Transport in cells

• What is diffusion?

  Spreading out of particles in solution or gas
• What is the net movement of particles during diffusion?

  From higher concentration to lower concentration
• How do particles behave in a solution or gas?

  They move about randomly
• What happens when there are more particles in one area?

  There is a net movement to lower concentration
• Is diffusion an active or passive process?

  Diffusion is an entirely passive process
• What does the movement of particles in a fluid depend on?

  How much kinetic energy they have
• How does temperature affect particle movement in diffusion?

  Higher temperature increases kinetic energy
• What happens to the rate of diffusion as temperature increases?

  The rate of diffusion increases
• What is the relationship between kinetic energy and diffusion speed?

  More kinetic energy means faster diffusion
• What surrounds all cells in living organisms?

  Cell membrane
• What is the function of the cell membrane?

  It separates the cell's interior from the environment
• How do substances enter or exit the cell?

  They diffuse through the cell membrane
• What does it mean for a membrane to be 'partially permeable'?

  Some substances can pass, others cannot
• Why can oxygen and carbon dioxide diffuse across the cell membrane?

  They are small enough to pass through
• Which substance cannot diffuse across the cell membrane?

  Starch
• Why can't starch diffuse across the cell membrane?

  It is too big to pass through
• What are the key characteristics of diffusion in living organisms?

  • Movement of substances from high to low concentration
  • Passive process (no energy required)
  • Essential for gas exchange and nutrient uptake
  • Influenced by temperature and concentration gradient
• Why must oxygen diffuse into cells?

  For aerobic respiration
• What substance must diffuse into photosynthesizing plant cells?

  Carbon dioxide
• Why must carbon dioxide diffuse into plant cells?

  To be available for photosynthesis
• What waste product do liver cells produce from excess amino acids?

  Urea
• Why must urea diffuse out of liver cells?

  It is highly toxic and must be excreted
• What must diffuse out of cells as a result of aerobic respiration?

  Carbon dioxide
• What drives the movement of substances by diffusion into and out of cells?

  A concentration gradient
• What happens to the movement of a substance by diffusion if the concentration gradient changes?

  The movement of the substance will also change
• What are the two main categories of substances that diffuse in and out of cells?

  Substances entering and leaving cells
• What is the significance of a concentration gradient in diffusion?

  It drives the movement of substances
• What is diffusion?

  Diffusion is the net movement of particles from an area of high concentration to an area of low concentration.
• What surrounds all cells in living organisms?

  Cell membrane
• Why must oxygen diffuse into cells?

  For aerobic respiration
• Why must carbon dioxide diffuse into photosynthesising plant cells?

  For photosynthesis
• Why can the cell membrane be described as 'partially permeable'?

  Not all substances are able to diffuse through the cell membrane

  for e.g. oxygen and carbon dioxide can diffuse across the cell membrane during gas exchange whereas the carbohydrate starch cannot as it's too big
• What are some examples of substances that leave cells by diffusion?

  • Liver cell breaks down excess amino acids into the waste product urea which is highly toxic and must diffuse out of cells to be excreted by the kidneys
  • Carbon dioxide produced by aerobic respiration must diffuse out of cells
• In both transport of substances in or out of the cell what is the movement driven by?

  a concentration gradient, if the direction of that concentration gradient changes, the movement of the substance by diffusion will also change
• How does a difference in concentration(concentration gradient)affect diffusion?

  The greater the difference in concentration between two regions, the faster the overall rate of diffusion
• How does temperature affect diffusion?

  The higher the temperature, the more kinetic energy the particles of that substance will have. They will move/spread faster compared to when at a lower temperature when they have less kinetic energy
• How does a surface area of a membrane separating two regions affect diffusion?

  A membrane with a greater surface area will have a greater rate of diffusion across it - more entry and exit points for the particles to cross
• What does the surface area to volume ratio affect?

  Affects how easily substances can be exchanged between it, and its environment
• Bacteria and surface area to volume ratio

  • Most bacteria are single-celled organisms and they have a relatively large surface area compared to their volume
  • This means that the distance between the cell membrane at a bacterial cell's surface, and the centre of the cell, is relatively low
  • substances do not have to travel very far to get where they are needed, so transport by diffusion, osmosis or active transport alone is sufficient for the cell to meet it needs
• Diffusion in multicellular organisms

  • Large, multicellular organisms like humans have relatively small surface areas (SA) in comparison to their volumes (in comparison to single-celled organisms)
  • This means that the distance between the surface of the organism to its centre is relatively large
  • This is why larger organisms usually have exchange surfaces and transport systems; as diffusion, osmosis and active transport cannot happen sufficiently to meet a larger organism’s needs otherwise