released in photosynthesis comes from CO₂. Correct: It comes from H₂O. Van Niel inferred this and it was later proved with radioisotopic techniques.
• Misconception: PS I acts first because it is numbered first. Correct: The photosystems are named in the order of their discovery. In the light reaction PS II acts first, then PS I.
• Misconception: P680 is the reaction centre of PS I. Correct: P700 is the reaction centre of PS I, and P680 is the reaction centre of PS II.
• Misconception: Cyclic photophosphorylation produces ATP and NADPH. Correct: It produces only ATP. NADPH + H⁺ is made only in non-cyclic flow.
• Misconception: In the chloroplast, protons accumulate in the stroma. Correct: They accumulate in the thylakoid lumen. Protons in the stroma decrease in number.
• Misconception: The dark reaction takes place only in darkness. Correct: It does not need light directly, but it depends on the ATP and NADPH of the light reaction and stops soon after light is removed.
• Misconception: The primary acceptor of CO₂ in C₃ plants is a 2-carbon compound. Correct: It is the 5-carbon RuBP. Scientists expected a 2-carbon compound and did not find one.
• Misconception: C₄ plants do not have the Calvin cycle. Correct: The Calvin pathway is common to all plants. In C₄ plants it runs only in the bundle sheath cells.
• Misconception: Photorespiration makes ATP, like respiration. Correct: It makes neither sugar, ATP nor NADPH. It releases CO₂ and uses ATP.
• Misconception: Light is usually the limiting factor for plants in nature. Correct: Light saturation occurs at 10 per cent of full sunlight. CO₂ is the major limiting factor.

Exam-style questions with model answers

Q1. Name the alga used by Engelmann, and state what he described with it. [1 mark]

1. Engelmann used the green alga Cladophora, with aerobic bacteria to detect the sites of O₂ evolution, and described the first action spectrum of photosynthesis.

Q2. Why is the enzyme RuBP carboxylase more correctly called RuBisCO? [2 marks]

1. The enzyme catalyses the carboxylation of RuBP, in which CO₂ is fixed to form two molecules of 3-PGA.
2. The same enzyme also has an oxygenation activity, because its active site can bind O₂ as well as CO₂. It is therefore called RuBP carboxylase-oxygenase, or RuBisCO.

Q3. Why does cyclic photophosphorylation not produce NADPH? Where is it likely to occur? [2 marks]

1. Only PS I is functional. The excited electron does not pass on to NADP⁺ but is cycled back to the PS I complex through the electron transport chain, so only ATP is synthesised.
2. It possibly occurs in the stroma lamellae, whose membranes lack PS II as well as the NADP reductase enzyme.

Q4. What are accessory pigments? State their two roles. [3 marks]

1. Accessory pigments are the thylakoid pigments other than chlorophyll a: chlorophyll b, xanthophylls and carotenoids. They absorb light and transfer the energy to chlorophyll a.
2. They enable a wider range of wavelengths of incoming light to be utilised for photosynthesis.
3. They protect chlorophyll a from photo-oxidation.

Q5. How many ATP and NADPH molecules are needed to make one molecule of glucose through the Calvin pathway? Show the working. [3 marks]

1. For each CO₂ fixed, the reduction stage uses 2 ATP and 2 NADPH, and the regeneration stage uses 1 ATP. So each CO₂ needs 3 ATP and 2 NADPH.
2. One molecule of glucose needs the fixation of 6 CO₂, that is 6 turns of the cycle.
3. ATP = 6 × 3 = 18 and NADPH = 6 × 2 = 12. So 18 ATP and 12 NADPH are required.

Q6. What is photorespiration? Why does it not occur in C₄ plants? [3 marks]

1. Photorespiration is the pathway in C₃ plants in which RuBP binds with O₂, instead of CO₂, to form one molecule of phosphoglycerate and one of phosphoglycolate.
2. No sugar, ATP or NADPH is synthesised. CO₂ is released and ATP is utilised.
3. In C₄ plants the C₄ acid from the mesophyll is broken down in the bundle sheath cells to release CO₂. This raises the intracellular CO₂ concentration, so RuBisCO functions as a carboxylase and its oxygenase activity is minimised.

Q7. State Blackman's Law of Limiting Factors and illustrate it with one example. [3 marks]

1. 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.
2. It is the factor which directly affects the process if its quantity is changed.
3. Example: despite a green leaf and optimal light and CO₂ conditions, a plant may not photosynthesise if the temperature is very low. Given the optimal temperature, the leaf starts photosynthesising.

Q8. Explain the chemiosmotic hypothesis of ATP synthesis in the chloroplast. [5 marks]

1. ATP synthesis is linked to the development of a proton gradient across the thylakoid membrane, with protons accumulating in the lumen.
2. Water is split on the inner side of the membrane, so the protons produced accumulate within the lumen.
3. As electrons move through the photosystems, an H carrier removes a proton from the stroma and releases it into the lumen. Protons are also removed from the stroma when NADP reductase, on the stroma side, reduces NADP⁺ to NADPH + H⁺.
4. The gradient is broken down when protons move to the stroma through the transmembrane channel of the CF₀ of ATP synthase, by facilitated diffusion.
5. The energy released causes a conformational change in the CF₁ particle, which makes the enzyme synthesise ATP. Chemiosmosis thus requires a membrane, a proton pump, a proton gradient and ATP synthase.

Q9. Describe the Hatch and Slack pathway of CO₂ fixation, naming the cells and enzymes involved. [5 marks]

1. The primary CO₂ acceptor is the 3-carbon phosphoenol pyruvate (PEP) in the mesophyll cells. The enzyme is PEP carboxylase (PEPcase). Mesophyll cells lack RuBisCO.
2. The 4-carbon acid OAA is formed in the mesophyll cells, and forms other 4-carbon compounds such as malic acid or aspartic acid.
3. These C₄ acids are transported to the bundle sheath cells, where they are broken down to release CO₂ and a 3-carbon molecule.
4. The 3-carbon molecule is transported back to the mesophyll and converted to PEP again, completing the cycle.
5. The CO₂ released in the bundle sheath cells enters the Calvin pathway. The bundle sheath cells are rich in RuBisCO but lack PEPcase.

Key takeaways

• The oxygen released in photosynthesis comes from water, not from carbon dioxide; van Niel inferred this from his studies of purple and green bacteria.
• Light reactions occur on the membrane system of the chloroplast and make ATP and NADPH; the carbon reactions occur in the stroma and make sugar.
• Chlorophyll a is the chief pigment; chlorophyll b, xanthophylls and carotenoids are accessory pigments that widen the range of light used and protect chlorophyll a.
• PS I has the reaction centre P700 and PS II has P680; PS II acts first, and water splitting is associated with PS II.
• Non-cyclic photophosphorylation uses both photosystems and gives ATP and NADPH; cyclic photophosphorylation uses only PS I and gives only ATP.
• ATP is made when protons accumulated in the thylakoid lumen move back to the stroma through the CF₀ channel of ATP synthase.
• The Calvin cycle has three stages, carboxylation, reduction and regeneration, and needs 18 ATP and 12 NADPH for one molecule of glucose.
• In C₄ plants PEPcase fixes CO₂ in the mesophyll cells as OAA, and the Calvin cycle runs only in the bundle sheath cells.
• Photorespiration occurs in C₃ plants when RuBisCO binds O₂; it makes no sugar, ATP or NADPH, and C₄ plants avoid it.
• By Blackman's law the factor nearest its minimal value sets the rate; CO₂ is the major limiting factor, and light saturates at 10 per cent of full sunlight.

Test yourself

Which plant did Priestley place in the bell jar, and what did he hypothesise?

Priestley used a mint plant. He hypothesised that plants restore to the air whatever breathing animals and burning candles remove.

What are the four pigments shown by a chromatographic separation of leaf pigments?

The four pigments are chlorophyll a, chlorophyll b, xanthophylls and carotenoids.

What are the reaction centres of PS I and PS II called?

The reaction centre of PS I is called P700 and the reaction centre of PS II is called P680, after their absorption peaks at 700 nm and 680 nm.

With which photosystem is the splitting of water associated?

The splitting of water is associated with PS II, which is located on the inner side of the thylakoid membrane.

Which parts of the chloroplast lack PS II and NADP reductase?

The stroma lamellae membranes lack PS II as well as the NADP reductase enzyme, so cyclic photophosphorylation possibly occurs there.

What is the primary acceptor of CO₂ in the Calvin cycle?

The primary acceptor of CO₂ in the Calvin cycle is ribulose bisphosphate (RuBP), a 5-carbon ketose sugar.

How many ATP and NADPH are required for every CO₂ molecule entering the Calvin cycle?

For every CO₂ molecule entering the Calvin cycle, 3 molecules of ATP and 2 molecules of NADPH are required.

What is the first stable product of CO₂ fixation in C₄ plants?

The first stable product of CO₂ fixation in C₄ plants is oxaloacetic acid (OAA), a 4-carbon organic acid formed in the mesophyll cells.

Which enzyme do the mesophyll cells of C₄ plants lack?

The mesophyll cells of C₄ plants lack the enzyme RuBisCO; they fix CO₂ using PEP carboxylase instead.

At what CO₂ concentrations do C₄ and C₃ plants show saturation?

C₄ plants show saturation at about 360 µl L⁻¹, while in C₃ plants saturation is seen only beyond 450 µl L⁻¹.