DigestionOxford AQA IGCSE Biology: Revision notes

The human digestive system is a long tube called the alimentary canal, with associated organs that break down food and absorb nutrients.

Journey through the digestive tract:

1. Mouth — Mechanical and chemical digestion begins; teeth break down food into smaller pieces whilst salivary amylase starts breaking down carbohydrates
2. Oesophagus — A muscular tube that uses peristalsis (coordinated muscle contractions) to push food down to the stomach
3. Stomach — A muscular bag that churns food and secretes gastric juices containing protease to begin protein digestion; the stomach also stores food temporarily
4. Small intestine — Divided into two regions:
   • Duodenum — The first part where bile and pancreatic enzymes enter to continue digestion
   • Ileum — The second part where most nutrient absorption occurs through the intestinal wall
5. Large intestine — Absorbs water from the remaining undigested food, making faeces more solid
6. Rectum — Stores faeces before they are eliminated
7. Anus — The opening through which faeces are expelled

Associated organs:

These organs work together to ensure complete digestion and efficient nutrient absorption.

Enzymes are biological catalysts that speed up the breakdown of large food molecules into smaller, absorbable units. Each enzyme is specific to one type of food.

The three main digestive enzymes:

Key points about enzyme action:

• Amylase begins in the mouth with saliva and continues in the small intestine with pancreatic amylase. This converts starch into maltose and then glucose, which can be absorbed
• Protease (called pepsin in the stomach) begins protein breakdown in the acidic stomach and continues in the small intestine with pancreatic protease to produce amino acids
• Lipase only works in the small intestine where the environment is alkaline; it requires bile salts to be effective because they emulsify fats into smaller droplets, increasing surface area for enzyme action

Without these enzymes, food molecules would be too large to be absorbed through the intestinal wall.

When partially digested food (called chyme) enters the small intestine from the stomach, it is very acidic. The small intestine must be neutralised for the enzymes to work effectively.

The role of bile:

• Produced by the liver and stored in the gall bladder
• Released into the duodenum when food arrives
• Neutralises stomach acid by being slightly alkaline, raising the pH to around 7-8
• Emulsifies fats by breaking large fat droplets into smaller ones, increasing surface area for lipase to act on
• Contains bile salts that coat fat droplets, preventing them from rejoining and keeping them dispersed
• Note: Bile does not contain enzymes — it is not a digestive enzyme itself

The role of pancreatic secretions:

• The pancreas produces three main enzymes: amylase, protease, and lipase
• The pancreas also produces bicarbonate, which further neutralises stomach acid
• Bicarbonate raises the pH to allow pancreatic enzymes to work optimally

Why neutralisation matters:

Enzymes only work in specific pH ranges. Proteases and lipases require a near-neutral pH to function, so if the acid from the stomach were not neutralised, digestion would stop and nutrient absorption would fail.

The small intestine is the main site of nutrient absorption. Its structure is highly specialised to maximise the uptake of digested food products.

The role of villi:

The small intestine is lined with millions of villi — tiny finger-like projections that extend into the intestinal cavity. Villi dramatically increase the surface area available for absorption. Without villi, the surface area would be insufficient for efficient nutrient uptake.

Structure of a single villus:

Each villus is composed of:

• Epithelial cells — Form the outer layer and absorb nutrients through their cell membranes
• Capillary network — Blood vessels that carry absorbed glucose and amino acids to the liver via the hepatic portal vein
• Lacteal — A lymphatic vessel that absorbs fatty acids and glycerol (products of fat digestion) and transports them via the lymphatic system
• Connective tissue — Supports the structure inside the villus

How villi are adapted for absorption:

1. Large surface area — Millions of villi provide an enormous area for absorption; each villus also has microvilli on its epithelial cells, further increasing surface area
2. Thin walls — The epithelial layer is only one cell thick, minimising the distance nutrients must diffuse to enter the blood
3. Good blood supply — Dense capillary networks around each villus rapidly carry away absorbed nutrients, maintaining a concentration gradient for continued absorption
4. Lacteal presence — The lymphatic vessel ensures that lipid products are transported separately from water-soluble nutrients

These adaptations work together to ensure that by the time food leaves the small intestine, most nutrients have been absorbed.

By the time undigested food reaches the large intestine (also called the colon), most nutrients have been absorbed. The large intestine has a different role: water reabsorption.

The function of the large intestine:

• Absorbs water from the remaining food material, making faeces more solid and concentrated
• Absorbs ions (particularly sodium) that were in the digestive secretions
• Hosts beneficial bacteria (gut flora) that can ferment any remaining carbohydrate and produce some vitamins
• Stores faeces temporarily before defecation

Why water absorption is essential:

• The digestive system secretes large volumes of water in saliva, gastric juice, bile, pancreatic juice, and intestinal secretions
• If this water were not reabsorbed, the body would become severely dehydrated
• Reabsorbing water also concentrates faeces, making them easier to expel
• The final faeces contain mainly indigestible material (fibre), bacteria, and shed intestinal cells

Key difference from the small intestine:

The small intestine absorbs nutrients (glucose, amino acids, fatty acids, glycerol, minerals, vitamins) whereas the large intestine absorbs water and minerals only. No enzymes are secreted in the large intestine because digestion is already complete.