transport in cells

• diffusion is the spreading out of particles from an area of higher concentration to an area of lower concentration

  • the bigger the concentration gradient, the faster the diffusion rate
  • a higher temperature will also give a faster diffusion rate because the particles have more energy, so move around faster
• Dissolved substances can move in and out of cells by diffusion. Only very small molecules can diffuse through cell membranes - things like oxygen, glucose, amino acids, & water. Just like with diffusion in air, particles flow through the cell membrane from where there's a higher concentration to where there's a lower concentration. If there are a lot more particles on one side of the membrane, there's a net (overall) movement from that side. The larger the SA of the membrane, the faster the diffusion rate, because more particles can pass through at once
• osmosis is the movement of water molecules across a partially permeable membrane from a region of higher water concentration to a region of lower water concentration.

  • because there are more water molecules on one side than on the other, there's a steady net flow of water into the region with fewer water molecules, i.e. into the stronger sugar solution
  • this means the strong sugar solution gets more dilute
• OSMOSIS PRACTICAL:

  • cut up a potato into identical cylinders, & get some beakers with different sugar solutions in them. One should be pure water and another should be a very concentrated sugar solution.
  • measure the mass of the cylinders, then leave one cyclinder in each beaker for 24 hrs or so
  • take them out, dry them with a paper towel, & measure their masses again
  • if the cylinders have drawn in water by osmosis, they'll have increased in mass. If water has been drawn out, they'll have decreased in mass. You can calculate % change in mass and then plot a graph
• ACTIVE TRANSPORT IN ROOT HAIR CELLS:

  plants need mineral ions for healthy growth; as the concentration of minerals is usually higher in the root hair cells than in the soil around them, they can't use diffusion to take up minerals from the soil.

  Active transport allows the plant to absorb minerals from a very dilute solution, against a concentration gradient. Active transport needs energy to make it work.
• Active transport is used in the gut when there is a lower concentration of nutrients in the gut, but a higher concentration of nutrients in the blood.

  Active transport allows nutrients to be taken into the blood - this means that glucose can be taken into the bloodstream when its concentration in the blood is already than in the gut. It can then be transported to cells, where it's used for respiration.
• osmosis practical conclusion:

  • dependant variable - chip mass
  • independant variable - concentration of the sugar solution
  • control variable - volume of solution, temperature, time, type of sugar used

  Errors may occur when carrying out the method, e.g. if some potato cylinders were not fully dried, the excess water would give a higher mass, or if water evaporated from the beakers, the concentration of the sugar solutions would change.

  You can reduce the effect of these errors by repeating the experiment and calculating a mean % change at each concentration