Cell Physiology

• Epithelial cells have an apical side, facing the lumen (inside of a hollow organ) and a basolateral side, facing the extracellular fluid. This confers polarity to the membrane.
• Epithelial cells contain tight junctions, which create a nearly impermeable barrier separating tissue spaces and preventing leakage of solutes. Some molecules, like glucose, may enter/exit through these junctions.
• Cells are mostly composed of water, followed by organic molecles, and then inorganic ions (less than 1% of mass).
• Desmosomes are adhesive intracelluar junctions that are used for anchoring cells to each other.
• The relative permeability/diffusion through the plasma membrane of molecules can be ordered in the following way, from most to least permeable:

  1. Hydrophobic molecules (ex: O2)
  2. Small, uncharged polar molecules (ex: glycerol)
  3. Large, uncharged polar molecules (ex: glucose)
  4. Ions (ex: Na+)
  5. Charged polar molecules (ex: ATP)
• Water cannot pass through the plasma membrane via simple diffusion because the hydrophobic internal section of the phospholipid bilayer. It requires aquaporins to facilitate its passive transport via the concentration gradient, in a process called osmosis.
• Osmolarity is the total concentration of solutes in a solution. It depends on the total number of molecules and not on the individual types of moelcules.
• Saltwater fish have developed systems that produce concentrated urine (otherwise, their cells would shrivel).

  Freshwater fish produce dilute urine (otherwise, their cells would burst).
• Gap junctions between cells are used primarily for communication.
• Osmotic pressure is generated by water moving from one area to another based on osmolarity, while hydrostatic pressure is caused by gravity.
• Molecules that are too large and/or polar to diffuse are transported

  across plasma membrane by mediated transport mechanisms such as facilitated diffusion, active transport, and bulk transport.
• Aquaporins are constitutive channels, as they are always open.
• Facilitated diffusion does NOT require energy input because it uses existing gradients.
• Mechanically-gated channels are regulated by a physical change, like pressure.
• Facilitated diffusion can occur either via channels or permease/carrier proteins. The latter bind the substrate before undergoing a conformation change, and then releasing the substrate to the other side.
• Glucose transporter (GLUT) proteins are an example of a permease/carrier protein.
• Two examples of primary active transport are the Ca2+ ATPase/pump as well as the Na+/K+ ATPase/pump.
• The gradient created by the Na+/K+ ATPase (3 Na+ export, 2 K+ import) has several functions:

  • involved in electrochemical impulses
  • promotes osmostic flow
  • regulates resting calorie expenditure and basal metabolic rate
  • provides energy for "coupled transport"
• Secondary active transport does not require direct input of energy, but it does depend on the electrochemical gradient to transport molecules up their concentration gradients.
• Bulk transport can either occur via exocytosis or endocytosis and is the simultaneous movement of many large molecules that cannot be transported via carriers.