Lesson focus: photosynthesisThe reaction in which plants use light energy, absorbed by chlorophyll, to turn carbon dioxide and water into glucose and oxygen. transfers light energy into chemical energy stores while carbon dioxide and water provide the atoms used to make glucoseA sugar that living things use for energy; plants make it in photosynthesis..
Explain photosynthesis as an endothermicDescribes a reaction that takes in heat energy from the surroundings, so the temperature of the surroundings decreases. chemical process and use its word and symbol equations without confusing matter with energy.
Build on earlier learning
Plant and animal cellsYear 7 identified chloroplasts in photosynthetic cells. This lesson explains the chemical process associated with chloroplasts.
You will be successful when you can…
- State the word equation for photosynthesis.
- Balance the symbol equation.
- Explain the roles of chlorophyll and light.
- Distinguish matter inputs from energy transfer.
Words that carry the science
- photosynthesis
- An endothermic process that uses light energy to make glucose from carbon dioxide and water.
- chlorophyll
- A group of pigments that absorb light energy in chloroplasts.
- chloroplast
- An organelle in many plant and algal cells where photosynthesis occurs.
- glucose
- A soluble sugar that stores chemical energy and supplies carbon for other substances.
- endothermic
- Describes a process that takes in energy from the surroundings.
- chemiosmosis
- ATP production driven by protons moving down a gradient through ATP synthase.
Track atoms and energy through photosynthesis
01The equation accounts for matter
The word equation is carbon dioxide + water → glucose + oxygen. A balanced symbol equation is 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. The same numbers of carbon, hydrogen and oxygen atoms appear before and after the reaction.
Light is written over the arrow because it supplies energy; it is not a substance whose atoms become part of glucoseA sugar that living things use for energy; plants make it in photosynthesis.. chlorophyllThe green pigment in chloroplasts that absorbs light energy for photosynthesis. absorbs some wavelengths and begins energy-transfer reactions.
02Glucose is an intermediate, not the final destination
Plants use glucose in respirationChemical reactions in all living cells that release energy from glucose, either using oxygen (aerobic) or without it (anaerobic)., convert it to insoluble starch for storage, build cellulose for cell walls, combine it with nitrate-derived nitrogen to make amino acids, and make lipids for membranes and stores.
Oxygen is released as a product. At a deeper level, the oxygen gas produced in photosynthesisThe reaction in which plants use light energy, absorbed by chlorophyll, to turn carbon dioxide and water into glucose and oxygen. originates from water molecules rather than carbon dioxide.
03Stretch: light reactions build an electrochemical gradient
In chloroplastAn organelle containing chlorophyll, which absorbs light energy so the cell can make glucose by photosynthesis. thylakoid membranes, light-driven electron transfer helps move protons and create a proton electrochemical gradient. Protons then flow through ATP synthaseThe enzyme that makes ATP, the molecule that carries energy around cells., coupling chemiosmosisThe way cells make ATP, using a flow of tiny charged particles through a special enzyme, like water turning a water wheel. to ATP production.
This extension explains one mechanism by which absorbed light energy becomes usable chemical energy. It is enrichment beyond the core equation, not a replacement for it.
Photosynthesis and light intensity
Move the lamp and count the bubbles of oxygen from the pondweed. The rate rises with light until something else runs short: the limiting factor.
Controls
Halving the distance gives four times the light. The model keeps the water temperature fixed, as a heat shield or LED lamp would.
Balance the photosynthesis equation
Balance CO₂ + H₂O → C₆H₁₂O₆ + O₂.
- Glucose contains six carbon atoms, so place 6 before CO₂.
- Glucose contains twelve hydrogen atoms, so place 6 before H₂O.
- The left side now contains eighteen oxygen atoms.
- Glucose contains six oxygen atoms, leaving twelve for oxygen gas, so place 6 before O₂.
How to make the reasoning visible: The coefficients conserve atoms: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂.
Account for photosynthetic inputs and outputs
Use the equation and biological context to classify each item.
| Item | Matter or energy? | Role | Evidence or consequence |
|---|---|---|---|
| Carbon dioxide | Matter | Supplies carbon and oxygen atoms | Enters leaves mainly through stomata |
| Water | Matter | Supplies hydrogen and oxygen atoms | Delivered through xylem |
| Light | Energy transfer | Drives endothermic reactions | Rate can change with light intensity |
| Glucose | Matter with a chemical energy store | Product used or converted | Can be converted to starch |
| Oxygen | Matter | Product released or used in respiration | Can be detected from aquatic plants |
Why is light not included as a reactant coefficient?
Why can a plant release oxygen while also using oxygen?
Check your understanding
Answer from memory, then check against the suggested answer.
State the word equation for photosynthesis.
Where does photosynthesis occur in plant cells?
Why is photosynthesis endothermic?
A sealed plant gains dry mass while carbon dioxide concentration falls. Explain the link.
Explain why chemiosmosis is a better description than saying chlorophyll directly makes ATP.
Exam Question Practice
Exam Question Practice
Mark points shown after marking
Plants increase their biomass by photosynthesis.
State the word equation inputs and the energy source for photosynthesis.
(2)Explain how photosynthesis increases the biomass of the plant.
(3)Total for this question: 5 marks
Command word: For explainA command word: say how or why something happens, giving reasons with words like "because" or "so"., connect each input to the process and then connect glucose to new biological material.