Each cell might only respond to only a few hormones. Cells only respond to a hormone if they have the right receptor, which is like a molecular antenna tuned to that specific hormone. The surfaces of cells are studded with thousands of these receptors. Each one receives a signal from just one type of signal molecule. Once they bind to the signal molecule, they pass the signal into the cell to trigger a specific response.

Why this is important… The body regulates cells by sending out chemical signals, whether that is from endocrine glands, nerves or other cells. Drugs can simulate that process as well. That’s how pharmaceuticals help adjust blood pressure, metabolic rate, water balance, or other body functions.

Foundations

Every cell in the body sends and receives chemical signals. The endocrine system has been our focus here, but other organs including the stomach, kidneys and liver, act like endocrine glands as well. The nervous system may send electrical signals, but this only causes the release of a chemical to signal the target cell.

Receiving signals requires the right receptor. This is usually a protein embedded in the cell membrane. Each cell has its own unique mix of receptors on the surface, listening for signals from other cells. But receiving that signal is not enough. The next step is to get the signal into the cell to start a specific action. The response could be to contract a muscle cell, activate an enzyme, add transport proteins to the membrane, or any other cellular process.

We won’t try to understand all the possible responses. We will just explore a general model for how signals are received and transmitted into the cell. You’ll find that many of the processes are already familiar, like enzyme reactions and ATP, but each is used in a new way.

Key terms

Receptor – protein that receives a specific signal molecule and initiates the cell response

Signal transduction – the process of getting a signal from the outside of the cell to the inside

Signal amplification – increasing the strength of a signal by increasing the number of signal molecules

Details

Most signal molecules are proteins. These signal molecules can be hormones, neurotransmitters, or others. Since proteins cannot pass through the cell membrane, the cell places receptors on the surface. Receptors are also proteins, embedded in the cell membrane like miniature antennas waiting for signals (Figure 9-12).

Each receptor is shaped to receive one particular signal molecule. This shape-matching is similar to enzymes and substrates. But in this case, the signal molecule does not get changed, it just activates the receptor. The model we will look at is important for many hormones and neurotransmitters, and pharmaceutical drugs – the G protein-coupled receptor (Figure 9-12).

Diagram of molecules involved in the G protein-coupled receptor signal pathway. Some proteins are in the membrane while others are in the cytoplasm. These activate an enzyme which then acts on its substrate.

FIGURE 9-12. Cell signaling model using a G protein-coupled receptor. Letters in black refer to stages of the process, which are described in the text. (“Cell signaling using GPCR” by J. Buegge is licensed under CC BY-SA 4.0)

In the model pictured in Figure 9-12, we can see these steps in action. Follow the images in the diagram with the summary of steps below.

Signal molecule binds to receptor at the cell surface
Receptor activates G proteins
Each G protein activates an adenylate cyclase enzyme
Adenylate cyclase converts ATP to cyclic AMP (cAMP)
cAMP is released into the cytoplasm as a second messenger
Each cAMP activates another enzyme – protein kinase A
Protein kinase A, using ATP, adds a phosphate to other proteins (such as the enzyme pictures) which changes their activity level. This is the cell response.

As the signal moves into the cell, it also grows. Getting the signal into the cell (signal transduction) happens by setting off a series of chemical reactions that produce new signal molecules within the cell. In this case, the signal also grows (signal amplification) because enzymes are involved. Remember that enzymes repeatedly convert a substrate to a product, which means that once they are activated each will produce many new signal molecules.

VIDEO 9d: Cell signaling

After watching this video, you should be able to:

explain the role of receptors in the body
describe how signals are passed into the cell and how they are amplified
recognize the roles of G proteins, adenylate cyclase, cAMP and protein kinase

Many of the processes in this diagram are familiar to you already – enzymes facilitating chemical reactions, ATP, phosphates and enzyme regulation.

The use of ATP is unusual in this example. Normally ATP powers processes within the cell. However, we have seen it act as a signal molecule before, such as when it inhibits enzymes involved in glycolysis. This is another case of it acting as a signal molecule. Video 9d explains the transformation of ATP into cAMP. The process is easier to understand when you learn the chemical details, like how ATP and AMP are related.

This is just one model for cell signaling, but there are others. Some signal molecules pass through the membrane to target genes directly. This usually applies to steroid hormones. Since steroid hormones are fat-soluble (or hydrophobic) they are able to pass through the cell membrane and don’t need a receptor on the surface. Instead, they can pass right into the nucleus and affect the promoter of specific genes that need to be expressed more. Examples of steroid hormones include testosterone, progesterone, and cortisol. (Hint: many steroid hormones end in -one or -ol.)

Stop & Think #12: Which signal molecule would take longer to produce a response – one that targets a cell-surface receptor or one that targets gene expression? Explain.

Learning objectives

Describe how protein hormones create a response inside target cells using the G protein-coupled receptor model.

Stop & Think Questions Explained

12. Targeting cell-surface receptors usually produces a quicker response since the proteins we need to work are already made. Gene expression requires both mRNA and a protein to be made, then folded, before it can begin to work. Nonetheless, both are very fast processes for how complex they seem.