Cell Physiology

• Multicellular organisms


  A system of cells whose individual functions contribute to the success of the organism as a whole
• Body has 100 trillion cells classified into ~200 types based on variations in structure and function
• Plasma membrane


  Forms a semipermeable barrier between the intracellular and extracellular compartments
• Plasma membrane composition


  Made from phospholipids that create impermeability due to hydrophilic/hydrophobic tails
• Integral proteins


  Embedded in the cell membrane, crossing it to perform functions that require transmission across the membrane
• Peripheral proteins


  Attached to the surface of the membrane, but do not enter the hydrophobic spaces in between the membrane
• Modes of Transformation


  • Simple diffusion
  • Facilitated Diffusion
  • Active Transport
• Simple diffusion


  Passive transport of lipid-soluble molecules directly across the lipid bilayer
• Facilitated Diffusion


  Passive transport of impermeable solutes by protein carriers or channels
• Active Transport


  Active transport of impermeable solutes by protein carriers
• Diffusion


  Net diffusion occurs from regions of higher concentration to regions of lower concentration
• Simple diffusion


  • Directly proportional to the transmembrane concentration gradient of transported molecule
  • Directly proportional to membrane surface area available for diffusion
• Facilitated Diffusion


  • Channels are classified by their selectivity, conductance, and gating mechanism
  • Open channels are accessible from both sides of the membrane
  • Undergo conformational changes that reorient their "sidedness"
  • Uniports carry a single solute
  • Rate of carrier-mediated diffusion is directly proportional to the transmembrane concentration gradient of the transported molecule until the carriers are saturated
  • Rate can be meditated by altering the number of functionally active carriers
• Primary Active transport


  • Maintain characteristic non-equilibrium ion distributions between the ICF and ECF
  • Uses chemical energy to move solutes across a membrane against their concentration gradient, creating a non-equilibrium, net accumulation of the solute
• Na+/K+ ATPase


  1. E1 conformation (accessible from intracellular fluid, high affinity for Na+ and low affinity for K+)

  2. E2 conformation (accessible from ECF, low affinity for Na+ and high affinity for K+)

  3. Transporter binds 3 Na+ from cytosol

  4. Phosphorylation by ATP favors conformational change

  5. Na+ is released, K+ binds

  6. Dephosphorylation favors original conformation

  7. K+ is released to cytosol. Cycle repeats
• Secondary Active Transport


  • Couples the dissipation of an existing transmembrane gradient to the production of a second transmembrane gradient
  • Uses the energy stored in electrochemical gradients to move other substances against their own gradient. ATP is not directly coupled to the molecule of interest here
• Bioenergetics


  The transfer/transformation of energy within living organism
• Metabolism


  The sum of all chemical reactions in a biological entity
• The fate of common metabolic intermediates

  Depends on the energy status of the cell (ATP/ADP ratio)
• Glycolysis


  • Low-efficiency, high-velocity metabolic pathway
  • Takes place in the cytoplasm of most cells
  • Metabolic pathway that converts glucose into pyruvate. The free energy released in this process is used to form the high-energy molecules ATP and NAD
  • First step in breaking down glucose to extract energy for cell metabolism. Responsible for breaking down enzymes and releasing energy
  • In mammalian red blood cells, glycolysis is the only source of ATP, this if it stopped, these cells would lose their ability to maintain their sodium-potassium pumps (and in turn membrane potential) and eventually die
• ATP synthesis in glycolysis

  • Via substrate level phosphorylation
  • Involves one redox reaction where oxidation of a metabolic intermediate is coupled to the reduction of NAD+. NAD+ must be regenerated for glycolysis to continue
• Kreb's Cycle


  • Final central pathway for oxidation of carbon fuels to CO2
  • Involves four redox reactions where oxidation of a metabolic intermediate is coupled to reduction of an electron carrier
  • Main function is to produce energy, which is stored and transported as ATP or GTP. Also provides precursors for certain amino acids and the reducing agent NADH
  • If removed, the concentration of pyruvate, NADH, and intermediate H+ would decrease, as it is responsible for producing these molecules. The cycle stopping would also make all useful cellular metabolic activity cease. Every muscle and nerve would instantly stop functioning
• Electron Transport System (ETS)


  Couples electron transfer along a favorable redox span to the pumping of protons into the intermembrane space creating a proton gradient