UNIT 2: Metabolic Processes
Chapter 3: An Introduction to Metabolism
pg. 124 - 165
3.2: ATP: Energy Currency of the Cell
pg. 141 - 145
There are thousands of chemical reactions occurring in the cell, many of the
reactions require an input of energy for the assembly of complex molecules
from simple reactants.
This energy comes from a compound called Adenosine Triphosphate, (ATP).
ATP drives all the chemical reactions that occur in the cell and is also
known as the universal energy ‘currency’.
Table 1: Types of Work Performed by ATP
Mechanical Work
Transport Work
Chemical Work
- beating of cilia or
movement of flagella
- contraction of muscle
fibres
- movement of chromosomes
during mitosis/meiosis
- process of pumping
substances across
membranes against their
concentration gradient
- process of supplying
chemical potential energy
for non-spontaneous
endergonic reactions,
including protein synthesis
and DNA replication
ATP Hydrolysis and Free Energy
The structure of ATP molecule consists of three parts; one nitrogenous base
(adenine), a five carbon sugar group (ribose), and three phosphate groups.
The free energy is found in the bonds of the three phosphate molecules.
ATP molecule is broken down during a hydrolysis reaction, releasing the
first phosphate from the other two, releasing free energy to do work. The
products are one phosphate group (Pi) and adenosine di-phosphate (ADP).
ATP + H2O → ADP + Pi + (H+)
∆G = -30.5 KJ/mol
The phosphate group (Pi) does not remain in solution, but quickly bonds to
another molecule.
Water is used (H2O) is used in the reaction, hydroxyl group (OH-) bonds to
the terminal end inorganic phosphate ion and hydrogen ion (H+) is released
into the solution. (Not usually shown in the reaction)
Figure 2: ATP releases large amounts of free energy during a hydrolysis reaction as new
bonds form in the products. The reaction results in the addition of a new OH group to a
released phosphate as well as the addition of an electron to the terminal oxygen on the
ADP and the release of an H- ion into solution. pg. 142
ATP and Energy Coupling
Phosphorylation – is the transfer of a phosphate group, usually from ATP,
to another molecule.
Phosphorylation results in a molecule gaining free energy and becoming
more reactive. During this coupling reaction, an enzyme must bring ATP
closer to the reactant molecule.
Regeneration of ATP
For all cells to keeping functioning, ATP must be regenerated. ATP
coupling reactions occur regularly in living cells.
ATP cycle – is the cyclic and ongoing breakdown and re-synthesis of ATP.
Cells create ATP by combining ADP with Pi, this is an endergonic reaction,
requiring an input of energy. Energy is supplied by an exergonic reaction,
where free energy is released, from large molecules (carbohydrates, lipids,
and proteins).
ATP is a cycle because it is hydrolyzed and re-synthesized in a normal cell.
Figure 4 The ATP cycle couples reactions that release free energy (exergonic) to
reactions that require free energy (endergonic).
ATP is the Universal Energy Currency
ATP is not found in our diet, yet our cells require a large supply of ATP to
survive. The energy from food is use to re-synthesize ATP in the cell. ATP
has a specific biochemical property; that allows the cell to readily use ATP
as an energy source for cell metabolism.
There are many large molecules which could supply chemical potential
energy for the cell, yet the glucose is the primary molecule which is used to
synthesize ATP in a coupling reaction. Yet this molecule is not a direct
source of energy for the cell (cellular respiration).
It is easier for the cell to utilize ATP immediately when energy is required
then any other source.