D2.3-Water-Potential

Solution

A homogeneous mixture where a solute is dissolved in a solvent (e.g., salt water).

Solvent

The substance that does the dissolving in a solution. In biology, this is almost always water.

Solute

The substance that is dissolved in a solvent (e.g., salt, sugar, ions).

Solvation

The process where solvent molecules surround and interact with solute particles, helping them dissolve (in water, it’s called hydration).

Polar Molecules

Molecules with an uneven distribution of charge (partial positive and partial negative ends), allowing them to dissolve well in water (e.g., water, glucose).

Hydration Shell

When ions or polar molecules dissolve in water, water molecules surround them, forming a "shell" where the partially charged ends of water orient toward the charged solute.
This stabilizes the solute in solution and prevents particles from re-clumping.

Hydrogen Bonds

Weak attractions between a slightly positive hydrogen atom in one polar molecule and a slightly negative atom (like O or N) in another. These bonds give water many of its unique properties.

Osmosis

The passive movement of water across a semipermeable membrane from a region of higher water potential (lower solute concentration) to lower water potential (higher solute concentration).

Hypertonic Solution

A solution with higher solute concentration (and lower water potential) than the cell. Water moves out of the cell, causing the cell to shrink.

Hypotonic Solution

A solution with lower solute concentration (and higher water potential) than the cell. Water moves into the cell, causing it to swell or burst.

Isotonic Solution

A solution with equal solute concentration to the cell. There is no net movement of water, and cell volume stays constant.

Standard Deviation

A measure of how spread out data values are around the mean. A small standard deviation means data points are close to the mean.

Standard Error (SE)

An estimate of how far the sample mean is likely to be from the true population mean. Often shown as error bars; smaller SE = more reliable mean.

Crenation

Shriveling of animal cells (like red blood cells) placed in a hypertonic solution as water leaves the cell.

Cytolysis

Bursting of animal cells in a hypotonic solution as too much water enters and the cell membrane can’t withstand the pressure.

Contractile Vacuole

An organelle in some freshwater protists (e.g., Paramecium) that pumps excess water out of the cell to prevent cytolysis.

Osmoregulation

The control of water and solute levels in an organism or cell to maintain a stable internal environment (homeostasis).

Plasmolysis

Shrinking of the plant cell cytoplasm and membrane away from the cell wall due to water loss in a hypertonic solution.

Turgor Pressure

The pressure of the cell contents against the cell wall in plant cells when water enters by osmosis; it keeps plants firm and upright.

Turgid

A plant cell that is full of water, with high turgor pressure; the cell is firm and swollen (usually in a hypotonic or pure water environment).

Flaccid

A plant cell that has lost some water and has low turgor pressure. The cell is soft/limp (often in an isotonic solution).

Intravenous Fluids (IV Fluids)

Sterile solutions given directly into a vein. They must be isotonic to blood plasma to avoid crenation or cytolysis of blood cells.

Water Potential (Ψ)

A measure of the potential energy of water in a system, compared to pure water. Determines the direction water will move. Water moves from higher Ψ to lower Ψ.

Solute Potential (Ψs)

The component of water potential related to solute concentration. More solute → lower (more negative) solute potential → less free water.

Pressure Potential (Ψp)

The component of water potential due to physical pressure on a solution (e.g., turgor pressure in plants). Can be positive (pushing) or negative (tension).