Transport Proteins: Definition, Types, Function & Examples

Transport proteins are used in various ways to move substances back and forth across the cell membrane. This lesson will discuss the different types, their functions, and examples of each.

Transport Proteins: The Doors to the Cell

Your cells are constantly shipping and receiving different types of molecules, similar to how a post office handles various letters and packages.

Each cell has a plasma membrane, or a filter, that helps regulate materials moving in and out of the cell. Each plasma membrane has different transport proteins embedded within it, which are used to help with this process. Each transport protein only allows a certain molecule to enter or exit the cell.

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Think of these transport proteins as specialized doors of the post office. Each type of parcel – each letter, each package, and so on – may only enter the post office through a specific doorway designated for it. Letters enter through the letter door; packages enter through the package door, and first-class mail comes through the first-class door.

Passive and Active Cellular Transport

There are two main kinds of cellular transport: passive transport and active transport.

When passive transport occurs, molecules are moving from a concentrated area to a less concentrated area. This doesn’t require any energy; the pressure in the concentrated area will naturally push molecules to the area of lower pressure. The opposite of this process, active transport, moves molecules from a less concentrated area to a more concentrated area. This requires an input of energy in the form of ATP, adenosine triphosphate.

Let’s go back to the post office. Imagine there are 100 postal workers inside the post office and 20 workers outside. That’d be 100 molecules inside the cell and 20 outside. When workers from inside leave and go outside of the post office, they are moving from an area with more workers (a higher concentration) to an area with less workers (lower concentration). There’s a lot more room outside, so leaving the crowded post office requires very little energy. This is exactly how passive transport works.

On the other hand, if some of the workers on the outside decide to go into the post office, they would be moving from an area with less workers (lower concentration) to an area with more workers (higher concentration). They would need to exert energy to cram themselves back into the crowded building. That’s why active transport requires energy from the cell.

Functions of Transport Proteins

Transport proteins function in both active and passive transport to move molecules across the plasma membrane. Two main groups of transport proteins can be found within the plasma membrane, and each helps water-loving molecules pass across the plasma membrane.

A channel protein serves as a tunnel across the membrane into the cell. More specifically, channel proteins help molecules across the membrane via passive transport, a process called facilitated diffusion. These channel proteins are responsible for bringing in ions and other small molecules into the cell. It’s important to remember that each channel protein can only bring in a specific molecule. For example, a calcium channel can only be used to transport calcium in and out of the cell. There are different types of channel proteins for different molecules, including ones for sodium, potassium, and chloride. In fact, there’s even a channel specifically for transporting water across the plasma membrane.

The other type of transport protein is called a carrier protein. Carrier proteins have to change shape in order to pass the molecule across the plasma membrane. Think of it as a revolving door to the post office. As you enter the revolving door, the door has to move in order to get you to the inside of the post office. A carrier protein acts in a similar way.

Similar to channel proteins, carrier proteins are specific to the molecules they are trying to move across the membrane. Some carriers can only move one substance at a time, while others can move two substances at the same time. Like channel proteins, carrier proteins can be used in facilitated diffusion. For instance, several glucose carrier molecules assist in the facilitated diffusion of glucose across a membrane. However, they can also be used during active transport as well. For example, the sodium potassium pump transports three sodium ions into the cell, while pumping two potassium ions out of the cell.

Lesson Summary

Transport proteins act as doors to the cell, helping certain molecules pass back and forth across the plasma membrane, which surrounds every living cell. In passive transport molecules move from an area of high concentration to an area of low concentration. Active transport, on the other hand, moves molecules from a low concentration to a higher concentration.

There are two main classes of transport proteins that are found within the plasma membrane. Channel proteins aid in passive transport, a process called facilitated diffusion. During this process, they serve as a tunnel for certain ions and small molecules. Examples of channel proteins include chloride, sodium, calcium, and potassium ion channels. Carrier proteins are used in both passive and active transport and change shape as they move their particular molecule across the membrane. Examples of carrier proteins within our cells include the sodium potassium pump and glucose transporters.

Notable Vocabulary Terms ; Definitions

transportproteins

Vocabulary Definitions
Transport Proteins Doors to the cell; they help molecules to move back and forth across the plasma membrane
Passive Transport Molecules move from high to low concentration
Active Transport Moves molecules from a low concentration to a higher concentration
Channel Proteins (i.e. chloride, calcium) Act as a tunnel that carries ions and small molecules during facilitated diffusion
Carrier Proteins (i.e. glucose transporters) Works both in active and passive transport; they change shape while moving molecules across membranes

Learning Outcomes

Use this lesson to expand your knowledge of transport proteins in preparation to subsequently:

  • Interpret the actions of transport proteins
  • Contrast active and passive cellular transport
  • Note the functions of channel and carrier proteins
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