Photosynthesis Trivia
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100 free Photosynthesis trivia questions with answers — science & nature quiz, new questions added Oct 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.
30 of 100 questions with answers and explanations. Play the quiz
Q 01Which ocean organisms carry out about half of all photosynthesis on Earth?
Phytoplankton
Phytoplankton are too small to see one at a time, yet these drifting specks together match the output of every forest, crop and grassland on land.
Q 02Which pigment do plants mainly use to absorb light?
Chlorophyll
Chlorophyll is so strong a colour that it hides the yellow and orange pigments sitting beside it, which is why they only show once it breaks down in autumn.
Q 03Why do most plants appear green?
Their pigment reflects green light
Their pigment reflects green light: it soaks up red and blue to power the leaf, so the colour we see is the part of sunlight the plant uses least.
Q 04Which plant tissue carries the sugars made in leaves to the roots, fruits and growing tips?
Phloem
Phloem sap is so sugary that aphids tap it for a living, drinking more than they can use and excreting the surplus as sticky honeydew.
Q 05What are the stacks of flattened thylakoid discs inside a chloroplast called?
Grana
Grana look like piles of coins under an electron microscope, and thin membrane bridges join each pile to its neighbours so that all of them work as one system.
Q 06What is the fluid that surrounds the thylakoids inside a chloroplast called?
Stroma
Stroma is where the day's spare sugar is stockpiled as starch grains, a reserve the plant then draws down through the night until dawn.
Q 07Which two energy-carrying molecules do the light-dependent reactions of photosynthesis make?
ATP and NADPH
ATP and NADPH carry the captured light energy to the second stage, which spends both to turn carbon dioxide into sugar.
Q 08What is the light-independent series of reactions that builds sugars in photosynthesis called?
The Calvin cycle
The Calvin cycle is nicknamed the dark reactions, yet it stalls in darkness, because it runs on the energy carriers that the light reactions keep supplying.
Q 09Which crusty growths on rocks and bark are a fungus living off a photosynthetic partner?
Lichens
Lichens are a fungus wrapped around algae: the algae make sugar from sunlight and feed their host, a deal that lets the pair settle on bare rock where plants cannot.
Q 10Which rainforest is nicknamed the 'lungs of the Earth', though it uses up almost all the oxygen it makes?
The Amazon
The Amazon's trees pour out oxygen by day, but the forest's animals, microbes and the trees themselves breathe nearly all of it back in, so its net gift to the air is about zero.
Q 11Which giant-flowered parasite of Southeast Asian rainforests has no leaves and cannot photosynthesise at all?
Rafflesia
Rafflesia lives as threads inside a jungle vine and steals all its food, showing itself only as a bloom up to a metre wide that reeks of rotting meat.
Q 12Which enzyme of photosynthesis is thought to be the most abundant protein on Earth?
RuBisCO
RuBisCO is a slowcoach, fixing only a few carbon dioxide molecules a second where many enzymes manage thousands, so leaves make up for it by producing it in bulk.
Q 13What wasteful side reaction occurs when the carbon-fixing enzyme binds O2 instead of CO2?
Photorespiration
Q 21Plants turn the sugar from photosynthesis into which fibre, the most abundant organic compound on Earth?
Cellulose
Cellulose is the main stuff of cotton, paper and wood, yet no human can digest it; cows and termites manage only because microbes in their guts do the job.
Q 22Which sea animals turn white and can starve when heat drives out the photosynthetic partners living in their tissues?
Corals
Corals get most of their food from the algae in their tissues, so a bleached reef has not merely lost its colour: it is going hungry.
Q 23Which 17th-century scientist grew a willow in a pot and credited its mass to water?
Jan van Helmont
Jan van Helmont found the soil had lost only about two ounces in five years; ironically the gas he overlooked, carbon dioxide, was one he had identified himself.
Photorespiration is worst on hot, dry days: a leaf sealed up to save water runs short of carbon dioxide inside, so the enzyme grabs oxygen far more often.
Q 14What are the pores in a leaf's surface that let gases in and out called?
Stomata
Stomata each sit between two guard cells that swell with water to open the pore and go limp to shut it, so a thirsty plant seals itself against further loss.
Q 15Which photosynthetic pathway do maize and sugarcane use to concentrate carbon dioxide?
C4 photosynthesis
C4 photosynthesis is named after the four-carbon acid these plants make first; the extra step costs energy but wastes far less water, which is why such crops thrive in heat.
Q 16Which fossil fuel is mostly the remains of ancient swamp forests, making it sunlight stored by photosynthesis?
Coal
Coal is swamp forest pressed into rock, so burning it releases energy that plants captured from sunlight hundreds of millions of years ago.
Q 17When do most cacti open their pores to take in carbon dioxide?
At night
At night the desert air is cooler and damper, so far less water escapes; the cactus banks the gas as an acid and turns it into sugar behind closed pores the next day.
Q 18Which metal lies at the heart of the green pigment in leaves, as iron does in the red of blood?
Magnesium
Magnesium is why a plant starved of it yellows between the leaf veins: with no metal for the pigment's core, it cannot stay green.
Q 19At sunless deep-sea vents, bacteria make food with energy from vent fluids rather than light, by what process?
Chemosynthesis
Chemosynthesis feeds giant tube worms that have no mouth or gut: bacteria packed inside their bodies make the food, in a food chain that never sees the Sun.
Q 20Oxygen released by photosynthesis went on to form which shield high in the atmosphere?
The ozone layer
The ozone layer forms when ultraviolet light splits oxygen molecules and the freed atoms join others in threes, so the shield could only build up once photosynthesis had put oxygen in the air.
Q 24Which English chemist showed a plant could restore air 'injured' by a candle or a mouse?
Joseph Priestley
Joseph Priestley later isolated oxygen itself, but it took a Dutch doctor repeating his plant experiments to show that the trick only works in sunlight.
Q 25A Venus flytrap makes its own sugar by photosynthesis, so what does it mainly gain from the insects it catches?
Nitrogen
Nitrogen is scarce in the boggy soil where the plant grows, so it takes from its prey what it needs to build proteins, while its energy still comes from sunlight.
Q 26What are organisms that make their own food, as plants do by photosynthesis, called?
Autotrophs
Autotrophs start every food chain: a grazing deer and the wolf that eats it are both living on food that a plant first made.
Q 27Which single-celled green alga, now sold as a health supplement, was used to trace how photosynthesis turns carbon dioxide into sugar?
Chlorella
Chlorella was given radioactive carbon dioxide for only a second or two before the reaction was stopped, so the very first compound the carbon entered could be caught.
Q 28Which sugar do plants use to ship the output of photosynthesis around their bodies?
Sucrose
Sucrose is what flows from the leaves to roots and fruit, and sugarcane stalks and sugar beet roots hoard so much of it that we refine them to fill the sugar bowl.
Q 29Photosystem II acts before photosystem I in the light reactions, so what were the two numbered by?
Order of discovery
Order of discovery: photosystem I was found first, and only later did experiments show that the one named second actually starts the chain.
Q 30Ruben and Kamen's heavy-oxygen tracer showed that the oxygen released in photosynthesis comes from what?
Water
Water, not carbon dioxide as was once thought: plants given water made with heavy oxygen gave off heavy oxygen gas, and plants given ordinary water did not.