Adenosine Triphosphate - ATP
- All living things, plants and animals, require a continual supply of energy in order to function.
The energy is used for all the processes which keep the organism alive. Some of these processes
occur continually, such as the metabolism of foods, the synthesis of large, biologically important
molecules, e.g. proteins and DNA, and the transport of molecules and ions throughout the
organism. Other processes occur only at certain times, such as muscle contraction and othercellular movements. Animals obtain their energy by oxidation of foods, plants do so by trapping
the sunlight using chlorophyll. However, before the energy can be used, it is first transformed
into a form which the organism can handle easily. This special carrier of energy is the molecule
adenosine triphosphate, or ATP. ATP is a nucleotide molecule that performs many essential roles
in the cell. It is the major energy currency of the cell, providing the energy for most of the
energy-consuming activities of the cell. When the third phosphate group of ATP is removed by
hydrolysis, a substantial amount of free energy is released. For this reason, this bond is known as
a "high-energy" bond . The bond between the first and second phosphates is also "high-energy".
- The ATP molecule is composed of three components. At the center is a sugar molecule, ribose.
Attached to one side of this is a base called adenine. The other side of the sugar is attached to a
string of three phosphate groups. These phosphates are the key to the activity of ATP. Each of the
phosphates are attached by what are termed high energy chemical bonds.
- ATP works by losing the endmost phosphate group when instructed to do so by an enzyme.
This reaction releases a lot of energy, which the organism can then use to build proteins, contact
muscles, etc. The reaction product is adenosine diphosphate (ADP), and the phosphate group
either ends up as orthophosphate (HPO4) or attached to another molecule (e.g. an alcohol). Even
more energy can be extracted by removing a second phosphate group to produce adenosine
monophosphate (AMP).
- When the organism is resting and energy is not immediately needed, the reverse reaction takes
place and the phosphate group is reattached to the molecule using energy obtained from food or
sunlight. Thus the ATP molecule acts as a chemical 'battery', storing energy when it is not
needed, but able to release it instantly when the organism requires it.
- .Synthesis of ATP - requires energy: 7.3 kcal/mole, occurs in the cytosol by glycolysis, occurs
in mitochondria by cellular respiration, occurs in chloroplasts during photosynthesis
- Glucose Catabolism - In most animal cells the major direct incoming source of energy is
simple sugar molecules such as glucose. The break down of glucose or oxidation is also called
cellular respiration. The complete oxidation of glucose to C02 and H20 involves three complex
chemical pathways: glycolysis, the Krebs cycle, and the electron transport chain.
- Glycolysis - Glycolysis refers to the breakdown of glucose into two molecules of pyruvic acid.
When oxygen is in short supply, pyruvic acid is converted to lactic acid; under aerobic
conditions, pyruvic acid enters the Krebs cycle. As a result of glycolysis, there is a net production
of 2 molecules of ATP
- Krebs Cycle - Pyruvic acid is prepared for entrance into the Krebs cycle by conversion to a
two-carbon compound (acetyl group) followed by the addition of coenzyme A to form acetyl
coenzyme A. The Krebs cycle involves decarboxylations and oxidations and reductions of
various organic acids. Each molecule of pyruvic acid that enters the Krebs cycle produces 3
molecules of C02, 4 molecules of NADH2, 1 molecule of FADH2, and I molecule of GTP. The
energy originally in glucose and then pyruvic acid is primarily in the reduced coenzymes NADH2
and FADH2.
- Electron Transport Chain - The electron transport chain is a series of oxidation-reduction
reactions in which the energy in NADH2 and FADH2 is liberated and transferred to ATP for
storage. The carrier molecules involved include FAD, coenzyme Q, and cytochromes. The
electron transport chain yields 32 molecules of ATP and H20. 10. Consumption of ATP - ATP powers most of the energy-consuming activities of cells, such
as: Most anabolic reactions. Examples: joining transfer RNAs to amino acids for assembly into
proteins, synthesis of nucleoside triphosphates for assembly into DNA and RNA, synthesis of
polysaccharides, synthesis of fats, active transport of molecules and ions, nerve impulses,
maintenance of cell volume by osmosis, adding phosphate groups (phosphorylation) to many
different proteins, e.g., to alter their activity in cell signaling, muscle contraction, beating of cilia
and flagella (including sperm), and bioluminescence.