Reviewing Photosynthesis: Exercises on C3 and C4 Plants, Pigments and Limiting Factors Quiz

Reviewing Photosynthesis: C3 and C4 Plants, Pigments and Limiting Factors

This quiz closes the course by turning the whole account of photosynthesis into a working review. Instead of new material, it tests whether the earlier lessons have settled into genuine understanding — the pigments and antenna complexes, the two photosystems, the Calvin cycle, the C4 pathway, photorespiration, and Blackman’s Law of Limiting Factors.

Expect three kinds of investigation:

Discrimination — telling a C3 plant from a C4 plant by external appearance or by internal leaf anatomy.
Reasoning about paradoxes — why a C4 plant is highly productive although only a few of its cells run the Calvin pathway, why RuBisCO behaves as both carboxylase and oxygenase, and what would happen to a plant with plenty of chlorophyll b but no chlorophyll a.
Observation and interpretation — why leaves kept in darkness turn yellow, why shaded leaves are often darker green, and how to read the light-intensity curve.

Each question is a small investigation. Answer without returning to earlier lessons and you have met the standard the course sets.

Q1. A student is handed two potted plants and asked to decide which one is C3 and which is C4 using only external appearance. What is the correct conclusion?

External appearance alone cannot reliably distinguish them, because the diagnostic difference lies in internal cell arrangement

The plant with narrower, more upright leaves must be the C4 plant

The plant with a more branched vein network must be the C3 plant

C4 plants can be recognised externally by the absence of stomata

Leaves of C3 and C4 plants share the same basic organisation — blade, veins, stomata and chloroplast-rich mesophyll. Narrow upright leaves are only an unreliable tendency among C4 grasses, and the real difference is the spatial division of labour visible only in a cross-section.

Q2. Which internal feature of a leaf cross-section is used to identify a C4 plant?

A ring of bundle sheath cells around the veins, with the Calvin cycle confined to them

Mesophyll cells containing no chloroplasts at all

Stomata sunk into deep pits on the lower surface

A single layer of epidermal cells with thickened cuticle

In C4 plants the Calvin cycle is confined to bundle sheath cells arranged in a ring around the vascular bundles, while the mesophyll performs the initial 4-carbon fixation. This spatial division of labour is the anatomical signature of the C4 pathway.

Q3. In a C4 plant, where does the first product of ##\ce{CO2}## fixation form, and what kind of compound is it?

In the mesophyll cells, as a 4-carbon compound

In the bundle sheath cells, as a 3-carbon compound

In the guard cells, as a 5-carbon compound

In the epidermal cells, as a 6-carbon compound

The summary states that in C4 plants the first product of ##\ce{CO2}## fixation, which takes place in the mesophyll, is a 4-carbon compound; the Calvin pathway then runs in the bundle sheath cells to make carbohydrates.

Q4. Only a small fraction of the cells in a C4 leaf run the Calvin pathway, yet C4 plants are highly productive. Which explanation is correct?

The bundle sheath cells are supplied with concentrated ##\ce{CO2}## by the mesophyll, so RuBisCO works efficiently and photorespiration is suppressed

The few bundle sheath cells divide rapidly to increase their numbers

C4 plants avoid the Calvin cycle entirely and make sugar in the mesophyll

C4 plants use chlorophyll b instead of chlorophyll a in the Calvin cycle

The mesophyll fixes ##\ce{CO2}## into a 4-carbon compound and delivers it to the bundle sheath, keeping ##\ce{CO2}## high around RuBisCO. Carboxylation therefore outcompetes the wasteful oxygenation reaction, so the limited number of Calvin-cycle cells still delivers high productivity.

Q5. RuBisCO is described as acting both as a carboxylase and an oxygenase. What does the oxygenase activity produce in C3 plants?

A wasteful oxygenation reaction known as photorespiration

Additional carbohydrate through the Calvin cycle

A 4-carbon compound for the bundle sheath

Extra ATP through cyclic photophosphorylation

The summary states that RuBisCO also catalyses a wasteful oxygenation reaction in C3 plants, called photorespiration. Carboxylation adds ##\ce{CO2}## to ##\ce{RuBP}##, whereas oxygenation diverts the enzyme into the wasteful pathway.

Q6. Why does RuBisCO carry out more carboxylation than oxygenation in C4 plants?

Because ##\ce{CO2}## is concentrated at the bundle sheath, favouring carboxylation over oxygenation

Because C4 plants contain a chemically different RuBisCO with no oxygenase site

Because C4 plants lack oxygen entirely inside their leaves

Because C4 plants carry out only cyclic photophosphorylation

The C4 pathway pumps ##\ce{CO2}## into the bundle sheath cells, raising its concentration around RuBisCO. With ##\ce{CO2}## abundant relative to ##\ce{O2}##, the carboxylase reaction dominates and photorespiration is minimised.

Q7. Suppose a plant had a high concentration of chlorophyll b but completely lacked chlorophyll a. What would be the outcome?

Photosynthesis would fail, because the reaction centre chlorophylls ##\ce{P680}## and ##\ce{P700}## are chlorophyll a molecules

Photosynthesis would proceed normally, since chlorophyll b absorbs light equally well

Photosynthesis would proceed but produce no oxygen at all

Photosynthesis would shift entirely to the C4 pathway

Light energy absorbed by antenna pigments is funnelled to special chlorophyll a molecules at the reaction centres — ##\ce{P700}## in PS I and ##\ce{P680}## in PS II. Without chlorophyll a there are no reaction centres, so the light reaction cannot run.

Q8. Why do plants possess chlorophyll b and other accessory pigments in addition to chlorophyll a?

They absorb light at wavelengths chlorophyll a absorbs poorly and funnel the energy to the reaction centre chlorophyll a

They replace chlorophyll a in the reaction centres when light is dim

They catalyse the carboxylation of ##\ce{RuBP}## in the stroma

They split water molecules at photosystem II

Accessory pigments broaden the range of wavelengths harvested and pass the energy on to the reaction centre chlorophyll a. They assist light capture; they do not replace chlorophyll a at the reaction centres or perform the chemistry of the Calvin cycle.

Q9. A leaf kept in complete darkness for several days turns yellow or pale green. What is the best explanation?

Chlorophyll is less stable and breaks down, while the more stable pigments such as carotenoids persist and show their colour

Chlorophyll is converted into chlorophyll b, which is yellow

The leaf begins fixing ##\ce{CO2}## in the dark and loses its green colour

Stomata close permanently and water loss bleaches the leaf

In darkness chlorophyll degrades while the more stable accessory pigments remain, so the leaf loses its green colour and appears yellow or pale green. This shows that chlorophyll is the less stable of the pigments.

Q10. Comparing leaves on the shady side of a tree with leaves on the sunny side, which are darker green and why?

The shaded leaves, because they build a higher chlorophyll content to capture the limited light available

The sunlit leaves, because strong light destroys their accessory pigments

The shaded leaves, because they contain no carotenoids at all

Both are identical, since light intensity does not affect pigment content

Leaves in shade are typically darker green because they accumulate more chlorophyll to harvest the weaker light. The same comparison holds for potted plants kept in shade versus those in sunlight.

Q11. On the light-intensity curve for the rate of photosynthesis, at which point is light a limiting factor?

Point A, in the steeply rising region of the curve

Point C, where the curve has levelled off

Point D, beyond the plateau

At every point on the curve equally

In the rising region (point A) the rate increases with light intensity, so light is the limiting factor. Once the curve plateaus at C and D, light is no longer limiting and other factors take over.

Q12. In region A of the light-intensity curve, apart from light itself, which factors could be limiting the rate of photosynthesis?

##\ce{CO2}## concentration and temperature

The number of stomata on the upper epidermis only

The wavelength of the moonlight

The rate of photorespiration in the bundle sheath

Blackman's Law of Limiting Factors states that the slowest factor governs the rate. In region A light is the principal limit, but ##\ce{CO2}## concentration and temperature are the other factors that can hold the rate down.

Q13. What do points C and D represent on the light-intensity curve?

The plateau where the rate levels off and light is no longer the limiting factor

The points where photosynthesis is zero

The points where light is the only limiting factor

The points where chlorophyll a is absent

At C the curve flattens and D lies on the same plateau. Further increases in light intensity no longer raise the rate, so light has ceased to be limiting and some other factor controls photosynthesis.

Q14. Which statement correctly compares the C3 and C4 pathways?

In C3 the first stable product is a 3-carbon compound formed in mesophyll cells, whereas in C4 the first product is a 4-carbon compound formed in mesophyll and the Calvin cycle runs in bundle sheath cells

In C3 the Calvin cycle runs in bundle sheath cells, whereas in C4 it runs in mesophyll cells

In C4 the first product is a 3-carbon compound, whereas in C3 it is a 4-carbon compound

Both pathways fix ##\ce{CO2}## first into a 5-carbon compound called ##\ce{RuBP}##

In the C3 pathway RuBisCO adds ##\ce{CO2}## to ##\ce{RuBP}## to give two molecules of the 3-carbon PGA inside mesophyll cells. In the C4 pathway the mesophyll first makes a 4-carbon compound, and the Calvin cycle is carried out in the bundle sheath cells.

Q15. Which comparison between cyclic and non-cyclic photophosphorylation is correct?

Non-cyclic photophosphorylation involves both PS I and PS II, splits water and releases ##\ce{O2}##, whereas cyclic photophosphorylation involves PS I only and produces ATP without ##\ce{O2}## release

Cyclic photophosphorylation involves both photosystems and releases ##\ce{O2}##, whereas non-cyclic involves PS I only

Both forms involve only PS II and both release ##\ce{O2}##

Neither form produces ATP; ATP is made only in the Calvin cycle

In non-cyclic photophosphorylation electrons flow through PS II and PS I, water is split at PS II releasing ##\ce{O2}##, protons and electrons, and both ATP and NADPH are formed. Cyclic photophosphorylation uses PS I alone and yields ATP without water splitting.