Photosynthesis and Plant Physiology

Vocabulary and terminology regarding the process of photosynthesis, including pigments, leaf anatomy, light-dependent and light-independent cycles, and limiting factors.

Photosynthesis

The process by which plants, some bacteria, and some protistans use sunlight energy to produce glucose from carbon dioxide and water.

Photosynthesis Word Equation

$$\text{carbon dioxide} + \text{water} \rightarrow \text{glucose} + \text{oxygen}$$

Chlorophyll a

The primary pigment used by all photosynthetic organisms that absorbs energy from violet-blue and reddish orange-red wavelengths.

Accessory pigments

Pigments that absorb light energy chlorophyll a cannot, including chlorophyll b, c, d, e, xanthophylls, and carotenoids such as beta-carotene.

Chlorophyll structure

Consists of a lipid-soluble hydrocarbon tail ($C_{20}H_{39}-$) and a flat hydrophilic head with a magnesium ion at its centre, linked by an ester bond.

Xylem vessels

Specialized plant cells that transport water from the roots up to the leaves.

Stomata

Specialized structures (singular: stoma) flanked by two guard cells that allow carbon dioxide to enter and oxygen/water vapor to leave the leaf.

Cuticle

A protective waxy layer covering the leaf that prevents carbon dioxide from passing through, necessitating the use of stomata.

Thylakoid

The structural unit of photosynthesis consisting of flattened sacs or vesicles containing photosynthetic chemicals.

Grana

Collective term for the stacks of thylakoids found within chloroplasts.

Stroma

The areas between the grana in a chloroplast where light-independent reactions occur.

Photoactivation

The process where chlorophyll a absorbs light, resulting in an excited electron, the oxidation of the chlorophyll, and the splitting of water molecules.

Phosphorylation

The addition of a phosphate group to an organic compound.

Light-dependent reactions

A series of reactions occurring in the grana that require direct light energy to produce ATP and NADPH while releasing oxygen.

Photolysis

The splitting of water into oxygen, hydrogen ions, and free electrons using light energy: $2H_2O \rightarrow 4H^{+} + O_{2} + 4e^{-}$.

NADP+

Nicotinamide adenine dinucleotide phosphate, an electron carrier that is reduced to NADPH during the light-dependent reactions.

Photoionisation

The process where light energy ionises a chlorophyll molecule, freeing an electron and leaving a positively charged ion.

Photosystem

A core unit consisting of a chlorophyll molecule, an electron acceptor, and an electron donor.

Photosystem II (PSII)

Also known as P680, this photosystem occurs first in the Z scheme and was named for its chronological order of discovery.

Photosystem I (PSI)

Also known as P700, this photosystem absorbs energy most efficiently at $700\,nm$ and is involved in both cyclic and non-cyclic phosphorylation.

Z scheme

The electron transfer process named for the Z-shaped pattern formed by the energy changes accompanying light-dependent reactions.

Non-cyclic phosphorylation

The process in the thylakoid membranes where both ATP and NADPH are produced using both PSII and PSI.

Chemiosmosis

The production of ATP driven by the diffusion of H+ ions down an electrochemical gradient from the thylakoid compartment to the stroma.

Cyclic phosphorylation

A process involving only PSI that generates extra ATP by cycling electrons back to the system rather than forming NADPH.

Carbon fixation

The incorporation of carbon dioxide from the atmosphere into organic compounds during the light-independent reactions.

Ribulose 1,5-biphosphate (RuBP)

A five-carbon sugar that combines with carbon dioxide at the start of the Calvin cycle.

Glycerate 3-phosphate (GP)

The three-carbon molecule formed when the unstable six-carbon sugar resulting from RuBP and CO2 breaks down.

Glyceraldehyde 3-phosphate (GALP)

A 3-carbon molecule formed from GP using ATP and NADPH; it can be used to make glucose or reform RuBP.

Phosphoglycerate (PGA)

The first stable 3-C product of the Calvin cycle.

Limiting factors of photosynthesis

The three main variables that can restrict the rate of photosynthesis: light intensity, carbon dioxide concentration, and temperature.