# AHL: Cell signaling

> IB Biology HL · Theme C: Interaction and Interdependence
> Source: https://www.owlsprep.com/study/ib-biology-hl-u3-ahl-cell-signaling/

This AHL module covers core cell signaling concepts for IB Biology HL, including GPCR structure and function, signal transduction, phosphorylation cascades, signal amplification, and pathway disruptions that cause disease.

**Prerequisites:** [SL cell communication and membrane structure](https://www.owlsprep.com/study/ib-biology-sl-u2-cell-membranes/)

## Learning objectives

- Classify different types of cell signaling based on distance between cells
- Describe the structure and mechanism of activation of G protein-coupled receptors (GPCRs)
- Explain how phosphorylation cascades transduce and amplify extracellular signals
- Analyze how disruptions to signaling pathways cause disease

## 1. Classification of Cell Signaling

Cell signaling is categorized based on the distance between the signaling cell and its target cell. SL content introduces basic types, while AHL requires deeper understanding of the functional differences between categories.

**Ligand** — A small signaling molecule that binds specifically to a receptor protein on or in a target cell to trigger a response

*Example:* Epinephrine, insulin, growth factors

- - **Local signaling**: Acts on adjacent or nearby cells, includes autocrine (signaling to the same cell) and paracrine (signaling to nearby different cells)
- - **Juxtacrine signaling**: Requires direct physical contact between the signaling and target cell, common in embryonic development
- - **Long-distance signaling**: Almost always endocrine signaling, where hormones are released into the bloodstream to reach distant targets

**Worked example:** A T-immune cell releases a signaling molecule that binds to receptors on its own surface to stimulate its own cell division. Identify this type of signaling and explain its biological purpose.

1. The signaling cell is also the target cell, so this is autocrine signaling, a type of local signaling.
2. The purpose of this autocrine signaling is to amplify the immune response: stimulating T-cell proliferation increases the number of cells available to fight the invading pathogen.

> **Exam tip:** IB exams often give you an example of signaling and ask you to classify it, so always link the distance between cells to the type of signaling.

## 2. G Protein-Coupled Receptors (GPCRs)

GPCRs are the largest family of cell-surface receptors, and are a core AHL requirement for IB Biology HL. All GPCRs share a common structure and activation mechanism.

**G Protein-Coupled Receptor (GPCR)** — A transmembrane receptor that works with a G protein to relay extracellular signals to intracellular signaling pathways

*Notation:* GPCR

*Example:* Epinephrine receptor on liver cells

All GPCRs have seven transmembrane α-helices that span the phospholipid bilayer. The extracellular N-terminus contains the ligand binding site, while the intracellular C-terminus binds the G protein.

**Worked example:** Outline the steps of GPCR activation after epinephrine binds to its receptor on a liver cell.

1. Epinephrine (ligand) binds to the extracellular binding site of the GPCR, causing a conformational change in the receptor structure.
2. The activated GPCR binds to an inactive G protein, which is bound to GDP in its inactive state.
3. GDP is released from the G protein and replaced by GTP, which activates the G protein.
4. The activated G protein dissociates from the receptor and diffuses along the intracellular membrane surface to bind and activate the downstream enzyme adenylyl cyclase.

> **Exam tip:** You may be asked to label or draw a GPCR, so always remember the key feature of seven transmembrane helices.

## 3. Signal Transduction and Amplification

After the receptor activates an intracellular protein, the extracellular signal is transduced through a series of steps to produce a final cellular response. A key advantage of multi-step pathways is signal amplification.

**Phosphorylation Cascade** — A sequential signaling pathway where each activated kinase enzyme phosphorylates and activates multiple downstream kinase enzymes, amplifying the original signal at every step.

Second messengers are small, non-protein water-soluble molecules that diffuse rapidly through the cytoplasm to spread the signal. The most common second messenger is cyclic AMP (cAMP), produced from ATP by adenylyl cyclase.

**Worked example:** Explain how a single epinephrine ligand can produce a very large response in a liver cell.

1. One epinephrine ligand activates one GPCR, which goes on to activate multiple G protein molecules.
2. Each activated G protein activates an adenylyl cyclase enzyme, which produces hundreds of cAMP second messenger molecules.
3. Each cAMP activates a protein kinase A (PKA), which phosphorylates and activates hundreds of phosphorylase kinase molecules.
4. Each phosphorylase kinase activates hundreds of glycogen phosphorylase, which each breaks down thousands of glycogen molecules into glucose.
5. The final result is that one epinephrine molecule leads to production of approximately 10^6 glucose molecules, demonstrating massive signal amplification.

> **tip**
>
> Amplification occurs at every step of the cascade, not just the first step, because each activated enzyme acts on multiple downstream targets.

## 4. Disruption of Cell Signaling

Any change to a component of a signaling pathway can alter or block the cellular response, leading to disease. Common disruptions include mutations that change receptor structure, toxins that modify G protein function, and overproduction of growth factor ligands.

**Worked example:** Explain how cholera toxin produced by *Vibrio cholerae* disrupts cell signaling to cause disease.

1. Cholera toxin chemically modifies the G protein that regulates adenylyl cyclase in intestinal epithelial cells.
2. The modification prevents the G protein from hydrolyzing GTP to GDP, leaving it permanently locked in its active state.
3. Permanently active G protein keeps adenylyl cyclase continuously active, leading to extremely high levels of cAMP in the cell.
4. High cAMP triggers excessive secretion of salt and water into the intestinal lumen, causing the severe dehydration and diarrhea that is the hallmark of cholera.

## Common pitfalls

- **Wrong:** Stating that G protein activation occurs when GDP replaces GTP
  - Why it fails: Inactive G proteins are bound to GDP, so activation requires GTP to replace GDP, not the reverse
  - Correct: When the GPCR binds the inactive G protein, GDP is released and GTP binds, activating the G protein
- **Wrong:** Claiming that GPCRs are intracellular receptors
  - Why it fails: Most GPCR ligands are hydrophilic and cannot cross the phospholipid bilayer, so receptors are located on the cell surface
  - Correct: GPCRs are transmembrane cell-surface receptors; only hydrophobic ligands like steroid hormones bind intracellular receptors
- **Wrong:** Claiming signal amplification only occurs at the receptor step
  - Why it fails: Amplification occurs at every step where one activated enzyme activates multiple downstream targets
  - Correct: Signal amplification occurs at each step of a phosphorylation cascade, leading to an exponential increase in response size
- **Wrong:** Classifying endocrine signaling as local signaling
  - Why it fails: Endocrine signaling relies on hormone transport through the bloodstream to reach distant targets, so it is long-distance
  - Correct: Endocrine = long-distance, autocrine/paracrine/juxtacrine = local/direct contact

## Cheatsheet

| Category | Key Information for Exam |
| --- | --- |
| Signaling Types | Autocrine = self, Paracrine = local nearby, Juxtacrine = direct contact, Endocrine = long distance |
| GPCR Structure | 7 transmembrane α-helices, extracellular ligand binding site |
| GPCR Activation | Ligand binds → conformation change → GDP → GTP → G protein activated |
| Signal Amplification | 1 ligand → ~10^6 response via multi-step phosphorylation cascade |
| Cholera Disruption | G protein cannot hydrolyze GTP → permanently active → high cAMP → diarrhea |

## What's next

Mastering AHL cell signaling provides a foundation for understanding many other key IB Biology HL topics, including the role of dysregulated signaling in cancer development, immune system activation and regulation, and hormonal control of homeostasis. Many extended response questions ask you to connect cell signaling concepts to disease mechanisms or drug action, so a solid understanding of this topic will help you earn full marks on these high-weight questions.

- [AHL: Neurobiology](https://www.owlsprep.com/study/ib-biology-hl-u3-ahl-neurobiology/)
- [AHL: Plant hormone interactions](https://www.owlsprep.com/study/ib-biology-hl-u3-ahl-plant-hormone-interactions/)
- [AHL: Ecological niches](https://www.owlsprep.com/study/ib-biology-hl-u3-ahl-ecological-niches/)

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