Question 2 of 9: Water Relations in Plant and Animal Cells

Question 2: Water Relations in Plant and Animal Cells (20 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Water movement into and out of cells. Water crosses cell membranes primarily by osmosis — diffusion of water down its own chemical-potential (water-activity) gradient, through the lipid bilayer and through membrane water channels (aquaporins). The direction and rate of net flux is set by the difference in water potential Ψ between the cell interior and its surroundings, where Ψ = Ψ_osmotic + Ψ_pressure (+ Ψ_matric in a porous solid matrix). Dissolved solutes lower the osmotic component (make Ψ more negative), so water moves toward the higher-solute (more negative Ψ) side until either equilibrium is reached or a counteracting pressure builds up.

Retention in plant cells. The plant cell has a rigid cellulose–hemicellulose cell wall surrounding the plasma membrane and a large central vacuole. Water entering the vacuole by osmosis pushes the protoplast against the wall, and the wall pushes back with an equal and opposite turgor pressure; equilibrium is reached when the pressure potential exactly balances the (negative) osmotic potential, i.e. Ψ_cell = Ψ_outside with Ψ_p > 0. Turgor pressure is what keeps non-woody plant tissue firm; loss of turgor (plasmolysis, wilting) occurs when the surrounding medium becomes more concentrated than the cell (e.g. salting, drying) and water leaves the vacuole.

Retention in animal cells. Animal cells have no rigid wall, so they cannot develop a compensating wall pressure; instead they must be kept in an isotonic environment (interstitial fluid actively regulated to match intracellular osmolarity) or they will swell and lyse (hypotonic surroundings) or shrink/crenate (hypertonic surroundings). Water retention in animal tissue instead depends on macromolecular hydration — proteins (e.g. actin–myosin, collagen) and glycosaminoglycans bind water through hydrogen bonding and electrostatic interaction with charged/polar side chains, and on the Gibbs–Donnan effect from fixed intracellular macromolecular charge, which pulls in mobile counter-ions and, with them, water.

Free water vs. bound water. Free water behaves like bulk liquid water: it has the vapor pressure, freezing point, and solvent properties of pure water and is readily removed by mild drying, freezing, or pressing. Bound water is water held so tightly by hydrogen bonding to polar/charged sites on macromolecules (or by capillary forces in fine pores) that its properties are measurably altered — a lower freezing point (it may not freeze at all at typical freezer temperatures), reduced solvent capacity, and much lower vapor pressure than free water at the same temperature. Only a few molecular layers around a macromolecule are truly "bound" in this sense; the rest of the tissue's water content is free or loosely (multilayer) bound.

Molecular adsorption mechanisms. Water molecules adsorb onto biological macromolecules through (i) hydrogen bonding to polar groups (–OH, –COOH, –NH₂, carbonyl oxygens), (ii) ion–dipole (electrostatic) attraction to charged side chains (Gibbs–Donnan-active groups), and (iii) capillary condensation in the sub-micron pores of a porous solid matrix (cell wall, connective tissue), where surface tension in a curved meniscus lowers the local vapor pressure below the bulk value (Kelvin equation). The first monolayer is adsorbed most strongly (highest binding energy, the "BET monolayer"); subsequent layers are progressively more free-water-like.

Water activity (a_w). Water activity is defined as a_w = p/p₀ = ERH/100, the ratio of the vapor pressure of water in the material to the vapor pressure of pure water at the same temperature (equivalently, the equilibrium relative humidity divided by 100). It is a thermodynamic measure of how "available" the water is for microbial growth, enzymatic reaction, and chemical spoilage reactions — not simply how much water is present. A moisture–sorption isotherm (moisture content vs. a_w at constant T) is sigmoidal: the low-a_w region (mono/multilayer adsorption, BET region) holds bound water very tightly with only a small moisture-content change per unit a_w, while the high-a_w region (capillary condensation and free water) shows moisture content rising steeply with a_w.

Influence on material properties. Because a_w (not total moisture) controls microbial and enzymatic activity, foods and biomaterials with the same moisture content but different a_w (e.g. due to added solutes/humectants) can have very different shelf lives — most bacteria are inhibited below a_w ≈ 0.90–0.91, most fungi below a_w ≈ 0.80. Bound water does not freeze at normal freezer temperatures, so it does not contribute to freeze-concentration damage or ice-crystal-driven texture loss, while free water does. Mechanically, loss of free (turgor or capillary) water is what drives shrinkage, case hardening, and textural toughening/staling during drying, whereas loss of even a small fraction of bound water can denature proteins and cause irreversible textural and functional changes.