What Is Stem and Leaf Photosynthesis?
Looking up at a tree, two distinct colors may be apparent: the green of the stems and leaves and the brown or gray of the bark. The color difference denotes places where photosynthesis occurs. Green portions absorb light and carry out energy producing reactions that are important to all living organisms.
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Photosynthesis
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Plants use photosynthesis to convert light energy and carbon dioxide into carbohydrates that act as a food source, enabling a plant to grow, reproduce, and perform cellular respiration. The energy from the light source travels through several mechanisms, in which energy-rich electrons are removed and used to produce carbohydrates, mostly in the form of glucose, using carbon dioxide and water as raw material.
Chloroplast
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Cells in the green portions of plants, stems, and leaves, contain chloroplast. Chloroplasts are organelles in plant cells, where photosynthesis occurs. These structures contain a protein liquid called stroma. Within the stroma a series of membranes called thylakoids is organized into stacks or grana. Thylakoids host many of the reactions necessary for photosynthesis to occur.
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Photosystems
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Light energy is captured in the chloroplast by pigment molecules. Collectively, these pigments form photosystems, where electrons from light waves are removed with the help of enzymes. Two types of pigments are found in photosystems. The most prominent, with 200 molecules, is chlorophyll. Chlorophyll absorbs high-energy red wavelengths and reflects low-energy green wavelengths. This is what gives leaves and stems their green color. The other pigment accounts for much less of the energy absorbed. With 50 molecules, carotenoids absorb blue and green wavelengths, which contain the least energy.
Energy-Fixing
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The first reaction in the photosynthetic process is the energy-fixing reaction. During this phase, photosystems remove electrons from the chlorophyll and convert them into chemical energy in the form of energy-storing molecules. This is done through a series of reductions and oxidations, which produce a charge imbalance between the different sides of membranes. Balance is restored through chemiosmosis, in which positively charged hydrogen molecules, taken from water, are pumped across the membrane, equalizing the charges and releasing energy simultaneously.
Carbon-Fixing
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The chemical energy created in energy-fixing reactions is used during the carbon-fixing reaction, also called the Calvin cycle, to convert carbon dioxide to glucose. The glucose produced in this reaction will be used as a food source for the plant. This process occurs in the stroma of the plant and uses the energy-storing molecules produced in the photosystems to synthesize the reaction. Because the energy is in a stored form, no light is necessary for the Calvin cycle to function. With enough stored energy, a plant can produce glucose from carbon dioxide independent of a light source.
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References
- Photo Credit Bambou image by Loic LE BRUSQ from Fotolia.com
