Chemical Composition in a Cell, revision notes

Overview

Cells are made of a small set of chemical building blocks: water, carbohydrates, proteins, lipids and nucleic acids, each present in different proportions depending on the type of cell and its function. This chapter covers what each biomolecule is made of at the molecular level, the role it plays in keeping a cell and an organism alive, and the food tests a student uses in the laboratory to detect its presence in an unknown sample.

Knowing the monomers, the general shape of each polymer, and the exact test results precisely is essential for Paper 3, where a sample of unknown composition is often analysed step by step. Marks are frequently lost not from a lack of knowledge but from mixing up which colour change belongs to which test, so precise recall of the reagent-and-result pairs matters as much as understanding the underlying chemistry.

Every biomolecule in this chapter is built or broken by the same two reactions. Condensation joins monomers into a polymer and releases a water molecule at each new bond formed, so glucose units join into starch, amino acids join into a protein chain, and glycerol joins fatty acids into a fat.

Hydrolysis reverses this: water is added to split a polymer back into its monomers, which is exactly what digestion does to food in the gut before the body can absorb and use the smaller units. Recognising which of the two reactions is happening in a diagram or word equation, and stating correctly whether water is released or added, is a skill this chapter shares with the later chapters on digestion and enzymes.

Paper 3 questions on this chapter usually give a food sample of unknown content and ask which nutrients are present, so the four food tests must be known precisely: the reagent, the exact procedure, and the positive colour change for reducing sugar, starch, protein and lipid. A table comparing the four tests side by side, together with the biomolecule each test detects and the structural unit that biomolecule is built from, is one of the fastest ways to revise this chapter before an exam.

Students who lose marks here usually know the biology but mix up small wording, such as writing 'blue-black' for the wrong test or forgetting that only Benedict's solution requires heating.

Water (4.1)

Water makes up the majority of a living cell's mass and is the medium in which almost every biochemical reaction takes place. Its polar molecular structure, with a slightly negative oxygen atom and two slightly positive hydrogen atoms, allows it to form hydrogen bonds with other water molecules and to dissolve ionic and polar substances readily, which is why it is often called the universal solvent of the cell.

Because dissolved substances can move freely through water, it is the medium that transports nutrients, respiratory gases, hormones and waste products around the body of an organism, whether inside a single cell's cytoplasm or through the blood plasma and xylem sap of a larger organism. Water is also a reactant in hydrolysis reactions, where a water molecule is added to break down a polymer such as starch or a protein into its component monomers during digestion.

Water helps keep an organism's temperature stable because it has a high specific heat capacity, meaning it can absorb or release a considerable amount of heat energy with only a small change in its own temperature, buffering the cell against sudden temperature swings; evaporation of water, as in sweating or transpiration, also removes heat efficiently because breaking hydrogen bonds between water molecules to convert liquid to vapour absorbs a large amount of energy.

Carbohydrates (4.2)

Carbohydrates are compounds of carbon, hydrogen and oxygen, usually in the ratio (CH₂O)ₙ, and range in size from a single sugar unit to long branched chains of many thousands of units. Monosaccharides are the simplest carbohydrates, single sugar units such as glucose, fructose and galactose, and act as the immediate source of energy for respiration.

Disaccharides such as maltose, sucrose and lactose form when two monosaccharides join together by condensation, releasing one water molecule at the new bond.

Polysaccharides are long polymers of many monosaccharide units joined by condensation: starch and glycogen are compact, coiled storage polysaccharides found in plants and animals respectively, releasing glucose again by hydrolysis when energy is needed, while cellulose forms long, straight, cross-linked chains that give plant cell walls their tensile strength rather than serving as an energy store.

Proteins (4.3)

Proteins are polymers built from amino acid monomers joined end to end by peptide bonds formed through condensation, releasing a water molecule at each bond. Each amino acid has the same basic structure, an amino group, a carboxyl group and a variable side chain attached to a central carbon atom, and it is the sequence and identity of the side chains along the chain that gives every protein its unique three-dimensional shape.

This shape determines the protein's function: enzymes fold so that a precise active site can bind a specific substrate and catalyse a reaction, antibodies fold to recognise and bind a specific antigen, and structural proteins such as collagen and keratin form long fibres that give tissues strength. High temperature and extreme pH break the hydrogen bonds and other interactions holding this shape together, denaturing the protein so that it permanently loses its function even though the peptide bonds joining the amino acids remain intact.

Lipids (4.4)

A lipid molecule forms when one glycerol molecule condenses with three fatty acid molecules, releasing three water molecules and forming three ester bonds; this is why fats and oils are sometimes called triglycerides. Lipids store roughly twice as much energy per gram as carbohydrates because their molecules contain a much higher proportion of carbon-hydrogen bonds, making them the body's most efficient long-term energy store, and they also form the phospholipid bilayer of every cell membrane and provide thermal insulation and mechanical protection around organs.

Fatty acids are described as saturated if every bond between their carbon atoms is a single bond, which allows the molecules to pack tightly together into a solid at room temperature, as in most animal fats such as butter and lard; unsaturated fatty acids contain one or more carbon-to-carbon double bonds, which kink the chain and prevent tight packing, so they remain liquid oils at room temperature, as in corn oil, olive oil and other plant-derived fats.

Nucleic acids (4.5)

Nucleic acids are polymers of repeating monomers called nucleotides, and each nucleotide is built from three parts: a five-carbon sugar, a phosphate group and a nitrogenous base. DNA (deoxyribonucleic acid) consists of two nucleotide strands wound around each other into a double helix, held together by hydrogen bonds between complementary bases on opposite strands, and stores an organism's complete genetic instructions as a specific sequence of bases along its length.

RNA (ribonucleic acid) is usually single-stranded and carries a working copy of part of the genetic code from the DNA in the nucleus to the site of protein synthesis, where the sequence of bases is read to determine the sequence of amino acids joined together to build a specific protein. The order of bases along a strand of DNA or RNA is therefore the fundamental link between an organism's genetic information and the proteins that carry out its cell functions.

Food tests for biomolecules

Paper 3 questions on this chapter typically give a food sample of unknown composition and ask which nutrients are present, so each of the four standard food tests must be known with its exact reagent, procedure and positive result. Benedict's test for reducing sugar requires the sample to be heated with Benedict's solution in a water bath, and a positive result is shown by a colour change from blue through green and yellow to a brick-red precipitate.

The iodine test for starch needs no heating: a drop of iodine solution added to the sample turns from orange-brown to blue-black if starch is present.

The Biuret test for protein is carried out at room temperature by adding Biuret solution (or sodium hydroxide followed by dilute copper sulfate) to the sample, which turns purple (violet) if protein is present and stays blue if it is absent. The emulsion test for lipids involves shaking the sample with ethanol, then pouring the mixture into water; a cloudy, milky-white emulsion layer forming on top of the water indicates a lipid is present, while the Sudan III test instead adds a red dye that stains a fat or oil layer red on the surface of the mixture.

Condensation and hydrolysis across biomolecules

The same two opposite reactions build and break down carbohydrates, proteins and lipids. Condensation joins two smaller monomer molecules into a larger polymer molecule and releases one water molecule at every new bond formed, for example, two glucose molecules condense to form maltose, two amino acids condense to form a dipeptide, and glycerol condenses with a fatty acid to form an ester bond in a lipid.

Hydrolysis is the reverse process: a water molecule is added across a bond to split a polymer back into its monomers, which is exactly what digestive enzymes do to large food molecules in the gut, breaking starch into glucose, proteins into amino acids, and fats into glycerol and fatty acids so that the smaller units can be absorbed into the blood. Recognising whether a diagram or word equation shows a water molecule being released (condensation) or added (hydrolysis) is a skill this chapter shares directly with the later chapters on digestion and enzymes.

Key concepts to master

• Water, Water makes up most of the mass of a living cell and is the medium in which nearly all reactions occur. It dissolves ionic and polar substances so nutrients and wastes can be transported, acts as a reactant in hydrolysis reactions, and helps keep cell temperature stable because it absorbs heat without a large rise in its own temperature.
• Carbohydrates, Carbohydrates range from single sugar units (monosaccharides such as glucose and fructose) to two linked units (disaccharides such as maltose and sucrose) to long chains of hundreds or thousands of units (polysaccharides such as starch, glycogen and cellulose). Their main role is to supply and store energy, though cellulose instead gives plant cell walls structural strength.
• Proteins, Proteins are polymers of amino acids joined by peptide bonds and folded into a specific three-dimensional shape that determines their function. They form enzymes that catalyse reactions, antibodies that fight infection, and structural material such as keratin and collagen. Heat and extreme pH break the bonds holding the shape together, denaturing the protein and destroying its function.
• Lipids, Lipids are fats and oils formed when one glycerol molecule joins with three fatty acid molecules by condensation. They store roughly twice as much energy per gram as carbohydrates, form the phospholipid bilayer of cell membranes, and provide insulation and protection around organs; saturated fatty acids have only single bonds while unsaturated fatty acids contain one or more double bonds.
• Nucleic acids, Nucleic acids are polymers of nucleotides, each nucleotide made of a sugar, a phosphate group and a nitrogenous base. DNA stores the genetic instructions for an organism as a sequence of bases arranged along a double helix, while RNA carries a copy of part of this code to build proteins in the cell.
• Food tests, Four food tests identify the main biomolecules in a sample: Benedict's solution turns brick-red on heating with a reducing sugar, iodine solution turns blue-black with starch, Biuret solution turns purple in the presence of protein without heating, and the emulsion or Sudan III test shows a milky-white layer or red colouration for lipids.
• Condensation and hydrolysis, Monomers join to form a polymer through condensation, a reaction that releases one water molecule at every new bond formed; digestion reverses this process through hydrolysis, in which a water molecule is added to break a bond and split the polymer back into its monomers. The same two reactions build and break down carbohydrates, proteins and lipids.
• DNA structure, DNA is a double helix made of two strands of nucleotides twisted around each other, held together by base pairing between the two strands. The sequence of bases along one strand is the genetic code, and it determines the sequence of amino acids joined together when a cell builds a particular protein.
• Saturated and unsaturated fats, Saturated fatty acids contain only single bonds between their carbon atoms, pack closely together and are usually solid at room temperature, as in most animal fats. Unsaturated fatty acids contain one or more carbon-to-carbon double bonds, do not pack as closely, and are usually liquid oils from plant sources such as corn or olive oil.
• Biomolecules with structural roles, Beyond storing energy, three groups of biomolecules build physical structure: cellulose strengthens plant cell walls, structural proteins such as collagen and keratin form connective tissue, hair and nails, and phospholipids arrange into the bilayer of every cell membrane. Comparing this structural role against the energy-storage role of starch, glycogen and fat is a common way exam questions test understanding across the whole chapter.

Quick recall checklist

1. Can you define and explain Water?
2. Can you define and explain Carbohydrates?
3. Can you define and explain Proteins?
4. Can you define and explain Lipids?
5. Can you define and explain Nucleic acids?
6. Can you define and explain Food tests?
7. Can you define and explain Condensation and hydrolysis?
8. Can you define and explain DNA structure?
9. Can you define and explain Saturated and unsaturated fats?
10. Can you define and explain Biomolecules with structural roles?

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