Before you start, make sure you know: Photosynthesis (High School Year 2 (semesters 3 and 4))
In a cornfield in Jalisco, green leaves make sugar from air, water, and light. In the depths of the Gulf of California, next to the hydrothermal vents, there is life without a single drop of sunlight. How is energy obtained in each case, and where does the oxygen that plants release come from? You will answer that in this section.
The overall equation and the chloroplast
In the previous course you saw photosynthesis in broad strokes. Now we are going to review it as a set of redox reactions.
Photosynthesis takes place in the chloroplast. Its inner membrane encloses the stroma (a fluid) and, inside it, flattened sacs called thylakoids, stacked in grana. It has two stages: the light-dependent reactions, in the thylakoid membrane, produce ATP, NADPH, and $\mathrm{O_2}$; the Calvin cycle, in the stroma, uses that ATP and NADPH to build carbohydrates from $\mathrm{CO_2}$.
In the diagram, 1 marks the thylakoids (a granum) and 2 the stroma. Follow the arrows going in and out of each one.
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Write the overall equation for oxygenic photosynthesis and state which molecule is oxidised and which is reduced in the process.
Write the overall equation for oxygenic photosynthesis and state which molecule is oxidised and which is reduced in the process.
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…
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.
White light is a mixture of wavelengths. Each pigment absorbs some and reflects others. Chlorophyll a is the main pigment: it is found in the photosystems, inside the thylakoid membrane of the chloroplast. It absorbs mostly blue (~430 nm) and red (~660 nm) light and hardly absorbs green-yellow (500-550 nm), which it reflects: that is why leaves look green.
Accessory pigments capture wavelengths that chlorophyll a uses poorly and pass that energy on to the reaction center:
Chlorophyll b: absorbs blue (~460 nm) and red-orange (~640 nm). It is found in plants and green algae.
Carotenoids: absorb blue-green (450-500 nm) and protect against excess light. They give carrots and chili peppers their orange color, and leaves their yellow in autumn.
Phycobilins (such as phycoerythrin): absorb green-yellow. They are found in red algae and cyanobacteria.
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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…
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?
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…
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.
If you extract pure chlorophyll and illuminate it, it only emits a reddish glow and heat: it wastes the energy. In the living membrane, however, the pigments are organized to use that energy and knock electrons loose.
Photolysis of water and photophosphorylation
In 1941, Ruben and Kamen watered algae with water labeled with $^{18}\mathrm{O}$ and the $\mathrm{O_2}$ given off carried the isotope; when they labeled the $\mathrm{CO_2}$, the oxygen released was normal. The oxygen we breathe comes from water.
Fate of the products: the electrons pass into the transport chain and reach NADPH; the protons accumulate in the lumen and form a gradient; the oxygen leaves the cell (or is used by the mitochondrion).
Practice★★☆Not started
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 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.
The enzyme RuBisCOfixes $\mathrm{CO_2}$: it attaches it to ribulose-1,5-bisphosphate (RuBP, 5 carbons) and two molecules of 3-phosphoglycerate (3 C) are formed. These are reduced with ATP and NADPH to triose phosphates; most of them regenerate RuBP and a portion leaves to synthesize carbohydrates.
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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…
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.
A tomato greenhouse in Sinaloa is sometimes enriched with $\mathrm{CO_2}$ and lit in the evening. Why does the harvest increase, and until when does it help to give more light?
The net photosynthetic rate is the $\mathrm{CO_2}$ the plant assimilates minus the amount it releases through respiration. It is measured with a device that detects the $\mathrm{CO_2}$ in the air. It depends on several factors; the one in shortest supply at any moment is the limiting factor.
Compensation point: the light level at which photosynthesis equals respiration and the net rate is zero. Below it, the plant loses carbon.
Low light: the limiting factor is light. The light-dependent reactions produce little ATP and NADPH, and there is plenty of $\mathrm{CO_2}$. That is why, at the same light level, both curves are identical and rise with the same slope even when there is more $\mathrm{CO_2}$.
Saturation: as light increases, the light-dependent reactions already produce more ATP and NADPH than the Calvin cycle can use. Now $\mathrm{CO_2}$ is limiting (RuBisCO and the cycle work at maximum with that $\mathrm{CO_2}$) and more light no longer helps.
More $\mathrm{CO_2}$: the plateau rises, because RuBisCO fixes more $\mathrm{CO_2}$ and less $\mathrm{O_2}$ (less photorespiration).
Temperature: it affects the enzymes of the Calvin cycle, not the absorption of photons. With low light the rate hardly changes; with high light it does.
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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…
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.
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.
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.
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 hot, dry climates, RuBisCO can fix $\mathrm{O_2}$ instead of $\mathrm{CO_2}$ (photorespiration), and energy is wasted. Corn and sugarcane (C4 plants) first fix $\mathrm{CO_2}$ into a 4-carbon compound in one cell type and carry out the Calvin cycle in another: separation in space. Nopal cactus and agave (CAM plants) open their stomata at night, fix the $\mathrm{CO_2}$, and carry out the Calvin cycle during the day with the stomata closed: separation in time, which also saves water.
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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…
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 $\text{NH}_4^+$.
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.
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.
c) Compare oxygenic photosynthesis and nitrifying chemosynthesis with respect to the type of cells and biological taxa that carry them out.
Back to the beginning: plants get energy from light and the oxygen they release comes from water; the bacteria at the hydrothermal vents get energy from inorganic compounds, without light.
Summary
Let's practice! 3 medium exercises in this section