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50 free Photosynthesis trivia questions with answers — science & nature quiz, new questions added Aug 2026.
This photosynthesis trivia quiz covers how plants, algae and cyanobacteria turn light into food: chlorophyll and why leaves are green, chloroplasts, thylakoids and stroma, the light-dependent reactions and the Z scheme, ATP and NADPH, the Calvin cycle, RuBisCO and photorespiration, stomata, C4 and CAM plants, and the surprisingly low efficiency of it all. It also covers the history — van Helmont's willow, Priestley's mouse, Ingenhousz and sunlight, Senebier, Sachs and his iodine, Engelmann's bacteria, Van Niel, Emerson, Ruben and Kamen, Calvin's Nobel Prize and the Hatch-Slack pathway — plus the big picture: cyanobacteria and the Great Oxidation Event, endosymbiosis, purple bacteria, bacteriorhodopsin and a sea slug that steals chloroplasts. Questions run from easy to expert with the difficulty shown on each one. Every answer has been checked against a primary reference and each question carries its citation. Enjoy this one? Try our Biology and Plants quizzes next.
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Q 01Which form of the process, used by plants and algae, releases oxygen?
Oxygenic
The oxygen comes from splitting water, not carbon dioxide — a fact proved with radioactive isotopes by Ruben and Kamen.
Q 02Which pigment do plants mainly use to absorb light?
Chlorophyll
It absorbs red and blue light and reflects green, which is why leaves look green.
Q 03Why do most plants appear green?
Chlorophyll reflects that colour instead of absorbing it
Chlorophyll's absorption peaks are in the violet-blue and red parts of the spectrum.
Q 04In which organelle does photosynthesis take place in plants and algae?
Chloroplast
A typical plant cell holds 10 to 100 of them, and a square millimetre of leaf interior can pack up to 800,000.
Q 05What are the stacks of flattened thylakoid discs called?
Grana
Each disc is a thylakoid; the fluid around them is the stroma, where the Calvin cycle runs.
Q 06What is the fluid that surrounds the thylakoids called?
Stroma
It houses RuBisCO and the other enzymes of the light-independent reactions.
Q 07Which two energy-carrying molecules are made in the light-dependent reactions?
ATP and NADPH
They power the second stage, where carbon dioxide is captured and reduced to sugar.
Q 08What is the light-independent series of reactions that builds sugars called?
The Calvin cycle
Melvin Calvin, Andrew Benson and James Bassham traced the pathway using carbon-14 and paper chromatography.
Q 09Which enzyme captures carbon dioxide from the air in the light-independent reactions?
RuBisCO
It is probably the most abundant protein on Earth, yet it is remarkably slow, fixing only 3 to 10 CO2 molecules a second.
Q 10How many carbons has the sugar to which CO2 is first attached in the dark reactions?
Five
Ribulose 1,5-bisphosphate plus CO2 splits into two three-carbon molecules of 3-phosphoglycerate.
Q 11Roughly how many CO2 molecules per second can one carbon-fixing enzyme molecule fix?
3 to 10
Many enzymes manage thousands of reactions a second, which makes RuBisCO the rate-limiting step of the Calvin cycle by day.
Q 12Which of these is described as probably the most abundant protein on the planet?
RuBisCO
It fills the stroma of every chloroplast in C3 plants.
Q 13What wasteful side reaction occurs when the carbon-fixing enzyme binds O2 instead of CO2?
Photorespiration
It produces toxic phosphoglycolate and can lose the plant both carbon and nitrogen.
Q 21Roughly how much carbon do photosynthetic organisms turn into biomass each year?
100 to 115 billion tonnes
That is 91 to 104 petagrams, most of it by ocean plankton and land plants combined.
Q 22Who is credited with discovering photosynthesis in 1779 by showing plants need light?
Jan Ingenhousz
He repeated Priestley's experiments and found sunlight let a plant revive 'injured' air within hours.
Q 23Which 17th-century scientist grew a willow in a pot and credited its mass to water?
Jan van Helmont
The soil barely changed weight; he was half right, since much of the mass actually comes from carbon dioxide.
Q 14What are the pores in a leaf's surface that let gases in and out called?
Stomata
The word is Greek for mouth; each pore is flanked by a pair of guard cells that open and close it.
Q 15Which photosynthetic pathway do maize and sugarcane use to concentrate carbon dioxide?
C4 photosynthesis
They fix CO2 into a four-carbon acid in mesophyll cells and shuttle it to bundle-sheath cells where RuBisCO waits.
Q 16Which two scientists gave their names to the C4 pathway discovered in the 1960s?
Hatch and Slack
About 8,100 species, 3% of land plants, use it — but they account for about 23% of terrestrial carbon fixation.
Q 17Cacti and many succulents open their leaf pores only at night, using which pathway?
CAM photosynthesis
They store the night's CO2 as malic acid and process it by day, separating the two steps in time rather than space.
Q 18Which discovery in 1906 was the first detection of that element in living tissue?
Magnesium in chlorophyll
Richard Willstätter's work from 1905 laid the ground for Hans Fischer to solve chlorophyll a's structure in 1940.
Q 19With what efficiency do plants typically convert light into chemical energy?
3 to 6 percent
Mass-produced solar panels manage roughly 6 to 20 percent; the rest of the absorbed light is mostly lost as heat.
Q 20Global photosynthesis captures energy at about what average rate?
130 terawatts
That is roughly eight times humanity's total power consumption.
Q 24Which English chemist showed a plant could restore air 'injured' by a candle or a mouse?
Joseph Priestley
He isolated oxygen itself in 1774, but missed the crucial role of sunlight.
Q 25Which Swiss pastor showed in 1796 that green plants take in CO2 and give off O2 in light?
Jean Senebier
Nicolas-Théodore de Saussure soon added that water, not just CO2, contributes to a plant's growing mass.
Q 26Who won the 1961 Nobel Prize in Chemistry for working out the carbon-fixation cycle?
Melvin Calvin
The pathway is often called the Calvin-Benson-Bassham cycle after his collaborators.
Q 27Which organism did Calvin's team use to trace carbon fixation with carbon-14?
The alga Chlorella
Within a fraction of a second of light, the CO2 turned up in a three-carbon molecule, phosphoglyceric acid.
Q 28Who first showed the process is a light-driven redox reaction, via purple sulfur bacteria?
Cornelis Van Niel
His general equation put 'H2A' as the electron donor — water in plants, hydrogen sulfide in his bacteria.
Q 29What did Robert Emerson's experiments with red and blue light reveal?
That there are two photosystems
Red alone suppressed the reaction; red plus blue boosted it, pointing to PSII (up to 600 nm) and PSI (up to 700 nm).
Q 30What did Samuel Ruben and Martin Kamen prove using radioactive isotopes?
The oxygen released comes from water
Robert Hill's isolated-chloroplast experiments of 1937-39 had pointed the same way.