Diffusion

• • Diffusion is the spreading out of particles from an area of high to low concentration
• • Small molecules like oxygen, glucose, amino acids, water can pass through the cell membrane
  • Large molecules like starch and proteins cannot fit through the membrane
• Factors which affect diffusion are:

  • Difference in concentration - concentration gradient - bigger gradient, faster diffusion rate
  • Temperature - more energy to particles, faster diffusion rate
  • Surface area of membrane - more particles can pass
• • Oxygen and carbon dioxide are transferred between cells and the environment in gaseous exchange
  • Urea diffuses from cells into blood plasma for removal of the body
• • Surface area to volume ratio depends on how easy it is for an organism to exchange substances with the environment
• • A single celled organism has a large surface area to volume ratio
  • Gases or dissolved substances can diffuse directly across the cell membrane
  • Enough substances can be transported across the membrane to supply to meets the needs of the organism
• • Multicellular organisms have a small surface area to volume ratio
  • Not enough substances can diffuse across the membrane to meet the needs of an organism
  • They need exchange surfaces for efficient diffusion
• Exchange surfaces are adapted for effectiveness:

  • Having a thin membrane - short distance to diffuse
  • Large surface area - lots of substances can diffuse at once
  • Exchange surfaces in animals have many blood vessels for efficient blood supply and to get substances in and out of the cell quickly
  • Gas exchange surfaces in animals - alveoli - ventilated