Principles of Organisation (B2.1)

Levels of organisation

Living organisms are organised into a hierarchy:

Cell → Tissue → Organ → Organ system → Organism

• Cell — the basic structural and functional unit of life.
• Tissue — a group of similar cells working together to perform a specific function. Example: muscle tissue (cells that can contract).
• Organ — a structure made of several different tissues working together. Example: the stomach contains muscular tissue, glandular tissue and epithelial tissue.
• Organ system — a group of organs working together. Example: the digestive system.
• Organism — the whole living thing.

Key examples

The leaf (plant organ)

• Epidermis — thin, transparent layer; protects the leaf; upper epidermis allows light through.
• Palisade mesophyll — elongated cells packed with chloroplasts; main site of photosynthesis.
• Spongy mesophyll — loosely arranged cells with air spaces; allows gas exchange.
• Xylem and phloem — in the vascular bundle (midrib and veins); transport water and dissolved sugars.
• Guard cells — control opening/closing of stomata to regulate gas exchange and water loss.

The digestive system (organ system)

Organs: mouth, salivary glands, oesophagus, stomach, liver, gall bladder, pancreas, small intestine, large intestine, rectum, anus.

Function: breaks down large insoluble food molecules into small soluble molecules that can be absorbed into the blood.

Enzymes in digestion

Enzymes are biological catalysts. In digestion:

Bile (made in the liver, stored in gall bladder) is not an enzyme. It emulsifies fats (breaks large fat droplets into smaller ones), increasing surface area for lipase. Bile also neutralises stomach acid in the small intestine.

Common exam errors

1. Confusing enzyme with substrate (amylase digests starch, not the other way round).
2. Saying bile digests fats — it emulsifies, it doesn't chemically break down.
3. Missing the organ-system level in the hierarchy.