NCERT Solution Class 11th Biology Chapter – 13 Photosynthesis in Higher Plants Notes

NCERT Solution Class 11th Biology Chapter – 13 Photosynthesis in Higher Plants

NCERT Solution Class 11th Biology Chapter – 13 Photosynthesis in Higher Plants

?Chapter – 13?

✍Photosynthesis in Higher Plants✍

?Notes?

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

Photosynthesis takes place only in the green parts of the plants, mainly the leaves. Within the leaves, the mesophyll cells have a large number of chloroplasts that are responsible for CO₂ fixation.

Within the chloroplasts, the membranes are sites, for the light reaction, while the chemosynthetic pathway occurs in the stroma.

Photosynthesis has two stages: the light reaction and the carbon fixing reactions.

In the light reaction, the light energy is absorbed by the pigments present in the antenna and funneled to special chlorophyll a molecule called reaction center chlorophylls.

There are two photosystems, PSI and PSII. PSI has a 700nm absorbing chlorophyll-a P₇₀₀ molecule at its reaction center, while PSII has a P₆₈₀ reaction center that absorbs red light at 680 nm.

After absorbing light, electrons are excited and transferred through PSII and PSI and finally to NAD forming NADH.

During this process, a proton gradient is created across the membrane of the thylakoid. The breakdown of the gradient due to the protons being moved through the F₀ part of the ATP as the enzyme releases enough energy for the synthesis of ATP.

Splitting of water molecules is associated with PSII resulting in the release of O₂, protons, and transfer of electrons to PSII.

In the carbon fixation cycle, CO₂ is added by the enzyme Rubisco, which also catalyzes a wasteful oxygenation reaction in C₃ plants: photorespiration.

Some tropical plants show a special type of photosynthesis called the C₄ pathway. In these plants, the first product of CO₂, a fixation that takes place in the mesophyll, is a 4 Carbon compound. In the bundle sheath cells, the Calvin pathway is carried out for the synthesis of carbohydrates.

The rate of photosynthesis is affected by light intensity, carbon dioxide concentration, temperature, availability of water, and plant factors

Action spectrum – A first action spectrum of photosynthesis was thus described. It resembles roughly the absorption spectra of chlorophyll (a) and (b).

Light reactions – The former set of reactions since they are light-dependent are called light reactions.

Dark reactions – The latter, though they are dependent on products of light reaction, that is ATP and NADPH, can theoretically take place in the dark and are called dark reactions.

Chlorophyll a – Chlorophyll a is the chief pigment associated with photosynthesis.

Accessory pigments – Other thylakoid pigments like chlorophyll b, xanthophylls, and carotenoids, are called accessory pigments.

Photosystem I (PSI) and photosystem II (PSII) – The pigments are organized into two discrete photochemical light-harvesting complexes (LHC) called Photosystem I (PSI) and photosystem II (PSII).

Antennae – Each photosystem has all pigments (except one molecule of chlorophyll a) forming a light-harvesting system also called antennae.

P₇₀₀ and P₆₈₀ –  In PSI the reaction center chlorophyll a has an absorption peak at P₇₀₀ nm hence is called P₇₀₀ while in PSII it has absorption maxima at 680 nm, and is called P₆₈₀.

Z scheme – The whole scheme of transfer of electrons, starting from the PSII, Uphill to the acceptor down to the electron transport chain to PSI excitation of electrons, transfer to another acceptor, and finally downhill to NADP causing it to be reduced to NADPH⁺ H⁺. This is called the Z scheme, due to its characteristic shape.

Photorespiration – C₄ plants have a special type of leaf anatomy, they tolerate higher temperatures, they show a response to highlight intensities, they lack a process called photorespiration, and have greater productivity
of biomass.

Bundle sheath cells – The particularly large cells around the vascular bundles of the C₄, pathway plants are called bundle sheath cells.