GCSE · Biology · AQA · Spec 8461
Photosynthetic reaction
Carbon dioxide, water and light: ask what a plant needs to make glucose and you'll hear all three. Only two of them are ingredients. Which one isn't?
Build the equation
Where does each one go?
Pick an item, then choose where it belongs in the photosynthesis equation. Names and chemical symbols are mixed together, so decide what each symbol stands for as you go.
Still to sort
Reactant: goes in (0)
A substance that gets used up. Written before the arrow.
Where the line is: Reactants are substances, made of atoms. If something isn't a substance, it can't be used up to make new ones.
Product: comes out (0)
A new substance the reaction makes. Written after the arrow.
Where the line is: Products are made from the reactants' atoms, so anything here must be a substance too.
Over the arrow: powers it (0)
Not a substance at all. It's what makes the change happen.
Where the line is: This is for energy, not ingredients. Nothing here turns into glucose or oxygen.
Energy in
Reason it through
Why is photosynthesis called an endothermic reaction?
First link · your turn
Turning carbon dioxide and water into glucose and oxygen needs energy. Where does that energy come from?
WHAT YOU'VE LEARNED
A quick recap of today's lesson.
What goes in, what comes out, and what makes it happen
What you need to know
- Photosynthesis turns two substances into two new ones: carbon dioxide and water go in, glucose and oxygen come out.
- Light isn't an ingredient. It's how energy gets in, so it's written over the arrow.
- You need to recognise four chemical symbols: CO₂, H₂O, C₆H₁₂O₆ and O₂.
- The reaction takes energy in from its surroundings, which makes it endothermic.
The big picture
Photosynthesis is represented by the word equation carbon dioxide + water → glucose + oxygen, with light written over the arrow. Carbon dioxide (CO₂) and water (H₂O) are the reactants; glucose (C₆H₁₂O₆) and oxygen (O₂) are the products. Light isn't a reactant. It transfers energy from the environment to the chloroplasts, and because the reaction takes in energy from its surroundings, photosynthesis is an endothermic reaction.
Key points
Worked example
Problem
A student writes: "Carbon dioxide, water and light react to make glucose and oxygen. Oxygen is given out, so photosynthesis is exothermic." Find the two mistakes and write a corrected answer.
⚠ Watch out
Mixing up endothermic and exothermic. An endothermic reaction takes energy in from its surroundings; an exothermic reaction gives energy out. Photosynthesis takes energy in, so it's endothermic, even though it gives out oxygen.
Memory hook
Light rides OVER the arrow, never in the list. It brings ENergy IN, so the reaction is ENDothermic.
Check yourself
A friend says: "Plants turn light into glucose." What's right about that, and what's wrong? Fix the sentence so it's true.
Flashcards
(7)The word equation for photosynthesis?
Name the substances: CO₂, H₂O, O₂
What is C₆H₁₂O₆?
Why isn't light written with the reactants?
In photosynthesis, light transfers energy from where, to where?
Endothermic or exothermic: which one takes energy in?
Which elements is glucose made of, and were they already in the reactants?
Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.
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