Water and Inorganic Ions

You’ve probably heard that you should drink about 2 litres of water every day, but why is water so important? Water makes up most of your mass (around 60% of your body, and even more of each cell), and nearly every reaction in your body happens in water (e.g. respiration, protein synthesis, DNA replication).

On this page:

• Structure of water
• Water as a Metabolite and a Solvent
• Thermal Properties of Water
• Cohesion and Surface Tension
• Inorganic Ions
  • Hydrogen ions (H⁺)
  • Iron ions (Fe²⁺)
  • Sodium ions (Na⁺)
  • Phosphate ions (PO₄³⁻)
• Exam Question Practice

Water has some important properties that make it essential for cells.

Structure of water

A water molecule (H₂O) is polar.

In a water molecule:

• The oxygen atom is slightly negative (δ−)
• The two hydrogen atoms are slightly positive (δ+)

The slightly positive hydrogen of one water molecule is attracted to the slightly negative oxygen of another. This attraction is a hydrogen bond.

Although each one individually is weak, there are large numbers of them, and together they explain most of water’s properties. These are the properties you need to know:

• High heat capacity
• Large latent heat of vaporisation
• Cohesion (water molecules sticking to each other)
• Surface tension
• Solvent*
• Metabolite*

*These two aren’t caused by hydrogen bonds, but they’re still important: water is a good solvent because it’s polar, and it’s a metabolite because it takes part in reactions.

Water as a Metabolite and a Solvent

Water is a metabolite: this means it takes part in (is used or produced by) many metabolic reactions in cells.

• It’s removed in condensation reactions, e.g. joining glucose molecules to make starch
• It’s used in hydrolysis reactions, e.g. digestion and ATP hydrolysis
• It’s also used in photosynthesis and produced in respiration

Water is a solvent: because it’s polar (see above), ions and other polar molecules (like glucose) dissolve in it.

• Metabolic reactions happen in solution, e.g. in the cytoplasm
• Dissolved substances can be transported, e.g. in blood plasma, or in the xylem and phloem of plants

Thermal Properties of Water

High heat capacity: it takes a lot of energy to raise the temperature of water. This is because much of the energy goes into breaking hydrogen bonds first.

• This is important as water buffers changes in temperature: the temperature of cells, blood and lakes or seas stays fairly stable
• This keeps the temperature close to the optimum for enzymes

Large latent heat of vaporisation: it takes a lot of energy to evaporate water (to turn it from liquid to gas), again because hydrogen bonds have to be broken.

• So when water evaporates, it takes lots of heat with it, which has a cooling effect, e.g. sweating in mammals and transpiration in plants
• Because each gram of water removes so much heat, you get a big cooling effect with little loss of water

Cohesion and Surface Tension

Cohesion means water molecules stick together, because of the hydrogen bonds between them. This is important in two ways.

Columns of water in plants: in the xylem, water molecules stick together, so when water evaporates from the leaves, a continuous column of water is pulled up the plant (the transpiration stream). Think of drinking through a straw: the water comes up as one continuous column because the molecules stick together.

Surface tension: in the middle of the water, each water molecule is pulled equally in all directions by the molecules around it. But at the surface there are no water molecules above (just air), so the surface molecules are only pulled sideways and down, towards the rest of the water. This makes them cling tightly together, so the surface acts like a stretchy “skin”. It’s strong enough for small organisms, such as pond skaters, to stand on.

Don’t confuse cohesion (water molecules sticking to each other) with adhesion (water sticking to other surfaces, like the walls of the xylem).

Inorganic Ions

An ion is an atom or group of atoms with an electrical charge. Inorganic ions:

• Don’t contain carbon (organic molecules do)
• Are dissolved in the cytoplasm and body fluids, some in high concentrations and some in very low concentrations
• Each have a specific function, depending on their properties

These ions come up in lots of other topics, so each section links to the page where it’s covered in detail.

Hydrogen ions (H⁺)

The more H⁺ ions there are, the lower the pH (more acidic). pH matters because it affects enzymes: a change in pH changes the charges on the amino acids in the active site, so the substrate can’t bind as well, and a big change can denature the enzyme.

Iron ions (Fe²⁺)

• Iron ions are a component of haemoglobin
• Each of haemoglobin’s four polypeptide chains has a haem group containing an Fe²⁺ ion, and this is where oxygen binds

Sodium ions (Na⁺)

Sodium ions are needed to absorb glucose and amino acids in the ileum by co-transport: the co-transporter protein only carries glucose into the cell if Na⁺ comes in with it.

Phosphate ions (PO₄³⁻)

Your body recycles roughly its own mass in ATP every day, so cells need a constant supply of phosphate.

• Phosphate is a component of DNA and RNA nucleotides: phosphate groups link nucleotides together in the sugar-phosphate backbone
• Phosphate is a component of ATP: ATP has three phosphate groups, and hydrolysing ATP releases one as an inorganic phosphate (Pᵢ), releasing energy
• (Phosphate is also in phospholipids)

Exam Question Practice