Cell Transport

• Diffusion

  The spreading out of the particles of any substance in solution, or particles of a gas, resulting in a net movement of particles from an area of higher concentration to an area of lower concentration.

  Passive process; no energy is required.
• Diffusion in living organisms

  Substances can enter and exit cells by diffusing across the cell membrane.

  Molecules must be small enough to diffuse through the membrane; oxygen, glucose, amino acids and water can, but proteins and starch cannot.
• Substances that enter cells by diffusion

  Oxygen must diffuse into cells for aerobic respiration.

  Carbon dioxide must diffuse into plant cells for photosynthesis.
• Substances that leave cells by diffusion

  Liver cells break down amino acids into the waste product urea, which diffused out of the cells to be excreted by the kidneys.

  Carbon dioxide produced by aerobic respiration must diffuse out of the cell to be breathed out.
• Factors affecting rate of diffusion

  Concentration gradient (difference in concentrations).

  Temperature.

  Surface area of membrane.
• Diffusion in Multicellular organisms

  Large multicellular organisms have a small surface area to volume ratio, resulting in a large distance between the surface and centre of the organism and therefore they cannot rely on diffusion alone to meet their needs.
• How is the small intestine adapted for absorption of digested food molecules into the bloodstream?

  Each cell has a projection called a villus (plural villi).

  The villi are only covered with one layer of epithelial cells; reduces diffusion distance.

  The villi have a good blood supply; maintains a concentration gradient.

  The villi have a large surface area; increases rate of diffusion.
• How are the lungs adapted for gas exchange?

  Each lung contains millions of alveoli, which collectively provide a huge surface area; increases rate of diffusion.

  Wall of each alveolus is one cell thick; reducing diffusion distance.

  Alveoli have strong blood supply; maintains a concentration gradient.
• How are gills adapted for gas exchange?

  Each gill is made from smaller plates called filaments, which are covered in projections called lamellae – this increases SA.

  Dense capillary network ensures good blood supply which flows in the opposite direction to water passing through the gills – this maintains a concentration gradient.
• How are the roots adapted to absorb water and mineral ions from soil?

  The root network is highly branched - this increases SA.

  The surface of the roots are covered in root hair cells, which have a specialised structure with root hair projections – this increases SA.
• How are the leaves adapted to take in carbon dioxide?

  Stomata allow air to circulate inside the leaf – this decreases diffusion distance for carbon dioxide and oxygen.

  The lower layer of the leaf is made from spongy mesophyll cells which allow air to circulate inside the leaf.
• Exchange surfaces in multicellular organisms

  Multicellular organisms have surfaces and organ systems that maximise the exchange of materials in a number of ways:

  Large surface area; increases rate of transport.

  Thin barrier/membrane; provides short diffusion distance.

  Large network of blood vessels; maintains concentration gradients.

  Ventilated gas exchange surfaces; maintains concentration gradients.
• Osmosis

  Diffusion of water molecules from a dilute solution to a concentrated solution through a partially permeable membrane.

  Passive process; no energy required.
• Why do water molecules move from a dilute to concentrated solution?

  Dilute solutions have a high concentration of water (high water potential). Concentrated solutions have a low concentration of water (low water potential).

  Water moves from dilute to concentrated solutions because it is moving from area of high water potential to low water potential - down the concentration gradient.
• Why does water move into cells when they are placed in dilute solution?

  Cytoplasm of a cell contains sugars and salts, causing water molecules to enter the cell because water potential is lower in the cell than in the solution.
• Isotonic VS Hypertonic VS Hypotonic Solutions

  If the concentration of sugar in the external solution is same as internal, the solution is isotonic to the cell.

  If the concentration of sugar in the external solution is higher than internal, the solution is hypertonic to the cell.

  If the concentration of sugar in the external solution is lower than internal, the solution is hypotonic to the cell.
• Osmosis in animal cells

  If external solution is more dilute (higher water potential), water will move into the animal cell causing them to burst.

  If external solution is more concentrated (lower water potential), water will leave the animal cell causing it to become shrivelled.
• Osmosis in plant cells

  If external solution is more dilute (high water potential), water will move into the plant cell and the vacuole, causing it to swell, resulting in turgor (pressure).

  If external solution is concentrated (low water potential), water will leave plant cell, causing it to become soft. Eventually the membrane moves away from the cell (plasmolysis) and it will die.
• Osmosis PRACTICAL

  Cut five potato cylinders of same diameter.

  Trim each cylinder so they are all the same length.

  Measure the mass of each cylinder and record.

  Measure 10cm^3 of each solution and pour into each boiling tube. Label each boiling tube clearly.

  Add one cylinder to each tube and leave for specified amount of time.

  Remove potatoes, blot, dry and record final mass of each.
• Osmosis PRACTICAL: Analysis

  Percentage change in mass = (Final-Initial Mass)/Initial Mass X 100.

  The point at which the line of best fit crosses the X-axis is the concentration of sugar inside the potato.
• Active transport

  Active transport moves substances from a more dilute solution to a more concentrated solution (against a concentration gradient). This requires energy from respiration.
• Active transport in plants

  Root hair cells need to move mineral ions such as magnesium and nitrate ions from the dilute soil into the concentrated cytoplasm.

  Plants need magnesium ions to make chlorophyll for photosynthesis.

  Nitrate ions are needed to make amino acids and therefore for protein synthesis.
• Active transport in animals

  When the concentration of sugar molecules in the blood is higher than in the gut, they can be transported into the blood by active transport rather than diffusion because they need to move against the concentration gradient.