Homeostasis and Transport

Explain how an equilibrium is established as a result of diffusion.

  1. Passive Transport- the movement of substances across a membrane without the use of energy
  1. Diffusion- substances move down a concentration gradient (more to less) until equilibrium is reached
  1. Concentration gradient- difference in concentration between two areas
  2. Equilibrium- the same concentration of molecules throughout a space
  • Diffusion is driven by the kinetic energy of the molecules.
  • The ability of a molecule to diffuse across a membrane depends on the size and type of a molecule and the chemical nature of the membrane.

Distinguish between diffusion and osmosis.

  1. Osmosis- the diffusion of water across a membrane. The net direction of osmosis depends on the concentration gradient across the membrane. Water tends to move to a hypertonic environment.
  2. Hypertonic- describes a solution with a higher concentration of solute (compared to hypotonic)
  3. Hypotonic-describes a solution with a lower concentration of solute (compared to hypertonic)
  4. Isotonic- describes solutions with equal solute concentrations
  5. Contractile vacuoles are present in some freshwater organisms. They are organelles that fill with water and then squeeze to pump out excess water. (This requires energy)
  • Other cells work toward equilibrium by pumping solutes from the cytosol. Water is then less likely to diffuse into the cell.
  • Turgor pressure is the force exerted against the cell wall in plants as the cell swells with water. Plant roots are in a hypotonic environment.
  • Plasmolysisis what occurs when turgor pressure is lost. The cell membrane pulls away from the cell wall.

Explain how substances cross the cell membrane through facilitated diffusion.

  1. Facilitated diffusion- diffusion with a carrier protein. This is used for molecules that cannot diffuse rapidly across a membrane even with a concentration gradient.
  • Molecules may be too large to pass through pores (glucose) or may not be lipid soluble.
  • Facilitated diffusion can help molecules like glucose move wither into or out of a cell.
  • The carrier proteins are specific for a certain type of molecule.

Explain how ion channels assist the diffusion of ions across the cell membrane.

  1. Ion Channels- small passageways across the cell membrane through which ions can diffuse. They must be shielded because they are not lipid soluble.
  • Some ion channels are always open. Some have “gates” that will open in response to the stretching of the cell membrane, electrical signals or chemicals in the cytosol or the external environment.

Distinguish between active and passive transport.

  1. Active Transport- the movement of from an area of low concentration to an area of high concentration that require the cell to use energy.

Explain how the sodium potassium pump operates.

  1. Sodium Potassium Pump- works to maintain a higher concentration of Na+ outside the cell and a higher concentration of K+ inside the cell.
  2. 3 Na+ ions bind to a carrier protein inside the cytosol.
  3. A phosphate from ATP binds to protein changing it shape.
  4. The 3 Na+ ions are released into the external environment.
  5. 2 K+ ions bind to the carrier protein.
  6. The phosphate group is released causing the protein to change shape.
  7. The K+ ions are released into the cytosol.
  • At top speed, the pump can transport about 450 Na+ ions and 300K+ ions per second.
  • This exchange of ions creates an electrical gradient across the membrane.

Compare and contrast endocytosis and exocytosis.

  1. Endocytosis the process of ingesting macromolecules or large particles. The particles are enveloped by a cell membrane forming a pouch called a vesicle. The vesicles go on to fuse with other membrane bound organelles.
  1. pinocytosis- the transport of solutes or fluids
  2. phagocytosis- the movement of large particles or whole cells
  1. Exocytosis- when vesicles in the cytoplasm fuse with the cell membrane to release its contents into the cell’s external environment.