Digestive system
Section: Biology | Syllabus: Cambridge IGCSE Combined Science (0653)
The Digestive System
The digestive system consists of the alimentary canal - a continuous tube from mouth to anus - and associated organs that help process food.
The alimentary canal and its associated organs work together to digest food
Organs of the Digestive System
The alimentary canal:
- Mouth - ingestion of food; physical digestion by teeth; saliva begins chemical digestion of starch
- Oesophagus - transports food from the mouth to the stomach
- Stomach - physical digestion by muscular churning; produces gastric juice (hydrochloric acid and proteases), which kills harmful bacteria and begins protein digestion
- Small intestine (duodenum and ileum) - the duodenum receives bile and pancreatic juice, completing chemical digestion; the ileum absorbs digested nutrients into the blood
- Large intestine (colon) - absorbs water from the undigested material and compacts it into faeces
- Rectum - stores faeces until egestion
- Anus - the controlled release of faeces from the body (egestion)
The associated organs:
- Salivary glands - produce saliva, containing amylase (which digests starch) and mucus (which lubricates food for swallowing)
- Liver - produces bile, which is stored in the gall bladder and used to emulsify fats
- Gall bladder - stores bile and releases it into the duodenum when fatty food is present
- Pancreas - produces pancreatic juice, containing amylase, lipase and protease, which is secreted into the duodenum
Key Terms in Nutrition
- Ingestion - the taking of substances, e.g. food and drink, into the body (through the mouth)
- Digestion - the breakdown of food, both physically and chemically, into smaller, soluble molecules (in the mouth, stomach and small intestine)
- Absorption - the movement of digested nutrients from the intestines into the blood (mainly in the small intestine, with some water absorbed in the colon)
- Assimilation - the uptake and use of absorbed nutrients by cells, for energy, growth and repair
- Egestion - the removal of undigested food from the body as faeces (through the anus)
Common Mistakes
- ❌ Confusing egestion with excretion
- ✓ egestion is the removal of undigested food - material that was never absorbed into the body, so it is not a product of metabolism. Excretion is the removal of waste products of metabolism (e.g. CO₂ from respiration, urea from the breakdown of amino acids)
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