CHANNELS - EXAM A2Z

CHANNELS

Are the transmembrane protein that are found in plasma membrane
facilitated diffusion of small ions
does not require complex conformational changes
faster than transport mediated by protein carriers.
simply open holes in the membrane
they are capable to difference between different ions •channels have gates that can be regulated in response to appropriate stimuli

Types of Channels

Ion Channels

Ion channels are pores that allow specific charged particles to cross the membrane in response to an existing concentration gradient.
Proteins are capable of spanning the cell membrane, including its hydrophobic core, and can interact with the charge of ions because of the varied properties of amino acids found within specific domains or regions of the protein channel.
Hydrophobic amino acids are found in the domains that are opposed to the hydrocarbon tails of the phospholipids.
Hydrophilic amino acids are exposed to the fluid environments of the extracellular fluid and cytosol.
Additionally, the ions will interact with the hydrophilic amino acids, which will be selective for the charge of the ion.
Channels for cations (positive ions) will have negatively charged side chains in the pore.
Channels for anions (negative ions) will have positively charged side chains in the pore.
This is called electrochemical exclusion, meaning that the channel pore is charge-specific.
Ion channels can also be specified by the diameter of the pore.
The distance between the amino acids will be specific for the diameter of the ion when it dissociates from the water molecules surrounding it.

Because of the surrounding water molecules, larger pores are not ideal for smaller ions because the water molecules will interact, by hydrogen bonds, more readily than the amino acid side chains.
This is called size exclusion.
Some ion channels are selective for charge but not necessarily for size, and thus are called a nonspecific channel.
These nonspecific channels allow cations—particularly Na+, K+, and Ca2+—to cross the membrane, but exclude anions.

The sodium/potassium pump requires energy in the form of adenosine triphosphate (ATP), so it is also referred to as an ATPase.
The concentration of Na+ is higher outside the cell than inside, and the concentration of K+ is higher inside the cell than outside.
That means that this pump is moving the ions against the concentration gradients for sodium and potassium, which is why it requires energy.
In fact, the pump basically maintains those concentration gradients.
Ion channels do not always freely allow ions to diffuse across the membrane.
Some are opened by certain events, meaning the channels are gated.
So another way that channels can be categorized is on the basis of how they are gated.
Although these classes of ion channels are found in large numbers in the cells of nervous and muscular tissue, many also can be found in the cells of epithelial and connective tissues.

Ligand-Gated Channels

A ligand-gated channel opens because a signaling molecule, a ligand, binds to the extracellular region of the channel.
This type of channel is also known as an ionotropic receptor because when the ligand, known as a neurotransmitter in the nervous system, binds to the protein, ions cross the membrane changing its charge .
Ligand-gated channels are found in highest numbers in the dendrites and body of the neuron.

Ligand-Gated Channels

These two diagrams each show a channel protein embedded in the cell membrane. In the left diagram, there is a large number of sodium ions (NA plus) and calcium ions (CA two plus) in the extracellular fluid. Within the cytosol, there is a large number of potassium ions (K plus) but only a few sodium ions. In this diagram, the channel is closed. Two ACH molecules are floating in the extracellular fluid. Their label indicates that a neurotransmitter, a ligand, is required to open the ion channel. The neurotransmitter receptor site on the extracellular fluid side of the channel protein matches the shape of the ACH molecules. In the right diagram, the two ACH molecules attach to the neurotransmitter receptor sites on the channel protein. This opens the channel and the sodium and calcium ions diffuse through the channel and into the cytosol, down their concentration gradient. The potassium ions also diffuse through the channel in the opposite direction down their concentration gradient (out of the cell and into the extracellular fluid).

When the ligand, in this case the neurotransmitter acetylcholine, binds to a specific location on the extracellular surface of the channel protein, the pore opens to allow select ions through.
The ions, in this case, are cations of sodium, calcium, and potassium.

Mechanically-Gated Channels

A mechanically-gated channel opens because of a physical distortion of the cell membrane.
Many channels associated with the sense of touch (somatosensation) are mechanically gated.
For example, as pressure is applied to the skin, these channels open and allow ions to enter the cell.
Similar to this type of channel would be the channel that opens on the basis of temperature changes, as in testing the water in the shower.

Mechanically-Gated Channels

These two diagrams each show a channel protein embedded in the cell membrane. In the left diagram, there are a large number of sodium ions in the extracellular fluid, but only a few sodium ions in the cytosol. There is a large number of calcium ions in the cytosol but only a few calcium ions in the extracellular fluid. In this diagram, the channel is closed, as the extracellular side has a lid, somewhat resembling that on a trash can, that is closed over the channel opening. In the right diagram, the mechanically gated channel is open. This allows the sodium ions to flow from the extracellular fluid into the cell, down their concentration gradient. At the same time, the calcium ions are moving from the cytosol into the extracellular fluid, down their concentration gradient.

When a mechanical change occurs in the surrounding tissue, such as pressure or touch, the channel is physically opened.
Thermoreceptors work on a similar principle.
When the local tissue temperature changes, the protein reacts by physically opening the channel.

Voltage-Gated Channels

A voltage-gated channel is a channel that responds to changes in the electrical properties of the membrane in which it is embedded.
Normally, the inner portion of the membrane is at a negative voltage.
When that voltage becomes less negative, the channel begins to allow ions to cross the membrane .
Some voltage-gated channels have a set voltage (threshold) above which the channel is open and below which it is closed.
Other types of voltage-gated channels are more complicated, opening and closing at different voltages.
For example, the gate may open at a specific positive voltage, but once open will not close again until it reaches a specific negative voltage, or vice versa.

Making things even more complicated, some voltage-gated channels have more than one gate where each gate responds to different voltage properties.
In this type of channel, both gates must be open for the ions to move through the membrane.
Neurons have voltage-gated channels that are specific to certain ions.
Voltage-gated sodium and potassium channels are mostly found in the axon hillock and axon portions of the neuron.
Voltage-gated calcium channels are found at the axon terminal bulbs and activate vesicle fusion with the cell membrane and neurotransmitter release.

Voltage-Gated Channels

This is a two part diagram. Both diagrams show a voltage gated channel embedded in the lipid membrane bilayer. The channel contains a sphere shaped gate that is attached to a filament. In the first diagram there are several ions in the cytosol but only one ion in the extracellular fluid. The voltage across the membrane is currently minus seventy millivolts and the voltage gated channel is closed. In the second diagram, the voltage in the cytosol is minus fifty millivolts. This voltage change has caused the voltage gated channel to open, as the small sphere is no longer occluding the channel. One of the ions is moving through the channel, down its concentration gradient, and out into the extracellular fluid.

Voltage-gated channels open when the transmembrane voltage changes around them. Amino acids in the structure of the protein are sensitive to charge and cause the pore to open or close.

Leakage Channels

A leakage channel is randomly gated, meaning that it opens and closes at random, hence the reference to leaking.
There is no actual event that opens the channel; instead, it has an intrinsic rate of switching between the open and closed states.
Leakage channels are found throughout the neuron and contribute to the resting transmembrane voltage of the excitable membrane.
Leakage channels also are found in autorhythmic cells, where the membrane spontaneously depolarizes at a regular interval, for example pacemaker cells in the heart.

Leakage Channels

This is a two part diagram. Both diagrams show a leakage channel embedded in the lipid membrane bilayer. The leakage channel is cylindrical with a large, central opening. In the first diagram there are several ions in the cytosol but only one ion in the extracellular fluid. No ions are moving through the leakage channel because the channel is closed. In the second diagram, the leakage channel randomly opens, allowing two ions to travel through the channel, down their concentration gradient, and out into the extracellular fluid.

In certain situations, ions need to move across the membrane randomly. The particular electrical properties of certain cells are modified by the presence of this type of channel.