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How do cells convert external signals into responses?
Through signal transduction pathways that process and relay the signal inside the cell.
What are the two types of cell signaling?
Local signaling and long-distance signaling.
What are two examples of local signaling?
Direct cell-to-cell contact and secreting local regulators.
How does long-distance signaling work?
One cell type releases chemical signals (hormones) that travel through the body to reach target cells of a different type.
What are two examples of hormones used in long-distance signaling?
Testosterone and estrogen.
What are the three stages of cell signaling?
Reception, Transduction, and Response.
What happens during reception?
The target cell detects a signaling molecule (ligand) from outside the cell.
What happens during transduction?
The receptor changes, triggering a cascade that converts the signal into a form that leads to a cellular response.
What happens during response?
The cell performs a specific function in reaction to the signal.
What happens when a ligand binds to a receptor?
The receptor undergoes a conformational change, which is the first step in transduction.
Where can receptors be found?
Intracellular receptors (inside the cytoplasm or nucleus) and plasma membrane receptors (on the cell surface).
What type of molecules bind to intracellular receptors?
Hydrophobic molecules that can cross the plasma membrane (e.g., testosterone).
How does testosterone signal a response in a cell?
It enters the cell, binds to an intracellular receptor, and this complex moves into the nucleus to regulate gene expression.
What type of molecules bind to plasma membrane receptors?
Water-soluble ligands (e.g., epinephrine).
How does epinephrine trigger a response in liver cells?
It binds to a plasma membrane receptor, initiating a cascade that converts glycogen into glucose.
What is a G protein-coupled receptor (GPCR)?
A membrane receptor that works with the help of a G protein.
What happens when a ligand binds to a GPCR?
The receptor is activated, changing shape and allowing a G protein to replace GDP with GTP (activating it).
What does the activated G protein do?
It diffuses along the membrane and activates a specific enzyme to trigger the next step in the pathway.
Why are G protein changes temporary?
The response stops when GTP is hydrolyzed back to GDP, deactivating the G protein.
What do ligand-gated ion channels do?
They open or close in response to a ligand, regulating the flow of specific ions.
What happens when a ligand binds to a ligand-gated ion channel?
The channel opens, allowing ions to flow into the cell, altering ion concentration and affecting cellular activity.
What happens when the ligand dissociates from the receptor?
The channel closes, stopping ion flow.
What is signal transduction?
A cascade of molecular interactions relaying signals from receptors to target molecules.
What is a phosphorylation cascade?
A sequence where protein kinases add phosphate groups to proteins, activating them.
Why is phosphorylation important in signal transduction?
It amplifies the signal, leading to a large cellular response.
What do protein phosphatases do?
They remove phosphate groups, turning off the signal.
What are second messengers?
Small, non-protein molecules or ions that help relay signals inside the cell.
What is an example of a second messenger?
Cyclic AMP (cAMP), which amplifies signals like epinephrine in liver cells.
What are two main ways a cell can respond to a signal?
Regulating transcription or modifying cytoplasmic activities.
How does cell signaling regulate transcription?
It activates transcription factors that turn specific genes on or off.
How does cell signaling regulate cytoplasmic activities?
It modifies enzyme activity or cytoskeleton structure.
How can signal transduction affect an organism's phenotype?
Environmental changes trigger pathways that alter gene expression and cellular behavior.
What is apoptosis?
A controlled cell suicide process that prevents damage to neighboring cells.
What triggers apoptosis?
Signals that activate a cascade of "suicide" proteins inside the cell.
Why is apoptosis important?
It helps with nervous system development, immune system function, and morphogenesis (e.g., finger separation in humans).
What is homeostasis?
The maintenance of a stable internal environment despite external changes.
How do homeostatic control systems work?
A set point is maintained by sensors detecting changes and triggering physiological responses.
What is an example of homeostasis in animals?
Snakes move in and out of sunlight to regulate body temperature.
What is negative feedback?
A response that reduces the original stimulus to maintain balance.
What is an example of negative feedback?
Sweating when body temperature rises, which cools the body back to normal.
Why is negative feedback important?
It prevents extreme fluctuations and maintains homeostasis.
What is positive feedback?
A response that amplifies the original stimulus.
What is an example of positive feedback?
Childbirth contractions intensify as pressure increases.
How do positive and negative feedback differ?
Negative feedback stabilizes (more gets you less), while positive feedback amplifies (more gets you more).
