diffusion and osmosis

• what is diffusion?

  the gradual movement of particles from places where there are lots of them to places where there are fewer of them
• what is the proper definition of diffusion?

  the spreading out of particles from an area of higher concentration to an area of lower concentration
• where does diffusion happen?

  in both solutions and gases, because the particles in these substances are free to move about randomly
• what is the simplest type of diffusion?

  when gases diffuse through each other, this is what happens when the smell of perfume diffuses through the air for example
• if the concentration gradient is bigger, how will the diffusion rate be affected?

  the diffusion rate will be faster
• what molecules can diffuse through cell membranes?

  small molecules like oxygen, glucose, amino acids and water
• what are examples of big molecules that can't fit through the membrane?

  starch and proteins
• if the surface area of the membrane is larger, how is the diffusion rate affected?

  the diffusion rate is faster because more particles can pass through at once
• what is the definition of osmosis?

  the movement of water molecules across a patially permable membrane from a region of higher water concentration to a region of lower water concentration
• what is a partially permeable membrane?

  a membrane with very small holes in it, so small that only tiny molecules can pass through them
• why do the water molecules pass both ways in osmosis?

  because water molecules move about randomly all the time
• how is osmosis a type of diffusion?

  the water molecules moves from an area of higher water concentration to an area of lower water concentration
• how can you carry out an osmosis practical, observing how sugar solutions affect plant tissue?

  • cut up a potato into identical cylinders, get beakers with different sugar solutions in them. one should be pure water and another should be very concentrated sugar solution
  • measure the mass of potato cylinders, leave one cylinder in each beaker for 24+ hours
  • take them out, dry them with paper towel and measure masses again
  • will have increased in mass if water was absorbed by osmosis, decreased if water was drawn out
• what is the dependent variable in this experiment?

  the chip mass
• what is the independent variable in this experiment?

  the concentration of the sugar solution
• what are some examples of the controlled variables in this experiment?

  volume of solution, temperature, time
• how could errors occur in this experiment?

  • potato cylinders not fully dried means higher mass
  • if water evaporated, concentrations of sugar solutions would change
• how can you reduce the effect of errors?

  by repeating the experiment and calculating a mean percentage change at each concentration