Chapter 15: TRANSPORT ACROSS CELL MEMBRANES

Membrane permeability:

• Classes of molecules and their permeabilities (15-1).
• Types of membrane transport proteins (15-3); transporter mechanisms ’Äî uniport, symport and antiport (15-3).

Uniport transport:

• Differences from diffusion/passive transport (15-5). (1) faster rate of transport. (2) transport is specific. (3) transport occurs via limited transporter molecules.
• Glucose transport; GLUT1 dependent transport mechanism (15-7).

Ion channels and maintenance of ion concentrations and membrane potential:

• Ion concentrations inside and outside the cell (T15-1).
• Typical membrane potential is ~-70mV; selective permeability determines membrane potential (15-8); calculating membrane potential using the Nernst equation.
• Ion movement is goverened by the membrane potential and the ion gradient (15-9).

Active ion transport via ATP driven pumps:

• Characteristics ’Äî ATP dependent (up to half of the cellular ATP generated); transport K⁺, Ca⁺², Na⁺, H⁺ and Cl^-; comprised of P, F/V ABC classes of pumps (15-10; T15-2).
• The P-class plasma membrane (PM) Ca+2 ATPase exports Ca⁺² ions; the muscle Ca⁺² pump moves Ca⁺² ions into the sarcoplasmic reticulum (similar to the ER); mechanism of muscle Ca⁺² ATPase (15-11).
• The P-class Na⁺/K⁺ ATPase maintains intracellular concentrations of these critical ions; movement of 2 K⁺ in and 3 Na⁺ out; mechanism of action (15-13).
• The V-class H⁺ ATPase pumps protons across lysosomal, vacuolar membranes and certain acid secreting cells; required to maintain the proton gradient that keeps lysosomes and vacuoles at low (~5) pH.
• ABC pumps can transport ions and other small molecules (amino acids, etc.); multi drug resistance (MDR) transporter causes resistance to cancer therapies; possible MDR pump mechanisms (15-17); found mainly in liver and kidney, MDR pumps naturally remove toxins from cells; cystic fibrosis transmembrane regulator (CFTR) pump defects cause CF (a buildup of mucous that can suffocate); CFTR encodes a Cl^- channel; activated by cAMP phosphorylation.

Co-transport via symporters and antiporters:

• General ’Äî transport small molecules (e.g. ions, glucose, amino acids) using energy from the electrochemical gradient across the membrane.

• Symport ’Äî two-Na+/one glucose transporter; mechanism of transport (15-19).
• Antiport ’Äî AE1 antiporter exchanges Cl^- and HCO₃^-; important for O₂ and CO₂ exchange in erythrocytes (15-20).

Epithelial transport:

• Epithelia are polarized, with the apical surface (facing the lumens of organs) having different membrane proteins than the basal surface (facing underlying tissue).
• Polarization of epithelial cells is due to limiting the movement of membrane proteins by tight junction; structure of tight junctions (15-28); tight junctions stop diffusion of aqueous solutions through an epithelial layer (15-29).
• Transport of glucose in the intestinal epithelium (15-25).
• Acidification of the stomach lumen by parietal cells (15-26).

Osmosis:

• Defined as the movement of water from a low solute concentration to a region of high solute concentration; hypotonic, hypertonic and isotonic media; water channels (aquaporins) control bulk water flow across membranes (15-33).