Forces Acting Across Membrane 2 Flashcards
Why is there a difference in charge across the plasma membrane?
The ions creating the concentration gradients are charged particles creating an electrical gradient
What does the electrochemical gradient drive?
The direction of passive movement.
What are 5 mechanisms of movement across the cell membrane?
- Endocytosis/ exocytosis
- Diffusion
- Mediated transport
- Osmosis
- Filtration
What occurs in endocytosis?
There is invagination of the membrane to form a vesicle which eventually separates on the cytoplasmic surface of the membrane and migrates within the cell to its destination.
What is exocytosis?
The reverse process of endocytosis
What is diffusion?
Process by which a gas or substance in solution expands to fill all the available volume, therefore molecules spread from regions of high concentration to regions of low concentration until the concentration is uniform throughout the volume.
What must be true of the barrier between 2 compartments for diffusion to take place?
It must be permeable to the diffusing substance
How can the magnitude of net flux be described?
F=kpA(C1-C2)
The magnitude of diffusion is equal to the permeability coefficient multiplied by the surface area across which diffusion occurs multiplied by the difference in concentration
What is the permeability coefficient?
A measure of the ease with which a molecule can pass through a given membrane
What are cell memebrane effectively impermeable to?
Intracellular proteins and organic anions as they cannot diffuse in any capacity and so stay inside the cell.
In order to diffuse through the lipid bilayer molecules need to be…
Small
Hydrophobic
Uncharged
Why do ions such as Na, K and Cl cross membranes at a much faster rate than predicted?
The ions use transport proteins
What are channel proteins?
Transmembrane IMPs that act as an aqueous route through the membrane for the diffusion of ions.
What can pass through channels?
mineral ions such as Na, Cl, Ca, H and H2O
Why can’t glucose pass through channels?
The channels are too small
What does water pass through?
Aquaporins, a family of water channels
What do channels consist of?
Multiple protein subunits
How are gated channels opened?
They remain closed until a stimulus causes them to open.
(stimulus can be chemical or a change in electrical charge across membrane)
What are voltage gated channels reliant on?
Open/close in response to alterations in the membrane electrical potential (charge difference either side of the membrane)
Where are voltage gated channels found mainly.
Muscle and nerve cells
What are ligand gated channels reliant on?
Open/close when they bind a chemical such as a neurotransmitter or hormone to a receptor binding site on the channel protein
What does the stimulus of a gated channel cause?
A conformational change of the channel protein causing it to open or close their channel.
What is the direction of diffusion dependent on?
The concentration gradient and in the cases of charged particles, the electrical gradient.
What is the membrane potential?
The inside of a cell usually carries a negative charge with respect to the outside if the cell.
What does the membrane potential create?
A potential gradient down which ions will flow
When is electrochemical equilibrium reached?
When the chemical and electrical gradients are in balance
Facilitated diffusion
Movement of molecules through transport proteins down their electrochemical gradients
Active Transport
Movement of molecules through transport proteins against their electrochemical gradients requiring energy (ATP)
What happens when a solute binds to a carrier-mediated protein?
The carrier undergoes a change in shape which exposes the binding site on the other side of the membrane. The solute moves away and the carrier returns to its original conformation
What is the function of the Na/K ATPase pump?
Occurs in all cells to help maintain the concentration gradient.
What does the Na/K ATPase pump do?
Continuously pumps our 3 Na ions and pump 2 K ions into the cell
What is produced by the Na/K ATPase pump?
A net movement of positive charges out of the cell
Electrogenic Pump
A pump which creates a charge difference across the membrane
How much of the resting energy of the body is used by the Na /K ATPase pump?
40%
