Ch 12Photosynthesis — Class 11 Biology, concept-first.

Photosynthesis is the only process on Earth by which solar energy is trapped by green plants and converted into food.

Photosynthesis is the only process on Earth by which solar energy is trapped by green plants and converted into food.

Light is a form of electromagnetic radiation with a dual character: quantum theory describes it as a stream of discrete particles called photons, each photon carrying one 'quantum' (packet) of light e…

In 1931, Van Niel studied photosynthetic bacteria that use hydrogen sulphide (H2S) instead of water to make carbohydrate:

In the light reaction, solar energy trapped by chlorophyll is stored chemically as ATP and as the reducing agent NADPH2, with oxygen released as water is split.

Photosystem II is the photosystem responsible for splitting water. Light absorbed by PS-II ejects electrons from its reaction centre P680, leaving P680 oxidised; the electrons released ultimately end…

Photosystem I is where the cell's reducing power is generated. When light quanta are absorbed by P700, energy-rich electrons are ejected from its reaction centre and passed down a chain of electron ca…

Photophosphorylation is simply the formation of ATP inside the chloroplast in the presence of light, and it happens in two distinct forms depending on how far the excited electron travels before its e…

In cyclic photophosphorylation, only Photosystem I is involved. When PS-I is illuminated, its reaction centre continuously ejects electrons that pass through a chain of carriers — ferredoxin, cytochro…

Non-cyclic photophosphorylation, by contrast, uses both photosystems together and produces both ATP and NADPH2.

Carbon fixation, the dark reaction, occurs in the stroma through a series of enzyme-catalysed steps that use up the ATP and NADPH2 the light reaction produced in the thylakoids.

Carboxylation is the CO2-fixing step that starts the Calvin cycle. The 5-carbon sugar ribulose-1,5-bisphosphate (RuBP) — a pentose carrying two phosphate groups — reacts with a molecule of CO2, cataly…

The reduction step, described in the textbook as 'glycolytic reversal', converts each molecule of 3-PGA into 1,3-diphosphoglyceric acid, using up a molecule of ATP; this intermediate is then reduced,…

Out of every 12 molecules of 3-phosphoglyceraldehyde (3-PGAL) produced in one turn of the cycle (from fixing 6 CO2 onto 6 RuBP), only 2 are drawn off to build a single molecule of glucose; the remaini…

Photorespiration is a wasteful process, tightly linked to the C3 (Calvin) cycle, that switches on under conditions of high temperature, bright light, high oxygen concentration and low CO2 concentratio…

M. D. Hatch and C. R. Slack, working on sugarcane, discovered that in some plants the very first stable product of CO2 fixation is not the 3-carbon compound of the ordinary Calvin cycle but a 4-carbon…

Crassulacean Acid Metabolism (CAM) is a further alternative pathway of carbon fixation, first described in the plant family Crassulaceae (from which it takes its name) and typically found in desert pl…

As with every other physiological process in a plant, photosynthesis does not proceed at a fixed rate; it is shaped by a number of external factors (conditions in the plant's environment) and internal…

Light supplies the energy that drives photosynthesis, and both its quality (wavelength/colour) and its intensity matter.

Temperature also governs the rate: with plenty of light and CO2 already available, photosynthesis speeds up as temperature rises, up to a maximum rate, after which the rate falls or declines again.

Internally, the presence of chlorophyll is essential for photosynthesis to happen at all, but the rate of photosynthesis is not strictly proportional to how much chlorophyll is present — because chlor…

Blackman's law of limiting factors states that when a process depends on the combined action of several separate factors, the rate of the process at any moment is governed by the pace of whichever one…

Photosynthesis is described as one of the most significant processes on Earth precisely because it is an anabolic (building-up) process that converts simple inorganic substances into food, supporting…