Science Practice Challenge Questions

8.2 The Light-dependent Reaction of Photosynthesis


On a hot, dry day, plants close their stomata to conserve water. Explain the connection between the oxidation of water in photosystem II of the light-dependent reactions and the synthesis of glyceraldehyde-3-phosphate (G3PA) in the light-independent reactions. Predict the effect of closed stomata on the synthesis of G3PA and justify the prediction.


The emergence of photosynthetic organisms is recorded in layers of sedimentary rock known as a banded iron formation. Dark-colored and iron-rich bands composed of hematite (Fe2O3) and magnetite (Fe3O4) only a few millimeters thick alternate with light-colored and iron-poor shale or chert. Hematite and magnetite can form precipitates from water that has a high concentration of dissolved oxygen. Shale and chert can form under conditions that have high concentrations of carbonates (CO3-2). These banded iron formations appeared 3.7 billion years ago (and became less common 1.8 billion years ago). Justify the claim that these sedimentary rock formations reveal early Earth conditions.


The diagram summarizes the light reactions of photosynthesis.

A diagram shows the light reactions of photosynthesis. The first reaction takes place in photosystem 2, which contains the P 680 reaction center. When light with a wavelength of 680 nanometers strikes the P680 reaction center, 2 water molecules are split to form 1 oxygen molecule, 4 hydrogen ions, and 2 electrons. The 2 electrons, which initially each possess 77 kilojoules of energy, are excited by the light and gain energy. The excited electrons slowly lose energy as they are shuttled to the cytochrome c
Figure 8.24

The diagram shows light-dependent reactions of photosynthesis, including the reaction centers, electron transport chains, and the overall reactions within each of these. The free energy per electron is shown for the oxidation-reduction reactions. The free change of the captured radiant energy is shown.

2NAD P + +2 H + +2 H 2 O+3ADP+3 P i O 2 +4 H + +2NADPH+3ATP 2NAD P + +2 H + +2 H 2 O+3ADP+3 P i O 2 +4 H + +2NADPH+3ATP
  1. In the overall mass balance equation for the light reactions shown above, identify the source of electrons for the synthesis of NADPH.
  2. Calculate the number of electrons transferred in this reaction.
  3. Using the free energies per electron displayed, calculate the free energy change of the light-dependent reactions.
  4. Given that the free energy change for the hydrolysis of ATP is -31.5 kJ/mole and the free energy change for the formation of NADPH from NADP+ is 18 kJ/mole, calculate the total production of free energy for the light reactions.
  5. Using this definition of energy efficiency, calculate the efficiency of the light reaction of photosynthesis: energy efficiency = free energy produced/energy input.

Algae can be used for food and fuel. To maximize profit from algae production under artificial light, researchers proposed an experiment to determine the dependence of the efficiency of the process used to grow the algae on light intensity (“brightness”) that will be purchased from the electric company.

The algae will be grown on a flat sheet that will be continuously washed with dissolved carbon dioxide and nutrients. Light-emitting diodes (LEDs) will be used to illuminate the growth sheet. Photodiodes placed above and below the sheet will be used to detect light transmitted through and reflected from the algal mat. The intensity of light can be varied, and the algae can be removed, filtered, and dried. The amount of stored energy in the algal mats can be determined by calorimetry.

A. Identify a useful definition of efficiency for this study and justify your choice.

B. Frequencies of light emitted by the LEDs will not be variables but must be specified for the construction of the apparatus. Identify the frequencies of light that should be used in the experiment and justify your choice.

C. Evaluate the claim that the experiment is based on the assumption that there is an upper limit on the intensity of light used to support growth of algae. Predict a possible effect on algal growth if light with too great an intensity is used and justify the prediction.

D. Design an experiment by describing a procedure that can be used to determine the relationship between light intensity and efficiency.


The classical theory of evolution is based on a gradual transformation, the accumulation of many random mutations that are selected. The biological evidence for evolution is overwhelming, particularly when one considers what has not changed: core conserved characteristics.

A. Describe three conserved characteristics common to both chloroplasts and mitochondria.

Some hypotheses that have been proposed to account for biological diversity are saltatory, involving sudden changes, rather than gradualist. In defense of the classical gradualist theory of evolution, nearly all biologists in the late 1960s rejected the theory of endosymbiosis as presented by Lynn Margulis in 1967. 

B. Suppose that you want to disprove the theory of endosymbiosis.

Explain how the following evidence could disprove the theory:

i. a “transitional species” with cellular features that are intermediate cells with and without mitochondria

ii. a “transitional organelle” with some features, such as compartmentalized metabolic processes, but not other features, such as DNA

Explain how the following evidence supports the theory of endosymbiosis:

iii. bacteria live within your intestines, but you still have a separate identity

iv. no one has directly observed the fusion of two organisms in which a single organism results

8.3 Using Light Energy to Make Organic Molecules


Discovering the carbon-fixation reactions (or light-independent reactions) of photosynthesis earned Melvin Calvin a Nobel Prize in 1961. The isolation and identification of the products of algae exposed to 14C revealed the path of carbon in photosynthesis. 14C was fed to the algal culture in the form of bicarbonate ion (HCO3-). To agitate the culture, air, which contains CO2, was bubbled through the system, so there were two sources of carbon.

Since Calvin’s experiment, research has focused on the way carbon from a solution containing bicarbonate ions is absorbed by algae. In aqueous solution, the bicarbonate anion (HCO3-) is in equilibrium with dissolved CO2 as shown in the equation below:

H + +HC O 3 H 2 O+C O 2 H + +HC O 3 H 2 O+C O 2

In a later experiment, Larsson and Axelsson (1999) used acetazolamide (AZ), a carbonate anhydrase inhibitor, to inhibit enzymes that convert bicarbonate into carbon dioxide. They also used disulfonate (DIDS), an inhibitor of the transport of anions, such as the bicarbonate ion, through the plasma membrane.

A. Pose a scientific question that can be pursued with AZ and DIDS in terms of the path of carbon in photosynthesis.

B. The plasma membrane is permeable to the nonpolar, uncharged carbon dioxide molecule. However, the concentration of carbon dioxide in solution can be very small. Explain how the enzyme carbonate anhydrase can increase the availability of carbon dioxide to the cell.

C. Larsson and Axelsson conducted experiments in which the growth medium was fixed at two different pH levels and determined the effects of AZ and DIDS on the rate of photosynthesis by measuring oxygen concentrations at various times. The results are shown in the two graphs below. The arrows indicate the time points during which HCO3-, AZ, and DIDS were added to each system.

The figure shows the effects of A Z and D I D S on the rate of photosynthesis for two different systems, system A and system B. The left plot shows the change in oxygen concentration for system A. The oxygen concentration is initially very low but gradually begins to increase. After addition of H C O 3 minus, the oxygen concentration begins to rapidly increase. A Z addition reduces the rate of the rise in oxygen concentration. D I D S, which is added after A Z, has no effect. The right plot shows the chan
Figure 8.25

In which system, A or B, is there a strong reliance on the bicarbonate ion as the mechanism of carbon uptake by the cell? Justify your answer using the data.

D. If both systems are dosed with the same concentrations of bicarbonate ion, in which system, A or B, is the pH higher? Justify your answer using the data and the bicarbonate-carbon dioxide equilibrium equation.