Classes of Signaling Receptors: Study with Video Lessons, Practice Problems & Examples

Signaling receptors are categorized into two main classes: cell surface receptors and intracellular receptors. Cell surface receptors, embedded in the cell membrane, include G protein-coupled receptors (GPCRs), receptor tyrosine kinases (RTKs), and ligand-gated ion channels. Intracellular receptors, found within the cell, interact with small hydrophobic signaling molecules that diffuse across the membrane, triggering cellular responses. Understanding these receptors is crucial for grasping signal transduction pathways and their roles in cellular communication.

Classes of Signaling Receptors

In this video, we're going to begin our introduction to the classes of signaling receptors. And so recall from our previous lesson videos that receptors are biomolecules, typically proteins, that are going to change conformation when bound to a very specific ligand or signaling molecule. It turns out that receptors can be categorized into 2 major classes. The first class are cell surface receptors, and the second class are intracellular receptors. Moving forward in our course, we're going to talk about the cell surface receptors and the intracellular receptors in their own separate videos. But we'll start off with the cell surface receptors. So I'll see you all in our next videos to talk more about these cell surface receptors.

Cell-Surface Receptors

In this video, we're going to introduce cell surface receptors. Cell surface receptors, as their name implies, are receptor proteins found embedded in the cell membrane, or in other words, on the surface of the target cell. There are three major types of cell surface receptors that are involved in most signal transduction pathways. Each of these three major types of cell surface receptors operates in its own unique way.

The first major type of cell surface receptor that you all should be aware of is the G protein coupled receptors, or GPCRs for short. You can see GPCR stands for G Protein Coupled Receptors. Taking a look at the image down below, you can see that the GPCR here is in this tannish color, and the G protein portion is highlighted in green. The ligand here can bind to the GPCR, and ultimately, the GPCRs have their own unique way of operating.

The second major type of cell surface receptor that you all should be aware of is the receptor tyrosine kinases, or RTKs for short. The RTKs usually have two domains. You can see the two domains of the RTK. The RTKs, or receptor tyrosine kinases, also have their own unique way of operating and functioning.

The third and final major type of cell surface receptor that you all should be aware of are the ligand-gated ion channels. Down below, we are showing some ligand-gated ion channels, which you can see on the left-hand side are in a closed position. It is not until the ligand binds that the receptor changes conformation into its open position, allowing ions to flow through the membrane. It is the ligand that binds to cause the channel to open up and allow for ions to diffuse across the membrane.

To learn more about the details of each of these different types of receptors and exactly how they operate and function, that's something that you are not typically going to be held accountable for in an introduction to biology course. We do have videos here at Clutch Prep that cover the details of GPCRs, RTKs, and ligand-gated channels. But you'll be able to find those details in our more advanced biochemistry courses. For now, this concludes our brief introduction to the cell surface receptors, and we will be able to talk about the intracellular receptors in our next video. So, I'll see you all there.

Intracellular Receptors

So now that we've introduced cell surface receptors in our previous lesson video, in this video, we're going to introduce intracellular receptors. Intracellular receptors, as their name implies, are receptor proteins found on the inside of a target cell, which is what intracellular means—on the inside of a cell. Recall from our previous lesson videos that really small hydrophobic molecules or signaling molecules are going to be able to freely diffuse across the membrane and into the cell. If we take a look at our example image down below at this basic intracellular signaling pathway right here, notice that we have a membrane representing the cell membrane. Up above in the blue background, we have the outside, the extracellular fluid, and down below here with the yellow background, we have the inside of the cell or the cytoplasm of the cell. Notice here, in red, we have a really small hydrophobic signaling molecule or ligand. Because it's small and hydrophobic, it can freely diffuse across the cell surface or the plasma membrane, and so its receptor is not going to be found on the cell surface or the plasma membrane. Instead, the small hydrophobic molecule will diffuse through the membrane and into the cell where it can interact with an intracellular receptor, which is this blue molecule you see right here. When the small hydrophobic molecule binds to the intracellular receptor, it will lead to a cascade of events that ultimately generates a cellular response. Intracellular signaling and intracellular receptors will be found specifically on the inside of the cell, not in the cell surface or the plasma membrane. This concludes our introduction to intracellular receptors. As we move forward in our course, we'll be able to get some practice applying the concepts that we've talked about. I'll see you all in our next video.

The molecules that convert extracellular signals into intracellular signals are:

a) Neurotransmitters.

b) Hormones.

c) Cell surface receptors.

d) Intracellular receptors.

• A. Neurotransmitters.
• B. Hormones.
• C. Cell surface receptors.
• D. Intracellular receptors.

Which type of receptor leads directly to a change in the distribution of ions on opposite sides of the membrane?

a) Receptor tyrosine kinase.

b) G protein-coupled receptor.

c) Ligand-gated ion channel.

d) Steroid receptor.

e) Intracellular receptor.

• A. Receptor tyrosine kinase.
• B. Ligand-gated ion channel.
• C. Steroid receptor.
• D. Intracellular receptor.

Why does testosterone, a lipid-soluble / hydrophobic signaling molecule, not affect all cells in the body but only specific cells?

a) Only target cells have the cell surface receptor able to bind with testosterone.

b) Only target cells contain the genes regulated by testosterone.

c) Only target cells possess the phosphorylation cascade uniquely activated by testosterone.

d) Only target cells possess the intracellular receptor able to bind with testosterone.

• A. Only target cells have the cell surface receptor able to bind with testosterone.
• B. Only target cells contain the genes regulated by testosterone.
• C. Only target cells possess the phosphorylation cascade uniquely activated by testosterone.
• D. Only target cells possess the intracellular receptor able to bind with testosterone.

Here’s what students ask on this topic:

What are the two main classes of signaling receptors?

The two main classes of signaling receptors are cell surface receptors and intracellular receptors. Cell surface receptors are embedded in the cell membrane and include G protein-coupled receptors (GPCRs), receptor tyrosine kinases (RTKs), and ligand-gated ion channels. These receptors interact with signaling molecules outside the cell and initiate signal transduction pathways. Intracellular receptors, on the other hand, are located within the cell and interact with small hydrophobic signaling molecules that can diffuse across the cell membrane. These interactions trigger a cascade of events leading to a cellular response.

What are G protein-coupled receptors (GPCRs) and how do they function?

G protein-coupled receptors (GPCRs) are a type of cell surface receptor that play a crucial role in signal transduction. They are embedded in the cell membrane and interact with extracellular ligands. When a ligand binds to a GPCR, it causes a conformational change in the receptor, which then activates an associated G protein. The activated G protein can then interact with other intracellular signaling molecules, leading to a cascade of cellular responses. GPCRs are involved in various physiological processes and are a major target for pharmaceutical drugs.

How do receptor tyrosine kinases (RTKs) work?

Receptor tyrosine kinases (RTKs) are a type of cell surface receptor that have intrinsic enzymatic activity. When a ligand binds to an RTK, it causes dimerization and autophosphorylation of the receptor. This phosphorylation activates the kinase domain, which then phosphorylates specific tyrosine residues on the receptor itself and on downstream signaling proteins. These phosphorylated proteins can then initiate various signaling pathways that regulate cell growth, differentiation, and metabolism. RTKs are critical in many cellular processes and are often implicated in cancer when mutated or overexpressed.

What are ligand-gated ion channels and their role in cellular signaling?

Ligand-gated ion channels are a type of cell surface receptor that open or close in response to the binding of a specific ligand. These channels are embedded in the cell membrane and allow ions to pass through when activated. When a ligand binds to the receptor, it induces a conformational change that opens the channel, permitting ions such as Na⁺, K⁺, Ca²⁺, or Cl^- to flow across the membrane. This ion flow can generate electrical signals or trigger other cellular responses. Ligand-gated ion channels are essential for processes like neurotransmission and muscle contraction.

What are intracellular receptors and how do they function?

Intracellular receptors are located within the cell, typically in the cytoplasm or nucleus. They interact with small hydrophobic signaling molecules, such as steroid hormones, that can diffuse across the cell membrane. When a ligand binds to an intracellular receptor, it causes a conformational change that allows the receptor-ligand complex to enter the nucleus and bind to specific DNA sequences. This binding can regulate the transcription of target genes, leading to changes in protein synthesis and cellular function. Intracellular receptors play a key role in regulating gene expression and cellular responses to hormones.