Molecular and Cellular Bases- Pt 2.docx

Cell Transport Cell transport is the movement of substances across the cellular membrane. The phospholipid bilayer (cell membrane) is semi-permeable. Lipid soluble molecules are uncharged, non-polar, hydrophobic molecules. Lipid soluble molecules can readily cross through the phospholipid bilayer. W...

Cell Transport Cell transport is the movement of substances across the cellular membrane. The phospholipid bilayer (cell membrane) is semi-permeable. Lipid soluble molecules are uncharged, non-polar, hydrophobic molecules. Lipid soluble molecules can readily cross through the phospholipid bilayer. Water soluble molecules are charged, polar, hydrophilic molecules. Water soluble molecules can NOT readily cross through the phospholipid bilayer. Water can readily cross the phospholipid bilayer without assistance due to it’s small size. The rate of water crossing the bilayer is dependent on the cell type. Membrane Proteins Membrane proteins are embedded within the cell membrane and allow for an alternative form of cell transport. Membrane proteins can act as a channel or as a transport/carrier protein. Both channels and transport/carrier proteins are usually selective. Channels have an opening going through the internal and external part of the cell membrane, allowing for ion flow through this opening. Transport/Carrier proteins bind to molecules or ions and endure a conformational change that will move the molecule to the other side of the cell membrane. Passive Transport/Diffusion vs Active Transport In passive transport, ions travel down their concentration gradient (high to low concentration) without expending energy. Passive transport uses kinetic energy. Active transport moves molecules against the concentration gradient (low to high concentration) and requires the cell to expend energy for movement. ATP is typically the energy source used for active transport. Passive Transport/Diffusion Types Simple diffusion is the movement of molecules through the lipid bilayer without assistance (from proteins). An example of simple diffusion is osmosis, which is the movement of water. The rate of simple diffusion directly correlates to the lipid solubility of the substance. Examples of simple diffusion include: Oxygen Nitrogen Carbon dioxide Alcohols The rate of simple diffusion is proportional to the quantity of the diffusing substance. Facilitated diffusion involves the movement of a molecule through the use of carriers or transporters without the requirement of energy use for movement. The rate of facilitated diffusion is directly proportional to the quantity of carrier proteins available, meaning that the rate of transport can’t exceed Vmax. Diffusion, in general, involves the movement of molecules from a high to low concentration (or movement with the concentration gradient). Facilitated Diffusion Proteins Carrier Proteins Carrier proteins bind specifically and aid in the diffusion of sugar, amino acids, and nucleosides. Carrier proteins will undergo a conformational change to move the molecule. Channels Channels are pores within the cell membrane that allow for the transport of ions and small hydrophilic molecules via facilitated diffusion. Many channels are very selective for the following characteristics: Diameter Shape Electrical Charge Channels used in Facilitated Diffusion Aquaporins Aquaporins are water channels that aid water in crossing the cell membrane at a faster rate than they would using simple diffusion. Cells with high water reabsorption tend to have more aquaporins, increasing the rate of water reabsorption. Ion Channels Ion channels aid in ion passage through the cell membrane. Some ion channels have a gate, which controls ion permeability. These ion gated channels can be opened with the following stimuli: Voltage change (across the membrane) Ligand (intracellular or extracellular) Mechanical stimulus (like pressure)- this is not fully understood Some ion channels are always open, rather than having a gate and are referred to as “leak channels”. Ion Channel Types Voltage Gated (ion) Channel Voltage gated channels are present in all excitable cell types. These excitable cell types include: muscle, nerve, endocrine, and egg cells. Voltage gated channels allow for neurons to transmit signals along their length to release neurotransmitters. The channel threshold is the minimum membrane potential required to trigger an action potential. There is a various range of membrane potentials that can allow for the opening of the voltage gated channels. Ligand Gated (ion) Channel Ligand gated channels open in response to ligand binding. An example of a ligand gated channel is acetylcholine binding to a nicotinic receptor within skeletal muscle cells. ligand gated channels are found in skeletal muscle cells, and within neurons of the ANS (autonomic nervous system) or brain. Molecule Permeability Small hydrophobic molecules readily dissolve in the lipid bilayer and have a high permeability. Small uncharged, hydrophilic molecules will diffuse through the lipid bilayer, but slightly slower than small hydrophobic molecules. Ions are almost completely impermeable to the lipid bilayer, regardless of size, giving it a very low permeability. Osmosis Osmosis is the net movement of water based on the difference in water concentration across a membrane. Water can travel across a cell membrane through either simple diffusion or through the use of aquaporins. There are approximately 13 types of aquaporins within mammal cells. Osmotic pressure is the amount of pressure required to stop osmosis. In an isotonic solution, there is no osmotic flow, leaving the appearance of a normal cell. In a hypotonic solution, there is osmotic flow of water into the cell due to the high sodium concentration internally. Hypotonic solutions can result in cellular rupture due to expansion. In a hypertonic solution, there is osmotic flow of water out of the cell due to the high sodium concentration externally. Hypertonic solutions can result in cells that appear shriveled. Active Transport Types Primary Active Transport Primary active transport requires the utilization of ATP or something similar as an energy source for movement. In primary active transport, the ion is ALWAYS moved AGAINST the concentration gradient. An example of primary active transport is the sodium potassium pump, which pumps 3 sodium out of the cell and pumps 2 potassium into the cell. Secondary Active Transport Secondary active transport can move an ion (usually sodium or hydrogen) EITHER against or towards the concentration gradient. This movement is usually done via a cotransporter or counter-transporter. Secondary active transport does NOT rely on ATP for energy, rather it utilizes the concentration gradient for movement. Secondary Active Transport Types Co-Transport (AKA: Symport) A symporter is a protein that simultaneously transports 2 molecules in the same direction across a membrane using the concentration gradient. An example of a symporter is the SGLT-1 (AKA: Sodium D-glucose) transporter. Counter-Transport (AKA: Antiport) An antiporter is a protein that simultaneously transports 2 molecules in opposite directions across a membrane using the concentration gradient. An example of an antiporter is the Sodium Hydrogen Antiporter within the kidney. Transcellular (or Epithelial) Transport Transcellular (or epithelial) transport involves the movement of a substance across the cell from one ECF (extracellular fluid) compartment to another. An example of transcellular transport would be potassium using the sodium potassium pump (a form of active transport) to move from the bloodstream to the interstitial fluid, and then using passive transport to further move into the lumen of distal nephron for secretion. This example is part of the mechanism of the aldosterone effect. Transcellular transport steps: Step 1: Active transport is used for movement through the membrane in one part of the cell. Step 2: Either simple diffusion or facilitated diffusion is used for movement through the membrane on the opposite side of the cell. Endocytosis vs Exocytosis Both endocytosis and exocytosis utilize part of the membrane as a carrier for transport of otherwise impermeable molecules. Endocytosis Endocytosis can take place using any of the following mechanisms: Phagocytosis Phagocytosis is the cellular ingestion of large particles. Pinocytosis Pinocytosis is the cellular ingestion of small particles. Receptor mediated endocytosis Receptor mediated endocytosis is utilized by cholesterol. Exocytosis During exocytosis, vesicles fuse with the cell membrane while the components of the vesicle are released into the external environment. Exocytosis can take place using the following mechanisms: Consecutive secretion Proteins within consecutive secretion are incorporated into the plasma membrane, extracellular matrix, or are signaling proteins. Consecutive secretion can take place in all cells and does not have a signal sequence. Regulated secretion regulated secretion only takes place in specialized cells and does require a signal (sequence) to stimulate the fusion and release to the cell exterior.