Nutrition and Human Digestive System, practice questions

C, Salivary amylase is secreted by the salivary glands into the mouth; pepsin acts in the stomach and trypsin and pancreatic lipase are secreted into the duodenum by the pancreas.

C, A purple colour with biuret solution is the positive result for protein.

C, Microvilli project from the epithelial cells of the villus, increasing its surface area for absorption.

B, Fatty acids and glycerol are absorbed into the lacteal at the centre of the villus and enter the lymphatic system before reaching the bloodstream.

C, Assimilation refers to how body cells use absorbed nutrients, not to digestion or absorption themselves.

C, Defaecation removes undigested material such as fibre, dead cells and bacteria, not metabolic waste.

B, Fibre adds bulk and stimulates peristalsis; a lack of it is linked to constipation.

C, A gastric ulcer forms when the lining of the stomach or duodenum is damaged by excess acid, often linked to irregular eating.

B, Bile is not an enzyme; it emulsifies fat to increase the surface area lipase can act on.

C, A lack of vitamin C causes scurvy, characterised by bleeding gums and slow wound healing.

• Wall of the villus is one cell thick, giving a short diffusion distance for digested food
• Villus is covered in microvilli, and millions of villi line the intestinal wall, giving a large surface area for absorption
• Dense network of capillaries carries away absorbed glucose and amino acids quickly, maintaining a diffusion gradient
• Lacteal absorbs fatty acids and glycerol into the lymphatic system

• Sample P contains starch (iodine test, blue-black colour)
• Sample Q contains reducing sugar (Benedict's test, brick-red precipitate on heating)
• Sample R contains protein (biuret test, purple colour)
• Safety: use a water bath or heat gently, and wear eye protection, to avoid the boiling tube contents spurting out

• Student A is more likely to develop obesity (or diabetes)
• Regularly eating fatty and sugary food provides more energy than the body uses
• Excess energy intake combined with low exercise/energy expenditure is stored as fat in the body
• Over time this leads to excess body fat and associated health issues such as obesity or type 2 diabetes

The alimentary canal from mouth to anus, plus glands such as the salivary glands, pancreas and liver.

Large molecules are broken down by enzymes: carbohydrases to glucose, proteases to amino acids, lipases to fatty acids and glycerol.

Digested food is absorbed into the blood mainly through the villi of the small intestine.

Thin walls, a large surface area, many capillaries and a lacteal make absorption efficient.

Absorbed nutrients are used by cells to build tissues and release energy.

The right proportions of carbohydrates, proteins, lipids, vitamins, minerals, fibre and water for the person's needs.

Undigested material such as fibre, dead cells and bacteria moves from the small intestine into the large intestine. There, water and some mineral ions are reabsorbed back into the blood, and the remaining waste is compacted and expelled through the anus as faeces in a process called defaecation, distinct from excretion of metabolic waste.

Eating too much fatty or sugary food with too little exercise is linked to obesity and diabetes; eating irregularly or too quickly can lead to a gastric ulcer when stomach acid damages the stomach lining; and a diet low in fibre and fluids is linked to constipation.

Each digestive enzyme acts on only one type of substrate: amylase breaks down starch, protease breaks down protein, and lipase breaks down fat, and each enzyme works best at a particular pH and temperature matched to the part of the gut where it is released.

Iodine solution turns blue-black in the presence of starch, Benedict's solution forms a brick-red precipitate with a reducing sugar on heating, and biuret solution turns purple in the presence of protein, so a food test result identifies which nutrient, and therefore which enzyme's product, is present.

Peristalsis, the wave-like contraction and relaxation of muscle in the wall of the alimentary canal, pushes food from the oesophagus through to the rectum; it is a separate mechanical process from the chemical digestion carried out by enzymes.

A diet lacking specific vitamins or minerals causes a named deficiency disease: too little vitamin C causes scurvy with bleeding gums, too little iron causes anaemia and fatigue, and too little vitamin D or calcium weakens bones, causing rickets in children.

In the Paper 3 practical, the vitamin C content of a fruit juice is estimated by adding the juice drop by drop to a fixed volume of blue DCPIP dye until the dye just turns colourless; a juice that needs more drops to decolourise the same volume of dye contains less vitamin C, while one that needs fewer drops contains more.