IB Bio C2.1

C2.1.1 Receptors as Proteins with Specific Binding Sites

Receptors are proteins located on the cell surface membrane or inside the cytoplasm. Their shape allows them to bind only one type of signalling molecule, giving them specificity.

Ligands are signalling chemicals that bind to receptors. Binding triggers signal transduction, which changes the cell’s activity—activating enzymes, opening channels, or switching genes on/off.

Examples of ligand–receptor interactions

  • Insulin → liver cell receptor — increases glucose uptake

  • Adrenaline → muscle cell receptor — initiates fight‑or‑flight responses

  • Dopamine → neuronal receptors — regulates mood and movement

Key ideas

  • Receptors = proteins with specific binding sites

  • Ligands = signalling molecules (hormones, neurotransmitters, cytokines)

  • Binding is highly specific (lock‑and‑key)

  • Binding triggers a cellular response

C2.1.2 Quorum Sensing in Bacteria

Quorum sensing is bacterial communication based on population density.

Each bacterium releases autoinducers (chemical signals). As the population grows, autoinducer concentration rises. When it reaches a threshold, autoinducers bind to bacterial receptors and activate gene expression across the whole population.

Example: Vibrio fischeri bioluminescence

  • Lives in the Hawaiian bobtail squid

  • Produces light only when enough bacteria are present

  • Light helps the squid camouflage by eliminating its shadow

Why quorum sensing matters

  • Coordinates virulence in pathogens

  • Controls biofilm formation

  • Regulates antibiotic resistance

  • Used in biotechnology and synthetic biology

C2.1.3 Functional Categories of Signalling Chemicals in Animals

Cells use different signalling chemicals depending on distance, speed, and function.

1. Hormones

  • Long‑distance signals via bloodstream

  • Slower but long‑lasting

  • Examples: insulin, adrenaline

2. Neurotransmitters

  • Very fast, local signals across synapses

  • Examples: acetylcholine, dopamine

3. Cytokines

  • Immune signalling molecules

  • Regulate inflammation and immune responses

  • Examples: interleukins, interferons

4. Calcium ions (Ca²⁺)

  • Intracellular second messengers

  • Trigger rapid responses like muscle contraction

C2.1.4 Chemical Diversity of Hormones and Neurotransmitters

Different signalling chemicals have different structures, solubilities, and functions.

Hormone types

  • Amines — water‑soluble (adrenaline)

  • Peptides/proteins — water‑soluble (insulin)

  • Steroids — fat‑soluble (testosterone, cortisol)

Solubility determines whether a hormone can cross membranes and how it travels in blood.

Neurotransmitter types

  • Amino acids — glutamate, GABA

  • Peptides — endorphins, substance P

  • Amines — dopamine, serotonin

  • Gases — nitric oxide

Diversity allows precise control of timing, strength, and type of response.

C2.1.5 Local vs Distant Effects of Signalling Molecules

Signalling molecules differ in how far they travel and how long their effects last.

Neurotransmitters

  • Act locally across synapses

  • Very fast (milliseconds)

  • Short‑lived

  • Example: acetylcholine at neuromuscular junctions

Hormones

  • Travel through bloodstream

  • Slower (seconds to hours)

  • Long‑lasting

  • Example: insulin affecting cells throughout the body

Key contrast

  • Neurotransmitters = local, fast, short

  • Hormones = distant, slower, long‑lasting

C2.1.6 Differences Between Transmembrane and Intracellular Receptors

Where Receptors Are Found

Cells use two major receptor types depending on the chemical nature of the signalling molecule:

  • Transmembrane receptors — embedded in the plasma membrane

  • Intracellular receptors — located in the cytoplasm or nucleus

Their structure and amino acid composition reflect their location and the type of ligand they bind.

Transmembrane Receptors

These receptors span the lipid bilayer and detect hydrophilic ligands that cannot cross the membrane.

  • Ligands: peptides, protein hormones, neurotransmitters

  • Structure:

    • Extracellular domain binds ligand

    • Transmembrane region contains hydrophobic amino acids

    • Cytoplasmic domain initiates intracellular signalling

  • Function: Convert an external signal into an internal response

  • Examples: insulin receptor, adrenaline receptor, acetylcholine receptor

Intracellular Receptors

These receptors detect hydrophobic ligands that diffuse through the membrane.

  • Ligands: steroid hormones, thyroid hormones

  • Structure:

    • Entirely inside the cell

    • Mostly hydrophilic amino acids (soluble in cytoplasm)

    • Often contain DNA‑binding domains

  • Function: Directly regulate gene expression or enzyme activity

  • Examples: estrogen receptor, cortisol receptor, thyroid hormone receptor

Comparison Table

Key idea: The receptor’s structure matches the ligand’s chemistry and the receptor’s location.

C2.1.7 Initiation of Signal Transduction Pathways

Signal transduction is the sequence of events that converts an external signal into a cellular response.

How Signal Transduction Begins

  1. Ligand binding
    A signalling molecule binds to its specific receptor.

  2. Receptor activation
    Binding causes a conformational change that activates the receptor.

  3. Signal cascade
    Activated receptors trigger internal pathways involving:

    • relay proteins

    • enzymes

    • second messengers (cAMP, Ca²⁺)

  4. Cellular response
    The cell changes its activity, such as:

  • activating genes

  • altering metabolism

  • releasing chemicals

  • contracting or dividing

Example: Adrenaline Pathway in Liver Cells

  1. Adrenaline binds to a membrane receptor.

  2. Receptor activates an internal enzyme.

  3. cAMP is produced as a second messenger.

  4. Enzymes are activated to break down glycogen.

  5. Glucose is released into the bloodstream.

Ligand Types and Receptor Location

  • Hydrophilic ligands → bind to membrane receptors

  • Hydrophobic ligands → diffuse into cell → bind intracellular receptors

Key idea: Signal transduction is a chain reaction that amplifies a small external signal into a large cellular response.

C2.1.8 Transmembrane Receptors for Neurotransmitters and Membrane Potential

Neurotransmitters communicate across synapses by binding to receptors on the postsynaptic membrane. Many of these receptors are ligand‑gated ion channels.

The Acetylcholine (ACh) Receptor

A classic example of a transmembrane receptor that directly affects membrane potential.

  • Type: Ligand‑gated ion channel

  • Ligand: Acetylcholine

  • Action: Opens when ACh binds

How It Works

  1. A neuron releases ACh into the synaptic cleft.

  2. ACh binds to receptors on the postsynaptic cell.

  3. The receptor changes shape and opens an ion channel.

  4. Na⁺ ions rush into the cell, sometimes with K⁺ leaving.

  5. The membrane becomes less negative (depolarization).

  6. Possible outcomes:

  • action potential

  • muscle contraction

  • activation of downstream pathways

Why This Matters

  • Neurotransmitter‑gated channels convert chemical signals into electrical changes.

  • This is essential for movement, sensation, cognition, and reflexes.

Key idea: Neurotransmitter receptors rapidly change membrane potential by controlling ion flow.

C2.1.9 Transmembrane Receptors That Activate G Proteins

Overview of GPCRs

G protein‑coupled receptors (GPCRs) are a major family of transmembrane receptors that detect external signals and activate G proteins inside the cell. They are involved in processes such as vision, smell, taste, mood regulation, and heart rate.

Structural Features

  • Span the membrane seven times (7‑transmembrane receptors).

  • Extracellular domain binds the ligand.

  • Intracellular domain interacts with a G protein composed of α, β, γ subunits.

How GPCRs Work

  1. Ligand binding
    A signalling molecule (e.g., adrenaline, dopamine) binds to the GPCR.

  2. Receptor activation
    The receptor changes shape and activates the G protein.

  3. G protein activation
    The α‑subunit swaps GDP for GTP and separates from βγ.

  4. Signal relay
    Activated subunits interact with target proteins such as:

    • adenylate cyclase → produces cAMP

    • ion channels

    • other second messenger systems

  5. Signal termination
    GTP is hydrolyzed to GDP, and the subunits reassemble.

Why GPCRs Matter

  • Humans have ~800 GPCRs.

  • They regulate essential physiological functions.

  • Many medications (antihistamines, beta blockers) target GPCRs.

Example: Adrenaline and Heart Rate

Adrenaline binds to β‑adrenergic GPCRs → activates G proteins → increases cAMP → speeds up heart rate.

C2.1.10 Mechanism of Action of Epinephrine (Adrenaline) Receptors

What Epinephrine Is

A hormone and neurotransmitter released by the adrenal glands during stress. It increases heart rate, mobilizes glucose, and prepares the body for rapid action.

Naming Note (NOS)

  • Adrenaline (Latin roots)

  • Epinephrine (Greek roots)
    Both refer to the same molecule; naming differences reflect international scientific collaboration.

How Epinephrine Triggers a Response

This pathway is a classic example of GPCR signalling.

  1. Epinephrine binds to β‑adrenergic GPCR on the cell membrane.

  2. G protein activation

    • Receptor changes shape

    • α‑subunit swaps GDP for GTP

  3. Adenylate cyclase activation

    • GTP‑bound α‑subunit activates the enzyme

  4. Production of cAMP

    • ATP → cAMP (a second messenger)

  5. Activation of Protein Kinase A (PKA)

    • cAMP activates PKA

    • PKA phosphorylates target proteins

  6. Cellular responses

  • Increased heart rate

  • Glycogen breakdown → glucose release

  • Relaxation of airway smooth muscle

Signal Amplification

One epinephrine molecule → many G proteins → thousands of cAMP molecules → large physiological response.

C2.1.11 Transmembrane Receptors with Tyrosine Kinase Activity

What Tyrosine Kinase Receptors Are

These receptors have built‑in enzyme activity that phosphorylates tyrosine residues on proteins inside the cell. They are essential for growth, metabolism, and cell differentiation.

Insulin as the Classic Example

Insulin is a protein hormone released when blood glucose is high. Its receptor is a tyrosine kinase receptor.

How Insulin Signalling Works

  1. Insulin binds to its receptor on the plasma membrane.

  2. Receptor dimerization
    Two receptor units join together.

  3. Autophosphorylation
    The receptor phosphorylates its own tyrosine residues.

  4. Signal cascade begins
    Phosphorylated tyrosines act as docking sites for intracellular proteins.

  5. GLUT4 vesicle movement
    Vesicles containing GLUT4 transporters move to the membrane.

  6. Glucose uptake
    GLUT4 transporters allow glucose to enter the cell via facilitated diffusion.

Outcome

Blood glucose levels decrease as cells take up glucose.

C2.1.12 Intracellular Receptors That Affect Gene Expression

Where These Receptors Are Found

Intracellular receptors sit inside the cell, either:

  • in the cytoplasm, or

  • directly in the nucleus.

They bind lipid‑soluble hormones that can diffuse through the plasma membrane.

How Intracellular Receptors Work

  1. A steroid hormone (oestradiol, progesterone, testosterone) diffuses through the membrane.

  2. It binds to its specific intracellular receptor.

  3. The receptor becomes activated and changes shape.

  4. The hormone–receptor complex enters the nucleus (if not already there).

  5. It binds to hormone response elements on DNA.

  6. This activates or represses gene transcription, changing which proteins the cell makes.

Examples of Steroid Hormones

  • Oestradiol — regulates female secondary sex characteristics and menstrual cycle.

  • Progesterone — prepares and maintains the uterus for pregnancy.

  • Testosterone — regulates male secondary sex characteristics and sperm production.

Why This Matters

Steroid hormones act as gene switches, producing long‑lasting changes in cell behavior. Their effects are slower than neurotransmitters but more sustained because they alter protein synthesis.

C2.1.13 Effects of Oestradiol and Progesterone on Target Cells

What Target Cells Are

Target cells have specific receptors for a hormone. Binding triggers a response unique to that cell type.

Oestradiol and Hypothalamic Cells

  • Target: GnRH‑secreting cells in the hypothalamus

  • Effect:

    • Oestradiol increases GnRH secretion

    • GnRH stimulates the pituitary to release FSH and LH

    • Leads to ovulation

  • Summary: Oestradiol → ↑ GnRH → ↑ FSH & LH → ovulation

Progesterone and Endometrial Cells

  • Target: Cells of the uterine lining (endometrium)

  • Effect:

    • Thickens the lining

    • Increases blood vessel growth

    • Promotes nutrient secretion

    • Maintains the lining in early pregnancy

  • Summary: Progesterone → prepares and maintains uterine lining

Hormonal Coordination

C2.1.14 Regulation of Cell Signalling by Positive and Negative Feedback

What Feedback Means in Signalling

Feedback describes how the output of a signalling pathway influences its own activity. It keeps responses balanced or amplifies them when needed.

Positive Feedback

  • Definition: The response enhances the original signal.

  • Effect: Amplifies the pathway; often rapid or self‑reinforcing.

  • Example:

  • Oestradiol stimulates LH secretion.

  • Rising LH triggers ovulation.

  • This is a classic reproductive positive feedback loop.

Negative Feedback

  • Definition: The response reduces the original signal.

  • Effect: Maintains homeostasis and prevents overstimulation.

  • Example:

  • High blood glucose → insulin release

  • Insulin lowers glucose

  • Lower glucose reduces insulin secretion

Feedback Summary