exchange surfaces

• Single-celled organisms have a high SA:V ratio which allows for the exchange of substances to occur via simple diffusion
• Single-celled organisms


  • Large surface area allows for maximum absorption of nutrients and gases and secretion of waste products
  • Small volume means the diffusion distance to all organelles is short
• As organisms increase in size their SA:V ratio decreases
• There is less surface area for the absorption of nutrients and gases and secretion of waste products in larger organisms
• The greater volume results in a longer diffusion distance to the cells and tissues of the organism in larger organisms
• Large multicellular animals and plants

  • Have evolved adaptations to facilitate the exchange of substances between their environment
  • Have a large variety of specialised cells, tissues, organs and systems
• Organisms require ATP in order to carry out the biochemical processes required for survival. The majority of ATP is produced through aerobic respiration which requires oxygen
• Carbon dioxide is a toxic waste product of aerobic respiration
• If carbon dioxide accumulates in cells/tissues it alters the pH
• Diffusion is a viable transport mechanism for single-celled organisms


  But not for larger multicellular organisms
• The time taken for oxygen to diffuse from the cell-surface membrane to the tissues would be too long for larger multicellular organisms
• A single rhino consumes more oxygen within a given period of time compared to a single mouse
• Smaller animals have a greater SA:V ratio so they lose more heat, meaning they have to use up more energy to maintain their body temperature
• Plants have much lower metabolic rates than animals as they do not move around their habitat and don't have to maintain a high body temperature
• Effective exchange surfaces in organisms

  • Large surface area
  • Short diffusion distance (thin)
  • Good blood supply
  • Ventilation mechanism
• The maximum distance that oxygen molecules would have to diffuse to reach the centre of a Chlamydomonas is 10μm, which would only take 100 milliseconds
• If the diffusion distance increased to 15cm the diffusion time would increase substantially to 7 hours
• Root hair cells


  Specialised cells found in the roots of plants that play an important role in the absorption of water and mineral ions from the soil
• The root hair is an extension of the cytoplasm, increasing the surface area of the cell in contact with the soil to maximise absorption of water and minerals
• Alveoli


  • Walls are only one cell thick and these cells are flattened
  • Large number of alveoli
  • Extensive capillary network
• The thin wall of the alveolus means that oxygen and carbon dioxide don't have to travel a large distance between the air in the lungs and the blood in the capillaries
• Fish gills


  • Have a large capillary network
  • The extensive capillary system covers the gills and ensures that the blood flow is in the opposite direction to the flow of water - it is a counter-current system
• The counter-current system ensures the concentration gradient is maintained along the whole length of the capillary
• Ventilation (mass flow of gases) in the lungs helps to ensure that there is always a higher concentration of oxygen in the alveoli than in the blood
• Tissues of the mammalian gas exchange system

  • Ciliated epithelial cells, goblet cells and mucous glands play vital roles in maintaining the health of the gas exchange system
  • Cartilage, smooth muscle, elastic fibres and squamous epithelial tissue all play important structural roles in maintaining the gas exchange system
• Cartilage


  Strong and flexible tissue found in rings along the trachea, called Tracheal rings, that help to support the trachea and ensure it stays open while allowing it to move and flex while we breathe
• Ciliated epithelium


  Specialised tissue found along the trachea down to the bronchi, with small projections of cilia which sweep mucus, dust and bacteria upwards and away from the lungs
• Goblet cells


  Mucus-producing cells scattered throughout the ciliated epithelium in the trachea that secrete viscous mucus which traps dust, bacteria and other microorganisms and prevents them from reaching the lungs
• Cartilage micrograph


  Image showing cartilage tissue
• Ciliated epithelium


  Specialised tissue found along the trachea down to the bronchi
• Ciliated epithelium


  • Each cell has small projections of cilia which sweep mucus, dust and bacteria upwards and away from the lungs
  • The epithelium itself
• Ciliated epithelium showing microvilli, basement membrane, goblet cell and nucleus

  
• Ciliated epithelium – electron micrograph
• Goblet cells


  Mucus-producing cells that secrete viscous mucus which traps dust, bacteria and other microorganisms and prevents them from reaching the lungs
• Goblet cells


  1. The mucus is then swept along by the cilia of the ciliated epithelium upwards and is swallowed

  2. The mucus and any microorganisms will then be destroyed by the acid in the stomach
• Goblet cell – light micrograph
• Squamous epithelium


  • The alveoli have a lining of thin and squamous epithelium, that allows for gas exchange
  • The squamous epithelium forms the structure of the alveolar wall and so is very thin and permeable for the easy diffusion of gases
• Squamous epithelium – light micrograph
• Smooth muscle


  • Can be found throughout the walls of the bronchi and bronchioles
  • It helps to regulate the flow of air into the lungs by dilating when more air is needed and constricting when less air is needed
• Smooth muscle – light micrograph