Photosynthesis





Plants are one of the many wonders of the world. They are considered autotrophs; they do not rely on outside sources for their food. They create their own food by the process of photosynthesis. Photosynthesis is when green plants and certain other organisms use light energy to change carbon dioxide and water into the glucose. In so doing, photosynthesis provides the basic energy source for almost all organisms. An extremely important byproduct of photosynthesis is oxygen, on which most organisms depend.
Photosynthesis occurs in green plants, seaweeds, algae, and some bacteria. These organisms are virtual sugar factories, producing millions of new glucose molecules per second. Plants use most of this glucose, which is a carbohydrate, as an energy source to build leaves, flowers, fruits, and seeds [1]. They also convert the produced glucose into cellulose, which they use as structural material in their cell walls. Most plants produce more glucose than they use, however, and they store it in the form of starch and other carbohydrates in roots, stems (Figure 1), and leaves (Figures 2 & 3). The plants can then draw on these reserves for extra energy or building materials. Each year, photosynthesizing organisms produce about 170 billion metric tons of extra carbohydrates, about 30 metric tons for every person on earth [2].
Photosynthesis has far-reaching implications. Like plants, humans and other animals depend on glucose as an energy source, but they are unable to produce it on their own and must rely ultimately on the glucose produced by plants. Most importantly, the oxygen humans and other animals breathe is the oxygen released during photosynthesis. Humans are also dependent on ancient products of photosynthesis, known as fossil fuels, for supplying most of our modern industrial energy. These fossil fuels, including natural gas, coal, and petroleum, are composed of a complex mix of hydrocarbons, the remains of organisms that relied on photosynthesis millions of years ago [3]. Therefore, almost all life on earth, directly or indirectly, depends on photosynthesis as a source of food, energy, and oxygen, making it one of the most important biochemical processes known.
Plant photosynthesis occurs in leaves and green stems within cell structures called chloroplasts (Figure 4 & 5). One plant leaf is composed of tens of thousands of cells, and each cell contains 40 to 50 chloroplasts. The chloroplast, an oval-shaped structure, is divided by membranes into numerous disk-shaped compartments. These disklike compartments, called thylakoids, are arranged vertically in the chloroplast like a stack of plates or pancakes [4]. A stack of thylakoids is called a granum. The grana lie suspended in a fluid known as stroma [3] (Figure 6).
Embedded in the membranes of the thylakoids are hundreds of molecules of chlorophyll (Figure 7), a light-trapping pigment required for photosynthesis. Additional light-trapping pigments, enzymes, and other molecules needed for photosynthesis are also located within the thylakoid membranes [4]. The pigments and enzymes are arranged in two types of units, Photosystem I and Photosystem II (Figure 8). Because a chloroplast may have dozens of thylakoids, and each thylakoid may contain thousands of photosystems, each chloroplast will contain millions of pigment molecules.
Photosynthesis is a very complex process, and is divided into two stages. In the first stage, the light-dependent reaction, the chloroplast traps light energy and converts it into chemical energy contained in nicotinamide adenine dinucleotide phosphate (NADPH) and adenosine triphosphate (ATP) (Figure 9), two molecules used in the second stage of photosynthesis. In the second stage, called the light-independent reaction (formerly called the dark reaction), NADPH provides the hydrogen atoms that help form glucose, and ATP provides the energy for this and other reactions used to synthesize glucose [5]. These two stages reflect the literal meaning of the term photosynthesis, to build with light.
Photosynthesis relies on flows of energy and electrons initiated by light energy. Electrons are minute particles that travel in a specific orbit around the nuclei of atoms and carry a small electrical charge. Light energy causes the electrons in chlorophyll and other light-trapping pigments to boost up and out of their orbit; the electrons instantly fall back into place, releasing resonance or vibrating energy, as they go, all in millionths of a second. Chlorophyll and the other pigments are clustered next to one another in the photosystems, and the vibrating energy