AQA GCSE Biology: Principles of Organisation
A clear GCSE Biology revision guide to the principles of organisation: the levels from cells to organ systems, and the organs of the digestive system.
Living things are organised in levels, building up from single cells to whole organisms. This guide sets out those levels, uses the stomach to show what makes something an organ rather than a tissue, and introduces the digestive system as an example of an organ system. Getting these levels straight makes the rest of Topic 2 much easier to place.
The levels of organisation
The parts of an organism are organised into a hierarchy, and each level is built from the one below it.
| Level | What it is | Example |
|---|---|---|
| Cell | The smallest structural and functional unit | A muscle cell |
| Tissue | A group of cells with similar structure and function, working together | Muscle tissue |
| Organ | A group of different tissues working together to perform a function | The stomach |
| Organ system | A group of organs working together | The digestive system |
| Organism | A whole living individual, made of organ systems working together | A human |
Cells reach that hierarchy by differentiating: a cell becomes specialised for one job, and cells specialised for the same job group together as a tissue. That is why the levels build in the order they do.
What makes the stomach an organ
The stomach is the example the specification uses, and it is worth knowing properly, because it shows exactly what separates an organ from a tissue. The stomach is made of three different tissues, each doing a different job:
- muscular tissue contracts to churn the food and mix it with the stomach's juices;
- glandular tissue produces digestive enzymes and hydrochloric acid;
- epithelial tissue covers the inside and outside of the stomach, protecting the tissue underneath.
No one of those is an organ on its own. What makes the stomach an organ is that different tissues work together for a single function.
The digestive system
The digestive system is an organ system that breaks food down and absorbs it. Food passes through its organs in this order:
mouth → oesophagus → stomach → small intestine → large intestine
Some organs are not on that path but make substances that help digestion:
- the salivary glands release amylase into the mouth;
- the pancreas makes amylase, protease and lipase and releases them into the small intestine;
- the liver produces bile;
- the gall bladder stores bile and releases it into the small intestine.
Each organ on the path has one job worth being able to state. The mouth begins mechanical breakdown by chewing and starts starch digestion with amylase. The oesophagus carries food to the stomach. The stomach churns food and digests protein in acid conditions. The small intestine is where digestion is completed and where the soluble products are absorbed into the blood. The large intestine absorbs water from the remaining material, leaving faeces.
The division between the liver and the gall bladder catches people out. The liver produces bile; the gall bladder only stores it before releasing it. Swapping those two is a reliably lost mark.
Telling a tissue from an organ in the exam
The two definitions are easy to blur under pressure, so hold on to the difference:
- a tissue is similar cells doing one job;
- an organ is different tissues working together.
If a question asks why the stomach is an organ, naming the three tissues and what each does is the answer. If it asks why muscle is a tissue, the answer is that its cells are all similar and all contract.
A building is a fair comparison for the whole hierarchy. One brick is a cell. A wall of identical bricks is a tissue, since everything in it is the same and doing the same job. A room is an organ, because it takes several different materials, walls, a floor, pipes and wiring, working together for one purpose. A house is an organ system, and a street of houses is the organism.
The comparison also shows why the levels cannot be reordered. You cannot build a room without walls, and you cannot build a wall without bricks, which is the same reason an organ cannot exist without tissues or a tissue without differentiated cells.
How this fits together
The digestive system is where large food molecules are broken down, and the enzymes that do this are the subject of the enzymes and digestion guide.