Plant nutrition and photosynthesis

What you'll learn

This guide covers plant nutrition and photosynthesis as tested in CIE IGCSE Co-ordinated Science (Double Award). You'll learn how plants manufacture food using light energy, the leaf structure adaptations that enable this process, and the factors that affect photosynthesis rate. The content follows the specification exactly, focusing on what examiners actually test.

Key terms and definitions

Photosynthesis — the process by which plants synthesise glucose from carbon dioxide and water using light energy, releasing oxygen as a by-product

Chlorophyll — the green pigment found in chloroplasts that absorbs light energy for photosynthesis

Limiting factor — an environmental variable that, when in short supply, restricts the rate of photosynthesis

Stoma (plural: stomata) — a pore in the leaf epidermis through which carbon dioxide enters and oxygen and water vapour exit

Palisade mesophyll — the upper layer of leaf tissue containing densely-packed cells with numerous chloroplasts, where most photosynthesis occurs

Transpiration — the loss of water vapour from plant leaves through stomata

Glucose — the simple sugar (C₆H₁₂O₆) produced during photosynthesis and used by plants for respiration, storage, or conversion to other substances

Guard cells — pairs of specialised cells surrounding each stoma that control its opening and closing

Core concepts

The photosynthesis equation

Photosynthesis is a two-stage chemical process occurring in chloroplasts. The overall equation you must know is:

Word equation: carbon dioxide + water → glucose + oxygen

Symbol equation: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

The equation shows that:

• Six molecules of carbon dioxide react with six molecules of water
• Light energy is absorbed by chlorophyll (shown above the arrow)
• One molecule of glucose and six molecules of oxygen are produced
• The reaction is endothermic (requires energy input)

This equation reverses respiration. Plants photosynthesise during daylight and respire continuously, but photosynthesis rate exceeds respiration rate in adequate light.

Light-dependent and light-independent stages

Though not examined in detail at this level, understand that photosynthesis occurs in two stages:

Light-dependent stage:

• Occurs in chloroplast grana
• Light energy splits water molecules (photolysis)
• Oxygen is released as a by-product
• Energy is captured in chemical form

Light-independent stage:

• Occurs in chloroplast stroma
• Carbon dioxide is used to synthesise glucose
• Uses energy from the light-dependent stage
• Sometimes called the "dark reaction" but occurs during day and night

For IGCSE, focus on the overall equation and factors affecting rate rather than biochemical detail.

Leaf structure and adaptations for photosynthesis

The leaf is specially adapted to maximise photosynthesis efficiency:

External adaptations:

• Large surface area to capture maximum light
• Thin structure allows light penetration and short diffusion distances
• Network of veins delivers water via xylem and removes products via phloem
• Leaf mosaic arrangement on plant minimises overlap and shading

Internal structure (cross-section from top to bottom):

Upper epidermis:

• Transparent single cell layer
• Allows light to pass through
• Covered by waxy cuticle preventing water loss
• Usually no chloroplasts

Palisade mesophyll layer:

• Located directly below upper epidermis
• Column-shaped cells packed with chloroplasts
• Principal site of photosynthesis
• Cells arranged vertically to maximise light absorption

Spongy mesophyll layer:

• Irregular-shaped cells with fewer chloroplasts
• Large air spaces between cells
• Allows gas diffusion throughout leaf
• Moist cell surfaces for dissolving carbon dioxide

Lower epidermis:

• Single cell layer with numerous stomata
• Guard cells control stomatal opening
• Waxy cuticle reduces water loss

Vascular bundles (veins):

• Xylem vessels transport water and minerals from roots
• Phloem tubes transport glucose (as sucrose) to other plant parts
• Provide structural support

Factors affecting photosynthesis rate

Three main factors limit photosynthesis rate. Understanding limiting factors is crucial for exam success.

Light intensity:

• Increases photosynthesis rate as intensity increases
• More light energy available for light-dependent reactions
• Beyond a certain point, light is no longer limiting
• Other factors (CO₂ or temperature) become limiting
• Relationship approximately linear at low intensities

Carbon dioxide concentration:

• Atmospheric CO₂ is approximately 0.04%
• Increasing concentration increases photosynthesis rate
• CO₂ is a raw material for glucose synthesis
• Beyond 0.4-0.5%, rate plateaus
• Greenhouses often enrich CO₂ to increase crop yields

Temperature:

• Affects enzyme activity controlling photosynthesis reactions
• Rate increases with temperature (typically 0-35°C)
• Optimum temperature approximately 25-35°C for most plants
• Above optimum, enzymes denature and rate decreases rapidly
• At low temperatures, insufficient kinetic energy for reactions

The principle of limiting factors:

When one factor is in short supply, increasing it increases photosynthesis rate. Once adequate, that factor is no longer limiting. The factor in shortest supply determines overall rate.

Example: On a winter day with high light intensity but low temperature, increasing light further won't increase rate because temperature is the limiting factor.

Uses of glucose produced in photosynthesis

Plants don't simply store all glucose produced. They convert it for various purposes:

Respiration:

• Immediate energy release for plant processes
• Growth, active transport, protein synthesis
• Occurs in all plant cells continuously

Starch storage:

• Glucose converted to starch in chloroplasts (temporary storage)
• Starch stored in roots (e.g., potato tubers), seeds, fruits
• Starch is insoluble so doesn't affect water potential
• Converted back to glucose when needed

Cellulose synthesis:

• For cell wall construction
• Provides structural strength
• Permanent component, not reconverted

Protein synthesis:

• Glucose combined with nitrogen (from nitrates absorbed by roots)
• Forms amino acids then proteins
• Essential for enzymes and cell growth

Lipid synthesis:

• For cell membranes
• Energy storage in seeds (e.g., sunflower seeds, peanuts)
• More energy-dense than carbohydrates

Mineral requirements for healthy plant growth

Plants require minerals absorbed through roots:

Nitrates (containing nitrogen):

• Essential for amino acid and protein synthesis
• Needed for chlorophyll production
• Deficiency causes stunted growth and yellow older leaves (chlorosis)

Magnesium:

• Central component of chlorophyll molecules
• Deficiency causes yellow leaves (chlorosis)
• Affects older leaves first as magnesium is mobile within plant

Phosphates (containing phosphorus):

• Essential for DNA, RNA, and ATP synthesis
• Important for root growth and energy transfer
• Deficiency causes poor root development and purple leaf tints

Farmers add fertilisers containing these minerals to maintain soil fertility and maximise crop yields.

Investigating photosynthesis experimentally

Testing a leaf for starch (demonstrates photosynthesis has occurred):

Method:

1. De-starch plant by placing in darkness for 24-48 hours
2. Cover part of leaf with opaque material (e.g., foil)
3. Place plant in bright light for several hours
4. Remove leaf and place in boiling water (kills cells, stops reactions)
5. Place leaf in hot ethanol (removes chlorophyll, making colour changes visible)
6. Rinse in cold water (softens brittle leaf)
7. Add iodine solution

Results:

• Previously covered areas remain brown/orange (no starch)
• Exposed areas turn blue-black (starch present)
• Proves light is necessary for photosynthesis

Safety: Hot ethanol is highly flammable; heat using water bath, not direct flame.

Investigating rate of photosynthesis using aquatic plants:

Canadian pondweed (Elodea) or Cabomba are commonly used:

Method:

1. Cut pondweed stem at angle
2. Place stem cut-end upwards in test tube of water
3. Add sodium hydrogencarbonate (provides CO₂)
4. Position lamp at measured distance
5. Allow equilibration period (5 minutes)
6. Count oxygen bubbles produced per minute (or collect gas volume)
7. Repeat at different distances/light intensities

Variables:

• Independent: light intensity (altered by changing lamp distance)
• Dependent: bubble rate or oxygen volume
• Control: temperature, CO₂ concentration, time period

Light intensity ∝ 1/distance² (inverse square law)

Worked examples

Example 1: Interpreting a limiting factors graph

Question: The graph shows photosynthesis rate at different carbon dioxide concentrations and temperatures.

At 15°C, the rate increases from 0.04% to 0.10% CO₂, then plateaus. At 25°C, the rate increases more steeply and continues increasing at 0.10% CO₂.

(a) Identify the limiting factor at 0.04% CO₂ concentration. [1] (b) Explain why the rate plateaus at 15°C but not at 25°C when CO₂ reaches 0.10%. [3]

Model answer:

(a) Carbon dioxide concentration [1]

(b) At 15°C, once CO₂ increases beyond 0.10%, it is no longer in short supply [1]. Temperature becomes the limiting factor [1] because enzyme activity is low at 15°C, preventing faster reactions even with more CO₂ available [1].

At 25°C, enzymes work faster [1], so higher CO₂ concentrations continue to increase photosynthesis rate [1].

Mark scheme notes: Part (b) requires explanation (command word), so stating facts without reasoning loses marks. Link the factor to the mechanism (enzyme activity).

Example 2: Leaf structure and function

Question: Describe how the palisade mesophyll layer is adapted for photosynthesis. [4]

Model answer:

Located near upper surface / directly below upper epidermis, so receives maximum light intensity [1]. Cells are column-shaped / cylindrical, allowing many to pack into a small area [1]. Contains many/numerous chloroplasts [1], which contain chlorophyll to absorb light energy [1].

Mark scheme notes: "Describe" requires stating features and their relevance. Four marks available, so provide four distinct points. Simply stating "has chloroplasts" without quantifying ("many") or explaining their function may not score.

Example 3: Calculation involving photosynthesis rate

Question: A student investigated photosynthesis rate using pondweed. At 10 cm distance, the plant produced 45 bubbles in 3 minutes. At 20 cm distance, it produced 12 bubbles in 3 minutes.

(a) Calculate the rate of bubble production per minute at each distance. [2] (b) Explain why the rate decreased at greater distance. [2]

Model answer:

(a) At 10 cm: 45 ÷ 3 = 15 bubbles per minute [1] At 20 cm: 12 ÷ 3 = 4 bubbles per minute [1]

(b) Light intensity decreases with distance [1]. Light is a limiting factor, so lower intensity reduces photosynthesis rate / fewer reactions occur [1].

Mark scheme notes: Show working in calculations. Units are essential. In explanations, connect the variable to its effect on the process.

Common mistakes and how to avoid them

• Confusing photosynthesis with respiration. Photosynthesis produces glucose and oxygen; respiration breaks down glucose using oxygen. They are opposite processes. Plants do both, but photosynthesis requires light.

• Writing the photosynthesis equation incorrectly. The products are glucose and oxygen, not water. Carbon dioxide and water are reactants (on the left). Don't forget to show that light energy and chlorophyll are needed (write above the arrow).

• Saying stomata "breathe in" carbon dioxide. Stomata don't actively breathe. Gas exchange occurs by diffusion through stomata down concentration gradients. Use precise terminology.

• Mixing up starch test steps. The sequence matters: boiling water first (kills cells), then ethanol (removes chlorophyll), then rinse, then iodine. Remember ethanol must be heated in a water bath, not over a flame.

• Misunderstanding limiting factors. When a graph plateaus, one factor is no longer limiting—another has become limiting. Identify which factor is preventing a further increase in rate.

• Forgetting that plants need minerals. Plants don't get proteins or vitamins from soil. They absorb minerals (nitrates, magnesium, phosphates) and synthesise everything else from glucose.

Exam technique for "Plant nutrition and photosynthesis"

• Command word awareness. "State" requires a brief answer without explanation. "Explain" requires reasoning (use "because," "so," "therefore"). "Describe" needs features and observations without necessarily explaining why. Match answer length to marks available.

• Equation questions award marks for correct formulae, balanced equation, and state symbols if required. Write the word equation if you're unsure of symbols—partial marks are available. Always show that light energy and chlorophyll are needed.

• Graph interpretation questions. Read axes carefully, identify which variable is being changed (independent variable), and state what this shows about photosynthesis. Look for plateau regions indicating limiting factors have changed.

• Use correct biological terminology. "Palisade mesophyll" not "top cells," "chloroplast" not "green bit," "diffusion" not "movement." Precision gains marks; vague language loses them.

Quick revision summary

Photosynthesis is the process where plants produce glucose from carbon dioxide and water using light energy absorbed by chlorophyll. The equation is 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. Leaf structure is adapted with palisade mesophyll cells containing numerous chloroplasts. Three main limiting factors affect rate: light intensity, carbon dioxide concentration, and temperature. Plants use glucose for respiration, convert it to starch for storage, or synthesise cellulose, proteins, and lipids. Minerals like nitrates and magnesium are essential for healthy growth.