Chemical Composition in a Cell
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.
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Content standards in this chapter
- 4.1 Water as a Chemical Component of the Cell
- 4.2 Carbohydrates
- 4.3 Proteins
- 4.4 Lipids
- 4.5 Nucleic Acids
Key concepts
- 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.
How this chapter is examined
SPM Biology is assessed over three papers. Paper 1 has 40 objective questions (40 marks) in 1 hour 15 minutes; Paper 2 carries 100 marks across Sections A, B and C in 2 hours 30 minutes; Paper 3 is the practical, testing science process skills. Content from this chapter can appear in any of them, so lessons drill recall for Paper 1 and structured answers for Paper 2.
Common exam angles
- 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.
Common mistakes
What students write: Saying starch gives a brick-red colour with Benedict's.
What earns the mark: Starch turns blue-black with iodine; Benedict's brick-red result is for reducing sugars such as glucose.
What students write: Confusing the monomer of protein and carbohydrate.
What earns the mark: Proteins are built from amino acids; carbohydrates from monosaccharides such as glucose.
What students write: Writing that denaturation breaks the protein into amino acids.
What earns the mark: Denaturation changes the protein's three-dimensional shape so it stops working; it does not break the peptide bonds into separate amino acids.
What students write: Using the Benedict's test without heating.
What earns the mark: Benedict's solution must be heated in a water bath for the colour change to develop.
What students write: Saying the Biuret test must be heated before the colour appears.
What earns the mark: The Biuret test works in the cold and needs no water bath at all; only Benedict's test for reducing sugar must be heated for its colour change to develop.
What students write: Writing that DNA is made of amino acids.
What earns the mark: DNA is a nucleic acid made of nucleotides, each with a sugar, phosphate and nitrogenous base; amino acids are the units of protein, not DNA, and confusing the two loses easy marks in a structure question.
What students write: Saying starch and cellulose have different monomers because plants use one and animals store the other.
What earns the mark: Both starch and cellulose are polymers of glucose; they differ in the type of glycosidic bond linking the glucose units, and this single difference in bonding gives them very different shapes and completely different roles in the plant.
What students write: Assuming lipids and carbohydrates release the same amount of energy per gram.
What earns the mark: Lipids release roughly twice as much energy per gram as carbohydrates because they contain a higher proportion of carbon-hydrogen bonds, which is also why lipids are the body's most efficient long-term energy store.
Study this chapter
Frequently asked questions
What are the food tests I need for SPM Biology?
What does it mean when a protein is denatured?
What are the monomers of the main biomolecules?
What is the difference between condensation and hydrolysis?
Why do starch, glycogen and cellulose behave so differently if they are all made of glucose?
Source:SRC-DSKP-EN, SRC-FORMAT
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