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AP®BiologyCollege BoardExam QuestionsUnit 3: Cellular EnergeticsEnergy & Photosynthesis

Energy & Photosynthesis (College Board AP® Biology): Exam Questions

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Energy & Photosynthesis

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1
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Scientists use radioactively labeled carbon dioxide (¹⁴CO₂) to track carbon atoms in a plant during photosynthesis. They find that the labeled carbon ends up in glucose molecules.

Which of the following statements best explains this result?

  • Carbon dioxide is broken down in the light-dependent reactions.

  • Carbon dioxide is fixed into organic molecules during the Calvin cycle.

  • Carbon dioxide is used to generate ATP in the mitochondria.

  • Carbon dioxide is directly converted into chlorophyll.

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2
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Which molecule is the immediate energy source generated in the light-dependent reactions of photosynthesis?

  • NADPH

  • ATP

  • Glucose

  • Oxygen

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3
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A scientist disrupts ATP and NADPH production in chloroplasts. They observe that the Calvin cycle stops functioning.

Why does the Calvin cycle require ATP and NADPH?

  • ATP and NADPH are used to fix carbon dioxide into organic molecules.

  • ATP and NADPH provide energy to split water molecules.

  • ATP and NADPH are used to release oxygen.

  • ATP and NADPH generate chlorophyll pigments.

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4
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Scientists study cell metabolism and observe that heat is continuously released as cells carry out chemical reactions. They note that living organisms require a constant input of energy to survive.

Which of the following best explains why organisms need a continuous energy supply?

  • To prevent energy loss in biological reactions.

  • To maintain order and power cellular processes.

  • To store unlimited amounts of energy for future use.

  • To convert all energy into work without any heat loss.

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51 mark

Scientists study early Earth’s atmosphere and find that oxygen levels began rising around 3.5 billion years ago. They trace this change to the evolution of photosynthetic prokaryotes.

Which of the following best describes the role of photosynthesis in early life on Earth?

  • It created chemical energy for growth of primitive organisms

  • It directly created oxygen by splitting carbon dioxide.

  • It allowed organisms to store energy as heat.

  • It captured light energy and stored it in organic molecules.

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1
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The laws of thermodynamics are fundamental principles that govern the transfer of energy, including energy within living systems. A scientist is studying energy flow in an aquatic ecosystem.

Which of the following best applies the second law of thermodynamics to this biological system?

  • Living organisms in the ecosystem are always transferring energy to their surroundings, increasing the overall entropy of the universe.

  • Energy within the ecosystem cannot be created or destroyed.

  • The total energy of the ecosystem remains constant, even if converted between forms.

  • Living organisms in the ecosystem need energy to fuel metabolism, meaning that they decrease the entropy of the universe.

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2
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All organisms require a constant input of energy. Cells obtain energy through chemical means, such as feeding or photosynthesis.

Which of the following best explains why organisms need a constant input of energy?

  • To allow transfer of energy to the environment.

  • To create energy during metabolic reactions.

  • To maintain order, and power chemical processes inside cells.

  • To enable cells to decouple energy-requiring processes from energy-releasing processes.

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3
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Photosynthesis involves two main stages: the light-dependent reactions and the Calvin cycle. During the light-dependent reactions, light energy is used to produce ATP and NADPH.

Which of the following correctly describes the initial step in the light-dependent reactions?

  • CO₂ is fixed into organic molecules.

  • Light energy excites electrons in chlorophyll molecules within the thylakoid membrane.

  • High-energy electrons are passed from photosystem I to photosystem II.

  • A proton gradient is established across the thylakoid membrane.

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4
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The light-independent reactions of photosynthesis, also known as the Calvin cycle, occur in the stroma of the chloroplast and are responsible for producing complex organic molecules.

Which of the following correctly describes an event that occurs during the Calvin Cycle?

  • ATP is synthesised from ADP and inorganic phosphate.

  • NADP+ is reduced to NADPH.

  • Energy from NADH allows the production of glucose.

  • Energy from NADPH allows the production of glucose.

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5
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DCPIP is a chemical compound that is blue when oxidised and colourless when it accepts electrons during the light-independent reaction. Figure 1 shows the experimental set-up of a study that investigates the effect of ammonium hydroxide on the photosynthetic activity of chloroplasts that have been extracted from spinach leaves and suspended in an isolation medium.  All five tubes are placed in a water bath at 20 °C and illuminated from all sides during the investigation.

Five test tubes labelled 1 to 5, containing chloroplast suspension, DCPIP, water, isolation medium, and ammonium hydroxide, with varying contents.

Which tube in Figure 1 acts as a control that allows the effect of ammonium hydroxide to be clearly determined?

  • Tube 1

  • Tube 2

  • Tube 3

  • Tube 4

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61 mark

Photosynthetic pigments, such as chlorophyll a, chlorophyll b and carotenoids, absorb light energy during photosynthesis. Different pigments absorb light at different wavelengths, as shown in Figure 1.

Graph showing light absorption vs. wavelength for chlorophyll a, chlorophyll b, and carotenoids, with peaks around 450 nm and 670-700 nm.
Figure 1.

Which of the following correctly describes the absorption data shown in Figure 1?

  • The highest wavelength at which chlorophyll a has an absorption peak is higher than the equivalent for chlorophyll b.

  • The absorption peak for carotenoids is at 460 nm.

  • Chlorophyll b does not absorb light at a wavelength of 620 nm.

  • The lowest wavelength at which chlorophyll b has an absorption peak is lower than the equivalent for chlorophyll a.

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7
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The evolution of photosynthesis in prokaryotic organisms transformed the Earth's atmosphere and allowed for the development of complex life. Scientists claim that the first early photosynthetic organisms, known as cyanobacteria, evolved around 2.3 billion years ago.

Which of the following graphs provides evidence for this?

  • Graph showing atmospheric oxygen percentage over 4 billion years ago. Oxygen levels remain low until a significant increase near 1 billion years ago.
  • Graph showing atmospheric oxygen percentage over billions of years. Oxygen rises sharply around 2 billion years ago, stabilising thereafter.
  • Graph showing atmospheric oxygen percentage over time, billions of years ago. Oxygen stable at 10% from 4 to 3 billion years ago, then declines.
  • Graph showing atmospheric oxygen percentage over time, with a peak around 2 billion years ago. Oxygen levels rise sharply then fall again towards 1 billion years ago.

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1
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Every metabolic process inside cells involves a change in free energy, known as ∆G (delta G), e.g. for the formation of sucrose from glucose and fructose ∆G = +27, while for the breakdown of ATP into ADP and Pi ∆G = -30.

Which of the following represent two reactions that might be coupled together within a cell to ensure a net release of free energy?

  • ∆G = +15 and ∆G = +25

  • ∆G = -15 and ∆G = -25

  • ∆G = -15 and ∆G = -5

  • ∆G = -25 and ∆G = +22

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2
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Scientists analyze geological records and discover that banded iron formations (BIFs), which contain alternating iron oxide and silica layers, appeared in Earth's rock layers around 2.4 billion years ago. This corresponds to the Great Oxygenation Event (GOE), a period when atmospheric oxygen levels began to rise significantly.

To investigate the biological origins of this oxygen increase, researchers analyze isotopic signatures of carbon and sulfur in ancient rock samples and find evidence of increased biological activity in oceanic microbial mats.

Which of the following best explains the formation of banded iron deposits during this period?

  • Oxygen produced by early photosynthetic prokaryotes reacted with dissolved iron in the oceans, forming insoluble iron oxides that precipitated on the seafloor.

  • Aerobic respiration by early prokaryotic organisms released oxygen which reacted with iron in the oceans, forming iron oxides that later formed iron bands within rock formations.

  • Anaerobic respiration in deep-sea hydrothermal vents caused iron to oxidize, forming layered deposits in marine sediments.

  • Oxygen from the decomposition of organic matter in the deep ocean chemically reacted with iron, leading to the accumulation of iron oxides.

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3
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Scientists conduct an experiment to determine how different wavelengths of light affect photosynthetic efficiency. They measure oxygen production in a plant exposed to different wavelengths of light for 30 minutes.

The table shows their results.

Light wavelength (nm)

Oxygen production (µmol O₂/min)

400 (violet)

6.8

450 (blue)

7.5

550 (green)

2.1

600 (yellow)

4.3

680 (red)

8.2

The scientists hypothesize that light absorption by chlorophyll is directly related to photosynthetic efficiency.

Based on the data, which of the following best explains the combinations of wavelengths that would be most suited to growing green plants?

  • Red and blue light because these create the highest rate of oxygen production.

  • Green and yellow light because chlorophyll is green and would reflect this wavelength.

  • Violet and blue light because these have the most similar wavelengths.

  • Violet and red light because these have wavelengths covering the widest range.

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4
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A scientist studied the activity of ATP synthase in chloroplasts using competitive inhibition. Despite functional electron transport and the production of NADPH, no ATP is produced.

Which of the following is the most likely explanation?

  • The inhibitor blocks the active site of ATP synthase, preventing ADP and inorganic phosphate (Pi) from binding and forming ATP.

  • The inhibitor disrupts proton (H⁺) transport across the thylakoid membrane, preventing the formation of a proton gradient.

  • The inhibitor binds to a protein in the electron transport chain, preventing the transfer of electrons to ATP synthase.

  • The inhibitor prevents carbon fixation in the Calvin cycle, reducing the demand for ATP and stopping its synthesis.

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5
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A scientist grows plants under controlled conditions and inhibits ATP and NADPH production. They observe that the rate of glucose synthesis decreases significantly despite a sufficient supply of CO₂.

Which of the following best explains the scientist's observations?

  • Oxygen is not released, preventing glucose synthesis in the Calvin cycle.

  • The light-dependent reactions use carbon fixation to generate ATP and NADPH

  • The Calvin cycle relies on ATP and NADPH to convert CO₂ into organic molecules.

  • The Calvin cycle recycles ATP and NADPH to power the light-dependent reactions

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