Sun is the ultimate source of energy for almost all cells to accomplish their metabolism. Cells manage a wide range of functions in their tiny package – growing, moving, housekeeping, and so on – and most of those functions require energy.
Cells get this energy and use it in the most efficient manner. Cells, like humans, cannot generate energy without locating a source in their environment. However, whereas humans search for substances like fossil fuels to power their homes and businesses, cells seek their energy in the form of food molecules or sunlight. In fact, the Sun is the ultimate source of energy for almost all cells, because photosynthetic prokaryotes, algae, and plant cells harness solar energy and use it to make the complex organic food molecules that other cells rely on for the energy required to sustain growth, metabolism, and reproduction.
Metabolism is the set of chemical reactions that happen in the cells of living organisms to sustain life. Metabolism is the set of chemical reactions that happen in the cells of living organisms to sustain life. These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments.
The word metabolism can also refer to all chemical reactions that occur in living organisms, including digestion and the transport of substances into and between different cells. Metabolism is usually divided into two categories. Catabolism and anabolism.
Catabolism breaks down organic matter, for example to harvest energy in cellular respiration. Anabolism uses energy to construct components of cells such as proteins and nucleic acids. The chemical reactions of metabolism are organized into metabolic pathways, in which one chemical is transformed through a series of steps into another chemical, by a sequence of enzymes.
Enzymes are crucial to metabolism because they allow organisms to drive desirable reactions that require energy and will not occur by themselves, by coupling them to spontaneous reactions that release energy. As enzymes act as catalysts they allow these reactions to proceed quickly and efficiently. Enzymes also allow the regulation of metabolic pathways in response to changes in the cell's environment or signals from other cells.