Photosynthesis and primary productivity definitions

Chloroplast structure and the light-dependent reactions

Thylakoid

A flattened, membrane-bound sac inside a chloroplast; the membrane holds the pigments, the electron carriers and ATP synthase.

Granum

A stack of thylakoids, giving a large area of membrane within a small volume.

Stroma

The fluid surrounding the thylakoids, containing the enzymes of the light-independent stage, along with DNA and 70S ribosomes.

Photosystem

A cluster of pigment molecules held in a thylakoid membrane, funnelling absorbed energy to the chlorophyll a at its reaction centre.

Photoionisation

The loss of an electron from a chlorophyll molecule after it absorbs light energy, leaving the molecule positively charged. Cambridge International calls the step photoactivation.

Photolysis

The splitting of water using light energy, giving electrons, protons and oxygen.

Chemiosmosis

The movement of protons down a concentration gradient through ATP synthase, driving the synthesis of ATP.

Photophosphorylation

The synthesis of ATP from ADP and inorganic phosphate using energy that originally came from light.

The Calvin cycle

Calvin cycle

The light-independent reaction of photosynthesis, in the stroma. Rubisco fixes carbon dioxide to RuBP, the GP formed is reduced to triose phosphate using ATP and reduced NADP, and most of the triose phosphate regenerates RuBP.

RuBP

Ribulose bisphosphate, a five-carbon compound; the molecule that accepts carbon dioxide.

Rubisco

RuBP carboxylase-oxygenase, the enzyme that joins carbon dioxide to RuBP. Slow, able to bind oxygen in place of carbon dioxide, and probably the most abundant protein on Earth.

GP

Glycerate 3-phosphate, a three-carbon compound; the first stable product of fixation.

TP

Triose phosphate, a three-carbon sugar phosphate; the product of reducing GP, and the molecule that either leaves the cycle or rebuilds RuBP. Pearson Edexcel calls it GALP, glyceraldehyde 3-phosphate.

Pentose

A sugar whose molecules contain five carbon atoms. Ribose, in RNA, and deoxyribose, in DNA, are the pentoses of nucleotides.

Limiting factors and primary productivity

Limiting factor

The factor, of all those a process requires, whose shortage is holding the rate back at that moment.

Plateau

The region of a rate graph where increasing the factor on the horizontal axis no longer increases the rate, because something else has become limiting.

Photosynthometer

Apparatus that collects the gas given off by an aquatic plant at the mouth of a capillary tube, so its volume can be measured against a scale.

Inverse square law

Light intensity from a small source is proportional to 1/d², where d is the distance from the source.

Hydrogencarbonate indicator

A pH indicator that shows carbon dioxide concentration: yellow when it is raised, orange-red in equilibrium with air, purple when it is lowered.

Immobilised algae

Algal cells trapped in beads of sodium alginate, so equal numbers can be put into each tube of an experiment.

Compensation point

The light intensity at which the rate of photosynthesis exactly equals the rate of respiration, so there is no net exchange of carbon dioxide or oxygen.

Compensation period

The time taken from first light for a plant to reach its compensation point. None of the A-level specifications this site follows uses the term.

Trophic level

The feeding position an organism occupies in a food chain, such as producer, primary consumer or secondary consumer.

Pyramid of numbers

A diagram of the number of individual organisms at each trophic level in an area.

Pyramid of biomass

A diagram of the dry mass of organisms at each trophic level per unit area, at one time.

Pyramid of energy

A diagram of the energy passing through each trophic level per unit area per year; it is never inverted.