Digestive System

1. INGESTION — taking food and drink into the body via the mouth.

2. DIGESTION — breaking large molecules into smaller, SOLUBLE ones.

• Physical (mechanical) digestion: teeth chewing; stomach churning; bile emulsifying fat. Doesn't change the molecules — just breaks food into smaller pieces with more surface area.
• Chemical digestion: enzymes break the chemical BONDS in molecules. Carbs → glucose; proteins → amino acids; fats → fatty acids + glycerol.

3. ABSORPTION — small soluble molecules pass FROM the gut INTO the blood (or lymph for fats). Mostly in the small intestine.

4. ASSIMILATION — body cells take up and USE the absorbed nutrients (e.g. amino acids made into new proteins; glucose used for respiration).

5. EGESTION — passing UNDIGESTED material out as faeces.

Worked qualitative. Are these the same: egestion vs excretion?

• Egestion = undigested food OUT (faeces). The food never entered cells.
• Excretion = METABOLIC WASTE made by cells (CO₂, urea, salts) OUT. These were INSIDE cells.
• Faeces = mainly cellulose, dead gut bacteria, dead gut cells. Not "metabolic waste" of the body.

Cambridge tip. Memorise the five stages in ORDER. Cambridge often asks "place these in order" or "give the term for…".

1. Mouth (buccal cavity).

• Teeth CHEW food (mechanical digestion).
• Tongue mixes food with SALIVA.
• Saliva contains AMYLASE (starts digesting starch → maltose) and mucus (lubrication).
• Food formed into a BOLUS and swallowed.

2. Oesophagus (gullet).

• Tube from mouth to stomach.
• Muscles do PERISTALSIS — pushes the bolus along.

3. Stomach.

• Stretchy, muscular sac.
• Walls produce GASTRIC JUICE: hydrochloric acid (HCl, ~pH 2) + PEPSIN (protease).
• HCl kills bacteria + provides optimum pH for pepsin.
• Pepsin breaks proteins into smaller polypeptides.
• Stomach churns food into a thick liquid (CHYME).
• Sphincter at exit controls release into small intestine.

4. Small intestine (duodenum + ileum).

• Long, narrow tube (~6 m).
• Duodenum (first part): receives chyme + pancreatic juice + bile.
• Ileum (rest): completes digestion AND ABSORPTION via villi.
• Pancreatic juice (from pancreas) brings amylase, protease (trypsin), lipase + bicarbonate (neutralises stomach acid).
• Bile (from liver/gall bladder) emulsifies fat.
• Wall enzymes (e.g. maltase) finish off the small molecules.

5. Large intestine (colon + rectum + anus).

• Colon: absorbs WATER from undigested matter; bacteria here make some vitamin K.
• Rectum: stores faeces.
• Anus: opens to release faeces (egestion).

Accessory organs (food doesn't pass through, but they help).

• Liver: makes BILE (stored in gall bladder); processes absorbed nutrients (e.g. converts excess glucose to glycogen).
• Gall bladder: stores bile; releases it into the duodenum.
• Pancreas: makes pancreatic juice (and insulin/glucagon for blood sugar).

Worked qualitative. Why is the small intestine so long?

• Long length = more time for digestion AND a huge surface area for absorption.
• Combined with internal folds + VILLI + microvilli, surface area is ~250 m² (size of a tennis court).

Cambridge tip. When asked "where does digestion happen?", say BOTH stomach AND small intestine. Many students say "stomach only".

The two muscle layers in the gut wall:

• Circular muscle (inner): squeezes the gut from the sides.
• Longitudinal muscle (outer): runs along the gut.

The wave. Behind a bolus of food:

• Circular muscle CONTRACTS → gut narrows → squeezes the food forward.
• Longitudinal muscle relaxes.

In front of the bolus:

• Circular muscle relaxes.
• Longitudinal muscle contracts → gut shortens and WIDENS → makes space for the food.

The wave runs all the way from mouth to anus.

Why fibre helps.

• Fibre adds BULK to gut contents.
• Muscles can grip the bulk more easily.
• Without fibre, food is too soft to push → constipation.

Worked qualitative. Why can astronauts swallow food in zero gravity?

• Peristalsis is muscle action, not gravity-driven.
• It works in any orientation — even upside down.

Cambridge tip. Always describe BOTH muscles in BOTH conditions (contracted/relaxed) for full marks. "The muscles squeeze food along" earns 1; the full mechanism earns 3.