Principles of Photosynthesis

Photosynthesis is the process of converting light energy to chemical energy and storing it in the bonds of sugar. This process occurs in plants and some algae. Plants need only light energy, CO2, and H2O to make sugar. The process of photosynthesis takes place in the chloroplasts,
specifically using chlorophyll, the green pigment found in most leaves.

Photosynthesis takes place primarily in plant leaves, and little to none occurs in stems, or other plant parts. The parts of a typical leaf include the upper and lower epidermis, the mesophyll, the vascular bundles (veins), and the stomata . The upper and lower epidermal cells do not have
chloroplasts. They serve primarily as protection for the rest of the leaf. The stomata are cell bordered pores which occur primarily in the lower epidermis and are for air exchange: they let CO2 in and O2 out. The vascular bundles or veins in a leaf are part of the plant's transportation
system, moving water and nutrients around the plant as needed. The mesophyll cells contain chloroplasts in which photosynthesis occurs.

The parts of a chloroplast include the outer and inner membranes, intermembrane space, stroma, and thylakoids stacked in grana which have the appearance of stacked coins. The chlorophyll is built into the membranes of the thylakoids.

Chlorophyll looks green because it absorbs red and blue light, making these colors unavailable to be seen by our eyes. It is the green light which is not absorbed and reflected back which makes chlorophyll appear green. It is the energy from the red and blue light that are absorbed that is and thus able to be used in the process of photosynthesis. The green light is not absorbed by the plant, and cannot be used to do photosynthesis.

The overall chemical reaction involved in photosynthesis is: 6 CO2 + 6 H2O (In the presence of light and chlorophyll) = C6H12O6 + 6 O2.

The Two Stages of Photosynthesis:

The light reaction (light dependent) stage occurs in the thylakoid membrane and converts light energy to chemical energy. Chlorophyll and several other pigments such as beta-carotene are organized in clusters in the thylakoid membrane and are involved in the light reaction. Each of these differently-colored pigments can absorb a slightly different color of light and pass its energy to the central chlorphyll molecule to do photosynthesis. The central part of the chemical structure of a chlorophyll molecule is called a porphyrin ring, which consists of several fused rings of carbon and nitrogen with a magnesium ion in the center. It is the magnesium that accounts for the green color.

The energy harvested during the light reaction is stored by forming a chemical molecule called ATP (adenosine triphosphate), a compound used by cells for energy storage. This chemical is made of the nucleotide adenine bonded to a ribose sugar, and that is bonded to three phosphate groups. This molecule is very similar to the building blocks for DNA and RNA.

The dark reaction (light independent stage) takes place in the stroma within the chloroplast, and converts CO2 into sugar. This reaction doesn't directly require light in order to occur, but it does need the products of the light reaction (ATP and another chemical called NADPH). The dark reaction involves a cycle called the Calvin cycle in which CO2 and energy from ATP are used in the formation of sugar. The first product of photosynthesis is a three-carbon compound called glyceraldehyde 3-phosphate. Almost immediately, two of these join to form a glucose (simple sugar) molecule.

Most plants introduce CO2 directly into the Calvin cycle and the first stable organic compound formed is the glyceraldehyde 3-phosphate. Since that molecule contains three carbon atoms, these plants are called C3 plants. For all plants, hot summer weather increases the amount of
water that evaporates from the plant. Plants lessen the amount of water that evaporates by keeping their stomata closed during hot, dry weather. Unfortunately, this means that once the CO2 in their leaves reaches a low level, they must stop doing photosynthesis. Even if there is a tiny bit of CO2 left, the enzymes used to grab it and put it into the Calvin cycle just don't have enough CO2 to use. Typically the grass in our yards just turns brown and goes dormant. Some plants like crabgrass, corn, and sugar cane have a special modification to conserve water. These plants capture CO2 in a different way: they do an extra step first, before doing the Calvin cycle. These plants have a special enzyme that can work better, even at very low CO2 levels, to grab CO2 and turn it first into oxaloacetate, which contains four carbons. Thus, these plants are called C4 plants. The CO2 is then released from the oxaloacetate and put into the Calvin cycle. This is why crabgrass can stay green and keep growing when all the rest of your grass is dried up and brown.

There is yet another strategy to cope with very hot, dry, desert weather and conserve water. Some plants (for example, cacti and pineapple) that live in extremely hot, dry areas like deserts, can only safely open their stomata at night when the weather is cool. Thus, there is no chance for them to get the CO2 needed for the dark reaction during the daytime. At night when they can open their stomata and take in CO2, these plants incorporate the CO2 into various organic compounds to store it. In the daytime, when the light reaction is occurring and ATP is available
(but the stomata must remain closed), they take the CO2 from these organic compounds and put it into the Calvin cycle. These plants are called CAM plants, which stands for crassulacean acid metabolism after the plant family, Crassulaceae (which includes the garden plant Sedum) where
this process was first discovered.