gas exchange

• Gas Exchange


  The exchange of carbon dioxide and oxygen gases at cells and tissues through the process of diffusion
• Cells carrying out aerobic respiration


  • Require oxygen to enter the cell, and the waste product carbon dioxide to exit the cell
• Cells carrying out photosynthesis


  • Require carbon dioxide to enter the cell, and the waste product oxygen to exit the cell
• Large animals need specialized gas exchange systems, and transport systems to provide cells with sufficient oxygen for respiration
• Specialized Gas Exchange Surfaces

  Unicellular organisms have a small surface area to volume ratio, allowing them to exchange gases directly through the plasma membrane of cells
• As animals increase in size


  • The surface area to volume ratio of the animals decreases, meaning there is less surface area for gas exchange relative to the size of the organism
  • The cells of the organism cannot obtain sufficient oxygen for respiration in all of the cells
  • Gases are exchanged by diffusion, which is a slow process. As an animal becomes larger, it takes too much time for oxygen to diffuse to all cells
• Large animals require specialized gas exchange and transport systems to ensure they obtain sufficient oxygen for all of the cells
• Adaptations of gas exchange systems


  • Large surface area
  • Very thin tissue layers
  • Permeable membranes
  • Concentration gradient for diffusing gases
  • Exchange surfaces are covered in a layer of moisture
• Diffusion


  The passive transport of particles from a region of high concentration to a region of low concentration
• Adaptations to maintain high concentration gradients


  • A dense network of capillaries surrounding tissues involved in gas exchange
  • Continuous blood flow through the capillaries surrounding the tissues involved in gas exchange
  • Animals with lungs for gas exchange ventilate the lungs with air, bringing a high concentration of oxygen to the alveoli, and removing carbon dioxide from the alveoli
  • Animals with gills move water through the gills, providing a high concentration of oxygen, and moving carbon dioxide away from the gills
• Gills in fish are adapted for rapid exchange of gases through having a large surface area for gas exchange, a continuous supply of blood flowing through the gills, and water continually moving through the gills
• Adaptations of mammalian lungs for gas exchange


  • Branching bronchioles which connect to many alveoli
  • All of the alveoli in the lungs provide a very large surface area for gas exchange
  • Alveoli secrete a surfactant which prevents the walls of the alveoli adhering to each other, and provides a moist surface for gas exchange
  • Alveoli are surrounded by an extensive capillary bed, which maintains high concentration gradients for O2 and CO2 between the blood and alveoli
  • The capillaries provide a continuous supply of blood with low oxygen concentration and high carbon dioxide concentration to the alveoli
• Ventilation of the lungs

  1. Inspiration (breathing in)

  2. Expiration (breathing out)
• Inspiration


  • The diaphragm contracts and moves downwards
  • The external intercostal muscles contract, moving the ribcage up and out
  • The volume in the thorax increases, decreasing the pressure in the lungs
  • Air passively moves from the surrounding air (with high pressure) into the lungs where there is low pressure
• Expiration


  • The abdominal muscles contract and push the diaphragm upwards
  • The external intercostal muscles relax and the internal intercostal muscles contract, moving the ribcage down and inwards
  • The volume in the thorax decreases, increasing the pressure in the lungs
  • The high pressure in the lungs moves air out of the lungs to the surrounding air, where pressure is lower
• Tidal volume


  The volume of air that moves in and out of the lungs in a normal breath
• Inspiratory reserve


  The additional volume of air that can be inhaled with maximum effort
• Expiratory reserve


  The additional volume of air that can be exhaled with maximum effort
• Vital capacity


  The greatest volume of air that can be expelled from the lungs after the deepest possible breath
• Vital capacity = tidal volume + inspiratory reserve + expiratory reserve
• Breathing


  1. Costal muscles relax

  2. Internal intercostal muscles contract

  3. Ribcage moves down and inwards

  4. Volume in thorax decreases

  5. Pressure in lungs increases

  6. High pressure in lungs moves air out to surrounding air with lower pressure
• Spirometer


  Instrument used to measure air capacity of the lungs
• Leaves carry out respiration and photosynthesis, and therefore need to be adapted to exchange oxygen and carbon dioxide
• Leaves also need to be adapted to reduce water loss
• Adaptations of leaf structure


  1. Waxy cuticle reduces evaporation of water

  2. Epidermis provides protection and allows light to reach mesophyll

  3. Spongy mesophyll increases surface area for gas exchange

  4. Air spaces facilitate diffusion of gases

  5. Stomata allow gases to enter and exit

  6. Veins provide support and transport water, minerals, and nutrients
• Transpiration is the movement of water through a plant, and its evaporation from aerial parts of the plant such as leaves
• Transpiration is an inevitable consequence of gas exchange, as water in mesophyll cells evaporates, and diffuses through the open stomata
• Factors affecting transpiration


  1. Water diffuses from high concentration in air spaces to low concentration in atmosphere

  2. Light intensity increases, more stomata open, more water can diffuse out

  3. Temperature increases, water particles gain kinetic energy and move faster, evaporation rate increases

  4. Humidity increases, concentration gradient decreases, water diffuses slower

  5. Air flow (wind) moves water vapour away, concentration gradient increases, transpiration rate increases
• Stomatal density is the number of stomata per unit area of a leaf
• Quantitative data


  Any data which involves numbers
• How do multicellular organisms solve the problem of access to materials for all their cells?

  Gas exchange and metabolic processes in cells are related
• What is the relationship between gas exchange and metabolic processes in cells?


  Gas exchange provides the materials needed for metabolic processes in cells