The Sun is the ultimate source of energy for most living things on Earth. Producers such as plants and photosynthetic bacteria and algae absorb light energy emitted by the Sun and convert it into chemical potential energy, usually in the form of glucose. Glucose can be used by plants for their own energy needs or passed on to consumers that eat the plants.
Photosynthesizers are not the only organisms that can manufacture chemical energy, but they are the most important. In the deep ocean where sunlight doesn’t penetrate, microbes derive energy from the oxidation of hydrogen sulfide in the process of chemosynthesis. Photosynthetic and chemosynthetic species are known as producers (of energy) or autotrophs (literally meaning “self-feeding”).
The Basics of Photosynthesis
The overall process of photosynthesis can be described by a simple word equation:
carbon dioxide + water + sunlight ===> glucose + oxygen
Of course, this process only functions in the presence of a green pigment called chlorophyll, which resides inside chloroplasts in the mesophyll cells of leaves (leaves are the food factories for plants). Without chlorophyll, the reactants above will combine to give nothing more than warm fizzy water!
The balanced symbol equation for the reaction is:
6CO2 + 6H2O + sunlight ===> C6H12O6 + 6O2 (in the presence of chlorophyll)
How Plants Use Sunlight to Make Food
The reactants for photosynthesis are supplied to the plants in several ways. Water is absorbed from soil through plant roots and transported to the leaves through xylem vessels in the vascular bundle (vein). Carbon dioxide diffuses from the atmosphere into the air space inside the leaves through many stomata located on the underside of the leaves. Sunlight passes through the thin upper epidermis of the leaf and enters cells containing chloroplasts.
Chlorophyll captures light and converts the radiant energy into glucose through a two-step process. In the first step, known as light-dependent reactions, the Sun’s energy is used to drive some reactions that split water to form oxygen. The reactions also produce adenosine triphosphate (ATP) and a reducing agent known as NADPH. The original radiant energy is now stored in the chemical bonds in these compounds.
ATP is considered as a kind of energy currency for biological reaction. You could think of it as having cash to spend on groceries (readily available) rather than owning a car (which you would have to sell to get money to spend on groceries). NADPH acts like a shuttle bus that moves electrons that were freed up during light-dependent reactions to be used in light-independent reactions.
During the light-independent reactions, also known as dark reactions or the Calvin Cycle, carbon dioxide is transformed through several intermediates into glucose, a compound containing 6 carbon atoms. These reactions are powered by the ATP and NADPH formed during the light-dependent processes.
The products of photosynthesis, glucose and oxygen, have different fates. Oxygen gas diffuses out of the leaves and into the atmosphere through the stomata. Glucose may be used directly by the plant for its own metabolic needs during cellular respiration. Excess glucose also can be converted into other molecules (fiber, starch, protein and fat) for structural or energy-storage purposes.
Photosynthesis is Important for Life on Earth
Glucose and oxygen are essential to life on Earth. Plants produce the chemical energy at the base of virtually all food chains. They produce oxygen gas (ironically, a waste product of photosynthesis!) that we need to breathe. Plants also fix carbon dioxide (in compounds and organic matter) that otherwise would increase in abundance in the atmosphere and contribute to global warming.
Adrienne Larocque - Dr. Larocque is an award-winning scientist who has published more than 20 articles in peer-reviewed scientific journals.
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