Edexcel A Level Biology Topic 2 Exam Questions

Compare and contrast the process of transcription with the process of DNA replication. (4)

• M1: involve formation of {polynucleotide/ phosphodiester bonds}

• M2: Involve DNA helicase (unzipping of DNA)

• M3: Transcription uses RNA nucleotides whereas replication uses DNA nucleotides

• M4: Transcription uses RNA polymerase whereas replication uses DNA polymerase

• M5: transcription {produces single strand of mRNA/ only copies template strand of DNA} whereas replication {produces double strand of DNA/ copies both strands of DNA}

State what is meant by the term gene. (2)

• M1: Sequence of {bases/nucleotides} in DNA

• M2: That codes for a {sequence of amino acids/ protein/ polypeptide}

Explain how a change of one amino acid could lead to a change in the structure and properties of the haemoglobin protein.

• M1: Different sequence of {amino acids/ primary structure}

• M2: (A different amino acid) will have a different R group

• M3: Therefore secondary/tertiary/quaternary structure will change

• M4: Due to change in a named bond (holding molecule in its 3D shape) - hydrogen bonds, disulphide bridges, ionic bonds

• M5:: (Haemoglobin) may not bond to oxygen

Describe the role of tRNA in the production of a specific protein?

• M1: tRNA molecules {transports amino acids to the ribosome}

• M2: tRNA molecule has an anticodon that {binds to/ recognises} a codon on the mRNA

• M3: each tRNA carries a particular amino acid

Describe how mRNA is synthesised at a template strand of DNA. (2)

• M1: RNA nucleotides align with complementary bases on DNA

• M2: RNA nucleotides joined together by {RNA polymerase/ phosphodiester bonds}

Describe the differences between the structure of DNA and the structure of RNA. (3)

• M1: DNA is double stranded whereas RNA is single stranded

• M2: DNA contains deoxyribose whereas RNA contains ribose

• M3: DNA contains {thymine/T} whereas RNA contains {uracil/U}

Explain why the mutations deletion and substitution have different effects on protein structure. (4)

• M1: deletion could affect every codon on mRNA/ substitution will only affect one codon

• M2: deletion more likely to affect the position of {stop codon/ start codon}

• M3: deletion results in a different sequence of amino acids/ substitution may not affect the sequence of amino acids

• M4: substitution may code for the same amino acid

• M5: due to the degenerate nature of the genetic code

Describe how nucleotides join together to form DNA. (2)

• M1: condensation reaction

• M2: phosphodiester bonds

• M3: DNA polymerase

Describe the roles of transcription and translation in the syntehsis of a globular protein by a muscle cell. (5)

• M1: the {gene/ sequence of DNA} for globular protein is transcribed

• M2: complementary base pairing between RNA nucleotides and DNA to produce mRNA

• M3: mRNA leaves the nucleus and attaches to a ribosome

• M4: pairing between codons on mRNA and anticodons on tRNA
  M5: tRNA provides specific amino acids

• M6: the sequence of {bases/ codons} determines the sequence of amino acids/ primary structure of the protein

Explain why betalain molecules cannot move through intact cell membranes? (3)

• M1: betalain molecules are too large to move through the cell membrane

• M2: There are no {carrier/channel} proteins for betalain molecules to move through

• M3: Betalain molecules are polar and {are repelled by hydrophobic fatty acid tails/ cannot move through fatty acid tails}

Diffusion and active transport are mechanisms by which molecules can enter cells. Compare and contrast these two mechanisms. (3)

• M1: Both move molecules through the {phospholipid bilayer/ cell surface membrane}

• M2: (in both), molecules can move through proteins

• M3: diffusion occurs down a concentration gradient whereas active transport occurs against a concentration gradient

• M4: diffusion is {passive/ does not require ATP} whereas active transport requires ATP

Compare and contrast the processes of endocytosis and exocytosis. (3)

• M1: Both processes involve vesicles

• M2: Both processes involve energy from ATP

• M3: Exocytosis involves {molecules/ substances} leaving the cell whereas endocytosis involves {molecules/ substances} entering the cell

• M4: Exocytosis involves vesicles fusing with cell surface membrane whereas endocytosis involves the formation of vesicles from cell surface membrane.

Explain how the structure of a phospholipid molecule contributes to the partial permeability of a cell surface membrane. (3)

• M1: phospholipid molecules contains a {polar/hydrophilic} phosphate head and non {polar/hydrophobic} fatty acid tails

• M2: allows {fat soluble/non polar} molecules to pass through the membrane

• M3: {polar/ionic} molecules cannot pass through the phospholipid bilayer.

Explain how phospholipids form a cell surface membrane. (3)

• M1: Hydrophilic parts associate with water

• M2: Hydrophobic parts {associate with each other/ repel water}

• M3: A bilayer forms with hydrophobic parts pointing {in towards the centre of the bilayer/ towards each other}

Describe the function of carrier proteins in a cell surface membrane. (4)

M1: Involved in facilitated diffusion

M2: Movement of {large molecules/ polar molecules/ions}

M3: Facilitated diffusion from a high concentration to a low concentration

M4: Involved in active transport

M5: Need ATP to move molecules against concentration gradient

Describe the structure of the cell surface membrane. (3)

• M1: Cell membrane is mainly phospholipids and proteins

• M2: Phospholipids form a bilayer

• M3: Proteins float in the phospholipid/ change position/ fluid mosaic model

• M4: Proteins may span the bilayer or be located in only one layer

Explain how the structure of human lungs enables rapid gas exchange. (4)

• M1: Many alveoli provide a large surface area

• M2: {alveoli/ capillaries} have walls that are one cell thick providing a short diffusion distance

• M3: High concentration gradient maintained by {circulation/ ventilation}

• M4: Extensive capillary network around alveoli provides large surface area for gas exchange

Explain why enzymes are described as biological catalysts. (1)

• Proteins which reduce activation energy of biological catalysts

Compare and contrast the molecular structures of globular and fibrous proteins. (4)

• M1: Both are chains of amino acids joined by peptide bonds

• M2: Both contain hydrogen bonds/ disulphide bridges/ ionic bonds in its three dimensional shape

• M3: Globular proteins have hydrophilic groups on the outside whereas fibrous proteins have hydrophobic groups on the outside

• M4: Globular proteins have tertiary or quaternary structure whereas fibrous proteins have little or no tertiary structure

• M5: Globular and folded in compact shape whereas fibrous have long chains

Describe how a peptide bond is formed. (2)

• M1: by a condensation reaction

• M2: between the {amine/NH₂ group} and the {carboxyl group/COOH} of adjacent amino acids

* People with cystic fibrosis require a higher energy diet than people without cystic fibrosis. They are also more likely to develop problems in the pancreas. Men with cystic fibrosis are less likely to be able to release sperm.

Discuss why a person with cystic fibrosis could have these symptoms. (6)

• mucus {thicker/ stickier} than normal

• Pancreatic enzymes cannot enter intestine because pancreatic duct blocked with mucus

• Hence high energy diet is required as digestion is less efficient

• Pancreatic enzymes trapped behind mucus damage pancreatic cells such as those that produce insulin.

• Cysts form in pancreas

• Sperm cannot leave the testes because {sperm duct/ vas deferens/ tube} blocked with mucus

• Sperm duct/ vas deferens absent therefore sperm cannot pass through

• Gene mutation

• Causing a non functional CFTR protein channel

• Chloride ions cannot move out of the epithelial cells

• Accumulation of sodium and chloride ions in the cells {causing water to move out of the mucus by osmosis/ preventing water moving into the mucus}

Explain how cystic fibrosis affects the rate of oxygen uptake in lungs. (3)

• M1: {Thick/ sticky/ viscous} mucus

• M2: Accumulation of mucus which cannot be moved by cilia

• M3: Restriction of air flow through {bronchioles/ bronchi}

• M4: {increases diffusion distance/ reduces surface area for gas exchange} in the alveoli

Explain why thicker mucus is produced if the functioning of the CFTR channel protein is impaired. (2)

• M1: Chloride ion channels {leave the cell/ enter mucus} through CFRT channel proteins

• M2: Sodium ions do not move {out of the cells/ into the mucus}

• M3: Therefore water {moves into the cells/ moves out of the mucus} by osmosis

Describe the role of the CFTR protein in ensuring that the mucus produced in the lungs has the right consistency. (3)

• M1: Chloride ions leave cells through the CFTR channel proteins

• M2: Sodium ions leave the cells following the chloride ions

• M3: Increasing the solute concentration in the mucus

• M4: Water moves out of the cell by osmosis into the mucus

Explain what is meant by an inherited recessive disorder

• M1: A recessive disorder is the one which is caused by fault allele

• M2: That is only expressed in the {homozygous condition/absence of a normal allele}

State what is meant by the term allele. (2)

• M1: Alternate for of a gene

• M2: Found on the same locus on a chromosome

Mineral ions in soil are in lower concentration than in the vacuoles of root hair cells.

Describe how mineral ions are taken up by root hair cells. (3)

• M1: Mineral ions are taken up by active transport

• M2: Through carrier proteins

• M3: This requires {energy/ ATP}

Compare and contrast the use of pre-implantation genetic diagnosis (PGD) and amniocentesis.

Similarity:

• M1: (both) look for any genetic condition (so parents can make informed decisions)

Differences

• M2: PGD before implantation / amniocentesis after implantation

• M3: (increased) risk of miscarriage with amniocentesis

• M4: Embryo tested in PGD / amniotic fluid tested in amniocentesis

Describe two different social issues related to the use of PGD. (2)

• M1: Selecting one embryo over another / loss of a potential life

• M2: Risk of identifying other conditions / selecting for other characteristics

• M3: {costly / emotional strain } as not always accurate

Explain why cystic fibrosis affects digestion. (3)

• M1: (produces) sticky mucus

• M2: (which) { blocks / reduces } flow of {pancreatic juice / enzyme from pancreas }

• M3: (so) reducing digestion

• M4: {blocks / reduces } flow of bile

Explain what is meant by the term semi‑conservative DNA replication. (2)

• M1: Doubling of the DNA

• M2: With each original DNA strand acting as a template (onto which a new strand is formed)

• M3: (so) (each double-stranded) DNA formed contains one original strand of DNA and one new strand of DNA

Explain why ageing and smoking cause mutations. (3)

• M1: {Chemicals in smoke/ carcinogen} can damage DNA

• M2: By changing the {DNA base sequence/ chromosome number}

• M3: With age, there have been a greater number of cell divisions

• M4: Therefore, greater chance of an error being introduced in the base sequence of the DNA (during replication)