Form 4 · Worked answers

Chemical Composition in a Cell, worked answers

Fully worked answers for Chemical Composition in a Cell, original structured and essay questions with mark-scheme keywords highlighted.

Book a Trial Class

One-hour paid trial · Same-day reply

How this topic is examined

Matching a food test to its positive result, its exact colour change, and whether heating is required.
Naming the monomers of a carbohydrate, protein or lipid and describing how they join together.
Explaining why heating denatures a protein and why this change cannot usually be reversed.
Describing condensation and hydrolysis with a named example of each, stating which bond is formed or broken.
Interpreting a table of food-test results to identify the nutrients present in an unknown sample.
Explaining why starch, glycogen and cellulose behave differently even though all three are polymers of glucose.

Model answer structure

Read the command word and answer to the marks, one clear point per mark.
Define the key biological term precisely before you explain it.
Explain the process or reason in the correct sequence, using the right terms.
Where useful, add a labelled diagram or a worked example.
End with the link the question asks for (cause → effect, structure → function).

Fully worked answers

A food sample gives a positive result with Benedict's solution but a negative result with the iodine test. State what this indicates about the sample's carbohydrate content, and explain the colour change you would expect to see.

[4]

Show answer

A positive Benedict's test with a negative iodine test indicates the sample contains a reducing sugar (such as glucose or maltose) but no starch. When Benedict's solution is added and the sample is heated in a water bath, the mixture changes colour progressively from blue through green and yellow to form a brick-red precipitate, whereas adding iodine solution produces no colour change from orange-brown because there is no starch present to bind with the iodine.

reducing sugarno starchBenedict's solutionheated in water bathbrick-red precipitate

Explain, in terms of bonding, why boiling an egg white (a protein) permanently changes its texture, and state the technical term for this change.

[5]

Show answer

Egg white protein (albumin) is folded into a specific three-dimensional shape held together by hydrogen bonds and other weak interactions between amino acid side chains. Heating the egg white to a high temperature during boiling supplies enough energy to break these bonds, so the protein unfolds and its side chains form new, random interactions with neighbouring protein molecules, producing a solid, opaque texture. This permanent change in shape and loss of the protein's original function is called denaturation; the sequence of amino acids and the peptide bonds joining them remain unbroken.

hydrogen bondsthree-dimensional shapehigh temperaturedenaturationpeptide bonds unbroken

Compare a saturated fat and an unsaturated fat in terms of their carbon-to-carbon bonds and their physical state at room temperature.

[4]

Show answer

A saturated fat contains only single bonds between the carbon atoms in its fatty acid chains, which allows the molecules to pack tightly together, so saturated fats are usually solid at room temperature, as seen in most animal fats such as butter. An unsaturated fat contains one or more carbon-to-carbon double bonds in its fatty acid chains, which put a kink in the molecule and prevent tight packing, so unsaturated fats are usually liquid oils at room temperature, as seen in plant oils such as olive oil or corn oil.

single bondspack tightlysolid at room temperaturedouble bondsliquid oil

Starch and cellulose are both polysaccharides made entirely of glucose monomers, yet humans can digest starch but not cellulose. Explain this difference.

[5]

Show answer

Although both starch and cellulose are polymers built from glucose monomers joined by condensation, the type of glycosidic bond linking the glucose units in each is different, and this single difference in bonding gives the two molecules very different shapes: starch coils into a compact, branched structure suited to energy storage, while cellulose forms long, straight, cross-linked chains suited to providing structural strength in plant cell walls. Human digestive enzymes are shaped to hydrolyse the glycosidic bond found in starch but cannot bind to and break the different glycosidic bond in cellulose, so cellulose passes through the human gut largely undigested as dietary fibre.

glucose monomersdifferent glycosidic bondstarch coilscellulose straight chainsenzyme cannot hydrolyse cellulose

A DNA molecule and an RNA molecule are compared. State two structural differences between them and explain why DNA, rather than RNA, is suited to storing an organism's permanent genetic information.

[6]

Show answer

DNA is double-stranded, with two nucleotide strands wound into a double helix and held together by base pairing, while RNA is normally single-stranded; DNA also contains the sugar deoxyribose whereas RNA contains ribose. Because DNA is double-stranded, each strand carries a complementary copy of the genetic information on the other strand, which allows errors to be checked and repaired by comparing the two strands, and this double-stranded structure is also more chemically stable than a single strand, making DNA better suited to storing an organism's genetic information reliably over a long period, while RNA's simpler single-stranded structure suits its short-term role of carrying a working copy of the code to build a protein.

double-strandedsingle-strandeddeoxyribose vs ribosebase pairing allows repairmore stablelong-term storage

During digestion, a starch molecule is broken down into individual glucose molecules. Name the type of reaction involved, state what is added to the starch molecule, and explain why this same type of reaction also occurs when a protein is digested into amino acids.

[5]

Show answer

The reaction that breaks starch down into glucose molecules is hydrolysis, in which a water molecule is added across each glycosidic bond to split the polymer into its monomers. This same type of reaction, hydrolysis, also breaks down a protein into amino acids, because a water molecule is likewise added across each peptide bond joining the amino acids together; both starch and protein are polymers built from monomers joined by condensation, so both require hydrolysis, the reverse reaction, to be broken back down into their monomers during digestion.

hydrolysiswater addedglycosidic bondpeptide bondpolymer to monomer

Phrasing that earns marks

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.

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.