02 - Cell Membrane and Transport Flashcards
Describe the composition of a cell membrane and explain how the distribution of phospholipids and proteins influences the membrane permeability of ions, hydrophilic and hydrophobic compounds
The cell membrane is composed of a phospholipid bilayer. The bilayer is surrounded by phosphate attached to fatty acid tails pointing at each other. The polar phosphate portion is hydrophilic and can thus freely interact with the aqueous environment inside and outside of the cell. The nonpolar lipid portion forms a barrier between the inside and outside of the cell that is impenetrable to anything hydrophilic. These items require special effort (transport proteins) to cross membrane; hyrdrophobic molecules, like steroid, are able to freely pass through to exert intracellular effects.
Define “steady state” and differentiate from “equilibrium.” Relate the pump-leak model of steady-state ion content to cell solute gradients and cell volume maintenance
Equilibrium: A process or reaction occurs at the same rate in forward and backward directions. There is no net change in the composition over time (eg leak channels for water would be considered at equilibrium if there is no movement of water in or out of cell).
Steady state: Can be thought of homeostasis for a specific mechanism. For example, Na/K ATPase using energy from ATP hydrolysis to exchange sodium ions for potassium ions. This maintains a constant internal concentration of potassium putting potassium in a steady state.
Differentiate the following terms based on the source of energy driving the process and the molecular pathway: Diffusion
Molecules will move from areas of high concentration to low concentration using a protein channel. No energy required.
Differentiate the following terms based on the source of energy driving the process and the molecular pathway: Facilitated diffusion
Molecules will bind to a transporter protein in the plasma membrane, the protein will change configuration (shape), the molecule will enter/exit cell based on the properties of diffusion (from high conc. to low conc.). No energy required.
Differentiate the following terms based on the source of energy driving the process and the molecular pathway: Primary active transport
Using energy to move molecules “upstream” against concentration gradients. Eg, Na/K pump relies on hydrolysis of ATP to move potassium and sodium against concentration gradients. Requires energy.
Differentiate the following terms based on the source of energy driving the process and the molecular pathway: Secondary active transport
Uses concentration gradients enhanced by primary active transport to move other molecules against concentration gradient. Eg, Na/Glucose symport, using the increased concentration of sodium, from Na/K pump, outside of the cell, sodium binds with a transporter protein that also requires glucose. This allows glucose to move into the cell as well - against its concentration gradient. Does not directly require energy; does require actions of primary active transport.
Explain how energy from the Na and K electrochemical gradients across the plasma membrane can be used to drive the net “uphill” movement of other solutes
Primary active transport utilizes energy to move solutes against the concentration gradient, from low concentrations to high concentrations (eg Na/K pump). Other molecules (glucose, Ca) can “hitchhike” on this system via cotransport and countertransport.
Cotransport or Symport: When solute moves in same direction as sodium (into cell).
Countertransport or Antiport: When solute moves in opposite direction as sodium (out of cell).
Explain functional significance of polarized distribution of various transport proteins to the apical or basolateral membrane
Restricting certain transport proteins to either the apical or basolateral cell membrane is necessary for the movement of ions across the epithelial cell. Need ions/molecules to enter from lumen but must continue into extracellular space. Without polarity, there would be lack of movement.
The movement of water is driven by solute movement. Define osmosis and the implications of cell volume when placed in isoosmotic, hyperosmotic and hypoosmotic solutions
Osmosis: Movement of water molecules across a membrane. Water follows solutes.
Isoosmotic: Solute concentration is equal inside and outside of the cell. Net movement of water is zero, so no change in cell volume.
Hyperosmotic: Cell volume will decrease due to water leaving the cell into the extracellular space.
Hypoosmotic: Cell volume will increase due to water entering the cell from the extracellular space.
Describe the importance of endocytosis and exocytosis to the turnover of the plasma membrane. Define the time course of this turnover and one process that typifies its importance
Endocytosis: Regions of the plasma membrane invaginate and pinch off to form vesicles, enclosing a small amount of extracellular material into the cell.
Exocytosis: Intracellular vesicles fuse with the plasma membrane releasing the contents into the extracellular fluid.
Between the combination of endo- and exocytosis, the plasma membrane is replaced every hour. This is important when prescribing drugs that affect membrane proteins, as the membrane will have 100% turnover in an hour.
