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• Edexcel IGCSE Biology: Double Science
• Contents


  • 2.51 Unicellular Organisms & Diyusion
  • 2.52 The Need for a Transport System in Multicellular Organisms
  • 2.53 Role of the Phloem
  • 2.54 Role of the Xylem
  • 2.59 Compostion of the Blood
  • 2.60 Role of Plasma in Transport
  • 2.61 Adaptations of Red Blood Cells
  • 2.62 Immune Response to Disease
  • 2.65 Structure & Function of the Heart
  • 2.66 Heart Rate & Exercise
  • 2.67 Risk Factors for Coronary Heart Disease
  • 2.68 Blood Vessels: Structure & Function
  • 2.69 Circulatory System: General Structure
• Unicellular Organisms


  In order for any organism to function properly, it needs to exchange substances, such as food molecules and waste products, between itself and its environment
• Exchange of substances


  Occurs across the cell membrane
• Transport processes used by living organisms

  • Diyusion
  • Osmosis
  • Active transport
• Unicellular organisms


  • Have very large surface areas (SA) in comparison to their volumes
  • The distance between the surface of the organism to its centre is very small
  • Do not need specialist exchange surfaces or transport systems as diyusion, osmosis and active transport through the cell membrane occur at a suzcient rate to meet the organisms needs
• Multicellular Organisms


  • Have relatively small surface areas (SA) in comparison to their volumes
  • The distance between the surface of the organism to its centre is relatively long
  • Diyusion, osmosis and active transport cannot occur at a suzcient rate to meet the needs of the organism, so larger organisms usually have exchange surfaces and transport systems
• Exchange surfaces in animals

  • Lungs and alveoli for gas exchange
  • Small intestines and villi for absorption of digested food
• Transport systems in animals

  • Blood and circulatory system carries the necessary substances around the body
• Exchange surfaces in plants

  • Roots and root hairs where mineral ions and water are absorbed
  • Leaves for gas exchange
• Transport systems in plants

  • Xylem moves water and mineral ions from roots to shoots
  • Phloem moves sugars and amino acids to where they are needed in the plant
• Phloem


  Phloem vessels transport food materials (mainly sucrose and amino acids) made by the plant from photosynthesising leaves to non-photosynthesising regions in the roots and stem
• Phloem


  • The cells are living cells and are not hollow
  • Substances move from cell to cell through pores in the end walls of each cell
• The position of the xylem and phloem in the stem compared to the root
• Xylem


  Xylem vessels transport water and minerals from the roots to the stem and leaves
• Xylem


  • It is composed of dead cells which form hollow tubes
  • Xylem cells are strengthened by lignin and so are adapted for the transport of water in the transpiration stream
• Blood consists of red blood cells, white blood cells, platelets and plasma
• Over half of the volume of the blood is made up of plasma
• The majority of the other half is made up of red blood cells
• The remaining fraction consists of white blood cells and platelets
• Plasma


  Plasma is a straw coloured liquid which the other components of the blood are suspended within
• Substances transported in plasma

  • Carbon dioxide
  • Digested food and mineral ions
  • Urea
  • Hormones
  • Heat energy
• Red blood cells


  • They are full of haemoglobin, a protein that binds to oxygen to form oxyhaemoglobin
  • They have no nucleus which allows more space for haemoglobin to be packed in
  • The shape of a red blood cell is described as being a 'biconcave disc' - this shape gives them a large surface area to volume ratio to maximise diyusion of oxygen in and out
• Types of white blood cells

  • Phagocytes
  • Lymphocytes
• Phagocytes


  • Carry out phagocytosis by engul|ng and digesting pathogens
  • Have a sensitive cell surface membrane that can detect chemicals produced by pathogenic cells
  • Once they encounter the pathogenic cell, they will engulf it and release digestive enzymes to digest it
  • This is a non-speci|c immune response