Article On Photosynthesis

This article will explain the process of photosynthesis, where it occurs, why it is important, its products, and its variant forms.

Photosynthesis is the process by which plants produce sugar and oxygen from carbon dioxide, water, and the energy from the sun. Without photosynthesis, life as we know it would not be possible; we, as animals, depend entirely on the sugars produced by plants to provide us with necessary energy.

Photosynthesis occurs in two stages: 1) the photochemical reaction and 2) the thermochemical reaction. This article will discuss photosynthesis as a whole, as well as examining these two stages. A summary of the two variant forms of photosynthesis will also be presented.

Photosynthesis:

Photosynthesis is the single process which distinguishes between autrophes ("auto" = self, "trophe" = food) and heterotrophes ("hetero" = other, "trophe" = food). Only autotrophies, such as plants, algae, and some prokaryotic organisms, can produce their own food. The remainder of Earth's creatures rely on these autotrophies for their own sustenance. Photosynthesis occurs in the chloroplasts of the plants' cells, especially those located in the leaves. Chloroplasts are specialized organelles that contain chlorophyll, the green pigment which not only gives plants their color, but absorbs the light energy needed to drive photosynthesis as well. The general equation for photosynthesis is as follows: 6 molecules of carbon dioxide plus 6 molecules of water plus light energy yields 6 oxygen molecules plus one molecule of glucose.



(Please Note: The two stages of photosynthesis are discussed below. Unless you are majoring in a biological discipline, no high school or college class will require you to memorize each individual step in these two processes. Instructors are merely looking for a general understanding of the overall reaction and its two major stages.)

The photochemical reaction:

This reaction is also known as the "light dependent", or simply "light", reaction, because it requires the energy of the sun. The light reaction occurs in the thylakoid membrane of the chloroplasts. The light reaction relies upon two clusters of pigments, known as photosystems I and II, which each possess a different type of chlorophyll and several accessory pigments. These pigments help to make photosynthesis more efficient by absorbing different wavelengths of light. When light hits photosystem II, electrons gain more energy and are carried, via a chain of electron-carrying proteins, to photosystem I. When the light hits this second photosystem, the electrons are moved again, this time to a molecule of energy-rich NAD. The addition of these electrons reduces NAD to NADH, which will be used in the thermochemical reaction. Meanwhile, the electrons that were moved from photosystem II must be replaced. Water is split, donating its electrons to fill the vacancies in photosystem II and releasing its hydrogen atoms. This creates oxygen, one of the net products of photosynthesis. The electrons needed to replace those removed from photosystem I are provided by photosystem II. The hydrogen ions produced by the splitting of water, supplemented with additional ions from the surrounding area, are pumped back and forth across the thylakoid membrane. This creates a proton gradient, which provides enough energy to create several molecules of energy-packed ATP. Along with the NADH produced by the movement of electrons, the ATP will be used immediately in the thermochemical reaction.

The Thermochemical Reaction:

This reaction is often known as the Calvin Cycle, "light-independent" reaction, or the "dark" reaction, because it does not directly require light energy. It is important to note, however, that the light independent reactions occur for only a brief time after sunset, because they quickly use up their store of ATP and NADH produced in the light reaction. The light independent reaction occurs in the stroma of the chloroplast. The stroma is a thick, syrupy fluid found between the thylakoid membranes. Three "turns" of the Calvin Cycle are required to produce a single molecule of glucose. 18 ATP and 12 NADH are needed to help transform the carbon dioxide into sugar. Carbon dioxide enters the cycle and is fixed into glucose through a series of steps catalyzed by enzymes. ATP provides the energy for these reactions, while NADH is the reducing agent, attaching high-energy electrons to form the sugar. After being used, the ATP is converted to ADP and the NADH to NAD, both of which are immediately used in the light reaction.

Variant Forms of Photosynthesis:

Because of the climate they live in, some plants must slightly alter the process of photosynthesis. Desert plants, for example, must keep passages open to admit the needed carbon dioxide, but the hot temperatures means water will be evaporated quickly. The passages cannot be closed for too long, because oxygen will build up in the plant and the gas will be used, instead of carbon dioxide, in photosynthesis. This is a process known as photorespiration uses energy but produces no sugar. To compensate, a special class of plants known as C4 plants incorporate carbon dioxide into an intermediate enzyme known as PEP carboxylase, which has a very low affinity for oxygen but a very high affinity for carbon dioxide. This adaptation allows the passages to stay closed while ensuring photorespiration does not occur. Another group of desert plants called CAM plants, open their passages at night and close them during the day, which is opposite to normal plants. Carbon dioxide is stored at night in organic acids and utilized during the day, when sunlight is present to drive the light dependent reactions.

Photosynthesis is essential to all life on Earth. Not only do photosynthetic autotrophies produce the sugar needed for other organisms, they also produce the vital element oxygen that all animals must breathe to survive.

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