NutritionEdexcel IGCSE Biology: Revision notes

Photosynthesis is the process by which plants convert light energy into chemical energy, stored in glucose.

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

Balanced symbol equation: 6CO2 + 6H2O → C6H12O6 + 6O2

The rate of photosynthesis is affected by carbon dioxide concentration, light intensity and temperature — each can be a limiting factor if in short supply.

The leaf is adapted for photosynthesis: broad and thin (large surface area, short diffusion distance), palisade cells packed with chloroplasts near the upper surface, many stomata for gas exchange, and a network of veins (xylem and phloem) for water/mineral supply and sugar transport.

Plants also need mineral ions: magnesium ions for making chlorophyll, and nitrate ions for making amino acids (for proteins).

A balanced diet contains appropriate proportions of carbohydrate, protein, lipid, vitamins, minerals, water and dietary fibre.

Energy requirements vary with activity level (more active people need more energy), age (growing children and teenagers need more energy relative to body size) and pregnancy (extra energy needed for the developing fetus).

The alimentary canal includes the mouth, oesophagus, stomach, small intestine, large intestine and the pancreas (which adds digestive juices).

Food is moved along the gut by peristalsis — waves of muscle contraction behind the food and relaxation ahead of it.

Digestive enzymes break down large food molecules into small, soluble ones:

• Amylase and maltase digest starch to glucose
• Proteases digest proteins to amino acids
• Lipases digest lipids to fatty acids and glycerol

Bile, made in the liver and stored in the gall bladder, neutralises stomach acid (making conditions alkaline for intestinal enzymes) and emulsifies lipids (breaks fat into droplets, increasing surface area for lipase).

The small intestine is lined with millions of finger-like projections called villi, each covered in smaller microvilli. A villus is adapted for efficient absorption of digested food:

• Large surface area — increases the rate of diffusion/active transport
• Thin walls (one cell thick) — short diffusion distance
• Good blood supply (capillary network) — maintains a steep concentration gradient
• Lacteal (part of the lymphatic system) — absorbs fatty acids and glycerol

Photosynthesis can be investigated by showing:

• Evolution of oxygen from a submerged water plant (e.g. Elodea) — count bubbles or collect gas
• Production of starch in a leaf — using the iodine test after destarching and re-exposing to light, with parts covered/uncovered by foil
• The requirements of light, carbon dioxide and chlorophyll — using controls such as a destarched plant, a variegated leaf, and a leaf in a CO2-absorbing environment

The energy content of a food sample can be investigated by burning a known mass of food beneath a boiling tube of water and measuring the temperature rise of a known volume of water, then calculating energy released.