Learning objectives
Explain how the polarity of the water molecule produces hydrogen bonding, and link that bonding to cohesion, adhesion, surface tension, high specific heat and solvent behavior
Explain why these properties matter to living systems, using transpiration, temperature buffering and the transport of solutes in blood
Identify carbon, hydrogen, oxygen, nitrogen, phosphorus and sulfur as the main elements of biological molecules, and state where nitrogen and phosphorus appear
Justify a claim about water as a biological solvent, and explain why carbon forming four covalent bonds allows such varied macromolecules
Key definitions
Polar covalent bond
A shared pair of electrons held unequally, because one atom attracts the pair more strongly, leaving one end of the bond slightly negative and the other slightly positive.
Hydrogen bond
A weak attraction between the slightly positive hydrogen of one polar molecule and a slightly negative oxygen or nitrogen on another molecule or another part of the same molecule.
Cohesion
The attraction of water molecules to one another through hydrogen bonding, which holds a column of water together and produces surface tension.
Adhesion
The attraction of water molecules to a different polar or charged surface, such as the cellulose walls of a xylem vessel.
Specific heat
The energy needed to raise the temperature of one gram of a substance by one degree Celsius; for water it is 4.184 J per gram per degree.
Hydrophobic
Describes a non-polar substance that cannot form hydrogen bonds with water and is therefore excluded from the hydrogen-bonded network.
Explanation
Polarity comes first. In each O–H bond the oxygen nucleus attracts the shared electrons far more strongly than the hydrogen does, so the electrons spend more time near the oxygen. That gives oxygen a partial negative charge (δ−) and each hydrogen a partial positive charge (δ+). The molecule is also bent, with a bond angle near 104.5°, so the two bond dipoles do not cancel and the whole molecule is polar. Water is still electrically neutral overall: polarity is about where the charge sits, not about a net charge.
From polarity to hydrogen bonding. The δ+ hydrogen of one water molecule is attracted to the δ− oxygen of a neighbor. This hydrogen bond is roughly a twentieth the strength of the covalent bond inside the molecule, and it constantly breaks and re-forms, but each molecule can make up to four of them at once. Almost every property in this topic follows from the fact that the bonds are individually weak and collectively very numerous. Cohesion is hydrogen bonding of water to water; adhesion is hydrogen bonding of water to another polar surface.
Cohesion, adhesion and transpiration. Cohesion holds a column of water together, so when water evaporates from the spongy mesophyll of a leaf the whole column in the xylem is pulled upwards without breaking: this is the cohesion–tension explanation of transpiration. Adhesion to the cellulose lining of the xylem helps resist the pull of gravity and produces capillary rise in narrow tubes. At an air–water surface the molecules below pull inward with no molecules above to balance them, giving surface tension strong enough to support a pond skater.
Thermal properties. Heating water first goes into breaking hydrogen bonds rather than into faster molecular motion, so water has a high specific heat and its temperature changes slowly. That buffers the temperature of cells, of lakes and of coastal climates. For the same reason the heat of vaporization is high, near 2260 J per gram: only the fastest molecules escape, they carry away a lot of energy, and the liquid left behind cools. Sweating in mammals and evaporative cooling of a transpiring leaf both use this. Ice is also less dense than liquid water, because each molecule is locked into four hydrogen bonds in an open lattice, so ponds freeze from the top and organisms survive beneath.
Water as a solvent. Polar and ionic solutes dissolve because water molecules surround each ion or polar group with a hydration shell, with δ− oxygens facing cations and δ+ hydrogens facing anions. Blood plasma can therefore carry glucose, amino acids, mineral ions and dissolved gases, and the cytosol can hold the reactants of metabolism in solution where they collide and react. Non-polar molecules such as fats cannot hydrogen-bond with water, so the water network excludes them; this hydrophobic exclusion is what drives phospholipids into a bilayer and pushes non-polar amino acid R-groups into the core of a folded protein.
The elements of life. Six elements — carbon, hydrogen, oxygen, nitrogen, phosphorus and sulfur, often abbreviated CHNOPS — make up the great majority of the dry mass of a cell. Nitrogen appears in the amino group of every amino acid and in the nitrogenous bases of nucleotides, so nitrogen supply limits protein and nucleic acid synthesis. Phosphorus appears in the phosphate groups that link nucleotides into the sugar–phosphate backbone of DNA and RNA, in the phosphate head of a phospholipid, and in ATP. Sulfur appears in the R-groups of cysteine and methionine, and the disulfide bridges between cysteines help hold a tertiary structure in place.
Why carbon? A carbon atom has four outer electrons, so it forms four covalent bonds. It can bond to other carbon atoms as readily as to hydrogen, oxygen, nitrogen or sulfur, producing straight chains, branched chains and rings, with single or double bonds. Each bonding arrangement has a definite geometry, so the same set of atoms can be built into molecules of very different shape. Because function in biology follows shape, this versatility is why carbon skeletons carry the functional groups of every macromolecule in the next topic.
Formulae
Energy transferred as heat: q = m × c × ΔT, with c = 4.184 J g⁻¹ °C⁻¹ for liquid water
Heat of vaporization of water ≈ 2260 J g⁻¹ at 100 °C
Percentage change = (change ÷ starting value) × 100
Worked examples
q: A student heats 250 g of water in a calorimeter and its temperature rises from 21.0 °C to 33.0 °C. The specific heat of water is 4.184 J g⁻¹ °C⁻¹. (a) Calculate the energy absorbed. (b) The same energy is supplied to 250 g of a cooking oil with a specific heat of 2.00 J g⁻¹ °C⁻¹. Calculate its temperature rise and comment on what this shows about water.
steps: ΔT for the water = 33.0 − 21.0 = 12.0 °C.
q = m × c × ΔT = 250 × 4.184 × 12.0. First 4.184 × 12.0 = 50.208 J per gram.
q = 250 × 50.208 = 12 552 J, which is 12.552 kJ.
For the oil, ΔT = q ÷ (m × c) = 12 552 ÷ (250 × 2.00) = 12 552 ÷ 500 = 25.1 °C.
The same energy raises the oil more than twice as far, so water resists temperature change far better; its high specific heat buffers the temperature of cells and of bodies of water.
In context
A sweating athlete loses heat mainly through the high heat of vaporization of water: each gram evaporated from the skin removes about 2260 J.
Coastal towns have milder summers and winters than inland towns at the same latitude because the ocean absorbs and releases large amounts of heat with little change in its own temperature.
A 100 m redwood lifts water to its crown without a pump, using evaporation at the leaves pulling on a cohesive, adhesive column of water in the xylem.
Common misconceptions
Students often say water molecules are charged. In fact each molecule is neutral overall; polarity means the charge is unevenly distributed, giving partial charges.
Students often describe the hydrogen bond as the bond holding a water molecule together. In fact the bonds inside the molecule are polar covalent, and hydrogen bonds act between molecules.
Students often say oil is repelled by water. In fact water molecules are attracted to one another more strongly than to the non-polar oil, so the oil is excluded rather than pushed away by a repulsive force.
Students often claim water has a high specific heat because it is a liquid. In fact it is because energy goes into breaking hydrogen bonds before it increases molecular motion.
Study tips
When a question says "explain", give the causal chain: polarity → hydrogen bonding → the named property → the biological consequence. Naming the property alone earns little.
For a claim–evidence–reasoning item, state the claim in one sentence, quote the specific evidence, then say in a separate sentence why that evidence supports the claim.
Watch the units in a specific heat calculation: mass in grams with c in J g⁻¹ °C⁻¹, and use the temperature change rather than the final temperature.
Reasoning
The hydrogen bonding and carbon chemistry set out here are the foundation of the next topic, where monomers are joined into the four classes of macromolecule.
Summary
Unequal electron sharing plus a bent shape makes water polar; polarity produces hydrogen bonding.
Cohesion, adhesion and surface tension hold water columns together and drive transpiration.
High specific heat and high heat of vaporization buffer temperature and allow evaporative cooling.
Water dissolves polar and ionic solutes by forming hydration shells, and excludes non-polar molecules.
CHNOPS build biological molecules: nitrogen in amino acids and bases, phosphorus in nucleic acids, ATP and phospholipids.
Check your understanding
Question 1: Water molecules stick to the cellulose walls of a xylem vessel. This is…
1. cohesion
2. surface tension
3. adhesion
Question 2: Which element is present in every amino acid but absent from a triglyceride?
1. Carbon
2. Nitrogen
3. Oxygen
Question 3: Ice floats because…
1. hydrogen bonds hold the molecules in an open lattice
2. the covalent bonds lengthen on freezing
3. trapped air lowers the density