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Photosynthesis can power the world!
Photosynthesis

Photosynthesis: The process that feeds the biosphere!
Life on Earth is solar powered. The specialized organs of the cell known as "chloroplasts" of plants capture light energy that has traveled 150 million kilometers from the sun and convert it to chemical energy stored in sugar and other organic molecules. This conversion process is called photosynthesis. Let's begin by placing photosynthesis in its ecological context. Photosynthesis nourishes almost the entire living world directly or indirectly. An organism acquires the organic compounds it uses for energy and carbon skeletons by one of two major modes: autotrophic nutrition or heterotrophic nutrition. Autotrophs are "self-feeders"; they sustain themselves without eating anything derived from other organisms. Autotrophs produce their organic molecules from CO2 and other inorganic raw materials obtained from the environment. They are the ultimate sources of organic compounds for all non-autotrophic organisms, and for this reason, biologists refer to autotrophs as the producers of the biosphere.

Plants, as the base for ecological food chains, serve as the structural and functional foundation of natural and managed systems. Photosynthesis is arguably the most important biological process on earth. By liberating oxygen and consuming carbon dioxide, it has transformed the world into the hospitable environment we know today. Directly or indirectly, photosynthesis fills all of our food requirements and many of our needs for fiber and building materials.

Making the oxygen we breathe!
The mystery of Earth's oxygen!

The presence of plants on this planet Earth has enabled other life forms including humans to survive. Plants have diversified into 290,000 living species inhabiting all but harsh environments such as polar–regions. They supply oxygen and are the ultimate provider of food eaten by animals besides fuel, wood and medicine. Six crops namely wheat, rice, maize, potatoes, cassava and sweet potatoes account for 80% of all the calories consumed by humans. As the world's population increases, the need for plants to supply food fuel and medicine increases and thereby the importance of understanding how plants grow and develop.

Almost all plants are autotrophs; the only nutrients they require are water and minerals from the soil and carbon dioxide from the air. Particularly, plants are photoautotrophs, organisms that use light as a source of energy to produce organic substances. Photosynthesis also occurs in algae, certain other protists, and some prokaryotes. Heterotrophs obtain their organic material by the second major mode of nutrition. Unable to make their own food, they live on compounds produces by other organisms. Heterotrophs are the biosphere's consumers. The most obvious form of this "other-feeding" occurs when an animal eats plants or other animals. But heterotrophic nutrition may be more subtle. Some heterotrophs consume the remains of dead organisms by decomposing and feeding on organic litter such as carcasses, feces, and fallen leaves; they are known as decomposers. Most fungi and many types of prokaryotes get their nourishment this way. Almost all heterotrophs, including humans, are completely dependent on photoautotrophs for food – and also for oxygen, a by-product of photosynthesis.

Photosynthesis – Create energy from carbon dioxide and sunlight
How does photosynthesis work?

All green plants of a plant, including green stems and unripened fruit, have chloroplasts, but the leaves are the major sites of photosynthesis in most plants. There are about half a million chloroplasts per square millimeter of leaf surface. The color of the leaf is from chlorophyll, the green pigment located within the chloroplasts. It is the light energy absorbed by chlorophyll that drives the synthesis of organic molecules in the chloroplast. Chloroplasts are found mainly in the cells of the mesophyll, the tissue in the interior of the leaf. Carbon dioxide enters the leaf, and oxygen exists, by way of microscopic pores called stomata. Water absorbed by the roots is delivered to the leaves in veins. Leaves also use veins to export sugar to roots and other non-photosynthetic parts of the plant.

In the presence of light, the green parts of plants produce organic compounds and oxygen from carbon dioxide and water. Using molecular formulas, we can summarize photosynthesis with this chemical equation:

          6CO2 + 12 H2O + Light energy = C6H12O6 + 6CO2 + 6H2O

The numbers refer to how many molecules of each type take part in the reaction and how many atoms of each type (carbon, hydrogen, and oxygen) are in each molecule. A significant result of the shuffling of atoms during photosynthesis is the extraction of hydrogen from water and its incorporation into sugar. The waste product of photosynthesis, O2 is released to the atmosphere.

Wave length and absorption of light in photosynthesis
The nature of light

The process of photosynthesis occurs when green plants use the energy of light to convert carbon dioxide (CO2) and water (H2O) into carbohydrates. Light is a form of energy known as electromagnetic energy, also called electromagnetic radiation. Electromagnetic energy travels in rhythmic waves analogous to those created by dropping a pebble into a pond. Electromagnetic waves, however, are disturbances of electrical and magnetic fields rather than disturbances of a material medium such as water. The distance between the crests of electromagnetic waves is called the wave length. Wave lengths range from less than a nanometer (for gamma rays) to more than a kilometer (for radio waves). This entire range of radiation is known as the electromagnetic spectrum. The segment most important to life is the narrow band from about 380 nm to 750 nm in wave length. This radiation is known as visible light because it is detected as various colors by the human eye. Thus, the sun powers everything with the photoelectric effect

The model of light as waves explain many of light's properties, but in certain respects light behaves as though it consists of discrete particles, called photons. A photon is a particle of light defined as a discrete bundle (or quantum) of electromagnetic (or light) energy. It can also be described as photons are not tangible objects, but they act like objects in that each of them has a fixed quantity of energy. The amount of energy is inversely related to the wavelength of the light; the shorter the wavelength, the greater the energy of each photon of that light. Thus, a photon of violet light packs nearly twice as much energy as a photon of red light. Although the sun radiates the full spectrum of electromagnetic energy, the atmosphere acts like a selective window, allowing visible light to pass through while screening our a substantial fraction of other radiation. The part of the spectrum we can see – visible light – is also the radiation that drives photosynthesis.

Photosynthesis nourishes almost entire living world directly or indirectly
What are the fates of photosynthetic products?

The sugar made in the chloroplasts supplies the entire plant with chemical energy and carbon skeletons for the synthesis of all the major organic molecules of plant cells. About 50% of the organic material produced by photosynthesis is consumed as fuel for cellular respiration in the mitochondria of the plant cells. Technically, green cells are the only autotrophic parts of the plant. The rest of the plant depends on organic molecules exported from leaves via veins. In most plants, carbohydrate is transported out of the leaves in the form of sucrose, a disaccharide. After arriving at non-photosynthetic cells, the sucrose provides raw material for cellular respiration and a multitude of anabolic pathways that synthesize proteins, lipids, and other products. A considerable amount of sugar in the form of glucose is linked together to make a polysaccharide cellulose, especially in plant cells that are still growing and maturing.

Most plants manage to make more organic material each day than they need to use as respiratory fuel and precursors for biosynthesis. They stockpile the extra sugar by synthesizing starch, storing some in the chloroplasts themselves and some in storage cells of roots, tubers, seeds, and fruits. In accounting for the consumption of the food molecules produced by photosynthesis, let's not forget that most plants lose leaves, roots, stems, fruits, and sometimes their entire bodies to heterotrophs, including humans.

Thus, on a global scale, photosynthesis is the process that is responsible for the presence of oxygen in our atmosphere. Furthermore, in terms of food production, the collective productivity of the minute chloroplasts is prodigious; it is estimated that photosynthesis makes about 160 billion metric tons of carbohydrate per year! No other chemical process on the planet can match the output of photosynthesis. And no process is more important than photosynthesis to the welfare of life on Earth.