cell signalling

Autocrine

Cell targets itself (self signalling)

Eg. Cancer cells releasing growth signals to self-stimulate

Paracrine

Cell targets nearby cells

Eg. Neurotransmitters or local inflammation responses

Endocrine

Cell targets distant cells via the bloodstream

Eg. Hormones like insulin or adrenaline

Juxtacrine

Cell communicates via direct contact (often via membrane proteins)

Eg. Immune cells recognising antigens, development signalling

cAMP or cyclic AMP - Second messengers and proteins in signalling

A second messenger produced in response to a signal like a hormone. Activates protein kinase A (PKA), which phosphorylates target proteins to cause a response Eg. Adrenaline increases cAMP to trigger energy release

CDKs or cyclin-dependent kinases - Second messengers and proteins in signalling

Enzymes that regulate the cell cycle, they must bind to cyclines (regulatory proteins) to be active, CDKs phosphorylate specific targets to progress through the cycle

Ligand

A signalling molecule (eg hormone, neurotrasmitter, cytokine) that binds to a receptor on or inside a target cell

Hormone

A type of ligand made by endocrine glands that travels via the bloodstream to act on distant targets

Receptor

A protein (on cell membrane or inside cell) that recognises and binds ligands, triggering a response

Signal transduction

The process of converting the extracellular signal (ligand binding) into a functional change inside the cell

metabolism

sum of all chemical reactions that occur within a living organism to maintain life. reactions are involved in converting energy and matter to support cellular functions like growth, repair, reproduction and maintanence. (cell signalling regulates metabolic processes like hormonal regulation)

two types: catabolism and anabolism

signal reception

in the case of extracellular signals (like hormones or growth factors), the signalling molecules ligands typically bind to receptors located on the cell membrane. these receptors are often integral membrane proteins, which span the cell membrane and allow the cell to detect signals from outside

for intracellular, some signalling molecules (like steroid hormones or thyroid hormones) are lipid-soluble and can pass through the cell membrane and bind directly to receptors inside the cell, usually in the cytoplasm or nucleus

catabolism

the breakdown of larger molecules into smaller ones, releasing energy (exothermic) eg breakdown of glucose to produce ATP

anabolic

the building of larger molecules form smaller ones, requiring energy (endergonic) eg. protein synthesis from amino acids

hormonal regulation of metabolism

hormones such as insulin, glucagon, epinephrine and cortisol serve as signalling molecules that regulate anabolic and catabolic pathways.

eg insulin promotes anabolic processes like glycogen synthesis and protein synthesis when glucose is abundant in bloodstream

eg glycagon and epinephrine promote catabolic processes like glycogen breakdown and fat breakdown when the body needs more energy

signal transduction pathways and enzyme regulation

signalling molecules (like cAMP, calcium ions, protein kinases) activate signal transduction pathways, which can result in the activation or inhibition of enzymes involved in metabolic reactions

cAMP activates protein kinase A which can then phosphorylate and regulate enzymes in processes like glycogen metabolism (activating glycogen phosphorylase, for example)

cellular energy sensing and metabolism

AMP-activates protein kinase (AMPK) is a key regulator of cellular energy balance, acting as an energy sensor.

when energy levels are low (eg high AMP/ATP ratio, AMPK is activated and switches on pathways that increase ATP production (catabolic processes) and turn off ATP-consuming processes (anabolic processes)

crosstalk between metabolic pathways and sensing

metabolic intermediates like acetyl-CoA can also serve as signalling molecules, influencing gene expression and cell signalling pathways.

acetyl-CoA can influence histone acetylation, affecting chromatin remodelling and gene transcription, thus regulating processes like cell growth and differentiation

metabolic reactions linked to cell growth and division

cell division and growth are energy-intensive processes, so metabolic pathways (like glycolysis and the citric acid cycle) are regulated by cell signalling to ensure enough energy and resources are available

growth factors stimulate pathways that promote protein synthesis and cell growth, which are energy consuming activities

signal transduction (inside the cell) (signal reception is on cell membrane)

once ligand binds to the receptor, transduction happens, second messengers relay and amplify the signal from the receptor to intracellular targets, initiating cascade of reactions inside the cell