Animal tissues, organs and systemsOxford AQA IGCSE Biology: Revision notes

A tissue is a group of cells with a similar structure and function, working together. Animal bodies are built from several types of tissue, each specialised for a job.

These three tissue types combine, in different proportions, to build the organs of the body.

An organ is made of several tissues working together to perform a shared function; one organ may contain more than one tissue type. The stomach is the standard example:

• Muscular tissue in the stomach wall contracts to churn and move the contents through the digestive system
• Glandular tissue produces digestive juices, including enzymes and acid
• Epithelial tissue covers the outside of the stomach and lines the inside, protecting the tissues underneath

Notice that all three animal tissues from the previous section appear here, each doing the job it is specialised for.

An organ system is a group of organs that work together to perform a particular function. The digestive system is a system in which humans and other mammals exchange substances with the environment — food goes in, and the body breaks it down and absorbs what it needs.

The digestive system includes:

Each organ contributes a different stage, from breaking large food molecules down to absorbing the useful, small, soluble molecules into the bloodstream.

Multicellular organisms are organised in a clear hierarchy, with each level built from the level below:

1. Cells — the basic unit of life; in animals, cells differentiate (become specialised) to do different jobs
2. Tissues — groups of similar cells working together, e.g. muscular tissue
3. Organs — different tissues working together, e.g. the stomach
4. Organ systems — different organs working together, e.g. the digestive system
5. Organism — all the organ systems working together as a whole living animal

This increasing scale explains why a large, complex organism cannot simply rely on individual cells exchanging materials with their surroundings — it needs whole systems, such as the digestive system, dedicated to that job.

Food enters the mouth, where saliva (containing digestive juices from the salivary glands) begins digestion. It then passes to the stomach, where muscular tissue churns it and glandular tissue adds acid and enzymes. Partly digested food, along with bile from the liver, moves into the small intestine, where digestion is completed and the soluble products of digestion (such as glucose and amino acids) are absorbed into the blood through its lining. What remains passes into the large intestine, where water is absorbed, leaving faeces to be expelled from the body.

This sequence shows why the digestive system counts as a single organ system: each organ depends on the ones before it, and together they achieve one overall function that no single organ could achieve alone.