IB Biology HL: Unit C2.1 - Chemical Signaling

Receptors

Proteins embedded in the plasma membrane or located within the cytoplasm of the cell, containing a binding site for specific signaling molecules (ligands).

What is the primary function of receptors?

To detect extracellular or intracellular signals and initiate a response in the target cell.

How do receptors interact with ligands?

The interaction is highly specific, based on shape complementarity and noncovalent chemical interactions such as hydrogen bonds, ionic bonds, and hydrophobic interactions.

What happens to a receptor after it binds to a ligand?

The binding causes a conformational change in the receptor, initiating a signaling pathway, and the ligand is released unchanged.

What is quorum sensing?

Type of cell-cell signaling in bacteria where they monitor population density through chemical signals called autoinducers.

How does quorum sensing affect gene expression in bacteria?

Once the concentration of autoinducers reaches a threshold, they bind to intracellular receptors, activating simultaneous gene expression in all cells.

What role does Vibrio fischeri play in quorum sensing?

Vibrio fischeri uses quorum sensing to activate the lux operon, which controls bioluminescence in the Hawaiian bobtail squid.

What is the significance of the lux operon in Vibrio fischeri?

The lux operon utilizes a positive feedback loop to increase bioluminescence as more autoinducer proteins bind to receptors.

What are the major functional categories of signaling chemicals in animals?

Hormones, neurotransmitters, cytokines, and calcium ions.

How do hormones travel in the body?

Hormones are secreted from endocrine glands that travel through the bloodstream to target cells with specific receptors.

What is the primary difference between hormones and neurotransmitters?

Hormones can affect cells at a distance, while neurotransmitters have local effects by transmitting signals across synapses.

What are the three main categories of hormones based on chemical structure?

Peptide, steroid, and amine.

What are peptide hormones and how do they function?

Amino acid chains that are hydrophilic and bind to receptors on the surface of target cells.

How do steroid hormones differ from peptide hormones?

Steroid hormones are hydrophobic, derived from cholesterol, and can diffuse through cell membranes to bind to intracellular receptors.

What are amine hormones and their solubility?

Small molecules modified from amino acids that are water-soluble and cannot pass through the hydrophobic center of the plasma membrane.

What are some examples of peptide hormones?

Insulin and glucagon.

What are some examples of steroid hormones?

Estrogen, progesterone, and testosterone.

What are examples of amine hormones?

Epinephrine and thyroxine.

What types of signaling molecules are involved in neurotransmission?

Amino acids (like glutamate and GABA), amines (like dopamine and serotonin), peptides (like endorphins), and others (esters and gases).

What is the role of autoinducers in quorum sensing?

Autoinducers are chemical signals that bacteria use to monitor population density and initiate a collective response.

How does the binding of autoinducers to receptors affect bacteria?

It triggers a signaling pathway that leads to gene expression changes across the bacterial population.

What is the importance of shape complementarity in receptor-ligand interactions?

Shape complementarity ensures that receptors bind specifically to their corresponding ligands, initiating the correct cellular response.

What is the outcome of receptor activation by a ligand?

The receptor undergoes a conformational change that initiates a signaling pathway, leading to a cellular response.

What are amines and how do they interact with cell membranes?

Amines are water-soluble and cannot pass through the hydrophobic center of the plasma membrane; they bind to receptors on the surface of the target cell membrane.

How do neurotransmitters and hormones differ in their signaling mechanisms?

Neurotransmitters are involved in local signaling and rely on diffusion between adjacent cells, while hormones are involved in distant signaling and travel through the bloodstream to reach target cells.

What is the role of neurotransmitters in local signaling?

Neurotransmitters are released from the axon terminal of one neuron, diffuse across the synapse, and bind to receptors on another neuron or muscle cell membrane.

How do hormones achieve distant signaling?

Hormones are released by endocrine glands directly into the bloodstream, allowing them to reach target cells anywhere in the body.

What are the two main types of receptors based on ligand properties?

Transmembrane receptors for hydrophilic ligands that cannot cross the membrane, and intracellular receptors for hydrophobic ligands that can cross the membrane.

What characterizes transmembrane receptors for hydrophilic ligands?

They contain hydrophilic amino acids in regions interacting with extracellular and intracellular areas and hydrophobic amino acids interacting with the phospholipid bilayer.

What characterizes intracellular receptors for hydrophobic ligands?

They have hydrophobic amino acids at the core to bind with the ligand and hydrophilic amino acids on the outer surface to remain dissolved in the cytoplasm.

What is a signal transduction pathway?

A signal transduction pathway consists of chains of molecules inside a cell that relay signals from the binding of a ligand to a target cell.

What happens when a ligand binds to its transmembrane receptor?

A conformational change occurs in the receptor, which may activate it as an enzyme or allow it to bind to intracellular molecules, initiating a cascade of signaling events.

What is the role of the first and second messengers in signal transduction?

The ligand is the first messenger, and as the receptor initiates signal transduction, other non-protein molecules act as second messengers, such as calcium ions and cyclic AMP (cAMP).

What are the four examples of transmembrane receptors?

1. Receptors for neurotransmitters, 2. Receptors that activate G-proteins, 3. Receptors of epinephrine, 4. Receptors with tyrosine kinase activity.

How does Acetylcholine (ACh) function as a neurotransmitter?

ACh binds to a ligand-gated ion channel, causing the channel to open and allowing sodium ions to diffuse into the target cell.

What is the significance of the conformational change in a receptor during signal transduction?

The conformational change can activate the receptor as an enzyme or enable it to interact with intracellular molecules, leading to a cascade of signaling events.

What is the effect of neurotransmitters on target cells?

Neurotransmitters have localized effects due to their diffusion between adjacent cells.

What is the function of endocrine glands in hormone signaling?

Endocrine glands release specific hormones directly into the bloodstream for distant signaling.

What types of amino acids are found in transmembrane receptors interacting with the phospholipid bilayer?

Hydrophobic amino acids are found in the region of the receptor that interacts with the phospholipid bilayer.

What is the role of calcium ions in signal transduction?

Calcium ions can act as second messengers in signal transduction pathways, relaying signals within the cell.

What is the relationship between ligand binding and gene expression changes in a cell?

The cascade of signaling events initiated by ligand binding can lead to changes in gene expression within the cell's nucleus.

How do neurotransmitters achieve their effects at the synapse?

They diffuse across the synaptic cleft and bind to receptors on the dendritic end of another neuron or muscle cell.

What happens to a receptor after a ligand binds to it?

The receptor undergoes a conformational change that can activate it or allow it to interact with other molecules inside the cell.

What is the role of hydrophilic amino acids in intracellular receptors?

Hydrophilic amino acids on the outer surface of intracellular receptors allow them to remain dissolved in the aqueous environment of the cytoplasm.

What is the primary mechanism of action for neurotransmitters?

Neurotransmitters primarily act through localized signaling by diffusing across synapses.

What is the primary mechanism of action for hormones?

Hormones primarily act through distant signaling by traveling through the bloodstream to reach target cells.

What occurs when sodium ions enter the target cell?

Depolarization may occur, increasing the membrane potential and potentially triggering an action potential.

What is the threshold potential that triggers a new action potential in the post-synaptic neuron?

+55mV

What enzyme breaks down Acetylcholine (ACh) in the synaptic cleft?

Acetylcholinesterase (AChE)

What is the role of acetylcholinesterase (AChE) in neurotransmission?

It breaks down ACh into choline and acetate, allowing the receptor to reset for new ACh binding.

What are G-protein-coupled receptors (GPCRs)?

A diverse group of transmembrane receptor proteins that consist of a single polypeptide with seven segments spanning the membrane.

What happens to GPCRs when they are not bound by a ligand?

They are inactive and GDP is bound to the intracellular alpha subunit of the G-protein complex.

What triggers the activation of a G-protein by a GPCR?

Ligand binding causes a conformational change in the receptor.

What molecule replaces GDP when a GPCR is activated?

GTP (guanosine triphosphate)

What are the components of a G-protein?

Three subunits: alpha (𝛂), beta (𝛃), and gamma (𝛄).

What are some effects of GPCR activation?

Opening ion channels, altering metabolism, activating gene expression, or changing cell shape.

How many different types of GPCRs are found in humans?

Over 1,000 different types.

What are some examples of ligands that bind to GPCRs?

Epinephrine, serotonin, dopamine, histamine, glucagon, taste and odor molecules, pheromones, and ACh.

What is epinephrine and where is it produced?

Epinephrine (adrenaline) is an amine hormone produced by the medulla of the adrenal glands.

What type of receptors do epinephrine bind to?

Adrenergic receptors, which are a type of GPCR.

What enzyme does the alpha subunit of the G-protein complex activate in response to epinephrine?

Adenylate cyclase.

What does adenylate cyclase do?

It converts ATP in the cytoplasm into cyclic AMP (cAMP).

What is the role of cAMP in signal transduction?

cAMP acts as a second messenger, activating protein kinase A and leading to a cascade of phosphorylation events.

What are some physiological changes resulting from cAMP activation?

Glycogen breakdown, triglyceride breakdown, increased heart rate, and increased bronchodilation.

What are receptor tyrosine kinases (RTKs)?

Transmembrane receptors that bind the ligand insulin.

What triggers the production of insulin?

Rising blood sugar levels.

What happens to the intracellular domains of insulin RTKs in their inactive state?

The two intracellular domains containing the tyrosine kinases are held apart from each other.

What occurs upon the binding of insulin to its receptor?

A conformational change occurs, bringing the two intracellular domains close together, forming a compact T-shape.

What is triggered by the proximity of the two segments of the insulin receptor?

Autophosphorylation between the tyrosine kinases on each segment.

What role do phosphorylated tyrosines play in insulin signaling?

They allow intracellular signaling proteins, such as insulin receptor substrate (IRS), to dock, initiating the signal transduction pathway.

What is the sequence of activation in the insulin signaling pathway?

PI3K → PIP2 → PIP3, which act as second messengers activating kinases like PDK1 and Akt.

What is one important result of the insulin signaling cascade?

The movement of vesicles embedded with glucose-transport (GLUT4) proteins to the membrane, facilitating glucose uptake into the cell.

What type of ligands do intracellular receptors bind to?

Hydrophobic ligands capable of crossing the cell membrane, such as steroids.

What is formed when steroids bind to intracellular receptors?

A hormone-receptor complex that acts as a transcription factor.

What is the role of testosterone in cellular function?

It binds to an androgen receptor (AR) and acts as a transcription factor, influencing cell behavior, differentiation, and tissue growth.

How does estrogen affect the hypothalamus?

Estrogen binds to receptors in the hypothalamus, leading to the production of gonadotropin-releasing hormone (GnRH).

What is the outcome of GnRH production in the pituitary gland?

It triggers the release of follicle-stimulating hormone (FSH) and luteinizing hormone (LH), promoting follicle development and ovulation.

What effect does progesterone have on the endometrial cells of the uterus?

It triggers the transcription of the gene coding for Insulin-like Growth Factor (IGF), leading to cellular proliferation and thickening of the endometrium.

What is the function of positive feedback in cell signaling pathways?

It amplifies the signal, driving a response further.

How does negative feedback regulate signaling pathways?

It dampens the signal and helps maintain homeostasis by counteracting changes in the pathway.

What is an example of positive feedback in the cell cycle?

Activation of cyclin-CDK complexes that trigger the production of more proteins needed for cell division.

What is end-product inhibition in negative feedback?

Using the product of the signaling pathway to inhibit a component at the start of the pathway, reducing the cascade effect.

How is testosterone signaling regulated?

By negative feedback mechanisms.

What is the significance of GLUT4 proteins in glucose metabolism?

They facilitate glucose uptake into the cell through facilitated diffusion.

What is the role of transcription factors in gene expression?

They initiate the transcription of specific genes, resulting in the formation of proteins.

What are Androgen Response Elements?

Specific genes within the DNA that the hormone-receptor complex binds to after testosterone signaling.

What is the primary function of insulin in the body?

To regulate glucose uptake and metabolism.

What happens to GLUT4 proteins when insulin binds to its receptor?

Vesicles containing GLUT4 proteins fuse with the membrane, allowing glucose entry into the cell.

What are the two main types of feedback mechanisms in cell signaling?

Positive feedback and negative feedback.

What is the overall effect of positive feedback in biological systems?

It enhances and accelerates processes, often leading to a decisive outcome.