Cell Signalling, Membrane Transport, and Homeostasis

Essential vocabulary flashcards covering the foundations of physiology, body fluid compartmentation, homeostasis, membrane transport mechanisms, and cellular signalling pathways.

Physiology

The branch of biology that aims to understand the mechanisms of living things, from cell function at the ionic and molecular level to the integrated behaviour of the whole body and the influence of the external environment.

Homeostasis

The maintenance of an organism's internal environment in a dynamic steady state.

Dynamic Steady State

A state in which materials continuously move back and forth between compartments to maintain stable physiological conditions despite concentrations not being equal across compartments.

Set Point

The target value of a physiological variable, such as $90\,mg/dL$ for non-fasting blood glucose.

Acceptable Range

The range of values a physiological variable can remain within without triggering a homeostatic response, such as $80 - 110\,mg/dL$ for plasma glucose.

Regulated Variable

The physiological variable that is monitored and kept within the acceptable range by physiological control mechanisms, such as plasma glucose concentration.

Controlled Variable

The physiological variable that acts to bring the regulated variable back within the acceptable range, such as insulin secreted by pancreatic $\beta$-cells or glucagon secreted by pancreatic $\alpha$-cells.

Negative Feedback

A homeostatic response loop mechanism where a detected change in a regulated variable triggers an effector response that switches off the loop, preventing overshooting of the set point.

Positive Feedback

A non-homeostatic feedback loop that promotes continual amplification of the controlled variable until an outside event stops the cycle, such as cervical stretch stimulating oxytocin release during labour.

Feedforward Mechanism

An anticipatory, non-homeostatic response that prepares the body for an impending change, such as the salivation and gastric acid release reflexes triggered by food cues.

Compartmentation

The division of space into separate macroscopic cavities or microscopic domains separated by membranes, allowing isolation and specialisation of function.

Extracellular Fluid (ECF)

Body fluid situated outside cells, composed of blood plasma within blood vessels and interstitial fluid surrounding most cells.

Intracellular Fluid (ICF)

The fluid contained within the cell membrane that provides an internal environment subdivided into distinct intracellular compartments.

Fluid-Mosaic Model

The structural arrangement of the cell membrane consisting of a phospholipid bilayer interspaced with proteins, lipids, and attached carbohydrates.

Glycocalyx

A protective carbohydrate-rich layer on the external surface of the cell membrane, composed of glycoproteins and glycolipids, essential for cell recognition.

Lipid Rafts

Membrane microdomains composed of sphingolipids with longer fatty acid tails and cholesterol that compartmentalise cellular processes.

Simple Diffusion

The passive movement of molecules directly across the phospholipid bilayer from an area of higher concentration to lower concentration without requiring energy input.

Fick's Law of Diffusion

A principle stating that the diffusion rate of a substance increases with an increase in surface area, concentration gradient, or membrane permeability.

Ion Channel

A pore-forming membrane protein facilitating rapid passive movement ($10^8\text{ ions/second}$) of ions down their electrochemical gradient across the lipid bilayer.

Resting Membrane Potential

The steady-state electrical charge difference across the cell membrane at rest, typically $-70\,mV$, where the intracellular fluid is net negative relative to the extracellular fluid.

Depolarization

A decrease in the membrane potential difference, shifting the membrane potential closer to $0\,mV$, typically caused by an influx of positive ions into the cell.

Hyperpolarization

An increase in the membrane potential difference, pushing the electrical state further away from $0\,mV$ into more negative values, caused by cation efflux or anion influx.

Carrier Protein

A membrane transport protein with a slower conductance rate ($10^2 - 10^5\text{ molecules/second}$) that binds specific substrates and undergoes conformational changes to move them across the membrane.

Sodium-Potassium Pump ($Na^+/K^+$ ATPase)

A primary active antiport carrier that hydrolyzes ATP to export $3\,Na^+$ ions and import $2\,K^+$ ions against their concentration gradients.

Secondary Active Transport

Transport driven by the potential energy stored in an electrochemical gradient generated by primary active transport, such as the sodium-glucose cotransporter (SGLT).

Transport Maximum ($T_{max}$)

The maximum transport rate observed when all binding sites on a population of carrier proteins are fully occupied by substrate.

Phagocytosis

An actin-mediated, ATP-dependent vesicular transport process in which a cell engulfs a large particle or bacterium into a phagosome that subsequently fuses with lysosomes for destruction.

Endocytosis

An ATP-dependent vesicular mechanism by which cells internalise extracellular fluids, macromolecules, or receptor-ligand complexes into small membrane-bound vesicles.

Ligand

An endogenous or exogenous molecule, also termed a first messenger, that binds specifically to a target receptor protein.

Receptor

A target molecule located in the nucleus, cytosol, or cell membrane through which physiological mediators like hormones, neurotransmitters, or drugs produce their effects.

Binding Affinity

The degree or strength with which a molecule binds to a receptor or carrier protein; higher affinity indicates higher probability of binding and lower likelihood of displacement.

Specificity

The selective ability of a receptor or carrier protein binding site to distinguish and bind only a particular molecule or closely related family of molecules.

Autocrine Signalling

A chemical signalling method in which a cell secretes a messenger molecule that acts locally upon receptors on that same cell.

Paracrine Signalling

A form of local cell communication in which a cell secretes chemical messengers that diffuse through the interstitial space to act on neighbouring cells.

Endocrine Signalling

A long-distance communication system in which hormones are released by endocrine cells into the bloodstream to reach distant target cells possessing specific receptors.

Neurohormone

A chemical messenger synthesized and secreted by a neuron directly into the bloodstream to act on distant target tissues.

Neuromodulator

A chemical released by a neuron that acts slowly in an autocrine or paracrine fashion to alter neuronal activity.

Lipophilic Signalling Molecule

A non-polar molecule, such as a corticosteroid or statin, that crosses the phospholipid bilayer by simple diffusion to bind cytosolic or nuclear receptors and regulate gene expression over hours to days.

Lipophobic Signalling Molecule

A water-soluble molecule, such as insulin or epinephrine, that cannot pass through the phospholipid bilayer and instead binds integral membrane receptors to trigger rapid cellular responses.

Signal Transduction

The process by which an extracellular signal molecule activates a membrane receptor that in turn alters intracellular molecules to initiate a cellular response.

Amplifier Enzyme

A membrane or intracellular enzyme activated by a ligand-bound receptor that synthesizes or mobilizes numerous second messenger molecules to amplify the signal.

Second Messenger

An intracellular signalling molecule, such as cAMP, cGMP, $IP_3$, DAG, or $Ca^{2+}$, that relays and amplifies signals within the cell following receptor activation.

Receptor Agonist

A chemical substance, either endogenous or exogenous, that binds to and activates a receptor, initiating signal transduction.

Receptor Antagonist

A molecule that binds to a membrane receptor and prevents activation, thereby blocking agonist binding and inhibiting signal transduction.

G Protein-Coupled Receptor (GPCR)

A large receptor family characterized by 7 transmembrane-spanning domains that interacts with intracellular heterotrimeric G proteins ($\alpha, \beta, \gamma$) to regulate ion channels or amplifier enzymes.

$G\alpha_s$ Subunit

A stimulatory G-protein subunit that activates adenylyl cyclase, increasing intracellular cAMP levels and subsequently activating protein kinase A (PKA) and EPAC.

$G\alpha_i$ Subunit

An inhibitory G-protein subunit that inhibits adenylyl cyclase, decreasing intracellular cAMP concentration.

$G\alpha_q$ Subunit

A G-protein subunit that activates phospholipase C (PLC), stimulating the cleavage of $PIP_2$ into inositol trisphosphate ($IP_3$) and diacylglycerol (DAG).

Protein Kinase

An enzyme that catalyzes the transfer of a phosphate group from ATP to specific amino acid residues (serine, threonine, or tyrosine) on target proteins.

Protein Phosphatase

An enzyme that hydrolytically removes a phosphate group from a phosphorylated protein, reversing kinase-mediated protein activation or deactivation.

Receptor Downregulation

A regulatory decrease in the number of functional receptors on a target cell surface, achieved through receptor internalisation or decreased protein expression.

Receptor Desensitisation

A process that diminishes receptor responsiveness to sustained agonist stimulation, occurring via receptor phosphorylation, uncoupling from G proteins, or internalisation.