The energy that powers most organism on Earth ultimately comes from the
sun. Photosynthetic organisms use sunlight to produce organic molecules
from carbon dioxide and water. Animals obtain these organic molecules
by eating plants or by animals that eat plants.
Fermentation, occurring without oxygen, is the partial degradation of organic
molecules. Cellular respiration occurs in the mitochondria of eukaryotic
cells. Starting with glucose or other organic fuel and using oxygen,
respiration yields water, carbon dioxide, and energy in the form of ATP
and heat.
How Cells Make ATP:
An Introduction
The free energy released from exergonic hydrolysis of ATP to ADP + P drives
essential endergonic processes in cells by transferring unstable phosphate
bonds to various substrates, priming them to undergo some change that results
in work.
To keep on working, a cell must regenerate ATP. Cellular respiration
provides the energy needed to drive the endergonic phosphorylation of ADP.
Cellular respiration consists of glycolysis, the Krebs cycle, and the electron
transport chain and oxidative phosphorylation. Glycolysis occurs
in the cytosol; the other two stages occur inside mitochondria
For each molecule of glucose degraded to carbon dioxide and water in this
entire process about 36 molecules of ATP are made.
A small amount of ATP is formed by substrate level phosphorylation in glycolysis
and the Krebs cycle. A phosphate group is directly transferred to
ADP from phosphorylated organic compounds produced as intermediates in
the catabolism of glucose.
Oxidative phosphorylation produces ATP by chemiosmotic mechanism.
During electron transport, some molecules in the inner membrane of the
mitochondria generate a proton gradient by translocating H+
from the mitochondrial matrix to the intermembrane space. The hydrogen
ions
then diffuse back into the matrix through an enzyme complex called ATP
synthase, triggering the phosphorylation of ADP
Food molecules store energy in the form of electrons associated mainly
with hydrogen atoms. The cell taps this energy through oxidation-reduction,
or redox, reactions, in which one substance (the reducing agent) partially
or totally shifts electrons to another (the oxidizing agent). The
substance receiving the electrons is reduced and the substance losing electrons
is oxidized.
In cellular respiration, glucose (C6H12O6) is oxidized to CO2
and O2 is reduced to H2O. Electrons lose potential
energy during their transfer from organic compounds to water, and this
energy is used to drive ATP synthesis.
The initial acceptor of these high energy electrons removed from organic
compounds is usually NAD+, which functions as a coenzyme. NAD+ is
reduced by gaining electrons and a hydrogen nucleus from the substrate
to become NADH.
The cell uses NADH to carry the high energy electrons of food to the electron
transport chain.
Glycolysis is the splitting of sugar that occurs in the cytosol.
It is a metabolic pathway that occurs in every cell.
In glycolysis, the six carbon sugar glucose is oxidized to two three carbon
molecules of pyruvic acid, producing two molecules of ATP by substrate
level phosphorylation and reduction two molecules of NAD+.
In the presence of oxygen, glycolysis functions as the first stage of respiration.
Pyruvic acid moves into the mitochondrion, where it is completely oxidized
to CO2 in the Krebs cycle.
The link between glycolysis and the Krebs cycle is the conversion of pyruvic
acid to acetyl CoA by a multienzyme complex in the matrix of the mitochondrion
The acetic acid of acetyl CoA joins a four carbon molecule, oxaloacetic
acid, to form the six carbon citric acid molecule, which is subsequently
degraded back to oxaloacetic acid in a series of steps constituting one
turn of the cycle. In the process, carbon dioxide is given off, one
molecule of ATP is formed by substrate level phosphorylation, and high
energy electrons are passed to three molecules of NAD+ and one molecule
of FAD, another redox coenzyme.
Electron Transport
Chain and Oxidative Phosphorylation
Most of the ATP created from the energy stored in the glucose is produced
by oxidative phosphorylation when NADH and FADH2
donate their electrons to a system of electron carriers embedded in the
mitochondrial cristae.
The electron transport chain consists of a series of increasingly
electronegative components, starting with a flavoprotein progressing through
an iron-sulfur protein, then to ubiquinone and a series cytochrome proteins
with iron containing heme groups, and finally reaching oxygen which is
very electronegative.
The components of the chain receive electrons from NADH and FADH2
and shifts between reduced and oxidized states, passing electrons down
an energy gradient to oxygen, which then picks up a pair of hydrogen ions
and forms water. Two mobile components, Q and cytochrome c, transfer electrons
between the other electron carriers, which are located in three groups
of integrated complexes.
The structural order of the carriers causes electron transfers at three
steps along the chain to translocate H+ from the matrix to the
intermembrane space, storing energy in an electrochemical gradient known
as the proton-motive force. As hydrogen ions diffuse back the matrix
through ATP synthase complexes on the christae, the exergonic passage of
H+ drives the endegonic phosphorylation of ADP.
The effects of various respiritory poisons provide evidence for the chemiosmotic
model of ATP synthesis.
The complete oxidation of glucose to carbon dioxide during aerobic respiration
in eukaryotes produces a net yeild of about 36 molecules of ATP, compared
to only 2 for incomplete oxidation.
The actual yeild of ATP during respiration varies, owing to differences
in the permiability of the christae to H+ and to partial use
of the proton gradient to drive the active transport of certain solutes
across the outer mitocondrial membrane.
Glycolysis produces 2 ATPs per sugar molecule by substrate level phosphorlation,
whether under aerobic or anaerobic conditions. Fermentation is the
anaerobic catabolism of organic nutrients. It yields the two ATPs
from glycolsyis as long as NAD+ is regenerated to act as the
oxidizing agent.
The electrons from NADH are transferred to pyruvic acid, or some derivatice
of that glycolytic end product.
The ultimate electron acceptor for the the electrons removed in the oxidation
of gluecose differs for three major metabolic schemes. In aerobic
respiration, oxygen is the final electron acceptors. In anaerobic
respiration, sulfate or nitrate serves as the electron acceptor.
Both types of respiration use electron transport chains and oxidate phosphorylation
to make ATP. Fermentation uses substrate phosphorylationto make ATP,
using neither oxygen nor electron transport chains
Yeast and certain bacteria are faculative anaerobes, capable of making
ATP either by aerobic respiration or fermentation depending on the avaliability
of oxygen.
Glycolisis, the breakdown of glucose to pyruvic acid, is a catabolic pathway
common to fermentation and respiration. It occurs in the cytoplasm
of all organisms and probably evolved in ancient prokaryotesbefore oxygen
was avaliable in the atmosphere.
Fats, proteins, and carbohydrates, can all be consumed by celular respiration
to form ATP.
Carbohydrates can be enzymaticlly converted to glucose, as fule for respiration.
The amino aciids from protein digestion lose their amino groups and are
converted to various intermediates of glycolisis or the krebs cycle.
When fats are used for fule, the glycerol backbone is converted to an intermediate
of glycolysis and the fatty acids enter the Krebs CycleCoA.
Thus, glycolysis and the Krebs are catabolic pathways that funnel high-energy
electronws from all kinds of food molecules into the electron transport
chain, which powers ATP syntesis.
In addition to providing energy, food must also provide the carbon skeletons
needed by cells to make molecules for growth and repair.
Organic precursors for anabolisim either come directly from digestion or
come fromglycolysis and the Krebs cycle, which donate intermediates for
use in the synthesis of polymers or the conversion of one type of molecule
into another.
The rate of ATP synthesis in respiration is finely controlled controlled
by allosteric enzymes at key places in glycolysis and the Krebs cycle.
Regulation of the enzymes speeds up or slows down ATPsynthesis, based on
the the presence of specific molecules that signal the moment-to-moment
balance between cell catabolism and anabolism.