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When we think of the sun we think of many things, but we generally do not appreciate it's role in our lives. Indeed, it is much more important than it often seems. Without the sun, it is unlikely that life would have ever formed on Earth; nearly all organisms are highly dependant on the sun and its products for numerous reasons, a fact which has made Earth the lush, fertile planet that it is.
[ A Source of Energy ]
Sunlight is the fuel of life, it provides the energy that maintains nearly every manifestation of life on Earth. However, this energy is not always obtained directly. The process of energy dispersion begins with plants in a reaction called photosynthesis. The most important component of photosynthesis is light itself. Sunlight, or white light, consists of all parts of the visible spectrum, which contains all possible colors. Because only certain parts of this spectrum are absorbed by plant cells (usually blue and red), the reflected light gives them their color, which is generally greenish. In photosynthesis, light is converted into energy. This absorption and conversion of sunlight occurs within the chloroplast, a sac-like membrane found in plant cells. The chloroplast contains chlorophyll and other pigments which absorb light. The energy provided by the excitation of electrons in these pigments provokes a reaction that separates water molecules and produces oxygen. The chemical energy created by this process is used to produce glucose and other compounds. Glucose stores a large amount of energy and provides plants with what they need to survive. Yet the energy in glucose originates from compounds produced in photosynthesis, a process that acquires energy directly from light directly from the sun.
How then, is this energy manifested in organisms which don't obtain their energy directly from sunlight?
Just as plants acquire energy from the sun, animals acquire energy from plants. Using oxygen, glucose, and other sources, animals create the energy they need to survive. Whereas herbivores acquire this energy directly from plants, carnivores acquire it from other animals. Thus, the energy used by nearly all organisms on Earth comes directly or indirectly from the sun. If the sun did not provide this energy, it is unlikely that life would be so successfully exhibited here. It is the consistent, powerful array of energy provided by sunlight that has instigated the growth and expansion of life on Earth.
8,000 feet under the surface of the ocean there is virtually no sunlight. This area, the deep ocean, is very inhospitable to nearly all forms of life. However, a group of organisms, including the giant tube worm, thrive at such depths. They obtain energy from thermal vents on the sea-bottom and are some of the very few known organisms which obtain their energy from a source other than the sun.
[ Other Benefits ]
Reflected light, or light that is not absorbed by an object, is measured with great accuracy by humans and most animals. This detection occurs in the eye and is called visual perception, or sight. The measurement of reflected light is extremely useful in assessing one's environment, and most animals rely heavily on their eyes to survive. The reason that this sense is available to life on Earth is because the sun provides a great amount of light, making visual receptors a practical device. If the sun did not provide this light, we would have to depend more on other, less efficient means of perception.
The sun also acts as a structurizer for our solar system. Because it is so massive, the sun has a gravitational field that exhibits a great force on the planets orbiting it. This force is what keeps the planets from traveling in a straight line and prevents them from leaving their orbit. It is a result of this force and the nearly constant orbits created by it that days and years are also nearly constant, which means that reception of sunlight is predictable. This consistency of environment is a major requirement for life to be successful.
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