Plant Nutrition
How plants make food by photosynthesis; the role of chlorophyll; the five uses of carbohydrates produced; why nitrate and magnesium ions are essential; the investigations that test what photosynthesis requires; Extended limiting factors with graph interpretation; and how every layer of the leaf is structurally adapted for maximum photosynthesis.
Photosynthesis
CORE EXTENDEDPhotosynthesis is the fundamental process that puts energy into biological systems. Plants, algae, and cyanobacteria use light energy to convert inorganic molecules into glucose, making them the primary producers at the base of almost every food chain on Earth.
Definition and Equations
Photosynthesis is the process by which plants synthesise carbohydrates from raw materials (carbon dioxide and water) using energy from light, in the presence of chlorophyll.
| Level | Equation |
|---|---|
| CORE Word equation |
carbon dioxide + water → glucose + oxygen (in the presence of light and chlorophyll) |
| EXTENDED Balanced equation |
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ |
Reactants (inputs): carbon dioxide + water + light energy + chlorophyll
Products (outputs): glucose + oxygen
Oxygen is a product, not a reactant of photosynthesis. Students frequently write oxygen on the left side — this is always wrong.
The Role of Chlorophyll
Chlorophyll is a green pigment found inside chloroplasts. It absorbs light energy (mainly red and blue wavelengths, reflecting green — which is why plants look green).
Chlorophyll transfers energy from light into chemical energy in molecules — this energy drives the synthesis of carbohydrates from CO₂ and water. Without chlorophyll, light energy cannot be captured and photosynthesis stops.
Uses of Carbohydrates Made in Photosynthesis
Glucose produced in photosynthesis is not just stored — it is used in multiple ways. The syllabus requires all five:
| Product | Made from glucose by… | Function |
|---|---|---|
| Starch | Polymerisation of glucose | Energy storage in plant cells (e.g. in leaves, seeds, potato tubers) |
| Cellulose | Polymerisation of glucose (different bonding) | Building cell walls — structural support |
| Glucose (used directly) | No conversion needed | Cellular respiration to release energy for all metabolic processes |
| Sucrose | Combination of glucose + fructose | Transport in the phloem to growing regions and storage organs |
| Nectar | Secreted as concentrated sucrose/sugar solution | Attract pollinating insects to flowers |
Essential Mineral Ions
| Ion | What it is used for | Deficiency symptom |
|---|---|---|
| Nitrate ions (NO₃⁻) | Making amino acids (and therefore proteins). Glucose provides the carbon skeleton; nitrate provides the nitrogen atom needed to form the amino (-NH₂) group. | Stunted growth; pale/yellowing leaves; thin, weak stems — plant cannot make proteins for growth |
| Magnesium ions (Mg²⁺) | Making chlorophyll. Magnesium is at the centre of the chlorophyll molecule; without it, no chlorophyll can be made. | Chlorosis — yellowing of leaves (especially between veins), reduced photosynthesis, poor growth |
This is one of the most frequently mixed-up pairs in Topic 6. Use this memory rule: N for N — Nitrate makes Nitrogen-containing molecules (amino acids, proteins). Mg for green — Mgnesium makes chlorophyll (the green pigment). Magnesium deficiency = yellow leaves because chlorophyll cannot be made.
Investigating the Requirements for Photosynthesis
All three investigations use the iodine starch test as an indicator of photosynthesis. A leaf is destarched first (kept in the dark for 24–48 h to use up existing starch). Then the experimental treatment is applied. After several hours in light, the leaf is tested with iodine. Blue-black = starch present = photosynthesis occurred.
| Investigation | How to test for it | Positive result (starch present) | Control |
|---|---|---|---|
| Need for light | Cover part of leaf with black paper or foil; expose rest to light | Exposed area goes blue-black; covered area stays yellow-brown | Uncovered area in same leaf |
| Need for chlorophyll | Use a variegated leaf (green and white areas) | Green areas (with chlorophyll) go blue-black; white areas stay yellow-brown | Green portion of same leaf |
| Need for CO₂ | Enclose leaf in airtight bag/flask with NaOH (absorbs CO₂); compare with leaf in CO₂-rich bag | Leaf in CO₂-free bag stays yellow-brown; leaf in normal CO₂ goes blue-black | Leaf in CO₂-containing bag |
Leaf preparation procedure: Kill cells (dip in boiling water, 30 s) → remove chlorophyll (boil in ethanol in a water bath, NOT directly — flammable!) → wash in warm water (softens leaf, removes ethanol) → spread flat and add iodine solution.
Factors Affecting Rate of Photosynthesis
| Factor | Effect when increased (up to a point) | What limits further increase |
|---|---|---|
| Light intensity | Rate increases — more energy available for reactions | CO₂ concentration or temperature becomes limiting |
| CO₂ concentration | Rate increases — more raw material for glucose synthesis | Light intensity or temperature becomes limiting |
| Temperature | Rate increases — more enzyme activity (rubisco etc.) | At very high temperatures, enzymes denature; rate falls to zero |
Hydrogencarbonate indicator is used to detect CO₂ changes in solution. It is red/orange at normal CO₂ levels.
| Condition | CO₂ change | Indicator colour |
|---|---|---|
| Light (photosynthesis > respiration) | CO₂ removed from water | Turns purple/pink (less acidic) |
| Dark (only respiration) | CO₂ added to water | Turns yellow/orange (more acidic) |
| Sealed, no plant (control) | No change | Stays red/orange |
Limiting Factors — Extended
A limiting factor is the factor that is present at the lowest (most restrictive) level — the one that is preventing the rate of photosynthesis from increasing further, even if other factors are increased.
When you increase one factor and the rate rises, that factor was limiting. When you increase one factor and the rate does not rise (plateau), a different factor is now limiting instead.
If rate vs. light intensity shows a plateau at low CO₂ but continues rising when CO₂ is increased → CO₂ concentration was the limiting factor at the plateau.
If rate vs. light intensity plateaus even when CO₂ is raised → something else limits (e.g. temperature or enzyme capacity). Raising temperature would then increase the rate further.
Unlike CO₂ and light (where high values just plateau), excessively high temperatures cause enzyme denaturation → rate falls sharply. This is not just a limiting factor — it is active inhibition.
A farmer grows tomatoes in a greenhouse. A graph shows that increasing light intensity raises the rate of photosynthesis until a plateau is reached at a rate of 40 arbitrary units. A second curve shows that when CO₂ concentration is doubled, the plateau rate rises to 80 units at the same light intensities.
(a) Identify the limiting factor at the plateau of the first curve and explain how you know. [2 marks]
(b) Suggest one other factor the farmer could change to increase the rate beyond 80 units. Explain why. [2 marks]
- (a) CO₂ concentration was the limiting factor [1 mark]; because when CO₂ was increased, the plateau rate doubled — showing there was now sufficient light but insufficient CO₂ at the first plateau [1 mark]
- (b) Temperature (increase it, within the range that doesn’t denature enzymes) [1 mark]; higher temperature increases the kinetic energy of molecules and enzyme activity, increasing the rate of the light-independent reactions [1 mark]. Accept: supplying more artificial light if the current light intensity is already the plateau limit; the answer must include an explanation.
A plant is grown without magnesium ions in the soil. Which symptom would most likely be observed?
- A. Stunted growth and very thin stems
- B. Yellowing of leaves (chlorosis)
- C. Wilting due to loss of turgor pressure
- D. Failure to flower and produce seeds
Leaf Structure
COREA dicotyledonous leaf is a masterpiece of biological engineering, with every layer optimised for photosynthesis. Questions about leaf structure almost always ask you to explain how a structural feature aids photosynthesis — not just identify and describe it. Always link structure to function.
Overall Leaf Adaptations
| Feature | Adaptation to photosynthesis |
|---|---|
| Large surface area | Maximises absorption of light and CO₂ diffusion in |
| Thin | Short diffusion distance for CO₂ to reach all mesophyll cells; allows light to penetrate to lower layers |
Leaf Cross-section — Layer by Layer
| Structure | Description | How it adapts the leaf for photosynthesis |
|---|---|---|
| Waxy cuticle | Transparent waterproof layer on upper surface | Reduces water loss by evaporation; transparent so light can still pass through to the cells below |
| Upper epidermis | Single layer of flat, transparent cells; no chloroplasts | Transparent — allows maximum light transmission to the palisade layer below. Acts as a protective barrier. |
| Palisade mesophyll | Column-shaped cells packed tightly together, each containing many chloroplasts; positioned just below upper epidermis | High chloroplast density near the top of the leaf maximises light absorption. Column shape allows many cells per unit area. Chloroplasts can move within the cell to optimise light capture. |
| Spongy mesophyll | Loosely packed cells with large air spaces between them | Large air spaces create a large internal surface area for gas exchange (CO₂ in, O₂ out). Cells are in contact with air, reducing diffusion distance. |
| Air spaces | Interconnecting spaces in spongy mesophyll connected to stomata | Allow CO₂ to diffuse rapidly from the stomata to all mesophyll cells; allow O₂ to diffuse out |
| Guard cells & stomata | Pairs of guard cells form a pore (stoma) in the lower epidermis; can open and close | Open during the day to allow CO₂ entry and O₂ exit (gas exchange); close at night or when water-stressed to reduce water loss. Most stomata are on the lower surface (away from direct sun) to reduce evaporation. |
| Lower epidermis | Single layer of cells, mostly without chloroplasts; contains stomata | Stomata allow gas exchange; lower surface position limits direct solar heating and excessive water loss |
| Vascular bundles | Veins containing xylem and phloem, branching throughout the leaf | Xylem delivers water and mineral ions to every part of the leaf for photosynthesis; phloem removes sucrose produced by photosynthesis |
| Xylem | Hollow tubes (dead cells) within the vascular bundle | Transports water and dissolved mineral ions from roots to leaves for use in photosynthesis and other reactions |
| Phloem | Living tubes in the vascular bundle | Transports sucrose (product of photosynthesis) away from the leaf to other parts of the plant (translocation) |
| Chloroplasts | Organelles containing chlorophyll; abundant in palisade cells | Site of photosynthesis — chlorophyll absorbs light energy and drives carbohydrate synthesis |
Every mark scheme for leaf structure questions rewards a structure → function link. Simply naming the structure scores at most 1 mark. For full marks:
1. Name the structure → 2. Describe the relevant feature of its structure → 3. Explain how that feature helps photosynthesis.
Example: “The palisade mesophyll cells [1. name] are packed with many chloroplasts and positioned near the upper leaf surface [2. structural feature] so they absorb the maximum amount of light energy for photosynthesis [3. function].”
What is the function of the air spaces in the spongy mesophyll of a leaf?
- A. To store water for use in photosynthesis during dry periods
- B. To allow rapid diffusion of CO₂ to all mesophyll cells and O₂ to diffuse out
- C. To provide structural support and prevent the leaf from wilting
- D. To reflect excess light and prevent overheating of the leaf
Explain how the palisade mesophyll layer is adapted for photosynthesis. Give three adaptations. [3 marks]
- Cells are packed with many chloroplasts → more chlorophyll to absorb light energy [1 mark]
- Cells are positioned near the upper surface of the leaf → first to receive incoming light / maximum light absorption before it is scattered by lower layers [1 mark]
- Cells are column-shaped (tall and narrow) → more cells can fit per unit of leaf surface area / increases the total number of chloroplasts per leaf area [1 mark]
- Accept also: cells are transparent so light can pass through to chloroplasts in deeper layers; OR chloroplasts can move within the cell to optimise position relative to light source [1 mark each if stated with function link]
Comprehensive Practice Questions
Mixed questions across Topics 6.1 and 6.2, in the style of Cambridge IGCSE 0610 Papers 1–4.
Which of the following correctly identifies the reactants and products of photosynthesis?
- A. Reactants: glucose + oxygen Products: carbon dioxide + water
- B. Reactants: carbon dioxide + water Products: glucose + oxygen
- C. Reactants: carbon dioxide + oxygen Products: glucose + water
- D. Reactants: water + oxygen Products: glucose + carbon dioxide
A gardener notices that some plants have yellowing leaves even though they are well-watered and in a sunny position.
(a) Suggest which mineral ion may be deficient, and explain your reasoning. [2 marks]
(b) Explain why this deficiency also reduces the rate of photosynthesis. [2 marks]
(c) Name one other mineral ion essential for plants, state what it is used for, and describe the deficiency symptom. [3 marks]
- (a) Magnesium (Mg²⁺) is deficient [1 mark]; because yellowing (chlorosis) indicates insufficient chlorophyll production, and magnesium is required to make chlorophyll [1 mark]
- (b) Without chlorophyll, the plant cannot absorb light energy [1 mark]; therefore glucose cannot be synthesised from CO₂ and water / photosynthesis cannot occur at the normal rate [1 mark]
- (c) Nitrate (NO₃⁻) [1 mark]; used to make amino acids (and therefore proteins) [1 mark]; deficiency causes stunted growth / thin stems / pale leaves because proteins are needed for growth [1 mark]
A student places a green aquatic plant in three test tubes of hydrogencarbonate indicator:
Tube 1: plant, in bright light | Tube 2: plant, in complete darkness | Tube 3: no plant, in bright light (control)
After 2 hours, the indicator in Tube 1 turns purple, Tube 2 turns yellow, and Tube 3 stays red-orange.
(a) Explain the result in Tube 1. [2 marks]
(b) Explain the result in Tube 2. [2 marks]
(c) State the purpose of Tube 3 (the control). [1 mark]
- (a) In light, photosynthesis rate exceeds respiration rate → the plant removes CO₂ from the water → less CO₂ dissolved → indicator becomes less acidic → turns purple [2 marks]
- (b) In darkness, only respiration occurs (no photosynthesis) → the plant releases CO₂ into the water → more CO₂ dissolved → indicator becomes more acidic → turns yellow [2 marks]
- (c) To show that the colour change is caused by the plant (CO₂ changes due to the plant’s metabolic activity) and not by the light or the indicator itself changing colour without a biological cause [1 mark]
A graph shows the rate of photosynthesis vs. CO₂ concentration at two temperatures (20°C and 30°C). At 20°C, the rate plateaus at 30 units. At 30°C with the same CO₂ range, the rate plateaus at a higher value of 55 units.
(a) Identify the limiting factor at the plateau of the 20°C curve. Explain your answer. [2 marks]
(b) Explain why the plateau is higher at 30°C than at 20°C. [2 marks]
(c) Predict what would happen to the rate at 30°C if the CO₂ concentration were increased beyond the plateau point. [2 marks]
- (a) Temperature is limiting at the plateau [1 mark]; even though CO₂ is being increased, the rate cannot rise further because the enzymes controlling photosynthesis are working at their maximum rate at 20°C / temperature is restricting enzyme activity [1 mark]
- (b) At 30°C, enzyme molecules have more kinetic energy → more frequent effective collisions between enzymes and substrates → faster reaction rate [1 mark]; so the same CO₂ increase can drive a higher maximum rate before another factor becomes limiting [1 mark]
- (c) The rate would not increase beyond the plateau [1 mark]; because at that plateau, temperature (or another factor such as light intensity) is the limiting factor, not CO₂ → adding more CO₂ cannot increase the rate when a different factor is at its limit [1 mark]
High-Frequency Mistakes — Topic 6 Overall
- 💡Oxygen is written as a reactant of photosynthesisOxygen is a product, not a reactant. The reactants are CO₂ and water. Writing O₂ on the left side is one of the most common equation errors in the whole course.
- 🥴Confusing chlorophyll and chloroplastsChlorophyll is the green pigment molecule inside chloroplasts. Chloroplasts are the organelles that contain chlorophyll. Saying “chlorophyll is found in the cell” without specifying chloroplasts loses marks.
- 💊Mixing up nitrate and magnesium functionsNitrate → amino acids (N for Nitrogen in amino group). Magnesium → chlorophyll (Mg is the central atom of the chlorophyll molecule). Deficiency symptoms: nitrate = stunted growth; magnesium = yellow leaves (chlorosis).
- 🐘Sucrose vs glucose transportGlucose is not transported in the phloem — it is converted to sucrose first. The phloem transports sucrose (and amino acids) from source to sink. Glucose is used directly in respiration or converted to starch for local storage.
- 🌿Describing leaf structure without explaining the functionSaying “palisade cells contain many chloroplasts” only scores 1 mark. You must add “so that maximum light energy is absorbed for photosynthesis” to score full marks. Always complete the structure→function chain.
- 📈Saying a plateau in a rate vs. [CO₂] graph means CO₂ is the limiting factorOpposite: a plateau means CO₂ is no longer limiting. Another factor (light or temperature) has become the new limiting factor. Rising part of graph = CO₂ is limiting; plateau = something else limits.
- 🫕Forgetting that temperature can denature photosynthesis enzymesUnlike CO₂ and light (where more always at least maintains rate), very high temperatures cause enzyme denaturation → rate falls. On a temperature vs. rate graph, the curve must fall sharply above the optimum, not just plateau.
- 🌞Saying the waxy cuticle “prevents all water loss”The cuticle reduces water loss — not prevents it entirely. Water still exits through stomata. Also, the cuticle is transparent, which is also an adaptation (allows light through). State both functions.
Photosynthesis is tested across Papers 1–6. Highest-yield items: the word equation with “in the presence of light and chlorophyll” (many lose marks by omitting this); mineral ion functions (nitrate vs magnesium); the five uses of carbohydrates; and the hydrogencarbonate indicator experiment (very likely to appear in Paper 5/6 or Paper 3 as a results-interpretation question). For Extended candidates, limiting factor graph interpretation is a near-certain component of Paper 4 — practise identifying which factor is limiting at any point on a curve and explaining the plateau using enzyme kinetics.