Form 4 · Paper 2 essay guide

Chemical Composition in a Cell, Paper 2 essay guide

How Chemical Composition in a Cell appears in Paper 2 essays: themes, a planning grid, a model answer structure and the keyword list.

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Essay themes this chapter supports

Explain how condensation and hydrolysis build and break down the main biomolecules: Define condensation and hydrolysis and state which molecule is released or added in each → Give a named example of condensation for a carbohydrate, a protein and a lipid → Give a named example of hydrolysis, linking it to digestion in the gut → Explain why recognising these two reactions is useful across every biomolecule in the chapter → Conclude by linking condensation and hydrolysis to how the body builds and breaks down its own molecules
Compare the structure and function of carbohydrates, proteins and lipids: State the monomer of each biomolecule and how the monomers join together → Describe one structural feature that is unique to each biomolecule → Explain the main function of each biomolecule in an organism → Compare the energy value of lipids against carbohydrates per gram, giving a reason → Conclude by linking each biomolecule's structure directly to the function it performs
Describe the four food tests used to identify biomolecules in a sample: State the reagent and procedure for each of the four food tests → State whether heating is required for each test → State the positive result for each test precisely, including the colour or physical change → Explain how a table of results from an unknown sample can be used to identify which nutrients are present → Conclude by linking accurate recall of each test to success in Paper 3 practical questions
Explain the structure of DNA and its role in storing genetic information: Describe the structure of a nucleotide and how nucleotides join to form a strand → Describe the double helix structure of DNA and the role of base pairing → Explain how the sequence of bases forms the genetic code → Compare DNA briefly with RNA, noting one structural difference → Conclude by linking DNA's structure to why it is well suited to storing genetic information reliably

How to structure your essay

Open with one or two sentences that define the topic and set the scope.
Devote one paragraph to each key concept, in a logical order.
Use precise biological terms and, where relevant, a labelled diagram.
Give an example or state where the process happens in the body or plant.
Close by linking the ideas back to the question.

Planning grid

Question	Answer
Introduction	Introduce the five biomolecules covered in this chapter and state that the essay will explain how their structure relates to their function.
Point 1	Carbohydrates, monosaccharide monomers joined by glycosidic bonds; main role is energy supply and storage, e.g. glucose, starch, glycogen; cellulose instead gives structural strength.
Point 2	Proteins, amino acid monomers joined by peptide bonds, folded into a specific shape; roles include enzymes, antibodies and structural proteins such as collagen.
Point 3	Lipids, one glycerol joined to three fatty acids by condensation; store about twice the energy per gram of carbohydrates and form the cell membrane bilayer.
Point 4	Nucleic acids, nucleotide monomers made of a sugar, phosphate and base; DNA stores genetic information as a double helix, RNA carries a working copy to build proteins.
Conclusion	Summarise that each biomolecule's monomer and bond type directly explain its shape and, in turn, its function in the cell.

Key terms to include

carbohydrate
protein
lipid
nucleic acid
monomer
polymer
condensation
hydrolysis
peptide bond
glycosidic bond
denaturation
amino acid
monosaccharide
fatty acid
nucleotide

A model essay outline

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.

Frequently asked questions

What are the food tests I need for SPM Biology?

Benedict's test for reducing sugar (heated in a water bath, turns brick-red), iodine test for starch (turns blue-black at room temperature), Biuret test for protein (turns purple without heating), and the emulsion or Sudan III test for lipids (a milky-white layer or red colouration). You should know the reagent, method, whether heating is needed, and the positive result for each.

What does it mean when a protein is denatured?

Denaturation is a permanent change in a protein's three-dimensional shape, usually caused by high temperature or extreme pH breaking the bonds that hold the folded structure together. The protein loses its function, for example an enzyme can no longer bind its substrate, or an antibody can no longer recognise its antigen, but the sequence of amino acids in the chain is not broken apart.

What are the monomers of the main biomolecules?

Carbohydrates are made of monosaccharides such as glucose, proteins are made of amino acids joined by peptide bonds, and lipids are made of one glycerol molecule joined to three fatty acid molecules. Nucleic acids are made of nucleotides, each built from a sugar, a phosphate group and a nitrogenous base.