Ch 13Photosynthesis — Class 11 Botany, concept-first.

Every organic compound on Earth ultimately traces back to plants - directly or indirectly, all life depends on them as the primary source of the carbon-based molecules that build carbohydrates, lipids…

Photosynthesis was worked out gradually, across more than three centuries, and each generation of scientists corrected or extended the previous one's picture.

Photosynthesis sits at the centre of nearly all life on Earth: it is the process by which plants, algae and some bacteria convert solar energy into the chemical energy stored in carbohydrates, releasi…

Photosynthesis is defined as a photochemical oxidation-reduction process: using light energy captured by chlorophyll, water is oxidised (loses electrons) to release oxygen, while carbon dioxide is red…

Photosynthesis performs several jobs that go far beyond simply feeding the plant that carries it out. First, photosynthetic organisms are the food source, directly or indirectly, for essentially every…

Chloroplasts are the dedicated organelle of photosynthesis, and both of the process's two main phases - the energy-yielding light reaction and the carbon-fixing dark reaction - take place inside them.…

A photosynthetic pigment is any light-absorbing molecule housed in a chloroplast (or, in bacteria, in an equivalent photosynthetic structure) that captures the light energy photosynthesis runs on.

Chlorophyll a is the primary photosynthetic pigment - it alone sits at the reaction centre and is directly responsible for the photochemistry of converting light into chemical energy, while every othe…

Carotenoids are yellow-to-orange pigments, chemically classed as tetraterpenes (almost all built from 40 carbon atoms), and they absorb light strongly in the blue-to-violet part of the visible spectru…

Phycobilins are proteinaceous, water-soluble accessory pigments that differ from both chlorophylls and carotenoids in one key structural respect: they have an open (rather than closed-ring) tetrapyrro…

The full electromagnetic spectrum spans an enormous range of wavelengths, and the narrow band of visible light plants rely on is only its smallest slice - yet the entire living world depends on that s…

A quantasome is the physically observable structure that corresponds to the photosynthetic unit - the minimum functional pool of chlorophyll and accessory pigment molecules that must act cooperatively…

Two related but distinct graphs are used to characterise how a pigment or a photosynthesising leaf responds to light of different wavelengths.

Absorption, in this context, means the complete retention of incoming light energy by a pigment molecule, as opposed to that light being reflected away or transmitted straight through.

The action spectrum measures something functionally different from the absorption spectrum: instead of asking how much light a pigment absorbs, it asks how effective each wavelength of light actually…

Three classic pieces of experimental evidence, all covered in this section, together forced photosynthesis researchers to abandon a single-photosystem model of the light reaction and adopt the two-pho…

Emerson investigated Chlorella using monochromatic light - a single wavelength at a time - and measured the resulting quantum yield of oxygen evolution across a range of wavelengths, aiming to find ex…

To follow up on the red-drop puzzle, Emerson modified his experiment by supplying two wavelengths of light together rather than one at a time: a shorter wavelength of red light (650 nm) supplied along…

In 1937, Robert Hill isolated intact chloroplasts and illuminated them in the presence of a suitable artificial electron acceptor - such as ferricyanide - but with no carbon dioxide present at all.

The modern understanding of photosynthesis frames it as one coupled oxidation-reduction process split cleanly into two phases.

The photo-oxidation phase is the first half of the light reaction, and its job is simply to capture a photon and use its energy to excite a pigment molecule's electron.

The normal, unexcited condition of an atom or molecule is called its ground state (S0). When a photon of light collides with a chlorophyll molecule, one of its outermost electrons is knocked up into a…

The photosynthetic machinery of the thylakoid membrane is organised into two distinct photosystems, Photosystem I (PS I) and Photosystem II (PS II), and each occupies a different physical location wit…

The photochemical phase is the light reaction's second half, and it is where the excitation energy generated during the photo-oxidation phase (13.9) is actually converted into usable chemical products…

Photolysis - the light-driven splitting of water - is carried out by a dedicated protein complex on Photosystem II called the Oxygen Evolving Complex (OEC), also known as the water-splitting complex,…

Once water photolysis has supplied electrons to PS II, those electrons - and the electrons PS I separately releases on absorbing light - travel through a defined sequence of four functional complexes…

Photophosphorylation is the name given specifically to ATP synthesis that is driven by electrons set in motion by light, distinguishing it from the two other ways cells make ATP: oxidative phosphoryla…

Cyclic photophosphorylation involves Photosystem I only, and gets its name because the electrons ejected from its reaction centre eventually cycle all the way back to that same reaction centre rather…

Non-cyclic photophosphorylation involves both photosystems working cooperatively in series, moving electrons one-way from water all the way to NADP+, rather than in a closed loop.

The light reaction has a definite, quantifiable photon cost. Releasing a single electron from a photosystem's reaction centre requires two quanta of light in total during non-cyclic transport: one qua…

The chemiosmotic theory, proposed by Peter Mitchell in 1966, explains the crucial missing link in the light reaction's energy story: how does moving electrons through a membrane-bound carrier chain ac…

The dark reaction (also called the biosynthetic phase, or the Photosynthetic Carbon Reduction, PCR, cycle) is where the ATP and NADPH+H+ generated during the light reaction are finally put to use: fix…

Until 1965, the Calvin (C3) cycle was believed to be the only pathway of CO2 fixation in plants. That year, Kortschak, Hart and Burr, studying sugarcane, observed that the earliest-labelled products o…

The C4 pathway's first stage takes place in the mesophyll cells, specifically in their stroma. Here, the CO2 acceptor is not RuBP (as in the C3 cycle) but a 3-carbon compound, phosphoenolpyruvate (PEP…

The C4 pathway's second stage takes place inside the bundle sheath cells, once the mesophyll-derived malic acid has arrived there.

The C4 pathway's two-cell CO2-concentrating mechanism gives C4 plants several distinct physiological advantages over C3 plants, despite its higher direct ATP cost.

Crassulacean Acid Metabolism (CAM) is a specialised carbon-fixation pathway found in succulent plants adapted to semi-arid or xerophytic (dry) habitats; it takes its name from the family Crassulaceae,…

Photorespiration is an extra, light-dependent form of respiration that occurs specifically in photosynthetic cells; Decker (1959) first noted that respiration rate is measurably higher in light than i…

Even though it represents a net loss of fixed carbon and energy, photorespiration is not simply wasteful - it delivers three genuine benefits to the plant.

The carbon dioxide compensation point is the specific atmospheric CO2 concentration, measured at a given (non-limiting) light intensity, at which a plant's rate of photosynthetic CO2 fixation exactly…

In 1860, Julius Sachs proposed that any physiological process, including photosynthesis, is governed by three cardinal points - a minimum, an optimum, and a maximum value for each factor it depends on…

Several factors external to the plant, originating in its immediate environment, can each independently limit the rate of photosynthesis.

Beyond the external environment, several factors intrinsic to the plant itself also set a practical ceiling on how fast it can photosynthesise.

Although this chapter covers it last, bacterial photosynthesis is, in evolutionary terms, the oldest form of photosynthesis known - it predates the chloroplast-based photosynthesis of plants and algae…

This closing Evaluation set draws on every major theme covered in the chapter. It opens with an assertion-reason question testing the chemiosmotic basis of ATP synthesis and the correct location of th…

Sample papers and previous-year board questions for this subject.