Exercises
By looking at a plant externally, can you tell whether a plant is C3 or C4? Why and how?
Answer
No, it is not possible to distinguish between C3 and C4 plants simply by their external appearance, because both types look like ordinary green plants and share similar general morphological features. However, there are subtle differences in leaf appearance: C4 plants (like maize, sugarcane, sorghum) are often tropical plants with narrow, erect leaves adapted to high light and temperature, while C3 plants are more common in temperate conditions. But these external features are not definitive; one must examine internal anatomy for a reliable distinction.
By looking at which internal structure of a plant can you tell whether a plant is C3 or C4? Explain.
Answer
The internal leaf anatomy (cross-section under microscope) reveals the difference. C4 plants show Kranz anatomy: the vascular bundles are surrounded by a conspicuous layer of large, thick-walled bundle sheath cells packed with chloroplasts (containing starch). These are in turn surrounded by mesophyll cells arranged radially. C3 plants lack this distinctive bundle sheath arrangement; their mesophyll cells are loosely arranged without the prominent bundle sheath. The presence or absence of Kranz anatomy thus distinguishes C4 from C3 plants.
Even though very few cells in a C4 plant carry out the biosynthetic Calvin pathway, yet they are highly productive. Can you discuss why?
Answer
In C4 plants, the initial CO2 fixation takes place in mesophyll cells via the C4 pathway using PEP carboxylase (PEP Case) as the primary carboxylating enzyme. This enzyme has a very high affinity for CO2 and can fix CO2 even at low concentrations, without any oxygenase activity. The fixed CO2 is transported (as C4 acid: oxaloacetate/malate/aspartate) to the bundle sheath cells, where it is released and refixed by RuBisCO in the Calvin cycle. This concentrating mechanism delivers CO2 at high concentrations to RuBisCO, suppressing photorespiration entirely. The elimination of the energetically wasteful photorespiration means that virtually all the CO2 captured is used productively in carbohydrate synthesis, resulting in higher productivity.
RuBisCO is an enzyme that acts both as a carboxylase and oxygenase. Why do you think RuBisCO carries out more carboxylation in C4 plants?
Answer
RuBisCO can bind either CO2 or O2 at its active site; which reaction predominates depends on the relative concentrations of the two gases. In C4 plants, the C4 pathway (operating in mesophyll cells) acts as a CO2 pump, actively concentrating CO2 in the bundle sheath cells where RuBisCO is located. The locally high CO2 concentration strongly favours the carboxylation reaction over the oxygenation reaction. Additionally, bundle sheath cells have reduced O2 levels because O2 produced during light reactions does not accumulate there as readily. Thus, the elevated CO2/O2 ratio in bundle sheath cells ensures that RuBisCO predominantly acts as a carboxylase, minimising photorespiration.
Suppose there were plants that had a high concentration of Chlorophyll b, but lacked chlorophyll a, would they carry out photosynthesis? Then why do plants have chlorophyll b and other accessory pigments?
Answer
No, such plants would not be able to carry out photosynthesis. Chlorophyll a is the primary photosynthetic pigment present at the reaction centres of both photosystems (PS I and PS II) and is the only pigment that can directly convert light energy into chemical energy by releasing energised electrons. Chlorophyll b, carotenoids, and other accessory pigments cannot directly transfer electrons to the reaction chain.
Plants have chlorophyll b and other accessory pigments because they broaden the range (spectrum) of light wavelengths that can be absorbed for photosynthesis. Chlorophyll a absorbs mainly red (680 nm and 700 nm) and blue light. Chlorophyll b absorbs at slightly different wavelengths (mainly blue-green), and carotenoids absorb blue-violet light. These accessory pigments funnel the captured light energy to chlorophyll a at the reaction centre, enabling the plant to utilise a wider spectrum of sunlight and increasing photosynthetic efficiency.
Why does the colour of a leaf kept in the dark frequently become yellow or pale green? Which pigment do you think is more stable?
Answer
Leaves contain multiple pigments: chlorophyll a (blue-green), chlorophyll b (yellow-green), xanthophylls (yellow), and carotenoids (orange). Normally, the abundant chlorophylls mask the colours of the other pigments. When a leaf is kept in the dark, chlorophyll synthesis is blocked (chlorophyll synthesis requires light) and the existing chlorophylls degrade. The chlorophylls are less stable than carotenoids and xanthophylls, so as they break down, the yellow pigments become visible, turning the leaf yellow or pale green. Carotenoids are more stable pigments since they persist even as chlorophylls degrade.
Look at leaves of the same plant on the shady side and compare with the sunny side. Which of them has leaves that are darker green? Why?
Answer
Leaves on the shady side (or plants kept in shade) are darker green. Shade leaves have a higher concentration of chlorophyll per unit area to maximise light capture in low-light conditions. They tend to be larger and broader, with more layers of palisade cells containing densely packed chloroplasts. In contrast, sun leaves may have reduced chlorophyll per cell (to avoid photoinhibition from excess light) and often have higher carotenoid content. The greater chlorophyll concentration makes shade leaves appear a deeper green.
Figure 11.10 shows the effect of light on the rate of photosynthesis. Based on the graph, answer the following questions:
Answer
(a) At which point/s (A, B or C) in the curve is light a limiting factor?
Ans. At point A (and in the low light intensity region near A), light is the limiting factor because the rate of photosynthesis increases linearly with increasing light intensity, showing that light is insufficient for maximum photosynthesis.
(b) What could be the limiting factor/s in region A?
Ans. In region A (low light intensity), light itself is the main limiting factor. Other factors such as CO2 concentration and temperature are available at adequate levels, but insufficient light restricts the rate.
(c) What do C and D represent on the curve?
Ans. Point C represents the light saturation point — beyond this light intensity, further increase in light does not increase the rate of photosynthesis (the curve flattens). At this stage, another factor (CO2 concentration or temperature) becomes limiting. Point D (if beyond C on the graph) could represent a decline in photosynthesis due to photoinhibition, where excess light causes destruction of chlorophyll molecules.
Q9. Give a comparison between the following
(a) C3 and C4 pathways
Ans. In the C3 pathway (Calvin cycle), CO2 is directly fixed by RuBisCO onto RuBP (5-carbon), producing two molecules of 3-phosphoglycerate (PGA, 3-carbon) as the first stable product. It occurs in all photosynthetic plants in the stroma of mesophyll cell chloroplasts. Photorespiration is significant. In the C4 pathway, CO2 is first fixed in mesophyll cells by PEP carboxylase onto PEP (3-carbon), producing oxaloacetate (OAA, 4-carbon) as the first product. The C4 acid is transported to bundle sheath cells where it is decarboxylated, releasing CO2 which enters the Calvin cycle. Photorespiration is negligible, and C4 plants are more efficient under high light, high temperature, and low CO2 conditions.
(b) Cyclic and non-cyclic photophosphorylation
Ans. In cyclic photophosphorylation, only PS I is involved. Electrons from P700 are excited, travel through a chain of carriers, and return to PS I without reducing NADP+. ATP is synthesised but no NADPH is produced and no O2 is evolved. In non-cyclic photophosphorylation, both PS II and PS I are involved. Electrons from PS II (P680) pass through the ETS to PS I and ultimately reduce NADP+ to NADPH. Water is split (photolysis) releasing O2, and both ATP and NADPH are produced. The electrons do not return to PS II.
(c) Anatomy of leaf in C3 and C4 plants
Ans. In C3 plants, the leaf anatomy shows mesophyll cells (palisade and spongy) and vascular bundles without a prominent bundle sheath. Chloroplasts are present in mesophyll cells. In C4 plants, the leaf shows Kranz anatomy: vascular bundles are surrounded by large, thick-walled bundle sheath cells densely packed with chloroplasts (often without grana). Mesophyll cells arranged around the bundle sheath also contain chloroplasts (with grana for light reactions). This two-cell type arrangement is unique to C4 plants.
Important Extra Questions
What is the significance of the light-harvesting complex (LHC) in photosynthesis?
Answer
The light-harvesting complex consists of hundreds of accessory pigment molecules (chlorophyll b, carotenoids, xanthophylls) arranged around a reaction centre chlorophyll a. The LHC funnels light energy of various wavelengths to the reaction centre, significantly increasing the probability of a photon being absorbed and used. This broadens the absorption spectrum and maximises light-use efficiency.
What is photorespiration and why is it considered wasteful?
Answer
Photorespiration is the light-dependent uptake of O2 and release of CO2 catalysed by RuBisCO acting as an oxygenase. RuBP reacts with O2 forming phosphoglycolate (2-carbon), which is metabolised through peroxisomes and mitochondria, consuming ATP and releasing CO2 without producing ATP or sugar. This is wasteful as it reverses carbon fixation and reduces photosynthetic productivity in C3 plants.
Why is the Calvin cycle also called the C3 cycle?
Answer
In the Calvin cycle, CO2 is fixed onto RuBP (5-carbon) by RuBisCO, producing two molecules of 3-phosphoglycerate (PGA) containing 3 carbons. Since the first stable product of CO2 fixation is a 3-carbon compound (PGA), the pathway is called the C3 cycle. It was elucidated by Melvin Calvin, hence also named after him.
Distinguish between PS I and PS II.
Answer
PS I has a reaction centre chlorophyll a called P700 (absorbs at 700 nm). It is involved in cyclic and non-cyclic photophosphorylation; its excited electrons reduce NADP+ to NADPH. PS II has a reaction centre P680 (absorbs at 680 nm). It is responsible for the photolysis of water (releasing O2) and drives non-cyclic photophosphorylation, supplying electrons via the ETS to PS I. PS II acts before PS I in the Z-scheme.
What is meant by the "Z-scheme" of light reactions?
Answer
The Z-scheme describes the pathway of non-cyclic electron flow during light reactions. Electrons released from water by PS II travel through a series of carriers (plastoquinone, cytochrome b6f complex, plastocyanin) losing energy that drives ATP synthesis, then reach PS I. After re-energisation by light in PS I, electrons are transferred to ferredoxin and then reduce NADP+ to NADPH. When the redox potentials of the carriers are plotted against the electron transport sequence, the graph forms a Z-shape, giving the scheme its name.