You Light up My Food Chain: The Process of Photosynthesis

In the last post, the importance of water quality and water chemistry was reviewed in order to determine it’s relevance to the various life forms and life cycles in the pond. In this post, the chemistry that was discussed will be tied into the life processes involved in the pond. This type of science of relating chemistry to biological functions is known as biochemistry. Here, the specific biochemical process that will be elucidated is called photosynthesis.
Photosynthesis is the source of energy for nearly all life forms on earth. It fixes carbon from the atmosphere by taking carbon dioxide (CO2) and utilizes energy from the sun to create organic molecules, such as sugars, which will be utilized in food chains as a source of nutrients. Generally, photosynthesis uses CO2 and water (H2O) to create oxygen (O2) and a sugar (Figure 1). It is used by plants, algae, and cyanobacteria (mentioned in the post Phycology: The Forgotten Field of Study and the Pond). This is the main way that nutrients are created from inorganic molecules to supply food chains. Such an organism with the capability of supplying food chains is referred to as an autotroph. The organisms that cannot create organic materials, and thus get them from ingesting other organisms, are referred to as heterotrophs, which were elaborated in the post Phycology: The Forgotten Field of Study and the Pond. Organisms that photosynthesize are specifically referred to as photoautotrophs, due to their ability to use energy from the sun packaged as photons to synthesize nutrients.

Figure 1: The overall equation of photosynthesis. The reagents are on the left, which are six molecules of carbon dioxide and six molecules of water. Light is utilized to combine these reagents to produce six molecules of oxygen and a sugar molecule, or the products on the right.

Before one gets into the specifics of photosynthesis, one needs to learn a little about chemistry. Redox reactions are a type of chemical reaction that takes place often during photosynthesis and cellular respiration, which will also be explained in greater detail. Redox is a shortened name for reduction/oxidation reactions, which involve the transferring of electrons between atoms and molecules, the building blocks of all the substances in our world. The oxidation portion involves the loss of an electron, and the reduction involves the gain of an electron (Figure 2). It is easy to think of a reduction as a reduction of charge and the gain of an electron in the atom or molecule involved since an electron has a negative charge. In a chemical reaction, everything must balance out; therefore there cannot be an oxidation without its corresponding reduction, or vice versa. In a redox reaction, a reductant, or reducing agent will transfer electrons to an oxidant, and thus an oxidation will occur where the reductant lost an electron and is oxidized. An oxidant or oxidizing agent will gain an electron from a reducer and become reduced in a reduction reaction. This is where the oxidant gained an electron. In other words, an oxidant oxidizes other substances, or removes electrons from other substances, while reducing substances reduce other substances, or give electrons to other substances (Figure 2). Due to this, oxidants are often referred to as electron acceptors while reducing agents are referred to as electron donors. The redox pair in a reaction therefore consists of an oxidizing and reducing agent. A simple example of a redox reaction is the substitution reaction of iron (Fe+2) and copper sulphate (CuSO4) (Figure 3).

Figure 2: The two parts of a redox reaction. The oxidant oxidizes other substances or removes electrons from other substances. Therefore, the oxidant gains an electron and is reduced in the reduction half of the reaction at the top. The reductant reduces other substances. It therefore loses an electron and is oxidized in the oxidation portion of the redox reaction at the bottom.

Figure 3: The substitution reaction of iron and copper sulphate. Copper sulphate (CuSO4) has a copper atom (Cu+2) with a charge of positive two bound to a sulphate ion (SO4-2) with a corresponding negative charge. Copper is reduced or is the oxidant, and therefore accepts two electrons from the iron (Fe) atom. Iron, therefore, is oxidized and is the reducing agent which donates two electrons. Iron initially has a charge of zero, but then loses two electrons to have a charge of positive two and is now bound to the sulphate ion with a charge of negative two.

In this reaction, copper (Cu+2) is bound with sulphate (SO4-2) in one molecule of copper sulphate, but the copper atom in the molecule is substituted for iron and the new compound formed is iron sulphate (FeSO4). The two half equations, which consist of the redox reaction, are:

Figure 4: The redox reactions of the substitution of iron and copper sulphate. Iron (Fe) in the top reaction or the oxidation reaction loses two electrons, denoted by 2e, and is oxidized. Iron is therefore the reducing agent. Copper (Cu) in the bottom corresponding reduction reaction, is reduced and accepts two electrons from the copper atom and is the oxidizing agent.

where the iron is oxidized and the copper is reduced. Therefore, iron is the electron donor and reductant, while copper is the electron acceptor and oxidizing agent.
Photosynthesis involves the transfer of electrons and therefore consists of redox reactions. It begins when light is absorbed by proteins called photosynthetic reactor centres. These centres, in plants, will contain mainly the pigment chlorophyll, but can also contain other secondary pigments such as carotenes and xanthophylls. Algae, which are the other major photosynthesizers of the world, will contain the same pigments in addition to phycoerythrin in red algae, and fucoxanthin in brown algae and diatoms. In the photosynthetic cyanobacteria, the photosynthetic reactor centres are in the plasma membrane, whereas in plants and algae, these centres are located in the chloroplasts. Chloroplasts are organelles, or compartments within a cell, that contain the photosynthetic components and regulate photosynthesis (Figure 5). They are the energy centres of plant or algal cells. A typical plant cell will contain 10-100 chloroplasts, and although all plant cells will have chloroplasts, most of the energy a plant requires will be captured through its leaves.

Figure 5: Plant cells with visible chloroplasts. Chloroplasts are the organelles, or compartments within a cell, that regulate photosynthesis. They often contain the photosynthetic pigment chlorophyll, which is green.

In the chloroplasts, two sets of chemical reactions take place, the light-dependant and light-independent reactions. In the light-dependant reactions, chlorophyll captures energy and facilitates creating molecules with the capability of storing energy. These molecules include adenosine triphosphate (ATP) and nicotinamide adenine dinucleotide phosphate (NADPH), which are molecules with a higher energy state. Later on, when these molecules are converted, energy will be released and can be used for other biochemical processes. The light-independent reactions occur later in photosynthesis. These reactions do not require light like the light-dependant reactions. In these reactions, ATP and NADPH are utilized and the energy they release will assist in reducing CO2. Thus CO2 can be captured and fixed into organic compounds, such as sugars like glucose (Figure 6).

Figure 6: The light-dependant and light-independent reactions of photosynthesis. These two types of reactions are necessary to complete photosynthesis. Light is absorbed by chlorophyll to assist in the formation of the compounds ATP and NADPH, which are molecules high in energy. These energy containing compounds will be utilized in the light-independent reactions of the Calvin cycle. In order to complete the light-dependent reactions, water is used up and oxygen is formed. The Calvin cycle utilizes ATP and NADPH, formed in the light-dependant reactions, to fix carbon in carbon dioxide in order to make sugars.

When light first reaches the chloroplast, the chlorophyll pigment absorbs a unit of energy from the sun, otherwise known as a photon. This causes the chlorophyll to lose one electron and become oxidized and to pass it on to various proteins in the chloroplast thus starting a flow of electrons down an electron transport chain. The ultimate result of the electron transport chain is the reduction of NADP to NADPH, which is a molecule used by photosynthesizers to store energy. The chlorophyll molecule, which has lost an electron to begin the electron transport chain, regains its electron from H20 which plays the role of the reducing agent and donates an electron to the chlorophyll molecule. This releases O2 from the water and creates a proton (H+) gradient across the membrane of the chloroplast which creates a proton motive force. Protons are just another way of referring to a hydrogen atom obtained and separated from water that has lost an electron and now has a positive charge. Therefore, there is a difference in the concentration of positively charged hydrogen atoms across the chloroplast membrane. This gradient of protons is used by an enzyme called ATP synthase to produce the other energy storing compound, ATP (Figure 7). If one thinks about gradients and the natural entropic tendency for gradients to diminish and smooth out, it will become clear how a proton gradient can furnish energy to create ATP. This is seen on a larger scale when a water gradient is stored by a dam. It takes a lot of energy to make a dam and to store the water behind the dam; therefore, potential energy is stored as a water gradient. Consider the force of all the water if it were released from the dam all at once. This energy is used to make hydroelectricity. The same concept applies in chloroplasts to create ATP from a proton gradient (Figure 8).

Figure 7: The light-dependant photosynthetic reactions in the membrane of the chloroplast. On the left, light from the sun causes chlorophyll in PSII to lose an electron and become oxidized. The electron will pass through various proteins in an electron transport chain and will arrive at a point where the molecule NADP can be reduced to NADPH in PSI, which is an excellent chemical method of storing energy for other biochemical processes. Examples of other biochemical processes include the fixing of carbon dioxide in the light-independent reactions of photosynthesis. The chlorophyll in PSII, which has lost an electron, regains another from water (H20) which is oxidized and releases a proton (H+) and oxygen (O2). The proton gradient created is used by ATP synthase, on the right, to create another energy storing compound, ATP.

Figure 8: The proton motive force across a chloroplast membrane. Proton motive force relies on a varying concentration of protons (in blue), or positively charged hydrogen ions across a biological membrane. Like water behind a dam, a higher concentration of these ions on one side of the membrane will possess a potential energy value. When these protons pass through a membrane protein (in red), which plays a role similar to a dam that can be opened and closed, the energy can be used for certain biochemical processes, such as the synthesis of ATP by ATP synthase.

Once the electrons reach the end of the electron transport chain, as the reduced form NADPH, this compound can further provide electrons in order to carry out reductions for other chemical reactions. The reduction reactions that will take place in order to complete photosynthesis include the reduction of CO2 into sugars. The other half of the redox reaction is the oxidation of H20 into O2 as mentioned before (Figures 1, 6 and 7). The reduction of CO2 into sugars consists of the half of photosynthesis that does not rely on sunlight and is therefore light-independent, as described earlier. This part of photosynthesis is referred to as the Calvin cycle. This cycle involves fixing and reducing CO2 in order to form sucrose and starch. These products are then available to be utilized as nutrition by plants, and to produce other organic compounds needed for plant life, such as lipids (fats), cellulose, and amino acids. The Calvin cycle does not require light but it requires electrons, which are donated by NADPH, and it requires CO2 (Figure 6).
Another biochemical process of plants that does not require light is called cellular respiration. This set of chemical reactions, however, is employed by all living cells including animal cells and not just autotrophs. Cellular respiration is a way of utilizing organic nutrient molecules such as sugars to power and sustain cellular life. Cellular respiration is, in many ways, the reverse or opposite of photosynthesis. It is the oxidation of sugars such as glucose, and the reduction of O2 to produce CO2 and water (Figure 9).

Figure 9: The process of cellular respiration. Sugars, such as glucose (C6H12O6), are oxidized and oxygen (O2) is reduced, which are the reagents of cellular respiration. The products formed are carbon dioxide (CO2) and water (H2O). Cellular respiration is the utilization of sugars and O2 by many life forms to fuel countless biochemical processes. Cellular respiration is the reverse of photosynthesis, which takes CO2 and H2O to form O2 and sugars.

Cellular respiration has a similar goal as photosynthesis. It utilizes certain compounds in order to make compounds such as ATP and NADPH, which can transfer energy to make possible an array of biochemical processes.
Photosynthesis is a very important process of all ecosystems including the pond. This is because it will fix carbon, a component of all organic or biological molecules, and it furnishes O2, an important gas necessary for most forms of life because it is utilized for cellular respiration. In a pond, plants will photosynthesize, but an even more important source of O2 and photosynthesis is from the algae in a pond. An excess of algae can be very unsightly, however, some amount of algae is considered important to the pond ecosystem. In fact, an ideal pond will have an overlapping of algal and bacterial species to achieve a proper balance. Algae will fix carbon, from either the alkalinity of the H2O or from a solution of CO2 from the atmosphere, and will furnish carbon compounds and O2 for bacterial replication. The bacteria will then supply certain carbon and nitrogen compounds, which will be of use to the algae (William et al., 1953). Having both algal and bacterial species in a pond will also ensure that there will not be an excess of one or the other population, therefore, population numbers will be controlled. It is important to have a certain concentration of O2 in the pond to sustain life, and this is achieved either through aeration and circulation, or through photosynthesis (Elaborated in the last post “Water, water everywhere! So what are we to think? Water quality and the pond”).
In conclusion, photosynthesis, or utilizing light, CO2 and H2O to synthesize sugars and O2, is a very important process for life in the pond ecosystem among others. Aeration and circulation are methods to ensure proper O2 concentrations of the H2O in a pond, but photosynthesis from plants and algae also play a huge part in this. To ensure a properly balanced pond, some photoautotrophs are necessary. However, if you are contemplating the number of plants or the algal population in your pond, please give us a call at Village Pond and Garden and we will assist you in achieving the right balance of photosynthesizers in your pond.

In the next instalment →Green with Chlorophyll or What Makes Up Plants.
The next instalment shall come in spring. Now it is time for me to take a break and go back to school to study infection prevention and control. Who knows, maybe this might have some crossover knowledge for life in ponds. Either way, learning is fun and I am glad that I could learn and revise these topics with you fellow readers. Thank you!

Summary/Important Points

• Photosynthesis fixes carbon from the atmosphere by taking carbon dioxide and, with water and energy from the sun creates oxygen and sugar.
• Photosynthesis is a source of nutrients for nearly all life forms on earth. It is employed by plants, algae and cyanobacteria.
• Photosynthesis utilizes redox reactions, which involve the transfer of electrons from one compound to another.
• Redox is short for reduction/oxidation reactions. Since chemical reactions must all balance out, a reduction cannot occur without an oxidation or vice versa.
• An oxidation is the loss of an electron while the reduction is a gain of an electron. A reducing agent will transfer an electron to an oxidant and will become oxidized itself. An oxidant will remove an electron from a reducing agent and will become reduced itself. Oxidants are often referred to as electron acceptors, while reducing agents are referred to as electron donors.
• Photosynthesis commences when light is absorbed by the photosynthetic reactor centres, which contain chlorophyll and can contain other pigments depending on the photosynthesizer. The photosynthetic reactor centres are in the plasma membrane in cyanobacteria. In plants and algae, these centres are located in specialized subcellular compartments called chloroplasts. Chloroplasts are the organelles responsible for photosynthetic reactions in a cell.
• Two sets of chemical reactions take place in photosynthesis, the light-dependant and light-independent reactions. In the light-dependent reactions energy from the sun is used to create molecules high in energy. These compounds are called adenosine triphosphate (ATP) and nicotinamide adenine dinucleotide phosphate (NADPH). Water is degraded in these reactions and oxygen is released. The light-independent reactions require no light, but utilize ATP and NADPH to fix carbon from carbon dioxide to produce sugars.
• Cellular respiration is the opposite of photosynthesis. It utilizes sugars to power and sustain cellular life. It utilizes sugars and oxygen to produce carbon dioxide and water. All cells utilize this process, not just photosynthesizers.
• Proper oxygen concentrations are essential to maintain a pond’s ecosystem. Circulation and aeration assist in this balance, but photosynthesis also plays a huge part.

References/Further Reading
Copeland, B.J., Butler, J., L., and Shelton, W., L., 1961. Photosynthetic Productivity in a Small Pond. Proc. of the Acad. of Sci. for 1961, p22-26.
Oswald, W. J., Gotaas, H.B., Ludwig, H.F., and Lynch, V., 1953. Algae Symbiosis in Oxidation Ponds. Sewage and Industrial Wastes, 25(6):692-705.

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