Water, Hydrogen, and Carbon (OCR A-Level Biology A): Revision Notes

Water, Hydrogen, and Carbon

Constituents of cells

Approximately $99\%$ of all living organisms consist of just four elements: carbon, hydrogen, oxygen, and nitrogen. These elements combine with a small number of additional elements to form all the molecules found within living cells.

The resulting molecules are termed biological molecules, which include:

• Water, which constitutes a substantial proportion of living cells
• Carbon-based molecules combined with hydrogen and oxygen (e.g. carbohydrates and triglycerides)
• Carbon-based molecules containing nitrogen alongside carbon, hydrogen, and oxygen (e.g. proteins and nucleic acids)

Water and hydrogen bonding

Water is essential for life. The human body comprises approximately $80\%$ water, whilst certain plants and animals may contain up to $95\%$ water by mass.

Structure of water

A water molecule (H₂O) forms when two hydrogen atoms bond covalently with one oxygen atom. A covalent bond is a strong chemical bond created by sharing electrons between atoms.

The oxygen atom draws electrons away from the hydrogen atoms, creating an uneven distribution of charge within the molecule. This forms a polar molecule – one with regions of negative and positive charge. The oxygen carries a slight negative charge (δ⁻), whilst each hydrogen carries a slight positive charge (δ⁺). Overall, the molecule remains electrically neutral.

Hydrogen bonds

Water molecules attract one another through weak interactions called hydrogen bonds. These form between the slightly positive hydrogen of one water molecule and the slightly negative oxygen of an adjacent molecule.

This attraction causes water molecules to flow together as a liquid and to be drawn towards charged particles or charged surfaces. Other similar molecules behave differently due to their distinct properties. For instance, hydrogen sulfide (H₂S) exists as a gas at 0\,^{\circ}\mathrm{C} because of its very low boiling point, whereas water remains liquid at this temperature – accounting for many of water's unique characteristics.

Unique properties of water

Thermal properties

Hydrogen bonds between water molecules continuously form and break, permitting the molecules to flow freely.

Water exists as a liquid between $0^\circ\text{C}$ and $100^\circ\text{C}$, providing an ideal habitat for living organisms. Water functions as an efficient coolant due to its high specific heat capacity – it absorbs considerable heat energy relative to its mass.

Water molecules cannot readily escape unless heat energy is supplied to weaken the hydrogen bonds. At $100^\circ\text{C}$, water transforms into a gas called water vapour.

As water molecules cool, they lose kinetic energy and the hydrogen bonds become harder to break. The molecules form a more rigid structure – a solid (ice). At $0^\circ\text{C}$, solid water is less dense than liquid water and therefore floats on its surface. This creates an insulating layer that maintains a more constant temperature in the water beneath, allowing organisms to survive below the ice.

Water as a solvent

Water molecules (the solvent) surround solute molecules and maintain them in solution – for example, in salt or sugar solutions. A solution consists of a liquid (the solvent) containing dissolved solids, liquids, or gases (solutes).

The dissolved solute reacts more readily when in solution and can participate in metabolic processes. Because water is polar, other polar molecules dissolve readily in water.

Carbon-containing molecules with charged ionised groups – such as carboxyl groups (–COO⁻) or amino groups (–NH₃⁺) – dissolve easily by forming hydrogen bonds with water's hydroxyl groups (–OH).

Ionic substances like sodium chloride (NaCl) release positively charged ions (cations) or negatively charged ions (anions), such as Na⁺ and Cl⁻. Water molecules form clusters around these ions, keeping them dissolved.

Non-polar substances cannot dissolve in water and are repelled by it. They are termed hydrophobic (water-repelling), such as oils. This property is important in the phospholipid bilayer of cell membranes.

The role of water in living things

The thermostable properties of water and its capacity to act as a solvent perform vital roles in living organisms. These properties enable chemical reactions to occur within cells and allow molecules to dissolve in the cytosol.

Water serves as an ideal transport medium because it dissolves polar molecules and ions, which can then be transported efficiently around living organisms – for example, in the blood transport system of many animals.

Hydrogen bonds cause water molecules to be attracted to each other (cohesion) and to surfaces (adhesion). Cohesion is a property where water molecules are attracted to each other by hydrogen bonding, allowing them to move together. Adhesion is a property where water molecules are attracted to surfaces such as cell walls, vessels, or tubes.

The hydrogen bonds pull water molecules towards each other, causing them to stick together. At the surface, these bonds pull molecules inward, creating surface tension – a force that stretches the surface. This effect is visible in spherical water droplets on glass or waxy leaves. Some insects exploit this surface tension to move across water surfaces.

Due to cohesion and adhesion combined, water can move up narrow tubes (such as a straw or a xylem vessel in plant vascular tissue). Water also functions as a coolant for plants as it moves through the transpiration stream, and provides suitable habitats for organisms in both freshwater and saltwater environments.

An abundance of carbon

In living organisms, the most common element by mass is oxygen, followed by carbon.

Carbon as a building block

Atoms share electrons to achieve a complete outer shell, making them stable. Carbon makes an excellent building block because it is a small atom with four pairs of electrons in its outer shell. These electron pairs repel each other, pushing the atom into a tetrahedral shape.

These four electron pairs can be shared with other atoms to form covalent bonds, enabling atoms to join and create organic molecules. Carbon's four bond sites form stable, strong covalent bonds capable of building larger molecules.

Carbon atoms can bond to other carbon atoms, forming:

• Short chains (e.g. in the amino acid alanine)
• Long chains (e.g. in fatty acids)
• Branched chains (e.g. in the amino acid valine)
• Ring shapes (e.g. in α-glucose)

Carbon also forms covalent bonds with oxygen, hydrogen, and other atoms such as nitrogen and sulfur, creating numerous different large molecules.

Sometimes carbon forms two bonds with another atom, called a double bond. In hydrocarbon chains, two carbons may share two bonds (C=C). In glucose, one oxygen atom forms a double bond with an adjacent carbon atom (C=O).

Building large molecules from small – monomers and polymers

Monomers and polymers

A monomer is a single small molecule that can form covalent bonds with other similar small molecules to build a larger molecule. These bonds form through a condensation reaction.

A dimer consists of two monomers joined by a condensation reaction.

A polymer is a large molecule constructed from many similar monomers joined by covalent bonds to form a chain or branched chain. Each bond forms via a condensation reaction.

Biological molecules	Monomers	Polymers
Carbohydrates	Monosaccharides (e.g. glucose, fructose, ribose, triose)	Polysaccharides (e.g. starch/amylose, glycogen, cellulose)
Proteins	Amino acids (e.g. glycine, valine, alanine)	Polypeptides (e.g. amylase, lysozyme)
Nucleic acids	Nucleotides	Polynucleotides (e.g. DNA, RNA)

Condensation reactions

Each monomer joins to another by forming a specific covalent bond through a chemical reaction called a condensation reaction. In this reaction, one molecule of water (two hydrogen atoms and one oxygen atom) is released.

The same reaction repeats each time another monomer is added to form the polymer. Condensation reactions (also termed dehydration reactions) are involved in forming many macromolecules, including triglycerides.

Hydrolysis reactions

A hydrolysis reaction is the chemical reaction that splits molecules joined by covalent bonds. In hydrolysis, a water molecule must be added to break the covalent bond, releasing the smaller molecules.

Hydrolysis occurs when breaking down disaccharides such as maltose, sucrose, and lactose. In maltose, two glucose molecules are released. In sucrose, one glucose and one fructose are released. In lactose, one glucose and one galactose are released.

Stabilisation of polymers

Polymers are often very large molecules, stabilised by numerous hydrogen bonds. These bonds help molecules maintain their final shape. Shape is crucial to molecular function, so hydrogen bonds (alongside other bonds) are important in maintaining polymer function.