Almost every scrap of energy in every living thing traces back to this one reaction. A plant takes two of the simplest molecules going, adds sunlight, and builds sugar. Nothing else in biology does quite so much with so little.
📚 What you need to know
Photosynthesis converts light energy into chemical energy, which is then stored in biomass.
It turns simple inorganic compounds into complex organic ones.
It happens in autotrophs: plants, algae and cyanobacteria.
Reactants: carbon dioxide and water. Products: glucose and oxygen.
Water is split. The hydrogen from it is used to build glucose; the oxygen is released as waste.
The word equation is what you need for the exam — the symbol equation is useful but not required.
Photosynthesis and respiration are opposites: the products of one are the reactants of the other.
An energy conversion, not an energy source
Energy is never created. Photosynthesis does not make energy — it converts it, from light into chemical energy held in the bonds of glucose. Get that word right and several marks look after themselves.
Once the energy is in glucose, it is stored in biomass. That store is what feeds everything else in an ecosystem, because animals cannot make their own organic compounds and have to eat something that did.
Definition
photosynthesis = the conversion of light energy into chemical energy stored in organic compounds
Autotroph means “self-feeding”. Plants, algae and cyanobacteria build their own organic molecules from inorganic ones. Everything else is a heterotroph and has to get its organic molecules ready-made.
What goes in and what comes out
Chlorophyll is written above the arrow too. It is not used up, so it belongs with the light rather than with the reactants.
The word equation
This is the version you must be able to write from memory.
Learn this one — light and chlorophyll go above the arrow
carbon dioxide + water → glucose + oxygen
The symbol equation is not required at SL, but it does make one thing much clearer — where the atoms actually go:
Not required, but useful
6CO2 + 6H2O → C6H12O6 + 6O2
Where the oxygen comes from
This catches people out. The plant splits water molecules. That splitting does two jobs at once:
It supplies hydrogen, which is combined with carbon dioxide to build glucose.
It leaves oxygen over, which the plant does not need and releases as a waste product.
If a question asks where the released oxygen came from, the answer is the water. Saying “the carbon dioxide” is a very common and very costly slip.
The other half of the story
Put photosynthesis next to aerobic respiration and something neat appears: each one produces exactly what the other needs.
They are opposites in what they do to the molecules, but not mirror images in detail — photosynthesis stores energy, respiration releases it, and both are pathways of many enzyme-controlled steps.
Worked examples
WORKED EXAMPLE
A plant is given water containing a heavy isotope of oxygen. The carbon dioxide supplied contains only normal oxygen. Predict where the heavy oxygen will be found, and explain your answer. [2]
Step 1: identify which molecule is splitwater is split, releasing hydrogen and oxygenStep 2: follow the labelled atoms
The oxygen released as gas comes from that water, so it carries the heavy isotope.
The heavy oxygen appears in the oxygen gas given offthe glucose oxygen comes from the carbon dioxide, so it stays normal
WORKED EXAMPLE
Explain why photosynthesis is described as an energy conversion rather than a source of energy. [2]
Step 1: state the rule
Energy cannot be created or destroyed, only transferred or converted.
Step 2: name the two formslight energy → chemical energy in the bonds of glucoseThe light energy already exists; photosynthesis only changes its form“the plant makes energy” is the phrase to strike out of your answers
WORKED EXAMPLE
A sealed jar contains a healthy pondweed and is left in bright light for several hours. Explain what happens to the concentration of oxygen and carbon dioxide in the water. [3]
Step 1: which process is faster
In bright light, photosynthesis runs faster than respiration.
Step 2: what photosynthesis does to each gasCO₂ used as a reactant, O₂ released as a productStep 3: state the net result
More oxygen is made than is used, and more carbon dioxide is used than is made.
Oxygen concentration rises and carbon dioxide concentration fallssay “net”, because the plant is still respiring the whole time
💡 Exam tip
Write the word equation with light and chlorophyll above the arrow, not as reactants.
Use converted or transformed, never “produced” or “made”, when talking about energy.
Name the three groups of autotrophs: plants, algae and cyanobacteria. Questions often want more than just “plants”.
If asked about the oxygen released, say it comes from the splitting of water.
Link back to respiration whenever a question mentions gases. The two processes run at the same time.
“Stored in biomass” is a useful phrase for any question about where the energy ends up.
⚠ Common mix-up
Saying the oxygen comes from carbon dioxide. It comes from water.
Listing light as a reactant. Light is the energy input, not a substance being consumed.
Thinking plants only respire at night. They respire continuously; in daylight photosynthesis simply outpaces it.
Saying photosynthesis produces energy. It converts light energy into chemical energy.
Forgetting that chlorophyll is not used up. It is not a reactant and does not appear in the products.
Confusing glucose with starch. Glucose is the product; starch is made later for storage.
Up next: Separating Photosynthetic Pigments (Skills) — chlorophyll is not one substance but several, and chromatography is how you pull them apart.
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