BSC 314 Lecture Notes - Lecture 19: Adenine, Cellular Respiration, Ribose
27 Jun 2018
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Chemistry of Energy Use
The total of all the chemical reactions occurring in living organisms is metabolism and
the sequences in which they occur are the metabolic pathways. Cells can metabolize
because they are isolated systems, separated from their environment by membranes.
Organisms, and the cells of which they are composed, use enzymes to regulate the
reactions and utilize energy carriers to move energy among the parts of the system.
Metabolic reactions are linked so that exergonic reactions supply energy for the
endergonic.
Oxidation–reduction reactions
Most of the energy exchanges in plants are chemical reactions that involve the
exchange of energy between one set of chemical bonds and another. These, for the
most part, are oxidation reduction‐reactions (commonly called redox reactions). In
oxidation, electrons are lost from an atom or molecule, which is then said to
be oxidized. (The term “oxidation” is used because oxygen is often the electron
acceptor.) Reduction always accompanies oxidation and is the acceptance or gain of
electrons by another atom, which, thereby, is reduced. As electrons are lost, so is
energy, thus oxidized molecules have less energy than reduced molecules
that gain energy as they receive electrons.
In organisms, electrons rarely move alone and are usually accompanied by a proton (a
hydrogen atom and its single electron). Oxidation, accordingly, entails the removal of a
hydrogen atom, and reduction entails the addition of hydrogen atoms.
The collections of biological molecules within the cells are reduced and electron rich ‐
and so have relatively weak chemical bonds. In the surrounding environment, most of
the molecules are oxidized(and electron poor) and have much stronger bonds. Living ‐
organisms thus continually add energy to their systems, to prevent operation of the
Second Law of Thermodynamics, i.e. to avert becoming a disordered collection of
oxidized molecules.
Photosynthesis and respiration are both oxidation reduction processes. Photosynthesis ‐
requires the input of energy, while respiration releases energy; photosynthesis is thus
an endergonic process, respiration an exergonic.
Energy Regulators: Enzymes and ATP
Enzymes
If all the energy in a reaction were released at the same time, most of it would be lost as
heat—burning up the cells—and little could be captured to do metabolic (or any other
kind) of work. Organisms have evolved a multitude of materials and mechanisms—such
as enzymes—that control and permit the stepwise use of the released energy.
Enzymes control the state of energy a molecule must attain before it can release energy
and are the chief catalysts of biochemical reactions. They are neither consumed nor
changed in the reactions. Basically, enzymes reduce the activation energyneeded to
start a reaction by temporarily bonding with the reacting molecules and, in so doing,
weakening the chemical bonds.
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Document Summary
The total of all the chemical reactions occurring in living organisms is metabolism and the sequences in which they occur are the metabolic pathways. Cells can metabolize because they are isolated systems, separated from their environment by membranes. Organisms, and the cells of which they are composed, use enzymes to regulate the reactions and utilize energy carriers to move energy among the parts of the system. Metabolic reactions are linked so that exergonic reactions supply energy for the endergonic. Most of the energy exchanges in plants are chemical reactions that involve the exchange of energy between one set of chemical bonds and another. These, for the most part, are oxidation reduction reactions (commonly called redox reactions). In oxidation, electrons are lost from an atom or molecule, which is then said to be oxidized. (the term oxidation is used because oxygen is often the electron acceptor. )
