Levels of Organisation of Cells (Edexcel International AS Biology)

• Cell theory states that cells are the basic functional unit of all living organisms
• Cells can become specialised for specific functions, e.g.
  
  • Epithelial cells in the small intestine are specialised to absorb food efficiently
  • Red blood cells are specialised to transport oxygen
  • Xylem cells in plants are specialised to allow the transport of water around a plant
• In multicellular organisms specialised cells of the same type group together to form tissues
  • A tissue is a group of cells that work together to perform a particular function, e.g.
    
    • Epithelial cells group together to form epithelial tissue the function of which, in the small intestine, is to absorb food
    • Muscle cells group together to form muscle tissue, the function of which is to contract in order to move parts of the body
• Different tissues can group together to form organs
  • An organ is a group of tissues working together to perform a particular function, e.g.
    
    • Many different tissues, including cardiac muscle tissue, blood vessel tissues and connective tissue, group together to form the heart, enabling it to function to pump blood around the body
    • Tissues including palisade mesophyll, spongy mesophyll, and vascular tissue, group together in plants to form leaves, enabling them to perform photosynthesis effectively
• Different organs work together to form organ systems
  • An organ system is a group of organs working together to perform a particular function, e.g.
    • The heart and blood vessels work together to form the circulatory system, the job of which is to allow blood to circulate around the body
    • The stomach, pancreas, small intestine, and large intestine work together to form the digestive system, the job of which is to digest food and absorb nutrients

Levels of Organisation Table

Examples of Specialised Cells and their Associated Tissues, Organs and Organ Systems Table