Transportation in a Cell – Cars and Crosswalks
Now that most of what a cell does has been discussed, you can learn how things move into and out of a cell. After all, if just anything can get into and out of a cell membrane – the flexible wall that is used to created a border between a cell and what is outside of it – then there would be no point in having a membrane at all. Think of the membrane of a cell like the screen door of a home. It lets air throw a house, but it keeps bugs and small animals from getting in the house without having to have a big heavy door closed all the time. The membrane does something similar for a cell in an effort to balance what is inside and outside of it, but what passes in and out of the membrane isn’t as easy as air. Because of this, we have two different types of transport that allow something to get through a cell membrane. These types of transport are called active transport and passive transport.
Active transport, as its name suggests, means that it is a type of transport that requires energy. Instead of being able to pass through the cell membrane easily and without effort, things that use active transport need a bit of help getting through because they are too big or complex to fit through the natural channels, or small open spaces, in a membrane. The membrane of a cell is made up of layers of proteins and phospholipids. Phospholipids are lipids, or fats, with phosphate attached to them. The proteins in the membrane of a cell are important in active transport because they are what helps move objects through the membrane in active transport.
Thinking of these proteins like a car crossing a street will help us understand active transport. The battery of the car is the energy, and the protein is the car itself. The passenger in the car is the object trying to get into the cell, and the street is the membrane of the cell. You use the battery to start the car, and once you’ve done that is moves across the road to take the passenger across the street and into the cell. It sounds very simple! Unfortunately, it isn’t quite that easy. Proteins can only take certain objects across the membrane – a protein that can take sugar across the membrane may not be able to take sodium across. This is also like cars: if the passenger can’t fit in the car, then you can’t use that car to get them across the street. These proteins could be cars, motorcycles, or big trucks. For active transport, the correct protein must match with the correct object, and have the right amount of energy, to cross the membrane.
Passive transport is much easier. Instead of using a car, the object can just cross the street on its own because it is small and doesn’t need much energy to cross. However, passive transport can be broken down into a few different kinds of transport itself. These different kinds of passive transport are osmosis, diffusion, and facilitated diffusion. Osmosis is passive transport with water only – sometimes it is easier to move water through the membrane instead of objects, so osmosis is used. Diffusion is used for all objects that are moved into and out of a cell through the membrane. But what is facilitated diffusion? As mentioned before, objects that move passively through the membrane can get across on their own. But sometimes it is necessary for them to move faster. Facilitated diffusion uses proteins on a membrane to move these passive objects into and out of the cell more quickly. Think of this like an elderly person getting helped across the street so that they can cross before any cars start moving forward.
Active and passive transport are both involved in something called the concentration gradient, which is how much of something is on either side of the membrane. If there is more sodium outside of the membrane than inside it, and you need more sodium inside, the sodium could just use passive transport to enter the cell. But if there’s more sodium outside the membrane, and you need to get even more sodium outside, then the sodium would have to go against the grain and use active transport instead. Therefore active transport requires energy – while passive transport goes with the flow, active transport goes against it.
