The chemical formula describing the process of photosynthesis may be stated:
(6)CO2 + (6)H20 + minerals + radiant energy → C6 H12 06 + 02↑
- CO2 = carbon dioxide from the air
- H20 = water from the roots
- Minerals = from the soil and in with water from the roots
- Radiant energy = light from the sun
- C6 H12 06 = glucose, a type of carbohydrate (food)
- O2 = oxygen from the air
Do not be misled by the apparent simplicity of this equation. In photosynthesis a great deal happens from one side of that arrow to the other. A sequence of at least 80 chemical reactions makes up the complete process of photosynthesis. One does not get carbohydrates and oxygen simply by putting water and carbon dioxide together and shining a light on the mixture.
The Components of Photosynthesis
Raw Products. (or the components to the left of the horizontal arrow in the photosynthesis formula) As the ”recipe” illustrates, the carbon dioxide from the air, along with the minerals and the water from the roots, is broken down and rearranged to form the carbohydrates (food) produced in photosynthesis.
Radiant Energy. Life here on earth depends on the flow of radiant energy from thermonuclear reactions occurring at the core of the sun. The amount of radiant energy hitting the earth is an enormous number of calories (13 followed by 23 zeros) per year but only a paltry one percent of energy can actually be used to fuel nearly all life on this planet.
End Products. (or the components to the right of the horizontal arrow in the photosynthesis formula) The most important end product is the carbohydrates (sugars) produced. These sugars may be used for food immediately or stored as starch for later use as food.
The oxygen produced during photosynthesis is technically regarded as a waste product formed during the process. This designation certainly belies its critical importance in completing the carbon dioxide-oxygen cycle and providing breathable air for every animal on the planet.
The Site of Photosynthesis
The process of photosynthesis takes place within green plant cell organelles (“tiny organs”) known as chloroplasts. Found floating in the cytoplasm of plant cells only, chloroplasts are similar in structure, regardless of what plant or plant-like organism they are found in. While some algae may contain a single large chloroplast, each cell in the leaf of a vascular plant may house 50 or more.
Located within each chloroplast are a variety of pigments (any substance that absorbs the wavelengths of visible light), the most important of which is called chlorophyll. There are two common types of chlorophyll: chlorophyll a and chlorophyll b.
Chlorophyll a absorbs mainly red and orange wavelengths of light while chlorophyll b absorbs mainly blue wavelengths. Neither absorbs much green wavelength. Instead, they allow green light to be reflected or transmitted. That is why leaves and other plant structures containing chlorophyll (chloro literally means “green”) appear green to our eyes.
Chlorophyll b assists chlorophyll a in absorbing light energy and so is called an accessory pigment. Other accessory pigments are found in plant cells as well, including yellow, orange, red, and brown carotenoids. These pigments absorb light in the regions of the spectrum where chlorophyll does not, thus allowing more of the available light energy to be used.
The Stages and Steps of Photosynthesis
While we do not know all the details and certainly cannot duplicate this process in the best of our laboratories, we do know that photosynthesis occurs in two stages: the light reaction and the Calvin cycle.
The Light Reaction. This stage is so named because it happens only in the presence of light when the energy of sunlight is absorbed (captured) and used to make energy-storing compounds.
The chlorophylls and carotenoids in a chloroplast are grouped in clusters of a few hundred pigment molecules known as a photosystem. The light reaction begins when light is absorbed by a photosystem and proceeds from there in a series of steps:
- The absorbed light energy sets up flow of energy into the reaction center of the photosystem. In the reaction center high-energy electrons are produced.
- High-energy electrons are transferred along a series of carriers in the membrane of the chloroplast. At the end of the electron transport chain, the high energy electrons are transferred to several energy storage compounds – NADPH and ATP. The energy stored in these molecules is used to power the next stage – the Calvin cycle.
The Calvin cycle. In the Calvin cycle (sometimes referred to as the dark reaction even though the reactions take place in the light) carbon dioxide and the energy-storing compounds produced in the light reaction (NADPH and ATP) are used to form PGAL. PGAL is a chemical “building block” that can be used to make other organic compounds, such as the sugar glucose (food for the plant).
The process of photosynthesis is without doubt the single most important biochemical reaction on the planet. Simply stated, without photosynthesis nearly all life on earth would perish.
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