Carrier Proteins: Types & Functions

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  • 0:00 Definition of a…
  • 1:08 Crossing the Membrane
  • 1:49 Types of Carrier Proteins
  • 2:35 Functions
  • 3:10 Lesson Summary
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Lesson Transcript
Instructor: Nadine James

Nadine has taught nursing for 12 years and has a PhD in Nursing research

In this lesson, you'll learn the definition of a carrier protein. Also, you'll explore the types of carrier proteins and find out how they each assist molecules as they pass through the cell membrane.

Definition of a Carrier Protein

Proteins are a group of organic compounds made up of 20 different amino acid chains. Each protein has a specific role. The sequence of the chains of amino acids defines the specific shape and function of the protein. The carrier protein is located in the membrane of a cell.

A membrane is a thin layer that may or may not allow particles, ions, and water to cross into and out of the cell. Most cell membranes are semi permeable or have selective permeability -- meaning that only some particles, ions, and water can cross the membrane. However, the cell membrane is made up of fatty acids and lipid layers which repel water.

To assist you with this concept, imagine a measuring cup in your kitchen. You pour ¾ cup of oil into the measuring cup. Then you add ½ cup of water. What happens? The water and oil don't mix together do they? No, in fact the water is on the bottom and the oil is on the top. This is an example of the repelling that occurs at the fatty lipid cell wall when particles dissolved in water approach.

Crossing the Membrane

So what happens to allow particles and ions to cross the cell membrane? The process used is called facilitated diffusion. Facilitated diffusion is the process of transporting particles into and out of a cell membrane. Energy is not required because the particles move along the concentration gradient. This requires the use of a carrier or channel protein.

Concentration gradient is the process of particles, which are sometimes called solutes, moving through a solution or gas from an area of higher number of particles to an area of lower number of particles. The carrier that assists the particle to cross the membrane is either a carrier or channel protein.

Types of Carrier Proteins

Carrier proteins use a process called carrier mediated transport to assist molecules across the cell membrane. As stated earlier, the makeup of the amino acid chains in the protein determines the size and shape of the carrier protein. This, in turn, determines what molecule can be received by the carrier protein to cross the cell membrane. Remember that each molecule has a specific carrier protein that assists the molecule across the cell membrane.

That molecule can only cross the cell membrane if the particular carrier protein is available. There are two types of carrier proteins. The first type, facilitated diffusion, does not require energy to move the substance across the cell membrane. The second type, active transport, does use energy.


The carrier proteins facilitate diffusion of molecules across the cell membrane. The protein is imbedded in the cell membrane and covers the entire membrane. This is important because the carrier must transport the molecule in and out of the cell. How does this occur with the fatty lipid membrane?

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