Nutrition in Plants

What you'll learn

Plants manufacture their own food through photosynthesis, making them autotrophic organisms. This topic examines the process of photosynthesis, the structure and function of leaves, factors affecting the rate of photosynthesis, and the mineral nutrition requirements of plants. Understanding nutrition in plants accounts for significant marks across CIE IGCSE Biology Paper 2 and Paper 4, particularly in experimental design questions involving limiting factors.

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.

Chloroplast — the organelle in plant cells where photosynthesis occurs, containing thylakoid membranes with chlorophyll.

Limiting factor — an environmental condition that, when in short supply, restricts the rate of photosynthesis (light intensity, carbon dioxide concentration, or temperature).

Stoma (plural: stomata) — a pore in the leaf epidermis, surrounded by guard cells, that allows gas exchange.

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

Magnesium — a mineral ion required for chlorophyll synthesis; deficiency causes chlorosis (yellowing of leaves).

Nitrate — a mineral ion required for amino acid and protein synthesis; deficiency causes stunted growth and yellow older leaves.

Core concepts

The photosynthesis equation

The word equation for photosynthesis:

carbon dioxide + water → glucose + oxygen

The balanced chemical equation:

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

This equation must be memorised. Light energy (indicated above the arrow) is absorbed by chlorophyll and converted into chemical energy stored in glucose bonds. Photosynthesis is an endothermic reaction because it requires energy input.

Chloroplast structure and leaf anatomy

Chloroplasts contain:

• Thylakoid membranes stacked into grana where chlorophyll molecules are located
• Stroma — the fluid-filled matrix where some photosynthesis reactions occur
• A double membrane envelope surrounding the organelle

Leaf adaptations for efficient photosynthesis:

• Large surface area to absorb maximum light energy
• Thin structure ensures short diffusion distances for gases
• Waxy cuticle on upper epidermis reduces water loss but is transparent to light
• Upper epidermis is transparent, allowing light penetration to photosynthetic tissues
• Palisade mesophyll cells contain 50-100 chloroplasts and are positioned near the upper surface for maximum light absorption
• Spongy mesophyll has air spaces for efficient gas diffusion to all cells
• Stomata (mainly on lower epidermis) allow CO₂ to diffuse in and O₂ to diffuse out
• Guard cells control stomatal opening: they become turgid in light, opening the stoma; become flaccid in darkness, closing the stoma
• Xylem vessels deliver water for photosynthesis
• Phloem sieve tubes transport glucose (as sucrose) away from the leaf

Factors affecting the rate of photosynthesis

Three main factors limit photosynthesis rate:

Light intensity

• As light intensity increases, the rate of photosynthesis increases proportionally
• This continues until another factor becomes limiting
• At very high light intensities, the rate plateaus
• The relationship follows the inverse square law: doubling the distance from a light source quarters the light intensity

Carbon dioxide concentration

• Normal atmospheric CO₂ is approximately 0.04%
• Increasing CO₂ concentration increases photosynthesis rate up to approximately 0.4%
• Beyond this, CO₂ is no longer limiting and the rate plateaus
• Commercial greenhouses often enrich CO₂ to 0.1% to increase crop yields

Temperature

• Photosynthesis is controlled by enzymes
• As temperature increases from 0°C to an optimum (typically 25-35°C), the rate increases as enzyme and substrate molecules have more kinetic energy and collide more frequently
• Above the optimum temperature, enzymes begin to denature and the rate decreases rapidly
• At very high temperatures (above 45°C), enzymes are completely denatured and photosynthesis stops

The law of limiting factors states that the rate of photosynthesis is limited by whichever factor is in shortest supply. When interpreting graphs:

• A steep rising section indicates that factor is limiting
• A plateau indicates that factor is no longer limiting; another factor now restricts the rate
• Comparing curves at different conditions (e.g., two temperatures) shows which factor is limiting under each condition

Uses of glucose produced in photosynthesis

Glucose synthesised during photosynthesis has multiple fates:

1. Respiration — glucose is broken down to release energy for metabolic processes
2. Conversion to sucrose — the transport form of sugar, translocated in phloem to all plant organs
3. Conversion to starch — the storage form in leaves (temporarily) and in storage organs like potato tubers; starch is insoluble so does not affect water potential
4. Synthesis of cellulose — for cell wall construction during growth
5. Synthesis of lipids — for cell membranes and storage in seeds
6. Synthesis of amino acids — when combined with nitrate ions absorbed from soil, glucose is used to make amino acids for protein synthesis

Testing for starch in leaves

The standard procedure to demonstrate photosynthesis has occurred:

1. Destarch the plant by placing it in darkness for 24-48 hours so stored starch is used up
2. Expose the plant to light for several hours
3. Remove a leaf and kill it by placing in boiling water (this stops enzyme activity)
4. Place the leaf in hot ethanol in a water bath to extract chlorophyll (the leaf turns pale/white)
5. Soften the brittle leaf by dipping in warm water
6. Spread the leaf flat on a white tile and add iodine solution
7. A blue-black colour indicates starch is present; a brown/orange colour indicates absence of starch

Variations test specific requirements:

• Variegated leaves demonstrate chlorophyll is needed (only green areas turn blue-black)
• Leaves partially covered with aluminium foil show light is needed (only uncovered areas turn blue-black)
• Plant kept in a CO₂-free environment (e.g., with soda lime) shows no starch production, proving CO₂ is needed

Mineral nutrition

Plants require mineral ions absorbed through root hair cells from soil solution:

Nitrate ions (NO₃⁻)

• Required for synthesis of amino acids and therefore proteins
• Proteins needed for enzymes, cell structure, and growth
• Deficiency symptoms: stunted growth, older leaves turn yellow (chlorosis) as nitrogen is mobile and moved from old to young leaves

Magnesium ions (Mg²⁺)

• Essential component of chlorophyll molecules
• Without magnesium, chlorophyll cannot be synthesised
• Deficiency symptoms: leaves turn yellow (chlorosis), particularly older leaves, reducing photosynthesis rate and plant growth

Phosphate ions (PO₄³⁻)

• Required for synthesis of DNA, RNA, and ATP
• Essential for respiration and energy transfer
• Deficiency symptoms: poor root growth, purple younger leaves

Potassium ions (K⁺)

• Required for enzyme activation
• Essential for efficient photosynthesis and respiration
• Deficiency symptoms: yellow leaves with dead spots, poor fruit and flower development

Mineral ions are absorbed by active transport (not diffusion) because they are needed in higher concentrations inside root cells than in soil solution. This requires energy from respiration.

Worked examples

Example 1: Interpreting a limiting factor graph

Question: A student investigated the effect of light intensity on photosynthesis rate at two different temperatures. The graph shows the results.

[Graph shows two curves: both start at origin, rise steeply, then plateau; the 25°C curve plateaus higher than the 15°C curve]

(a) Explain the shape of the curve at 15°C between points A and B where the curve is rising steeply. [2 marks]

(b) Explain why the curve plateaus at point C. [2 marks]

(c) Explain why the curve at 25°C is higher than at 15°C beyond point C. [2 marks]

Mark scheme answers:

(a)

• Light intensity is the limiting factor [1]
• As light intensity increases, the rate of photosynthesis increases because more light energy is available for the light-dependent reactions / more chlorophyll molecules are activated [1]

(b)

• Light is no longer the limiting factor [1]
• Another factor is now limiting, such as carbon dioxide concentration or temperature [1]

(c)

• At 25°C, enzymes have more kinetic energy [1]
• Enzyme-substrate collisions occur more frequently, increasing the rate of reactions in photosynthesis [1]

Alternatively: Enzymes work faster / closer to optimum temperature at 25°C than at 15°C [2]

Example 2: Experimental design

Question: A student set up an experiment to investigate whether carbon dioxide is needed for photosynthesis.

Describe how the student could use two destarched plants to carry out this investigation. Include details of:

• how to set up the experiment
• what results would be expected
• how to test for the results [6 marks]

Mark scheme answer:

• Place one plant in a sealed container with soda lime / sodium hydroxide solution [1]
• Place the second plant (control) in a sealed container without soda lime / with a source of CO₂ [1]
• Expose both plants to light for several hours [1]
• Remove a leaf from each plant and test for starch using the iodine test [1]
• The leaf from the plant without CO₂ will remain brown/orange (no starch) [1]
• The leaf from the control plant will turn blue-black (starch present), showing CO₂ is necessary for photosynthesis [1]

Accept: boiling in ethanol to remove chlorophyll (method mark)

Example 3: Mineral deficiency

Question: A farmer notices that the older leaves on his crop plants are turning yellow, but the younger leaves remain green. Growth is also slower than expected.

(a) Identify which mineral deficiency is most likely causing these symptoms. [1 mark]

(b) Explain why this deficiency causes yellowing of leaves. [2 marks]

(c) Suggest why older leaves are affected before younger leaves. [1 mark]

Mark scheme answers:

(a) Nitrate / nitrogen deficiency [1]

(b)

• Nitrate is required to make amino acids / proteins [1]
• Without sufficient protein, the plant cannot make enough chlorophyll / enzymes for photosynthesis, causing chlorosis [1]

(c) Nitrate is mobile in the plant and is moved from older leaves to younger growing leaves / younger leaves have priority [1]

Common mistakes and how to avoid them

• Mistake: Writing "plants respire at night and photosynthesise during the day." Correction: Plants respire continuously, 24 hours a day. Photosynthesis only occurs in light. During daylight, plants both photosynthesise and respire; at night, they only respire.

• Mistake: Stating "chlorophyll is a food for the plant" or "chlorophyll is used up during photosynthesis." Correction: Chlorophyll is a pigment that absorbs light energy; it is not consumed in the reaction. Glucose is the food (product) made by photosynthesis.

• Mistake: Confusing the raw materials and products, e.g., "plants take in oxygen and release carbon dioxide in photosynthesis." Correction: In photosynthesis, plants take in carbon dioxide and release oxygen. In respiration, they take in oxygen and release carbon dioxide.

• Mistake: Writing "light intensity is directly proportional to distance from a light source." Correction: Light intensity follows the inverse square law: it is inversely proportional to the square of the distance. Doubling the distance quarters the light intensity.

• Mistake: Explaining a plateau on a graph by saying "the plant has died" or "the plant is tired." Correction: The plateau indicates a different factor has become limiting. Identify which factor is now in short supply (e.g., CO₂ concentration or temperature).

• Mistake: Describing magnesium deficiency as affecting only old leaves and nitrate deficiency as affecting only young leaves. Correction: Both deficiencies typically affect older leaves first because these minerals are mobile and transported to younger, growing tissues. However, nitrate deficiency is more commonly associated with stunted growth.

Exam technique for "Nutrition in Plants"

• Explain questions (2-3 marks): Always give a reason, not just a description. For limiting factors, state which factor is limiting AND explain how it affects the rate using enzyme kinetics or light energy availability. Link cause and effect explicitly.

• Describe an experiment questions (5-6 marks): Follow a logical sequence—starting conditions (destarching), experimental setup with control, variables controlled, duration, method of measurement/testing, expected results, and conclusion. The iodine test for starch requires the full method including boiling in ethanol.

• Graph interpretation questions: Identify the section (rising, plateau), name the limiting factor for that section, and explain the trend using collision theory for temperature or substrate availability for CO₂ and light. Compare curves by identifying where and why they differ.

• Calculate percentage change or rate questions: Show working clearly. Rate = change ÷ time. Use correct units (e.g., cm³/min for oxygen production). For percentage change: (change ÷ original) × 100.

Quick revision summary

Photosynthesis is the synthesis of glucose from carbon dioxide and water using light energy absorbed by chlorophyll in chloroplasts, releasing oxygen. The rate is limited by light intensity, CO₂ concentration, or temperature—whichever is in shortest supply. Leaves are adapted with large surface area, palisade cells rich in chloroplasts, stomata for gas exchange, and thin structure for short diffusion distances. Glucose is used for respiration, converted to starch (storage) or cellulose (cell walls), or combined with nitrate to make amino acids. Plants require mineral ions: nitrate for protein synthesis and magnesium for chlorophyll production. Deficiencies cause characteristic symptoms including chlorosis and stunted growth.