Organisation
Tissues, organs, organ systems, enzymes, and the digestive system.
In this topic
Levels of Organisation
Living organisms are organised in a hierarchy:
Cells → Tissues → Organs → Organ Systems → Organisms
- Tissue: a group of similar cells working together for a specific function (e.g., muscle tissue) - Organ: a group of different tissues working together (e.g., the stomach contains muscle tissue, glandular tissue, and epithelial tissue) - Organ system: a group of organs working together (e.g., the digestive system)
Key Points
- Each level builds on the one below
- Organs contain multiple types of tissue
- The digestive system includes: mouth, oesophagus, stomach, small intestine, large intestine, liver, pancreas
Example Questions
1Put these in order from smallest to largest: organ, tissue, organ system, cell.
Cell → Tissue → Organ → Organ system
[1 mark]
Enzymes
Enzymes are biological catalysts — they speed up chemical reactions without being used up. They are proteins with a specific 3D shape.
The lock and key model: the substrate fits into the enzyme's active site like a key in a lock. The enzyme is specific to its substrate.
Key digestive enzymes: - Amylase: breaks down starch → sugars (maltose). Produced in salivary glands and pancreas. - Protease: breaks down proteins → amino acids. Produced in stomach, pancreas, small intestine. - Lipase: breaks down lipids → glycerol + fatty acids. Produced in pancreas and small intestine. - Bile: not an enzyme, but emulsifies fats (increases surface area). Produced by liver, stored in gallbladder.
Key Points
- Enzymes have an optimum temperature and pH
- High temperatures denature enzymes — the active site changes shape
- pH extremes also denature enzymes
- Each enzyme works best at a specific pH (e.g., pepsin in stomach works at pH 2)
Example Questions
3Explain why enzymes stop working at high temperatures.
At high temperatures, the bonds holding the enzyme's 3D shape break. This causes the active site to change shape (denature), so the substrate can no longer fit into it. The enzyme can no longer catalyse the reaction.
[3 marks]
3Explain the role of bile in digestion.
Bile is produced by the liver and stored in the gallbladder. It emulsifies fats, breaking large fat droplets into smaller ones. This increases the surface area for lipase to act on, speeding up fat digestion. Bile also neutralises stomach acid, creating alkaline conditions for intestinal enzymes.
[3 marks]
The Heart and Circulatory System
The circulatory system transports substances around the body. Humans have a double circulatory system: 1. Right side pumps blood to the lungs (pulmonary circuit) 2. Left side pumps blood to the body (systemic circuit)
The heart has four chambers: - Right atrium: receives deoxygenated blood from the body (via vena cava) - Right ventricle: pumps blood to the lungs (via pulmonary artery) - Left atrium: receives oxygenated blood from the lungs (via pulmonary vein) - Left ventricle: pumps blood to the body (via aorta) — thickest wall
Blood vessels: - Arteries: carry blood AWAY from the heart (thick walls, high pressure) - Veins: carry blood TO the heart (thin walls, valves, low pressure) - Capillaries: exchange substances with tissues (one cell thick)
Key Points
- Left ventricle has the thickest wall — pumps blood furthest
- Valves prevent backflow of blood
- Arteries = Away from heart
- Coronary arteries supply the heart muscle with blood
Example Questions
3Explain why the left ventricle has a thicker muscular wall than the right ventricle.
The left ventricle pumps blood all around the body (systemic circulation), which requires a higher pressure than the right ventricle, which only pumps blood to the lungs. The thicker muscle wall generates a stronger contraction to create this higher pressure.
[3 marks]
3Explain how capillaries are adapted for the exchange of substances.
Capillaries have walls that are only one cell thick, providing a short diffusion distance. They have a very large surface area due to their extensive network. They carry blood at low pressure, allowing time for exchange.
[3 marks]