Sec 3 Biology Nutrition in Humans notes

The alimentary canal is a single tube about nine metres long, and every organ is a station on it. Ingestion (taking food in) happens at the mouth, where teeth grind the food and the salivary glands add amylase, starting starch digestion. The oesophagus transports the swallowed bolus to the stomach, which stores it, churns it, and runs protease in acid to begin protein digestion.

Chemical digestion then concentrates in the duodenum, the first loop of the small intestine, where bile from the gall bladder and pancreatic juice from the pancreas arrive together. The ileum, the long final stretch of small intestine, is the absorption stage. In the colon water and salts are reclaimed, the rectum stores the residue, and egestion of faeces ends the journey at the anus.

Peristalsis is the muscle engine of the canal: rings of circular muscle squeeze behind the food while longitudinal muscle relaxes ahead, sending rhythmic waves that both mix the contents with digestive juices and propel them along. It works in the oesophagus, stomach, small intestine and large intestine alike — which is why you can swallow upside down.

Fat gets one extra preprocessing step: bile, made in the liver and stored in the gall bladder, is not an enzyme. It emulsifies fat — breaking big globules into tiny droplets so lipase has a far larger surface to attack. Without bile, fat digestion crawls.

The ileum’s lining is folded into millions of finger-like villi, and each villus cell bristles with microvilli — folds upon folds that massively enlarge the absorbing surface. Each villus is one cell thick, so absorbed molecules cross a minimal distance, and its interior is packed with a blood capillary network plus a single central lacteal.

The two vessels split the cargo. Glucose and amino acids enter the capillaries, whose blood drains into the hepatic portal vein for direct delivery to the liver. Fatty acids and glycerol are rebuilt into fat inside the villus cells and enter the lacteal, travelling in lymph that rejoins the bloodstream further along — bypassing the liver on the first pass.

The hepatic portal vein exists because the liver must vet every absorbed meal before general circulation. Its jobs, per the syllabus: converting glucose to glycogen for storage (and back again when blood glucose dips); producing the bile that digests fats; deaminating excess amino acids — stripping their nitrogen and turning it into urea for the kidney to excrete; and breaking down alcohol and hormones.

Assimilation completes the journey: glucose is respired for energy or stored as glycogen; amino acids are built into the body’s own proteins (or deaminated if in excess); fats are stored or used in membranes. The liver sits at the centre of all three decisions.

Alcohol is a depressant: it slows the brain’s processing. Short term, that means slower reaction times, reduced self-control and clouded judgement — the reasons drink-driving laws exist. At the same time it irritates the stomach and increases urine production.

Long-term heavy use damages exactly the organs that handle it. The liver — which must break the alcohol down — can develop fatty change, then cirrhosis, in which working liver tissue is replaced by non-functioning scar tissue, wrecking glucose storage, urea production and detoxification. The brain suffers too: long-term abuse causes brain damage, with memory and cognitive loss.

The social implications complete the syllabus list: impaired driving and accidents, workplace errors, violence and crime, and strained families and finances. Biology, behaviour and society connect through one molecule — which is why the syllabus asks you to discuss all three.