B2.2 Tissues, Organs and Organ Systems

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Organisation of the Body

Large multicellular organsims such as humans have bodies that are organised into different organ systems. Parts of the body, increasing in size:

1. Differentiated cells
2. Tissues
3. Organs
4. Organ systems - these often exchange substances between the body and the environment e.g. the digestive system


During development, cells specialise to perform different functions. A group of specialised cells is a tissue, where they have a similar structure and function

  • Nerve tissue
    • Carries electrical impulses around the body and brain
    • Nerve cells have long fibres to carry electrical impulses
  • Glandular tissue
    • Secretes substances e.g. hormones or enzymes
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Tissues and Organs

  • Epithelial tissue
    • Covers the surface and lines all tubes
    • Epithelial cells make a continuous surface 
  • Skeletal tissue
    • Supports and protects body
    • Bone cells form hard bone
  • Muscle tissue
    • Helps body to move
    • Muscle cells contract

Organs are made of tissues. An organ may contain different kinds of issue that work together to carry out the function of the organ. The stomach, for example, contains:

  • Glandular tissue
    • Stomach wall secretes enzymes to break down food
  • Muscle tissue
    • Contracts to churn the food and mix it with the enzymes so digestion is more effective
  • Epithelial tissue
    • Lines the stomach and protects the other tissues from digestion by the enzymes
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Organ Systems

An organ system is a group of organs that work together to perform a particular function in the body, like the organs in the digestive system work together to digest and absorb substances from the environment


  • Salivary glands
    • Produce digestive juices
      • Amylase which breaks down starch into glucose
  • Stomach
    • Digestion occurs
      • Produces protease enzymes to break down proteins
      • Mixes with HCl acid to clean bacteria
  • Liver
    • Produces alkaline bile
      • Changes PH in small intestine to alkaline
        • Neutralises acid in food
    • Emulsifies fats
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Digestive System

  • Pancreas
    • Gland that produces digestive juices
      • Produces protease enzymes which breaks down proteins into amino acids
      • Produces lipase enzymes which breaks down lipids into fatty acids and glycerol
  • Small intestine
    • Digestion occurs and soluble food is absorbed
    • Uses vili to break down food into small, soluble food molecules that can be absorbed into blood stream
  • Large intestine
    • Water is absorbed from the undigested food, producing faeces
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Plant Organs

Plant organs include:

  • Stems
  • Roots
  • Leaves

Plant tissues include:

  • Epidermal tissue
    • To cover the plant
  • Mesophyll tissue in the upper part of the leaf
    • Carries out photosynthesis (when it includes cells that contain chlorophyll)
  • Xylem and phloem tissue
    • Transport substances around the plant


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