Types of Membrane Proteins - Online Tutor, Practice Problems & Exam Prep

Membrane proteins are crucial for cellular functions, categorized into integral and peripheral proteins. Integral proteins span the phospholipid bilayer, while peripheral proteins are located on the membrane's surface. Functions include cell recognition, anchorage to the extracellular matrix, signal transduction, transport of molecules, linkage between cells, and enzymatic activity. These roles are vital for maintaining cellular integrity and communication, highlighting the importance of membrane proteins in biological systems.

Types of Membrane Proteins

So now that we know that proteins play a major role in forming biological membranes, in this video we're going to introduce the different types of membrane proteins. Now, really, there are 2 main types of membrane-associated proteins, and we have those 2 listed down below. Now, the first are the integral membrane-associated proteins, and the second type are the peripheral membrane-associated proteins. Now, the integral membrane-associated proteins, as their name implies, are non-covalently integrated into the phospholipid bilayer as we can see down below. And usually, these integral membrane proteins span the entire lipid bilayer, so that they have an extracellular portion and an intracellular portion. And so of course, this purple protein that we see down below is our integral membrane protein, and again we can tell because it's completely integrated and spanning our phospholipid bilayer.

Now the next type, the peripheral membrane-associated proteins, stay on the perimeter of the lipid bilayer, and they do not span the entire lipid bilayer like integral membrane proteins do. And so notice down below in our image, this blue protein right here is on the periphery or on the perimeter of the lipid bilayer. And so this is going to be our peripheral membrane-associated protein. Now, there is a subcategory of peripheral membrane-associated proteins, and those are the lipid-linked membrane-associated proteins. And so again, the lipid-linked membrane-associated proteins are peripheral membrane-associated proteins, so they're going to be found on the periphery. But really what makes these lipid-linked ones so distinct is that these are peripheral membrane proteins that are covalently anchored to lipid groups within the bilayer. And of course, if you have a protein that is covalently linked to a lipid, that's going to make it a lipoprotein. And so these lipid-linked membrane-associated proteins are peripheral lipoproteins. And so if we take a look at our image down below right here, notice we're showing you a lipid-linked membrane-associated protein here. And notice that it is indeed on the periphery, and the protein is not really spanning the membrane like our integral membrane protein was. Instead, it is on the periphery, so that makes it a peripheral membrane protein. But also notice that it's covalently attached to this lipid group that we see right here, and that is really what distinguishes it as its own subcategory within the peripheral membrane proteins.

Now, as we move forward in our course, we're going to talk even more details about each of these different types of membrane-associated proteins. But for now, this here concludes our introduction to the types of membrane proteins, and I'll see you guys in our next video.

Which of the following are covalently attached to lipid molecules in the cell membrane? 

a) Integral proteins.

b) All membrane proteins.

c) Peripheral proteins.      

d) Lipid-anchored proteins.

• A. Integral proteins
• B. All membrane proteins
• C. Peripheral proteins
• D. Lipid-anchored proteins

Membrane Protein Functions

So in our last lesson video, we introduced the different types of membrane-associated proteins. But what in the world do these membrane-associated proteins even do for a cell? Well, in this video, we're going to talk about some membrane protein functions. And so it's important to note that membrane-associated proteins can actually perform a wide variety of functions. There are so many different functions that it's not possible to list all of the membrane-associated protein functions on this page. But what you'll notice is down below in this table, we're showing you guys some of the more common functions that membrane proteins can have.

And so the first function here that we're showing you guys is recognition. Some membrane-associated proteins can be used purely for recognition and can mark the cell for identification. Just like your ID can identify you, some membrane-associated proteins can be used to identify the cell, and that allows our immune cells to distinguish healthy cells from foreign invading cells. Now, other membrane-associated proteins can be used in anchorage, and this allows a cell and a cell cytoskeleton to be anchored to the ECM or the extracellular matrix.

Now, other membrane-associated proteins can be used in transduction, specifically signal transduction. They can act as signal molecule receptors and transduce a signal from the outside of the cell to the inside of the cell or vice versa, transduce a signal from the inside of the cell to the outside of the cell allowing for communication to occur. Here we have an image of a signal receptor to remind you of that function.

Now, other membrane-associated proteins can act in transport and allow for molecular transport across the membrane, either from the outside of the cell to the inside of the cell or vice versa, from the inside molecular cargo. Now, other membrane-associated proteins function in linkage and basically can connect two cells via a protein linkage. Just like two paper clips can be linked together in this fashion, protein linkages again can link together two different cells.

And then last but not least, other membrane-associated proteins can function as enzymes. And, of course, we know that enzymes will catalyze and speed up the rate of chemical reactions. Of course, there are many, many different types of enzymatic processes, and these membrane-associated proteins can act as many different types of enzymes to catalyze all of these different types of enzymatic processes. Here, we're showing you an image of an enzymatic process to remind you of that function.

And so what you'll notice here is that the first letter of all of these general functions here actually spells out the word "rattle". And so over here, we have a rattle to help remind you of these generalized functions. Now as we move forward in our course, we'll be able to see some examples of how membrane-associated proteins can function in these different ways. But for now, this is our general introduction to membrane protein functions, and I'll see you guys in our next video.

Here’s what students ask on this topic:

What are the main types of membrane proteins?

The main types of membrane proteins are integral and peripheral proteins. Integral membrane proteins are embedded within the phospholipid bilayer and often span the entire membrane, having both extracellular and intracellular portions. Peripheral membrane proteins, on the other hand, are located on the surface of the membrane and do not span the bilayer. A subcategory of peripheral proteins is lipid-linked proteins, which are covalently attached to lipid groups within the bilayer. These classifications are crucial for understanding the diverse roles that membrane proteins play in cellular functions.

What functions do membrane proteins perform in a cell?

Membrane proteins perform a variety of essential functions in a cell. These include:

• Recognition: Identifying the cell to the immune system.
• Anchorage: Connecting the cell to the extracellular matrix or cytoskeleton.
• Signal Transduction: Acting as receptors to transmit signals across the membrane.
• Transport: Facilitating the movement of molecules across the membrane.
• Linkage: Connecting cells together via protein linkages.
• Enzymatic Activity: Catalyzing chemical reactions.

These functions are vital for maintaining cellular integrity, communication, and overall function.

How do integral membrane proteins differ from peripheral membrane proteins?

Integral membrane proteins differ from peripheral membrane proteins in their location and structure. Integral proteins are embedded within the phospholipid bilayer and often span the entire membrane, having both extracellular and intracellular portions. They are non-covalently integrated into the bilayer. Peripheral membrane proteins, however, are located on the surface of the membrane and do not span the bilayer. They can be attached to the membrane or to integral proteins through non-covalent interactions. A subcategory of peripheral proteins, lipid-linked proteins, are covalently attached to lipid groups within the bilayer.

What is the role of lipid-linked membrane proteins?

Lipid-linked membrane proteins are a subcategory of peripheral membrane proteins. They are distinguished by being covalently attached to lipid groups within the phospholipid bilayer. This covalent attachment anchors them to the membrane, allowing them to perform their functions while remaining on the membrane's periphery. These proteins play roles in various cellular processes, including signal transduction, where they act as receptors or enzymes, and in maintaining the structural integrity of the cell membrane.

Why are membrane proteins important for cellular communication?

Membrane proteins are crucial for cellular communication because they facilitate the transmission of signals across the cell membrane. Signal transduction proteins, a type of membrane protein, act as receptors that bind to signaling molecules (ligands) outside the cell. This binding triggers a series of intracellular events that convey the signal into the cell, leading to a specific response. This process allows cells to respond to their environment, communicate with other cells, and coordinate various physiological processes, ensuring proper cellular function and homeostasis.