The Concentration Gradient: Transport of Molecules

Exploiting the Concentration Gradient

Once again, our body uses these concentration gradient exceptions to its advantage. In fact, every animal’s energy production capacity relies on this. You may be thinking, “but I thought ATP was the chemical source of energy and the bonds hold the energy”. This is correct, but the question then becomes, “how did the energy get into the bonds of ATP?” I will address this issue later in the post.

The unique thing about the membranes of a cell is that they all are selectively permeable. This means that only certain things can get in and out without assistance or energy. When such a molecule travels across the membrane, it is known as passive transport or diffusion. This leads to a buildup of something on one side of the membrane pushing onto the other side of the membrane, following the concentration gradient. This can be good and bad for the cell, depending on the substance. We will go over one example of each:

The Regulatory Pumps

The first example involves pushing specific molecules against their concentration gradient. Two ions that are present everywhere in the body are sodium and potassium. Cells have a very intimate relationship with these two chemical species. The first important thing to note is that neither of these ions can pass through the membrane. This triggers another phenomenon known as osmosis. When a molecule cannot pass through the membrane, there is a difference between the number of molecules on each side of the membrane. This results in different concentrations of the molecule on each side of the membrane. Think of a wall separating a crowded room from a nearly empty room.

Effects of Osmosis

In order to compensate for more “stuff” on one side, water is pulled through the membrane until both sides are equally concentrated. This means if there are a lot of molecules on the outside of a cell, water will leave and it will shrivel up and lose functionality. On the other hand, if there are relatively more molecules inside the cell, it will take in water, swell, and burst. (A cell with more molecules outside of it is considered hypotonic to the environment while a cell with more molecules inside is considered hypertonic to the environment. A cell with equal amounts of molecules inside and outside are isotonic.)

This balance is very energetically costly. In fact, roughly 30% of our daily ATP is dedicated to powering one protein present in every cell: the sodium- potassium pump. This is a lot of energy considering we generate enough ATP in one day to almost equal our body weight. Although the phenomenon of osmosis is not unique to these two molecules, they are the most numerous by far and thus the most important to regulate.

How the Sodium-Potassium Pump Works

What this pump does is very simple. Using one ATP, the sodium potassium pump pushes out 3 sodium ions and brings in 2 potassium ion. When energy is used to push something against its gradient, it is called primary active transport. This does two things: it makes sure there is an optimal water balance inside the cell while also creating two very steep gradients of sodium and potassium. These two gradients are facing opposite sides and can be imagines as stuck in a endless game of tug-of-war for water. However, at the same time that they try and pull water,, both of these ions are pushing towards the cell membrane, trying to get in by constantly bumping into the barrier.

This stress caused by the concentration gradient stores vast amounts of potential energy on both sides of the membrane. This energy storage drives all the processes essential to life, from nervous signals to generation of energy stored in ATP. In fact, most of the ATP inside your cells is generated by using a gradient of protons, slowly leaking through a membrane. The cells use the potential energy they carry to form ATP. This process deserves its own post, however. The second example explores another, simpler way this gradient can be used to let in molecules that can’t get in themselves.

The Glucose Transporter

One thing that cells need dearly is glucose. Specifically for intestinal cells, absorbing glucose from the intestine before it leaves is essential. If it doesn’t, the body may not have enough energy to survive. Thus, these cells need to take in as much glucose as possible and feed it into the blood. However, glucose is so big that it cannot pass through the membrane without energetic assistance. As a result, a unique form of transportation evolved.

Sugar Uptake and Release

Due to the large size of glucose, it would require a lot of ATP and a very complicated protein to let it into a cell against its concentration gradient. Instead, intestinal cells evolved to use the huge amount of potential energy stored in the sodium gradient to their advantage. A protein which would bind to both glucose and sodium accomplishes this. Because of the vast concentration of sodium trying to get in, like a crack in a river dam, the glucose is swept into the cell even though it doesn’t want to be there. Using this strategy, very little sodium is used to let in lots of glucose. This process is known as secondary active transport.

Once the glucose is built up in the cell, it needs to be slowly let into the blood. However, letting it all in would result in dangerous levels of blood sugar. This time, a protein that uses the glucose gradient to leak glucose out is utilized. The protein acts as a door and can only let glucose out so fast. This ensures glucose can only exit in a slow, steady stream. This is a lot like the subway entrance. No matter how many people are in line, only a few people can swipe through the door at a time, resulting in a steady flow of people into the station.