B1.4 Transport in Cells

• Define active transport

  Active transport moves substances from a more dilute solution to a more concentrated solution, requiring energy from respiration
• Define osmosis

  Osmosis is the diffusion of water from a dilute solution to a concentrated solution, through a partially permeable membrane
• Name the process by which water moves across cell membranes

  Osmosis
• Explain which type of organism can meet its needs through transport of substances into and out of the organism by diffusion

  A single celled organism, because it has a relatively large surface area : volume ratio
• Explain how the surface area of the membrane affects the rate of diffusion

  Larger surface area of the membrane increases the rate of diffusion because there is more space for the particles if move
• Describe how to use a graph of salt concentration against % mass change to determine the salt concentration in the tissue sample

  Determine the concentration of salt that causes no change in the mass (0%) by seeing where the line crosses the x-axis
• State which type(s) of transport can reach equilibrium

  Diffusion, osmosis
• Define diffusion

  Diffusion is the spreading out of the particles of any substance (in solution, or particles of a gas,) resulting in a net movement from an area of high concentration to an area of low concentration
• State how to calculate percentage mass change

  % mass change = ((final mass - original mass) / original mass) × 100
• Explain how the temperature affects the rate of diffusion

  Higher temperatures increase the rate of diffusion because the particles have more kinetic energy and so move faster
• Explain how the concentration gradient affects the rate of diffusion

  Steeper concentration gradients increase the rate of diffusion because the overall movement from the higher to lower concentration is larger
• Explain why absorption of sugar from the gut occurs through active transport 


  Active transport allows sugar to be absorbed from the gut into the blood, which has a higher sugar concentration
• Explain why % mass change is calculated when investigating the effect of salt solutions on tissue mass

  Percentage mass change is calculated to control for different starting masses
• Give example of substances that are transported across cell membranes by diffusion

  Some of the substances transported in and out of cells by diffusion are:

  • oxygen

  • carbon dioxide in gas exchange, and of the waste product

  • from cells into the blood plasma for excretion in the kidney
• Explain why absorption of ions into plant root hairs occurs through active transport 


  Active transport allows mineral ions to be absorbed from very dilute solutions in the soil into plant root hairs (where the concentration is higher)