Daily Newsletter

September 18, 2013 Active Transport

In active transport, cells are moving substances across the membrane, but against the chemical concentration gradient. Chemicals are move from areas of low concentration to areas of higher concentration. To move against a concentration gradient requires energy to overcome the inherent Brownian motion of the molecule. This always requires protein (enzyme) pumps. The word pump implies an active process that moves against a natural gradient or flow. (consider: passive transport proteins are called channels, pores or carriers, while active transport proteins are called pumps)
The most commonly discussed pumps will be the ion pumps. There are two pumps that all biology students must become familiar with as they are critically important and are discussed in many different biology courses.

Sodium-Potassium ATPase (also known as the Na⁺/K⁺ pump).
- This helps to establish and maintain the Sodium and Potassium gradients of a cell.
- Sodium should be at high concentrations outside of the cell (extracellular)
- Potassium should be a high concentrations inside of the cell (intracellular)
- This combined gradient helps to establish the Resting Membrane Potential of many cells (an electrical charge across the membrane).
Proton Pumps
- This pump system help to establish and maintain a proton (hydrogen ion) gradient.
- In Eukaryotes, this will be found along the inner mitochondrial membrane.
- In Prokaryotes, this will be found along the cell membrane.
- This is a critical electrochemical gradient for cellular energy.

With both of these pump systems, we are creating electrochemical gradients, and both represent potential energy.

All active transport systems require energy to work. Primary active transport will utilize ATP, while secondary active transport will utilize either reducing power (redox reactions) or an established electrochemical gradient.

Primary Active Transport: The addition of the phosphate causes a conformational change in the proteins structure. Remember, you are adding a -2 charge to a specific location on the protein; this will change the electrical profile of the protein, causing the proteins folding pattern to change. Note that both Na⁺ and K⁺ are moving against their electrochemical gradients.

Secondary Active Transport: As with active transport, there will be a conformational change in the protein (aka, the folding pattern, and hence shape, will change). The cause of this shape alteration will either result from a redox reaction or an established electrochemical gradient. In the image below, we see the entry of one ion (sodium) causing the expulsion of a second ion (amino acid) in an antiport system. How do you know that sodium is the "motive force"? Look at the Na⁺concentration gradient; sodium is moving down the electrochemical gradient.

Important Memes

When an ion moves down it's electrochemical gradient, across a membrane, work (kinetic energy) is done. The electrochemical gradient is a seperation of charged particles (ions) on either side of the membrane. When we speak of a Sodium (Na⁺) gradient, we are actually referring to an electrochemical gradient. Movement of ions down an electro chemical gradient is akin to hooking up a battery in a circuit, you get kinetic genery (work is done). Until then, the electrochemical gradient acts a potential energy.
When you add something to a protein, the protein changes shape.
Whether it is the substrate of an enzyme, an ion, or a phosphate, as things are added, the protein's shape changes.

Daily Challenge: Concentration and Electrochemical Gradients

Today, reflect on the idea of concentration and electrochemical gradients. In living system, we do not see the typical end point of diffusion in which you get equal concentrations on either side of the membrane. Instead, we are constantly adjusting concentrations by using membrane proteins (Such as the Na⁺/K⁺ pump above). Why is it necessary for cells to do this? Why is it that disruption of the membrane (and therefore the possibility of concentration equilibrium) means the death of a cell?

This article from Scitable may help: Why Are Cells Powered by Proton Gradients?