This section of the AP Biology curriculum focuses on the many different ways that cells communicate. We’ll start by taking a look at the process of signal transduction. This process starts when a receptor protein is activated by a ligand or physical signal, causing it to catalyze a reaction on the inside of the cell membrane. A cascade of reactions then takes place as signal transduction leads from the initial receptor protein to a cellular response. After we look at this process, we’ll take a look at how different forms of cellular communication are classified by the distance the signal must travel. We’ll see how cell-to-cell contact is responsible for many immune system reactions. Then, we’ll take a look at some paracrine signals – like neurotransmitters – that only affect nearby cells. Finally, we’ll see how the endocrine system sends endocrine signals throughout the body to coordinate an organism-wide response to a stimulus!

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Resources for this Standard

• Overview & Video Tutorial (This article)
• Quick Test Prep
• Crossword Puzzle

• PowerPoint

ENDURING UNDERSTANDING
IST-3
Cells communicate by generating, transmitting, receiving, and responding to chemical signals.

LEARNING OBJECTIVE
IST-3.A
Cells communicate with one another through direct contact with other cells or from a distance via chemical signaling
IST-3.B
Explain how cells communicate with one another over short and long distances.

ESSENTIAL KNOWLEDGE
IST-3.A.1
Cells communicate with one another through direct contact with other cells or from a distance via chemical signaling —

1. Cells communicate by cell-to-cell contact

IST-3.B.1
Cells communicate over short distances by using local regulators that target cells in the vicinity of the signal-emitting cell —

1. Signals released by one cell type can travel long distances to target cells of another type

4.1 Cell Communication Overview

Our cell phones can communicate through touch and NFC protocols that allow us to “tap-and-pay.” They can conduct medium-range communication through Bluetooth signals and WiFi networks that allow us to use headphones and connect to the internet. Plus, they can connect to a cell tower miles away to transmit a video call to someone on the other side of the world. This is all pretty amazing. But, did you know that cells have been communicating through very similar mechanisms for billions of years??

There are many different ways that cells can communicate with one another. Some cells communicate with their neighbors through contact. Others use short-range signals to communicate with nearby cells. Some cells are even responsible for creating hormone molecules that travel throughout the entire body and affect cells in many different tissues and organs. This information will definitely be incorporated into the AP test. So, stick with us as we cover the basics of cell communication!

Let’s start by taking a look at how cell signaling actually works. The process of a signal activating a certain response in another cell is called signal transduction. Though we will cover signal transduction pathways in much more detail over the next few sections, there is a basic structure that most signal transduction pathways use.

Most signals start when a protein on the surface of the cell receives a signal. The signal could be a chemical, or it could be a physical stimulus like touch, light, or sound waves. Each receptor protein has evolved to receive a very specific signal, known as a ligand if it is a chemical.

Once the receptor protein receives the signal, it catalyzes a reaction on the inside of the cell. The product of this reaction starts a cascade of other reactions, a process known as signal transduction. This chain reaction eventually leads to a cellular response. Responses can vary greatly, depending on the cell signal received.

For instance, an immune cell may initiate phagocytosis when receptors on the cell surface notify the immune cell that it is in contact with an invasive bacterial cell. Or, the insulin receptors on a muscle cell could trigger a cascade of reactions upon receiving insulin that cause the cell to send glucose importers to the cell membrane and import as much glucose as possible.

Since there are literally millions of different specific reactions that cell communication can illicit, the easiest way to classify cell communication is to break it down by the distance the signal has to travel. Some signals can only be sent when two cells are literally touching. Most often, this involves surface proteins on two cells binding with one another. Other times, this includes cells passing chemical messengers through pores like the plasmodesmata in plants that connect two or more cells together.

While touch does account for some cell signaling, the large majority of cell signals are sent via chemical messenger. Chemical messages that affect the same cell they originated in are known as autocrine signals. Local regulator molecules like neurotransmitters are known as paracrine signals because they only affect nearby cells. Finally, endocrine signaling covers the most distance of all the chemical signaling methods. Glands that are part of the endocrine system send hormones and other signaling molecules through the bloodstream to affect target cells across the body!

Think about this… the cells in your body need to communicate for the same reason the ants in a colony need to communicate. Each ant needs to know when to collect food, when they need to engage in defense, and when they can rest. As we continue into the many methods of cell communication, try to remember that the ultimate reason cells communicate is to coordinate their actions in support of the organism as a whole!

Cell-to-cell contact is used by a large number of cells in order to send and receive some of the most important signals that cells within organisms need to transmit. Essentially, this form of cell communication is like a handshake. While there are many, many examples of this, we’re going to cover a few of the most important cellular signals that travel via cell-to-cell contact.

First, consider what happens as normal cells are growing in your body. As these cells hit other cells around them, proteins on the cell’s surface are activated and tell the cell to stop growing. This is how healthy tissues have such an orderly and standardized arrangement. These signal transduction pathways are broken in many cancer cells, which is why they tend to continue cell division in a disorderly fashion – growing into misshapen tumors and invading other parts of your body!

Now, consider one of the most important cell communication pathways in your body – the pathway that allows your immune cell to recognize and kill invading pathogens like viruses and bacteria. Almost all parts of your immune system – from the white blood cells that catch and kill invading bacteria to the cells that build and release antibodies to fight infections – rely on cell-to-cell contact signal transduction pathways to carry out their work.

While it is not necessary to memorize the entire immune system here, it is important to realize that different cells can react to encountering another cell in different ways. For example, helper T-cells learn what the antigen of an invading virus or bacteria looks like when it is trained by a phagocytic antigen-presenting cell. Then, through another cell-to-cell contact, helper T-cell can “train” B-cells, which go on to produce the antibodies an organism needs to fight off an infection!

There are many different examples of local regulator molecules that affect only the cells within a close proximity. Sometimes, these are referred to as paracrine signals. The most popular paracrine molecules used in your body are neurotransmitters. Neurotransmitters are molecules that allow a nerve signal to pass between nerve cells.

This complex process starts when the nerve impulse traveling down the first neuron reaches a voltage-gated ion channel. This allows calcium ions to flood the sending neuron. This influx of calcium triggers synaptic vesicles to merge with the cell membrane, causing the release of neurotransmitters into the synaptic space. These neurotransmitters bind to ligand-gated channels, causing an influx of sodium ions into the receiving neuron. This rapid influx of ions depolarizes the membrane, causing a cascade of ion channels to open down the length of the neuron that propagate the signal.

However, this is not the only good example of local regulator molecules. In fact, a really interesting form of local regulator communication comes from the world of bacterial cells. Bacterial cells are more likely to survive when they form biofilms with other bacterial cells. But, biofilms can only form when bacterial cells are spaced out just right. This means that the bacterial cells have to stop dividing and have to work together to excrete substances to create a stable biofilm that will protect them. So, as they grow, each bacterium secretes a small amount of a local regulator molecule. When the bacterial cells reach a certain density, the concentration of local regulator molecules causes the bacterial cells to switch their metabolism slightly to grow more slowly and secrete biofilm molecules!

The last category of cell communication happens over long distances. Within an organism, these are typically called endocrine signals, and they are controlled and released by the endocrine system. The endocrine system is incredibly complex because it is constantly coordinating the activities of your body by releasing a wide variety of hormones from different organs. These hormones circulate through your bloodstream and bind to receptor proteins on many target cells in different tissues and organs throughout the body.

For example, let’s look at the complex role that the pancreas plays in regulating the body’s blood glucose levels. When blood glucose levels are high, glucose importers move glucose into beta cells within the pancreas. This activates a signal transduction pathway that causes insulin to be excreted from the beta cells. The insulin travels through the blood, where it binds to insulin receptor proteins on cells across the body. When insulin binds to liver cells, it transduces a signal that causes the liver to store glucose as glycogen. When insulin binds to receptors on other tissues throughout your body, the receptors start a series of reactions that cause vesicles loaded with glucose-importers to bind to the cell membrane. This causes loads of glucose to be imported into each cell from the bloodstream.

Then, when your glucose levels start to drop in your bloodstream, your pancreas secretes glucagon. This hormone is specific to receptors on liver cells and directs the liver to convert glycogen back into glucose. This helps regulate your blood glucose levels until you eat your next meal.

Keep in mind that all of these complex forms of cell communication between multiple organ systems and tissues are required just to maintain your blood glucose levels in a specific range. Your endocrine system is constantly secreting hormones for many different purposes that cause a huge variety of reactions in different cell types around your body.