Photosynthesis and chemosynthesis

Write the overall equation for oxygenic photosynthesis and state which molecule is oxidized 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.

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

What are thylakoids and what are grana? Explain why the thylakoid membranes are the right place for the light-dependent reactions of photosynthesis.

In the diagram of a chloroplast, two structures are labeled 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.

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.

The absorption spectrum of chlorophyll $a$ has maxima in the blue ($\sim 430\text{ nm}$) and red ($\sim 660\text{ nm}$) regions, while it barely absorbs in the green-yellow band (500–550 nm). Considering that seawater rapidly absorbs red light and that at a depth of 25 meters the available radiation is almost exclusively blue-green:

1. a) Reasonably predict how the rate of $\text{O}_2$ evolution will vary if two samples of equivalent biomass are illuminated with monochromatic light of 530 nm: one of the green alga Ulva (with chlorophylls $a$ and $b$) and another of the brown alga Laminaria (with chlorophyll $a$ and fucoxanthin).
2. b) Given that fucoxanthin is located in the antenna complexes and not in the reaction centers, explain how the light energy it absorbs is transferred to make the photolysis of water possible in photosystem II.
3. c) A student states that, to adapt to life in deep waters, it would be enough for brown algae to replace all their chlorophyll $a$ with fucoxanthin. Refute this statement based on the essential physicochemical function that chlorophyll $a$ performs in photochemical reaction centers.

A researcher illuminates isolated chloroplasts and adds a herbicide that blocks the flow of electrons at the exit of photosystem II. Predict, with reasoning, what will happen to the production of $\text{O}_2$, of ATP and of NADPH, and explain why the plant treated with that herbicide ends up dying even though it receives light and $\text{CO}_2$.

Almost all the oxygen in the atmosphere comes from photosynthesis. Explain from which molecule that $\text{O}_2$ is released and reason why the appearance of oxygenic photosynthesis was a milestone in the history of the Earth.

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.

For each $\text{O}_2$ molecule released in the light phase, $4$ electrons are released from water. Knowing that forming one glucose molecule requires fixing $6$ $\text{CO}_2$ molecules and consuming $12$ NADPH, calculate how many water molecules must be photolyzed at a minimum and relate both calculations.

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-dependent reactions the Calvin cycle uses and what it uses 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 catalyzes 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.

Name three environmental factors that influence the rate of photosynthesis and explain briefly how one of them acts.

In $\text{C}_4$ plants (corn, 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.

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.

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.

In greenhouses the air is enriched with $\text{CO}_2$ and supplemented with artificial light. a) Reason, from the concept of the limiting factor, why these measures only increase the harvest up to a point. b) State which factor could become limiting after applying them. c) Explain why raising the temperature too much can reduce production even though light and $\text{CO}_2$ are abundant.

A species of photosynthetic amoeba possessing photosynthetic organelles called chromatophores is analyzed in the lab. Experimental assays reveal the following:

1. Chloramphenicol (an antibiotic that blocks translation on bacterial 70S ribosomes) arrests protein synthesis inside the chromatophore, whereas cycloheximide (which blocks eukaryotic 80S ribosomes) only inhibits translation in the amoeba's cytoplasm.

2. DNA sequencing of the chromatophore demonstrates that it is circular and phylogenetically much closer to modern cyanobacteria than to the nucleus of the amoeba itself.

1. a) Interpret these results in light of the endosymbiotic theory and deduce what mutual selective advantages allowed this association to be consolidated during the evolution of the first photosynthetic eukaryotes.
2. b) A student claims that the appearance of the first cyanobacteria with oxygenic photosynthesis caused the immediate and massive oxygenation of the primitive atmosphere. Refute their claim by explaining what geochemical process occurred in the hydrosphere before $\text{O}_2$ could accumulate freely in the atmosphere, and what biological consequence this oxygen crisis had for pre-existing anaerobic organisms.