Thursday, July 26, 2012

What if a Concentration Gradient Doesn't Exist?

Active Transport and Concentration Gradients - Energy

In the nervous system and muscle system small particles with electrical charges use active transport carriers to recover the functioning capability of each body system.

Remember what the purpose of glucose is to a living cell? It provides a large molecule, glucose, that is broken up and acted upon in a cycle that produces usable chemical energy.

The energy produced is stored by the cells as ATP. It is in the form of a high energy bond associated with ATP. When a cell needs energy for one or more functions the ATP provides that energy.

There are two types on active transport mechanisms. One is associated with small particles like Sodium Ions (+), Chloride Ions (-), Potassium Ions (+), Hydrogen Ions (+) Calcium Ions (+) and a few more complex ions like NH4 (+), HCO3 (-) and Phosphate Ions (-). The other with larger protein molecules that I will post on tomorrow.

Example of a small particle active transport:

When a nerve is ready to receive an impulse that eventually reaches the brain for interpretation, it prepares by fixing its cell membrane to carry an electrical charge.

A nerve cell membrane has a large concentration of Sodium Ions (+) on the outside of the membrane and a large concentration of Potassium ions (+) on the inside of the cell membrane. 

The cell, when it has a larger concentration of positive ions on one side (outer membrane surface) and a lower concentration of positive ions on the inside of the membrane (both positive) the cell membrane is polarized (ready to create a electrical charge by depolarizing.}

Think of a nerve as a railroad track. On the track is a single railroad car. Wherever the railroad car is a depolarization is taking place. All the Sodium Ions are rushing into the cell through the selectively permeable membrane and all the Potassium Ions rush out of the same membrane. 

After the railroad car has passed over a section of track it begins the process of depolarization. The nerve cell calls upon its supply of ATP to provide energy to push the Sodium ions back out of the nerve cell against its concentration gradient. There are many more Sodium Ions out there even after a depolarization.

A small carrier molecule picks up a Sodium Ion, carries it to the external membrane surface and releases it.

Instead of wasting its return trip to pick up another Sodium Ion the carrier molecule picks up a Potassium Ion and returns (against its concentration gradient) it to the cell interior by active transport. Its like a two way pump that re-establishes the polarity across the nerve cell membrane.

All this activity needs energy to function. Once the nerve cell is depolarized it is ready to receive a new nerve impulse. 

In review, active transport uses energy to push a particle (ion) through a selectively permeable membrane against its concentration gradient.

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