AP Biology: Membrane Transport, Tonicity, and Water Potential

Comprehensive vocabulary flashcards covering passive and active membrane transport, carrier and channel mechanisms, pumps, bulk transport, tonicity, and water potential calculations from the lecture notes.

Selective Permeability

The property of cell membranes that regulates what enters and exits the cell, creating and maintaining concentration gradients of solutes due to a hydrophobic interior of tails that prevents polar and charged molecules from passing through.

Passive Transport

The transport of a molecule across a membrane that does not require energy from the cell because the solute moves with its concentration or electrochemical gradient.

Diffusion

A spontaneous passive transport process resulting from the constant motion of molecules, wherein substances move down their concentration gradient from an area of high concentration to an area of low concentration.

Osmosis

The diffusion of water down its concentration gradient across a selectively permeable membrane, moving from areas of low solute concentration to areas of high solute concentration (hypotonic to hypertonic).

Facilitated Diffusion

The passive diffusion of molecules down their concentration gradient through the membrane via channel or carrier proteins without requiring cellular energy.

Channel Proteins

Transmembrane proteins that form open pores across the cell membrane to move specific charged ions (such as $Na^+$ and $K^+$); many are gated and open or close in response to stimuli.

Aquaporins

Specialized channel proteins that facilitate the passage of water across the cell membrane.

Carrier Proteins

Transmembrane proteins that alternate between two conformations to transport large polar molecules, such as glucose and other sugars, amino acids, and nucleosides.

Active Transport

The transport of a molecule across a membrane that requires cellular energy because it moves a solute against its concentration gradient.

Adenosine Triphosphate (ATP)

The energy source used by cells to power active transport; it can transfer its terminal phosphate group to a transport protein to induce a conformational change.

Membrane Potential

The electrical charge across a membrane resulting from unequal concentrations of ions, where the cytoplasm is relatively negative compared to the extracellular fluid.

Electrogenic Pumps

Membrane proteins that generate voltage across membranes, which can be stored as an energy source in electrochemical gradients for cellular processes.

Sodium-Potassium Pump

An electrogenic pump in animal cells that hydrolyzes $ATP$ to pump $3\,Na^+$ out of the cell and $2\,K^+$ into the cell, creating a $+1$ net charge in the extracellular fluid.

Proton Pump

An integral membrane protein used by plants, fungi, and bacteria that pumps $H^+$ out of the cell to build up a proton gradient.

Cotransport

The coupling of the favorable downhill diffusion of one substance with the unfavorable uphill movement of another substance against its concentration gradient using stored electrochemical energy.

Sucrose-$\text{H}^+$ Cotransporter

A plant transport protein that allows sucrose to move into the cell against its concentration gradient only when coupled with $H^+$ diffusing down its electrochemical gradient.

Exocytosis

The active secretion of intracellular molecules via vesicles that fuse to the plasma membrane and release their contents into the extracellular fluid.

Endocytosis

The active uptake of extracellular molecules and matter where the plasma membrane folds inward, pinches off, and forms a vesicle inside the cell.

Phagocytosis

A type of endocytosis where a cell engulfs large particles by surrounding them with pseudopodia and packaging them into a food vacuole to be digested by lysosomes.

Pinocytosis

A type of endocytosis characterized by the nonspecific cellular uptake of extracellular fluid containing dissolved molecules.

Receptor-Mediated Endocytosis

A specific type of endocytosis where solutes bind to plasma membrane receptors and cluster into coated vesicles to allow the cell to take up large quantities of a target substance.

Tonicity

The ability of an extracellular solution to cause a cell to gain or lose water, depending on the concentration of nonpenetrating solutes that cannot pass through the membrane.

Osmoregulation

The process by which cells regulate their solute concentrations and maintain water balance relative to their external environment.

Isotonic Solution

A solution where the concentration of nonpenetrating solutes outside the cell equals that inside the cell, resulting in no net movement of water.

Hypertonic Solution

A solution where the concentration of nonpenetrating solutes outside the cell is higher than inside, causing cells to lose water, shrivel, and potentially die.

Plasmolysis

The process occurring in plant cells immersed in a hypertonic solution wherein the vacuole shrinks and the plasma membrane pulls away from the cell wall.

Hypotonic Solution

A solution where the concentration of nonpenetrating solutes outside the cell is lower than inside, leading to water uptake that lyses animal cells but allows plant cells to function optimally.

Turgor Pressure

The physical force exerted by the vacuole fluid against the plant cell wall when immersed in a hypotonic environment, maintaining structural support and optimal function.

Water Potential ($\Psi$)

A physical property measured in megapascals ($\text{MPa}$) or bars ($1\,\text{MPa} = 10\,\text{bars}$) that predicts the direction water will flow, combining solute concentration and physical pressure.

Solute Potential ($\Psi_s$)

Also called osmotic potential; the component of water potential that becomes negative as solutes bind free water, decreasing overall water potential ($0\,\text{MPa}$ for pure water).

Pressure Potential ($\Psi_p$)

The physical pressure exerted on a solution relative to atmospheric pressure; it can be positive or negative, and equals $0\,\text{MPa}$ ($0\,\text{bars}$) in open air.

Solute Potential Formula

The equation $\Psi_s = -iCRT$, where $i$ is the ionization constant, $C$ is molar concentration, $R$ is the pressure constant ($0.0831\,\text{liter bars/mol}\cdot\text{K}$ or $0.00831\,\text{liter MPa/mol}\cdot\text{K}$), and $T$ is temperature in Kelvin ($K = 273 + ^\circ\text{C}$).

Ionization Constant ($i$)

The factor in the solute potential formula representing the number of particles formed when a solute dissolves in water, equaling $1$ for non-ionizing molecules (such as sucrose) and $2$ for $NaCl$.