B2.1 Tissues, Organs and Organ Systems

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.


Multicellular 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.


Animal organs


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

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.