Edexcel A Level Biology B Topic 1: Biological Molecules Exam Questions

About Edexcel A Level Biology B Topic 1: Biological Molecules

Topic 1 of Edexcel A Level Biology B lays the biochemical foundations for the whole course. You study the structure and functions of carbohydrates, lipids and proteins, how monomers join by condensation and separate by hydrolysis, and how enzymes work, including the factors that affect their rate and the effect of inhibitors. The topic also covers the roles of inorganic ions and the properties of water that make it essential for life. Finally you learn the structure of DNA and RNA, semi-conservative replication and protein synthesis by transcription and translation. Questions often ask you to link molecular structure to function and to analyse enzyme rate data from practical work.

What you need to know

• Carbohydrates Alpha and beta glucose form glycosidic bonds. Starch (amylose and amylopectin) and glycogen store energy; cellulose forms straight chains with hydrogen bonds between them, giving strength.
• Lipids Triglycerides contain glycerol and three fatty acids joined by ester bonds; saturated and unsaturated fatty acids differ in C=C double bonds. Phospholipids have a hydrophilic head and hydrophobic tails.
• Proteins Amino acids join by peptide bonds. Primary to quaternary structure is held by hydrogen, ionic and disulfide bonds. Compare globular proteins such as haemoglobin with fibrous proteins such as collagen.
• Enzymes Enzymes lower activation energy. Rate depends on temperature, pH, enzyme and substrate concentration. Competitive inhibitors bind the active site; non-competitive inhibitors bind elsewhere and change its shape.
• Inorganic ions and water Ions such as nitrate, phosphate, calcium and iron have specific roles. Water's polarity and hydrogen bonding make it a solvent with high specific heat capacity and cohesion.
• DNA and protein synthesis DNA replication is semi-conservative using DNA helicase and DNA polymerase. Transcription produces mRNA; translation at ribosomes uses tRNA to join amino acids in the order set by codons.

Common mistakes in Topic 1: Biological Molecules

• Writing that enzymes are denatured at low temperatures; low temperature reduces kinetic energy but does not denature the enzyme.
• Confusing hydrolysis and condensation reactions.
• Saying a non-competitive inhibitor competes for the active site; it binds at an allosteric site.
• Describing the genetic code as overlapping; it is non-overlapping and degenerate.
• Forgetting to measure the initial rate from the steepest part of the curve at the start of an enzyme reaction.

Try three Topic 1: Biological Molecules exam-style questions