Photosynthesis in Higher Plants

Generally, No. It is difficult to distinguish C3 and C4 plants purely by external appearance.

However: C4 plants are adapted to dry tropical regions. If a plant is thriving in very hot, dry conditions (like Maize, Sorghum, Sugarcane) without wilting, it is likely a C4 plant. But this is not a definitive test without examining internal anatomy.

You can tell by observing the Leaf Anatomy (Transverse Section).

C4 plants are more productive because:

1. CO2 Concentrating Mechanism: The mesophyll cells pump $CO_2$ (as malic acid) into the bundle sheath cells. This keeps the intracellular $CO_2$ concentration very high at the site of RuBisCO.
2. Prevention of Photorespiration: High $CO_2$ ensures RuBisCO acts only as a carboxylase (fixing carbon) and not as an oxygenase. This prevents the wasteful process of photorespiration.
3. Efficiency: They can tolerate higher temperatures and high light intensity.

RuBisCO has an active site that can bind to both $CO_2$ and $O_2$. Its activity depends on the relative concentration of these gases.

In C4 plants, the unique C4 pathway (Hatch & Slack pathway) actively transports $CO_2$ from mesophyll cells to bundle sheath cells. This creates a locally high concentration of $CO_2$ around the enzyme RuBisCO. This high $CO_2:O_2$ ratio forces the enzyme to function purely as a carboxylase, minimizing oxygenase activity.

No. They would not be able to carry out photosynthesis.

• Chlorophyll a is the primary pigment and forms the Reaction Centre where the primary photochemical reaction (ejection of electron) occurs.
• Role of Accessory Pigments (Chl b, Xanthophylls, Carotenoids):
  1. They absorb light at different wavelengths and transfer the energy to Chlorophyll a.
  2. They protect Chlorophyll a from photo-oxidation.

Yellowing (Etiolation): In the absence of light, plants cannot synthesize new Chlorophyll, and existing Chlorophyll degrades. The yellow/orange pigments (Carotenoids) are unmasked.

Stability: Carotenoids are more stable than Chlorophyll. They do not degrade as quickly in the absence of light, which is why the leaf appears yellow.

Shade Leaves (or plants in shade) are Darker Green.

Reason: Plants adapt to low light intensity by increasing the amount of chlorophyll and the size of chloroplasts to maximize light absorption. Leaves in direct sunlight often have thicker cuticles and less chlorophyll per surface area to prevent photo-oxidation, appearing lighter green.

• (a) Light is limiting: At Region A (the linear part of the curve) and partially at B. Here, increasing light increases the rate.
• (b) Limiting factors in A: Only Light Intensity is the limiting factor because the rate is directly proportional to it.
• (c) C and D:
  • C: Represents the point where light saturation is approached.
  • D: Represents the maximum rate (saturation) where light is no longer limiting. Other factors (like $CO_2$ concentration or Temperature) have become limiting.