Science Weblecture for Unit 58
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Cells need energy to perform all their functions. As we learned recently, every chemical reaction requires some small amount of activation energy in order to get started, even if the net result of the reaction releases energy. In lab experiments and industrial chemical processes, this activation energy usually comes from heat energy: the reactants or reacting molecules are heated up until they collide with sufficient energy to break apart and reform into new combinations or products.
In plants and animals, though, heating up the reactants isn't possible: high temperatures will destroy cell structure and kill the organism. So all energy reactions must take place at comparatively low temperatures; in the human body, at about 98.6 degrees Fahrenheit. Even a small temperature change of two to four degrees is enough to disrupt metabolic processes for invading cells, which is why you run a fever as part of the immune reaction to infection.
To get around this problem, all cells use enzymes to promote reactions. An enzyme is a protein molecule made of many amino acids in a long chain. The particular order of the amino acids gives the enzyme a specific shape. This shape allows it to grab onto one type of molecule and pull it apart so that the pieces can combine with other molecular bits to form new molecules. Even if the enzyme doesn't break apart its target molecule, it can hold it in place and force it into position to combine with a new partner. This means the two combining parts don't have to have high velocities and collision energies, and the reactions can take place with less energy. The enzyme is unchanged by the reaction; as soon as the desired products are formed, it releases the new molecules and the whole reaction process starts again.
Note, however, that although enzymes lower the amount of activation energy necessary for the reaction, they don't eliminate it, nor do they change the overall release or absorption of energy. Many biological reactions are endothermic, that is, the cell has to put energy into the reactants to cause the products to form. This energy comes from chemical bonds formed through photosynthesis by plants and some bacteria and single-celled protists. Multi-celled animals cannot make their own stored energy molecules, and so must consume these directly from plants or other animals in the form of starches and sugars. When the bonds of the molecules in starches and sugars are broken during cellular respiration, the energy in the bonds is released and made available to the cell to use in forming other molecules to build new cells for growth, or carry out necessary processes.
Photosynthesis is the process by which the chloroplasts within plant cells change light energy from the sun into chemical energy stored in sugars. The overall reaction is really a combination of many steps making up a light reaction followed by the Calvin cycle reactions, and each step has its own special enzyme to reduce the activation energy level for the particular reaction in the step. The overall reaction is:
The formula C6H12O6 means six carbon atoms combined with 12 hydrogen atoms and six oxygen atoms. This makes the simplest sugar, a monosaccharide (meaning "one sugar molecule"). Depending on how the atoms are organized in the molecule, it can take the form of either maltose, glucose, or fructose. These monosaccharides can join together to form a two-sugar molecule (called a disaccharide) like sucrose or galactose; the latter forms chains to make up starch. These products are stored in plants in the stems and roots.
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