Lecture 2 : Movement Of Ions Flashcards
What is responsible for maintaining Ion concentrations inside and outside of cells (2)
Cell membranes and Ion transfer proteins
what are cell membranes made of (2)
Made mostly of protein and lipids
The present day modes is a “fluid mosaic” where the proteins are afloat on a sea of lipids
Fluid mosaic model (3)
Integral proteins are embedded in the phospholipid bilayer
Carbohydrate chains bond to membrane proteins (framing Glycoproteins) or lipids (forming glycolipids) and extend into the extracellular fluid outside the cell membrane
Peripheral proteins bind to the integral proteins but are not in the bilayer. They help anchor the cell membrane to the cytoskeleton
Movement of molecules across the cell membrane (2)
Small non-polar, small uncharged polar, and lipophilic molecules can pass through
Large uncharged polar, and charged molecules can not
Two types of protein mediated transport
Channel proteins and carrier proteins
Channel proteins (4)
-Hundreds of known channels
-open to both sides of the cell membrane (=pores)
-can be gated or non-gated
-can allow many kinds of molecules to pass through the cell membrane
Ways to open and channel protein (4)
-voltage across cell membranes
-neurotransmitters
-phosphorylation
-mechanical forces
Carrier proteins (2)
-hundreds of different carrier proteins, all specific to different ions and molecules
-NEVER form an open channel, work like the two door system of a jewelry store
Three types of movement across carrier proteins
-Facilitated diffusion
-Primary active
-secondary active
Facilitated diffusion (5)
-does not require ATP
-follows diffusion across electrochemical gradient
-movement is faster than diffusion alone
-requires solutes to bind then unbind to the carrier protein
-Example is glucose which uses GLUT proteins (form of carrier protein)
Primary active transport (3)
-Uses energy from hydrolysis of ATP to move solutes across membranes
-This establishes gradients
-sometimes called pumps
Na+/k+/ATPase (sodium potassium pump!) (5 steps)
-3 internal Na+ ions bind to carrier from inside the cel
-ATPase is phosphorylated, closing the gate
-proteins change formation, releasing the sodium into the outside of the cell membrane
-2 K+ ions bind to the open gate from the outside
-ATPase unbinds from the carrier, releasing the potassium into the cell
Sodium potassium pump results (2)
-3 positive ions are pumped out for every 2 pumped in which
-establishes ionic gradients and resting membrane potential
-because more positive ions are going out then in, membrane potential becomes slightly negative
Secondary active transport (3)
-Does not directly use ATP as an energy source
-uses concentration gradient of one ion to move another against its own gradient
-an example is the movement of glucose using sodium, where the binding of sodium (diffusing down it’s gradient) to the carrier creates a binding location for glucose, allowing it to move up its own gradient
Membrane potential (6)
-due to the electrical gradient across cell membranes (which is due to the unequal distribution of ions)
-established by APTase and leak channels
-measure in mV
-not consist! Ions can move so membrane potential changes
-steady-state, not equilibrium! The overall amount of ions (inside and out) does not change, but the location of the ions is not at equilibrium
-varies between -20mV and -90mV
How resting membrane potential is established
-equilibrium potential (one species of ion)
-resting membrane potential (considered multiple species of ions)
What is equilibrium potential?
Take a hypothetical cell that functions much like a real one . It has a sodium potassium pump constantly supplying K+, and K+ leaks out of the cell as it diffuses down its gradient. if you were to stop the pump, K+ would constantly leak out until it reaches equilibrium with the exterior environment, so that [K in] = [K out]. If you were to add + or - charges (- in this case) to the inside of the cell to reach a point where the inside and out are electrically balanced, K would stop leaking. The voltage required to reach this point is K+’s equilibrium potential. In other words, the equilibrium potential of an ion is the membrane potential that exactly opposes the concentration gradient of an ion.
Resting membrane potential
-if there was only one ion in a cell, then RMP would be equal to that ions equilibrium potential
-however all real cells are permeable to K+, Cl-, and Na+ (they leak in/out of the cell based on their electrochemical gradients)
-Resting membrane potential is an equilibrium potential that accounts for all three of these ions present in the cell
Movement of Ions through channels (3)
-Ion channels allow ions to move across membranes down their electrochemical gradient
-when ion channels open, the ions always moves to make membrane potential the same as equilibrium potential
-this can be deceptive because most of the time it appears that the ions simply diffuses down its electrochemical gradient however its about the CHARGE not the amount of ions in/out. For example if there is a ton of K+ inside the cell but RMP is - and the equilibrium potential of k+ is positive, then more k+ will want to flow into the cell
