Nutrition
Biology· 2.18–2.33B (2017 Spec Issue 3)· 45 min read
1. Photosynthesis in Flowering Plants★★☆☆☆⏱ 10 min
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Photosynthesis
The process by which green plants use light energy trapped by chlorophyll to convert carbon dioxide and water into glucose (stored chemical energy) and oxygen, released as a waste product.
Photosynthesis is essential for all ecosystems: it converts light energy to chemical energy that passes up food chains, and produces oxygen for aerobic respiration in all living organisms.
Word equation: carbon dioxide + water glucose + oxygen (conditions: light, chlorophyll)
The rate of photosynthesis is controlled by three main limiting factors: light intensity, carbon dioxide concentration, and temperature. When one factor is in shortest supply, it limits the rate, and increasing it will raise the rate until another factor becomes limiting (seen as a plateau on rate graphs).
Key leaf adaptations for photosynthesis: waxy cuticle (transparent, reduces water loss), upper epidermis (transparent to let light pass), palisade mesophyll (packed with chloroplasts near the top of the leaf to capture maximum light), spongy mesophyll (air spaces for gas diffusion), stomata and guard cells (control gas exchange and water loss), vascular bundle (xylem delivers water, phloem transports glucose away).
Plants require mineral ions for growth: magnesium ions to make chlorophyll (deficiency causes yellowing leaves, chlorosis), and nitrate ions to make amino acids for protein production (deficiency causes stunted growth).
A student measures the rate of photosynthesis in tomato plants as they increase CO₂ concentration at 20°C with constant high light intensity. The rate increases up to 0.1% CO₂, then plateaus. Explain why the rate plateaus.
- 1
Before the plateau, CO₂ concentration was the limiting factor, so increasing CO₂ raised the rate of photosynthesis.
- 2
At 0.1% CO₂, CO₂ is no longer the limiting factor, so further increases do not raise the rate.
- 3
The new limiting factor is most likely temperature, as 20°C is below the optimum temperature for plant enzymes involved in photosynthesis.
Exam tip:
Always name the new limiting factor at the plateau in graph questions to get full marks, as per marking scheme requirements.
2. Core Photosynthesis Required Practical★★★☆☆⏱ 8 min
The photosynthesis practical tests three key concepts: 1) evolution of oxygen from water plants (e.g. Elodea) to measure rate changes with different limiting factors, 2) production of starch as evidence of photosynthesis, and 3) requirements for light, CO₂ and chlorophyll for photosynthesis.
Destarch plants first by leaving them in the dark for 24 hours to remove all stored starch.
Test for starch by boiling the leaf to kill cells, boiling in ethanol to remove chlorophyll, rinsing, and adding iodine: blue-black = starch present, brown = no starch.
Test for light requirement: cover part of a leaf with foil, leave in light, test for starch (only uncovered parts will have starch).
Test for CO₂ requirement: place a plant in a sealed container with soda lime (absorbs CO₂), leave in light, test for starch (no starch produced).
Test for chlorophyll requirement: test a variegated leaf (only green parts have chlorophyll), only green areas will produce starch.
A student tests a variegated leaf from a destarched plant that has been left in sunlight for 6 hours. Predict the iodine test result and explain your answer.
- 1
The green parts of the leaf (containing chlorophyll) will turn blue-black, showing starch is present.
- 2
The non-green parts of the leaf (no chlorophyll) will stay brown, showing no starch is produced.
- 3
This confirms chlorophyll is required for photosynthesis to produce starch.
Exam tip:
Always mention destarching as the first step of any photosynthesis practical testing starch production, to prove starch detected is made during the experiment, not pre-stored.
3. Human Balanced Diet & Energy Requirements★★☆☆☆⏱ 8 min
Balanced Diet
A diet containing the correct proportions of carbohydrate, protein, lipid, vitamins, minerals, water and dietary fibre to meet the body's functional needs.
Component | Sources | Key Function | Deficiency Symptom |
|---|---|---|---|
Carbohydrate | Bread, rice, pasta | Main energy source | Fatigue, weight loss |
Protein | Meat, fish, pulses | Growth, tissue repair, enzyme production | Kwashiorkor (stunted growth, swollen abdomen) |
Lipid | Butter, oil, nuts | Long-term energy store, insulation | Fat-soluble vitamin deficiency |
Vitamin A | Carrots, liver | Vision, immune function | Night blindness |
Vitamin C | Citrus fruit, veg | Collagen production, immune function | Scurvy (bleeding gums, slow healing) |
Vitamin D | Sunlight, oily fish | Calcium absorption, bone health | Rickets (soft deformed bones) |
Calcium | Milk, green leafy veg | Bone/teeth health, muscle contraction | Rickets, osteoporosis |
Iron | Red meat, spinach | Haemoglobin production for oxygen transport | Anaemia (tiredness, shortness of breath) |
Water | Drinks, fruit/veg | Transport, chemical reactions | Dehydration |
Dietary fibre | Whole grains, fruit/veg | Aids peristalsis, prevents constipation | Constipation, increased bowel cancer risk |
Human energy requirements vary with three key factors: age (children/teens need more energy for growth), activity levels (active people need more energy for muscle function), and pregnancy (extra energy for fetus growth, tissue development and breastfeeding preparation).
Explain why a 30-year-old pregnant woman has a higher energy requirement than a non-pregnant 30-year-old woman with the same activity level.
- 1
The pregnant woman needs extra energy to support the growth and development of the fetus.
- 2
Additional energy is needed for the growth of supporting tissues including the placenta, enlarged uterus and increased blood volume.
- 3
Extra energy is also stored in preparation for breastfeeding after birth.
Exam tip:
When comparing energy requirements, always link differences directly to a specific biological need to get full marks, rather than giving general statements.
4. Structure & Function of the Alimentary Canal★★★☆☆⏱ 10 min
The human alimentary canal is the sequence of organs food passes through during digestion: mouth, oesophagus, stomach, small intestine (duodenum, ileum), large intestine (colon, rectum). Associated organs include the pancreas, liver and gall bladder.
Food is moved through the gut by peristalsis: waves of contraction of circular and longitudinal muscle layers in the gut wall, which push food along even when you are upside down.
Digestive Enzymes
Biological catalysts that break down large insoluble food molecules into small soluble molecules that can be absorbed into the bloodstream.
Enzyme(s) | Substrate | End Product |
|---|---|---|
Amylase (mouth, pancreas) | Starch | Maltose |
Maltase (small intestine lining) | Maltose | Glucose |
Proteases (stomach, pancreas, small intestine) | Protein | Amino acids |
Lipases (pancreas, small intestine) | Lipids | Fatty acids + glycerol |
Bile is a yellow-green alkaline fluid produced by the liver, stored in the gall bladder, and released into the duodenum. It has two key functions: 1) neutralises acidic stomach acid to provide the optimum alkaline pH for pancreatic and small intestine enzymes to work, 2) emulsifies lipids, breaking large fat droplets into small droplets to increase surface area for lipase action. Bile is not an enzyme.
A patient has their gall bladder removed. Explain why they are advised to eat a low-fat diet after surgery.
- 1
The gall bladder stores bile produced by the liver, so without it, bile is released continuously in small amounts rather than in large amounts after a meal.
- 2
Less bile is available to emulsify large fat droplets, so lipase has a smaller surface area to act on, leading to incomplete fat digestion.
- 3
Incomplete fat digestion causes diarrhoea and discomfort, so a low-fat diet reduces these symptoms.
Exam tip:
Never state that bile is made in the gall bladder: this is a common marking point error. Always specify it is made in the liver and stored in the gall bladder.
5. Small Intestine Absorption & Biology-only Calorimetry Practical★★★☆☆⏱ 9 min
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The small intestine is highly adapted for absorption of digested food molecules: it is very long (gives time for digestion and absorption), covered in thousands of villi and microvilli (massively increases surface area), villi have a thin one-cell thick epithelium (short diffusion distance), a dense capillary network (carries away absorbed glucose and amino acids to maintain a concentration gradient), and a lacteal (lymphatic vessel that absorbs fatty acids and glycerol).
The food calorimetry practical measures the energy content of a food sample by burning it to heat a known mass of water. The energy released is calculated using: Energy (J) = mass of water (g) × 4.2 × temperature rise (°C). Energy per gram of food = total energy released ÷ mass of food burned. Common sources of error include heat loss to surroundings, incomplete combustion of the food, and evaporation of water from the food sample, leading to lower measured values than the actual energy content.
A student burns 0.5g of a peanut to heat 20g of water, which rises in temperature from 20°C to 70°C. Calculate the energy content of the peanut per gram, using the specific heat capacity of water = 4.2 J/g°C.
- 1
Calculate total energy released: J
- 2
Divide by the mass of the peanut burned: J/g = 8.4 kJ/g
- 3
This value is lower than the actual energy content of the peanut due to heat loss to the surroundings during the experiment.
Exam tip:
Always show all working for calorimetry calculations: marks are awarded for correct substitution into the formula even if your final answer is incorrect.
6. Common Pitfalls
Wrong move:
Writing light or chlorophyll as reactants in the photosynthesis equation, instead of conditions above the arrow.
Why:
Marking schemes explicitly reject equations that include these as reactants, as they are not consumed in the reaction.
Correct move:
Write light and chlorophyll above the reaction arrow, and only include CO₂, H₂O as reactants, glucose and O₂ as products.
Wrong move:
Stating that bile is an enzyme, or that it breaks down lipids into fatty acids and glycerol.
Why:
Bile has no enzyme activity, it only emulsifies lipids to increase surface area for lipase action. Lipase is the enzyme that digests lipids.
Correct move:
Always specify bile is an alkaline fluid that neutralises stomach acid and emulsifies lipids, and is not an enzyme.
Wrong move:
Mixing up the functions of magnesium and nitrate ions in plants, e.g. stating magnesium is needed for protein production.
Why:
Marking schemes strictly assign magnesium to chlorophyll production, and nitrate to amino acid/protein production, so mixing these loses all marks for the question.
Correct move:
Remember: Mg = chlorophyll (yellow leaves if deficient), Nitrate = protein (stunted growth if deficient).
Wrong move:
Forgetting to destarch a plant before photosynthesis practicals testing starch production.
Why:
If stored starch is present, you cannot prove that starch detected was produced during the experiment, so results are invalid.
Correct move:
Always destarch plants by leaving them in the dark for 24 hours before any photosynthesis practical measuring starch production.
Wrong move:
Stating villi absorb all food molecules into blood capillaries, including fatty acids and glycerol.
Why:
Fatty acids and glycerol are absorbed into the lacteal (lymphatic vessel) inside the villus, not directly into blood capillaries.
Correct move:
Specify glucose and amino acids go into capillaries, while fatty acids and glycerol go into the lacteal.
7. Quick Reference Cheatsheet
Topic | Key Exam Facts |
|---|---|
Photosynthesis Equations | Word: CO₂ + water → glucose + oxygen (light/chlorophyll); Balanced: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ |
Limiting Factors | Light intensity, CO₂ concentration, temperature; plateau = new limiting factor |
Plant Minerals | Mg = chlorophyll (deficiency: chlorosis); Nitrate = protein (deficiency: stunted growth) |
Digestive Enzymes | Amylase→starch→maltose, maltase→maltose→glucose, protease→protein→amino acids, lipase→lipid→fatty acids+glycerol |
Bile | Made in liver, stored in gall bladder; neutralises acid, emulsifies lipids (not enzyme) |
Villus Adaptations | Large surface area, thin wall, good blood supply, lacteal for lipid absorption |
Calorimetry Formula | Energy (J) = mass water × 4.2 × temp rise; per gram = total energy ÷ mass of food burned |
8. Frequently Asked
Is bile an enzyme?
No, bile is not an enzyme. It is an alkaline fluid produced by the liver, stored in the gall bladder, that neutralises stomach acid and emulsifies lipids to increase surface area for lipase action.
What are the products of digestion for major food groups?
- Starch: digested by amylase and maltase to glucose
- Protein: digested by proteases to amino acids
- Lipids: digested by lipases to fatty acids and glycerol
Why must plants be destarched before photosynthesis practicals?
Destarching (leaving plants in the dark for 24h) removes all stored starch from leaves, so any starch detected after the experiment is confirmed to be produced during photosynthesis, rather than being pre-stored.
Going deeper
What's Next
Now you have mastered the Nutrition topic, you can move on to closely linked topics in Edexcel IGCSE Biology Section 2. Next, study respiration, the process by which living organisms break down glucose to release energy for cell processes, using the oxygen produced by photosynthesis. You will also explore gas exchange in plants and humans, which is closely tied to both photosynthesis and respiration, as well as transport systems that move the products of nutrition around organisms. If you are taking the full Biology IGCSE, you can also revise human health and disease topics, which build on your knowledge of balanced diets and deficiency symptoms. Make sure to practice past paper questions on this topic to familiarise yourself with common marking points, especially for limiting factor and adaptation questions that appear frequently in exams.