IB Biology Test Study Guide: Chemical Signaling
Concept One: Chemical Signaling
If Ligand is the chemical message, then the Receptor is the molecule that receives the
chemical message.
Basic Pathway of a Chemical Signal:
1.) Signal is received to the receptor by the ligand
2.) The initial signal gets converted into a signal that can be passed on– called transduction.
3.) The signal reaches its target (effector)
4.) The appropriate reaction is made
Types of Ligands:
1.) Hormones - Secreted by endocrine glands which travel through the bloodstream.
2.) Cytokines - Small signaling proteins, produce effects impacting the whole body such as a
fever.
3.) Calcium Ions - Signaling within muscle fibers and neurons.
4.) Neurotransmitters - Travel across the synapses between two neurons.
When the ligand binds to the receptor, the receptor’s conformation changes which signals
another chain of process to produce an appropriate response. The ligand on the other hand
leaves unchanged.
Hormones are secreted by endocrine glands through the bloodstream. There are three kinds of
hormones: steroids, amines, and peptides.
Amine Hormones are small molecules synthesized by the modification of amino acids. Peptide
and protein hormones are both made from chains of amino acids. Steroid Hormones are derived
from lipids, more specifically cholesterol, thus they have a similar structure to it.
More Detail: Pathway of Chemical Signaling
The first three steps of chemical signaling are reception, transduction, and response.
Reception
First, extracellular ligands (which act as the first messenger) bind to the receptor causing
conformational change.
Transduction
The changed receptor is activated by the ligand. The activated receptor acts out in four different
ways.
1.) Intracellular receptors that directly alter gene expression (Testosterone receptor)
, 2.) Transmembrane receptors that are ligand-gated ion channels which let specific ions flow
in or out of the cell. (Nicotine acetylcholine receptor)
3.) Transmembrane receptors that activate g binding proteins to invoke a process of
reactions to convey a cell response (Epinephrine)
4.) Transmembrane receptors with enzymatic activity (Insulin receptors)
Usually after the first contact, another contact is made to send the cell into a signaling cascade.
The second messengers include molecules such as cAMP (g-coupled receptors), nitric oxide, and
Ca^2+ Ions (Ligand-gated ion channels).
Response: The response is produced by alteration of gene expression, changes in cell
metabolism, shape or movement.
G- Coupled Protein Receptors
The G-Coupled Protein Receptors are formed of a single polypeptide chain folded into
seven lopes, embedded into the plasma membrane. The G Protein is attached to this polypeptide
chain. The G-Protein consists of three subunits: alpha, gamma and beta. Where alpha and
gamma are attached. This is a primary protein. The G protein binds with a special molecule
called GTP. When GTP loses a phosphate it becomes GDP. When the G-protein binds with GTP
its at high energy level, when the G-protein binds with GDP its at low energy level. GTP is
present in the cell, in the cytosol. GTP binds with the alpha subunit of the G-Protein which
activates the G-Protein– the alpha subunit is activated, this marks the start of GPCR Signaling.
The moment GTP is hydrolyzed into GDP, the G-Protein gets unactivated. There are two
separate pathways of GPCR: Via formation of Cyclic AMP (cAMP) involving an enzyme called
adeline cyclase and IP3 Pathway involving an enzyme called Phospholipase (PLC).
Active G-Protein has multiple roles. In this study guide, we will mainly focus on the
cAMP pathway. G-Protein will activate an enzyme called adenylyl cyclase (Job: ATP —> Cyclic).
Adenylyl cyclase is an enzyme that produces cAMP by hydrolyzing ATP to produce cAMP. The
cAMP will activate another enzyme called protein kinase A (PKA). PKA will further activate
other transcription factors that will lead to the formation of protein which leads to cell growth
etc.
When the ligand binds, it activates the receptor. The space where the ligand binds to the
receptor is called ligand binding domain. Once the ligand binds, the intracellular part starts
sending signals to the cell– the intracellular part is called the effector domain.
Catecholamines like epinephrine use G-Coupled Protein Receptors aka 7-pass receptors.
Concept One: Chemical Signaling
If Ligand is the chemical message, then the Receptor is the molecule that receives the
chemical message.
Basic Pathway of a Chemical Signal:
1.) Signal is received to the receptor by the ligand
2.) The initial signal gets converted into a signal that can be passed on– called transduction.
3.) The signal reaches its target (effector)
4.) The appropriate reaction is made
Types of Ligands:
1.) Hormones - Secreted by endocrine glands which travel through the bloodstream.
2.) Cytokines - Small signaling proteins, produce effects impacting the whole body such as a
fever.
3.) Calcium Ions - Signaling within muscle fibers and neurons.
4.) Neurotransmitters - Travel across the synapses between two neurons.
When the ligand binds to the receptor, the receptor’s conformation changes which signals
another chain of process to produce an appropriate response. The ligand on the other hand
leaves unchanged.
Hormones are secreted by endocrine glands through the bloodstream. There are three kinds of
hormones: steroids, amines, and peptides.
Amine Hormones are small molecules synthesized by the modification of amino acids. Peptide
and protein hormones are both made from chains of amino acids. Steroid Hormones are derived
from lipids, more specifically cholesterol, thus they have a similar structure to it.
More Detail: Pathway of Chemical Signaling
The first three steps of chemical signaling are reception, transduction, and response.
Reception
First, extracellular ligands (which act as the first messenger) bind to the receptor causing
conformational change.
Transduction
The changed receptor is activated by the ligand. The activated receptor acts out in four different
ways.
1.) Intracellular receptors that directly alter gene expression (Testosterone receptor)
, 2.) Transmembrane receptors that are ligand-gated ion channels which let specific ions flow
in or out of the cell. (Nicotine acetylcholine receptor)
3.) Transmembrane receptors that activate g binding proteins to invoke a process of
reactions to convey a cell response (Epinephrine)
4.) Transmembrane receptors with enzymatic activity (Insulin receptors)
Usually after the first contact, another contact is made to send the cell into a signaling cascade.
The second messengers include molecules such as cAMP (g-coupled receptors), nitric oxide, and
Ca^2+ Ions (Ligand-gated ion channels).
Response: The response is produced by alteration of gene expression, changes in cell
metabolism, shape or movement.
G- Coupled Protein Receptors
The G-Coupled Protein Receptors are formed of a single polypeptide chain folded into
seven lopes, embedded into the plasma membrane. The G Protein is attached to this polypeptide
chain. The G-Protein consists of three subunits: alpha, gamma and beta. Where alpha and
gamma are attached. This is a primary protein. The G protein binds with a special molecule
called GTP. When GTP loses a phosphate it becomes GDP. When the G-protein binds with GTP
its at high energy level, when the G-protein binds with GDP its at low energy level. GTP is
present in the cell, in the cytosol. GTP binds with the alpha subunit of the G-Protein which
activates the G-Protein– the alpha subunit is activated, this marks the start of GPCR Signaling.
The moment GTP is hydrolyzed into GDP, the G-Protein gets unactivated. There are two
separate pathways of GPCR: Via formation of Cyclic AMP (cAMP) involving an enzyme called
adeline cyclase and IP3 Pathway involving an enzyme called Phospholipase (PLC).
Active G-Protein has multiple roles. In this study guide, we will mainly focus on the
cAMP pathway. G-Protein will activate an enzyme called adenylyl cyclase (Job: ATP —> Cyclic).
Adenylyl cyclase is an enzyme that produces cAMP by hydrolyzing ATP to produce cAMP. The
cAMP will activate another enzyme called protein kinase A (PKA). PKA will further activate
other transcription factors that will lead to the formation of protein which leads to cell growth
etc.
When the ligand binds, it activates the receptor. The space where the ligand binds to the
receptor is called ligand binding domain. Once the ligand binds, the intracellular part starts
sending signals to the cell– the intracellular part is called the effector domain.
Catecholamines like epinephrine use G-Coupled Protein Receptors aka 7-pass receptors.
