The Stomach (OCR GCSE Biology A (Gateway Science Suite)): Revision Notes

2.1.2 The Stomach

Structure of the Stomach:

• Muscular Walls:
  • The stomach has thick, muscular walls that contract to mix food with digestive juices. This process is called churning.
  • The contractions help break down food mechanically into smaller pieces, increasing the surface area for digestive enzymes to act.
• Gastric Glands:
  • The inner lining of the stomach contains gastric glands, which secrete gastric juice.
  • Gastric juice is a mixture of hydrochloric acid (HCl), pepsin (a digestive enzyme), and mucus.
• Hydrochloric Acid (HCl):
  • HCl creates an acidic environment in the stomach, which helps to kill bacteria and pathogens that may be present in food.
  • The acid also provides the optimal pH for pepsin to function effectively.
• Pepsin:
  • Pepsin is a protease enzyme that begins the digestion of proteins by breaking them down into smaller peptides.
  • Pepsin is secreted in an inactive form called pepsinogen, which is activated by the acidic environment.
• Mucus:
  • The stomach lining is protected by a layer of mucus, which prevents the stomach from digesting itself and protects against the corrosive effects of the acid.

Function of the Stomach:

1. Mechanical Digestion:

• The stomach's muscular walls mix and churn food, breaking it down into a semi-liquid form known as chyme.

2. Chemical Digestion:

• Gastric juice, containing HCl and pepsin, starts the chemical breakdown of proteins into smaller peptides.
• The acidic environment helps denature proteins, making them easier to digest.

3. Temporary Storage:

• The stomach serves as a temporary storage site for food, releasing it gradually into the small intestine for further digestion and absorption.

4. Protection:

• The stomach's acidic environment helps kill harmful bacteria and pathogens that may be ingested with food.

The Stomach in the Digestive System:

• After food is swallowed, it enters the stomach from the oesophagus through a valve called the lower oesophageal sphincter.
• Food is then mixed with gastric juices in the stomach.
• The partially digested food (chyme) is gradually released into the duodenum (the first part of the small intestine) through another valve called the pyloric sphincter.