Action potential and transport across membranes
Definition: Action
Potential is a mode generated by the neurons due to the flow of certain ions
across the neuron which is caused by a change in the voltage across the
membrane. This takes place in neurons, muscle cells and endocrine cells.
The sequence of Action Potential is
often termed as a ‘spike train’.
- The voltage-gated ion
channels that are present in a plasma membrane are closed, when the resting
potential is achieved.
- This results in the
opening of the channels if the potential increases.
- This leads to the inward
flow of sodium ions. This may end in the rising of the membrane potential.
- This sequence continues
until all the channels open up and the sodium ions reverses the polarity of the
membrane.
- The ion channels
inactivate and the Na+ ions are actively transported out of the plasma membrane.
- Then, K+ ions are
activated, resulting in the resting state of the electrochemical gradient.
- When the sequence gets
over, the negative shift occurs which is called as Refractory period.
Refractory
period is of two phases. Absolute and Relative.
In the former one, Na+ channels cannot be opened by a stimulus, while in the
latter one, spikes are initiated. The
hyperpolarized state remains higher than the resting state of K+, for which
more voltage is required for reaching the threshold which is also higher due to
few Na+ channels that are still inactivated. Due
to the ‘spike train’, transport of information is faster and the temporal
resolution being high than before. These spikes propagate along axons, muscle
fibres and also heart muscle fibers.
Transport
across membranes:
As
we saw in the previous articles, biological membranes are selectively permeable
to the membrane solutes. They employ three distinct mechanisms for the
transport of metabolites across the membrane.
- Passive diffusion
- Facilitated diffusion
- Active transport
Passive
Diffusion:
It does not require energy. Water and
Gas transportation across the membrane occurs through this process. Those process is based on the concentration gradient of a particular substance that
is to be passed through the membrane.
Facilitated Diffusion:
This
diffusion process also does not require energy. They move from a higher
concentration to a lower concentration. This process is accomplished with the
help of Carrier proteins. Special carrier proteins are isolated for the
transport of Glucose, Galactose, Leucine, Phenylalanine. FD involves
a mechanism which employs a ping-pong model. This explains that the carrier
protein exists in two different forms. So, the carrier protein is in higher
concentration initially, exposing themselves to bind with the solutes at
specific sites. Then, they shift to the lower concentration,
where the solute molecule is released. This process is regulated by the
hormones.
Active Transport:
It
acts against a concentration gradient and is based on the supply of ATP. Primary
Active Transport systems includes ion-pumps which is very important for the
regulation of transportation process.
Sodium - Potassium pump:
The cells have Na+ and K+
concentrations. Na+ is of lower concentration and K+ is of higher
concentration. And higher concentration
of K+ is required for optimal glycolysis. They are also required for
transmission of Nerve impulses.- Na+ - K+ pump is caused
by an integral plasma membrane protein, an enzyme, Na+-K+ ATPase.
- This pumps 3 Na+ ions
from internal to external cell environment and 2K+ ions from external to
internal environment.
- 3Na+ ( in) + 2K+ ( out) + ATP = 3Na+
(out) + 2K+ (in) + ADP+ Pi
- Major amount of ATP is
used by the Na+- K+ pump in order to maintain the cytosolic Na+- K+ levels.
- The voltage-gated ion
channels that are present in a plasma membrane are closed, when the resting
potential is achieved.
- This results in the
opening of the channels if the potential increases.
- This leads to the inward
flow of sodium ions. This may end in the rising of the membrane potential.
- This sequence continues
until all the channels open up and the sodium ions reverses the polarity of the
membrane.
- The ion channels
inactivate and the Na+ ions are actively transported out of the plasma membrane.
- Then, K+ ions are
activated, resulting in the resting state of the electrochemical gradient.
- When the sequence gets over, the negative shift occurs which is called as Refractory period.
Refractory
period is of two phases. Absolute and Relative.
In the former one, Na+ channels cannot be opened by a stimulus, while in the
latter one, spikes are initiated.
The
hyperpolarized state remains higher than the resting state of K+, for which
more voltage is required for reaching the threshold which is also higher due to
few Na+ channels that are still inactivated.
Due
to the ‘spike train’, transport of information is faster and the temporal
resolution being high than before. These spikes propagate along axons, muscle
fibres and also heart muscle fibers.
Transport across membranes:
As we saw in the previous articles, biological membranes are selectively permeable to the membrane solutes. They employ three distinct mechanisms for the transport of metabolites across the membrane.
- Passive diffusion
- Facilitated diffusion
- Active transport
Passive Diffusion:
It does not require energy. Water and Gas transportation across the membrane occurs through this process. Those process is based on the concentration gradient of a particular substance that is to be passed through the membrane.
Facilitated Diffusion:
This
diffusion process also does not require energy. They move from a higher
concentration to a lower concentration. This process is accomplished with the
help of Carrier proteins. Special carrier proteins are isolated for the
transport of Glucose, Galactose, Leucine, Phenylalanine.
FD involves
a mechanism which employs a ping-pong model. This explains that the carrier
protein exists in two different forms. So, the carrier protein is in higher
concentration initially, exposing themselves to bind with the solutes at
specific sites.
Then, they shift to the lower concentration,
where the solute molecule is released. This process is regulated by the
hormones.
Active Transport:
It
acts against a concentration gradient and is based on the supply of ATP. Primary
Active Transport systems includes ion-pumps which is very important for the
regulation of transportation process.
Sodium - Potassium pump:
The cells have Na+ and K+
concentrations. Na+ is of lower concentration and K+ is of higher
concentration. And higher concentration
of K+ is required for optimal glycolysis. They are also required for
transmission of Nerve impulses.
- Na+ - K+ pump is caused
by an integral plasma membrane protein, an enzyme, Na+-K+ ATPase.
- This pumps 3 Na+ ions from internal to external cell environment and 2K+ ions from external to internal environment.
- 3Na+ ( in) + 2K+ ( out) + ATP = 3Na+ (out) + 2K+ (in) + ADP+ Pi
- Major amount of ATP is used by the Na+- K+ pump in order to maintain the cytosolic Na+- K+ levels.
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