As
an
open system , cells require
a constant source of energy to carry out their life functions. The
main source of energy for most living systems is the sun.
Photosynthetic organisms capture sunlight and transform it into a
useable source of energy via the chemical bonds in the organic
compounds it produces. Cells uses some of this chemical bond
energy to make ATP, the energy source for cellular work.
Much of this energy is released as unusable heat. The products of
respiration are the raw materials of photosynthesis. Chemical
elements essential for life are recycled but energy is not. In
this section we are going to examine the cellular respiratory
process.
Catabolism: Chemical
pathways that break down materials and release energy.
Fermentation.
An ATP producing pathway in which both electron donors and
acceptors are organic compounds.
The process can be
anaerobic and results in partial degradation of
sugars.
Cellular
Respiration. An ATP
producing process in which the ultimate electron acceptor is an
inorganic
molecule, such as
oxygen. It is the most prevalent and efficient catabolic process.
It is an exergonic process producing a delta G of -686
kcal/mol.
The catabolic process of
respiration transfers the energy stored in food molecules to ATP.
If the energy of sugar molecules was released at one time the body
could not use it efficiently. Organisms use ATP molecules to
capture and release small amounts of energy to fuel various bodily
functions. ATP is unique in its function as an energy transferrer.
The molecule contains the nitrogenous base adenine connected to
three molecules of phosphorous. The bonds between the last 2
phosphates are high energy bonds. When ATP releases the terminal
(end) phosphate, energy is released while forming a new compound
ADP. ADP can be refitted with another phosphate
(phosphorylation)
to form ATP again. Cellular respiration contains the steps to
allow this process to occur.
In order for the most
efficient production of ATP to occur the cell must transfer this
energy from the chemical bonds of the organic compounds to the ATP
molecule with minimal loss. The process involved is found in basic
inorganic chemistry, the
REDOX
reactions. Redox is short for oxidation-reduction which involves
partial or complete transfer of
electrons
from one reactant to
another.
Oxidation
= Partial or complete
loss of electrons.
Reduction
= Partial or complete gain of electrons.
Basic rule to
remember: Electrons lose
potential energy when they shift toward more electronegative
atoms, redox reactions that move electrons closer to oxygen
release energy.
Cellular respiration is a
redox process that transfers hydrogen atoms from sugar to
oxygen.
In order to allow these
hydrogen atoms to be transfered, an intermediate organic compound
is called into use by the cell. This coenzyme is called
NAD+short for
nicotinamide adenine
dinucleotide. During
the oxidation of glucose NAD+ functions as an oxidation
agent by trapping energy rich electrons from glucose or food.
These reactions are catalyzed by enzymes called
dehydrogenases.
The end results are the
removal of 2 hydrogen atoms from the substrate (glucose) with the
delivery of one to
NAD+forming NADH and
the other into solution. The hydrogen attached to the NADH will be
passed down a series of protein receptors in the inner
mitochondrial membrane (electron transport chain) to produce an
abundant supply of ATP.
Cellular respiration can be
divided up into 3 stages:
Glycolysis
Krebs Cycle (Citric Acid
Cycle)
Electron Transport Chain (ETC)
and oxidative phosphorylation
Glycolysis: harvests
chemical energy by oxidizing glucose to pyruvate.
Glycolysis is a catabolic
pathway during which six-carbon glucose is split into 2 three-
carbon sugars,which are then oxidized and rearranged to produce
two pyruvate molecules.
Each reaction is catalyzed by
specific enzymes dissolved in the cytosol.
No carbon dioxide is released
as glucose is oxidized to pyruvate. All the carbon can be
accounted for in the 2 molecules of pyruvate.
It occurs under aerobic or
anaerobic conditions.
The process occurs in two
phases: The energy investment phase and the energy yielding
phase.
The Krebs Cycle: completes the energy
yielding oxidation of organic molecules.
Most of the chemical energy originally stores
in glucose still remains in the two pyruvate molecules produced by
glycolysis. The fate of pyruvate depends upon the presence or
absence of oxygen. If oxygen is
present, pyruvate enters the mitochondrion where it is completely
oxidized by a series of enzyme-controlled reactions. The junction
between glycolysis and the Krebs Cycle is the formation of
Acetyl-CoA. The Acetyl-CoA combines with oxaclacetic acid to begin
the cycle. This process occurs in the mitochondrial
matrix.
The Electron Transport Chain: is made
of electron carrier molecules embedded in the inner mitochondrial
membrane.
Each successive carrier in the chain has a
higher electronegativity than the carrier befor it, so the
electrons are pulled down hill toward the oxygen.
Except for ubiquinone (Q), most of the
carriers are protein containing a non -protein
cofactor.
The cofactors alternate between an oxidized
and reduced state as they accept and donate electrons.
Protein Electron
Carriers
CoFactor
Groups
Flavoproteins
flavin mononucleotide
(FMN)
iron-sulfur
proteins
iron and sulfur
cytochromes
heme group
Quick Overview of
Cellular Respiration
Fermentation: a cell process that can
produce ATP without the presence of oxygen. Fermentation recycles
NAD+ from NADH. The two most common forms of
fermentation are: alcoholic and lactic acid
fermentation.
