Do plants convert carbon dioxide and water into carbohydrates?

Most life on Earth depends on photosynthesis.The process is carried out by plants, algae, and some types of bacteria, which capture energy from sunlight to produce oxygen (O2) and chemical energy stored in glucose (a sugar). Herbivores then obtain this energy by eating plants, and carnivores obtain it by eating herbivores.

The process

During photosynthesis, plants take in carbon dioxide (CO2) and water (H2O) from the air and soil. Within the plant cell, the water is oxidized, meaning it loses electrons, while the carbon dioxide is reduced, meaning it gains electrons. This transforms the water into oxygen and the carbon dioxide into glucose. The plant then releases the oxygen back into the air, and stores energy within the glucose molecules.

Chlorophyll

Inside the plant cell are small organelles called chloroplasts, which store the energy of sunlight. Within the thylakoid membranes of the chloroplast is a light-absorbing pigment called chlorophyll, which is responsible for giving the plant its green color. During photosynthesis, chlorophyll absorbs energy from blue- and red-light waves, and reflects green-light waves, making the plant appear green.

Light-dependent reactions vs. light-independent reactions

While there are many steps behind the process of photosynthesis, it can be broken down into two major stages: light-dependent reactions and light-independent reactions. The light-dependent reaction takes place within the thylakoid membrane and requires a steady stream of sunlight, hence the name light-dependent reaction. The chlorophyll absorbs energy from the light waves, which is converted into chemical energy in the form of the molecules ATP and NADPH. The light-independent 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 assemble carbohydrate molecules, like glucose, from carbon dioxide.

C3 and C4 photosynthesis

Not all forms of photosynthesis are created equal, however. There are different types of photosynthesis, including C3 photosynthesis and C4 photosynthesis. C3 photosynthesis is used by the majority of plants. It involves producing a three-carbon compound called 3-phosphoglyceric acid during the Calvin Cycle, which goes on to become glucose. C4 photosynthesis, on the other hand, produces a four-carbon intermediate compound, which splits into carbon dioxide and a three-carbon compound during the Calvin Cycle. A benefit of C4 photosynthesis is that by producing higher levels of carbon, it allows plants to thrive in environments without much light or water.

Carbohydrates are essential for sustaining countless life forms on Earth, and carbon dioxide, CO2, is a necessary ingredient to make carbohydrates. Plants serve as factories for its production. If all plants and other photosynthesizing organisms died out, all animal life would die as well, as oxygen and food sources become depleted.

Carbon Dioxide

Carbon dioxide is produced by the cells of organisms as a byproduct of the breakdown of nutrients. Humans and other animals exhale excess CO2 into the air. Plants release unneeded CO2 through pores in the leaf surface. Carbon dioxide is also created by events in the Earth's crust, such as volcano emissions, which release up to 255 million tons of CO2 into the atmosphere each year, according to U.S. Geological Survey.

Photosynthesis

Photosynthesis is the process plants use to produce oxygen and carbohydrates. Plants are both producers and consumers of oxygen and carbohydrates. The process is divided into two parts: the light-dependent reactions and the light-independent reactions. All of these reactions take place in a small structure in plant cells called chloroplasts. The light-dependent and light-independent reactions take place concurrently during the daytime.

Light-Dependent Reactions

The light-dependent reactions take place in structures inside the chloroplast called thylakoids. Plants need sunlight and water for the light-dependent reactions. The energy of the Sun is used to create the high-energy molecules adenosine triphosphate, ATP, and reduced nicotinamide adenine dinucleotide phosphate, NADPH. Photons of light hit photosynthetic pigments in the thlyakoid membrane, which excites electrons and begins the process of energy transfer. The water molecules are split, creating oxygen, which can be used by the plant or released to the atmosphere, and hydrogen ions. Hydrogen ions and electrons are used to convert NADP+ to NADPH. An accumulation of hydrogen ions on one side of the thylakoid membrane also provides the energy needed to produce ATP from the lower energy form ADP.

Light-Independent Reactions

The light-independent reactions are also known as the Calvin cycle, which is the process that produces carbohydrates. The high-energy products of the light-dependent reactions, ATP and NADPH, are required ingredients to drive the carbohydrate building process. A third required ingredient is CO2, which plants get from their own metabolic processes or from taking it up from the atmosphere. Carbon dioxide reacts with the molecule ribulose-1,5-bisphosphate, RuBP, with the help of the catalyst ribulose bisphosphate carboxylase oxygenase, RuBisCO. This starts three rounds of a cyclical series of reactions in which three CO2 molecules are converted into a compound called glyceraldehyde-3-phosphate, which is converted into various nutrients -- fats, proteins and carbohydrates. RuBP is replenished at the end of each cycle. The used up ATP and NADPH are converted to ADP and NADP+, respectively, which are used for the light-dependent reactions.

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Writer Bio

Nathan Cranford is a writer who began pursuing the craft professionally in 2010, specializing in education, music and the arts. He holds a Master of Arts in comparative literature and is the creator of Fantasiestücke, an online journal for literature and the arts.

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