IGCSE Biology / Food and digestion

The parts of the digestive system revision notes

Edexcel International GCSE Biology (4BI1), chapter 4: Food and digestion, specification point 2.27. Free lesson.

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The digestive system is a continuous tube from the mouth to the anus, with each region doing a particular job. The five stages of nutrition explain how food enters the body, is processed and used, then leaves as faeces.

How food moves through the alimentary canal

The alimentary canal is the continuous tube that food passes through, from the mouth to the anus. Each region has a different role, so learn the parts in order. The mouth breaks food up with the teeth. Saliva moistens it, and the tongue rolls it into a ball called a bolus. The oesophagus is a muscular tube. It pushes food towards the stomach by peristalsis, which is the movement of muscles that squeezes food along. Nothing is absorbed here. The stomach churns food with its muscular walls and holds it for a few hours. Acid and pepsin are present here. The duodenum is…

How the liver and pancreas supply the gut

The liver and pancreas sit beside the alimentary canal. Food does not pass through them. Instead, they make digestive substances that travel into the duodenum, where much digestion happens. The liver makes bile, which is stored in the gall bladder between meals. The bile duct carries bile to the duodenum. The pancreas makes digestive enzymes. The pancreatic duct carries them to the duodenum. Bile and pancreatic enzymes arrive at the same part of the gut, the duodenum.

What the five stages of nutrition mean

The five stages describe what happens to food. Keep them in order, because each name refers to a different point in the process.

How absorption differs from assimilation

Both stages involve digested food, but they describe different destinations. Absorption is food entering the blood. Assimilation is food entering a cell and being used there. For example, glucose from bread is absorbed into the blood in the ileum. If that glucose then enters a muscle cell and is respired, its entry into the cell is assimilation. Egestion is not excretion. Faeces is undigested food that was never absorbed, so it was never inside a body cell and is not a waste product made by metabolism.

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