What general type of plant food is produced by photosynthesis?

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Cells get nutrients from their environment, but where do those nutrients come from? Virtually all organic material on Earth has been produced by cells that convert energy from the Sun into energy-containing macromolecules. This process, called photosynthesis, is essential to the global carbon cycle and organisms that conduct photosynthesis represent the lowest level in most food chains (Figure 1).

What Is Photosynthesis? Why Is it Important?

Most living things depend on photosynthetic cells to manufacture the complex organic molecules they require as a source of energy. Photosynthetic cells are quite diverse and include cells found in green plants, phytoplankton, and cyanobacteria. During the process of photosynthesis, cells use carbon dioxide and energy from the Sun to make sugar molecules and oxygen. These sugar molecules are the basis for more complex molecules made by the photosynthetic cell, such as glucose. Then, via respiration processes, cells use oxygen and glucose to synthesize energy-rich carrier molecules, such as ATP, and carbon dioxide is produced as a waste product. Therefore, the synthesis of glucose and its breakdown by cells are opposing processes.

However, photosynthesis doesn't just drive the carbon cycle — it also creates the oxygen necessary for respiring organisms. Interestingly, although green plants contribute much of the oxygen in the air we breathe, phytoplankton and cyanobacteria in the world's oceans are thought to produce between one-third and one-half of atmospheric oxygen on Earth.

What Cells and Organelles Are Involved in Photosynthesis?

Chlorophyll A is the major pigment used in photosynthesis, but there are several types of chlorophyll and numerous other pigments that respond to light, including red, brown, and blue pigments. These other pigments may help channel light energy to chlorophyll A or protect the cell from photo-damage. For example, the photosynthetic protists called dinoflagellates, which are responsible for the "red tides" that often prompt warnings against eating shellfish, contain a variety of light-sensitive pigments, including both chlorophyll and the red pigments responsible for their dramatic coloration.

What Are the Steps of Photosynthesis?

Photosynthesis consists of both light-dependent reactions and light-independent reactions. In plants, the so-called "light" reactions occur within the chloroplast thylakoids, where the aforementioned chlorophyll pigments reside. When light energy reaches the pigment molecules, it energizes the electrons within them, and these electrons are shunted to an electron transport chain in the thylakoid membrane. Every step in the electron transport chain then brings each electron to a lower energy state and harnesses its energy by producing ATP and NADPH. Meanwhile, each chlorophyll molecule replaces its lost electron with an electron from water; this process essentially splits water molecules to produce oxygen (Figure 5).

Once the light reactions have occurred, the light-independent or "dark" reactions take place in the chloroplast stroma. During this process, also known as carbon fixation, energy from the ATP and NADPH molecules generated by the light reactions drives a chemical pathway that uses the carbon in carbon dioxide (from the atmosphere) to build a three-carbon sugar called glyceraldehyde-3-phosphate (G3P). Cells then use G3P to build a wide variety of other sugars (such as glucose) and organic molecules. Many of these interconversions occur outside the chloroplast, following the transport of G3P from the stroma. The products of these reactions are then transported to other parts of the cell, including the mitochondria, where they are broken down to make more energy carrier molecules to satisfy the metabolic demands of the cell. In plants, some sugar molecules are stored as sucrose or starch.

Conclusion

Photosynthetic cells contain chlorophyll and other light-sensitive pigments that capture solar energy. In the presence of carbon dioxide, such cells are able to convert this solar energy into energy-rich organic molecules, such as glucose. These cells not only drive the global carbon cycle, but they also produce much of the oxygen present in atmosphere of the Earth. Essentially, nonphotosynthetic cells use the products of photosynthesis to do the opposite of photosynthesis: break down glucose and release carbon dioxide.

Photosynthesis is the name given to the set of chemical reactions performed by plants to convert energy from the sun into chemical energy in the form of sugar. Specifically, plants use energy from sunlight to react carbon dioxide and water to produce sugar (glucose) and oxygen. Many reactions occur, but the overall chemical reaction for photosynthesis is:

• 6 CO2 + 6 H2O + light → C6H12O6 + 6 O2
• Carbon Dioxide + Water + Light yields Glucose + Oxygen

In a plant, the carbon dioxide enters via leaf stomates by diffusion. Water is absorbed through the roots and is transported to leaves through the xylem. Solar energy is absorbed by chlorophyll in the leaves. The reactions of photosynthesis occur in the chloroplasts of plants. In photosynthetic bacteria, the process takes place where chlorophyll or a related pigment is embedded in the plasma membrane. The oxygen and water produced in photosynthesis exit through the stomata.

• In photosynthesis, energy from light is used to convert carbon dioxide and water into glucose and oxygen.
• For 6 carbon dioxide and 6 water molecules, 1 glucose molecule and 6 oxygen molecules are produced.

Actually, plants reserve very little of the glucose for immediate use. Glucose molecules are combined by dehydration synthesis to form cellulose, which is used as a structural material. Dehydration synthesis is also used to convert glucose to starch, which plants use to store energy.

The overall chemical equation is a summary of a series of chemical reactions. These reactions occur in two stages. The light reactions require light (as you might imagine), while the dark reactions are controlled by enzymes. They don't require darkness to occur -- they simply don't depend on light.

The light reactions absorb light and harness the energy to power electron transfers. Most photosynthetic organisms capture visible light, although there are some that use infrared light. Products of these reactions are adenosine triphosphate (ATP) and reduced nicotinamide adenine dinucleotide phosphate (NADPH). In plant cells, the light-dependent reactions occur in the chloroplast thylakoid membrane. The overall reaction for the light-dependent reactions is:

• 2 H2O + 2 NADP+ + 3 ADP + 3 Pi + light → 2 NADPH + 2 H+ + 3 ATP + O2

In the dark stage, ATP and NADPH ultimately reduce carbon dioxide and other molecules. Carbon dioxide from the air is "fixed" into a biologically usable form, glucose. In plants, algae, and cyanobacteria, the dark reactions are termed the Calvin cycle. Bacteria may use different reactions, including a reverse Krebs cycle. The overall reaction for the light-independent reaction of a plant (Calvin cycle) is:

• 3 CO2 + 9 ATP + 6 NADPH + 6 H+ → C3H6O3-phosphate + 9 ADP + 8 Pi + 6 NADP+ + 3 H2O

During carbon fixation, the three-carbon product of the Calvin cycle is converted into the final carbohydrate product.

Like any chemical reaction, the availability of the reactants determines the amount of products that can be made. Limiting the availability of carbon dioxide or water slows the production of glucose and oxygen. Also, the rate of the reactions is affected by temperature and the availability of minerals that may be needed in the intermediate reactions.

The overall health of the plant (or other photosynthetic organism) also plays a role. The rate of metabolic reactions is determined in part by the maturity of the organism and whether it's flowering or bearing fruit.

If you're asked about photosynthesis on a test, you may be asked to identify the products of the reaction. That's pretty easy, right? Another form of the question is to ask what is not a product of photosynthesis. Unfortunately, this won't be an open-ended question, which you could easily answer with "iron" or "a car" or "your mom." Usually this is a multiple choice question, listing molecules which are reactants or products of photosynthesis. The answer is any choice except glucose or oxygen. The question may also be phrased to answer what is not a product of the light reactions or the dark reactions. So, it's a good idea to know the overall reactants and products for the photosynthesis general equation, the light reactions, and the dark reactions.

• Bidlack, J.E.; Stern, K.R.; Jansky, S. (2003). Introductory Plant Biology. New York: McGraw-Hill. ISBN 978-0-07-290941-8.
• Blankenship, R.E. (2014). Molecular Mechanisms of Photosynthesis (2nd ed.). John Wiley & Sons. ISBN 978-1-4051-8975-0.
• Reece J.B., et al. (2013). Campbell Biology. Benjamin Cummings. ISBN 978-0-321-77565-8.