Exercises: 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.

State and explain three reasons why photosynthesis is essential for life on Earth, considering matter, energy, and the atmosphere.

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.

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.

Name three environmental factors that influence the photosynthetic rate (light intensity, $\text{CO}_2$ concentration and temperature) and explain briefly how each one acts when it is scarce or excessive.

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.

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.

What is the light compensation point? Explain what happens when photosynthesis equals respiration, what happens to a plant that lives for weeks below that point (where does it get energy from?) and why understorey plants have lower compensation points than full-sun ones.

Design an experiment with an aquatic plant (Elodea) to measure how photosynthesis varies with the distance to a lamp: hypothesis, independent variable, how the rate is measured (bubbles of $\text{O}_2$ per minute), two necessary controls and one limitation of the method.

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 variegated leaf has white zones without chlorophyll. After a few hours of light, it is decolourised and stained with iodine. Predict which zones will stain blue-black, explain what the result demonstrates (what does iodine detect?) and why the chlorophyll must be removed from the leaf beforehand.

The concentration of atmospheric $\text{CO}_2$ rises every year, but with a seasonal oscillation (the "saw teeth" of the Keeling curve). Explain what causes this annual oscillation, why it mainly reflects the photosynthesis of the northern hemisphere, and what it says about the scale of the biosphere's activity.

A commercial tomato greenhouse in north County Dublin installs supplemental LED lighting to increase winter production. When irradiance is increased from $400$ to $800\ \mu\text{mol}\times\text{m}^{-2}\times\text{s}^{-1}$, 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.

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.

The Keeling curve shows atmospheric $\text{CO}_2$ concentration at Mauna Loa since 1958, combining a continuous upward trend with an annual periodic "sawtooth" oscillation (maximum in May, minimum in October).

1. a) Interpret the global biological cause of the annual oscillation and reason why the $\text{CO}_2$ minimum is recorded at the beginning of the boreal autumn (October) and not at the summer solstice, and why the Southern Hemisphere seasons do not neutralize this oscillation.
2. b) At the South Pole measuring station, the underlying upward trend is identical, but the annual oscillation has an almost negligible amplitude (less than $1\ \text{ppm}$ compared to $\sim 6\ \text{ppm}$ in Hawaii). Justify this discrepancy biogeographically.
3. c) Knowing that terrestrial and oceanic biological sinks currently absorb about $50\,\%$ of anthropogenic $\text{CO}_2$, predict and justify how the slope of the Keeling curve would change if heat stress were to transform boreal forests from net carbon sinks into net carbon emitters.

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 enzyme RuBisCO is bifunctional: it catalyzes both the carboxylation (photosynthesis) and the oxygenation of ribulose-1.5-bisphosphate (photorespiration). In a plant physiology experiment, identical soybean plants (C3 plant) are grown at $30\ ^\circ\text{C}$ in two chambers with identical light and the same concentration of $\text{CO}_2$ ($0.04\,\%$), but varying oxygen: Chamber 1 ($21\,\%$ $\text{O}_2$) and Chamber 2 ($2\,\%$ $\text{O}_2$).

1. a) Predict in which chamber the net photosynthetic rate and biomass gain will be higher. Justify your deduction by explaining the energy and carbon balance of photorespiration.
2. b) If the temperature in both chambers is raised from $30\ ^\circ\text{C}$ to $40\ ^\circ\text{C}$, the photosynthetic yield in Chamber 1 drops drastically, whereas in Chamber 2 it remains almost constant. Justify this result considering the enzymatic affinity of RuBisCO and the relative solubility of both gases as water warms up.
3. c) Biotechnology aims to introduce C4 pathways into C3 crops such as wheat or rice. Argue whether this modification would provide a clear adaptive advantage in cold and temperate latitudes or if, on the contrary, it could reduce their yield compared to unmodified varieties.

An air transport company offers to "offset" the carbon footprint of a route that emits $132\ \text{t}$ of $\text{CO}_2$ per month by funding reforestation. The project report states that one hectare of young pine forest accumulates about $6\ \text{t}$ of elemental carbon ($C$) per year in net biomass.

1. a) Perform the stoichiometric mass balance: calculate how many tonnes of atmospheric $\text{CO}_2$ one hectare of that forest removes per year and determine the minimum plantation area required to offset the annual emissions of that air route ($M_m(C) = 12\ \text{g/mol}$; $M_m(\text{CO}_2) = 44\ \text{g/mol}$).
2. b) From the perspective of the global biogeochemical cycle, justify why replacing the reduction of fossil emissions with forest sinks presents a critical asymmetry regarding the geological fixation time and the vulnerability of the reservoir.
3. c) As the reforested forest progresses toward a mature climax community, predict whether it will continue to remove net $\text{CO}_2$ from the atmosphere at the same rate per hectare. Justify your answer by analyzing the relationship between gross photosynthesis ($P$) and ecosystem respiration ($R$).

Although marine phytoplankton represents less than $1\,\%$ of the global photosynthetic biomass, it is responsible for nearly $50\,\%$ of the net primary production of the biosphere.

1. a) Resolve this apparent ecological paradox by comparing the residence time of organic matter and the turnover rate (production/biomass ratio, $P/B$) of marine microalgae versus the trees of a temperate forest.
2. b) In tropical seas where solar radiation and temperature are optimal year-round, photosynthetic productivity is typically extraordinarily low. Justify the role of the permanent thermocline in this phenomenon and predict the impact of ocean surface warming on the extent of these oligotrophic areas.
3. c) Explain how the ocean's "biological carbon pump" works and predict the consequences of a phytoplankton decline on both global warming and the higher trophic levels of the pelagic marine web.