Reading_15 Photosynthesis

Carbon Flow in Ecosystems

Photosynthesis is one of the central processes that connects cellular metabolism to the global carbon cycle. At the scale of an ecosystem, photosynthesis is the major entry point for carbon into living systems (Figure 1). Autotrophs, including plants, algae, and many bacteria, take carbon dioxide from the environment and convert it into organic molecules. Those organic molecules then move through food webs as organisms eat, grow, reproduce, and are eaten by others. Heterotrophs return much of this carbon to the atmosphere as carbon dioxide through respiration. In this way, photosynthesis and respiration form a biological cycle of carbon flow. Carbon must keep moving: without photosynthetic carbon fixation, ecosystems would lose the supply of reduced organic carbon that supports nearly all trophic levels.

Figure 1. Ecosystem carbon flow and the role of photosynthesis. Photosynthesis by plants and other primary producers fixes atmospheric CO₂ into organic molecules, introducing carbon into the ecosystem. This fixed carbon then moves through the food web by consumption and biological transfer among herbivores, carnivores, omnivores, detritivores, decomposers, and soil biota. Respiration by organisms releases CO₂ back to the atmosphere, while some carbon is retained in soil organic matter and other biological pools. Together, these processes show that ecosystem carbon flow depends on a balance between carbon fixation, biological transfer, storage, and CO₂ release. Attribution: Satoshi H. Namekawa

Where Photosynthesis Happens

Photosynthesis occurs in chloroplasts in plants and algae. At the tissue level, photosynthesis begins in leaves, where carbon dioxide enters through stomata and light reaches mesophyll cells (Figure 2). Inside these cells, chloroplasts contain internal membranes called thylakoids. The light reactions occur in the thylakoid membrane. This membrane contains photosystems, electron carriers, and ATP synthase. The Calvin cycle occurs in the stroma, the fluid-filled space surrounding the thylakoids. This physical separation matters because the light reactions generate ATP and NADPH, while the Calvin cycle consumes ATP and NADPH to fix carbon dioxide.

Figure 2. Location and organization of photosynthesis from the leaf to the photosystem. Photosynthesis occurs in chloroplasts within mesophyll cells of the leaf. CO₂ enters the leaf through stomata, while O₂ produced during photosynthesis exits through the same openings. Inside chloroplasts, thylakoid membranes are organized into stacks called grana. Photosystems are embedded in the thylakoid membrane and consist of an antenna complex and a reaction center. Pigments in the antenna complex, including chlorophylls and carotenoids, absorb light energy and transfer excitation energy to the reaction center, where two chlorophyll a molecules initiate the light-dependent reactions. Attribution: Satoshi H. Namekawa

Light Energy and Excited Electrons

Light behaves as electromagnetic radiation. Different colors of visible light have different wavelengths, and wavelength is related to energy. Shorter wavelengths, such as violet and blue light, carry more energy than longer wavelengths, such as orange and red light. Biological molecules do not absorb all wavelengths equally. Instead, photosynthetic organisms use pigments that absorb specific ranges of light. When a pigment absorbs a photon, one of its electrons can move from a lower-energy ground state to a higher-energy excited state. This excited electron is unstable, but its energy can be transferred in biologically useful ways.

Chlorophyll and Accessory Pigments

The most important photosynthetic pigment in plants is chlorophyll. Chlorophyll molecules have a light-absorbing ring structure that contains magnesium, as well as a hydrophobic tail that helps anchor the molecule in the thylakoid membrane (Figure 3). Chlorophyll absorbs red and blue light strongly but absorbs green light poorly. Because green light is mostly reflected or transmitted, leaves appear green to our eyes (Figure 4). Plants also contain accessory pigments such as carotenoids. These pigments broaden the range of wavelengths that can be captured and can also help protect the photosynthetic apparatus from excess light energy. Having multiple pigments is useful because natural light conditions vary. A leaf in full sun, a leaf shaded by other plants, and an aquatic photosynthetic organism living below the water surface experience different light environments.

Figure 3. Photosynthetic pigments and their absorption spectra. Chlorophyll a, chlorophyll b, and β-carotene are examples of pigments found in photosynthetic organisms. Chlorophylls contain a light-absorbing ring and a hydrophobic tail that helps anchor them in the thylakoid membrane, while β-carotene contains a long conjugated double-bond system. Because these pigments differ in chemical structure, they absorb different wavelengths of visible light. Chlorophylls absorb strongly in the blue and red regions, while β-carotene absorbs mainly in the blue-green region. Using multiple pigments broadens the range of light that plants can capture and also helps protect photosynthetic cells from excess light energy. Attribution: Satoshi H. Namekawa

Figure 4. Why leaves look green. Chlorophyll absorbs visible light most strongly in the blue and red regions of the spectrum, but absorbs relatively little green light. As a result, much of the green light that reaches a leaf is reflected or transmitted rather than absorbed. The reflected and transmitted green wavelengths are what make chlorophyll-containing leaves appear green to our eyes. Attribution: Satoshi H. Namekawa

Photosystems: Antenna Complex and Reaction Center

A photosystem is a protein-pigment complex made of two major parts: an antenna complex and a reaction center. The antenna complex contains many pigment molecules. These pigments absorb photons and pass the excitation energy from one molecule to another. Importantly, during this transfer through the antenna complex, energy is being transferred, not electrons. Eventually, the energy reaches a special pair of chlorophyll molecules in the reaction center. At the reaction center, light energy is converted into a high-energy electron that can enter an electron transport chain. When a compound absorbs a photon of light, the compound is said to leave its ground state and become "excited" (Figure 5). This is the key step that turns light energy into chemical energy.

Figure 5. A diagram depicting what happens to a molecule that absorbs a photon of light. Attribution: Marc T. Facciotti (original work)

Photosystem II and the Start of Noncyclic Flow

Plants use two photosystems in the noncyclic light reactions: photosystem II and photosystem I. This pathway is called noncyclic photophosphorylation because electrons move in one direction rather than returning to their original chlorophyll molecule (Figure 6). In this process, electrons begin with water, pass through photosystem II, the electron transport chain, and photosystem I, and eventually reduce NADP⁺ to NADPH. Because the electrons do not cycle back, they must be replaced by electrons taken from water.

Although photosystem II is named “II,” it acts first in the linear pathway. Photosystem II absorbs light and uses that energy to remove electrons from water. Splitting water replaces electrons lost from photosystem II and produces oxygen as a by-product. This is why oxygenic photosynthesis releases O₂. The electrons from photosystem II are passed through an electron transport chain that includes mobile carriers and the cytochrome b₆f complex. As electrons move through this chain, energy is used to move protons into the thylakoid lumen. Water splitting also adds protons to the lumen. Together, these processes build a proton gradient across the thylakoid membrane.

Figure 6. Noncyclic and cyclic photophosphorylation. In noncyclic photophosphorylation, electrons move from water through photosystem II, the electron transport chain, and photosystem I, eventually reducing NADP⁺ to NADPH. This linear electron flow releases O₂ from water, contributes to a proton gradient across the thylakoid membrane, and produces both ATP and NADPH. In cyclic photophosphorylation, electrons excited by photosystem I cycle back through the electron transport chain instead of reducing NADP⁺. This pathway produces ATP only, without making NADPH or releasing O₂, and helps the chloroplast adjust ATP production to match the cell’s energy needs. Attribution: Satoshi H. Namekawa

Photophosphorylation: Building a Proton Gradient and ATP

Photosystem I and NADPH Production

After electrons pass through the electron transport chain, they reach photosystem I. Photosystem I absorbs light and re-energizes the electrons. These high-energy electrons are then passed to carriers including ferredoxin and finally to NADP+ reductase, which reduces NADP+ to NADPH. NADPH provides reducing power for carbon fixation. In noncyclic photophosphorylation, electrons move in one direction: from water, through photosystem II, through the electron transport chain, through photosystem I, and finally to NADP+. The major products are ATP, NADPH, and oxygen. Water is the original electron donor, and NADP+ is the final electron acceptor.

The Z-Scheme

The Z-scheme is a useful way to visualize this electron flow (Figure 7). It shows electron energy rising twice, once after light absorption by photosystem II and once after light absorption by photosystem I. Between these two boosts, electron energy decreases as electrons move through the transport chain, and that energy drop is used to help build the proton gradient. The Z-shape emphasizes why two photosystems are needed. Photosystem II is strong enough, after light activation, to take electrons ultimately from water and feed them into the transport chain. Photosystem I then gives the electrons another energy boost so that they can reduce NADP+ to NADPH. Without both photosystems, plants could not efficiently use water as the electron donor while also producing NADPH. To help you connect photophosphorylation with respiration, Table 1 provides a redox tower showing common electron donors and acceptors involved in photosynthetic electron flow.

Figure 7. The Z-scheme of photosynthetic electron flow. The Z-scheme summarizes how electron energy changes during the light reactions of oxygenic photosynthesis. Light first excites electrons in photosystem II, allowing them to move through plastoquinone, the cytochrome b₆f complex, and plastocyanin. As electrons move through this transport chain, their energy decreases, and part of that energy is used to help build the proton gradient that drives ATP synthesis. Light then excites electrons again in photosystem I, allowing them to pass to ferredoxin and ultimately reduce NADP⁺ to NADPH. The inset compares redox potentials and shows why oxidized photosystem II, P680⁺, can pull electrons from water: P680⁺ is a stronger oxidant than the H₂O/O₂ redox pair, so water is oxidized and O₂ is released.  Attribution: Satoshi H. Namekawa

The Calvin Cycle Uses ATP and NADPH

The ATP and NADPH produced by the light reactions support the Calvin cycle. The Calvin cycle is sometimes called the light-independent reaction, but this name can be misleading. It does not directly require photons, but it depends on ATP and NADPH produced by the light reactions. The cycle begins when the enzyme Rubisco adds carbon dioxide to ribulose-1,5-bisphosphate, or RuBP. This carbon fixation step produces 3-phosphoglycerate, often abbreviated 3-PGA, as the first stable product. ATP and NADPH are then used to convert 3-PGA into glyceraldehyde-3-phosphate, or G3P. Some G3P leaves the cycle and can be used to build sugars and other organic molecules. The rest of the G3P is used to regenerate RuBP, allowing the cycle to continue.

Figure 8. Overview of the Calvin cycle. The Calvin cycle occurs in the stroma of the chloroplast and uses ATP and NADPH produced by the light reactions to fix CO₂ into organic molecules. In the carbon fixation step, RuBP accepts CO₂, and Rubisco catalyzes the formation of 3-phosphoglycerate (3-PGA). During the reduction step, ATP and NADPH are used to convert 3-PGA into glyceraldehyde-3-phosphate (G3P). Some G3P exits the cycle and can be used to build sugars and other organic molecules, while the remaining G3P is used to regenerate RuBP so the cycle can continue. Attribution: Satoshi H. Namekawa

Rubisco and the Regulation of Carbon Fixation

Rubisco is important because it starts the process that brings inorganic carbon into organic form. It is often described as one of the most abundant proteins on Earth because so many photosynthetic organisms rely on it. However, Rubisco also has limitations. It can react with oxygen as well as carbon dioxide, and its activity depends on environmental conditions. This point helps explain why photosynthesis is not just a simple chemical equation. The cell must coordinate light capture, electron flow, ATP production, NADPH production, and carbon fixation under changing environmental conditions.

Balancing ATP, NADPH, and Carbon Flow

One major challenge in photosynthesis is balancing the production of ATP and NADPH with the demands of the Calvin cycle. Noncyclic photophosphorylation is the main pathway that supplies both molecules. In this pathway, electrons move from water through photosystem II, the electron transport chain, and photosystem I, eventually reducing NADP⁺ to NADPH (Figure 6). This process also helps build the proton gradient used to make ATP and releases O₂ from the splitting of water.

However, the Calvin cycle does not always need ATP and NADPH in exactly the same proportions produced by noncyclic electron flow. Environmental conditions can also change the demand for these molecules. For example, during drought, plants often close their stomata to reduce water loss. This limits CO₂ entry into the leaf, slowing the Calvin cycle and reducing the use of NADPH. At the same time, light may continue to strike the leaf, so the chloroplast must still manage incoming light energy even when carbon fixation is limited.

From Cellular Photosynthesis to Ecosystem Function

At the ecosystem level, these cellular processes scale up. Photosynthesis fixes carbon dioxide into biomass, creating the organic matter that supports food webs. Energy captured from sunlight does not cycle in the same way carbon does. Instead, energy flows through trophic levels and much of it is lost as heat at each transfer. Only a fraction of the energy stored in biomass at one trophic level becomes available to the next. This is why higher trophic levels usually support less biomass than lower trophic levels. Photosynthesis is therefore both a cellular process and an ecosystem-level foundation: it introduces reduced carbon into biological systems and captures energy that can move through food webs.

Key Take-Home Message

In summary, photosynthesis links light energy, electron flow, ATP production, reducing power, and carbon fixation. Pigments capture light and pass energy to reaction centers. Noncyclic electron flow uses photosystem II and photosystem I to move electrons from water to NADP+, producing oxygen, ATP, and NADPH. The Calvin cycle uses ATP and NADPH to fix carbon dioxide and build organic molecules. Cyclic electron flow provides extra ATP and helps balance energy needs when conditions change. Understanding photosynthesis therefore requires thinking across scales, from excited electrons in chlorophyll to carbon flow through entire ecosystems.

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