BIO UNIT 3

Cell signaling

How cells communicate by sending, receiving, and responding to chemical signals.

Ligand

A signaling molecule that binds to a specific receptor on or inside a target cell.

Target cell

A cell that has a receptor capable of recognizing and binding a particular signaling molecule.

Paracrine signaling

Short-distance signaling in which a cell releases signals that affect nearby cells.

Autocrine signaling

Signaling in which a cell releases a signal that binds to receptors on its own surface.

Endocrine signaling

Long-distance signaling in which hormones travel through the bloodstream to target cells.

Synaptic signaling

Signaling in which neurons release neurotransmitters across a synapse to communicate with nearby target cells.

Reception

The first stage of cell signaling, when a signaling molecule binds to a receptor and activates it.

Transduction

The second stage of cell signaling, when an activated receptor triggers a series of molecular changes inside the cell.

Response

The final stage of cell signaling, when the cell carries out a specific action, such as changing gene expression or enzyme activity.

G-protein-coupled receptor (GPCR)

A membrane receptor that activates a G protein when a signaling molecule binds to it.

G protein

A protein that relays signals from a GPCR to other proteins inside the cell; it becomes active when GDP is replaced by GTP.

GTP

A molecule that binds to and activates a G protein; the G protein hydrolyzes GTP into GDP to switch itself off.

GDP

The inactive form of the nucleotide bound to an inactive G protein.

Receptor tyrosine kinase (RTK)

A membrane receptor that activates itself by adding phosphate groups to tyrosines when a signaling molecule binds.

Dimerization

The joining of two receptor proteins together, commonly occurring when a ligand activates a receptor tyrosine kinase.

Kinase

An enzyme that transfers phosphate groups from ATP to another molecule, often changing that molecule's activity.

Phosphorylation

The addition of a phosphate group to a protein or other molecule, which can activate or deactivate its function.

Phosphorylation cascade

A series of proteins activated through phosphorylation, allowing a signal to be amplified and passed through a cell.

Protein kinase

A kinase that adds phosphate groups to proteins, often changing their activity.

Protein phosphatase

An enzyme that removes phosphate groups from proteins, reversing phosphorylation.

Signal amplification

The process in which one signaling molecule activates many molecules at the next step, producing a larger cellular response.

Specificity

The ability of a signaling molecule to produce a particular response only in cells with the appropriate receptors and signaling proteins.

Second messenger

A small, nonprotein molecule or ion that relays signals inside a cell, such as cAMP or calcium ions.

cAMP

A second messenger made from ATP that activates proteins such as protein kinase A during signal transduction.

Calcium ions (Ca2+)

Second messengers that help regulate processes such as muscle contraction, secretion, and enzyme activity.

Calcium storage

The endoplasmic reticulum stores calcium in many cells; muscle cells store calcium in the specialized sarcoplasmic reticulum.

Signal termination

The process of stopping a signaling pathway through mechanisms such as GTP hydrolysis, ligand removal, or protein dephosphorylation.

Advantage of multi-step transduction

Multiple steps allow signal amplification, greater control, regulation, and different responses from the same signal.

Homeostasis

The maintenance of relatively stable internal conditions despite changes in the internal or external environment.

Negative feedback

A regulatory mechanism in which a change triggers responses that reverse the original change and return conditions toward a set point.

Insulin

A hormone produced by beta cells of the pancreas that lowers blood glucose by promoting glucose uptake and storage.

Beta cells

Cells in the pancreas that produce and release insulin when blood glucose levels rise.

Glucagon

A hormone produced by alpha cells of the pancreas that raises blood glucose by stimulating glycogen breakdown and glucose production in the liver.

Alpha cells

Cells in the pancreas that produce and release glucagon when blood glucose levels fall.

Glycogen

A storage form of glucose found primarily in the liver and skeletal muscles.

Glycogenesis

The process of converting glucose into glycogen for storage.

Glycogenolysis

The breakdown of glycogen into glucose.

Calcitonin

A hormone produced by the thyroid gland that lowers blood calcium levels by inhibiting osteoclast activity.

Parathyroid hormone (PTH)

A hormone produced by the parathyroid glands that raises blood calcium levels by increasing calcium release from bone, kidney reabsorption, and intestinal absorption.

Osteoblasts

Bone-forming cells that build new bone and help deposit calcium into bone tissue.

Osteoclasts

Cells that break down bone tissue, releasing calcium into the bloodstream.

Antidiuretic hormone (ADH)

A hormone produced in the hypothalamus and released by the posterior pituitary that increases water reabsorption in the kidneys, reducing water loss in urine.

Osmolarity

The concentration of dissolved particles in a solution.

Osmoreceptors

Sensors in the hypothalamus that detect changes in blood osmolarity and help regulate ADH release.

Aquaporins

Channel proteins that allow water to move across cell membranes, including in kidney collecting ducts.

Renin

An enzyme released by the kidneys when blood pressure or blood volume is low; it begins the renin-angiotensin-aldosterone system.

Angiotensinogen

A protein produced by the liver that is converted into angiotensin I by renin.

Angiotensin I

A molecule produced when renin acts on angiotensinogen; it is converted into angiotensin II.

Angiotensin II

A hormone that causes vasoconstriction and stimulates aldosterone release, increasing blood pressure.

Aldosterone

A hormone released by the adrenal cortex that increases sodium reabsorption by the kidneys; water follows sodium, increasing blood volume and pressure.

Renin-angiotensin-aldosterone system (RAAS)

A hormonal pathway activated by low blood pressure that increases blood volume and pressure through vasoconstriction and sodium and water retention.

Baroreceptors

Pressure sensors located primarily in the carotid arteries and aortic arch that detect changes in blood pressure.

Vasoconstriction

The narrowing of blood vessels, which increases resistance and raises blood pressure.

Vasodilation

The widening of blood vessels, which decreases resistance and lowers blood pressure.

Sympathetic nervous system

The division of the autonomic nervous system responsible for the fight-or-flight response.

Fight-or-flight response

A response to stress that prepares the body for rapid action by increasing heart rate, blood flow to muscles, and available energy.

Epinephrine (adrenaline)

A hormone produced by the adrenal medulla that activates fight-or-flight responses, including increased heart rate and blood glucose.

Adrenal medulla

The inner region of the adrenal glands that releases epinephrine and norepinephrine during sympathetic activation.

Oxytocin

A hormone produced in the hypothalamus and released by the posterior pituitary that stimulates uterine contractions and milk ejection.

Milk ejection reflex

A positive feedback process in which oxytocin causes milk to be released, encouraging continued stimulation and further oxytocin release.

Positive feedback

A regulatory mechanism in which a response strengthens the original stimulus, continuing until a specific event ends the process.

Neuromuscular junction

The synapse between a motor neuron and a skeletal muscle fiber.

Motor neuron

A nerve cell that sends signals to skeletal muscle fibers to initiate contraction.

Acetylcholine (ACh)

A neurotransmitter released by motor neurons that binds receptors on muscle cells and initiates an action potential.

Acetylcholinesterase

An enzyme that breaks down acetylcholine in the synaptic cleft, stopping the signal.

Action potential

A rapid electrical signal that travels along a cell membrane due to changes in ion movement.

Sarcolemma

The cell membrane of a muscle fiber.

T-tubules

Extensions of the sarcolemma that carry action potentials deep into the muscle fiber.

Sarcoplasmic reticulum (SR)

A specialized endoplasmic reticulum in muscle cells that stores and releases calcium ions.

Voltage sensor (DHPR)

A protein in the T-tubule that detects an action potential and activates calcium release from the sarcoplasmic reticulum.

Ryanodine receptor

A calcium-release channel in the sarcoplasmic reticulum that releases calcium into the muscle cell cytoplasm.

Troponin

A regulatory protein that binds calcium, causing tropomyosin to move and expose binding sites on actin.

Tropomyosin

A protein that covers myosin-binding sites on actin when a muscle is relaxed.

Actin

The thin filament in muscle fibers that contains binding sites for myosin.

Myosin

The thick filament in muscle fibers that uses energy from ATP to pull actin filaments during contraction.

Cross-bridge

A temporary connection formed when a myosin head binds to an exposed site on actin.

Sliding filament model

The explanation of muscle contraction in which actin filaments slide past myosin filaments, shortening the sarcomere.

Sarcomere

The basic contractile unit of a muscle fiber, extending from one Z line to the next.

Calcium's role in muscle contraction

Calcium binds to troponin, causing tropomyosin to move and expose myosin-binding sites on actin.

ATP's role in muscle contraction

ATP allows myosin to detach from actin and reset its position; ATP also powers calcium pumps that return calcium to the SR.

Myosin power stroke

The movement of a myosin head that pulls actin toward the center of the sarcomere.

Muscle relaxation

Occurs when calcium is pumped back into the sarcoplasmic reticulum, allowing tropomyosin to cover actin's binding sites again.

Cell cycle

The series of events a cell goes through as it grows, copies its DNA, and divides into two daughter cells.

Interphase

The longest part of the cell cycle, consisting of G1, S, and G2, during which the cell grows and prepares for division.

G1 phase

The first stage of interphase, when the cell grows, performs normal functions, and produces proteins and organelles.

S phase

The stage of interphase when DNA is replicated, producing two identical sister chromatids for each chromosome.

G2 phase

The final stage of interphase, when the cell continues growing, makes proteins, and prepares for mitosis.

M phase

The stage of the cell cycle that includes mitosis and cytokinesis.

Mitosis

The division of the nucleus into two genetically identical nuclei.

Cytokinesis

The division of the cytoplasm, producing two separate daughter cells.

Cell cycle checkpoint

A control point that checks whether specific conditions have been met before the cell continues through the cell cycle.

G1 checkpoint

The checkpoint that determines whether the cell is large enough, has enough nutrients and growth signals, and has undamaged DNA before DNA replication.

G2 checkpoint

The checkpoint that checks whether DNA has been completely and correctly replicated before mitosis.

M checkpoint (spindle checkpoint)

The checkpoint that ensures chromosomes are properly attached to spindle fibers before sister chromatids separate.

Cyclin

A regulatory protein whose concentration changes throughout the cell cycle and helps control cell cycle progression.

Cyclin-dependent kinase (CDK)

An enzyme that regulates the cell cycle by phosphorylating target proteins; it becomes active when bound to a cyclin.

MPF (Maturation-promoting factor)

A complex of cyclin and CDK that triggers the cell's progression into mitosis.

Chromosome

A DNA molecule packaged with proteins that carries genetic information.

Sister chromatids

Two identical copies of a replicated chromosome that remain attached at the centromere.