Pearson Edexcel International GCSE Biology 4BI1: Nutrition – Study Guide

This guide teaches sub-topic 2(e) Nutrition of the Pearson Edexcel International GCSE Biology (4BI1) specification, Issue 3 (September 2024): statements 2.18 to 2.33B. The qualification is untiered. Only 2.33B (the food energy practical) is in bold with a B reference, so it is Paper 2 only. Everything else can be examined on Paper 1 (2 hours, 110 marks) or Paper 2 (1 hour 15 minutes, 70 marks). A calculator may be used in both.

Course links: 4BI1 Biology hub, printable checklist and all free 10-minute diagnostics. Enzymes as catalysts, and how temperature and pH change the active site (2.10 to 2.14B), are taught in the cell structure and biological molecules study guide.

What this sub-topic covers

Statements	What you must be able to do
2.18–2.20	Explain photosynthesis, write its equations, explain the effect of carbon dioxide, light and temperature on the rate
2.21–2.22	Explain leaf adaptations; state why magnesium and nitrate ions are needed
2.23	Practical: oxygen from a water plant, starch production, and the need for light, carbon dioxide and chlorophyll
2.24–2.26	Balanced diet; sources and functions of each component; how energy needs vary
2.27–2.29	Alimentary canal and pancreas; peristalsis; amylase, maltase, proteases, lipases
2.30–2.32	Bile; adaptations of the small intestine and villus for absorption
2.33B	Practical: energy content of a food sample (Paper 2 only)

Photosynthesis (2.18, 2.19)

In photosynthesis, chlorophyll in the chloroplasts absorbs light energy. That energy is used to join carbon dioxide and water into glucose, and oxygen is released as a by-product. Light energy has been converted into chemical energy stored in glucose.

Word equation:

carbon dioxide + water  →  glucose + oxygen
           (light energy, chlorophyll)

Balanced symbol equation:

6CO₂ + 6H₂O  →  C₆H₁₂O₆ + 6O₂

Check the balance: 6 C, 12 H and 18 O on each side.

Why it matters: almost every food chain starts with a photosynthesising producer, so the chemical energy in your food came from light captured by plants. Photosynthesis also releases the oxygen that aerobic organisms use.

Factors affecting the rate (2.20)

• Light intensity. Light supplies the energy. More light gives a faster rate, until something else runs short.
• Carbon dioxide concentration. Carbon dioxide is a raw material. More carbon dioxide gives a faster rate, until something else runs short.
• Temperature. The reactions are controlled by enzymes. Warming increases the rate up to an optimum. Above the optimum, the enzymes’ active sites change shape and the rate falls quickly.

A limiting factor is the one in shortest supply, holding the rate back. A graph of rate against light intensity rises, then levels off. On the rising part, light is limiting. On the flat part, carbon dioxide or temperature is.

Worked example: reading a limiting-factor graph

A graph shows rate against light intensity at 0.04% and at 0.4% carbon dioxide, both at 20 °C. The curves rise together at low light. The 0.04% curve levels off first; the 0.4% curve levels off higher.

1. At low light intensity, both curves are the same, so carbon dioxide is not limiting there. Light intensity is limiting.
2. On the flat part of the 0.04% curve, more light does nothing but more carbon dioxide raises the rate, so carbon dioxide is limiting.
3. On the flat part of the 0.4% curve, light and carbon dioxide are both high, so the likely limiting factor is temperature.

Leaf structure (2.21)

Feature	Adaptation for photosynthesis
Broad, flat shape	Large surface area to absorb light
Thin	Short diffusion distance for carbon dioxide to reach the mesophyll cells
Waxy cuticle	Transparent, so light passes through; reduces water loss
Upper epidermis	Transparent cells with no chloroplasts, so light reaches the cells below
Palisade mesophyll	Tall cells packed with chloroplasts, near the top surface where light is brightest
Spongy mesophyll	Air spaces let carbon dioxide diffuse to the cells and oxygen diffuse out
Stomata (mainly in the lower epidermis)	Let carbon dioxide in; guard cells open and close them
Vascular bundle	Xylem brings water; phloem carries sugars away

For 2.21, link each feature to light, carbon dioxide or water. Gas exchange in the leaf (2.40B–2.43B) is in sub-topic (g).

Mineral ions (2.22)

Plants also need mineral ions, absorbed from the soil by the roots.

• Magnesium ions are needed to make chlorophyll. Without enough, leaves turn yellow and photosynthesis slows.
• Nitrate ions are needed to make amino acids, which are joined into proteins. Without enough, growth is poor.

The photosynthesis practicals (2.23)

Oxygen from a water plant. Put pondweed in water with some sodium hydrogencarbonate (a carbon dioxide source) and a lamp at a measured distance. After a few minutes, count bubbles per minute, or collect the gas in an inverted measuring cylinder. Repeat at other distances. The gas relights a glowing splint, so it is oxygen. A beaker of water between lamp and plant keeps the temperature steady.

Testing a leaf for starch.

1. Boil the leaf in water for about a minute to kill it and let iodine in.
2. Put it in ethanol in a tube standing in hot water. Turn off any flame first: ethanol is flammable. Ethanol removes the chlorophyll so the colour change shows.
3. Rinse in warm water to soften it.
4. Spread it on a white tile and add iodine solution. Blue-black shows starch; orange-brown shows no starch.

Destarching. Keep the plant in the dark for about 48 hours first. It uses up its starch, so any starch found later was made during the experiment.

Showing what is needed.

• Light: cover part of a leaf with foil, leave in light, then test. Only the uncovered part goes blue-black.
• Carbon dioxide: enclose a plant with soda lime, which absorbs carbon dioxide; the control has none. Only the control makes starch.
• Chlorophyll: use a variegated leaf (green and white). Only the green parts go blue-black.

Worked example: processing bubble counts

A student counts bubbles from pondweed with the lamp at 10 cm (42, 45, 39 per minute) and at 25 cm (24, 27, 21 per minute).

1. Mean at 10 cm = (42 + 45 + 39) ÷ 3 = 126 ÷ 3 = 42 bubbles per minute.
2. Mean at 25 cm = (24 + 27 + 21) ÷ 3 = 72 ÷ 3 = 24 bubbles per minute.
3. Conclusion: moving the lamp further away lowers the light intensity, so the rate of photosynthesis falls.
4. Limitation: bubbles vary in size, so measuring gas volume is more accurate.

A balanced diet (2.24, 2.25)

A balanced diet contains carbohydrate, protein, lipid, vitamins, minerals, water and dietary fibre in the right proportions for the person. It is not just “enough food”.

Component	Good sources	Function
Carbohydrate	Bread, rice, pasta, potatoes	Main energy source for respiration
Protein	Meat, fish, eggs, beans	Growth and repair; making enzymes and other proteins
Lipid (fats and oils)	Butter, oils, nuts	Energy store; insulation; cell membranes
Vitamin A	Liver, carrots, dairy foods	Healthy eyes (vision in dim light) and skin
Vitamin C	Citrus fruit, peppers	Healthy skin and gums; lack causes scurvy
Vitamin D	Oily fish, eggs; made in skin in sunlight	Helps absorb calcium; strong bones and teeth; lack causes rickets
Calcium	Milk, cheese	Strong bones and teeth; blood clotting; muscle contraction
Iron	Red meat, liver, spinach	Making haemoglobin in red blood cells; lack causes anaemia
Water	Drinks, food	Solvent for reactions and transport; cooling by sweating
Dietary fibre	Vegetables, wholegrain cereals	Adds bulk so food moves by peristalsis; prevents constipation

Energy requirements (2.26)

• Activity. More activity means more respiration in muscles, so more energy is needed. A builder needs more than an office worker of the same age and size.
• Age. Children and teenagers need a lot of energy for their size because they are growing. Older adults are usually less active and need less.
• Pregnancy. A pregnant woman needs extra energy for the growing fetus and placenta and for carrying the extra mass.

The alimentary canal (2.27, 2.28)

Part	Function
Mouth	Teeth chew food into smaller pieces (larger surface area); salivary glands release amylase
Oesophagus	Carries food to the stomach by peristalsis
Stomach	Churns food; protease digests protein; hydrochloric acid kills bacteria and suits the stomach protease
Pancreas	Makes amylase, protease and lipase and releases them into the duodenum
Duodenum (first part of small intestine)	Receives pancreatic juice and bile; digestion continues
Ileum (rest of small intestine)	Digestion is completed; soluble products are absorbed into the blood
Colon (large intestine)	Absorbs water from undigested material
Rectum	Stores faeces until they are egested through the anus

Peristalsis moves food along the gut. The gut wall has circular and longitudinal muscles. Behind the food, circular muscle contracts and narrows the gut; in front, it relaxes. Waves of contraction and relaxation push the food along. Fibre gives the muscles something to push against.

Digestive enzymes (2.29)

Digestion turns large insoluble molecules into small soluble ones.

Food	Enzyme(s)	Product
Starch	Amylase, then maltase	Maltose, then glucose
Protein	Proteases	Amino acids
Lipid	Lipases	Fatty acids and glycerol

Amylase (from the salivary glands and pancreas) breaks starch into maltose. Maltase, on the lining of the small intestine, breaks maltose into glucose. Proteases act in the stomach and small intestine; lipases act in the small intestine.

Bile (2.30, 2.31)

Bile is produced by the liver and stored in the gall bladder. It passes along the bile duct into the duodenum. Bile is not an enzyme. It does two jobs.

1. Neutralising stomach acid. Bile is alkaline. It neutralises acid from the stomach, giving the slightly alkaline conditions the small intestine’s enzymes work best in.
2. Emulsifying lipids. Bile breaks large fat drops into tiny droplets, giving a larger surface area for lipase, so lipids are digested faster. No bonds are broken: the droplets are still lipid.

Absorption in the small intestine (2.32)

The ileum is long and lined with millions of finger-like villi, covered in cells with microvilli.

• Large surface area: the length, the villi and the microvilli all increase it.
• Thin wall: the villus surface is one cell thick, so the diffusion distance is short.
• Good blood supply: a dense capillary network carries glucose and amino acids away, keeping a steep concentration gradient.
• Lacteal: a lymph vessel in the centre of each villus absorbs fatty acids and glycerol.

Practical: energy content of food (2.33B, Paper 2 only)

Put a measured volume of water in a boiling tube and record its temperature. Weigh a piece of food, light it on a mounted needle and hold it under the tube until it is burnt completely. Record the highest temperature. Use 1 cm³ of water = 1 g, and 4.2 J to raise 1 g of water by 1 °C.

energy released (J) = mass of water (g) × 4.2 × temperature rise (°C)
energy per gram     = energy released ÷ mass of food (g)

Worked example

A 0.50 g piece of peanut heats 20 cm³ of water from 19.0 °C to 51.0 °C.

1. Temperature rise = 51.0 − 19.0 = 32.0 °C.
2. Energy = 20 × 4.2 × 32.0 = 2688 J.
3. Energy per gram = 2688 ÷ 0.50 = 5376 J/g = 5.38 kJ/g (3 s.f.).

The result is too low: heat escapes to the air and glass, and burning may be incomplete.

Common errors

• Saying the flat part of a rate graph means photosynthesis has stopped; it is still going on at its maximum rate for those conditions.
• Writing that magnesium “is chlorophyll”; it is needed to make chlorophyll.
• Leaving out “in the right proportions” when defining a balanced diet.
• Calling bile an enzyme, or saying it digests fat.
• Saying amylase makes glucose; amylase makes maltose and maltase makes glucose.

Next steps

Use the nutrition revision notes, then the nutrition practice questions. For enzyme action, see the cell structure revision notes and cell structure practice.

Official syllabus

Pearson Edexcel International GCSE in Biology (4BI1), Specification, Issue 3, September 2024, published by Pearson Education Limited. Topic 2: Structures and functions in living organisms, sub-topic (e) Nutrition, statements 2.18–2.33B.