100 Fun Facts About Photosynthesis

Which ocean organisms carry out about half of all photosynthesis on Earth?

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.

Which pigment do plants mainly use to absorb light?

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.

Why do most plants appear green?

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.

Which plant tissue carries the sugars made in leaves to the roots, fruits and growing tips?

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.

What are the stacks of flattened thylakoid discs inside a chloroplast called?

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.

What is the fluid that surrounds the thylakoids inside a chloroplast called?

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.

Which two energy-carrying molecules do the light-dependent reactions of photosynthesis make?

ATP and NADPH carry the captured light energy to the second stage, which spends both to turn carbon dioxide into sugar.

What is the light-independent series of reactions that builds sugars in photosynthesis called?

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.

Which crusty growths on rocks and bark are a fungus living off a photosynthetic partner?

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.

Which rainforest is nicknamed the 'lungs of the Earth', though it uses up almost all the oxygen it makes?

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.

Which giant-flowered parasite of Southeast Asian rainforests has no leaves and cannot photosynthesise at all?

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.

Which enzyme of photosynthesis is thought to be the most abundant protein on Earth?

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.

What wasteful side reaction occurs when the carbon-fixing enzyme binds O2 instead of CO2?

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.

What are the pores in a leaf's surface that let gases in and out called?

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.

Which photosynthetic pathway do maize and sugarcane use to concentrate carbon dioxide?

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.

Which fossil fuel is mostly the remains of ancient swamp forests, making it sunlight stored by photosynthesis?

Coal is swamp forest pressed into rock, so burning it releases energy that plants captured from sunlight hundreds of millions of years ago.

When do most cacti open their pores to take in carbon dioxide?

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.

Which metal lies at the heart of the green pigment in leaves, as iron does in the red of blood?

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.

At sunless deep-sea vents, bacteria make food with energy from vent fluids rather than light, by what process?

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.

Oxygen released by photosynthesis went on to form which shield high in the atmosphere?

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.

Plants turn the sugar from photosynthesis into which fibre, the most abundant organic compound on Earth?

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.

Which sea animals turn white and can starve when heat drives out the photosynthetic partners living in their tissues?

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.

Which 17th-century scientist grew a willow in a pot and credited its mass to water?

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.

Which English chemist showed a plant could restore air 'injured' by a candle or a mouse?

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.

A Venus flytrap makes its own sugar by photosynthesis, so what does it mainly gain from the insects it catches?

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.

What are organisms that make their own food, as plants do by photosynthesis, called?

Autotrophs start every food chain: a grazing deer and the wolf that eats it are both living on food that a plant first made.

Which single-celled green alga, now sold as a health supplement, was used to trace how photosynthesis turns carbon dioxide into sugar?

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.

Which sugar do plants use to ship the output of photosynthesis around their bodies?

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.

Photosystem II acts before photosystem I in the light reactions, so what were the two numbered by?

Order of discovery: photosystem I was found first, and only later did experiments show that the one named second actually starts the chain.

Ruben and Kamen's heavy-oxygen tracer showed that the oxygen released in photosynthesis comes from what?

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.

The word photosynthesis joins Greek words meaning 'light' and what?

Putting together is the sense of 'synthesis'. The American botanist who called for a new name in 1893 also put forward 'photosyntax', but that version never caught on.

Which stain did Julius von Sachs use to show that sunlit leaves make starch?

Iodine turns starch blue-black, so a leaf that has been in sunlight stains dark while one kept in the dark stays pale: proof that light is what makes the starch.

What do oceanographers call the top layer of the sea, where enough light arrives for photosynthesis?

The photic zone rarely reaches below 200 metres, a thin skin on a sea several kilometres deep, yet it is home to the great majority of ocean life.

Which organisms produced the oxygen that transformed Earth's early atmosphere?

Cyanobacteria's oxygen first rusted the iron dissolved in the seas, laying down the banded ore that is mined for steel today, and only then began to build up in the air.

The first oxygen photosynthesis added to the air, some 2.4 billion years ago, is thought to have triggered what?

An ice age is thought to have set in as the new oxygen destroyed methane, the greenhouse gas that had kept the young planet warm; the upheaval is known as the Great Oxidation Event.

Which layered rock mounds, built up by mats of photosynthetic microbes, still grow in Shark Bay, Australia?

Stromatolites like these are among the oldest traces of life, found in rocks about 3.5 billion years old, so Shark Bay's shallows show roughly what the first living shores looked like.

According to endosymbiotic theory, what were chloroplasts originally?

Photosynthetic bacteria engulfed by early cells never quite lost their independence: chloroplasts still carry their own loop of DNA, and a cell that loses them cannot make new ones.

Which kind of animal is Elysia chlorotica, which steals chloroplasts from the algae it eats and then photosynthesises for months?

A sea slug: it pierces an algal thread and sucks out the contents as if through a straw, then keeps the green parts working in the lining of its own gut, turning bright green itself.

Which gas do purple bacteria in airless lake water use instead of water when they photosynthesise?

Hydrogen sulfide gives up its hydrogen to feed the reaction, and the sulfur left behind is stored by many of these bacteria as tiny grains inside their cells.

Purple microbes in salt lakes harvest sunlight with a pigment that works much like one humans use for what?

Vision: both pigments hold retinal, a molecule that changes shape when light strikes it. Our eyes read that twitch as a signal, while the microbes use it to pump protons for energy.

The pigment phycoerythrin soaks up blue-green light, giving which group of seaweeds its colour?

Red algae are often found deeper than other seaweeds, and the pigment has long been given as the reason: blue and green light reach furthest down, while red is the first colour the water absorbs.

What are the yellow, orange and red accessory pigments that help plants harvest light called?

Carotenoids are named after the carrot, and flamingos owe their pink to the ones in the plankton they eat: zoo birds given too little in their feed fade to pale.

Which six-carbon sugar is the end product in the textbook equation for photosynthesis?

Glucose takes patience to build: the sugar-making reactions take in a single carbon dioxide molecule per turn, so one six-carbon molecule needs six turns.

White asparagus is the same plant as green asparagus, so what keeps its spears white?

Growing under mounds of soil keeps the spears away from sunlight, and a shoot that never sees light never starts photosynthesis, so it never turns green.

Which famous record of carbon dioxide in the air, kept in Hawaii since 1958, dips each northern summer?

The Keeling Curve saw-tooths because most land plants grow in the Northern Hemisphere: their spring and summer photosynthesis pulls the gas out of the air, and winter decay puts it back.

Which process is, in its overall chemistry, essentially photosynthesis run in reverse?

Cellular respiration burns sugar with oxygen and gives back carbon dioxide and water, and plants do it too: in dim light a leaf breathes out as much carbon dioxide as it takes in.

Rare 'ghost' redwoods have pure white needles and cannot photosynthesise, so how do they stay alive?

Tapping a parent tree's roots works because redwoods sprout new trunks from their own base, so the white shoot stays plumbed into a green tree that feeds it sugar for life.

Where do most cacti carry out nearly all of their photosynthesis?

In their stems: a cactus's spines are what is left of its leaves, shrunk to shade the plant and slow the drying air, so the swollen green stem has to do a leaf's job.

Young sunflowers follow the sun across the sky each day, but which way do the mature flower heads face?

East is where they settle: facing the sunrise warms the flowers early and draws more pollinators, while the daily tracking in youth helps the plant put on more growth.

Since 2008 an international project has tried to give which staple crop maize's more efficient photosynthesis?

Rice was chosen because billions of people depend on it, yet it photosynthesises the less efficient way; the hope is that rewired leaves would turn the same sunlight into more grain.

From where does a growing oak tree absorb the carbon it builds into its trunk, branches and roots?

The air supplies it: leaves take in carbon dioxide and photosynthesis turns the gas into sugars that become wood. Soil provides water and minerals rather than carbon.

Which ocean organisms perform about half of Earth's photosynthesis yet make up only 1% of plant biomass?

Phytoplankton are microscopic drifters, yet they release at least half of the planet's oxygen. Their name comes from Greek words meaning 'plant drifter'.

Which sea animals are fed by photosynthetic algae living inside their tissues, and bleach white when they expel them?

Corals take most of their energy from the sugars these algae make in sunlight. Heat stress makes the polyps expel their lodgers, and the colour goes with them.

Which crop do Yorkshire growers raise in pitch-dark sheds and pick by candlelight, so it grows without photosynthesis?

Rhubarb forced in darkness lives on carbohydrate stored in its roots, which turns to sugar and sweetens the tender stalks. Strong light would stop the growth, hence the candles.

Which leafless plant with the world's largest single flower steals its food from a vine instead of photosynthesising?

Rafflesia has no leaves, roots or green pigment at all, only threads growing inside a tropical vine. Its bloom can span more than a metre and smells of rotting meat.

Which tropical fruit saves water like a cactus by taking in carbon dioxide only at night and storing it as acid?

Pineapple opens its leaf pores in the cool of night and banks the gas as malic acid, then shuts them by day and feeds the stored carbon to photosynthesis. Far less water escapes.

Which part of the atmosphere could form only once photosynthesis had filled the air with oxygen?

The ozone layer is built from three-atom oxygen molecules, so it had to wait for photosynthesis. It now absorbs 97–99% of the Sun's medium-frequency ultraviolet light.

What process lets bacteria at sunless deep-sea vents build food using energy from sulfur compounds rather than light?

Chemosynthesis feeds whole communities of giant tube worms, clams and shrimp that never see the Sun. Scientists first found these vent ecosystems in 1977.

Which crusty growths on rocks and tree bark are partnerships between a fungus and a photosynthetic alga?

Lichens work because the alga makes sugar from sunlight while the fungus holds water and gathers minerals for it. Together they live on bare rock, beyond the range of either partner alone.

Which simple sugar, with the formula C6H12O6, is the main product plants make from carbon dioxide and water?

Glucose can be used as an energy source by all living organisms, which is why photosynthesis feeds almost every food chain. Plants also chain it into starch for storage.

Which plant tissue, made largely of dead hollow cells, carries water from the roots up to the leaves for photosynthesis?

Xylem takes its name from the Greek for 'wood', and a tree trunk is mostly old layers of it. Its water-carrying cells are dead by maturity, so the flow costs them no energy.

What term describes trees such as oak that drop every leaf in autumn rather than photosynthesise through winter?

Deciduous trees break down their green pigment as the days shorten, unmasking yellows and oranges that were in the leaf all along. The word means 'falling off at maturity'.

Which sugar, sold in shops as table sugar, do plants load into their phloem to ship food from leaf to root?

Sucrose is the main sugar in phloem sap, carried from the leaves to roots, fruits and storage organs. Sugar cane and sugar beet are grown to harvest that cargo.

Which swollen underground organs does a potato plant pack with starch made from the sugars its leaves produce?

Tubers are the swollen tips of underground shoots, not roots, which is why a potato's 'eyes' can sprout. The starch inside began as sugar piped down from the leaves.

Which lab technique did botanist Mikhail Tsvet invent while trying to separate the coloured pigments of green leaves?

Chromatography means 'colour writing' in Greek, after the bands of pigment Tsvet's leaf extract left in a column of calcium carbonate. Fittingly, his surname is Russian for colour.

Which North American amphibian is the only vertebrate known to carry photosynthetic algae inside its own cells?

Spotted salamander embryos take in a green alga while still inside the egg. The alga supplies oxygen and feeds on the carbon dioxide the embryo breathes out.

Which tropical disease is caused by a parasite still carrying the relic of a photosynthetic organelle from red algae?

Malaria parasites cannot photosynthesise, yet they die without this leftover organelle, the apicoplast. That dependence makes it a target for antimalarial drugs.

Which pigments, made fresh from sugars in the dying leaf, give autumn foliage its reds and purples?

Anthocyanins also colour cranberries, red apples, blueberries and plums. Bright autumn days mean more of them in the leaf, and a more brilliant display.

Which process sends over 97% of the water a plant's roots take up out through the leaves as vapour?

Transpiration leaves only a sliver of that water for photosynthesis and growth. The escaping vapour also draws fresh water and dissolved minerals up from the roots.

What do biologists call organisms such as green plants that build their own food from carbon dioxide?

Autotrophs are the producers at the base of every food chain. A few bacteria power the job with chemical reactions instead of light, but all turn simple carbon into living matter.

Which pair of structures swells with water to open each tiny pore on a leaf and goes limp to close it?

Guard cells are made in pairs, with the pore as the gap between them. Light is their main trigger, so most leaves open up for carbon dioxide by day.

Because it must take in energy from light to proceed, photosynthesis is classed as which type of chemical reaction?

Endothermic reactions absorb energy, and here the light ends up stored in sugar. Burning a log is the exothermic opposite, giving the stored sunshine back as heat.

What term describes leaves patterned with white patches, which cannot photosynthesise because they lack green pigment?

Variegated leaves are often chimeras, with some cell layers able to make the green pigment and others not. A cutting with no green tissue at all may be impossible to propagate.

Plants link sugar units from photosynthesis into which tough fibre, the most abundant organic polymer on Earth?

Cellulose gives plant cell walls their strength: cotton fibre is about 90% of it and wood 40–50%. Humans cannot digest it, which is why it counts as dietary fibre.

Which tiny underground outgrowths absorb most of a plant's water by greatly increasing the surface in contact with soil?

Root hairs are outgrowths of single skin cells near the tip of each root. They also release acids that free up minerals for the plant to absorb.

What are the packets of light energy called, each of which knocks a single electron loose from a leaf's green pigment?

Photons got their name only in 1926, when the chemist Gilbert Lewis popularised the term. In a leaf, each one absorbed frees one electron to start the chain of reactions.

To which group does giant kelp belong, a non-plant that can grow half a metre a day by photosynthesis?

Brown algae such as kelp evolved their blades and stalks separately from land plants. Giant kelp can reach 30 to 80 metres, forming underwater forests.

Which breakfast topping is boiled down from tree sap carrying sugar stored from last summer's photosynthesis?

Maple syrup starts as starch the trees store in their trunks and roots before winter. In early spring it turns back into sugar and rises in the sap, which is only 2–5% sugar until boiled.

Artificial photosynthesis copies leaves by using sunlight to split water, hoping to yield which clean-burning fuel?

Hydrogen burns to leave nothing but water, which makes it the simplest solar fuel to aim for. One 'artificial leaf' made it with a thin sheet of silicon dipped straight into water.

Which single-celled pond organism can photosynthesise like a plant yet also take in food like an animal?

Euglena means 'good eyeball', after the eyespot its namer saw through an early microscope. When it blooms, it can turn the surface of a pond or ditch green.

To turn sugars from photosynthesis into amino acids and proteins, plants must absorb which mineral ions from the soil?

Nitrates supply the nitrogen that sugars lack and every amino acid needs. Nitrogen compounds make up 40–50% of the dry matter of a cell's living contents.

Which microscopic algae, each sealed in a glassy silica shell, generate at least a fifth of Earth's oxygen every year?

Diatoms build their intricate shells, called frustules, from silicon drawn out of the water. They make up nearly half of the organic material found in the oceans.

Rare white-needled albinos of which Californian tree cannot photosynthesise and live on sugar from a parent's roots?

Coast redwood albinos, nicknamed ghosts of the forest, tap the roots of the green tree they sprouted from. No albino of any other conifer is known to have survived to maturity.

In the classic school starch test, a leaf is boiled in which flammable liquid to strip out its green colour?

Ethanol dissolves the green pigment and leaves the leaf pale, so the blue-black of a positive result shows clearly. It is heated in a hot water bath, never over a Bunsen flame.

Which leaf tissue, a row of tall cells packed just under the upper skin, carries out most of the photosynthesis?

Palisade mesophyll sits right under the upper surface, where the light is strongest. Sun leaves grow several layers of it, while shade leaves make do with one.

Which waxy film coats the outer skin of a leaf, sealing in water so that gases must pass through the pores instead?

The cuticle is made by the leaf's own skin cells from waxy polymers. It was one of the innovations that let plants leave the water for dry land more than 450 million years ago.

Which condition, common on iron-poor soils, turns leaves pale yellow because they cannot make enough green pigment?

Chlorosis leaves a plant with little or no ability to make carbohydrates, so it may die untreated. Grapevines on limestone-rich soils are prone to it because they struggle to take up iron.

Satellites gauge plant health from orbit because healthy leaves strongly reflect which invisible band of radiation?

Near-infrared light carries too little energy to build organic molecules, so leaves bounce it away while soaking up visible red. Comparing the two bands gives the NDVI greenness index.

Moving a lamp twice as far from a water plant quarters the light for photosynthesis, following which physical law?

The inverse-square law says light intensity falls with the square of the distance, so tripling the gap leaves just a ninth. GCSE biologists use it in lamp-and-pondweed experiments.

What name is given to the splitting of water molecules that replaces the electrons a leaf's pigments lose in sunlight?

Photolysis also releases protons inside the cell's membrane sacs, and that build-up drives the machinery that makes its energy-carrying molecules.

What is the light level called, typically met at dawn and dusk, at which photosynthesis exactly balances respiration?

The compensation point is where a leaf takes up carbon dioxide exactly as fast as its own respiration releases it. In any dimmer light, the plant is living off its reserves.

Which type of graph plots the rate of photosynthesis against the wavelength of light, peaking in the red and blue?

An action spectrum shows which colours actually drive the reaction. Theodor Engelmann plotted the first by shining a prism's rainbow on an alga and watching where oxygen-loving bacteria gathered.

Which ancient rocks, now the main ore for steel, are thought to have formed as photosynthetic oxygen rusted the seas?

Banded iron formations are thought to have built up as the oxygen combined with iron dissolved in seawater and sank as rust. They hold over 60% of the world's iron reserves.

What is the energy diagram of electron flow through photosystems II and I called, after the letter it resembles?

The Z-scheme shows light lifting each electron twice, once at each photosystem, with a downhill run in between. Turned on its side, the rise, fall and rise trace the letter.

Which blue pigment, extracted from spirulina to colour sweets and ice cream, helps the microbe harvest orange light?

Phycocyanin soaks up orange light that the main green pigment largely misses, then passes the energy on. Food makers sell it as the natural colouring 'Lina Blue'.

Which marine microbe, not discovered until 1986, is probably the most abundant photosynthetic organism on Earth?

Prochlorococcus is also the smallest known photosynthesiser, just 0.5 to 0.7 micrometres across. It thrives in the sunlit surface waters of the world's tropical oceans.

Which British biochemist showed in 1937 that green organelles isolated from leaves still give off oxygen when lit?

Robin Hill's isolated organelles made oxygen but could not fix carbon dioxide, so the two jobs had to be separate steps. He had to add a chemical to accept the electrons.

Which weedkiller, banned in the EU but long among the most used in the US, kills by jamming photosystem II?

Atrazine binds a photosystem protein that animals do not have, so the electrons freed by light have nowhere to go. It can seep from soil into groundwater, where it breaks down slowly.

Which festive evergreen is only a part-time parasite, tapping a host tree for water while photosynthesising for itself?

Mistletoe plugs into its host's plumbing through a structure called the haustorium, taking water and nutrients. Because it still makes some sugar of its own, botanists class it as a hemiparasite.

Which technology has NASA tested for growing food crops in space because it drives photosynthesis so efficiently?

Light-emitting diodes can be made to emit just the colours plants use, which makes them highly efficient. NASA's Veggie unit on the International Space Station has grown romaine lettuce under red, blue and green ones.