Plant Nutrition
Summary: Plants are autotrophs — they produce their own food through photosynthesis. The process uses light energy to convert carbon dioxide and water into glucose and oxygen, with chlorophyll acting as the catalyst. The leaf is a specialised organ for photosynthesis, with adaptations at the tissue and cellular level. The rate of photosynthesis is limited by light intensity, carbon dioxide concentration, and temperature. Tags: igcse biology photosynthesis plant-nutrition leaf-structure limiting-factors Created: 2026-07-16 Last Updated: 2026-07-16
1. Photosynthesis
Definition: Photosynthesis is the process by which plants manufacture carbohydrates (glucose) from raw materials (carbon dioxide and water) using light energy, with chlorophyll acting as the catalyst.
Word equation:
Carbon dioxide + Water —(light energy, chlorophyll)⇒ Glucose + Oxygen
Balanced chemical equation:
6CO2 + 6H2O —(light energy, chlorophyll)⇒ C6H12O6 + 6O2
Key points:
- Photosynthesis is an endothermic reaction — it absorbs energy (from sunlight)
- Chlorophyll is the green pigment found in chloroplasts that absorbs light energy. It is NOT used up in the reaction — it acts as a catalyst
- Light energy is converted into chemical energy stored in the bonds of glucose
- The products are glucose (used for respiration, storage, or conversion to other substances) and oxygen (released as a by-product or used for respiration)
2. Uses of Glucose in Plants
The glucose produced by photosynthesis is used in several ways:
| Use | Details |
|---|---|
| Respiration | Glucose is broken down in cells (aerobic respiration) to release energy (ATP) for metabolism, growth, and active transport |
| Starch storage | Excess glucose is converted to starch for long-term energy storage. Starch is insoluble — it does not affect water potential and is compact. Stored in leaves, stems, and roots |
| Cellulose synthesis | Glucose is converted to cellulose — the structural polysaccharide used to build cell walls |
| Sucrose transport | Glucose is converted to sucrose for transport in the phloem to other parts of the plant (translocation) |
| Amino acid and protein synthesis | Glucose is combined with nitrate ions (absorbed from the soil) to form amino acids, which are then used to synthesise proteins |
| Lipid synthesis | Glucose is used to make lipids (fats and oils) for storage and cell membranes |
Mineral requirements:
- Nitrate ions (NO3-) — needed for making amino acids and proteins. Deficiency: stunted growth, yellowing of older leaves
- Magnesium ions (Mg2+) — needed for making chlorophyll. Deficiency: yellowing between leaf veins (chlorosis), reduced photosynthesis
3. Leaf Structure
The leaf is a specialised organ adapted for photosynthesis. It consists of several distinct tissues:
Cross-section of a Leaf (top to bottom)
| Layer | Description | Adaptations for Photosynthesis |
|---|---|---|
| Waxy cuticle | Thin, transparent, waterproof layer on the upper surface | Transparent to allow light through; waterproof to reduce water loss by evaporation |
| Upper epidermis | Single layer of transparent cells on the upper surface | Transparent — allows light to pass through to the palisade layer below; no chloroplasts |
| Palisade mesophyll | Tightly packed, cylindrical cells containing many chloroplasts | Located near the upper surface to receive maximum light; many chloroplasts for maximum light absorption; tightly packed — no wasted space |
| Spongy mesophyll | Loosely packed, irregularly shaped cells with fewer chloroplasts | Air spaces between cells allow rapid diffusion of CO2 and O2; some chloroplasts for photosynthesis |
| Vascular bundle (xylem + phloem) | Xylem vessels (upper part of vein) transport water and minerals; phloem (lower part) transports sucrose | Located in the centre of the leaf — close proximity to all cells for efficient transport |
| Lower epidermis | Single layer of cells on the lower surface; contains guard cells and stomata | Guard cells control opening/closing of stomata to regulate gas exchange and water loss |
Stomata and Guard Cells
Stomata (singular: stoma) are pores in the lower epidermis that allow gas exchange.
Structure of guard cells:
- Two guard cells surround each stoma
- Guard cells contain chloroplasts (unlike other epidermal cells)
- The inner wall (facing the pore) is thicker than the outer wall
How stomata open and close:
| Condition | What Happens | Result |
|---|---|---|
| Day / High light intensity | Guard cells photosynthesise → produce glucose → water enters by osmosis (lowered water potential) → guard cells become turgid and curve (thinner outer wall stretches more) | Stoma opens — CO2 enters for photosynthesis |
| Night / Low light intensity | No photosynthesis → water leaves guard cells → guard cells become flaccid | Stoma closes — reduces water loss when no photosynthesis is occurring |
Functions of stomata:
- Allow carbon dioxide to diffuse into the leaf for photosynthesis
- Allow oxygen (by-product of photosynthesis) to diffuse out of the leaf
- Allow water vapour to escape (transpiration) — this is inevitable but regulated
4. Limiting Factors of Photosynthesis
The rate of photosynthesis is affected by several factors. A limiting factor is the factor that is in shortest supply, preventing the rate from increasing further.
Light Intensity
- As light intensity increases, the rate of photosynthesis increases proportionally
- Eventually, the rate plateaus — light is no longer the limiting factor (something else, e.g. CO2 or temperature, is now limiting)
Carbon Dioxide Concentration
- Atmospheric CO2 concentration is ~0.04% — this is often the limiting factor in natural conditions
- Increasing CO2 concentration increases the rate of photosynthesis
- Eventually plateaus when another factor becomes limiting
Temperature
- Photosynthesis is controlled by enzymes (e.g. rubisco in the Calvin cycle)
- As temperature increases, enzyme activity increases (more kinetic energy, more collisions)
- Above the optimum temperature (~25-30 degrees C for most plants), enzymes denature and the rate falls sharply
Interpreting limiting factor graphs:
- The plateau region indicates the current limiting factor is no longer the factor on the x-axis
- To increase rate beyond a plateau, increase the factor that is actually limiting (not the one on the x-axis)
- At the very start of a curve, the factor on the x-axis IS the limiting factor (rate increases proportionally with it)
5. Testing a Leaf for Starch
This experiment tests whether photosynthesis has occurred (starch is a product of photosynthesis).
Method:
- Boil the leaf in water for 2 minutes — kills the leaf (stops metabolic reactions) and softens it
- Transfer the leaf to a test tube of ethanol (alcohol) and place in a hot water bath — the ethanol removes chlorophyll (decolourises the leaf). Never heat ethanol directly with a flame — it is highly flammable
- Wash the leaf in water — softens it again and removes ethanol
- Spread the leaf flat on a white tile and add a few drops of iodine solution
Results:
- Blue-black colour → starch is present → photosynthesis has occurred
- Orange-brown colour (unchanged) → no starch → no photosynthesis
Control variables: The leaf must first be destarched (placed in darkness for 24-48 hours) to ensure any starch present at the start of the experiment does not affect results.
Common investigations using this technique:
- Testing whether light is needed (cover part of leaf with black paper/aluminium foil)
- Testing whether chlorophyll is needed (use a variegated leaf with green and white areas)
- Testing whether CO2 is needed (use soda lime to absorb CO2 in one setup)
Pondweed Experiment — Measuring Rate of Photosynthesis
- Place a piece of aquatic plant (e.g. Elodea/pondweed) in a beaker of water
- Place a light source at a measured distance
- Count the number of oxygen bubbles released per minute as a measure of the rate of photosynthesis
- Vary the distance of the lamp to change light intensity, or use sodium hydrogen carbonate (NaHCO3) in the water to vary CO2 concentration
- A gas syringe or inverted measuring cylinder can be used to collect and measure gas volume — more accurate than counting bubbles
Sources
- BBC Bitesize GCSE Biology — Photosynthesis, BBC (free educational resource)
- OpenStax Biology 2e — Ch. 8 Photosynthesis, Rice University (free, CC BY 4.0)
- Cambridge IGCSE Biology 0610 — Syllabus 6: Plant nutrition, Cambridge Assessment International Education
- CK-12 Biology for High School — Photosynthesis, CK-12 Foundation (free, CC BY-NC 3.0)
Related Notes
- Biological Molecules — Structure of starch, cellulose, glucose, and lipids produced by plants
- Transport in Plants — Xylem and phloem transport water, minerals, and sucrose
- Enzymes — Enzymes like rubisco catalyse the reactions of photosynthesis
- Organisms and their Environment — Photosynthesis as the foundation of food chains; carbon cycle
- Movement Into and Out of Cells — Diffusion of CO2 and O2 through stomata; osmosis in guard cells
- IGCSE-Bio-Index — Full IGCSE Biology index
Common Misconceptions
| Misconception | Reality |
|---|---|
| ”Plants get their food from the soil” | Plants make their own food (glucose) through photosynthesis. They absorb mineral ions (not food) from the soil |
| ”Photosynthesis produces energy” | Photosynthesis converts light energy into chemical energy stored in glucose — energy is not “produced” or “created" |
| "Photosynthesis only happens in leaves” | Photosynthesis occurs in any green part of the plant (e.g. green stems), not just leaves |
| ”Plants photosynthesise and respire — it’s the same thing” | Photosynthesis builds glucose (anabolic, endothermic); respiration breaks down glucose (catabolic, exothermic). They are opposite processes |
| ”Plants only respire at night” | Plants respire 24 hours a day — they need energy continuously. Photosynthesis only occurs during daylight |
| ”The waxy cuticle is mainly for preventing water loss” | True, but it is also transparent — an adaptation that allows light through to the palisade layer |
| ”Starch is a sugar” | Starch is a polysaccharide (polymer of glucose). It is insoluble and used for storage, unlike glucose which is soluble |