Nucleic acids, ATP, water and inorganic ionsAQA A-Level Biology: Topic test

1. 1
   A biochemist extracts a nucleic acid from a cell. Chemical analysis shows that its nucleotides contain ribose, phosphate and four bases: adenine, cytosine, guanine and uracil.

   (a)

   Which two components of this nucleic acid are not found in DNA?

   [1 mark]

   • APhosphate and adenine
   • BRibose and uracil
   • CRibose and cytosine
   • DUracil and guanine

   (b)

   In a cell, RNA transfers genetic information from DNA to which structures?

   [1 mark]

   • ALysosomes
   • BMitochondria
   • CRibosomes
   • DGolgi vesicles

   (c)

   Describe how a phosphodiester bond forms between two nucleotides.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A molecule of double-stranded DNA is replicated in a test tube that contains DNA helicase, DNA polymerase, a supply of free DNA nucleotides and ATP.

   (a)

   After one round of replication, what does each of the two DNA molecules contain?

   [1 mark]

   • AOne original strand and one new strand
   • BTwo original strands
   • CTwo new strands
   • DShort sections of original and new DNA mixed along each strand

   (b)

   Which enzyme joins adjacent free nucleotides to form the new strand?

   [1 mark]

   • ADNA helicase
   • BATP hydrolase
   • CATP synthase
   • DDNA polymerase

   (c)

   A second tube contains everything in the first tube except DNA helicase. Explain why no replication occurs.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Cells lining the proximal tubule of a kidney nephron reabsorb glucose from the filtrate. A carrier protein in the cell-surface membrane on the filtrate side moves sodium ions and glucose into the cell together. A different protein on the opposite side of the cell uses ATP to move sodium ions out of the cell.

   (a)

   Explain how sodium ions allow glucose to enter the cell even when the glucose concentration inside the cell is higher than in the filtrate.

   [3 marks]

   (b)

   A poison stops ATP synthase working in the mitochondria of these cells. Explain why less glucose is reabsorbed from the filtrate.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A cyclist rides for two hours on a hot day. She sweats heavily, and her muscle cells hydrolyse ATP continually to release energy for muscle contraction.

   (a)

   Explain how the properties of water help the cyclist to stay cool, and why water is important for the reactions in her cells.

   [6 marks]

   (b)

   Describe the structure of ATP and explain how its hydrolysis and resynthesis supply energy to her muscle cells.

   [6 marks]

   Total for question 4: 12 marks

5. 5
   A student heats 100 g of water and 100 g of ethanol, in identical beakers with identical heaters, for the same length of time. The temperature of the ethanol rises much more than the temperature of the water.

   (a)

   Which property of water explains this result?

   [1 mark]

   • AHigh latent heat of vaporisation
   • BStrong cohesion between molecules
   • CBeing a good solvent
   • DHigh heat capacity

   (b)

   How does this property of water benefit organisms living in a lake?

   [1 mark]

   • AIt allows the lake to support organisms on its surface
   • BThe temperature of the lake changes slowly, so organisms experience small temperature changes
   • CIt cools the organisms through evaporation
   • DIt allows reactions to take place in solution

   (c)

   Explain why the fact that water is a solvent is important for the metabolic reactions in the cytoplasm of a cell.

   [2 marks]

   Total for question 5: 4 marks

6. 6
   A dietician reviews the roles of three inorganic ions in the human body: iron ions, phosphate ions and hydrogen ions.

   (a)

   Which row correctly gives a role for each ion (iron ions, phosphate ions, hydrogen ions)?

   [1 mark]

   • AComponent of ATP, component of haemoglobin, determine pH
   • BComponent of haemoglobin, determine pH, component of DNA
   • CComponent of haemoglobin, component of ATP, determine pH
   • DDetermine pH, component of haemoglobin, component of DNA

   (b)

   A solution has a high concentration of hydrogen ions. Which statement is correct?

   [1 mark]

   • AIt has a low pH
   • BIt has a high pH
   • CIt is neutral
   • DIts pH is not related to hydrogen ion concentration

   (c)

   Explain how iron ions enable haemoglobin to carry oxygen.

   [2 marks]

   Total for question 6: 4 marks

7. 7
   A double-stranded DNA fragment from a bacterium is 30 base pairs long. In total, 12 of the bases in the fragment are adenine.

   (a)

   Calculate the number of thymine, cytosine and guanine bases in the fragment. Show your working.

   [3 marks]

   (b)

   The fragment is replicated once. Calculate the number of new phosphodiester bonds formed, and name the enzyme that catalyses their formation.

   [4 marks]

   Total for question 7: 7 marks

8. 8
   A synthetic biology team is building a minimal artificial cell inside a droplet of water. The cell must be able to copy its DNA, take in glucose and amino acids, and keep its internal chemistry stable.

   (a)

   Describe how the DNA in the artificial cell would be replicated, and explain how this ensures genetic continuity.

   [6 marks]

   (b)

   Explain the roles of phosphate ions, sodium ions and hydrogen ions in the artificial cell.

   [6 marks]

   Total for question 8: 12 marks

End of questions