• Some hormones act inside cells by causing the formation of a second messenger, such as cAMP, inside the cell membrane.
• The cAMP then produces the hormone’s effects inside the cell.
• Therefore, the hormone’s direct action is mainly to activate one specific membrane receptor.
• The second messenger carries out the remaining intracellular effects.
• cAMP is not the only second messenger used by hormones.
• Two other important second messengers are:
  • Calcium ions (Ca²⁺) and calmodulin
  • Products formed from membrane phospholipid breakdown
• In some cases, one hormone can activate more than one second-messenger system in the same target tissue.

KEY CONCEPT

• Hormone → membrane receptor → second messenger → intracellular effects.
• Important second messengers include cAMP, Ca²⁺ with calmodulin, and membrane phospholipid breakdown products.

CONCEPTUAL EXAMPLES

• cAMP: Hormone activates its membrane receptor → cAMP is formed → cAMP produces intracellular effects.
• Ca²⁺/calmodulin: A hormone can use calcium ions and calmodulin as a second-messenger system.
• Multiple systems: One hormone may activate more than one second-messenger system in the same target tissue.

Adenylyl Cyclase–cAMP Second Messenger System

• Table 75.3 lists some hormones that use the adenylyl cyclase–cAMP mechanism, while Fig. 75.7 shows how this system works.

• When a hormone binds to its receptor, the receptor connects with a G protein.
• If this G protein stimulates adenylyl cyclase, it is called a Gs protein (stimulatory G protein).
• Gs activates adenylyl cyclase, a membrane-bound enzyme.
• Adenylyl cyclase converts a small amount of cytoplasmic ATP into cAMP inside the cell.
• cAMP activates cAMP-dependent protein kinase, which adds phosphate groups to specific cell proteins.
• These protein changes start biochemical reactions that produce the cell’s response to the hormone.
• Once cAMP is formed, it usually starts an enzyme cascade.
• One enzyme activates another, which activates another, and so on.
• This creates amplification, so a very small amount of hormone can produce a powerful response throughout the cell.
• If the hormone receptor is connected to an inhibitory G protein (Gi), adenylyl cyclase is inhibited.
• This reduces cAMP formation and usually produces an inhibitory effect in the cell.
• Therefore, Gs increases cAMP, whereas Gi decreases cAMP.
• Changes in cAMP can also change the phosphorylation of important proteins inside the cell.
• The final effect of cAMP depends on the intracellular machinery of the target cell.
• Different cells contain different enzymes, so the same cAMP signal can produce different effects.
• These effects may include:
  • Synthesis of specific intracellular chemicals
  • Muscle contraction or relaxation
  • Secretion by cells
  • Changes in cell permeability
• In thyroid cells, cAMP stimulates formation of thyroxine and triiodothyronine.
• In adrenocortical cells, cAMP causes secretion of adrenocortical steroid hormones.
• In some epithelial cells of the distal and collecting tubules of the kidney, cAMP increases water permeability.

KEY CONCEPT

• Hormone → receptor → G protein → adenylyl cyclase → cAMP → protein kinase → protein phosphorylation → cell response.
• Gs → ↑ adenylyl cyclase → ↑ cAMP → stimulation.
• Gi → ↓ adenylyl cyclase → ↓ cAMP → inhibition.
• Small hormone signal → enzyme cascade → large cellular response.

CONCEPTUAL EXAMPLES

• Gs example: Hormone binds receptor → Gs activates adenylyl cyclase → cAMP increases → cell is stimulated.
• Gi example: Hormone binds receptor → Gi inhibits adenylyl cyclase → cAMP decreases → cell activity is inhibited.
• Thyroid cell: cAMP → formation of thyroxine and triiodothyronine.
• Adrenocortical cell: cAMP → secretion of steroid hormones.
• Kidney epithelial cell: cAMP → increased permeability to water.

Absolutely. Let’s turn this Guyton Figure 75.7 — cAMP second-messenger system into a very simple story.

🧠 The BIG IDEA

Think of a hormone as a person knocking on the outside door of a house.

The hormone cannot enter the cell.

So it gives a message to a receptor on the cell membrane.

The receptor then activates an internal messenger system that carries the message inside the cell.

The whole pathway in ONE line:

Hormone → Receptor → G-protein → Adenylyl cyclase → cAMP → Protein kinase → Protein phosphorylation → Cell response

That is the entire figure. Now let’s understand every part.

1️⃣ Hormone = The Outside Messenger 📩

At the top of the figure:

Extracellular fluid → Hormone → Receptor

The hormone is outside the cell.

Why?

Because many hormones are water-soluble and cannot freely cross the lipid cell membrane.

So they need a receptor on the outside surface of the cell membrane.

👉 Think:

Hormone = WhatsApp message arriving at the cell’s door.

2️⃣ Receptor = The Doorbell 🔔

The green structure crossing the membrane is the hormone receptor.

When the hormone binds to it:

Hormone + receptor → receptor changes shape

This starts the signaling process.

Important:

The hormone usually does NOT go inside.

Instead, the receptor communicates the message inward.

3️⃣ The Cell Membrane = The Wall 🧱

The orange/pink structure is the cell membrane.

It separates:

Outside → Extracellular fluid

from

Inside → Cytoplasm

So:

OUTSIDE   ↓Hormone   ↓Receptor──────────── CELL MEMBRANE ────────────   ↓G-protein   ↓Adenylyl cyclase   ↓cAMP   ↓Protein kinase   ↓Cell responseINSIDE

4️⃣ G-Protein = The Middleman 🚶

Look near the adenylyl cyclase:

α, β, γ

These are the three components of a G-protein.

The important one here is mainly the α-subunit.

Normally:

α has GDP

When the hormone activates the receptor:

GDP is replaced by GTP

So:

Inactive G-protein → Active G-protein

Easy memory:

GDP = OFF

GTP = ON

5️⃣ What does the α-subunit do?

The activated α-subunit moves toward adenylyl cyclase.

Think:

Receptor says: “Hey G-protein, we’ve received a message!”

G-protein says:

“I’ll tell adenylyl cyclase.”

6️⃣ Adenylyl Cyclase = The Factory 🏭

This yellow structure is adenylyl cyclase.

Its job is extremely important:

It converts:

ATP → cAMP

So:

ATP → cAMP

cAMP stands for:

Cyclic adenosine monophosphate

You don’t need to memorize the entire chemical name initially.

Just remember:

Adenylyl cyclase makes cAMP.

7️⃣ cAMP = The SECOND Messenger 🏃‍♂️

This is the most important concept in the figure.

The hormone is the:

FIRST messenger

cAMP is the:

SECOND messenger

Why?

Because the hormone delivered the original message.

Then cAMP carries that message inside the cell.

So:

Hormone = First messenger

cAMP = Second messenger

ELI5:

Mom calls you:

“Clean your room!”

Mom = first messenger

You tell your brother:

“We need to clean!”

Your brother = second messenger

😂

8️⃣ Why does the cell need a second messenger?

Because the hormone often cannot enter the cell.

Therefore:

Hormone stays outside

but

cAMP carries the signal inside

This is why the system is called a:

Second-messenger system

9️⃣ cAMP Activates Protein Kinase

Now look at the middle-bottom part of the figure.

cAMP acts on:

cAMP-dependent protein kinase

This enzyme is commonly called:

Protein Kinase A (PKA)

So:

cAMP → activates PKA

Very important.

Remember:

cAMP = activates Protein Kinase A

🔟 What does Protein Kinase do?

This is where the cell actually starts changing its activity.

Protein kinase transfers a:

Phosphate (PO₄)

onto another protein.

This process is called:

Phosphorylation

So:

Protein + ATP → Protein–PO₄ + ADP

The protein gets a phosphate group.

1️⃣1️⃣ Why phosphorylate a protein?

Because adding phosphate can change the protein’s activity.

It may:

• activate the protein
• inhibit the protein
• change its function
• alter enzyme activity
• produce a cellular response

Therefore:

Phosphorylation → changes protein activity → cell response

1️⃣2️⃣ The final result = CELL RESPONSE 🎯

At the bottom of the figure:

Protein → Cell’s response

This is the final objective.

The cell may respond by changing things such as:

• enzyme activity
• metabolism
• ion transport
• secretion
• contraction
• other cellular functions

The exact response depends on which hormone, receptor, and intracellular proteins are involved.

🔥 NOW READ THE WHOLE FIGURE LIKE A STORY

Imagine this happening:

Step 1

A hormone arrives outside the cell.

⬇️

Step 2

Hormone binds to its receptor.

⬇️

Step 3

The receptor activates a G-protein.

⬇️

Step 4

The G-protein’s α-subunit binds GTP.

⬇️

Step 5

The α-subunit activates adenylyl cyclase.

⬇️

Step 6

Adenylyl cyclase converts:

ATP → cAMP

⬇️

Step 7

cAMP acts as the second messenger.

⬇️

Step 8

cAMP activates cAMP-dependent protein kinase (PKA).

⬇️

Step 9

PKA phosphorylates proteins.

Protein + ATP → Protein–PO₄ + ADP

⬇️

Step 10

The modified protein produces the:

CELLULAR RESPONSE

🧠 SUPER-SHORT MEMORY CHAIN

Memorize this:

H → R → G → AC → cAMP → PKA → P → Response

Where:

Abbreviation	Meaning
H	Hormone
R	Receptor
G	G-protein
AC	Adenylyl cyclase
cAMP	Second messenger
PKA	Protein kinase A
P	Phosphorylation
Response	Cellular effect

⭐ What is the role of ATP?

The figure shows ATP twice conceptually.

First:

ATP → cAMP

This is done by:

Adenylyl cyclase

Second:

ATP → ADP

This happens when protein kinase transfers phosphate to a protein.

So don’t confuse them.

Adenylyl cyclase:

ATP → cAMP

Protein kinase:

Protein + ATP → Protein–PO₄ + ADP

⚡ Why can a tiny amount of hormone cause a BIG response?

This is a very important conceptual point.

The signal can be amplified.

One hormone-receptor interaction can activate G-proteins.

Those can activate adenylyl cyclase.

Adenylyl cyclase can generate many cAMP molecules.

cAMP can activate protein kinase.

Protein kinase can modify many proteins.

Therefore:

Small outside signal → large intracellular response

Think of it like:

One person → starts a chain reaction → many people become involved.

🎯 The MOST IMPORTANT EXAM POINTS

1. Hormone

Usually remains outside the cell in this pathway.

2. Receptor

Located in the cell membrane.

3. G-protein

Acts as the link between receptor and adenylyl cyclase.

4. GTP

Activates the relevant G-protein α-subunit.

5. Adenylyl cyclase

Converts:

ATP → cAMP

6. cAMP

Is the:

SECOND MESSENGER

7. Protein kinase A

Is activated by:

cAMP

8. Protein kinase

Causes:

Phosphorylation

9. Phosphorylation

Changes protein activity.

10. Final result

Cellular response

🧩 ONE VISUAL STORY

Think of a restaurant:

Hormone = customer placing an order
↓
Receptor = waiter receiving order
↓
G-protein = waiter carrying the message
↓
Adenylyl cyclase = kitchen machine
↓
ATP → cAMP = order ticket produced
↓
cAMP = internal message
↓
Protein kinase A = chef activating workers
↓
Phosphorylation = workers get instructions
↓
Cell response = food gets prepared

🚨 Don’t make these common mistakes

❌ Hormone enters the cell
✅ Hormone binds membrane receptor.

❌ G-protein directly makes the cellular response
✅ G-protein activates the signaling pathway.

❌ cAMP is the hormone
✅ cAMP is the second messenger.

❌ Adenylyl cyclase produces ATP
✅ Adenylyl cyclase uses ATP to produce cAMP.

❌ Protein kinase produces cAMP
✅ Adenylyl cyclase produces cAMP; cAMP activates protein kinase.

❌ Phosphorylation means removing phosphate
✅ Phosphorylation means adding phosphate.

🏆 FINAL 10-SECOND REVISION

Hormone knocks → receptor hears → G-protein switches ON → adenylyl cyclase works → ATP becomes cAMP → cAMP activates PKA → PKA phosphorylates proteins → proteins change → CELL RESPONDS.

That is the entire Guyton figure.

🔑 Golden sentence:

“The hormone gives the message outside; cAMP carries the message inside; protein kinase converts the message into a cellular action.”

Cell Membrane Phospholipid Second Messenger System

• Some hormones activate transmembrane receptors, which activate phospholipase C (PLC) on the inner side of the receptor (Table 75.4).

• PLC breaks down membrane phospholipids, especially phosphatidylinositol bisphosphate (PIP₂).
• PIP₂ is broken down into two second messengers:
  • Inositol triphosphate (IP₃)
  • Diacylglycerol (DAG)
• IP₃ releases Ca²⁺ from mitochondria and the endoplasmic reticulum.
• Ca²⁺ then acts as a second messenger, producing effects such as:
  • Smooth muscle contraction
  • Changes in cell secretion
• DAG activates protein kinase C (PKC).
• PKC phosphorylates many proteins, which produces the cellular response (Fig. 75.8).
• The lipid portion of DAG is arachidonic acid.
• Arachidonic acid is a precursor of prostaglandins and other local hormones.
• These substances produce multiple effects in tissues throughout the body.

KEY CONCEPT

Hormone → transmembrane receptor → PLC → PIP₂ → IP₃ + DAG

• IP₃ → Ca²⁺ release → cellular effects
• DAG → PKC → protein phosphorylation → cellular response
• Arachidonic acid → prostaglandins and other local hormones

CONCEPTUAL EXAMPLES

• Smooth muscle: PIP₂ → IP₃ → Ca²⁺ → smooth muscle contraction.
• Cell secretion: PIP₂ → IP₃ → Ca²⁺ → changes in secretion.
• Protein response: PIP₂ → DAG → PKC → protein phosphorylation → cell response.

🧠 THE BIG PICTURE

This pathway is basically:

Hormone → Receptor → G-protein → Phospholipase C → PIP₂ → IP₃ + DAG → Ca²⁺ + PKC → Cell response

The most important thing is that PIP₂ gets split into TWO messengers:

🔵 IP₃ → releases Ca²⁺

🟠 DAG → activates Protein Kinase C

1️⃣ PEPTIDE HORMONE 📩

At the very top:

Peptide hormone

It is outside the cell.

Why doesn’t it simply enter the cell?

Because peptide hormones are generally water-soluble and cannot easily cross the lipid cell membrane.

So they need a receptor.

Think:

Hormone = person knocking at the door 🚪

2️⃣ RECEPTOR = THE DOORBELL 🔔

The peptide hormone binds to the:

Receptor

The receptor is embedded ( Attached ) in the cell membrane.

When the hormone binds:

Receptor becomes activated

Then the receptor sends the message toward the inside.

3️⃣ G-PROTEIN = THE MESSENGER 🚶

Immediately below the receptor is:

G protein

The receptor activates the G protein.

Think:

Receptor: “Hey G-protein, we received a hormone message!”

G-protein:

“Got it. I’ll pass the message along.”

4️⃣ PHOSPHOLIPASE C = THE SCISSORS ✂️

G-protein activates:

Phospholipase C (PLC)

This is one of the most important enzymes in the diagram.

Think:

PLC = scissors

Its job is to cut a membrane molecule.

That molecule is:

PIP₂

5️⃣ PIP₂ = THE ORIGINAL RAW MATERIAL 🧱

PIP₂ stands for:

Phosphatidylinositol bisphosphate

Don’t get stuck on the long name.

For exams, remember:

PIP₂ is the molecule that PLC cuts.

PLC cuts:

PIP₂ → IP₃ + DAG

🔥 This is the central reaction of the whole figure.

6️⃣ TWO NEW MESSENGERS ARE CREATED

Once PLC cuts PIP₂, you get:

IP₃

and

DAG

Now the pathway splits into two branches.

🔵 BRANCH 1: IP₃ → Ca²⁺

Follow the IP₃ arrow downward.

IP₃ travels toward the:

Endoplasmic reticulum (ER)

The ER acts like a:

Calcium storage warehouse 🏪

It stores lots of Ca²⁺.

IP₃ basically says:

“Open the calcium warehouse!”

So the ER releases:

Ca²⁺

7️⃣ Ca²⁺ = ANOTHER SECOND MESSENGER ⚡

The calcium released into the cytoplasm now acts as a signaling molecule.

So:

IP₃ → Ca²⁺ release

Then:

Ca²⁺ → cellular response

The response can include things such as:

• smooth muscle contraction
• secretion
• changes in enzyme activity
• other cellular functions

Easy memory:

IP₃ = Calcium releaser

🟠 BRANCH 2: DAG → Protein Kinase C

Now go back to the other product:

DAG

DAG activates:

Protein Kinase C (PKC)

In the figure you can see:

Inactive protein kinase C

⬇️

Active protein kinase C

So:

DAG → activates PKC

8️⃣ PKC = THE PHOSPHORYLATION MACHINE ⚙️

Once PKC becomes active, it acts on proteins.

It adds a phosphate group:

Protein + ATP → Protein–PO₄ + ADP

This is:

PHOSPHORYLATION

The phosphorylated protein changes its activity.

That produces:

Cell’s response

🎯 NOW FOLLOW THE ENTIRE FIGURE

Let’s read it from top to bottom:

PEPTIDE HORMONE       ↓    RECEPTOR       ↓   G-PROTEIN       ↓PHOSPHOLIPASE C       ↓      PIP₂       ↓ ┌─────┴─────┐ ↓           ↓IP₃         DAG ↓           ↓ER         PKC ↓           ↓Ca²⁺     Phosphorylation ↓           ↓CELL       CELLRESPONSE   RESPONSE

That’s the entire figure! 🎯

🧠 WHY TWO PATHWAYS?

Because one hormone signal can produce multiple effects simultaneously.

IP₃ branch:

IP₃ → Ca²⁺ → response

DAG branch:

DAG → PKC → phosphorylation → response

So one hormone can activate different intracellular mechanisms.

🔥 THE MOST IMPORTANT DIFFERENCE

Don’t mix up these two:

Molecule	Main job
IP₃	Releases Ca²⁺ from ER
DAG	Activates Protein Kinase C
Ca²⁺	Produces cellular effects
PKC	Phosphorylates proteins

🔑 One-line memory:

“IP₃ opens the Calcium door; DAG turns PKC ON.”

🧩 WHAT IS THE ER DOING?

The purple structure at the bottom is:

Endoplasmic reticulum

Here, think of ER as:

Calcium storage tank 🛢️

Normally:

Ca²⁺ stored inside ER

IP₃ arrives:

IP₃ → opens Ca²⁺ release channels

Then:

Ca²⁺ moves into cytoplasm

Then Ca²⁺ helps create the cellular response.

🧠 WHY IS Ca²⁺ CALLED A SECOND MESSENGER?

Because the hormone started the message.

The hormone itself doesn’t have to enter the cell.

Instead:

Hormone → receptor → signaling pathway → Ca²⁺

Ca²⁺ then carries the message inside the cell.

Therefore:

First messenger:

Hormone

Second messengers:

IP₃, DAG

And:

Ca²⁺ can also function as a second messenger.

⚡ COMPARE WITH THE PREVIOUS cAMP FIGURE

You just learned the cAMP system.

Now put them side-by-side:

cAMP SYSTEM

Hormone
↓
Receptor
↓
G-protein
↓
Adenylyl cyclase
↓
ATP → cAMP
↓
PKA
↓
Phosphorylation
↓
Cell response

PHOSPHOLIPID SYSTEM

Hormone
↓
Receptor
↓
G-protein
↓
PLC
↓
PIP₂
↓
IP₃ + DAG

Then:

IP₃ → Ca²⁺ → response

and

DAG → PKC → phosphorylation → response

🚨 EXAM TRAPS

❌ PLC converts ATP into cAMP

No.

✅ PLC breaks down PIP₂.

❌ IP₃ activates PKC

No.

✅ DAG activates PKC.

❌ DAG releases Ca²⁺ from ER

No.

✅ IP₃ causes Ca²⁺ release.

❌ PKC releases Ca²⁺

No.

✅ PKC phosphorylates proteins.

❌ PIP₂ is the final messenger

No.

✅ PIP₂ is the substrate that gets broken down to produce IP₃ and DAG.

🏆 10-SECOND GUYTON REVISION

If you’re in an exam and see this pathway, immediately think:

“PLC CUTS PIP₂.”

Then:

PIP₂ → IP₃ + DAG

And:

IP₃ → Ca²⁺

DAG → PKC

PKC → Phosphorylation

Ca²⁺ + phosphorylation → Cell response

🧠 ULTIMATE MEMORY FORMULA

H → R → G → PLC → PIP₂ → IP₃ + DAG

Then:

🔵 IP₃ → ER → Ca²⁺ → Response

🟠 DAG → PKC → Protein phosphorylation → Response

⭐ Golden sentence:

“Hormone activates receptor, receptor activates G-protein, G-protein activates PLC, PLC cuts PIP₂ into IP₃ and DAG; IP₃ releases Ca²⁺, while DAG activates PKC.”