Glossary
Adenylyl cyclase
An enzyme that converts ATP into cyclic AMP (cAMP) in response to a signal, often activated by a G protein.
Example:
In response to stress, the activation of adenylyl cyclase leads to a rapid increase in cAMP, which helps mobilize energy reserves.
Amplification
The process by which a signal is strengthened during a signal transduction pathway, allowing a small number of signaling molecules to elicit a large cellular response.
Example:
A single hormone molecule binding to a receptor can trigger the release of millions of glucose molecules from a liver cell, demonstrating signal amplification.
Cascade Effect
A phenomenon in signal transduction where a small initial signal is amplified through a series of sequential activation steps, leading to a large cellular response.
Example:
In the immune system, a single activated T cell can trigger a cascade effect, leading to the rapid proliferation of many more immune cells to fight an infection.
Cell communication
The process by which cells detect, interpret, and respond to signals from other cells and their environment.
Example:
During embryonic development, cells use cell communication to coordinate their growth and differentiation into specialized tissues.
Cell surface receptors
Receptor proteins embedded in the plasma membrane that bind to hydrophilic signaling molecules that cannot cross the membrane.
Example:
Many growth factors bind to cell surface receptors to stimulate cell division and tissue repair.
Cyclic-AMP (cAMP)
A common secondary messenger derived from ATP that activates protein kinases and other signaling molecules within the cell.
Example:
When a cell needs to break down glycogen, cyclic-AMP (cAMP) acts as a crucial internal signal, activating enzymes for glucose release.
G protein
A protein that binds GTP (guanosine triphosphate) and is involved in transmitting signals from G-protein-coupled receptors to other effector proteins.
Example:
When a G protein is activated by a GPCR, it can then activate an enzyme like adenylyl cyclase, propagating the signal within the cell.
G-Protein-Coupled Receptors (GPCRs)
A large family of cell surface receptors that work with the help of a G protein, often initiating a signal transduction pathway involving secondary messengers.
Example:
Many sensory experiences, such as smell and taste, rely on G-Protein-Coupled Receptors to detect specific molecules and initiate a cellular response.
Hydrophilic heads
The water-loving, phosphate-containing heads of phospholipids that face the aqueous environments inside and outside the cell.
Example:
The hydrophilic heads of the cell membrane interact with the watery cytoplasm and the extracellular fluid, forming the outer surfaces of the membrane.
Hydrophobic tails
The water-fearing, fatty acid tails of phospholipids that face inward, forming the core of the plasma membrane.
Example:
The hydrophobic tails create a nonpolar barrier that prevents most water-soluble molecules from freely crossing the cell membrane, maintaining cellular integrity.
Ion Channel Receptors
Transmembrane proteins that open or close a gate, allowing or blocking the flow of specific ions, when a ligand binds to them.
Example:
In nerve cells, Ion Channel Receptors open in response to neurotransmitters, allowing sodium ions to rush in and generate an electrical impulse.
Ligand
A molecule that specifically binds to another molecule, often a larger one, such as a receptor protein, initiating a cellular response.
Example:
Neurotransmitters like dopamine act as ligands when they bind to specific receptors on nerve cells, influencing mood and movement.
Lipid Hormones
Signaling molecules, such as steroid hormones, that are lipid-soluble and can pass directly through the plasma membrane to bind to intracellular receptors.
Example:
Testosterone, a lipid hormone, can easily diffuse into muscle cells to promote protein synthesis and muscle growth.
Phospholipid bilayer
The double layer of phospholipids that forms the plasma membrane, acting as a selective barrier around the cell.
Example:
The flexible nature of the phospholipid bilayer allows cells to change shape and move, like an amoeba engulfing food.
Phosphorylation
The addition of a phosphate group to a protein, typically by a protein kinase, which often changes the protein's shape and activates or deactivates it.
Example:
The phosphorylation of a protein can act like an on/off switch, controlling its activity in a signaling pathway.
Reception
The first stage of cell signaling, where a target cell detects a signaling molecule (ligand) when it binds to a specific receptor protein.
Example:
When a scent molecule enters your nose, its binding to a receptor on an olfactory neuron is the reception step that initiates the sense of smell.
Receptor protein
A protein, typically on the cell surface or inside the cell, that binds to a specific signaling molecule (ligand), initiating a cellular response.
Example:
The taste buds on your tongue contain specialized receptor proteins that bind to specific molecules, allowing you to perceive different flavors.
Response
The third and final stage of cell signaling, where the transduced signal triggers a specific cellular activity or change.
Example:
When a plant cell detects a growth hormone, its response might be to elongate, causing the plant to bend towards light.
Secondary Messengers
Small, non-protein, water-soluble molecules or ions that relay signals from cell surface receptors to target molecules inside the cell, amplifying the signal.
Example:
Calcium ions (Ca2+) can act as secondary messengers in muscle cells, triggering the contraction of muscle fibers.
Signal transduction pathway
A series of steps linking a mechanical or chemical stimulus to a specific cellular response, often involving multiple relay molecules.
Example:
The intricate process by which adrenaline prepares your body for 'fight or flight' involves a complex signal transduction pathway within various cell types.
Transcription factors
Proteins that bind to specific DNA sequences, thereby controlling the rate of transcription of genetic information from DNA to messenger RNA.
Example:
When activated by a steroid hormone, a transcription factor might enter the nucleus and turn on genes responsible for developing secondary sexual characteristics.
Transduction
The second stage of cell signaling, where the binding of the signaling molecule changes the receptor protein, initiating a series of molecular changes within the cell.
Example:
After light hits a photoreceptor in the eye, a complex transduction pathway converts that light energy into an electrical signal the brain can interpret.