Joseph Priestley's 1771 experiment in which a sprig of mint 'restored' air a candle had spoiled (1771)
The first clear demonstration that plants release something animals need.
Science · Biology
Photosynthesis
Biology — plant physiology and bioenergetics
Photosynthesis is the process by which plants, algae and cyanobacteria use light energy to convert carbon dioxide and water into carbohydrate, releasing oxygen as a by-product. It runs in two halves: light-dependent reactions in the thylakoid membranes, which split water and make ATP and NADPH, and the light-independent Calvin cycle in the stroma, which fixes carbon dioxide using the enzyme RuBisCO. It is the source of virtually all the oxygen in the atmosphere and of nearly all the energy in the food chain.
How to spot them
- ▸Overall equation 6CO2 + 6H2O → C6H12O6 + 6O2, driven by light
- ▸Happens in the chloroplast: thylakoid membranes stacked into grana, surrounded by stroma
- ▸Depends on the magnesium-containing green pigment chlorophyll
- ▸The oxygen released comes from water, not from carbon dioxide
- ▸Responsible for the Great Oxidation Event about 2.4 billion years ago
Key facts
Jan Ingenhousz showed in 1779 that the process needs sunlight and happens only in the green parts of a plant (1779)
He also showed plants respire in the dark, releasing carbon dioxide.
Van Helmont's willow tree, which gained 74 kg over five years while the soil lost almost nothing
He wrongly concluded the mass came from water alone; in fact most of it comes from carbon dioxide in the air.
Melvin Calvin traced the carbon-fixing pathway with carbon-14 and the 'lollipop' apparatus, winning the 1961 Nobel Prize in Chemistry (1961)
The cycle bears his name; it is also called the Calvin–Benson–Bassham cycle.
The process powered by the enzyme RuBisCO, reckoned the most abundant protein on Earth
Roughly 0.2% of all protein on the planet, and notoriously slow and error-prone.
Its waste product transformed the atmosphere 2.4 billion years ago, poisoning most life then alive
The Great Oxidation Event, caused by cyanobacteria.
C4 and CAM are the two workarounds plants evolved for its wasteful side reaction, photorespiration
CAM plants such as cacti open their stomata at night to save water.
Key ideas
- The light reactions run the 'Z-scheme': photosystem II strips electrons from water (releasing oxygen), passes them down an electron transport chain that pumps protons, and photosystem I re-energises them to reduce NADP+.
- RuBisCO's flaw is that it also reacts with oxygen instead of carbon dioxide, producing nothing useful; C4 plants such as maize and sugarcane concentrate CO2 around it to prevent this.
Quick-fire facts
- •From the Greek phos (light) and synthesis (putting together); the English term dates from the 1890s.
- •Chlorophyll absorbs red and blue light and reflects green, which is why vegetation looks green.
- •Chlorophyll's molecular structure centres on a magnesium ion; haemoglobin's very similar ring centres on iron.
- •Cornelis van Niel's work on purple sulphur bacteria predicted that the oxygen released comes from water, later confirmed with the isotope oxygen-18 by Samuel Ruben and Martin Kamen.
- •Robert Huber, Hartmut Michel and Johann Deisenhofer shared the 1988 Nobel Prize in Chemistry for the structure of the photosynthetic reaction centre.
- •Chloroplasts, like mitochondria, are endosymbionts: descendants of free-living cyanobacteria, per Lynn Margulis's theory.
- •Global primary production fixes on the order of 100 billion tonnes of carbon a year; about half of it happens in the oceans.
- •Efficiency is low — typically 1–2% of incident sunlight for crops, against a theoretical maximum of around 11%.
- •Common misconception: plants do not stop respiring in the light. They do both at once; the compensation point is where the two exactly balance.
Drill it — 9 cards
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Sources
- Peter H. Raven, Ray F. Evert and Susan E. Eichhorn — Biology of Plants
- Nick Lane — Oxygen: The Molecule that Made the World