Photosynthesis · System
Why almost all life depends on it
One reaction, running in leaves and in water you cannot see into, is the doorway through which almost all the energy in the living world arrives — and the source of every breath you have ever taken.
Start here
Every meal you have ever eaten was once a leaf.
A steak is a cow, and a cow is grass. That one is easy. The awkward cases are the ones that look nothing like a plant — a fish finger, a slice of cheese, a mushroom — and they are where the rule is either true or not.
Which of these did not begin with photosynthesis?
All of them did. Every chain runs back to something that took in carbon dioxide and built a sugar out of it — grass, wheat, or microscopic algae drifting in the sea. The mushroom is the interesting one: fungi cannot photosynthesise at all, so they feed on material that something else built, which puts them further down the same chain rather than outside it.
At the bench · follow it back
Trace a meal to a leaf
step 1 of 4
Pick something off the plate and step backwards through the chain that supplied it. Count the steps — and notice that the number changes while the destination never does.
On the plate
Steak
follow it back
- Steak on the plateMuscle tissue, built from protein the animal assembled.
- The cowEverything in it was built from what it ate. A cow eats no meat at all.
- GrassAround ten kilograms of grass for every kilogram of cow — most of what the cow eats is respired away to keep the cow alive.
- Photosynthesis in the grass leafCarbon dioxide from the air, water from the soil, energy from sunlight falling on a field.
Three steps from a field to a plate, and every gram of it was carbon dioxide before the grass got hold of it.
Bread
follow it back
- A slice of breadMostly starch, plus the protein that makes the dough stretch.
- Wheat grainA seed — a store of starch the plant packed for its own seedling, which we intercept.
- Photosynthesis in the wheat leafGlucose made in the leaves, moved to the developing grain and stored as starch.
Two steps, the shortest chain on the plate. Eating a producer directly is the most efficient thing you can do with a field.
Salmon
follow it back
- A salmon filletFrom the sea, and nothing green anywhere near it.
- Small fish and shrimp-like animalsWhat a salmon hunts.
- ZooplanktonTiny drifting animals, each a fraction of a millimetre across, eaten in enormous numbers.
- PhytoplanktonMicroscopic algae drifting in the sunlit surface water. They photosynthesise exactly as a leaf does, with chlorophyll and the same reaction.
- Sunlight on the top hundred metres of oceanBelow that depth there is not enough light, which is why almost all sea life lives in a thin layer at the top.
Four steps, and the producer at the end is invisible. Roughly half the world’s photosynthesis is done by organisms like these.
Cheese
follow it back
- CheeseMilk protein and fat, curdled and matured — bacteria do the maturing.
- Milk from a cowProduced from what the cow ate, by an animal that eats only plants.
- Grass and silageSilage is preserved grass, which is how a dairy herd is fed in winter.
- Photosynthesis in the grass leafThe same field as the steak, and the same reaction.
Three steps. The bacteria that ripen the cheese are living on the same sugars and proteins, one further down the chain again.
Mushroom
follow it back
- A mushroomNot a plant. Fungi have no chlorophyll and cannot photosynthesise at all.
- Threads through compost or dead woodThe mushroom is the fruiting body; the organism is a network of threads digesting dead material from the outside in.
- Dead plant materialStraw, wood, leaf litter — built by a plant that is no longer alive.
- Photosynthesis, some time agoThe molecules the fungus is living on were assembled in a leaf, possibly years earlier.
Three steps, and it is the case that looks like an exception and is not. A decomposer is not outside the chain — it is at the end of it.
Honey
follow it back
- HoneyConcentrated sugar, and almost nothing else.
- BeesThey collect nectar and evaporate the water off it. They add enzymes; they do not add sugar.
- Nectar in a flowerA sugar solution the plant manufactures deliberately, to pay pollinators for a delivery service.
- Photosynthesis in the plantThe sugar in nectar is glucose the plant made from carbon dioxide.
Three steps, and the shortest chemical journey of the six: the sugar you taste is close to the sugar the leaf built.
Three things it does for everything else
Food, air, and the balance between them.
Job one · The way in for energy
Energy enters almost every food chain on Earth at one point: a photosynthetic organism capturing sunlight. Everything above that is spending what a producer captured. Without it: nothing above the producers has any source of energy at all, and the chains empty from the bottom up.
Job two · The oxygen in the air
The atmosphere is about a fifth oxygen and all of it is biological in origin — released over billions of years as a waste product of splitting water. Without it: no new oxygen is added, while every respiring organism keeps consuming what is there.
Job three · Taking carbon back out
Respiration, decay and burning all put carbon dioxide into the air. Photosynthesis is the only large-scale biological process that removes it, locking carbon into wood, roots and ocean sediment. Without it: carbon dioxide only ever rises, because the living world's one route out of the atmosphere has been closed.
Key fact
Photosynthetic organisms — plants on land, algae and plankton in the sea — are the producers. They build the organic molecules that almost every other living thing eats, they released the oxygen in the atmosphere, and they take carbon dioxide back out of it.
Think again
“Plants do the photosynthesising — trees are the lungs of the planet.”
Roughly half of all photosynthesis on Earth happens in the sea, carried out by algae and phytoplankton, most of them single cells too small to see without a microscope. A drifting organism a hundredth of a millimetre across, in numbers no one can count, matches every forest and grassland on land put together. Rainforests are irreplaceable for a long list of reasons, and being the planet's only oxygen supply is not one of them — a mature forest also consumes much of what it produces, since every one of its cells respires day and night. Keep the vocabulary honest and this stays clear: the group that matters is photosynthetic organisms, which includes plants, algae and some bacteria. Restricting the idea to green things with roots leaves out half the planet's production and most of its producers.
“Plants make oxygen for us to breathe.”
Not for us, and not for anything. Oxygen is a waste product: the plant is building glucose, and oxygen is what is left over from splitting water to do it. There was no oxygen worth mentioning in the atmosphere for the first two billion years of Earth's history, and the organisms that eventually filled it with the stuff were bacteria releasing a gas that was, at the time, poisonous to almost everything alive. Breathing animals appeared afterwards, in an atmosphere that was already there — the sequence runs the other way round from the way the sentence implies. Nothing in biology is done for another organism's benefit, and a plant that gave away useful material on purpose would be out-competed by one that did not. Plants also respire continuously, which is the subject of Stomata and gas exchange in plants and worth having straight before this claim is repeated.
Mastery ladder
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1 · Producers
What does it mean to call a plant a producer?
Rung 2 · The one that catches people
A mushroom is not green and cannot photosynthesise. Does it depend on photosynthesis?
Rung 3 · Follow the energy
Explain how the energy in a beef burger got there, starting from the Sun. Use the words producer, photosynthesis, glucose and respiration.
Rung 4 · Take it somewhere new
Coal is often described as "ancient sunlight". Explain what that means, where the carbon in a lump of coal came from, and what burning it does to the atmosphere.
Key note
Photosynthetic organisms are producers: they build organic molecules from carbon dioxide and water using light, and everything that cannot do this depends on them, directly or through a chain. They also released the oxygen in the atmosphere and continue to take carbon dioxide out of it. Roughly half of the world's photosynthesis happens in the sea.
Going further
The word almost in this lesson's title is doing real work. Two and a half kilometres down, where no light has ever reached, hot water loaded with dissolved chemicals pours out of cracks in the sea floor and whole communities live around it — tube worms two metres long, crabs, clams. At the base of those food chains are bacteria that build sugars using energy released from chemical reactions instead of energy from light, a process called chemosynthesis. They were found in 1977, and they mattered immediately: until then, sunlight was assumed to be the only way energy could enter a living system. It is a useful thing to know when people speculate about life on a moon with a frozen ocean and no daylight at all.
Before this lesson
Connects to
At GCSE this becomes
- Trophic levels and biomass transfer, the carbon cycle, and the evidence behind human effects on atmospheric carbon dioxide.
Where to next
Ask Mr Badmus AI
Want to trace something that is not on the plate above?
The chains are simplified to one line each: real diets are webs, a farmed salmon eats feed made from several sources, and a cow's rumen bacteria sit between the grass and the cow. The land-and-sea split of global photosynthesis is an approximate figure that varies between estimates and with the season.
Lesson content © MrBadmusAI.