Photosynthesis and chemosynthesis

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

What is a photosystem and what does it contain in its reaction centre? Explain what happens to chlorophyll when it absorbs the energy of light.

Describe the journey of an electron from a water molecule to $\text{NADP}^+$ during the light phase (photosystems II and I and the electron transport chain), and state at what moment the $\text{O}_2$ is released and where the protons end up accumulated.

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.

Explain what photophosphorylation consists of and compare it with the oxidative phosphorylation of the mitochondrion: initial source of electrons, final electron acceptor and formation and use of the proton gradient.

In a recreation of Robert Hill's experiment, spinach chloroplasts are isolated and placed in an isotonic solution with DCPIP (an artificial electron acceptor that turns from dark blue in the oxidized state to colorless in the reduced state). Four tubes are tested without $\text{CO}_2$:

• Tube 1: In the dark.

• Tube 2: Exposed to light.

• Tube 3: Exposed to light, after adding DCMU (an inhibitor of photosystem II).

• Tube 4: Exposed to light, but having previously boiled the chloroplasts at 100 °C.

1. a) Predict in which tube(s) there will be loss of blue color and release of $\text{O}_2$ bubbles. Justify your prediction by indicating which physiological acceptor DCPIP replaces.
2. b) From the result of Tube 2, refute the hypothesis that the oxygen released during photosynthesis comes from $\text{CO}_2$.
3. c) Reason whether the chloroplasts in Tube 2 synthesize carbohydrates or not under these conditions, and what this fact demonstrates regarding the interdependence between the light-dependent phase and the biosynthetic phase (Calvin cycle).

State which two products of the light phase the Calvin cycle uses and what it employs them for in the synthesis of carbohydrates.

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.

The Calvin cycle has three phases: $\text{CO}_2$ fixation, reduction and regeneration of ribulose-1,5-bisphosphate. Describe them briefly and state how many $\text{CO}_2$ molecules must be fixed to form one glucose molecule.

In $\text{C}_4$ plants (maize, sugar cane), $\text{CO}_2$ is first fixed into a four-carbon compound which then releases it near the Rubisco enzyme. Explain what problem this mechanism solves (the competition between $\text{O}_2$ and $\text{CO}_2$ for Rubisco, or photorespiration) and why these plants perform especially well in warm climates.

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.

In a study with crassulacean acid metabolism (CAM) plants kept in an arid environment, stomatal opening, leaf vacuolar juice pH, and malate concentration are measured every 4 hours throughout the day.

1. a) At 04:00 (night), the stomata are open and leaf pH drops to 3,6 with a maximum of malate; at 14:00 (day), the stomata are closed, pH rises to 6,0, and malate almost disappears. Interpret these data by explaining the biochemical sequence that occurs in darkness and during daylight hours.
2. b) If during a warm night the storage capacity of malate in the vacuoles is saturated halfway through the night, $\text{CO}_2$ fixation stops completely until the following day. Justify how this physical-chemical limit explains the low growth rate of CAM plants compared to $\text{C}_3$ plants.
3. c) Compare the adaptive strategy of CAM plants with that of $\text{C}_4$ plants (such as corn), explaining how each manages to avoid photorespiration and how they differ with respect to the type of compartmentalization (temporal versus spatial) of $\text{CO}_2$ fixation.