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GCSE Biology Revision

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GCSE Biology revision

The human digestive system

Animal tissues, organs and organ systems

AQA 4.1.1.1
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AQA student objectives

Learning pathway

All · Most · Some

ALL 🎯

Identify and describe The Two Main Roles of the Digestive System, The Mouth: Where Digestion Begins and The Oesophagus and Stomach in the context of the human digestive system.

MOST 🎯🎯

Explain the relationships between The Two Main Roles of the Digestive System, The Mouth: Where Digestion Begins and The Oesophagus and Stomach and how they contribute to the human digestive system.

SOME 🎯🎯🎯

Apply and analyse knowledge of The Two Main Roles of the Digestive System, The Mouth: Where Digestion Begins and The Oesophagus and Stomach to interpret unfamiliar information about the human digestive system.

Revision summary

Key knowledge

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The Two Main Roles of the Digestive System

  • Digestion is the process of breaking down large food molecules into smaller ones, such as starch into glucose, proteins into amino acids, and lipids into glycerol and fatty acids
  • Absorption is the process by which small molecules, along with vitamins, minerals, and water, are taken from the digestive tract into the bloodstream for use around the body

The Mouth: Where Digestion Begins

  • The teeth physically break down food by chewing, increasing its surface area so enzymes can act on it more efficiently and making it easier to swallow
  • The salivary glands release saliva, which contains the enzyme amylase that begins the digestion of starch into maltose
  • Enzymes such as amylase are called biological catalysts because they reduce the energy needed for reactions, allowing nutrients to be broken down more quickly

The Oesophagus and Stomach

  • After swallowing, food travels down the oesophagus (also called the gullet), a muscular tube that carries food to the stomach
  • The stomach's muscular walls contract to churn and mix the food thoroughly
  • The stomach produces pepsin, a protease enzyme that breaks proteins down into amino acids
  • The stomach also produces hydrochloric acid, which kills swallowed bacteria and provides the correct acidic pH for pepsin to function

The Pancreas and Its Digestive Juices

  • The pancreas releases pancreatic juice into the small intestine at the same time as food arrives there
  • Pancreatic juice is a liquid mixture containing a range of digestive enzymes, including carbohydrases (such as amylase), proteases, and lipases, which continue the breakdown of food

Bile: Production, Storage, and Function

  • Bile is produced in the liver and stored in the gall bladder until it is needed in the small intestine
  • Bile neutralises the acid arriving from the stomach, creating a more alkaline pH that is ideal for digestive enzymes to function
  • Bile emulsifies lipids, breaking large fat and oil droplets into tiny droplets, which greatly increases the surface area available for lipase enzymes to act on

The Small Intestine: Digestion and Absorption

  • The small intestine is where the majority of digestion takes place, as it releases its own digestive enzymes including carbohydrases, proteases, and lipases
  • The small intestine is also the primary site of absorption, where digested nutrients pass across the intestinal lining into the bloodstream
  • The inner surface of the small intestine is covered in tiny finger-like projections called villi (singular: villus), which massively increase the surface area for absorption
  • Villi have a single layer of surface cells, meaning there is only a very short diffusion distance for nutrients to cross into the blood
  • Villi have a rich blood supply, which maintains a steep concentration gradient to ensure efficient absorption of nutrients by diffusion
  • The cells of the villi also have their own tiny projections called microvilli, which further increase the surface area available for absorption

The Large Intestine, Rectum, and Anus

  • After nutrients are absorbed in the small intestine, the remaining material passes into the large intestine, where excess water is absorbed back into the body
  • The relatively dry material left behind is called faeces, which is stored in the rectum until it is expelled from the body through the anus

Key Digestive Enzymes and Where They Act

  • Carbohydrases, such as amylase, break down carbohydrates like starch into smaller sugars such as maltose and are produced in the salivary glands, pancreas, and small intestine
  • Proteases, such as pepsin, break down proteins into amino acids and are produced in the stomach, pancreas, and small intestine
  • Lipases break down lipids (fats and oils) into glycerol and fatty acids and are produced by the pancreas and small intestine

Summary of the Digestive Journey

  • Food enters at the mouth, where mechanical digestion by teeth and chemical digestion by salivary amylase begin the process
  • The stomach, pancreas, gall bladder, and small intestine all contribute enzymes or secretions that continue breaking food down into absorbable molecules
  • Nutrients are absorbed into the bloodstream mainly through the villi of the small intestine, while water is reclaimed in the large intestine before faeces are excreted