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The key difference
Ingestion is taking food into the mouth. Digestion breaks it into small soluble molecules. Absorption is the passage of these molecules into the blood. Egestion is the removal of undigested material as faeces.
These four terms describe the stages of food processing in the alimentary canal. Examiners test precise definitions: confusing any two (especially egestion with excretion) is a common way to lose easy marks. This guide nails the exact wording you need.
Feature
Ingestion
Digestion
Absorption
Egestion
Definition
Taking food into the body through the mouth
Breaking down large, insoluble food molecules into small, soluble molecules
Passage of small, soluble molecules through the wall of the small intestine into the blood (or lymph)
Passing out undigested food material as faeces through the anus
Where it occurs
Mouth
Mouth, stomach, and small intestine
Small intestine (mainly ileum)
Large intestine and anus
How it works
Biting and placing food in the mouth
Mechanical (teeth, churning) and chemical (enzymes)
Diffusion and active transport through villi into blood capillaries and lacteals
Peristalsis moves undigested material through the large intestine; water is absorbed; faeces are expelled
What is involved
Teeth, tongue, jaw muscles
Enzymes (amylase, protease, lipase), teeth, stomach muscles, bile
Villi with large surface area, blood capillaries, lacteals
Muscular walls of the large intestine and rectum
Egestion vs Excretion: The Classic Confusion
This is one of the most commonly tested distinctions in IGCSE Biology:
Egestion: removal of undigested food that was never absorbed into the body. Faeces are mainly undigested fibre, dead bacteria, and dead cells from the intestine lining. This material was never part of the body's metabolism.
Excretion: removal of metabolic waste products that were produced by chemical reactions inside cells. Examples include CO₂ (from respiration, removed via the lungs) and urea (from deamination of amino acids in the liver, removed via the kidneys).
The key distinction: egested material was never absorbed or used by the body; excreted material is waste from metabolism.
How Absorption Is Maximised
The small intestine (particularly the ileum) is highly adapted for absorption:
Villi: finger-like projections that increase the surface area enormously
Microvilli: tiny projections on the surface of each villus cell, further increasing surface area
Thin walls: each villus is one cell thick, providing a short diffusion distance
Rich blood supply: a dense network of capillaries maintains a steep concentration gradient for diffusion
Lacteals: lymph vessels inside each villus absorb fatty acids and glycerol
Try a comparison question (4 marks)
Distinguish between egestion and excretion. Give one example of each. [4]
Show the answer and marking guidance
Egestion is the removal of undigested food / material that has not been absorbed into the body
Example: faeces passed out through the anus (containing undigested fibre, dead bacteria)
Excretion is the removal of metabolic waste products produced by chemical reactions in cells
Example: carbon dioxide removed via the lungs (from respiration) or urea removed via the kidneys (from deamination of amino acids)
Original Cambridge-style practice written for this site.
Common comparison mistakes
When answering a compare question, write matched pairs rather than two separate descriptions.
Confusing egestion with excretion: egestion = undigested food out; excretion = metabolic waste out
Saying absorption occurs in the stomach: it mainly occurs in the small intestine (ileum)
Calling faeces "excretory waste": faeces are egested, not excreted, because the material was never metabolised
Forgetting that digestion includes both mechanical AND chemical processes