In people are shoulder-to-shoulder carrying cotton candy

In this lesson, we’ll learn how substances are transported across the cell membrane against the concentration gradient. This might seem like an uphill battle for the cell, but all it takes is a little chemical energy and a few integral membrane proteins to kick off some active transport!

Transport Against a Concentration Gradient

Passive transport occurs when cells transport something along a concentration gradient.
High to low concentration transport

Have you ever been to the fair when it’s really crowded? Maybe you wanted to go, but it’s too crowded for you.

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The lines are too long, and the people are shoulder-to-shoulder carrying cotton candy and spilling popcorn all over the place. Perhaps you wait in the busy parking lot, observing the throngs of people and wondering if you shouldn’t wait ’til it’s not so busy. You’re not a fan of the crowds, but you are a huge fan of the fair. Finally, you decide, ‘Let’s face it – it’s a fair.

A fair is always busy.’ There’s only one thing to do in a situation like this: you muster up all your energy, and you fight your way through the admission gate to get yourself a roll of tickets and some fried dough.Is a cell like the town fair? Well, there sure are a lot of things going on in a cell at the same time, so it might seem busy like a carnival. There’s also many times when a cell needs to transport something in that goes against the concentration gradient. So, let’s think of the fair like a concentration gradient.Remember that a concentration gradient is the gradual difference in solute concentration between two areas – much like you standing outside the fair while there are thousands of people inside.

There is a higher concentration of people inside the fair. When a cell needs to transport something along a concentration gradient, or from an area of high concentration to an area of low concentration, it does this by passive transport. This would be like people leaving the fair because it’s crowded; that’s easy.However, when a cell needs to transport something against a concentration gradient – or you decide to move from an area of low concentration of people to an area of high concentration of people – this is going to take some willpower. Or, in the case of the cell, some chemical energy.

Proteins in Active Transport

Active transport of ions or molecules is achieved through the use of an integral membrane protein.
Integral Membrane Protein

Active transport is transport against a concentration gradient that requires chemical energy. Active transport moves ions or molecules in a specific direction through the use of an integral membrane protein.

Think of these like different gates at the fair. There are three types of membrane proteins that use energy to push these substances against the concentration gradient.A uniport is an integral membrane protein that moves an ion or molecule in one direction. ‘Uni’ can remind you of the word ‘one,’ such as one substance moving in one direction. This would be like an admission gate that people could only enter or an exit gate through which people could only leave.An antiport is an integral membrane protein that moves one ion or molecule in one direction while moving a second substance in an opposite direction.

The ‘anti’ in antiport means ‘against,’ and can remind you of two substances moving against each other, like an admission gate that only lets a few people enter if a few people exit at the same time.A symport is an integral membrane protein that moves two ions or molecules in the same direction. ‘Sym’ in symport stands for the word ‘same,’ as in two substances moving in the same direction. This would be like an admission gate that had two lines of people to enter the fair at the same time, in the same direction.Both antiports and symports are a type of coupled transport, because they transport two different substances through the same integral membrane protein. Importantly, all three different types of integral membrane proteins use chemical energy to make this active transport happen. This allows a cell to maintain specific concentrations of substances different than in its surroundings.

Symports and antiports are coupled transports.
Symports and antiports

Sodium-Potassium Pump

Let’s talk about a specific example of active transport that is going on in all of your nerve cells right now. Your nerve cells require that a specific concentration gradient of sodium and potassium be maintained in order to properly conduct your signals.There is a higher concentration of sodium ions outside a cell than inside a cell. There is also a higher concentration of potassium ions inside the cell than outside the cell.

The sodium-potassium pump ensures this gradient exists by constantly pumping sodium ions and potassium ions against the concentration gradient using active transport.First, the sodium-potassium pump in the cell membrane binds three sodium ions from inside the cell. Next, with the help of chemical energy, the sodium-potassium pump changes shape and exports these sodium ions outside the cell. This is against the concentration gradient of sodium.

Two potassium ions from outside of the cell then bind to the pump and are imported into the cell. Again, this is against the concentration gradient. Because two substances are transported in opposite directions, like the fair gate that is both an entrance and an exit, the sodium-potassium pump is a type of antiport.

Lesson Summary

In summary, we have learned that active transport is transport that requires chemical energy to move substances against a concentration gradient.

There are three types of integral membrane proteins that aid in active transport. Uniports are proteins that move one substance in one direction. Antiports are proteins that move two substances in opposite directions. Symports are proteins that move two substances in the same direction.

Learning Outcomes

After watching this lesson, you should be able to:

  • Define active transport
  • List three types of integral membrane proteins involved in active transport
  • Explain how the sodium-potassium pump operates
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