2.1Unit 1 · Nutrition, Respiration & Gas Exchange

Nutrition — Edexcel IGCSE Biology Modular - UNIT 1 Revision Notes & Topical Questions

10 min read 7 parts Lesson 2 of 4 1 quiz

📚 Edexcel IGCSE Biology Modular - UNIT 1 Hub / Topical Revision

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Lesson summary

Describe photosynthesis with word and balanced symbol equations.Word equationcarbon dioxide + water → glucose + oxygen (light energy, chlorophyll).Balanced equation6CO2 + 6H2O → C6H12O6 + 6O2.Photosynthesis is an endothermic processlight energy becomes chemical energy in glucose.…

Three limiting factors
Starch is tested with iodine
Bile is not an enzyme
Process of photosynthesis
Factors affecting the rate
Leaf structure and adaptations

Part 1 of 7Key ideas

Word equation

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

Balanced equation

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂.

Photosynthesis is an endothermic process

Photosynthesis is an endothermic process light energy becomes chemical energy in glucose.

Three limiting factors

Three limiting factors light intensity, carbon dioxide concentration and temperature.

Starch is tested with iodine

Starch is tested with iodine orange-brown → blue-black; leaves are first boiled in ethanol to remove chlorophyll.

Energy needs vary with age, activity level and pregnancy.

Amylase

Amylase starch → maltose; maltase: maltose → glucose; protease: protein → amino acids; lipase: lipids → fatty acids + glycerol.

Bile is not an enzyme

Bile is not an enzyme it emulsifies fat to increase surface area for lipase.

Part 2 of 7Learning objectives

Describe photosynthesis with word and balanced symbol equations.

Explain how light intensity, CO₂ concentration and temperature affect the rate of photosynthesis.

Relate the structure of a leaf and the role of mineral ions to plant growth.

Describe the components of a balanced diet and the functions of the alimentary canal.

Explain how peristalsis moves food through the gut.

Part 3 of 7Essential knowledge

Process of photosynthesis

Process of photosynthesis chlorophyll in chloroplasts absorbs light energy, which is used to join carbon dioxide and water into glucose; oxygen is given off.

Glucose may be respired, stored as insoluble starch, used to make cellulose for cell walls, converted to sucrose for transport in the phloem,
or combined with nitrate ions to make amino acids.

Factors affecting the rate

Factors affecting the rate increasing light intensity or CO₂ concentration raises the rate until another factor becomes limiting, when the graph levels off. Temperature affects the enzymes that control photosynthesis: the rate rises to an optimum, then falls as enzymes denature.

Leaf structure and adaptations

Leaf structure and adaptations

leaves are broad and flat (large surface area for light) and thin (short diffusion distance for gases).
- A waxy cuticle reduces water loss
the transparent upper epidermis lets light through
the palisade mesophyll is packed with chloroplasts near the top surface
the spongy mesophyll has air spaces for gas diffusion
stomata controlled by guard cells allow gas exchange
vascular bundles bring water (xylem) and remove sugars (phloem).

Plants and mineral ions

Plants and mineral ions nitrate ions are needed to make amino acids and proteins for growth; magnesium ions are needed to make chlorophyll. Shortage of nitrate causes stunted growth and yellow leaves; shortage of magnesium causes yellowing between the veins.

Practical — investigating photosynthesis

Practical — investigating photosynthesis

destarch a plant by leaving it in the dark for 48 hours,
- then test leaves for starch after exposure to different conditions: cover part of a leaf with foil (light needed), use a variegated leaf,
  - or enclose a leaf with sodium hydroxide / soda lime to absorb CO₂ (CO₂ needed).
  - (chlorophyll needed)
The rate can be measured by counting oxygen bubbles from pondweed (e.g. Elodea) at different lamp distances, keeping temperature constant with a water bath.

Balanced diet

Balanced diet carbohydrates and lipids provide energy; proteins are for growth and repair; vitamin A (vision), vitamin C (prevents scurvy), vitamin D (calcium absorption, strong bones); calcium (bones and teeth) and iron (haemoglobin, prevents anaemia); fibre helps peristalsis and prevents constipation; water is the solvent for reactions.

Energy requirements depend on activity level, age and pregnancy.

Alimentary canal structure and function

Alimentary canal structure and function the processes are ingestion, digestion, absorption, assimilation and egestion.

Food passes mouth → oesophagus → stomach → small intestine → large intestine (colon) → rectum → anus.
- (duodenum and ileum)
The salivary glands, pancreas and liver add secretions.
Villi in the ileum have microvilli, thin walls one cell thick and a rich capillary network plus a lacteal, giving fast absorption.

Peristalsis

Peristalsis the gut wall has circular and longitudinal muscles. Circular muscles contract behind the food bolus (narrowing the gut) while longitudinal muscles relax, and in front the reverse happens, so a wave pushes food forward. Dietary fibre gives the muscles something to push against.

Practical – energy in food

Measure a known volume of water into a boiling tube (1 cm³ = 1 g) and record its temperature. Weigh the food, ignite it and hold it under the tube until fully burnt, then record the final temperature. Energy per gram (J/g) = mass of water × temperature rise × 4.2 ÷ mass of food.

Energy in food – limitations

Values come out lower than the true energy content because heat escapes to the air and the food may not burn completely. Relight the food until it will not burn, keep it close to the tube, insulate the tube, or use a calorimeter. Example: 20 g water rising 15 °C from 2.0 g food gives 630 J/g.

Testing a leaf for starch – steps

Place the leaf in boiling water to kill cells and stop reactions; warm it in ethanol in a hot water bath (no flame, ethanol is flammable) to remove chlorophyll; rinse in warm water to soften; spread on a white tile and add iodine. Leaves are tested for starch because glucose is quickly converted.

Showing oxygen is produced

Place pondweed under an upturned funnel in water with a water-filled tube over the funnel stem. Gas bubbles from the plant collect and displace the water; the gas relights a glowing splint, proving it is oxygen. The volume collected in a set time can measure rate.

Water and chlorophyll as factors

Water is not treated as a limiting factor because the amount used in photosynthesis is tiny compared with the amount lost in transpiration. The amount of chlorophyll can limit the rate; it is reduced by magnesium deficiency, viral disease, leaf loss or variegation.

Where the digestive enzymes are made

Salivary glands and pancreas make amylase; maltase is made in the small intestine.

Pepsin in the stomach breaks proteins into shorter polypeptides,
which proteases from the pancreas and small intestine complete to amino acids.
The pancreas releases amylase, protease and lipase in alkaline juice into the duodenum.

Mechanical digestion

Chewing, stomach churning and emulsification by bile break food into smaller pieces without breaking chemical bonds, increasing surface area for enzymes. Food is formed into a bolus lubricated by saliva; the stomach turns it into liquid chyme, and mucus reduces friction along the gut.

Blood or lymph

Glucose and amino acids are absorbed into the blood capillaries of the villi,

while fatty acids and glycerol enter the lacteal and are carried in the lymph.
Absorption is by diffusion and active transport.
Most water is absorbed by osmosis in the small intestine, with the rest taken up in the colon.

Accessory organs

The liver, gall bladder and pancreas help digestion but food does not pass through them. The liver makes bile and breaks down excess amino acids to form urea (deamination); the gall bladder stores bile and releases it into the duodenum.

Deficiency diseases

Lack of vitamin D or calcium causes rickets in children (weak, poorly formed bones); low calcium later in life contributes to osteoporosis. Vitamin A is needed to make the light-sensitive pigment in the retina. Calcium is also needed for blood clotting.

Part 4 of 7Process & sequence

Light energy is absorbed by chlorophyll in chloroplasts of palisade cells.

Carbon dioxide diffuses into the leaf through open stomata and air spaces.

Water is absorbed by root hairs and carried up the xylem to the leaf.

Light energy is converted to chemical energy, building glucose from CO₂ and H₂O.

Oxygen is released as a by-product and diffuses out through stomata.

Glucose is used for respiration or converted to starch, cellulose, sucrose, proteins and lipids.

From leaf to glucosewrite these steps in this order; each step is a marking point.

Light absorbed by chlorophyll

CO₂ enters through stomata

Water arrives through xylem

Glucose made in chloroplast

Oxygen released

Glucose stored as starch

Part 5 of 7Worked examples

A student moves a lamp from 10 cm to 20 cm away from pondweed. Using the inverse square law, how does light intensity change, and what happens to the bubble rate if light is limiting?

Light intensity is proportional to 1/d².
- Doubling the distance gives (1/2)² = 1/4 of the original intensity.
- If light is the limiting factor, fewer oxygen bubbles are produced per minute because the rate of photosynthesis falls.

Explain why the rate of photosynthesis levels off at high light intensity.

At high light intensity light is no longer limiting.
- Another factor, such as carbon dioxide concentration or temperature, is now in short supply and limits the rate,
- so increasing light further has no effect.

Part 6 of 7Summary

Photosynthesis turns light energy into chemical energy stored in glucose.

Rate is controlled by the limiting factor: light, CO₂ or temperature.

Leaves are adapted for gas exchange and light absorption.

A balanced diet supplies all nutrients in the right amounts.

Enzymes digest food; villi absorb the products in the small intestine.

Part 7 of 7Compare: Compare

Mineral ions needed by plants
Ion	Used for	Deficiency symptom
Nitrate (NO₃^-)	Making amino acids and therefore proteins	Stunted growth, yellow older leaves
Magnesium (Mg²⁺)	Making chlorophyll	Yellowing between the veins of leaves (chlorosis)
Phosphate	Making DNA and cell membranes; respiration	Poor root growth, purple older leaves
Potassium (K⁺)	Helping enzymes for photosynthesis and respiration	Yellow leaves with dead edges

Ions are absorbed from soil water by root hair cells by active transport.

Parts of the alimentary canal
Region	Main function
Mouth	Chewing (mechanical digestion); salivary amylase starts starch digestion
Stomach	Churning; pepsin (protease) digests protein; HCl kills bacteria (pH about 2)
Small intestine: duodenum	Receives pancreatic juice and bile; digestion continues
Small intestine: ileum	Final digestion and absorption through villi
Large intestine: colon	Absorbs water from undigested food
Rectum and anus	Stores faeces; egestion

Key terms

Command terms

Give / State / Name

Recall a fact; one idea per mark.

Define

Give the exact meaning using the specification keywords.

Describe

Give an account of what happens / the pattern in data, quoting values; no reasons needed.

Explain

Give reasons with linked statements (cause → effect). Each linked step is usually a separate marking point.

Suggest

Use knowledge in a new context; more than one answer may be accepted.

Compare

Similarities and differences; each point must refer to both items.

Calculate

Show working; include units; check significant figures. Method marks are given even if the final answer is wrong.

Discuss

Explore the issue from different angles and link the points.

Evaluate / Assess

Weigh strengths and weaknesses and reach a supported judgement.

Deduce

Draw a conclusion from the information given.

Justify

Support a statement/choice with evidence.

Exam technique

Keywords the mark scheme looks for in this lesson

Photosynthesis · Chlorophyll · Limiting factor · Stomata · Balanced diet · Digestion · Absorption · Peristalsis · Bile · Villus

Learn each definition in the Key words tab exactly; a missing keyword in a definition usually loses the mark.

How to get full marks (examiner guidance)

Use specification keywords exactly (e.g. eukaryotic/prokaryotic, enzyme–substrate complex, active site, complementary).

Link ideas

Link ideas for “explain” each marking point is usually a cause → consequence step; unlinked facts often score nothing.

Use the information in the question

“discuss/evaluate” answers must use the data provided and match the number of marks.

Convert units carefully (mg → g, cm³ → dm³) and show working for calculations.

Avoid vague words

“it”, “stuff”, “things”, “goes up”, “energy is made”. Name the substance and the process.

Do not extrapolate a graph line beyond the data or to zero unless asked.

Six-mark extended responses are marked on content AND a logical, linked line of reasoning.

Last updated: 14 October 2026