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Intro to Anatomy 3: Energy Flow in the Life of a Cell
Intro to Anatomy 3: Energy Flow in the Life of a Cell
The Lone Ranger
Published by The Lone Ranger
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An Introduction to Human Anatomy and Physiology
Chapter Three: Energy Flow in the Life of a Cell

Chemical reactions either release or absorb energy as they proceed. Whether a reaction releases or absorbs energy as it progresses is determined by whether there is more energy stored in the chemical bonds of the reactants or the products.

Among the fundamental characteristics of living organisms is their ability to regulate and control the various chemical reactions within their cells. These chemical reactions allow organisms to generate and utilize the energy they need in order to maintain themselves.

The ultimate source of energy for living organisms is almost always sunlight. Plants, algae and some bacteria are known as autotrophs (“self-feeders”) because they can use molecules such as chlorophyll and rhodopsin to absorb solar energy. The captured solar energy is then used to build organic molecules (especially the monosaccharide carbohydrate glucose) from carbon dioxide and water. Thus, autotrophs are organisms that can make their own food.

This process, in which plants and similar organisms capture solar energy and use the captured energy to make food, is known as photosynthesis. (“Photosynthesis” means “to make with light.”) In a typical photosynthesis reaction, solar energy is used to combine six carbon dioxide molecules and six water molecules, forming a single molecule of glucose. Six molecules of diatomic oxygen are left over after the reaction is complete, and so photosynthesis releases oxygen as a byproduct.

Photosynthesis: 6CO2 + 6H2O + sunlight energy → C6H12O6 + 6O2

Where does the energy go that was used to assemble the glucose molecule? Well, since photosynthesis is an endothermic reaction, it absorbs energy (in this case, solar energy) from its surroundings as it occurs. The absorbed energy is used to assemble glucose molecules, and so it is stored in the chemical bonds of the glucose.

When a plant needs to use energy (for repairing damaged tissues, for example, or for building molecules more complex than glucose), it can decompose the glucose that was produced during photosynthesis back into CO2 and water. Decomposition of glucose is an exothermic reaction, of course, because there is more energy stored in the bonds of the original glucose than is present in the bonds of the CO2 and water that result from its decomposition. So, the captured solar energy that had been stored in the chemical bonds of the glucose is released when the glucose is decomposed, and the plant can use the released energy to do work.


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