Diffusion

• Gas Exchange


  The exchange of oxygen and carbon dioxide between an organism and its environment
• Respiration by plants and animals


  1. Taking up oxygen (O2)

  2. Releasing carbon dioxide (CO2)
• Photosynthesis


  1. CO2 uptake

  2. O2 release
• Respiratory surfaces


  • Highly convoluted to increase the area available for gas exchange
  • Examples: spongy mesophyll of leaves, alveoli of lungs
• Respiratory system


  • Complex, specialized tissues for efficient gas exchange
  • Varies based on organism size and habitat

  
• Primary function of respiratory system


  To deliver oxygen to the cells of the body's tissue and remove carbon dioxide (cell waste product)
• Complexity of the Respiratory System


  • Greatly determines how an animal respires
  • As animal size increases, diffusion distance increases, and the ratio of surface area to volume drops
• Diffusion


  • A slow, passive transport process
  • The natural movement of particles from an area of higher concentration to an area of lower concentration, due to random molecular motion
• Oxygen movement in diffusion

  1. Moves from the alveoli to the blood through the capillaries lining the alveolar walls

  2. Picked up by the hemoglobin in red blood cells
• Respiratory Media


  The conditions for gas exchange vary considerably, depending on whether the respiratory medium (source of oxygen (O2)) is air or water
• Oxygen is plentiful in air, and air is much less dense and less viscous than water, so it is easier to move and force through small passageways
• • Water is a much more demanding gas exchange medium than air
  • The amount of oxygen (O2) dissolved in a given volume of water varies but is always less than an equivalent volume of air
  • Water's lower oxygen content, greater density, and greater viscosity mean that aquatic animals must expend considerable energy to carry out gas exchange
• Adaptations have evolved to enable most aquatic animals to be very efficient in gas exchange
• Respiratory Surfaces


  • The movement of oxygen (O2) and carbon dioxide (CO2) across respiratory surfaces takes place by diffusion
  • The rate of net diffusion is proportional to the surface area across which it occurs and inversely proportional to the square of the distance through which molecules must move
  • Gas exchange is fast when the area for diffusion is large and the path for diffusion is short
• The rate of oxygen uptake must match the rate of diffusion across the membrane
• Direct Diffusion


  • Diffusion for organisms less than 1 mm in diameter
  • Every cell in the body is close enough to the external environment that gases can diffuse quickly between any cell and the environment
  • Cells are kept moist, and gases diffuse quickly via direct diffusion
  • The shape of these organisms increases the surface area for diffusion, ensuring that each cell within the body is close to the outer membrane surface and has access to oxygen
• Simple Animals


  • Sponges, Cnidarians, Flatworms