Photosynthesis in Higher Plants

Yes, sometimes. By external appearance alone, it is not definite, but plants adapted to dry tropical regions are often C₄ plants, and many C₄ plants generally show higher productivity and are often better adapted to high light and higher temperatures. However, to be sure, external appearance is not enough; internal leaf anatomy is needed.

Yes. By looking at the leaf anatomy, especially the bundle sheath and mesophyll arrangement, we can tell. C₄ plants have Kranz anatomy: large bundle sheath cells around the vascular bundles, usually in several layers, with many chloroplasts, thick walls, and no intercellular spaces. C₃ plants do not show this Kranz anatomy. So the internal structure used is the cross-section of the leaf.

C₄ plants are highly productive because they have a mechanism that concentrates CO₂ in bundle sheath cells. This increases the amount of CO₂ around RuBisCO, so RuBisCO works mainly as a carboxylase and not as an oxygenase. Therefore, photorespiration is absent in C₄ plants, and the loss of energy is reduced. Also, they tolerate high light intensity and higher temperatures, which further increases productivity.

RuBisCO carries out more carboxylation in C₄ plants because CO₂ is released in the bundle sheath cells, increasing the local concentration of CO₂ near the enzyme. This makes the CO₂:O₂ ratio high at the site of RuBisCO. Since RuBisCO has a greater affinity for CO₂ when CO₂ is relatively high, it acts mainly as a carboxylase and its oxygenase activity is minimized.

No, photosynthesis would not occur without chlorophyll a, because chlorophyll a is the chief pigment and the reaction-centre pigment directly associated with photosynthesis. Chlorophyll b and other accessory pigments do not replace chlorophyll a; instead, they absorb light of additional wavelengths and transfer the energy to chlorophyll a. They also help plants use a wider range of light and protect chlorophyll a from photo-oxidation.

A leaf kept in the dark often becomes yellow or pale green because chlorophyll breaks down in the absence of light, so the green colour fades. The carotenoids and xanthophylls are more stable pigments, so when chlorophyll decreases, their yellow colour becomes more visible. Thus, carotenoids/xanthophylls are more stable than chlorophyll.

The leaves on the sunny side are usually darker green. This is because they receive more light, so they generally have more chlorophyll and a greater capacity for photosynthesis. Leaves in shade are often lighter because of lower light intensity and less chlorophyll development.