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NCERT Solutions · Class 11 Biology Photosynthesis in Higher Plants

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Exercises 11.1–11.9

  1. Exercise 11.1

    By looking at a plant externally, can you tell whether a plant is C3\displaystyle C_{3} or C4\displaystyle C_{4}? Why and how?

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    No — you cannot tell a \(\displaystyle C_{3}\) plant from a \(\displaystyle C_{4}\) plant by looking at it externally.
    The features that separate the two groups are internal: the special 'Kranz' anatomy of the \(\displaystyle C_{4}\) leaf, which is visible only in a vertical section of the leaf under a microscope.
    The chapter's own test is exactly this — cut vertical sections of leaves of different plants and look for the bundle sheath around the vascular bundles; its presence identifies a \(\displaystyle C_{4}\) plant.
    The remaining \(\displaystyle C_{4}\) characters are physiological and equally invisible from outside: tolerance of higher temperatures, response to high light intensities, absence of photorespiration and greater productivity of biomass.
    At most the habitat is a weak hint — \(\displaystyle C_{4}\) plants are those adapted to dry tropical regions (maize, sorghum) — but this is not a reliable external test.

  2. Exercise 11.2

    By looking at which internal structure of a plant you can tell whether a plant is C3\displaystyle C_{3} or C4\displaystyle C_{4}? Explain.

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    The internal structure to look at is the leaf anatomy in vertical section — specifically the 'Kranz' anatomy of the \(\displaystyle C_{4}\) leaf.
    Bundle sheath cells: in \(\displaystyle C_{4}\) plants the cells around the vascular bundles are particularly large, may form several layers, and are characterised by a large number of chloroplasts, thick walls impervious to gaseous exchange, and no intercellular spaces.
    'Kranz' means 'wreath', and the name reflects this wreath-like arrangement of cells around the vascular bundle.
    A \(\displaystyle C_{3}\) leaf has no such specialised sheath; its \(\displaystyle CO_{2}\) is fixed in one cell type only, the mesophyll (palisade and spongy), whereas the \(\displaystyle C_{4}\) leaf has two \(\displaystyle CO_{2}\)-fixing cell types, mesophyll and bundle sheath.
    In \(\displaystyle C_{4}\) plants the Calvin cycle runs only in the bundle sheath cells; in \(\displaystyle C_{3}\) plants it runs in all the mesophyll cells.
    To see it for yourself, section a leaf of maize or sorghum and observe the bundle sheath and the distribution of mesophyll cells.

  3. Exercise 11.3

    Even though a very few cells in a C4\displaystyle C_{4} plant carry out the biosynthetic - Calvin pathway, yet they are highly productive. Can you discuss why?

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    Because in \(\displaystyle C_{4}\) plants those few cells — the bundle sheath cells — run the Calvin cycle at a high internal \(\displaystyle CO_{2}\) concentration, so photorespiration does not occur and almost nothing that is fixed is wasted.
    The mesophyll cells act as a \(\displaystyle CO_{2}\) pump: PEPcase fixes \(\displaystyle CO_{2}\) into \(\displaystyle C_{4}\) acids (malic or aspartic acid), which are transported into the bundle sheath and broken down there to release \(\displaystyle CO_{2}\).
    This raises the intracellular \(\displaystyle CO_{2}\) concentration at the enzyme site, so RuBisCO functions as a carboxylase and its oxygenase activity is minimised.
    Photorespiration would otherwise make neither sugar nor ATP — it releases \(\displaystyle CO_{2}\) and consumes ATP. Avoiding it means the whole output of the bundle sheath cells goes into sugar.
    The thick walls of the bundle sheath cells are impervious to gaseous exchange and there are no intercellular spaces, so the released \(\displaystyle CO_{2}\) stays concentrated where RuBisCO is.
    Added to this, \(\displaystyle C_{4}\) plants tolerate higher temperatures and respond to high light intensities — together these give them greater productivity of biomass and better yields.

  4. Exercise 11.4

    RuBisCO is an enzyme that acts both as a carboxylase and oxygenase. Why do you think RuBisCO carries out more carboxylation in C4\displaystyle C_{4} plants?

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    Because \(\displaystyle C_{4}\) plants concentrate \(\displaystyle CO_{2}\) at the site of the enzyme, so RuBisCO almost always meets \(\displaystyle CO_{2}\) rather than \(\displaystyle O_{2}\).
    The active site of RuBisCO binds both \(\displaystyle CO_{2}\) and \(\displaystyle O_{2}\), and this binding is competitive — it is the relative concentration of \(\displaystyle O_{2}\) and \(\displaystyle CO_{2}\) that decides which one binds. RuBisCO has a much greater affinity for \(\displaystyle CO_{2}\) when \(\displaystyle CO_{2}\):\(\displaystyle O_{2}\) is nearly equal.
    In a \(\displaystyle C_{4}\) plant the \(\displaystyle C_{4}\) acid brought from the mesophyll is broken down in the bundle sheath cells to release \(\displaystyle CO_{2}\), raising the intracellular \(\displaystyle CO_{2}\) concentration there.
    With \(\displaystyle CO_{2}\) in excess, the equilibrium of the competition shifts: RuBisCO works as a carboxylase and its oxygenase activity is minimised.
    The bundle sheath cells help hold this advantage — their thick walls are impervious to gaseous exchange and they have no intercellular spaces, so the \(\displaystyle CO_{2}\) released inside is not lost.
    Consequence: photorespiration does not occur in \(\displaystyle C_{4}\) plants, whereas in \(\displaystyle C_{3}\) plants some \(\displaystyle O_{2}\) does bind RuBisCO and \(\displaystyle CO_{2}\) fixation is decreased.

  5. Exercise 11.5

    Suppose there were plants that had a high concentration of Chlorophyll b, but lacked chlorophyll a, would it carry out photosynthesis? Then why do plants have chlorophyll b and other accessory pigments?

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    No — such a plant could not carry out photosynthesis, because chlorophyll a is the chief pigment associated with photosynthesis and it would be missing.
    Each photosystem has all its pigments forming the light harvesting antenna except one molecule of chlorophyll a, and that single molecule forms the reaction centre — P700 in PS I (absorption peak $\displaystyle 700$ nm) and P680 in PS II ($\displaystyle 680$ nm).
    Chlorophyll b is only an accessory pigment: it absorbs light and transfers the energy to chlorophyll a. With no chlorophyll a there is no reaction centre to receive that energy, no electron is excited, and the light reaction — and hence ATP and NADPH formation — cannot begin.
    Why plants nevertheless keep chlorophyll b, xanthophylls and carotenoids:
    They absorb wavelengths that chlorophyll a does not, so a wider range of wavelengths of incoming light can be used for photosynthesis — which is why some photosynthesis occurs outside the blue and red regions.
    They protect chlorophyll a from photo-oxidation.
    This is also why the absorption spectrum of chlorophyll a and the action spectrum of photosynthesis do not show a complete one-to-one overlap.

  6. Exercise 11.6

    Why is the colour of a leaf kept in the dark frequently becomes yellow, or pale green? Which pigment do you think is more stable?

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    The leaf turns yellow or pale green because the green chlorophyll is lost in the dark, leaving the yellow pigments of the leaf unmasked.
    The colour of a leaf is not due to one pigment but to four: chlorophyll a (bright or blue-green), chlorophyll b (yellow-green), xanthophylls (yellow) and carotenoids (yellow to yellow-orange).
    Normally the abundant chlorophylls dominate and hide the yellow pigments; as chlorophyll disappears, the leaf fades first to pale green and then to the yellow of the remaining pigments.
    The more stable pigments are the carotenoids and xanthophylls — they survive when chlorophyll has gone. Chlorophyll is the unstable one; the chapter notes that it is broken down even by an increase in incident light beyond a point.
    Note: the chapter does not itself describe what happens to chlorophyll in darkness — the stability comparison here follows from its account of the four leaf pigments and their colours.

  7. Exercise 11.7

    Look at leaves of the same plant on the shady side and compare it with the leaves on the sunny side. Or, compare the potted plants kept in the sunlight with those in the shade. Which of them has leaves that are darker green ? Why?

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    The leaves on the shady side (and the potted plants kept in the shade) are the darker green.
    In shade, light is at a sub-optimal level — light saturation occurs at only about $\displaystyle 10$ per cent of full sunlight, so light is rarely a limiting factor in nature except for plants in shade or in dense forests. A shade leaf compensates by holding more chlorophyll, which deepens the green.
    On the sunny side the opposite happens: an increase in incident light beyond a point causes the breakdown of chlorophyll and a decrease in photosynthesis, so sun leaves carry less chlorophyll and look paler.
    The amount of chlorophyll is itself one of the plant (internal) factors that affect the rate of photosynthesis, along with the number, size, age and orientation of leaves.
    Darker green does not mean a higher rate: by the law of limiting factors, the shaded plant's rate is set by the factor nearest its minimum, which for it is light.

  8. Exercise 11.8

    Figure 11.10\displaystyle 11.10 shows the effect of light on the rate of photosynthesis. Based on the graph, answer the following questions:
    NCERT_Question_Class11_Biology_Ch11_Q11-8
    (a)
    At which point/s (A, B or C) in the curve light is a limiting factor?
    (b)
    What could be the limiting factor/s in region A?
    (c)
    What do C and D represent on the curve?

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    (a) A. Light is the limiting factor only along A, the straight rising limb of the curve. Here the chapter's linear relationship between incident light and \(\displaystyle CO_{2}\) fixation rates at low light intensities holds — every rise in light intensity is matched by a rise in the rate, which is what "limiting" means.
    Not B: at B the curve has already begun to bend away from that straight line, so light is ceasing to limit. Not C: C is the flat stretch, where the rate no longer rises with light at all.
    (b) Light itself — its intensity — is the limiting factor in region A. A is the double-headed arrow spanning the linear rising limb, so the rate over that stretch is set by how much light is available.
    By Blackman's Law of Limiting Factors the rate is determined by the factor nearest its minimal value; in region A that factor is light, while \(\displaystyle CO_{2}\), temperature and water are not the ones holding the rate back.
    (c) C is the light-saturation plateau — the stretch over which increasing the light intensity gives no further increase in the rate, because, in the chapter's words, other factors have become limiting: chiefly \(\displaystyle CO_{2}\) concentration, the major limiting factor for photosynthesis at only $\displaystyle 0.03$–$\displaystyle 0.04$ per cent of the atmosphere, and also temperature and water.
    D is a point on the light-intensity (x) axis, not a point on the curve. The dashed vertical dropped from the knee of the curve meets the axis at D, so D is the light intensity at which light saturation is reached — the chapter puts this at about $\displaystyle 10$ per cent of full sunlight, which is why light is rarely limiting in nature except for plants in shade or in dense forests.
    The companion label E (not asked for) sits where the dashed horizontal from the plateau meets the rate axis: the saturated, maximum rate of photosynthesis.
    The chapter adds that an increase in incident light beyond a point causes the breakdown of chlorophyll and a decrease in photosynthesis; the graph is not drawn far enough to the right to show that fall.

  9. Exercise 11.9

    Give comparison between the following:
    (a)
    C3\displaystyle C_{3} and C4\displaystyle C_{4} pathways
    (b)
    Cyclic and non-cyclic photophosphorylation
    (c)
    Anatomy of leaf in C3\displaystyle C_{3} and C4\displaystyle C_{4} plants

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    (a) \(\displaystyle C_{3}\) and \(\displaystyle C_{4}\) pathways
    Primary \(\displaystyle CO_{2}\) acceptor: RuBP, a $\displaystyle 5$-carbon ketose sugar, in \(\displaystyle C_{3}\); PEP (phosphoenol pyruvate), a $\displaystyle 3$-carbon molecule, in \(\displaystyle C_{4}\).
    Enzyme of the first fixation: RuBisCO in \(\displaystyle C_{3}\); PEPcase in \(\displaystyle C_{4}\) (the \(\displaystyle C_{4}\) mesophyll lacks RuBisCO).
    First stable product: $\displaystyle 3$-PGA, a $\displaystyle 3$-carbon acid, in \(\displaystyle C_{3}\); OAA, a $\displaystyle 4$-carbon acid, in \(\displaystyle C_{4}\).
    Site of the Calvin cycle: all the mesophyll cells in \(\displaystyle C_{3}\); only the bundle sheath cells in \(\displaystyle C_{4}\).
    Cell types that fix \(\displaystyle CO_{2}\): one (mesophyll) in \(\displaystyle C_{3}\); two (mesophyll and bundle sheath) in \(\displaystyle C_{4}\).
    Names: the \(\displaystyle C_{4}\) route is the Hatch and Slack pathway; the Calvin cycle itself is common to both groups.
    Photorespiration: present in \(\displaystyle C_{3}\) plants, where \(\displaystyle O_{2}\) competes with \(\displaystyle CO_{2}\) at RuBisCO; absent in \(\displaystyle C_{4}\) plants, which raise the \(\displaystyle CO_{2}\) concentration at the enzyme site.
    Temperature: \(\displaystyle C_{4}\) plants respond to higher temperatures (optimum about $\displaystyle 30$–$\displaystyle 40$ °C); \(\displaystyle C_{3}\) plants have a much lower optimum (about $\displaystyle 20$–$\displaystyle 25$ °C).
    \(\displaystyle CO_{2}\) response: \(\displaystyle C_{4}\) plants saturate at about $\displaystyle 360$ μ\(\displaystyle lL^{-1}\), \(\displaystyle C_{3}\) plants only beyond $\displaystyle 450$ μ\(\displaystyle lL^{-1}\) — so present atmospheric \(\displaystyle CO_{2}\) is limiting to \(\displaystyle C_{3}\) plants (which is why greenhouse tomato and bell pepper are grown in \(\displaystyle CO_{2}\)-enriched air).
    Productivity: greater biomass and yield in \(\displaystyle C_{4}\) plants, e.g. maize and sorghum, which are plants adapted to dry tropical regions.
    (b) Cyclic and non-cyclic photophosphorylation
    Photosystems involved: only PS I works in cyclic; PS II and then PS I in series (the Z scheme) work in non-cyclic.
    Path of the electron: it is cycled back to the PS I complex through the electron transport chain in cyclic; it flows one way from water to \(\displaystyle NADP^{+}\) and is not returned to its donor in non-cyclic.
    Products: only ATP in cyclic; both ATP and NADPH + \(\displaystyle H^{+}\) in non-cyclic.
    Splitting of water: does not occur in cyclic, so no \(\displaystyle O_{2}\) is released; occurs in non-cyclic, where water is split into \(\displaystyle 2H^{+}\), [O] and electrons and \(\displaystyle O_{2}\) is evolved.
    Source of the electron: PS I's own electron in cyclic; water in non-cyclic.
    Site: the stroma lamellae, which lack PS II and the NADP reductase enzyme, for cyclic; the grana lamellae, which have both PS I and PS II, for non-cyclic.
    When it happens: cyclic photophosphorylation also occurs when only light of wavelengths beyond $\displaystyle 680$ nm is available for excitation; it makes up the extra ATP needed, since the Calvin cycle uses $\displaystyle 3$ ATP but only $\displaystyle 2$ NADPH per \(\displaystyle CO_{2}\) fixed.
    (c) Anatomy of leaf in \(\displaystyle C_{3}\) and \(\displaystyle C_{4}\) plants
    \(\displaystyle C_{4}\) leaves show 'Kranz' anatomy; \(\displaystyle C_{3}\) leaves do not. 'Kranz' means 'wreath' and describes the arrangement of cells around the vascular bundle.
    Bundle sheath: in \(\displaystyle C_{4}\) plants the cells around the vascular bundles are particularly large, may form several layers, and have a large number of chloroplasts, thick walls impervious to gaseous exchange and no intercellular spaces. A \(\displaystyle C_{3}\) leaf has no such chloroplast-rich, specialised sheath.
    Mesophyll: in a \(\displaystyle C_{3}\) leaf it is differentiated into palisade and spongy tissue and every mesophyll cell carries out the whole Calvin cycle; in a \(\displaystyle C_{4}\) leaf the mesophyll performs only the initial carboxylation by PEPcase and hands \(\displaystyle C_{4}\) acids to the bundle sheath.
    Distribution of enzymes: \(\displaystyle C_{4}\) mesophyll has PEPcase but lacks RuBisCO, while the bundle sheath is rich in RuBisCO but lacks PEPcase; in \(\displaystyle C_{3}\) leaves RuBisCO is present in the mesophyll cells.
    Chloroplasts: confined to the mesophyll in \(\displaystyle C_{3}\) leaves; present in both the mesophyll and the bundle sheath cells in \(\displaystyle C_{4}\) leaves.
    The presence of the bundle sheath around the vascular bundles, seen in a vertical section under the microscope, is the practical way to identify a \(\displaystyle C_{4}\) plant such as maize or sorghum.