1.6.1 Pathway of A Signal From Outside To Inside

Introduction

Cells receive signals from their environment and from other cells. These signals must be transmitted from outside the cell to the inside, where they can trigger a response. The process by which a signal on a cell's surface is converted into a specific cellular response is a series of steps called a signal transduction pathway.

• Protein Signaling (for water-soluble ligands)
• Steroid Signaling (for lipid-soluble ligands)

Definition

Signal Transduction Pathway: The process by which a signal on a cell's surface is converted into a specific cellular response is a series of steps called a signal transduction pathway.

In simple words: A signal transduction pathway is the route that a signal takes from outside the cell to inside the cell, where it causes a response.

Core Concept

• Protein and peptide hormones are water-soluble. They cannot pass through the plasma membrane.
• These hormones bind to receptors on the plasma membrane of the target cell.
• This starts a series of events in the cell that generates a second messenger.
• Steroid hormones are lipid-soluble (lipophilic). They can freely diffuse across the plasma membrane.
• They bind to receptors inside the cell (in the cytoplasm or nucleus).

Detailed Explanation

Protein Signaling

Protein and peptide hormones are water-soluble. So they cannot pass through the plasma membrane of the cell.

Steps in Protein Signaling:

• These hormones or environmental stimuli are the first messenger.
• They bind with their receptors on the plasma membrane of the target cell.

2. This starts a series of events in the cell. - The binding of the first messenger to the receptor activates a series of relay molecules.

3. A second messenger is generated. - The series of events generates a second messenger, e.g., cAMP (Cyclic adenosine monophosphate).

• These changes include:
• Activation of enzymes
• Gene activation
• Other cellular responses

• The first messenger (hormone) cannot enter the cell.
• It binds to a receptor on the cell surface.
• This triggers the production of a second messenger inside the cell.
• The second messenger causes the cellular response.

Example: - cAMP (Cyclic adenosine monophosphate) is a common second messenger.

Steroid Signaling

Small hydrophobic ligands can directly diffuse through the plasma membrane and interact with internal receptors.

• Important members of this class of ligands are the steroid hormones.
• Steroids are lipids that have a hydrocarbon skeleton with four fused rings.

Steroid hormones are lipophilic (fat-loving). It means they can freely diffuse across the plasma membrane of a cell.

Steps in Steroid Signaling:

1. The steroid hormone diffuses through the plasma membrane. - Because it is lipid-soluble, it can pass through the phospholipid bilayer.

2. It binds to receptors in either the cytoplasm or nucleus of the target cell. - The receptor may be in the cytoplasm or already in the nucleus.

3. An active receptor-hormone complex is formed. - The binding of the hormone to the receptor activates the receptor.

4. The activated complex moves into the nucleus. - The receptor-hormone complex moves into the nucleus.

5. It binds directly to DNA. - The complex binds to specific sites on the DNA.

• This means it turns specific genes on or off.
• This leads to the production of specific proteins.
• This is the cellular response.

• The steroid hormone enters the cell directly.
• It binds to a receptor inside the cell.
• The hormone-receptor complex enters the nucleus.
• It binds to DNA and controls gene expression.
• This leads to a cellular response.

• Estrogen
• Progesterone
• Testosterone

Intermediate Level

Comparison of Protein Signaling and Steroid Signaling

Protein Signaling in Detail

• They are water-soluble.
• The plasma membrane is made of a phospholipid bilayer, which is hydrophobic in the interior.
• Water-soluble molecules cannot pass through the hydrophobic interior.

• The hormone binds to a receptor on the cell surface.
• The receptor changes shape and activates a series of relay molecules.
• These relay molecules generate a second messenger (e.g., cAMP).
• The second messenger triggers the cellular response.

• cAMP stands for Cyclic adenosine monophosphate.
• It is a common second messenger.
• It is made from ATP by the enzyme adenylyl cyclase.
• It activates other enzymes (e.g., protein kinase A).
• These enzymes then cause the cellular response.

Steroid Signaling in Detail

• They are lipid-soluble.
• The plasma membrane is made of a phospholipid bilayer.
• Lipid-soluble molecules can pass through the hydrophobic interior.

• The steroid hormone binds to a receptor in the cytoplasm or nucleus.
• The receptor-hormone complex moves into the nucleus.
• It binds to specific DNA sequences.
• It acts as a transcription factor.
• This turns specific genes on or off.
• This leads to the production of specific proteins.
• This is the cellular response.

• Because it involves gene expression.
• Gene expression takes time (transcription and translation).
• But the effects last longer because the proteins produced remain in the cell for some time.

Important Points

• 1. Signal transduction pathway is the process by which a signal on a cell's surface is converted into a specific cellular response.
• 2. Protein and peptide hormones are water-soluble.
• 3. They cannot pass through the plasma membrane.
• 4. These hormones bind to receptors on the plasma membrane of the target cell.
• 5. They are the first messenger.
• 6. The binding starts a series of events in the cell.
• 7. This generates a second messenger, e.g., cAMP (Cyclic adenosine monophosphate).
• 8. The second messenger triggers various changes in the cell, including:
• - Activation of enzymes
• - Gene activation
• - Other cellular responses
• 9. Steroid hormones are lipid-soluble (lipophilic).
• 10. They can freely diffuse across the plasma membrane.
• 11. They bind to receptors in either the cytoplasm or nucleus of the target cell.
• 12. This forms an active receptor-hormone complex.
• 13. The activated complex moves into the nucleus.
• 14. It binds directly to DNA.
• 15. It acts as a transcription factor for gene expression.
• 16. Examples of steroid hormones: Estrogen, progesterone, testosterone.
• 17. Steroids are lipids that have a hydrocarbon skeleton with four fused rings.

Remember

• - Protein signaling = water-soluble hormone → binds to cell surface receptor → second messenger → cellular response
• - Steroid signaling = lipid-soluble hormone → diffuses into cell → binds to internal receptor → hormone-receptor complex → binds to DNA → gene expression → cellular response
• - First messenger = the hormone
• - Second messenger = cAMP (in protein signaling)
• - Protein signaling = fast, short-lived
• - Steroid signaling = slow, long-lasting
• - Examples of steroid hormones: Estrogen, progesterone, testosterone

Common Mistakes

• 1. Thinking protein hormones can enter the cell – Protein hormones are water-soluble and cannot pass through the plasma membrane. They bind to receptors on the cell surface.
• 2. Thinking steroid hormones bind to cell surface receptors – Steroid hormones are lipid-soluble and bind to receptors inside the cell (cytoplasm or nucleus).
• 3. Confusing first and second messenger – The first messenger is the hormone itself. The second messenger is the molecule generated inside the cell (e.g., cAMP).
• 4. Forgetting that steroid signaling directly affects gene expression – Steroid hormones act as transcription factors and directly affect gene expression.
• 5. Thinking the response is always fast – Protein signaling is fast; steroid signaling is slow.
• 6. Forgetting the examples of steroid hormones – Estrogen, progesterone, testosterone.
• 7. Thinking cAMP is the only second messenger – cAMP is a common example, but there are other second messengers (e.g., IP₃, DAG, calcium ions).
• 8. Confusing the location of receptors – Protein hormone receptors are on the plasma membrane; steroid hormone receptors are in the cytoplasm or nucleus.
• 9. Thinking the steroid-receptor complex stays in the cytoplasm – It moves into the nucleus and binds to DNA.
• 10. Forgetting that steroids have four fused rings – The steroid nucleus consists of 17 carbon atoms arranged in four attached rings.

Quick Revision

• Signal transduction pathway is the process by which a signal on a cell's surface is converted into a specific cellular response.
• Protein Signaling:
• Protein and peptide hormones are water-soluble.
• They cannot pass through the plasma membrane.
• They bind to receptors on the plasma membrane of the target cell.
• They are the first messenger.
• This starts a series of events in the cell.
• This generates a second messenger, e.g., cAMP (Cyclic adenosine monophosphate).
• The second messenger triggers various changes in the cell, including activation of enzymes, gene activation, etc.
• Steroid Signaling:
• Small hydrophobic ligands can directly diffuse through the plasma membrane and interact with internal receptors.
• Important members of this class of ligands are the steroid hormones.
• Steroids are lipids that have a hydrocarbon skeleton with four fused rings.
• Steroid hormones are lipophilic (fat-loving). They can freely diffuse across the plasma membrane of a cell.
• They bind to receptors in either the cytoplasm or nucleus of the target cell, forming an active receptor-hormone complex.
• This activated complex moves into the nucleus and binds directly to DNA, acting as a transcription factor for gene expression.
• Examples: Estrogen, progesterone, testosterone.

60-Second Revision

• Protein signaling = water-soluble hormone → cell surface receptor → second messenger (cAMP) → cellular response
• Steroid signaling = lipid-soluble hormone → diffuses into cell → internal receptor → hormone-receptor complex → binds to DNA → gene expression → cellular response
• First messenger = hormone
• Second messenger = cAMP
• Protein signaling = fast, short-lived
• Steroid signaling = slow, long-lasting
• Steroid hormones: Estrogen, progesterone, testosterone
• Steroids = four fused rings
• Warning: Protein hormones cannot enter the cell. Steroid hormones bind to receptors inside the cell, not on the surface. Do not confuse first and second messengers.