Like all living things, plants need energy to carry out the processes that keep them alive. They get this energy from food. Humans and most other animals are heterotrophs, meaning we have to consume other organisms—plants, other animals, or some combination of the two—for food. However, plants are autotrophs, meaning they create their own food.
Plants make sugar by using energy from sunlight to transform carbon dioxide (CO2), a gas absorbed from the air, and water (H20) taken from the ground by roots into glucose (C6H12O6) and oxygen (O2). This process is called photosynthesis and occurs in the chloroplast of the plant cell.
Plants take in carbon dioxide through tiny openings or pores in their leaves called stomata. Special cells in the leaves of plants called guard cells open and close the stomata. Cellular respiration is a process that occurs in the mitochondria of all organisms.
In this process, both plants and animals break down simple sugars into carbon dioxide and water and release energy in the form of adenosine triphosphate (ATP). The ATP is used for all the processes that occur within a cell that need energy.
Chemical reaction is a process in which one or more substances, the reactants, are converted to one or more different substances, the products.
- Reactants are substances that start a chemical reaction. (ingredients)
- Products are substances that are produced in the reaction. (finished results)
Chemical equations typically show the molecules that enter the reaction (the reactants) to the left and the molecules that result from the reaction (the products) to the right, separated by an arrow that indicates a reaction taking place.
(Reactants) → (Products)
You can think of the reactants as the ingredients for preparing a meal and the products as the different dishes in that meal.
With that in mind, let’s take a look at the chemical equation for photosynthesis:
6 CO2 + 6 H2O + light → C6H12O6 + 6 O2
Carbon Dioxide + Water + Light yields Glucose + Oxygen
- CO2 = carbon dioxide
- H2O = water
- C6H12O6 = glucose
- O2 = oxygen
Therefore, to produce one molecule of glucose (and 6 molecules of oxygen gas), a plant needs 6 molecules of carbon dioxide and 6 molecules of water.
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.
What Is Products of Photosynthesis?
Photosynthesis converts carbon dioxide and water into oxygen and glucose. Glucose is used as food by the plant and oxygen is a by-product. Cellular respiration converts oxygen and glucose into water and carbon dioxide. Water and carbon dioxide are byproducts and ATP is energy that is transformed from the process.
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 take place within the thylakoid membrane and requires a steady stream of sunlight, hence the name light-dependent reaction. Chlorophylls absorb this light energy, which is converted into chemical energy through the formation of two compounds, ATP—an energy storage molecule—and NADPH—a reduced (electron-bearing) electron carrier. In this process, water molecules are also converted to oxygen gas—the oxygen we breathe! 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, also known as the Calvin Cycle, takes place in the stroma, the space between the thylakoid membranes and the chloroplast membranes, and does not require light, hence the name light-independent reaction.
During this stage, energy from the ATP and NADPH molecules is used to fix carbon dioxide and produce three-carbon sugars—glyceraldehyde-3-phosphate, or G3P, molecules—which join up to form glucose. 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.
Not only carbohydrates, as was once thought, but also amino acids, proteins, lipids (or fats), pigments, and other organic components of green tissues are synthesized during photosynthesis. Minerals supply the elements (e.g., nitrogen, N; phosphorus, P; sulfur, S) required to form these compounds.
Chemical bonds are broken between oxygen (O) and carbon (C), hydrogen (H), nitrogen, and sulfur, and new bonds are formed in products that include gaseous oxygen (O2) and organic compounds. More energy is required to break the bonds between oxygen and other elements (e.g., in water, nitrate, and sulfate) than is released when new bonds form in the products.
This difference in bond energy accounts for a large part of the light energy stored as chemical energy in the organic products formed during photosynthesis. Additional energy is stored in making complex molecules from simple ones.
What Is Not a Product of Photosynthesis?
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.