Introduction to Physiology: Homeostasis, Membrane Transport, and Cell Communication

Comprehensive practice vocabulary flashcards covering basic principles of homeostasis, passive and active transport mechanisms, osmosis, tonicity, epithelial transport, and receptor signaling.

Homeostasis

The process of maintaining the composition of the extracellular fluid within a specific range required to sustain life.

Negative Feedback

A homeostatic control mechanism that maintains equilibrium by opposing changes, where an initial increase in a variable triggers pathways that decrease it back to its set point.

Set Point

The desired reference value maintained by a control system, such as a normal body temperature of approximately $37^\circ\text{C}$.

Integrator

A component of a feedback control system, typically the brain or hypothalamus, that receives inputs from sensors, compares controlled variables to a set point, and initiates an effector response.

Positive Feedback

A mechanism that reinforces movement further away from a set point by self-stimulating a process until an endpoint is reached, such as during the depolarization phase of an action potential.

Feed Forward

A physiological response initiated in anticipation of a change in a variable, such as producing saliva and digestive enzymes upon smelling or seeing food.

Phospholipid

A major membrane lipid molecule composed of a glycerol backbone, two non-polar hydrophobic fatty acid chains, and a polar hydrophilic head group.

Hydrophilic

A chemical property describing polar or charged molecules or groups that attract water.

Hydrophobic

A chemical property describing non-polar molecules or groups, such as fatty acid chains, that push away or fear water molecules.

Transmembrane Proteins

Proteins that span entirely through the lipid bilayer and are exposed on both extracellular and intracellular sides of the plasma membrane.

Peripheral Proteins

Proteins associated with the inner or outer surface of the plasma membrane that do not penetrate the hydrophobic core.

Integral Proteins

Proteins permanently bound to specific regions of the plasma membrane, such as nicotinic receptors localized at high concentrations in postsynaptic muscle membranes.

Tight Junctions

Intercellular membrane structures that encircle epithelial cells to form a selective barrier that controls or completely prevents the passage of solutes and water between cells.

Transcellular Pathway

The movement of solutes or water across an epithelial cell layer by sequentially passing through two plasma membranes (luminal and basolateral).

Paracellular Pathway

The movement of substances through the spaces between adjacent cells via tight junctions.

Gap Junctions

Intercellular channels formed by connexins (such as connexin 43) that allow direct bidirectional passage of electrical signals, water, and small molecules between adjacent cells for synchronized action.

Simple Diffusion

The passive net movement of solutes down their concentration gradient driven by continuous thermal Brownian motion without requiring cellular energy.

Aquaporins

Selective transmembrane protein pores that remain constantly open to provide rapid transport of water molecules across plasma membranes.

Ion Channels

Transmembrane protein structures that facilitate the movement of specific ions across plasma membranes and are controlled by gating mechanisms.

Carrier Proteins

Transmembrane proteins that undergo cyclical conformational changes to transport specific large polar molecules, such as glucose or urea, down their concentration gradient.

Facilitated Diffusion

Passive transport of specific molecules across a cell membrane down their concentration gradient via carrier proteins without energy consumption.

Primary Active Transport

A transport mechanism that directly hydrolyzes ATP via ATPase pumps to move ions across a plasma membrane against their concentration gradient.

Sodium-Potassium ATPase

A primary active pump present in cell membranes that hydrolyzes ATP to export $3\,\text{Na}^+$ ions and import $2\,\text{K}^+$ ions against their concentration gradients.

Secondary Active Transport

A transport system that moves solutes or ions against their concentration gradient using the energy stored in an established ion concentration gradient, typically sodium.

Cotransporters

Secondary active transport proteins that move an driving ion and a solute in the same direction across the plasma membrane.

Countertransporters

Secondary active transport proteins that move an driving ion and a solute in opposite directions across the plasma membrane, such as the sodium-calcium exchanger.

Endocytosis

An active mechanism that internalizes extracellular fluids (pinocytosis), solid particles (phagocytosis), or specific ligands into the cell by forming membrane-bound vesicles.

Exocytosis

An active transport process triggered by an elevation in cytosolic calcium where intracellular vesicles fuse with the plasma membrane to secrete large amounts of particles into the extracellular fluid.

Osmosis

The net diffusion of water across a semipermeable membrane from an area of higher water concentration to an area of lower water concentration.

Osmolarity

The concentration of a solution expressed as the total number of osmotically active solute particles per liter of solution ($\text{osmol/L}$ or $\text{mOsm/L}$).

Osmolality

A temperature-independent measure of solute concentration expressed as the number of osmotically active solute particles per kilogram of solvent ($\text{osmol/kg}$).

Penetrating Solutes

Solutes capable of crossing the plasma membrane (such as gases, steroids, ethanol, glucose, and urea) that do not cause sustained net water movement or long-term cell volume changes.

Non-Penetrating Solutes

Solutes unable to freely cross the plasma membrane (such as $\text{Na}^+$, $\text{K}^+$, and $\text{Cl}^-$ ions) that create osmotic gradients driving net water movement and altering cell volume.

Toxicity

The measure of a solution's ability to alter cell volume by driving osmotic water movement, determined strictly by the concentration of non-penetrating solutes.

Isotonic Solution

A solution containing a concentration of non-penetrating solutes equal to that of the intracellular fluid (approximately $290\,\text{mOsm/L}$ to $310\,\text{mOsm/L}$), resulting in zero net water movement.

Hypertonic Solution

A solution with a higher concentration of non-penetrating solutes than the cytoplasm, causing water to diffuse out of the cell and leading to cell shrinkage.

Hypotonic Solution

A solution with a lower concentration of non-penetrating solutes than the cytoplasm, causing water to diffuse into the cell and leading to cell swelling or bursting.

Agonist

A chemical messenger or exogenous drug that binds to a specific receptor and activates it to produce a biological response.

Antagonist

A substance or drug that binds to a receptor without activating it, occupying the site and blocking access to agonists.

Metabotropic Receptors

Plasma membrane receptors linked to intracellular metabolic pathways, often via G-proteins, that alter cellular biochemistry and function.

Ionotropic Receptors

Plasma membrane receptors that function directly as ligand-gated ion channels, rapidly altering ion permeability and membrane potential upon ligand binding.

Down-Regulation

A reduction in the total number or responsiveness of target cell receptors caused by chronic overstimulation by an agonist.

Up-Regulation

An increase in the total number or sensitivity of target cell surface receptors resulting from chronic lack of agonist stimulation or long-term exposure to an antagonist.