This mini-lesson covers the CCEA topic Photosynthesis & plants: the word and balanced symbol equations, the raw materials and conditions photosynthesis needs, how we test a leaf for starch, how the leaf is adapted, what plants do with the glucose made, the limiting factors that control the rate — and the minerals plants need to grow.
Work through each screen, answer the questions as you go (some are wordy, some are calculations) and collect ⭐ stars. Press Start when you're ready.
Photosynthesis · the equation
What is photosynthesis?
Photosynthesis is how green plants make their own food. They take in carbon dioxide and water and use light energy (trapped by chlorophyll in the chloroplasts) to make glucose, releasing oxygen.
carbon dioxide + water → glucose + oxygen(condition: light energy trapped by chlorophyll)
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂the balanced symbol equation — light & chlorophyll shown above the arrow
Raw materials: carbon dioxide (from the air) and water (from the soil).
Needed:light, carbon dioxide and chlorophyll — if any is missing, photosynthesis stops.
Where: in the chloroplasts, which contain the green pigment chlorophyll.
Products:glucose (a sugar) and oxygen (given off as a waste gas).
Watch out: chlorophyll traps the light energy but is not used up — it is not a raw material. The two raw materials are carbon dioxide and water.
Quick check
What does it need?
?A plant is kept in the dark. Which condition needed for photosynthesis is now missing, so photosynthesis stops?
Photosynthesis · prescribed practical
Testing a leaf for starch
A photosynthesising leaf turns its glucose into starch for storage, so testing for starch shows where photosynthesis has happened. We use iodine solution: it stays orange-brown where there is no starch and turns blue-black where starch is present.
Blue-black with iodine = starch present = photosynthesis happened here.
Destarch the plant first — leave it in the dark for a day or two so any stored starch is used up.
Then set up the test condition, expose to light, and test a leaf: boil in water, then in ethanol to remove chlorophyll, then add iodine.
Variegated leaf (part green, part white): only the green parts turn blue-black — this shows chlorophyll is needed.
Sodium hydroxide in a sealed flask absorbs the carbon dioxide — that leaf gives no starch, showing CO₂ is needed.
Quick check
What does the iodine show?
?After testing a leaf, part of it turns blue-black with iodine. What does the blue-black colour tell you about that part of the leaf?
Plants · leaf structure
How the leaf is adapted
A leaf is a photosynthesis machine. It is thin and flat — a large surface area to absorb light — and each layer is adapted to its job.
Palisade cells (top) are packed with chloroplasts; the spongy layer has air spaces; stomata on the lower surface let gases in and out.
Waxy cuticle — clear waterproof layer that reduces water loss and lets light through.
Upper epidermis — thin and transparent so light passes to the cells below.
Palisade mesophyll — tall cells packed with chloroplasts, near the top for maximum light; most photosynthesis happens here.
Spongy mesophyll — loose cells with intercellular air spaces for gas exchange.
Guard cells & stomata — pores (usually on the lower surface) that control gas exchange and water loss; guard cells open and close the stomata.
Quick check
Where does most photosynthesis happen?
?Which layer of the leaf is packed with the most chloroplasts and carries out most of the photosynthesis?
Sort it
Raw material, product or adaptation?
Tap an item, then tap the group it belongs to.
🔵 Needed FOR photosynthesis
🟢 Product OF photosynthesis
🍃 Leaf adaptation
Plants · uses of glucose
What plants do with the glucose
The glucose made in photosynthesis is used in several ways:
Respiration — glucose is broken down to release energy for the plant's living processes.
Storage — glucose is turned into starch (an insoluble store) in leaves, roots and tubers.
Cellulose — glucose is used to make cellulose for building strong cell walls.
Proteins — glucose is combined with nitrates (absorbed from the soil) to make amino acids, which build proteins for growth.
Key link: plants can make glucose from air, water and light, but to make proteins they also need a mineral — nitrate — absorbed from the soil by the roots.
Quick check
Which mineral for proteins?
?A plant makes glucose by photosynthesis. Which mineral, absorbed from the soil, must it combine with glucose to make amino acids and proteins for growth?
Photosynthesis · limiting factors
Limiting factors & growing crops
The rate of photosynthesis is controlled by three limiting factors: light intensity, carbon dioxide concentration and temperature. As you increase one, the rate rises — until a different factor runs short and becomes limiting, so the graph levels off (plateaus).
Increasing light raises the rate until CO₂ or temperature becomes the new limiting factor.
If temperature is too high, the enzymes controlling photosynthesis are denatured and the rate falls.
Commercial growers in greenhouses control these factors to raise the yield: adding artificial lighting, CO₂ enrichment, fertilisers and keeping an optimum temperature.
But there is a cost: growers must weigh the price of extra light, heating and CO₂ against the value of the extra crop grown.
Quick check
Why has the graph levelled off?
?On a graph, the rate of photosynthesis rises as light intensity increases, then reaches a plateau where more light makes no difference. What is the best explanation for the plateau?
Calculate
Your turn — rate of photosynthesis
1A pondweed gives off bubbles of oxygen. A student counts 40 bubbles in 5 minutes. Calculate the rate of photosynthesis in bubbles per minute.
bubbles/min
Hint: rate = bubbles ÷ time = 40 ÷ 5.
Match it
Match each leaf part to its function
Tap a leaf part on the left, then its matching function on the right.
Leaf part
Function
Recap
The big ideas to know
Word equation: carbon dioxide + water → glucose + oxygen (light + chlorophyll)
Symbol equation: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
Needs: light, carbon dioxide and chlorophyll · happens in chloroplasts
Starch test: destarch first · iodine turns blue-black where starch (photosynthesis) is present
Leaf: palisade = most chloroplasts · stomata & guard cells = gas exchange · waxy cuticle = less water loss