the energy from ATP (adenosine triphosphate), these precursor metabolites are
used to construct or build larger molecules. ATP is the short term energy storage
molecule of the cell. Think of it as the battery pack of the cell. Cells use energy
from ATP and enzymes to connect smaller molecules to form macromolecules.
The cell grows as macromolecules are linked together and continue to grow into
cellular structures such as organelles, plasma membranes, and cell walls.Catabolic and Anabolic:
The Only Reactions You Need
A biochemical reaction is called a metabolic reaction. Metabolic reactions fall
into one of two classifications. These are catabolicreactions (catabolism)and
anabolic reactions (anabolism).
Acatabolic reactionis a metabolic reaction that releases energy as large
molecules that are broken down (metabolized) into small molecules. An example is
when triglycerides and diglycerides are metabolized into glycerol and fatty acids.
An anabolic reactionrequires energy as small molecules are combined to
form large molecules. This type of reaction is called endergonicbecause it uses
free energy. For example, an anabolic reaction is the synthesis of phospholipids
from glycerol and fatty acids in order to build the cell plasma membrane.A Little Give and Take: Oxidation-Reduction
Metabolic reactions sometime involve the transfer of electrons from one mole-
cule to another. One molecule donates an electron and another molecule
acceptsthe electron. This transfer of electrons is called oxidation-reductionor
redox reaction. A redox reaction is comprised of two events. The first event
happens when a molecule donates an electron. This is called oxidation. The sec-
ond event happens when another molecule accepts the donated electron. This is
called reduction.
The cell uses electron carrier moleculesto carry electrons between areas within
the cell. Think of these carrier molecules as “shuttle buses.” Carrier molecules are
necessary because the cytoplasm of the cell does not contain free electrons.(^88) CHAPTER 5 The Chemical Metabolism