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Flashcards in Membranes Deck (20)
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Membranes acts as barriers to...

diffusion of substances and regulate their movement


What are the three major routes of diffusion?

i) diffusion through aqueous pores
ii) diffusion in the lipid portion
iii)Carrier-mediated diffusion (facilitated).
All three are passive processes and require no energy, they happen continually until equilibrium is reached.


What is diffusion through aqueous pores?

Simple diffusion where polar molecules pass through channels in lipid layer down a concentration gradient. These channels are 10 Angstroms wide. Rate is proportional to concentration gradient.


What is diffusion in the lipid portion?

When a substance can dissolve into and out of the layer of phospholipids down a concentration gradient. Substances include gases such as O2 and CO2, steroids, fatty acids. Rate is proportional to concentration gradient.


What is carrier-mediated diffusion?

Diffusion whereby carrier proteins embedded in the phospholipid layer help to carry substances from one side to another. Rate is dependent on the concentration gradient only in so far as the carrier-proteins do not become saturated.


Despite the three passive processes of diffusion, substance concentrations in cells are not usually in equilibrium - how?

Due to the active processes across the membrane that use ATP.


What are these active processes called?

Active Transport


What are Active Transport systems called and name an example?

Membrane pumps. (example is the sodium-potassium pump)


What does the Sodium-Potassium pump do?

It pumps sodium ions (Na+) out of the cell and potassium ions (K+) into the cell at a rate of three sodium for every two potassium.


Describe the normal sodium and potassium concentrations inside and outside of a cell.

Inside - Sodium low, potassium high
Outside - sodium high, potassium low


Potassium ions (K+) by diffusion leak out of the cell meaning the cell loses...

positive charge. Nearly every cell sits at around -70mV


What is the normal -70mV called?

The resting membrane potential.


Where are the excitable cells?

Nerve and muscle cells


Excitable cells can alter their membrane potential using...

gated ion channels.


Describe the process that occurs when an excitable cell is activated. (see notes for graph)

Na+ gates open up
Sodium ions rush into cell down concentration gradient adding positive charge to cell.
Cell becomes positively charged to around 30mV while the outside of the cell becomes negatively charged.
This is called depolarisation
Na+ gates begin to close and K+ gates begin to open
K+ ions rush out of cell
The cell repolarises
Gates close.


What is this change in the membrane potential called.

Action potential


What do action potentials do?

Code information in nerve and muscle cells.


What is the threshold and the threshold potential?

The minimum amount of stimulus required to initiate and AP


What is the action potential absolute refractory period?

The time between an initial action potential and the relative refractory period where it is impossible regardless of stimulus size to generate a secondary AP.


What is the relative refractory period?

The time between the absolute refractory period and the point where the threshold potential returns to normal. During this period a secondary AP is possible id the stimulus of a significant size.