Cell signaling
1. Why Cells Need Signaling
Cells are constantly responding to their environment. Even single-celled organisms detect signals like nutrients or toxins, while multicellular organisms coordinate millions or billions of cells.
Signals control things like:
cell growth
cell division
metabolism
movement
differentiation
apoptosis (programmed cell death)
Without signaling, cells would act independently instead of as a coordinated organism.
Example:
When blood sugar rises after eating, pancreatic cells release insulin, which signals muscle and liver cells to absorb glucose.
2. The Three Stages of Cell Signaling
Almost every signaling pathway follows the same three major steps.
1. Reception
A signaling molecule called a ligand binds to a receptor protein.
The receptor recognizes the ligand through shape and chemical compatibility, similar to enzyme–substrate interactions.
This binding causes the receptor to change shape, which activates it.
2. Transduction
The activated receptor triggers a series of intracellular signaling events.
These events usually involve:
protein phosphorylation
second messenger molecules
enzyme activation
Each step passes the signal along, like a relay race.
3. Response
The pathway eventually produces a specific cellular effect, such as:
turning genes on or off
activating enzymes
opening ion channels
reorganizing the cytoskeleton
triggering cell division
The response depends on the type of cell and signaling pathway.
3. Direct Cell-to-Cell Communication
Some cells communicate through direct physical connections instead of releasing signaling molecules.
Examples include:
Gap Junctions (animals)
These are protein channels connecting neighboring cells.
They allow:
ions
small molecules
to pass directly between cells.
This allows cells to coordinate activity extremely quickly.
Example: heart muscle cells contracting together.
Plasmodesmata (plants)
These are channels through plant cell walls connecting cytoplasm between cells.
They allow movement of:
signaling molecules
nutrients
RNA and proteins.
Septal Pores (fungi)
Similar to plasmodesmata but found in fungal cells.
4. Types of Cell Signaling Based on Distance
Cells communicate over different distances.
Contact-Dependent Signaling
The signaling molecule remains attached to the surface of the signaling cell.
The target cell must physically touch it to receive the signal.
Often uses:
membrane proteins
glycoproteins
glycolipids.
Example:
immune cell recognition.
Paracrine Signaling
Cells release signaling molecules that diffuse to nearby cells.
These signals are called paracrines.
Because they diffuse locally, they usually degrade quickly and do not travel far.
Examples:
growth factors
inflammatory signals.
Endocrine Signaling
Endocrine signals are hormones released into the bloodstream.
They travel long distances to reach target cells throughout the body.
Examples:
insulin
cortisol
thyroid hormone.
Important concept:
Hormones reach many cells, but only cells with the correct receptor respond.
Neuronal Signaling
Neurons transmit signals very quickly.
Steps:
electrical signal travels down the axon
calcium enters the neuron terminal
vesicles release neurotransmitters
neurotransmitters diffuse across the synapse
receptors on the next cell detect them.
Because synapses are extremely small, this signaling is very fast and precise.
5. Receptors
Receptors are proteins that detect signaling molecules.
They are highly specific.
There are two main locations:
Cell-Surface Receptors
These are embedded in the plasma membrane.
They detect signals that cannot cross the lipid membrane, such as:
peptides
neurotransmitters
proteins.
Intracellular Receptors
These are inside the cell.
They detect molecules that can pass through the membrane, such as:
steroid hormones
thyroid hormone
small hydrophobic molecules.
6. Intracellular Receptor Signaling
Small hydrophobic hormones can diffuse across the membrane.
Examples:
estrogen
testosterone
cortisol.
Once inside the cell, they bind intracellular receptors.
The hormone–receptor complex often enters the nucleus and binds DNA.
This acts as a transcription factor, turning specific genes on or off.
This type of signaling is slower but produces long-lasting effects.
7. Three Major Types of Cell-Surface Receptors
The three major classes are:
ligand-gated ion channels
G-protein coupled receptors
receptor tyrosine kinases.
8. Ligand-Gated Ion Channels
These receptors act as ion channels that open when a ligand binds.
Steps:
ligand binds receptor
channel opens
ions flow across membrane.
Important: the ligand itself does not pass through the channel.
Only ions move.
Example: neurotransmitters in the brain.
Common ions involved:
Na⁺
K⁺
Ca²⁺
Cl⁻.
Ion flow changes the membrane potential, which can trigger nerve signals.
9. G-Protein Coupled Receptors (GPCRs)
GPCRs are the largest receptor family in humans.
They are involved in:
vision
smell
taste
hormone signaling
neurotransmission.
These receptors activate G proteins, which act as molecular switches.
10. Structure of G Proteins
G proteins have three subunits:
alpha (α)
beta (β)
gamma (γ).
The alpha subunit binds GDP or GTP.
GDP = inactive
GTP = active.
11. GPCR Activation Process
Step-by-step:
ligand binds GPCR
receptor changes shape
GPCR causes GDP to be replaced by GTP on the alpha subunit
alpha subunit separates from beta–gamma complex
both parts activate downstream signaling proteins.
Eventually the alpha subunit hydrolyzes GTP to GDP, which turns the signal off.
12. Second Messengers
Second messengers are small molecules inside the cell that relay signals.
They allow signals to spread quickly and amplify the response.
Examples include:
cAMP
Ca²⁺
IP₃
DAG.
13. cAMP Signaling Pathway
One of the most common signaling pathways.
Steps:
GPCR activates Gs protein
Gs activates adenylyl cyclase
adenylyl cyclase converts ATP → cAMP.
cAMP activates protein kinase A (PKA).
PKA phosphorylates target proteins, which changes their activity.
This leads to cellular responses such as:
metabolism regulation
gene expression.
14. Turning Off the cAMP Signal
Cells must stop signals to prevent overactivation.
Phosphodiesterase breaks down:
cAMP → AMP.
This stops activation of PKA.
15. Calcium Signaling (IP3/DAG Pathway)
Another GPCR pathway uses Gq proteins.
Steps:
GPCR activates Gq
Gq activates phospholipase C
phospholipase C splits PIP₂ into:
IP₃
DAG.
IP₃ Function
IP₃ travels to the endoplasmic reticulum.
It opens calcium channels.
Calcium floods into the cytoplasm.
Calcium as a Second Messenger
Calcium ions activate many proteins.
One major protein is calmodulin.
Calcium signaling controls:
muscle contraction
secretion
metabolism
gene expression.
16. Receptor Tyrosine Kinases (RTKs)
RTKs are receptors that function as enzymes that add phosphate groups to tyrosine residues.
Steps:
ligand binds receptor
two receptors join together (dimerize)
receptors phosphorylate each other.
This is called autophosphorylation.
Phosphorylated tyrosines create binding sites for signaling proteins, activating multiple pathways.
RTKs often control:
cell growth
cell division
differentiation.
17. Signal Amplification
Signaling pathways amplify signals.
Example:
1 ligand → activates receptor
1 receptor → activates many G proteins
1 enzyme → produces thousands of second messengers.
This allows tiny amounts of hormone to produce huge effects.
18. Signal Termination
Cells must stop signals to maintain balance.
Mechanisms include:
ligand dissociation
receptor internalization
phosphorylation of receptors
GTP hydrolysis
second messenger degradation.
Big Picture
Cell signaling converts external information into cellular action.
Flow of information:
signal molecule → receptor → intracellular signaling → cellular response.