Plants, also known as autotrophs, are an important element of food webs. Photosynthesis allows autotrophs to produce their food, which is then used to sustain other organisms in the food webs. The myriad of food webs that we see in the entire world is based on the abundance of autotrophs. The ratio of oxygen to carbon dioxide in the environment depends on autotrophs keeping up with all the carbon dioxide we provide. The inability to maintain and replenish oxygen supplies brings about terrifying possibilities for every animal that depends on external sources for replenishing of oxygen supplies. But then, how do autotrophs contribute oxygen and reduce carbon dioxide? Well, through the process of photosynthesis.

What is photosynthesis, you ask? Photosynthesis is a process by which plants use carbon dioxide, water, and energy from the sun to make their food in the form of sugars (Figure 1). Plants, in turn, produce oxygen as a by-product of photosynthetic processes, which is released into the environment. So, where does photosynthesis happen, you ask? Photosynthesis occurs within organelles called the chloroplasts that are abundant on the leaf of a plant, giving it its signature green pigment. Of course, the leaf is not the only photosynthetic part of the plant; it’s just where most of it happen most of the time. Here we will go through the equation for photosynthesis, photosynthesis cycle, a diagram of photosynthesis and the exact sites where different parts of photosynthesis happen.

Before we explore this topic in detail, take a look at the equation for photosynthesis:

6CO₂ + 6H₂O_→ C₆H₁₂O₆ + 6O₂

This equation makes all life possible. Sounds like a lot of pressure on just one equation, doesn’t it? Don’t worry; it can handle it. Below, we will go through the explanation of how this is possible. More specifically, we will go through what the chloroplast is and what the different parts of it are. After which, we will go through the light phase, the dark phase and the details of each including cyclic photophosphorylation, chemiosmosis and non-cyclic photophosphorylation.

Now, let’s talk chloroplast (Figure 2)—the center of photosynthesis (Figure 1). The chloroplast is an organelle that is thought to have been acquired by plants through the process of endosymbiosis because much like mitochondria, this organelle has its own DNA, genes and can, therefore, make its protein. Essentially, the chloroplast is an independent fully functional organelle. Although all the green parts of a plant can carry out photosynthesis, it mainly occurs in the leaves due to the high abundance of chloroplasts. A chloroplast contains stroma, fluid, and stack of thylakoids called grana. The chloroplast contains three main pigments that absorb light energy, chlorophyll a, chlorophyll b, and carotenoids. Chlorophyll is most abundant in the thylakoids.

Photosynthesis fundamentally has two phases, the light-dependent and light independent reactions. What this essentially means is that the former happens during the day when there is a constant supply of light and the latter that happens at night in the absence of light. These two phases happen in different parts of the chloroplast. In prokaryotes, the light reaction occurs in the inner (plasma) membrane in the invaginations called the chromatophores. In eukaryotes, it occurs in the thylakoid membrane of the chloroplast. In eukaryotes, the dark reaction happens in the stroma of the chloroplast.

Light-dependent Reaction

The light-dependent reaction (light reaction) happens in the thylakoids of the chloroplast. Here, the chlorophyll traps the energy from the light (Figure 3). Once trapped this light excites and kicks out the electron (e-) into the electron transport chain. This chain is a series of proteins in the thylakoid membrane. At every step the electron goes through, energy is lost. This “lost energy” goes on to recharge ADP to make ATP. During the electron transport chain, NADPH is also produced, it then stores energy until it can be transferred to the stroma. This NADPH plays an important role in the light-independent reaction (to be discussed later).

The light-dependent reaction converts light energy into chemical energy with ATP as a by-product which will then be used as a source of energy for the light-independent reaction. During this process, the water molecules are split to produce Oxygen and 4 Hydrogens. Photolysis is the process whereby the lost electrons are replaced and in the process water is split. Are you slightly confused? Take a look at Figure 4; it should tie things up nicely for you.

2 H₂O _→ O₂ + 4 [H·]

So to recap, at the end of the light reaction, these are the byproducts:  ATP, NADPH, O₂.

Light-independent Reaction

The by-product of the light reaction, ATP, powers the light-independent reaction producing simple sugars. This reaction is also called the Calvin Cycle, so named for Melvin Calvin who discovered this system along with James Bassham and Andrew Benson. It is also referred to as the dark reaction because it can occur in the absence of light energy. NADPH and ATP produced from the light reaction power the synthesis if CH₂O from CO₂ and H+ (the by-product of the light reaction). So really, it is not that it is independent of light because the ingredients (NADPH, ATP, and H+) would not be available for the dark reaction without the light reaction. So, one should rather say it is indirectly dependent on light.

4 [H·] + CO_2→(CH₂O) + H₂O

Let’s talk more about the Calvin Cycle. Once again, remember that this reaction does not require light energy to proceed. It occurs in the stroma of the chloroplast, and it requires carbon dioxide to run and needs ATP and NADPH to fuel the reaction. When all these ingredients are available, this reaction makes glucose sugar as a by-product. The NADPH provides the electrons needed to reduce carbon dioxide to glucose.

In the first phase; three carbon dioxide molecules enter the Calvin Cycle. Catalyzed by Rubisco, here it reacts with ribulose-1,5-bisphosphate (RuBP) to make two copies of 3-phosphoglycerate. Phosphoglycerate gets phosphorylated by the six ATPs made in the light reaction and 1, 3-Bisphosphate is made. This by-product is then converted into glyceraldehyde-3-phosphate (G3P), during this reaction six NADPH lose their Hydrogen protons and the six phosphates (P_i) molecules are released. After this, the cycle enters into the second phase which is the reduction phase. One G3P exits the cycle and is used to make glucose and other organic compounds. One G3P is used to regenerate the CO₂ acceptor RuBP in the preparation of the next CO₂ input.

Alright, now you should be ready to follow the process of photophosphorylation (photo=light; phosphorylation= addition of phosphate). As the name suggests, this is the process of generating ATP.

Background Stories of ADP →ATP and NADPH and NADP+

Remember: In the light reactions the electron transport chains generated ATP, NADPH, and O₂. What we didn’t delve into are the nitty gritty of how exactly the electron transport chain works. There are two photosystems located in the thylakoid membranes of the chloroplast. Photosystems are functional protein units tasked with the primary photochemistry of photosynthesis; absorbing light and transferring electrons and energy. They can be identified based on the wavelength at which they are most active. Photosystem I (PSI) and photosystem II (PSII) are at their optimum activity at 700 and 680 nanometres, respectively.

Cyclic photophosphorylation occurs in the thylakoid membrane. The electron starts its path in PSI and goes through to a two proteins and an enzyme complex before returning to chlorophyll. Well, what’s the point then, you might ask? The point of this cycle is to create a concentration gradient of H+ ions to power ATP synthase during chemiosmosis (discussed next). During this process oxygen and NADPH are not produced at all, unlike in the non-cyclic counterpart. Based on this paragraph, formulate a concise definition of cyclic photophosphorylation as an exercise to make sure you understand what it is about.

In the light reaction, chemiosmosis enables ATP synthesis. Chemiosmosis—so named for its close relation to osmosis—is the process where ions more down their electrochemical gradient through a selectively permeable membrane (thylakoid membrane in this case). As a result, ATP is generated as a result of the electrochemical gradient of protons as the H+ moves across the thylakoid membrane during photosynthesis. ATP synthase powers the ATP synthesis reaction. In the stroma of the chloroplast, the H+ ions react with NADP+ to make NADPH.

During the noncyclic photophosphorylation, PSII regains the electrons by splitting water and freeing oxygen as a by-product.The non-cyclic photophosphorylation has two fundamental stages involving both PSI and PSII.The non-cyclic photophosphorylationhappens during the light reaction in the stroma lamellae. Water breaks down to form 2H+ + 1/2 O₂ + 2e−. These two electrons stay in PSII while the 2H+ and 1/2O₂ await a different fate. Through the chlorophyll pigments close to the reaction core a photon is absorbed.

The electrons from the pigments are excited in preparation for the electron transport chain. At the end of their journey through this chain, the electrons end up in the core of PSII. The electrons are transferred through a series of reactions until they are passed to plastocyanin where they power the transfer of hydrogen ions (H+) to the thylakoid space. The resulting gradient allows H+ ions to flow back into the stroma of the chloroplast, where they serve as energy sources for the regeneration of ATP.

Photosystem II replenishes the lost electrons from an external source, but the remaining electrons still do not return to PSII as it happens in the cyclic pathway. Instead, these electrons are transferred to the PSI complex, which, together with a second solar photon, increases the energy levels. The highly excited electrons are then involved in the following reaction:

NADP+ + 2H+ + 2e− → NADPH + H+

When the supply of ATP in the chloroplast is running low and Calvin Cycle reactions cannot be sustained, NADPH accumulates and the cyclic phosphorylation flow may be preferred.

Conclusion

Photosynthesis is not entirely light dependent, occurs in different parts of the chloroplast and completes the photosynthesis cycle by recycling its own products to power its processes. However, it is energy demanding and expensive, since it requires a lot of ATP to generate sugars (in the Calvin Cycle alone nine ATP are used for 3CO₂). The ability to partition roles, recycle its products, and go on in the absence of light is what ensures that the photosynthetic equation and reactions are competent and productive enough to support life forms.

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