AP Biology

## Learning Outcomes

- Explain why second messengers are needed in many signal transduction pathways.
- Describe how cAMP is produced from ATP by adenylyl cyclase.
- Predict the effects of altered adenylyl cyclase, G proteins, phosphodiesterase, or protein kinase activity.
- Describe how calcium ions function as intracellular second messengers.
- Explain how second messengers contribute to signal amplification.
- Connect receptor activation, transduction, and cellular response in AP Biology-style questions.

## AP Biology Syllabus

For AP Biology Unit 4, second messengers are part of the broader idea that cells receive, process, and respond to information. You should be able to follow a pathway from ligand binding to receptor activation, through intracellular signaling molecules such as cAMP or calcium ions, to a cellular response. Questions often ask you to predict the effect of a mutation, toxin, inhibitor, or added enzyme on the pathway.

- Hydrophilic ligands usually bind receptors on the plasma membrane.
- Ligand binding changes the receptor’s shape and starts transduction inside the cell.
- Second messengers are small intracellular signaling molecules or ions.
- cAMP is made from ATP by adenylyl cyclase.
- Calcium ions can act as second messengers when their cytosolic concentration changes.
- Protein kinases often relay signals by phosphorylating target proteins.
- Phosphatases and enzymes such as phosphodiesterases help turn pathways off.
- Signal transduction pathways can amplify a small external signal into a large cellular response.

## Test Your Knowledge

Attempt these questions before reading this article. If you find some difficult or cannot remember the answers, look more closely at that area during your revision.

1. Why do hydrophilic ligands usually require receptors on the plasma membrane?
2. What enzyme produces cAMP, and what molecule is used to make it?
3. How would a nonfunctional adenylyl cyclase affect a cAMP-dependent pathway?
4. Why can one ligand molecule produce a large cellular response?
5. How can calcium ions act as second messengers inside a cell?

## Introduction

Signal transduction is how a cell converts an external message into an internal response. A signaling molecule, or ligand, binds to a receptor on or in a target cell. That binding changes the receptor’s shape, starting a chain of intracellular events. The final response might include altered gene expression, secretion of a molecule, opening of ion channels, enzyme activation, cell division, or programmed cell death.

Second messengers are common in pathways where the original ligand cannot enter the cell. For example, many hydrophilic signaling molecules cannot pass through the hydrophobic interior of the phospholipid bilayer. Instead, they bind a plasma membrane receptor. The receptor then triggers production or release of small internal signaling molecules, such as cyclic AMP or calcium ions, that carry the message through the cytoplasm.

> **Key Term: second messenger**
> A small intracellular molecule or ion that relays a signal from an activated receptor to target molecules inside the cell.

### Why second messengers are used in signal transduction

Many signaling pathways can be divided into three broad stages: reception, transduction, and response. During reception, a ligand binds to a specific receptor. During transduction, intracellular molecules relay and process the message. During response, the cell changes its activity in a specific way.

Second messengers are mainly involved in the transduction stage. They are useful because they are small, can move quickly through the cell, and can be produced or removed rapidly. This allows a cell to respond quickly when a ligand appears and stop responding when the signal is no longer present.

A key AP Biology idea is receptor specificity. A cell only responds to a ligand if it has the correct receptor. If a cell lacks the receptor, the ligand may be present in the environment, but the cell will not detect it. If the receptor is mutated so that the ligand no longer binds, the pathway may fail before second messengers are even produced.

> **Key Term: receptor**
> A protein that binds a specific ligand and starts a cellular response, often by changing shape after ligand binding.

Plasma membrane receptors are especially important for hydrophilic ligands. These ligands cannot easily cross the membrane, so the receptor must transmit the signal inward without the ligand entering the cell. Common membrane receptor types include ligand-gated ion channels, enzyme-linked receptors, and G protein-coupled receptors.

> **Key Term: G protein-coupled receptor**
> A plasma membrane receptor that activates an intracellular G protein after ligand binding, often leading to second messenger production.

In a typical G protein pathway, a ligand binds to the receptor outside the cell. The receptor changes shape and activates a G protein inside the cell. Many G proteins act like molecular switches. In the inactive state, the G protein is associated with GDP. Activation usually involves exchange of GDP for GTP. The activated G protein can then regulate another protein, such as an enzyme that makes a second messenger.

> **Test Tip:**
> In AP Biology questions, do not simply say “the cell responds.” Trace the pathway: ligand binds receptor, receptor changes shape, transduction molecules are activated, second messenger levels change, and a cellular response occurs.

### cAMP as a second messenger

Cyclic AMP, usually written as cAMP, is one of the most common second messengers in AP Biology examples. It is made from ATP by the enzyme adenylyl cyclase. Adenylyl cyclase is often activated downstream of a G protein-coupled receptor.

> **Key Term: cyclic AMP (cAMP)**
> A second messenger made from ATP that relays signals inside the cell, often by activating protein kinases.
>
> **Key Term: adenylyl cyclase**
> An enzyme that converts ATP into cyclic AMP during some signal transduction pathways.

A simplified cAMP pathway looks like this:

1. A ligand binds to a G protein-coupled receptor on the plasma membrane.
2. The receptor changes shape.
3. A G protein is activated.
4. The activated G protein stimulates adenylyl cyclase.
5. Adenylyl cyclase converts ATP into cAMP.
6. cAMP activates one or more target proteins, often protein kinases.
7. Activated proteins produce the cellular response.

The formation of cAMP can be summarized as:

$$
ATP \rightarrow cAMP + PP_i
$$

In many textbook pathways, cAMP activates protein kinase A, often abbreviated PKA. Protein kinases transfer phosphate groups to target proteins. Phosphorylation can activate or inactivate a protein depending on the protein and pathway. The AP Biology exam usually cares more about the pattern than the name of every kinase: a second messenger activates a kinase, and the kinase phosphorylates target proteins, leading to a response.

> **Key Term: protein kinase**
> An enzyme that transfers a phosphate group to a target molecule, often changing that molecule’s activity.

One ligand molecule can lead to the production of many cAMP molecules. Each cAMP molecule can help activate kinase molecules, and each kinase can phosphorylate many targets. This creates signal amplification.

> **Key Term: signal amplification**
> An increase in signal strength during transduction, where one activated molecule leads to activation of many molecules downstream.

Signal amplification explains how a very small amount of hormone can produce a large cellular response. For example, hormones such as epinephrine and glucagon are associated with energy mobilization. They can trigger pathways that help cells access stored energy molecules. You do not need to memorize every step of these hormone pathways for AP Biology, but you should understand that a ligand outside the cell can cause major internal changes through second messengers and enzyme cascades.

> **Exam Warning:**
> cAMP is not usually the original ligand. In most AP Biology pathway questions, cAMP is produced inside the cell after receptor activation. The ligand binds the receptor; cAMP relays the message internally.

cAMP pathways are common targets for experimental questions. Suppose a mutation makes adenylyl cyclase ineffective. If the pathway depends on cAMP, then little or no cAMP will be produced. A cAMP-dependent protein kinase will not be activated properly, so the final response will be reduced or absent. This is a stronger answer than simply saying “signaling stops” because it identifies the specific affected step.

Another common experimental setup involves a molecule that interferes with G protein activation. If the G protein remains inactive, adenylyl cyclase may not be stimulated. As a result, cAMP production decreases, and downstream kinase activity decreases. In a graph, you might see low cAMP concentration or a reduced cellular response even when the ligand is present.

### Turning off cAMP signals

Cells must be able to stop signaling as well as start it. If a pathway stayed active after the ligand disappeared, the cell would waste energy and might make harmful responses. Several mechanisms help terminate or reduce signaling.

One major mechanism is breakdown of the second messenger. In cAMP pathways, cAMP-dependent phosphodiesterase breaks down cAMP. If cAMP is removed, cAMP-dependent kinases are less likely to remain active.

> **Key Term: phosphodiesterase**
> An enzyme that breaks down cyclic nucleotides such as cAMP, helping terminate second messenger signaling.

If phosphodiesterase is added to a cell or becomes more active, cAMP concentration will usually decrease. In a pathway where cAMP activates an inactive protein kinase, adding phosphodiesterase would most directly affect the step in which cAMP activates the kinase. The receptor may still bind ligand, and adenylyl cyclase may still make cAMP, but cAMP will be degraded more quickly.

Cells also use protein phosphatases to reverse phosphorylation. If a kinase adds phosphate groups to target proteins, a phosphatase can remove them. This helps return the cell to its original state.

> **Key Term: protein phosphatase**
> An enzyme that removes phosphate groups from target molecules, often reversing the effect of protein kinases.

In AP Biology, distinguish between kinases and phosphatases:

- Kinases add phosphate groups.
- Phosphatases remove phosphate groups.
- Adenylyl cyclase makes cAMP.
- Phosphodiesterase breaks down cAMP.

> **Test Tip:**
> When a question asks which step is affected by an inhibitor, identify the direct target first. For example, phosphodiesterase directly lowers cAMP, so the most immediate effect is on cAMP-dependent steps, such as activation of a cAMP-dependent protein kinase.

Signal termination is also relevant to disease. Some toxins, drugs, or mutations can interfere with normal signaling. Cholera toxin is a classic example of disruption in a G protein-related pathway. The result is abnormal signaling in intestinal cells, which can contribute to severe water loss and dehydration. For AP Biology, the exam focus is usually not on memorizing every molecular detail of the toxin, but on explaining how altered signaling can change cell function.

### Calcium ions as second messengers

Calcium ions, written as Ca²⁺, are another major second messenger. Cells maintain a low calcium ion concentration in the cytosol compared with the extracellular fluid and some internal stores. Because cytosolic calcium is normally kept low, a sudden increase in cytosolic Ca²⁺ can act as a clear intracellular signal.

> **Key Term: calcium ion (Ca²⁺)**
> A positively charged ion that can function as a second messenger when its concentration changes inside the cytosol.

Calcium signaling often depends on opening calcium channels. Calcium may enter the cytosol from outside the cell through plasma membrane channels, or it may be released from internal stores such as the endoplasmic reticulum. Once cytosolic Ca²⁺ rises, calcium can bind to target proteins and alter their activity.

One common calcium-binding protein is calmodulin. When Ca²⁺ binds calmodulin, the calcium-calmodulin complex can activate other proteins, including protein kinases. This gives calcium pathways the same general logic as cAMP pathways: a small intracellular signal activates proteins that produce a larger response.

> **Key Term: calmodulin**
> A calcium-binding protein that changes shape when it binds Ca²⁺ and can regulate other proteins inside the cell.

Calcium ion signaling is used in many cell types. In muscle cells, calcium helps control contraction. In neurons and endocrine cells, calcium can be involved in secretion. In many cells, calcium-dependent pathways affect enzyme activity, gene expression, or cytoskeletal changes.

For exam purposes, the most useful idea is that calcium is not just a structural ion. It can carry information. A change in cytosolic Ca²⁺ concentration can tell the cell that a receptor has been activated or that a membrane event has occurred.

> **Exam Warning:**
> Do not assume all ions are second messengers in every context. Calcium ions act as second messengers when a pathway uses a change in Ca²⁺ concentration to relay information inside the cell.

Calcium pathways can interact with other second messenger systems. Some receptor pathways produce molecules that cause calcium release from the endoplasmic reticulum. For example, inositol trisphosphate, often abbreviated IP₃, can trigger Ca²⁺ release in many cells. AP Biology may mention other second messengers such as cyclic GMP, calcium ions, or IP₃, but cAMP and calcium are the most likely ones in this subtopic.

### Predicting pathway changes in AP Biology questions

AP Biology questions often give you a diagram of a pathway and ask you to predict what happens when one component is changed. The best approach is to move step by step from the point of disruption to the final response.

Consider this pathway:

Ligand → receptor → G protein → adenylyl cyclase → cAMP → protein kinase → cellular response

If the receptor is missing, the ligand cannot trigger the pathway. The G protein is not activated, adenylyl cyclase is not stimulated, cAMP is not produced at normal levels, and the response decreases or disappears.

If the G protein cannot exchange GDP for GTP, it may remain inactive. Even if the ligand binds the receptor, the signal may not reach adenylyl cyclase. cAMP production will be low, so the kinase will not be activated normally.

If adenylyl cyclase is nonfunctional, ATP will not be converted into cAMP at the expected rate. The steps before adenylyl cyclase may still occur, but downstream cAMP-dependent events will be reduced.

If phosphodiesterase activity increases, cAMP will be broken down more quickly. The pathway may turn off sooner or fail to reach enough cAMP to activate the downstream kinase strongly.

If a protein kinase is inactive, cAMP might still be produced, but phosphorylation of target proteins will not occur normally. This distinction matters: cAMP concentration could be high, yet the response could still be low because the downstream kinase cannot act.

> **Test Tip:**
> For prediction-and-justification free-response questions, write both the predicted result and the mechanism. A strong answer says, “cAMP concentration will decrease because adenylyl cyclase cannot convert ATP into cAMP,” rather than only “the pathway will not work.”

The same logic applies to calcium pathways. If calcium channels fail to open, cytosolic Ca²⁺ may not rise. If Ca²⁺ does not rise, calcium-binding proteins such as calmodulin may not activate their targets. If a calcium pump is overactive, Ca²⁺ may be removed from the cytosol too quickly, shortening the signal.

Signal pathways can also affect the cell cycle. Growth factors can bind plasma membrane receptors and trigger signal transduction pathways that promote cell division. If signaling pathways are overactive, cells may divide when they should not. If pathways fail, cells may not divide when growth or repair is needed. This connection helps explain why mutations in signaling proteins can contribute to cancer, especially when they alter normal control of growth and division.

Second messengers therefore sit at a central position in cell communication. They do not usually receive the external signal directly, and they are not usually the final response. Instead, they link receptor activation to internal changes. Understanding that middle part of the pathway is essential for interpreting AP Biology diagrams, experimental data, and free-response prompts.

## Key Point Checklist

This article has covered the following key knowledge points:

- Signal transduction usually includes reception, transduction, and response.
- Hydrophilic ligands commonly bind plasma membrane receptors because they cannot easily cross the phospholipid bilayer.
- Second messengers are small intracellular molecules or ions that relay signals inside the cell.
- G protein-coupled receptors can activate G proteins, which may regulate second messenger-producing enzymes.
- Adenylyl cyclase converts ATP into cAMP.
- cAMP can activate protein kinases, leading to phosphorylation of target proteins.
- Protein kinases add phosphate groups, while protein phosphatases remove phosphate groups.
- Phosphodiesterase breaks down cAMP and helps terminate cAMP signaling.
- Calcium ions can act as second messengers when cytosolic Ca²⁺ concentration changes.
- Calcium can bind proteins such as calmodulin to regulate other proteins.
- Signal amplification allows a small number of ligand molecules to produce a large response.
- AP Biology questions often ask you to predict how mutations, toxins, or inhibitors affect specific pathway steps.

## Key Terms and Concepts

- second messenger
- receptor
- G protein-coupled receptor
- cyclic AMP (cAMP)
- adenylyl cyclase
- protein kinase
- signal amplification
- phosphodiesterase
- protein phosphatase
- calcium ion (Ca²⁺)
- calmodulin


Signal transduction pathways - Second messengers (cAMP, calcium)