Photosynthesis

Write the overall chemical equation for photosynthesis and identify the reactants, products, and energy source of the process.

Which pigment captures light during photosynthesis and in which cell organelle is it located? Explain why leaves are green and what happens to this color in autumn in deciduous species.

Name the main photosynthetic pigments (chlorophylls a and b, carotenoids) and explain what the absorption spectrum is: which colours of light they use and which they reflect, relating this to the colour we see.

Explain where photosynthesis occurs inside the chloroplast: what the thylakoids and the grana are and what the stroma is, indicating which phase of the process occurs in each compartment.

Distinguish between the light-dependent stage and the light-independent stage (Calvin cycle) of photosynthesis: in which part of the chloroplast each occurs, what substances enter, and what is obtained in each stage.

Which molecule does the oxygen released by photosynthesis come from: $\text{CO}_2$ or water? Explain how this was tested using labelled isotopes ($\text{O-18}$) and why the result was decisive.

Explain what the photolysis of water is and which three products it generates (electrons, protons and oxygen), indicating what the plant uses the first two for.

In 1882, T. W. Engelmann illuminated a filament of the green alga Spirogyra with a visible light spectrum obtained using a prism, using motile aerotactic bacteria to map oxygen production.

1. a) Identify the independent variable, the dependent variable, and the biosensor in this experimental design, justifying the biological basis of the bacterial movement.
2. b) Predict along which wavelengths of the spectrum ($400\text{--}700\ \text{nm}$) the densest bacterial aggregates were observed and justify why this distribution correlates mainly with the absorption spectrum of chlorophylls and cannot be explained by that of accessory carotenoids alone.
3. c) Imagine that the experiment is repeated replacing Spirogyra with a deep benthic cyanobacterium equipped with phycobilins (pigments with optimal absorption between $500$ and $600\ \text{nm}$). Predict how the pattern of bacterial aggregation would change and deduce the ecological advantage of these pigments in aquatic habitats.

The overall equations for photosynthesis and cellular respiration appear symmetrical. Reason why, despite this, they are not reverse processes: compare organelles, mechanisms, and the net energy direction of each.

The diagram shows the exchange of matter in a leaf during photosynthesis. a) Indicate which substance is carried by the arrow marked "?" that exits through the vein and to which parts of the plant it travels. b) Write the overall equation for the process and relate each arrow to the reactants or products. c) Name the epidermal structure through which $\text{CO}_2$ enters.

To reforest a degraded hillside, a sapling with an initial dry mass of $50\ \text{g}$ is planted. After two years, its dry biomass (primarily made up of cellulose, a polymer of glucose) reaches $2{,}5\ \text{kg}$, while the surrounding soil has only lost $15\ \text{g}$ of net mineral salts.

1. a) Refute the common misconception that plant mass comes from the soil: justify the origin of the $2450\ \text{g}$ of gained organic matter and calculate what minimum mass of atmospheric $\text{CO}_2$ the plant must have assimilated to synthesize that equivalent amount of glucose ($M_m(\text{glucose}) = 180\ \text{g/mol}$; $M_m(\text{CO}_2) = 44\ \text{g/mol}$).
2. b) Measurements with gas analyzers indicate that the plant actually fixed a significantly higher mass of $\text{CO}_2$ than that calculated in the previous section. Biologically explain this discrepancy based on the balance between gross photosynthesis, net photosynthesis, and cellular respiration.
3. c) If the tree were watered with water labeled with the heavy isotope $^{18}\text{O}$ ($\text{H}_2^{18}\text{O}$) and supplied with ordinary $\text{CO}_2$, predict with justification in which of the photosynthesis products (in glucose or in the emitted $\text{O}_2$) this isotope will be detected, detailing the photosynthetic phase involved.

Greenhouses enrich the air with $\text{CO}_2$ to increase the harvest. Explain why it works by resorting to the law of the minimum (the limiting factor), whether the increase in production that can be achieved is unlimited, and which other factors become limiting once $\text{CO}_2$ is corrected.

A commercial tomato greenhouse in Almería installs supplemental LED lighting to increase winter production. When irradiance is increased from $400$ to $800\ \mu\text{mol}\cdot\text{m}^{-2}\cdot\text{s}^{-1}$, the $\text{CO}_2$ fixation rate only increases by $4\,\%$, reaching an unexpected plateau.

1. a) Justify at the physiological and molecular level why, beyond a certain light intensity, the photosynthetic rate saturates even though more photons are available.
2. b) Formulate a hypothesis regarding what alternative environmental factor is limiting the process in the greenhouse and design an experiment with controlled variables to test it.
3. c) Reasonably predict how the light saturation curve and the maximum photosynthetic rate will change if the greenhouse atmosphere is enriched with gaseous $\text{CO}_2$.

In an agronomic trial in a semiarid area subjected to a heat wave ($38\ ^\circ\text{C}$) and summer drought, three adjacent plots are monitored: wheat (C3), sorghum (C4), and prickly pear (Opuntia, CAM).

1. a) Vacuolar acidity (malate concentration) and stomatal opening are measured at 03:00 in the morning and at 14:00 at midday. Predict which crop will show maximum acidity at 03:00 and justify how this adaptation allows it to survive with minimal water expenditure.
2. b) During the central hours of the day, wheat drastically reduces its net carbon assimilation rate to near-zero values, while sorghum maintains a very high positive net rate. Justify this contrast at the enzymatic level (RuBisCO versus PEP carboxylase) in relation to stomatal closure and photorespiration.
3. c) In an experimental scenario with artificially enriched $\text{CO}_2$ but without water deficit, predict which of the two crops (C3 or C4) will experience a greater percentage increase in its photosynthetic rate and reason why.