Membranes and Transport

• Plasma membranes are made of a phospholipid bilayer. It is partially permeable allowing lipid soluble substances (like some hormones) and very small molecules like carbon dioxide, oxygen and water through.
• Facilitated diffusion is passive and uses channel (filled with water and open in presence of certain ions) or carrier (bind with molecule, changes tertiary structure releasing molecule on other side) proteins.
• A hypotonic solution has a more positive water potential, a hypertonic has a more negative water potential. The more solutes in solution, the more negative the water potential.
• Sodium potassium pump

  1 - sodium ions actively transported out of epithelial cell into the blood so the concentration of sodium in the epithelial cells is decreased.

  2 - sodium can diffuse from the ileum into the epithelial cell through a co-transport protein which requires either glucose or an amino acid.

  3 - glucose or amino acid co-transported against concentration gradient.

  4 - glucose moves by facilitated diffusion from epithelial cell into the blood.
• Co-transport uses the hydrolysis of ATP. Ion and co-transporter molecule binds to the protein and it causes a change to the tertiary structure which allows the ion to move across the membrane.