B2.2 Tissues, organs and organ systems

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. Examples of 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.

Organ systems are groups of organs that perform a particular function. 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.

• In the mouth, salivary glands produce the enzyme amylase which can break down carbohydrates like starch.
• The oesophagus (gullet) is a tube that connects the mouth to the stomach and its lined with muscles that help move the balls of food along.
• In the stomach the food is where your food is broken down.
• The liver produces alkaline bile, which neutralises excess stomach acid and bile helps to emulsify oils/fats.
• The gall bladder stores bile before its released into the small intestine to help with digestion.
• The pancreas gland produces digestive juices containing the enzymes (i) protease pepsin (breaks down proteins), (ii) amylase (breaks down starches) and (iii) lipase (breaks down oils/fats), which are released into the small intestine.
• The small intestine is where digestion process continues with the release of the enzymes from the pancreas. Here the absorption of soluble food into the blood stream occurs from the digestive system
• In the large intestine excess water is absorbed from the undigested food, producing faeces which are initially stored in the rectum before release through the anus!

• Plant organs include stems, roots and leaves.

  • Palisade cells contain chlorophyll and are adapted for photosynthesis.

  • Guard cells are adapted to open and close the pores which allow gas exchange and water evaporation.

• Examples of plant tissues including:

  • epidermal tissues, the outer layers which cover the whole plant,

  • mesophyll, between two epidermis layers, where most photosynthesis happens,

  • xylem and phloem, which transport substances around the plant e.g. sugars like sucrose and glucose, minerals and water.

The organs of the digestive system:

e.g. salivary glands, stomach, gall bladder, liver, large intestine, pancreas, small intestine, rectum