Ch 11Photosynthesis in Higher Plants — Class 11 Biology, concept-first.

Before the numbered sections begin, the NCERT chapter opens with a short profile of the biochemist Melvin Calvin (born 1911), who earned a Ph.D.

Before going further, it is worth pausing to recall what earlier classes have already established about photosynthesis through a few simple, familiar experiments.

It is fascinating to trace the simple experiments that led, step by step, to our present understanding of photosynthesis.

The obvious answer to where photosynthesis happens is 'in the green leaf' or 'in the chloroplasts', based on what was learnt in Chapter 8.

Looking at plants, you may have wondered why there are so many shades of green in their leaves, even on the same plant.

Light reactions, also called the 'photochemical' phase, include several events: light absorption, water splitting, oxygen release, and the formation of the high-energy chemical intermediates ATP and N…

Everything starts at PS II, where the P680 reaction-centre pigment absorbs a photon of 680 nm red light.

A natural question arises: how does PS II supply electrons continuously, when its own electrons keep leaving for the electron transport chain? The electrons that were moved out of photosystem II must…

Living organisms have the capability of extracting energy from oxidisable substances and storing it in the form of bond energy. Special substances like ATP carry this energy in their chemical bonds.

Let us try to understand how ATP is actually synthesised in the chloroplast. The chemiosmotic hypothesis has been put forward to explain the mechanism.

We have learnt that the products of the light reaction are ATP, NADPH and O2. Of these, O2 diffuses out of the chloroplast, while ATP and NADPH are used to drive the processes leading to the synthesis…

Let us ask the question that puzzled the early scientists studying the 'dark reaction': how many carbon atoms would a molecule need so that, after accepting (fixing) CO2, it would have three carbons (…

Calvin and his co-workers worked out the whole pathway and showed that it operates in a cyclic manner, with RuBP being regenerated at the end so the cycle can continue.

Plants that are adapted to dry tropical regions have the C4 pathway mentioned earlier. Though these plants form the C4 acid oxaloacetic acid as the first CO2 fixation product, they still use the C3 pa…

Let us understand one more process that creates an important difference between C3 and C4 plants — photorespiration.

An understanding of the factors that affect photosynthesis is necessary, because the rate of photosynthesis is very important in determining the yield of plants, including crop plants.

While discussing light as a factor that affects photosynthesis, we need to distinguish between light quality, light intensity and the duration of exposure to light.

Carbon dioxide is the major limiting factor for photosynthesis. The concentration of CO2 is very low in the atmosphere — between 0.03 and 0.04 per cent.

The dark reactions, being enzymatic, are temperature controlled. Though the light reactions are also temperature sensitive, they are affected to a much lesser extent.

Even though water is one of the reactants in the light reaction, its effect as a factor is felt more through its effect on the plant than directly on photosynthesis itself.

Green plants make their own food by photosynthesis. During this process, carbon dioxide from the atmosphere is taken in by the leaves through stomata and used to make carbohydrates, principally glucos…

Higher-order thinking problems from the NCERT Exemplar.