This mini-lesson covers Topic 6 — Plant nutrition: the word and symbol equations for photosynthesis, the limiting factors, the structure and adaptations of a leaf, and the plant's mineral needs for nitrate and magnesium ions.
Work through each screen, answer the questions as you go and collect ⭐ stars. Watch for the Supplement flag. Press Start when you're ready.
Photosynthesis
Photosynthesis
Photosynthesis is the process by which plants make carbohydrates from raw materials, using energy from light. Chlorophyll (a green pigment in chloroplasts) transfers the light energy into chemical energy to build carbohydrates.
carbon dioxide + water → glucose + oxygen(in the presence of light and chlorophyll)
The carbohydrates made are then used and stored in several ways:
Starch — an energy store;
Cellulose — to build cell walls;
Glucose — used in respiration to release energy;
Sucrose — for transport in the phloem;
Nectar — to attract insects for pollination.
Quick check
The raw materials
?Which two substances are the raw materials (reactants) of photosynthesis?
Match it
What is each carbohydrate used for?
Tap a substance on the left, then its use on the right.
Substance
Use in the plant
Limiting factors
Limiting factors
The rate of photosynthesis is affected by three environmental factors: light intensity, carbon dioxide concentration and temperature. A limiting factor is whichever one is in shortest supply and so holds back the rate.
As a factor increases the rate rises, then levels off when a different factor becomes limiting.
Supplement: identify and explain limiting factors in different conditions — e.g. on a warm, bright day the rate may be limited by CO₂ concentration; at dawn it is often limited by light.
Supplement · Quick check
What is limiting the rate?
?On a warm, brightly lit afternoon, giving a plant extra carbon dioxide makes it photosynthesise faster, but extra light makes no difference. What is the limiting factor?
Leaf structure
The leaf & its adaptations
Most leaves are broad (large surface area) and thin — adaptations that make them efficient at photosynthesis. Key tissues of a dicotyledonous leaf:
Palisade mesophyll (packed with chloroplasts) sits near the top; stomata and guard cells control gas exchange.
Palisade mesophyll — column cells near the top, packed with chloroplasts to absorb light.
Spongy mesophyll — loosely packed with air spaces for gases to diffuse.
Stomata & guard cells — pores (mostly on the lower epidermis) that let CO₂ in and O₂ out.
Xylem brings water; phloem carries sugars away (in the vascular bundles).
Waxy cuticle — a waterproof layer that reduces water loss.
Sort it
What does each leaf part do?
Tap a leaf part, then tap its job.
☀️ Absorbs light
💨 Gas exchange
🚚 Transport
Mineral nutrition
Nitrate & magnesium ions
As well as making glucose, plants need mineral ions absorbed from the soil:
Nitrate ions are needed for making amino acids (which build proteins). A shortage causes poor growth and stunted plants.
Magnesium ions are needed for making chlorophyll. A shortage causes yellow leaves (chlorosis), because the plant cannot make enough green pigment.
Exam tip: link the deficiency to the job — no magnesium → no chlorophyll → yellow leaves; no nitrate → no amino acids/proteins → stunted growth.
Quick check
Why the yellow leaves?
?A plant grown in soil low in magnesium ions develops yellow leaves. Why?
Calculate
Your turn — rate of photosynthesis
1A pondweed produces 90 bubbles of oxygen in 3 minutes under bright light. Calculate the rate of photosynthesis in bubbles per minute.
bubbles/min
Hint: rate = bubbles ÷ time = 90 ÷ 3.
Supplement · Quick check
The balanced equation
?Which is the correct balanced symbol equation for photosynthesis?
Recap
The big ideas to know
Word equation: carbon dioxide + water → glucose + oxygen (light & chlorophyll)
Balanced (Supp): 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
Limiting factors: light intensity · CO₂ concentration · temperature