IB Biology 2025 - Cell Signaling

Signal molecules bind to

receptor proteins

ligand

A signal molecule that binds specifically to another molecule, usually a larger one.

Signal molecules bind to

receptor proteins

ligand

A signal molecule that binds specifically to another molecule, usually a larger one.

The second-messenger mechanism of hormone action operates by ________.

cAMP

Endocrine

long distance signaling occurring in the body

autocrine

term for hormones that act on same cells that secrete them

paracrine signaling

Signal released from a cell has an effect on neighboring cells.

transduction pathway

A series of molecular changes that converts a signal on a target cell's surface to a specific response inside the cell.

reception

The target cell's detection of a signal molecule coming from outside the cell.

Insulin released by the pancreas signals the liver to store glucose.

endocrine response

Stage of cell signaling when a transcription protein is activated for gene regulation.

cellular response

steroid

type of cell signaling molecule that can bind to an intracellular receptor.

enzymes that transfer the terminal phosphate from ATP to an amino acid residue on the target protein.

protein kinases

Apoptosis

process of programmed cell death

target cells

cells that have receptors for a particular hormone

The antagonistic functions of the direct and indirect pathways are often regulated by

hormones

negative feedback system

a process that results in a response that reverses the original stimulus

positive feedback mechanism

Feedback that tends to cause the level of a variable to change in the same direction as an initial change (more gives more, less gives less)

G-protein coupled receptors

A special class of membrane receptors with an associated GTP binding protein; activation of a G protein-coupled receptor involves dissociation and GTP hydrolysis

G-protein-linked receptor

A plasma membrane receptor that works with the help of a G-protein.

Example of antagonistic effect

insulin and glucagon

Yeast cells release mating factors that

bind to receptors on membranes of opposite sex only

Signaling across gap junctions

a cell targets a cell connected by gap junctions (cell signals to cell that is connected to it)

The order of cell signaling from shortest to longest distance.

autocrine --> signaling across gap junctions --> paracrine --> endocrine

functional categories of signaling chemicals

Hormones, neurotransmitters, cytokines and calcium ions

Hormones

Chemical messengers, mostly those manufactured by the endocrine glands, that are produced in one tissue and affect another

Cytokines

Hormone-like chemicals- small proteins - facilitating communication between brain and immune system.

Neurotransmitters

chemical messengers that cross the synaptic gaps between neurons

Chemical categories of hormones

steroids, amines, polypeptides, glycoproteins

chemical categories of neurotransmitters

amino acids, amines, peptides

structure receptor proteins in the cell membrane

intracellular receptors

hydrophobic Ligands move directly across the membrane to bind to the intracellular receptor.

Steps in signaling pathway

1. Recognition of hormone signal

2. Transduction of signal across membrane

- receptor conformational change (form weak interactions)

3. transmission to intracellular components

- receptor activates adaptor proteins

4. Modulation of the effector

- effector takes the inner molecule + makes it active

- 2nd messengers produced, target proteins activated

5. Response of the cell to the signal

6. Termination of the signal

- degrade 2nd messenger

- turn off target proteins

first messenger

A water soluble hormone that binds to its receptor at the outer surface of the plasma membrane.

transduction (cell signaling)

the binding of the signaling molecule alters the receptor and initiates a signal transduction pathway; transduction often occurs in a series of steps

signaling cascade/pathway

a series of chemical reactions that occur within a biological cell when initiated by a stimulus

Epinephrine (adrenaline)

Produced by adrenal medulla. Targets liver, muscle, and adipose tissue to raise blood level of sugar and fatty acids; increases heart rate and force of contraction.

Melatonin function

maintains sleep/wake cycle; high during sleep

Melatonin

A hormone manufactured by the pineal gland that produces sleepiness.

Insulin

A protein hormone synthesized in the pancreas that regulates blood sugar levels by facilitating the uptake of glucose into tissues

tyrosine kinase receptors

- transmembrane receptors - bind EC ligand (conformational change outside first)

- then internal part of receptor phosphorylates tyrosine sidechains (conformational change) induces:

- auto-phosphorylation = first step in signalling cascade

- eg Insulin R = 4 subunits (2 alpha EC + 2beta IC) disulfide bonds = outside cell

- platelet derived growth factors = single pass protein

tyrosine kinase

An enzyme that catalyzes the transfer of phosphate groups from ATP to the amino acid tyrosine on a substrate protein.

Phosphorylation

The metabolic process of introducing a phosphate group into an organic molecule.

Kinase

an enzyme that catalyzes the transfer of a phosphate group from ATP to a specified molecule.

Outline how suprachiasmatic nuclei cells sense and respond to changes in light

Light Detection:

Retinal Photoreceptors: Specialized retinal ganglion cells (ipRGCs) containing melanopsin detect ambient light levels.

Signal Transmission:

Retinohypothalamic Tract (RHT): Signals from ipRGCs travel via the RHT directly to the SCN.

Neuronal Activity: Light exposure increases SCN neuronal firing rates, particularly during the daytime.

Gene Expression: Light induces expression of immediate-early genes in the SCN, influencing circadian rhythms

Outline the mechanism of action of epinephrine as a signaling molecule.

Receptor Binding:

Adrenergic Receptors: Epinephrine is a non-selective agonist, activating both alpha (α) and beta (β) adrenergic receptors.

Physiological Effects:

Cardiovascular System: Activation of β₁ receptors increases heart rate and contractility, enhancing cardiac output.

Respiratory System: Stimulation of β₂ receptors leads to bronchodilation, improving airflow in the lungs.

Vascular System: Binding to α₁ receptors causes vasoconstriction, which raises blood pressure and redirects blood flow to essential organs