Edexcel International A Level Biology Membranes, Proteins, DNA and Gene Expression Exam Questions

About Edexcel International A Level Biology Membranes, Proteins, DNA and Gene Expression

This topic is part of Unit 1 (Molecules, Diet, Transport and Health) of Edexcel International A Level Biology. It explains how the structure of cell membranes controls transport, how proteins fold into shapes that suit their functions and how enzymes work. You then study DNA and RNA structure, semi-conservative replication, the genetic code, transcription and translation, and how gene mutations can lead to conditions such as cystic fibrosis. The topic finishes with monohybrid inheritance and the ethics of genetic screening. It links to cell structure in Unit 2 and gene technology in Unit 5. Practical work on membrane permeability and enzyme initial rates is commonly examined.

What you need to know

• Membrane structure The fluid mosaic model describes a phospholipid bilayer with proteins, cholesterol and glycoproteins. Membrane permeability increases at high temperatures as proteins denature and the bilayer becomes more fluid.
• Transport Diffusion and facilitated diffusion (channel and carrier proteins) are passive; osmosis is the diffusion of water; active transport needs ATP; endocytosis and exocytosis move large molecules in vesicles.
• Proteins and enzymes Amino acids are joined by peptide bonds. The 3D shape depends on hydrogen, ionic and disulfide bonds. Enzymes lower activation energy; initial rates are affected by substrate and enzyme concentration.
• DNA and replication DNA nucleotides contain deoxyribose, phosphate and a base; complementary base pairs are joined by hydrogen bonds. Replication is semi-conservative, as shown by Meselson and Stahl.
• Protein synthesis A gene is a sequence of bases coding for a polypeptide. Transcription by RNA polymerase produces mRNA; translation at ribosomes uses tRNA anticodons matching mRNA codons.
• Mutation and inheritance Gene mutations alter the amino acid sequence. Cystic fibrosis is caused by recessive alleles of the CFTR gene. Genetic screening includes amniocentesis, chorionic villus sampling and preimplantation genetic diagnosis.

Common mistakes in Membranes, Proteins, DNA and Gene Expression

• Describing channel proteins as using ATP; channel proteins allow passive facilitated diffusion.
• Saying beetroot pigment leaks because cells burst; the pigment leaks because membranes are damaged at high temperature.
• Mixing up transcription and translation.
• Stating that a recessive allele is less common than a dominant one; dominance is about expression, not frequency.
• Forgetting to give both risks and benefits when evaluating genetic screening.

Try three Membranes, Proteins, DNA and Gene Expression exam-style questions