The Digestive System

What Is Digestion?

Digestion is the process of breaking down large, complex food molecules into small, soluble molecules that can be absorbed into the bloodstream and used by cells.

There are two types of digestion:

• Mechanical digestion — physical breakdown of food into smaller pieces (e.g., chewing, churning in the stomach). Increases surface area for enzymes to act on.
• Chemical digestion — enzymes break large molecules into smaller ones (e.g., starch → sugars; proteins → amino acids; fats → fatty acids + glycerol).

The Digestive Journey

Food travels approximately 9 metres through the digestive system over 24–72 hours:

1. Mouth

• Teeth break food into smaller pieces (mechanical digestion)
• Salivary glands secrete saliva containing salivary amylase
• Amylase begins breaking starch down into maltose (a sugar)
• The tongue forms food into a ball (bolus) for swallowing

2. Oesophagus

• A muscular tube connecting the mouth to the stomach
• Food is moved by peristalsis — wave-like contractions of circular and longitudinal muscles
• No digestion occurs here

3. Stomach

• A muscular bag that churns food (mechanical digestion)
• Gastric glands in the stomach wall produce:
  • Hydrochloric acid (HCl) — creates acidic environment (pH 1–2); kills bacteria; activates pepsin
  • Pepsin (a protease) — begins breaking proteins into smaller peptides
• Food is mixed into a semi-liquid paste called chyme
• The stomach empties slowly into the small intestine over several hours

4. Small Intestine

The small intestine is the main site of both chemical digestion and absorption. It is about 6–7 metres long, highly coiled.

Two accessory organs contribute:

Pancreas — secretes pancreatic juice containing:

• Amylase (starch → maltose)
• Lipase (fats → fatty acids + glycerol)
• Protease/trypsin (proteins → amino acids)

Liver — produces bile, stored in the gallbladder, released into the small intestine:

• Bile emulsifies fats — breaks large fat droplets into tiny droplets, greatly increasing surface area for lipase to work on
• Bile is alkaline — neutralises stomach acid entering the small intestine

5. Large Intestine

• Receives indigestible material (mainly fibre) and water
• Absorbs water back into the bloodstream
• Compacts remaining material into faeces
• Contains billions of bacteria that break down some remaining material

6. Rectum and Anus

• Faeces are stored in the rectum
• Expelled through the anus (egestion)

Key Digestive Enzymes

Enzyme	Where produced	Substrate	Product
Salivary amylase	Salivary glands (mouth)	Starch	Maltose (sugar)
Pancreatic amylase	Pancreas	Starch	Maltose
Protease (pepsin)	Stomach	Proteins	Peptides
Protease (trypsin)	Pancreas	Peptides	Amino acids
Lipase	Pancreas	Fats (lipids)	Fatty acids + glycerol
Maltase	Small intestine walls	Maltose	Glucose

All digestive enzymes are biological catalysts — they speed up digestion without being used up, and work best at specific pH and temperature conditions.

The Role of Bile

Bile is not an enzyme — it does not chemically break down fat. Instead, it emulsifies fat:

• Large fat droplets are physically broken into many smaller droplets
• This massively increases the surface area available for lipase to act on
• Lipase can then digest fat much more efficiently

Bile is produced by the liver, stored in the gallbladder, and released into the duodenum (first section of small intestine) via the bile duct.

Bile is also alkaline (pH 7–8), which neutralises the acidic chyme entering the small intestine from the stomach, creating the optimal pH for intestinal enzymes.

Absorption in the Small Intestine

After chemical digestion, small molecules are absorbed through the wall of the small intestine into the bloodstream.

The small intestine is highly adapted for absorption:

• Very long — provides large surface area
• Villi — finger-like projections of the intestinal wall that massively increase surface area
• Microvilli — tiny projections on the surface of each villus (brush border) — further increase surface area
• Good blood supply — capillaries inside each villus carry absorbed nutrients to the liver via the hepatic portal vein
• Lacteal — lymph vessel inside each villus; absorbs fatty acids and glycerol (as fat droplets called chylomicrons)

Nutrients absorbed:

• Glucose and amino acids — absorbed by active transport and diffusion into capillaries
• Fatty acids and glycerol — absorbed into lacteals, transported as lymph
• Vitamins and minerals — various mechanisms

What Happens to Absorbed Nutrients?

Once in the bloodstream, nutrients travel to the liver (via the hepatic portal vein) where they are:

• Glucose — regulated (stored as glycogen if excess)
• Amino acids — used to make proteins; excess deaminated and excreted as urea
• Fats — processed, stored, or used for energy

Key Terms

Term	Definition
Digestion	Breakdown of large food molecules into small, soluble molecules
Mechanical digestion	Physical breakdown of food (e.g., chewing, churning)
Chemical digestion	Enzymatic breakdown of food molecules
Peristalsis	Wave-like muscle contractions that move food through the gut
Enzyme	Biological catalyst that speeds up chemical reactions
Bile	Alkaline liquid produced by liver; emulsifies fats
Emulsification	Breaking large fat droplets into smaller ones
Villi	Finger-like projections in small intestine that increase surface area
Absorption	Movement of digested nutrients from gut into bloodstream

Worked Example

Q: Explain why bile is important for the digestion of fats, even though it is not an enzyme.

Bile is produced by the liver and released into the small intestine. It emulsifies fats — breaking large fat globules into much smaller droplets.

This increases the surface area of fat that is exposed to lipase (the fat-digesting enzyme). With a larger surface area, lipase can bind to and break down fat much more efficiently.

Without bile, fat digestion would be much slower because lipase could only act on the outer surface of large fat droplets.

Common Mistakes

• Saying bile digests fat — bile emulsifies fat (physical breakdown); lipase chemically digests fat into fatty acids and glycerol.
• Confusing the large intestine with the small intestine — the small intestine is where digestion and nutrient absorption occur; the large intestine absorbs water.
• Saying the stomach digests all proteins — the stomach begins protein digestion with pepsin, but most protein digestion is completed in the small intestine.
• Forgetting where amylase is produced — two places: salivary glands (mouth) and pancreas.

Tips and Tricks

• Digestion summary: starch → sugars (amylase); protein → amino acids (protease); fat → fatty acids + glycerol (lipase).
• Remember bile = from Liver, stored in gaLLbladder (two L's in both).
• Villi increase surface area — this is the single most important adaptation of the small intestine for absorption.
• The small intestine is small in diameter (about 2.5 cm) but very long (6–7 m); the large intestine is large in diameter but shorter.