Rucete ✏ AP Biology In a Nutshell
4. Movement of Water in Cells
This chapter explores water potential and osmolarity, explaining how water moves in and out of cells, how water potential is calculated, and how organisms regulate water balance to maintain homeostasis.
Water Potential
• Water potential (Ψ) describes the potential energy of water in a solution.
• Water moves from areas of higher water potential to areas of lower water potential.
• Hypotonic: less solute, more water → higher water potential.
• Hypertonic: more solute, less water → lower water potential.
• Isotonic: equal solute concentrations → no net water movement.
• Water potential is not a relative term—it can be calculated and compared numerically.
Water Potential Components
• Total water potential (Ψ) has two components: – Ψs = solute potential – Ψp = pressure potential
• Equation: Ψ = Ψs + Ψp
• Water flows like a ball rolling downhill—from higher to lower potential.
Solute Potential (Ψs)
• Calculated using the formula: Ψs = –iCRT
– i = ionization constant (e.g., 1 for glucose, 2 for NaCl, 3 for CaCl₂) – C = concentration (mol/L) – R = pressure constant (given) – T = temperature in Kelvin (K = °C + 273)
• More solute = lower solute potential = more negative Ψs.
• Distilled water has Ψs = 0 because C = 0.
Pressure Potential (Ψp)
• Positive Ψp: pressure pushes water out (e.g., turgid plant cells, Super Soaker toy).
• Negative Ψp: suction pulls water in (e.g., drinking through a straw).
• Most systems open to the atmosphere have Ψp = 0, so Ψ ≈ Ψs.
Water Movement and Examples
• Example 1: A 0.50 M glucose solution at 21°C, open to the atmosphere → Ψp = 0 – i = 1 (glucose is covalent) – T = 294 K → calculate Ψs using the equation.
• Example 2: A plant cell with Ψp = 1.0 bars and Ψs = –6.5 bars – Ψ = Ψs + Ψp = –5.5 bars
Osmolarity
• Osmolarity: total concentration of all solute particles in a solution.
• The higher the osmolarity, the lower the water potential.
• Water moves from lower osmolarity (fewer solutes) to higher osmolarity (more solutes).
• Osmolarity is key in comparing solutions to predict net water movement.
Homeostasis and Osmoregulation
• Homeostasis: the ability to maintain a stable internal environment.
• Osmoregulation: the process of regulating water and solute concentrations to maintain homeostasis.
• Organisms must control internal osmolarity to survive in varying external conditions.
Cell Responses to Tonicity
• Hypotonic solution: – Animal cells may burst (lysis). – Plant cells become turgid (ideal for plants).
• Isotonic solution: – No net water movement; animal cells are stable; plant cells are flaccid.
• Hypertonic solution: – Animal cells shrink (crenation). – Plant cells undergo plasmolysis (membrane pulls away from wall).
Applications
• Intravenous (IV) fluids must be isotonic to avoid damaging blood cells.
• Plants rely on turgor pressure (Ψp) for structural support; wilting occurs when water potential decreases.
• Marine vs. freshwater organisms exhibit different osmoregulatory strategies:
– Freshwater fish: excrete dilute urine, actively absorb ions.
– Saltwater fish: drink seawater, excrete excess salt via gills.
In a Nutshell
Water potential and osmolarity govern the movement of water in and out of cells. Cells respond to the tonicity of their environment, and organisms maintain internal balance through osmoregulation. Understanding these principles is key to explaining how cells and organisms adapt to diverse environmental conditions.