B2.1 Tissues, Organs and Organ Systems
The cells of multicellular organisms may differentiate and become adapted for specific functions. Tissues are aggregations of similar cells; organs are aggregations of tissues performing specific physiological functions. Organs are organised into organ systems, which work together to form organisms.
- Large multicellular organisms develop systems for exchanging materials.
- During the development of a multicellular organism, cells differentiate so that they can perform different functions.
- A tissue is a group of cells with similar structure and function.
- Organs are made of tissues.
- One organ may contain several tissues.
- Organ systems are groups of organs that perform a particular function.
Examples of animal tissues include:
- muscular tissue, which can contract to bring about movement
- glandular tissue, which can produce substances such as enzymes and hormones
- epithelial tissue, which covers some parts of the body.
The stomach is an organ that contains:
- muscular tissue, to churn the contents
- glandular tissue, to produce digestive juices
- epithelial tissue, to cover the outside and the inside of the stomach.
The digestive system is one example of a system in which humans and other mammals exchange substances with the environment.
The digestive system includes:
- glands, such as the pancreas and salivary glands, which produce digestive juices
- the stomach and small intestine, where digestion occurs
- the liver, which produces bile
- the small intestine, where the absorption of soluble food occurs
- the large intestine, where water is absorbed from the undigested food, producing faeces.
Plant organs include stems, roots and leaves.
Examples of plant tissues include:
- epidermal tissues, which cover the plant
- mesophyll, which carries out photosynthesis
- xylem and phloem, which transport substances around the plant.