Biological Molecules: Question 9
Syllabus 2.4
Many of water's biological roles arise from hydrogen bonding between its polar molecules.
(a) Explain how hydrogen bonding between water molecules gives rise to cohesion, and describe how this property helps transport water through the xylem of a flowering plant. [3]
(b) Explain how hydrogen bonding between water molecules gives water a high latent heat of vaporisation, and describe how this property benefits a mammal that cools itself by sweating. [3]
(c) State one reason why water's properties as a solvent are important for metabolic reactions inside a cell. [2]
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Worked solution
Part (a): Cohesion and transport in the xylem
Water is a polar molecule, with a slightly positive charge on its hydrogen atoms and a slightly negative charge on its oxygen atom. This polarity allows a hydrogen bond to form between the hydrogen atom of one water molecule and the oxygen atom of a neighbouring water molecule. Because these hydrogen bonds form throughout the liquid, water molecules are strongly attracted to one another, a property called cohesion.
In a flowering plant, water evaporates from the surfaces of mesophyll cells in the leaf and diffuses out through the stomata (transpiration). This creates tension that pulls more water molecules upward from the xylem vessel below. Because of cohesion, the water molecules in the xylem remain linked together as an unbroken column, so this pulling force is transmitted all the way down the plant to the roots, drawing more water up. Without cohesion, the water column would break, and water could not be transported over such long distances against gravity.
Part (b): High latent heat of vaporisation and evaporative cooling
For a water molecule to evaporate, to change from a liquid to a gas, the hydrogen bonds holding it to neighbouring water molecules must be broken. Because there are so many hydrogen bonds to break, a relatively large amount of energy is needed to evaporate even a small mass of water, giving water a high latent heat of vaporisation.
When a mammal sweats, water (sweat) on the surface of the skin absorbs heat energy from the body in order to evaporate. Because evaporating water removes a large amount of heat energy for each gram evaporated, only a small volume of sweat needs to evaporate to remove a substantial amount of heat from the body. This cools the skin and the blood flowing near it, helping to keep the mammal’s core body temperature stable despite heat generated by metabolism or a hot environment, which protects enzyme-controlled reactions from disruption by overheating.
Part (c): Water as a solvent
Because water is polar, it can form hydrogen bonds with other polar molecules and can surround (hydrate) charged ions, drawing them into solution. This solvent property allows many of the substances involved in metabolism (such as glucose, amino acids, inorganic ions and enzymes) to dissolve in the aqueous cytoplasm of a cell, so that metabolic reactions can take place in solution, and dissolved substances can be transported around the cell or around the whole organism (for example, in blood plasma).
Final answers
- (a) Hydrogen bonding gives water cohesion; cohesive water molecules form an unbroken column that can be pulled up through the xylem as transpiration occurs.
- (b) Breaking many hydrogen bonds during evaporation requires a large amount of energy (high latent heat of vaporisation); evaporating a small volume of sweat removes a large amount of heat, cooling the mammal.
- (c) Water’s polarity lets it dissolve polar and ionic solutes, allowing metabolic reactions to occur in solution and substances to be transported.