Photosynthesis: An Overview
Energy is needed for all living cells to function. They are needed for food digestion, growth, molecule formation, and reproduction. Everything would shut down without sunlight (include us humans), it is the original source of all this energy. The molecular process by which plants, algae (aquatic, photosynthesizing organisms), and certain bacteria use light energy to make food (sugar molecules) used by all living organisms from simple chemicals (carbon dioxide, CO2 and water, H2O) is called photosynthesis. DuTemple (2000) suggested that this process is essential to all living organisms on Earth since all food comes, either directly or indirectly, from it. Think about this for a moment: As human beings, we may not use or require sunlight, but we have to eat to stay alive. (Even if we are on some extremely strict diet here.) We do not only eat green plants and their grains and fruit, but also the animals. The meat we are eating got to our table only because it ate plants. The plants would not be here fueling the entire planet with energy if without sunlight.
The word photosynthesis means "putting together with light," in order for a cell to obtain usable energy, the radiant energy of light must be converted through a series of complex chemical reactions. (Bruno, and Carnagie, 2001) So what happens in photoynthesis? It took scientists hundreds of years to understand what really occurs. Photosynthesis occurs inside the leaf of a plant at the cellular level. Figure 6 (http://gened.emc.maricopa.edu/bio/bio181/BIOBK/BioBookPS.html, 2001) shows a picture of a leaf. In this process, chloroplasts which are the chlorophyll found in structures in green plants are stimulated by sun light. The sunlight reacts in the chloroplasts with carbon dioxide (CO2) that the plant breathes in though microscopic holes in its leaves (called stroma), and with water (H2O) that it absorbs through its roots. The photosynthetic process removes CO2 from the atmosphere (released by respiration) while releasing molecular oxygen (O2) in plants and algae, as by-product. Some photosynthetic bacteria use light energy to create organic compounds without the production of O2; other types of photosynthetic bacteria give off O2, functioning similar to algae and plants. The type of photosynthesis that releases O2 emerged early in Earth's history, more than three billion years ago, (Robinson, 2001) and is the source of O2 in our atmosphere. As what we can see, photosynthetic organisms not only provide the food we eat, but also the air we breathe, as well as the building blocks for the oil, natural gas, and coal that we currently rely on for our survival, which are produced from ancient photosynthesis. (Can't live without it for sure.)
For what is known, the overall photosynthetic process can be written as:
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Fig. 1 (note the equation is unbalanced) |
As what we can see from Fig. 1 (http://www.alienexplorer.com/ecology/topic3.html, 2001), in chlorophyll, carbon dioxide and water plus light energy produce carbohydrate and oxygen as wastes and are released into the atmosphere. This simple chemical equation (as what people say never judge an object based on its appearance), however, does not disclose all the reactions that must occur inside a plant to produce carbohydrate. For instance: We will end with what we have started (and gained a lesson), if we shine light on a CO2 and H2O mixture. Add a living plant, though, and we get sugar. The sugar is formed in plants by a series of molecular steps using a complicated machinery made up of proteins and other organic molecules. (Robinson, 2001)
It was demonstrated about 200 years ago noted by Curtis, and Barnes (1989), that light energy is needed for the process of photosynthesis. It is also known that this process takes place in two stages, and only one of the two requires light energy. The stage where light is required is called the light dependent reaction, where light is used to break water (photolysis). In this stage of photosynthesis, chlorophyll a molecules are struck by light, and electrons from the molecules are boosted to higher energy levels. In a series of reactions, the added energy is used to form ATP from ADP and to reduce NADP+, the electron carrier molecule. NADP+ closely resembles NAD+, and it too is reduced by the addition of two electrons and a proton, forming NADPH. (Curtis, and Barnes, 1989) In this stage of photosynthesis, water molecules are also broken down, supply electrons that replace those boosted from the chlorophyll a molecules.
The second stage of photosynthesis is called the dark reaction, the light independent reaction, or the Calvin-Benson cycle. Please do not get confused here. Although the dark reactions do not require light, they can occur in either dark or light environments. Here, the ATP (wooah, energy) and NADPH created from the light reaction are needed to reduce the carbon in carbon dioxide to a simple sugar. The chemical energy temporarily stored in ATP and NADPH molecules is transferred to other molecules suitable for transport and storage in the plant body of algal cell. Meanwhile, other organic molecules can only be built when a carbon skeleton is formed, carbon dioxide is then incorporate into organic compounds. Calvin Benson cycle occurs in the stroma of the chloroplast. (Curtis, and Barnes, 1989)
Knowing that plants must gather light energy, transport electrons between molecules, transfer protons across the phospholipid bilayers, and finally rearrange chemical bonds to create carbohydrate, in order for us to understand how exactly plants perform photosynthesis, it is essential to know where it takes place at. (Yeehoo, can't wait!) Let us first take a closer look at the structure of chlorophyll: the start of all.
