Transport in Cells

• Diffusion:

  the net movement of particles from an area of high concentration to an area of low concentration

  • passive process (no energy is required)
• Concentration gradient: The difference in the concentration of a substance between two areas.
• Factors that affect rate of diffusion:

  • temperature
  • more kinetic energy means that the particles more more quickly
  • concentration gradient
  • greater concentration gradient means that there are more particles diffusing per unit time
  • surface area
  • greater surface area means more particles can diffuse across the membrane at once
  • distance to diffuse
  • shorter distance means the the particles diffuse more quickly
• Diffusion and Breathing:

  Oxygen:

  • inhaled air though lungs and bloodstream
  • oxygen is transported throughout body

  Carbon Dioxide:

  • from body tissues into bloodstream
  • exhaled via lungs

  Alveoli (tiny airsacs at end of bronchioles):

  • very thin (one cell thick)
  • covered by network of fine capillaries - gas to pass directly
  • moist - encourages gas molecules to dissolve
  • large combines surface area
• Diffusion and Digestion:

  Diffused substances:

  • glucose, amino acids, fatty acids

  Villi:

  • increase surface area (maximum absorption)
  • good blood supply (absorb nutrients)
  • microvilli - increase surface area
• Diffusion and Nerve Impulses:

  • synapse: junction between 2 neurones (a nerve cell) across which electrical signals pass
  • neurotransmitter molecules diffuse from vesicles (a thin-walled sac filled with a fluid, usually clear and small) toward neurotransmitter receptors, moving from an are of high concentration to an area of low concentration
• Diffusion and The Placenta:

  • Diffused substances:
  • oxygen and nutrients
  • carbon dioxide and waste
  • Umbilical cord: connects mother and foetus
  • enables nutrients and oxygen to pass from mother to foetus by diffusion
  • enables waste and carbon dioxide to pass from mother to foetus by diffusion
• Diffusion and Photosyntheis:

  Diffused substances:

  • Carbon Dioxide + water → oxygen and glucose
  • CO2 diffuses in through stomata
  • O2 and H2O diffuse out through stomata
• The membrane surface:

  • critical interface between cell and environment
  • exchange of materials (often through diffusion) occurs here
• Organisms/ cells exchange:

  • gases (O2 and CO2)
  • nutrients (C6H1206, amino acids, minerals, vitamins, etc)
  • waste (urea, ammonia, CO2)
  • heat
• Surface Area to Volume Ratio:

  • high surface area to volume ratio: lot of surface is exposed compared to volume of object
  • low surface area to volume ratio: a small surface is exposed compared to volume of object

  the larger the body size of the organism, the smaller the surface area to volume ratio
• Osmosis: the net movement of water from an area of low solute concentration to an area of high solute concentration across a selectively permeable membrane
• Selectively permeable membrane: A membrane that allows some molecules to pass through it but not others.
• Increase in temperature, surface area and concentration gradient will increase speed of osmosis
• Water will move in/out of a cell depending on the concentration of its surrounding solution:

  ISOTONIC - solution has same concentration

  HYPERTONIC - solution has a lower concentration

  HYPOTONIC - solution has a higher concentration
• HYPOTONIC SOLUTION:

  • water concentration is lower OUTSIDE cell
  • solute concentration is higher INSIDE cell
  • water moves into cell
  • cell gets bigger

  
• ISOTONIC SOLUTION:

  • water and solute concentration is equal inside and outside cell
  • no net movement
  • cell size stays the same

  
• HYPERTONIC SOLUTION:

  • water concentration is lower INSIDE cell
  • solute concentration is higher OUTSIDE cell
  • water moves out of cell
  • cell gets smaller

  
• Active Transport:

  • the transport of a substance from a low concentration to a high concentration of that substance across a selectively permeable membrane
  • Energy is required the substance moves AGAINST the concentration gradient as it is not a passive process
  • the substance moves AGAINST the concentration gradient as it is not a passive process