Everything from the 5 lessons of this topic on a few pages: the key idea of each lesson, the diagram to remember, the points to learn and the traps examiners set. Read it, cover it, test yourself.
A balanced diet contains all the nutrients (carbohydrates, fats and oils, proteins, vitamins, mineral ions, fibre and water) in the correct proportions and amounts to stay healthy, and supplies enough energy for the person's needs. Scurvy is caused by a lack of vitamin C; rickets is caused by a lack of vitamin D or calcium.
A balanced diet is not just "eating healthy food" or "eating a bit of everything". You must say it contains all the nutrients in the correct proportions (amounts).
Rickets can be caused by a lack of vitamin D even when there is enough calcium in the diet, because vitamin D is needed to absorb the calcium. Name both possible causes.
Food passes along the alimentary canal: mouth, oesophagus, stomach, small intestine (duodenum then ileum) and large intestine (colon, rectum, anus). The salivary glands, pancreas, liver and gall bladder are associated organs that the food never passes through. Five processes happen on the way: ingestion, digestion, absorption, assimilation and egestion.
The liver, pancreas, gall bladder and salivary glands are not part of the alimentary canal: food never passes through them. They secrete juices into it.
Egestion is not excretion. Faeces are mostly undigested food that never entered the body's cells; excretion removes waste products of metabolism, such as urea and carbon dioxide.
Physical digestion is the breakdown of food into smaller pieces without chemical change to the food molecules. It increases the surface area of the food for the action of enzymes in chemical digestion. Teeth start it, the muscular stomach wall continues it by churning and (Extended) bile emulsifies fats into small droplets.
Physical digestion does not produce smaller molecules. Only chemical digestion by enzymes breaks large molecules into small ones.
Bile contains no enzymes and does not digest fat. It emulsifies fat, a physical change that increases the surface area for lipase.
Chemical digestion is the breakdown of large insoluble molecules into small soluble molecules, which can then be absorbed. Amylase breaks down starch to simple reducing sugars, proteases break down protein to amino acids and lipase breaks down fats and oils to fatty acids and glycerol. Hydrochloric acid in gastric juice kills harmful microorganisms in food and provides an acidic pH for the stomach protease.
Say "proteases break down protein to amino acids", not "to peptides" or "to glucose". And amylase produces simple reducing sugars (maltose), not glucose directly.
Hydrochloric acid is not an enzyme and does not digest protein itself. Its jobs are to kill microorganisms and to provide the optimum pH for the protease.
Pepsin and trypsin are both proteases, but each has a different optimum pH. Pepsin would be denatured or inactive in the alkaline duodenum, and trypsin in the acid stomach.
The small intestine is the region where nutrients are absorbed. Most water is absorbed from the small intestine, but some is also absorbed from the colon. Extended: villi and microvilli greatly increase the internal surface area of the small intestine; each villus has a one-cell-thick epithelium, a network of capillaries that absorb glucose and amino acids, and a central lacteal that absorbs fatty acids and glycerol.
Most water is absorbed in the small intestine, not the colon. The colon only absorbs some of the water that remains.
Fatty acids and glycerol go into the lacteal, not the blood capillaries. Glucose and amino acids go into the capillaries. Keep the two routes separate.