7. Human nutrition

The correct amount is not identical for everyone. Energy and nutrient requirements vary with age, sex, body size, activity, pregnancy and health.

Too little of a component can cause deficiency; too much energy intake relative to use can increase fat storage and body mass. Balance therefore means adequacy without harmful excess.

Vitamin-C-rich fruit and vegetables prevent scurvy. Vitamin D from diet and safe sunlight exposure, together with adequate calcium, supports normal bone development and helps prevent rickets.

Vitamin D is linked to rickets because it enables efficient calcium absorption; naming calcium alone without explaining vitamin D misses the principal syllabus deficiency link.

Trace the alimentary canal in order: mouth → oesophagus → stomach → small intestine (duodenum, then ileum) → large intestine (colon, then rectum) → anus.

A useful chain is: ingest food → digest it → absorb nutrients into blood → assimilate nutrients in cells → egest what remains undigested.

The pieces change in size and shape, but the molecules in the food remain the same. No new substances are made.

Chewing by teeth and churning by the stomach are physical digestion because they fragment or mix food mechanically.

smaller pieces → greater total surface area → more food exposed to enzymes at once → faster chemical digestion

In an unfamiliar diagram, use both position and crown shape. A pointed tooth near the front is a canine; a broad cusped tooth at the back is a premolar or molar, with molars larger and farther back.

Read a section from outside to inside: enamel on the crown (cement on the root) → dentine → pulp containing nerves and blood vessels.

Together, the teeth reduce food to smaller pieces, increasing surface area and making swallowing and enzyme action easier.

Repeated muscular contractions squeeze and turn the stomach contents. This makes a more uniform mixture and exposes more food surface.

Churning increases the surface area available for chemical digestion without changing the food molecules itself.

large fat globule → many small droplets → greater total surface area → more contact with lipase → faster chemical digestion of fat

Bile is made by the liver, stored in the gall bladder and released into the duodenum.

Chemical bonds are broken, so the products are different molecules from the original food molecules. Digestive enzymes catalyse these reactions.

Chemical digestion converts them into small, soluble molecules. These can dissolve and move across the epithelium of the small intestine, so they can be absorbed into the blood.

large and insoluble → enzymatic breakdown → small and soluble → crosses the intestinal wall → absorbed into blood

The enzyme name is not the product: amylase acts on starch, proteases act on protein, and lipase acts on lipids (fats and oils).

Pancreatic amylase, protease and lipase are secreted through the pancreatic duct into the duodenum, so the pancreas is a secretion site while the small intestine is their action site.

These are the two syllabus functions: antimicrobial action and an optimum acidic pH for enzyme activity.

Salivary amylase begins starch digestion in the mouth. Pancreatic amylase continues it in the small intestine; membrane-bound maltase then produces glucose at the absorptive surface.

Hydrochloric acid provides the acidic conditions for pepsin in the stomach. After the mixture enters the duodenum, bile helps create alkaline conditions suitable for trypsin.

Food leaving the stomach is mixed with acidic gastric juice. Bile neutralises this acidic mixture, raising its pH in the duodenum.

The resulting alkaline conditions provide a suitable pH for digestive enzymes acting in the small intestine, including trypsin and lipase.

The small intestine is the region of the alimentary canal where nutrients are absorbed. Its inner surface is specialised for moving digested nutrients out of the lumen and into transport systems in the villi.

digested nutrient in the lumen → crosses the small-intestinal epithelium → enters blood capillaries or a lacteal → transported around the body

When a question asks where most water is absorbed, choose the small intestine. The colon still has an important role, but it does not absorb the largest total amount.

A larger internal surface provides more membrane through which nutrient molecules can move at the same time. This increases the rate and efficiency of absorption.

Many villi line the small intestine. Each combines a large surface area, a short distance across the epithelium and nearby routes that carry absorbed substances away.

The capillary network carries water-soluble nutrients away from the villus. Continued blood flow helps maintain a concentration gradient between the intestinal lumen and the blood.