C2.1 Chemical Signaling

What is a receptor?

A receptor is a protein that detects and responds to specific signals. They bind to signalling molecules (ligands) to initiate a cellular response.

Why are receptors highly specific?

to ensure that cells respond accurately to the right signals

What is a ligand?

A ligand is a molecule that binds to a specific site on a receptor protein

Receptors are proteins found either:

1. on the cell membrane (for ligands that cannot pass through the membranes, like hydrophilic molecules)

2. Inside the cell (for ligands that are hydrophobic and can diffuse through the membrane, like steroid hormones).

What are the steps of interaction between receptors and ligands?

1. Binding; the ligand binds to the receptor’s ligand-binding site.

2. Conformational Change; the receptor changes shape, activating it.

3. Signal Transmission; the activated receptor triggers a response inside the cell

What is the difference between receptors and enzymes?

Receptors do NOT alter their ligands, they simply transmit signal.

Give two examples of ligands and their function

1. Hormone —> Insulin; travels in the bloodstream and binds to receptors on target cells.

2. Neurotransmitter —> Acetylcholine; released at synapses and binds to receptors on the next cell.

What is Quorum Sensing?

A communication system used by bacteria to coordinate group behaviors based on population density through the release and detection of signaling molecules.

How does Quorum Sensing work? 4 steps

1. Production of signaling molecules; each bacterium releases small chemical signals called autoinducers

2. Diffusion and accumulation; these molecules diffuse freely in the environment. As the bacterial population grows, the concentration of autoinducers increases.

3. Detection; when the concentration of autoinducers reaches a critical threshold, they bind to specific receptors in the bacteria.

4. Response activation; this binding triggers changes in gene expression, leading to coordinated behaviors like biofilm formation and bioluminescence.

What is an example of Quorum Sensing and describe

Vibrio Fischeri —> a marine bacterium, uses quorum sensing to produce light through bioluminescence. A Symbiotic relationship with bobtail squid.

What is the role of autoinducers in the case of high/low populations

1. Low population density; when vibrio fischeri bacteria are spparse, the concentration of autoinducers is too low to trigger a response.

2. High population density; in dense populations, such as those in the squid’s light organ, autoinducers accumulate and bind to a cytoplasmic receptor called LuxR.

Steps of the Lux system (7)

1. Luxl synthesizes autoinducer

2. Autoinducer diffuses out of the cell membrane

3. This causes a high concentration of autoinducers out of the cell membrane

4. Autoinducers re-enter the cell by diffusion

5. The autoinducer binds to LuxR (intracellular receptor protein)

6. This complex binds to promoter region of the lux operon, activating transcription of the genes involved with bioluminescence.

7. Products include Luciferase (enzyme), which catalyzes a redox reaction into bioluminescence.

What are hormones and what type of messengers are they?

Hormones are long-distance messengers. Hormones are chemical messengers secreted by endocrine glands into the bloodstream.

What are the key features of hormones? (4)

1. Wide reach; hormones can affect cells far from their origin

2. Specificity; only target cells with the right receptors respond to a hormone.

3. Long-lasting effects; hormones can remain active for hours or even days.

4. Slow action; hormonal responses are slower compared to other signals, as they rely on blood circulation.

What are neurotransmitters and what type of messengers are they?

Neurotransmitters are the rapid communicators. Neurotransmitters are chemical signals used by neurons to communicate across synapses, and tiny gaps between nerve cells or between nerve cells and tiny targets.

Key features of Neurotransmitters (4)

1. Short distance; neurotransmitters act across synapses, typically 20-40 nanometers wide

2. Fast action; signals are transmitted in milliseconds, enabling quick responses

3. Short-lived effects; neurotransmitters are quickly broken down or reabsorbed, ensuring precise control

4. Localized impact; they affect only the postsynaptic neuron or muscle fiber they bind to

What are cytokines and what type of coordinators are they?

Cytokines are the immune system coordinators. Cytokines are small proteins that mediate and regulate immune responses, inflammation, and cell signaling.

Key features of cytokines (4)

1. Local or short-range action; cytokines often act on nearby cells or the cell that produced them

2. Diverse effects; they regulate immune responses, cell growth, and even embryonic development

3. Multiple sources; cytokines can be produced by various cell types, including immune cells and epithilial cells.

4. Complex interactions; a single cytokine can have different effects depending on the target cell and receptor

What are calcium ions and what type of signal are they?

Calcium ions are the universal signal. Calcium ions are versatile signalling molecules involved in a wide range of cellular processes, from muscle contraction to neurotransmitter release.

Are calcium ions like traditional molecules?

No, however they play a crucial role as secondary messengers in cellular signaling.

Key features of calcium ions (4)

1. Intracellular and extracellular roles

2. Rapid and reversable

3. Wide range of functions; muscle contraction/ neurotransmitter release

4. Highly regulated; Ca2+ concentrations are tightly controlled by channels/pumps

Hormones and neurotransmitters are classified into various chemical groups based on their structure, including…

amines, proteins, steroids, amino acids, peptides, and nitrous oxide.

Hormones can be classified into three main chemical groups, which are..

amines, peptides, and steroids

What are amines?

Amines are hormones derived from amino acids, particularly those involving modifications to amino groups.

Amines are generally…

water-soluble and often have rapid, short-term effects.

What are proteins and peptides?

Proteins and peptides are chains of amino acids that act as signaling molecules.

Proteins and peptides are generally…

water-soluble and bind to receptors on the cell surface to initiate a cellular response.

What are steroids?

Steroid hormones are derived from cholesterol and are lipid-soluble, meaning they can pass through cell membranes and bind to intracellular receptors.

Steroids typically have… (what type of effect)

longer-lasting effects

What are amino acids?

Amino acids are the building blocks of proteins and are among the most common neurotransmitters

Amino acids are typically…(___ acting)

fast acting, can directly influence the postsynaptic membrane.

What are peptides?

Peptides, similar to their role as hormones, can also act as neurotransmitters.

Peptides typically have…(___ effects)

longer-lasting effects compared to amino acid neurotransmitters.

Between peptides and proteins, which have longer chains?

proteins

What are amines?

Important neurotransmitters derived from amino acids

What do amines have effects on? (3)

mood, alertness, attention

Give two examples of amines and their functions.

1. Dopamine: Involved in reward, motivation, and motor control.

2. Serotonin: Regulates mood, appetite, and sleep.

What is nitric oxide (NO)?

Nitric oxide is a gaseous neurotransmitter, which is highly unique compared to other neurotransmitters.

How does nitric oxide diffuse?

across membranes, rather than binding to specific receptors.

Between neurotransmitters and hormones, which can be broken down or reabsorbed easily?

neurotransmitters

The wide range of chemical structures among hormones and neurotransmitters is essential for their specialized roles, which are…(3)

1. Specificity of Action; Each signalling molecule must have a unique shape and chemical properties to bind selectively to its receptor. This ensures that signals are precise and do not interfere with one another.

2. Solubility and Transport; Water-soluble hormones (e.g., peptides) cannot cross cell membranes and must bind to surface receptors. Lipid-soluble hormones (e.g., steroids) can diffuse through membranes and act on intracellular receptors.

3. Speed and Duration of Action; The diversity in chemical structure also influences how quickly and how long a signalling molecule acts. Neurotransmitters work rapidly and have short-lived effects. Hormones often have slower, longer-lasting effects.

water-soluble hormones (amines, peptides) bind _____, while lipid-soluble hormones (steroids) _____________.

1. surface receptors

2. enter the cell and bind intracellular receptors

What are signalling molecules?

Signalling molecules are chemicals that transmit information between cells, triggering specific responses in target cells.

Cell signaling involves ___ that recognize signaling molecules and initiate a cellular response.

receptors

What can receptors be classified into? (2)

1. Transmembrane receptors (in the plasma membrane)

2. Intracellular receptors (in the cytoplasm or nucleus)

What are transmembrane receptors?

Transmembrane receptors are embedded in the plasma membrane and interact with signalling molecules that cannot cross the membrane.

Structure and distribution of amino acids in transmembrane receptors

1. Hydrophobic core; The part of the receptor that spans the membrane contains hydrophobic amino acids, allowing it to interact with the lipid bilayer.

2. Hydrophilic regions; The regions exposed to the extracellular and intracellular environments are composed of hydrophilic amino acids, enabling interaction with aqueous solutions.

Transmembrane receptors bind to ligands that are ___ and cannot cross the lipid bilayer, such as…

1. hydrophilic

2. peptides or neurotransmitters.

Binding of the ligand induces a ____ ___ in the receptor, activating intracellular signalling pathways.

conformational change

What are G-Protein-Coupled receptors?

G protein-coupled receptors are membrane receptors that activate a G protein inside the cell when a ligand binds.