Principles of OrganisationAQA GCSE Biology: Revision notes

Multicellular organisms are built up in a hierarchy, from the smallest working unit to the whole organism:

1. Cells — the basic building blocks of all organisms
2. Tissues — a group of cells with a similar structure and function, working together
3. Organs — an aggregation of different tissues performing specific functions
4. Organ systems — groups of organs that work together to perform particular functions for the whole organism

The digestive system is an example of an organ system in which several organs work together to digest and absorb food, so that nutrients can be used by the body's cells.

Organs involved include the stomach, small intestine, large intestine, liver and pancreas — each contains different tissues doing different jobs, but all contribute to the single overall function of digestion.

Organs such as those in the digestive system rely on enzymes — biological catalysts made of protein — to carry out their functions efficiently.

• Enzymes catalyse specific reactions because of the shape of their active site
• The lock and key theory models this: only a substrate with a complementary shape to the active site ('the key') fits into the enzyme ('the lock')
• Enzyme activity changes with temperature and pH: activity increases with temperature up to an optimum, then falls sharply as the enzyme denatures (active site shape is destroyed); each enzyme also has an optimum pH

The same hierarchy applies across the body. For example, the heart is an organ made of muscle tissue that pumps blood as part of the circulatory system, a double circulatory system: the right side pumps blood to the lungs, and the left side pumps blood around the rest of the body.

This shows the general principle: individual organs, each built from several tissues, combine into an organ system that carries out one overall life process for the whole organism.