Chapter 11 Solutions — Photosynthesis in Higher Plants

No, it is not possible to determine whether a plant is C3 or C4 by merely looking at its external appearance.

Both C3 and C4 plants look similar from the outside — they are green, bear leaves, and show no obvious structural difference visible to the naked eye.

However, there are certain external clues that may suggest C4 status:

C4 plants are typically adapted to dry, tropical, or high-light environments. Examples include maize (corn), sorghum, and sugarcane — tall, robust grasses that grow in hot, sunny regions.

C4 plants tend to have higher biomass productivity and may appear more vigorous under high-light, high-temperature conditions compared to C3 plants in the same environment.

Despite these tendencies, these are indirect indicators, not definitive proof. The only reliable way to classify a plant as C3 or C4 requires examining its internal leaf anatomy (Kranz anatomy) or biochemical tests.

When a leaf is kept in the dark for a prolonged period, it loses its green colour and becomes yellow or pale green.

Reason: Chlorophylls (both chlorophyll a and chlorophyll b) are relatively unstable pigments — they degrade and are broken down in the absence of light. Light is required for the continuous synthesis/replenishment of chlorophyll in leaves. Without light, the chlorophyll molecules are not regenerated as they break down.

Carotenoids and xanthophylls, on the other hand, are more chemically stable pigments. They do not break down as rapidly in the dark. Once chlorophyll degrades, the yellow-orange colours of carotenoids and xanthophylls are unmasked, giving the leaf its yellow or pale-yellow appearance.

This is the same process responsible for the yellowing of leaves in autumn — as chlorophyll breaks down faster than the carotenoids, the underlying yellow-orange pigments become visible.

The more stable pigment is the carotenoid (and xanthophyll group). These persist longer even when chlorophyll has broken down, revealing the yellow colour.

Leaves on the shady side (or plants kept in shade) are darker green compared to leaves on the sunny side (or plants in full sunlight).

Reason:

Plants in shade receive less light. As an adaptive response to low light availability, shade leaves develop a higher concentration of chlorophyll (and particularly chlorophyll b and accessory pigments) per unit area of leaf. This increases the leaf's light-harvesting capacity to capture as much of the available limited light as possible.

Shade leaves are also typically thinner and broader, with fewer but larger chloroplasts, maximising surface area for light absorption.

In contrast, sun leaves receive abundant light. They do not need to maximise chlorophyll content for light capture. Sun leaves tend to be smaller and thicker with more protective mechanisms (higher carotenoid content to protect against photooxidation), which dilutes the green colour.

The greater chlorophyll concentration in shade leaves gives them their characteristically darker green colour.