ATP and the light-dependent reactionsEdexcel A-Level Biology A: Subtopic test

1. 1
   A resting adult holds only about 50 g of ATP in the whole body at any moment, yet turns over roughly their own body mass of ATP in a day. A student reading about this wants to understand how a cell can use so much ATP while storing so little.

   (a)

   Which equation represents the hydrolysis of ATP?

   [1 mark]

   • AADP + Pi → ATP + H₂O
   • BATP + H₂O → ADP + Pi
   • CATP + Pi → ADP + H₂O
   • DADP + H₂O → ATP + Pi

   (b)

   ATP is described as an immediate source of energy. Which statement best explains this?

   [1 mark]

   • AIts hydrolysis is a single-step reaction that releases energy quickly in small amounts
   • BIt stores large amounts of energy that can be released over several days
   • CIt is made only in the cell surface membrane, close to where energy is needed
   • DIts hydrolysis is a series of reactions that releases energy slowly

   (c)

   Using the information given, explain why a cell must continually resynthesise ATP.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A student investigates the light-dependent reactions using isolated chloroplasts suspended in a cold, buffered solution. The suspension is mixed with DCPIP, a blue dye that is colourless when reduced and that can accept electrons in place of NADP. No carbon dioxide is supplied. One tube is placed in bright light and an identical tube is wrapped in foil.

   (a)

   What is the role of DCPIP in the experiment?

   [1 mark]

   • AIt supplies electrons to replace those lost from chlorophyll
   • BIt absorbs light energy in place of chlorophyll
   • CIt catalyses the splitting of water
   • DIt accepts electrons in place of NADP

   (b)

   The DCPIP in the tube wrapped in foil stays blue. Which is the best explanation?

   [1 mark]

   • AWater is not available to the chloroplasts in the dark
   • BDCPIP only changes colour at high temperatures
   • CNo light energy is absorbed, so electrons are not excited and none reach the DCPIP
   • DATP is needed to reduce DCPIP and none is made in the dark

   (c)

   Explain why the DCPIP in the illuminated tube changes from blue to colourless.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   In 1966 the biochemist André Jagendorf soaked isolated chloroplasts in an acidic buffer at pH 4 until the space inside the thylakoids was also at about pH 4. He then moved them, in complete darkness, into a buffer at pH 8 that contained ADP and inorganic phosphate (Pi). The chloroplasts made ATP for a short time and then stopped.

   (a)

   Suggest how the chloroplasts made ATP in the dark in this experiment.

   [3 marks]

   (b)

   Explain why the production of ATP in the experiment stopped after a short time, and how the gradient is maintained in a living chloroplast in the light.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A herbicide binds to an electron carrier in the electron transport chain of the thylakoid membrane, between photosystem II and photosystem I, and stops electrons passing along the chain. Weeds sprayed with the herbicide stay in bright light with plenty of water and carbon dioxide, but they turn yellow and die within a few days.

   (a)

   Describe how light energy is used in the light-dependent reactions to produce ATP, reduced NADP and oxygen.

   [6 marks]

   (b)

   Explain why the herbicide kills the weeds even though they have plenty of light, water and carbon dioxide.

   [6 marks]

   Total for question 4: 12 marks

End of questions