Exercises: Photosynthesis and chemosynthesis

Write the overall equation for oxygenic photosynthesis and state which molecule is oxidised and which is reduced in the process.

What main pigment captures light in green plants and in which cell structure is it located? Which light colors (wavelengths) does it preferentially absorb and which does it reflect?

Name the two main stages of photosynthesis, state where in the chloroplast each stage occurs, and what is produced in each stage.

In the diagram of a chloroplast, two structures are labelled with the numbers 1 and 2. Identify them and explain which photosynthetic processes occur in each, relating them to the input and output arrows in the diagram.

Explain what photolysis of water is, in which stage and location of the chloroplast it occurs, and what the fate of the generated electrons, protons, and oxygen is.

In the Calvin cycle, $\text{CO}_2$ is "fixed" in the stroma. Explain what fixing $\text{CO}_2$ means, which enzyme catalyses this fixation, and where the ATP and NADPH consumed by the cycle come from.

In a laboratory, two cultures of unknown autotrophic microorganisms (A and B) capable of fixing $^{14}\text{CO}_2$ in mineral medium are isolated. When subjected to different experimental conditions, the following data are obtained:

• In absolute darkness and in the presence of $\text{NH}_4^+$ and $\text{O}_2$, only culture A proliferates and incorporates $^{14}\text{C}$ into its organic biomass.

• In the presence of light and in the absence of reduced inorganic donors (except $\text{H}_2\text{O}$), culture B proliferates, but culture A shows no growth.

• If $\text{O}_2$ is removed from the medium in darkness, culture A completely stops fixing $^{14}\text{CO}_2$, despite having an abundant supply of $\text{NH}_4^+$.

1. a) Deduce the type of autotrophic metabolism of each culture (chemosynthesis or photosynthesis), identifying in each case the primary energy source, the carbon source, and the electron donor.
2. b) Justify why culture A is strictly dependent on $\text{O}_2$ to synthesize organic matter from $\text{CO}_2$, explaining the role of the electron transport chain in this process.
3. c) Compare oxygenic photosynthesis and nitrifying chemosynthesis with respect to the type of cells and biological taxa that carry them out.

The net photosynthetic rate ($\text{CO}_2$ assimilation) of a plant is measured at increasing light intensities under two environmental conditions: Condition 1 (air with 0.04 % $\text{CO}_2$ at 20 °C) and Condition 2 (air enriched with 0.15 % $\text{CO}_2$ at 20 °C). The results indicate that at low intensities (below 80 $\mu\text{mol}\times\text{m}^{-2}\times\text{s}^{-1}$), both curves are identical and increase linearly with the same slope; however, the saturation plateau is reached at a level 60 % higher in Condition 2.

1. a) Justify biochemically why at low light intensities the $\text{CO}_2$ concentration does not influence the photosynthetic rate, and indicate which processes of the light-dependent phase limit the rate in that section.
2. b) Explain why the photosynthetic rate stabilizes at a plateau at high light intensities and deduce which stromal enzyme acts as a bottleneck in Condition 1.
3. c) If in Condition 2 (saturating light and 0.15 % $\text{CO}_2$) the temperature is raised to 30 °C, the photosynthetic rate increases noticeably; in contrast, that same thermal increase produces no rise at low light intensities. Explain the different temperature sensitivity of both phases of photosynthesis.

Brown algae live tens of meters deep, where only blue-green light penetrates. Explain the role played by their accessory pigments (such as fucoxanthin) and why chlorophylls alone would not be sufficient to carry out photosynthesis at that depth.