MICROBIAL
METABOLISM
Now that you are familiar
with the structure of prokaryotic
cells, we can discuss the activities that enable
these microbes to thrive.
The life-support
processes of even the most structurally
simple organism involve a large number of complex biochemical reactions.
Most, although not
all, of the biochemical
processes of bacteria also occur in eukaryotic microbes and in the cells of multicellular organisms, including
humans,However, the reactions that are unique to bocteria are fascinating
because they allow microorganisms to do things
we cannot do.
For example, some
bacteria can live
on cellulose, while others can live on petroleum.Through their metabolism,
bacteria recycle elements after other
organisms have used them.
Chemoautotrophs can live on diets of such inorganic
substances as carbon dioxide, iron, sulfur, hydrogen gas, and ammonia.
This chapler
examines some representalive chemical re
actions that either produce energy (the catabolic reactions)he anabolic reactions) in
microorgan ganisms.
look at how these various reactions are integrated within the cell.
CATABOLIC
AND ANABOLIC REACTIONS
We use the term metabolism to refer to the
sum of all chemical
reactions within a living organism.
Because chemical reactions either release or
require energy, metabolism can be viewed as an energy-balancing act.
Accordingly,
metabolism can be divided into two classes of chemical reactions: those that release energy
and those that require energy In living cells, the enzyme-regulated chemical reactions
that release energy are generally the ones involved in catabolism, the breakdown of complex organic compounds
into simpler ones.
These reactions are
called catabolic, or degradative,
reactions. Catabolic reactions are generally hydrolytic reactions (reactions that use water and in which chemical bonds are broken), and they are exergonic
(produce more energy than they consume).
An example of catabolism
occurs when cells break down sugars into carbon dioxide and water.
The enzyme-regulated
energy-requiring reactions are mostly
involved in anabolism, the building of complex organic molecules from simpler ones.
These reactions are called anabolic, or biosynthetic, reactions.
Anabolic processesoften
involve dehydration synthesis reactions (reactions that release water), and they are endergonic
(consume more energy than they produce).
Examples of anabolic
processes are
the formation of proteins from amino acids, nucleic acids from nucleotides, and polysaccharides from
simple sugars.
These biosynthetic
reactions generate the materials or
cell grow Catabolic reactions provide building
blocks for anabolic reactions and furnish the energy needed to drive anabolic reactions.
This coupling of
energy requiring and energy releasing reactions is made possible
through the molecule adenosine triphosphate (ATP).
ATP stores energy derived from catabolic reactions and
releases it later to drive anabolic reactions and perform
other cellular work.
Recall from Chapter 2 that a molecule
of ATP consists of an adenine, a ribose, and three phosphate groups.
When the terminal phosphate group is
split from ATP, adenosine diphosphate (ADP) is formed, and energy is released to drive anabolic reactions.
Using ®to represent a phosphate group (® represents
inorganic Which molecule
facilitates the coupling of anabolic and catabolic reactions?
phosphate, which is not bound to any other
molecule),we write this reaction as:ATP-ADP i energy.Then, the energy from catabolic reactions is
used to combine ADP and a B to resynthesize ATP ADPenergy -> ATP Thus, anabolic reactions are coupled to ATP
breakdown and catabolic reactions are coupled to ATP
synthesis.
For now, you
should know that the chemical composition of a living cell
is constantly changing: some molecules are broken down while
others are being synthesized.
This balanced flow of
chemicals and energy maintains the life of a cell.
The role of ATP in coupling anabolic and
catabolic reactions is shown in Only part of
the energy released in catabolism is actually available for
cellular functions because part of the energy is lost to
the environment as heat.
Because the cell must use energy
to maintain life, it has a continuous need for new external
sources of energy Before we discuss how cells produce energy,
let's first consider the principal properties of a group of
proteins involved in almost all biologically important
chemical reactions.
It is important to understand that a
cell's metabolic pathways
(sequences of chemical reactions)are determined by its enzymes, which are in turn
determined by the cell's genetic makeup.
ENZYMES
COLLISION THEORY
when chemical bonds are formed or broken. In
order for reactions to take place, atoms, ions, or
molecules must he collision theory explains how chemical reactions occur and how certain factors affect the
rates of those reactions.
The basis of the collision theory is
that all atoms, ions, and molecules are continuously
moving and are thus continuously colliding with one
another.
The energy transferred by the particles in the
collision can disrupt their electron structures enough so that
chemical bonds
are broken or new bonds are formed Several factors determine whether a collision
will cause a chemical reaction: the velocities of the
colliding particles their energy, and their specific chemical
configurations.
Up to a point, the higher the particles' velocities,
the greater
the probability that their collision will cause
a reaction Also, each chemical reaction requires a specific
level of energy.
But even if colliding particles possess
the minimum energy needed for reaction, no reaction will
take place unless the particles are properly oriented
toward each other.
Let's assume that molecules of substance AB (the
reactant) are to be converted to molecules of
substances A and B (the products).
In a given population of
molecules of substance AB, at a specific temperature, some
molecules will possess relatively little energy; the
majority of the population will possess an average amount of
energy; and a small portion of the population will have high
energy.
If only the high-energy AB molecules are able to react
and be converted to A and B molecules, then only
relatively few molecules at any one time possess enough
energy to react in a collision. The collision energy
required for a chemical reaction is its activation energy,
which is the amount of energy needed to disrupt the stable
electronic configuration of any specific molecule so that
the electrons can be rearranged.
The reaction
rate-the frequency of collisions containing sufficient energy to bring about a
reaction depends on the number of reactant molecules at
or above the activation energy level.
One way to increase
the reaction rate of a substance is to raise its
temperature.
By causing the molecules to move faster, heat increases
both the frequency of collisions and the number of
molecules that attain activation energy.
The number of
collisions also increases when pressure is increased or when the
reactants are more concentrated (because the distance
between molecules is thereby decreased).
In living
systems, enzymes increase the reaction rate without raising the
temperature.
ENZYMES AND CHEMICAL REACTIONS
Substances that can speed up a chemical reaction
without being permanently altered themselves are called
catalysts In living cells, enzymes serve as biological
catalysts.
A catalysts, enzymes are specific.
Each acts stance, called the enzyme's substrate (or
substrates, when there are two or more reactants), and each
catalyzes only
one reaction.
For example, sucrose (table sugar)
is the substrate of the enzyme sucrase, which catalyzes
the hydrolysis of sucrose to glucose and fructose.
As catalysts, enzymes typically accelerate
chemical reactions. The three-dimensional enzyme molecule
has an active site, a region that will interact with a
specific chemical substance The enzyme orients the substrate into a position
that increases the probability of a reaction.
The
enzyme substrate complex formed by the temporary
binding of enzyme and reactants enables the collisions to
be more effective and lowers the activation energy of the
reaction.
The enzyme therefore speeds up the
reaction by increasing the number of AB molecules that
attain sufficient activation energy to react.
An enzyme's ability to accelerate a reaction
without the need for an increase in temperature is crucial
to living systems because a significant temperature
increase wouldestroy cellular proteins. The crucial function
of enzymes,therefore, is to speed up biochemical reactions
at a temperature that is compatible with the normal
functioning a specific substance.
ENZYME SPECIFICITY AND EFFICIENCY
The specificity of enzymes is made possible by
their structures. Enzymes are generally large globular
proteins that range in molecular weight from about 10,000 to
several million.
Each of the thousands of known enzymes
has a characteristic three-dimensional shape with a
specific surface
configuration as a result of its primary, secondary,and tertiary structures.
The unique configuration of each enzyme enables it
to "defind"the correct substrate from among the large
number of diverse molecules in the cell Enzymes are extremely efficient. Under optimum
conditions, they can catalyze reactions at rates
108 to 1010 times (up to 10 billion times) higher than those
of comparable reactions without enzymes.
The turnover
number(maximum number of substrate molecules an enzyme
molecule converts to product each second) is
generally between 1 and 10,000 and can be as high as
500,000.
For example, the enzyme DNA polymerase I, which
participates in the synthesis of DNA, has a turnover
number of 15, whereas the enzyme lactate dehydrogenase,
which removes hydrogen
atoms from lactic acid, has a turnover number of 1000 Many enzymes exist in the cell in both active
and inactive forms.
The rate at which enzymes switch
between these two forms is determined by the cellular
environment
NAMING ENZYMES
The names of enzymes usually end in ase. All
enzymes can be grouped into six classes, according to the
type of chemical reaction they catalyze Enzymes
within each of the major classes are named according to
the more specific types of reactions they assist.
For
example, the class called oxidoreductases is involved with
oxidation reduction reactions (described shortly). Enzymes
in the oxidoreductase class that remove hydrogen from a
substrate are called
dehydrogenases; those that add molecular oxygen are called oxidases.
As you will see later,
dehydrogenase and oxidase enzymes have even more specific
names,such as lactate dehydrogenase and cytochrome
oxidase,depending on the specific substrates on which
they act.
ENZYME COMPONENTS
Although some enzymes consist entirely of
proteins, most consist of both a protein portion called an
apoenzyme and a nonprotein component called a cofactor.
Ions
of iron, zinc, magnesium, or calcium are examples
of cofactors.
If the cofactor is an organic molecule, it
is called a coenzyme. Apoenzymes are inactive by themselves;
they must be activated by cofactors.
Together, the
apoenzyme and cofactor form a holoenzyme, or whole, active
enzyme If the cofactor is removed,
the apoenzyme will not function.
THE MECHANISM OF
ENZYMATIC ACTION
Enzymes lower the activation energy of chemical
reactions.
The surface of the substrate contacts a
specific region of the surface of the enzyme molecule called the active site.
A temporary intermediate compound forms,
called an enzyme-substrate complex The substrate molecule is transformed by the
rearrangement of existing atoms, the breakdown of
the substrate molecule, or in combination with
another substrate molecule.
The transformed substrate molecules-the products
of the reaction-are released from the enzyme
molecule because they no longer fit in the active site of
the enzyme
The unchanged enzyme is now free to react with
other substrate molecules.
As a result of these events, an enzyme speeds up
a chemical reaction As mentioned earlier, enzymes have specificity
for particular substrates.
For example, a specific
enzyme may be able to hydrolyze a peptide bond only between
two specific amino acids.
Other enzymes can hydrolyze
starch but not cellulose; even though both starch and
cellulose are polysaccharides composed of glucose subunits,
the orientations of the subunits in the two polysaccharides
differ.
Enzymes have this specificity because the
three-dimensional
shape of the active site fits the substrate
somewhat a lock fits with its key (Figure 5.4b). However,
the active site and substrate are flexible, and they change
shape somewhat as they meet to fit together more
tightly.
The substrate is usually much smaller than the
enzyme, and relatively few of the enzyme's amino acids make
up the active site.
A certain compound can be a substrate for
several different enzymes that catalyze different reactions,
so the fate of compound depends on the enzyme that acts upon cose 6-phosphate, a molecule important in cell
metabolism can be acted upon by at least four different
enzymes, and each reaction will yield a different product.
FACTORS INFLUENCING
ENZYMATIC ACTIVITY
TEMPERATURE
The rate of most chemical reactions increases as
the tem perature increases.
Molecules move more slowly
at lower temperatures than at higher temperatures and so
may not have enough energy to cause a chemical reaction.
For enzymatic reactions, however, elevation beyond a
certain
temperature (the optimal temperature)
drastically reduces the rate of reaction
The optimal
temperature for most disease producing bacteria in the
human body is between 35°C and 40°C.
The reduced rate
of reaction beyond the optimal temperature is due
to the enzyme's denaturation, the loss of its
characteristic three dimensional structure (tertiary configuration) Denaturation of a protein involves the breakage
of hydrogen bonds and other noncovalent bonds; a common
example is the transformation of uncooked egg white
(a proteincalled albumin) to a hardened state by heat.
Denaturation of an enzyme changes the
arrangement of the amino acids in the active site, altering
its shape and sing the enzyme to
lose its catalytic ability.
In some cases, denaturation is partially or fully
reversible. How if denaturation continues until the enzyme has
lost its s«o ubility and coagulates, the enzyme cannot regain
its original properties.
Enzymes can also be denatured by bases, heavy-metal ions (such as lead,concentrated acids,arsenic, or mercury), alcohol, and ultraviolet
radiationMost enzymes have an optimum pH at which their
activity is characteristically maximal.
Above or
below this pH value, enzyme activity, and therefore the
reaction rate decline.
When the H+
concentratio (pH) in the medium is changed drastically, the
protein's three-dimensional structure is altered.
Extreme
changes in pH can cause denaturation. Acids (and bases)
alter atein's three-dimensional structure because the H
(andOH-) compete with hydrogen and ionic bonds in an enzyme, resulting in the enzyme's denaturation.
SUBSTRATE CONCENTRATION
here is a maximum rate at which a certain amount
of enzyme can catalyze a specific reaction.
Only
when the concentration of substrate(s) is extremely high
can this maximum rate be attained.
Under conditions of high
substrate concentration, the enzyme is said to be in
saturation; that is, its active site is always occupied by
substrate or product molecules.
In this condition, a further increase
in substrate concentration will not affect the reaction rate
because all active sites are
already in use Under normal cellular conditions, enzymes are not saturated
with substrate(s).
At any given time, many of the enzyme
molecules are inactive for lack of substrate; thus, the
rate of reaction is likely to be influenced by the substrate
concentration.
INHIBITORS
An effective way to control the growth of
bacteria is to control their enzymes.
Certain poisons, such as
cyanide, arsenic, and mercury, combine with enzymes and
prevent them from functioning.
As a result, the cells
stop functioning and die Enzyme inhibitors are classified as either
competitive or noncompetitive inhibitors
Competitive inhibitors fill the active site of an enzyme and
compete with the normal substrate for the active site.
A
competitive inhibitor can do this because its shape and
chemical structure are similar to those of the normal
substrate
However, unlike the substrate, it
does undergo any reaction to form products. Some
competitive inhibitors bind irreversibly to amino acids in
the active site, preventing any further interactions with
the substrate.
Others bind reversibly, alternately
occupying and leaving the active site; these slow the enzyme's
interaction with the substrate.
Reversible competitive
inhibition carn be overcome by increasing the substrate
concentration.
As active sites become available, more substrate
molecules than competitive inhibitor molecules are
available to attach to the active sites of enzymes.
FEEDBACK
INHIBITION
Allosteric inhibitors play a role in a kind of
biochemical in
control called feedback inhibition, or
end-product hibition.
This control mechanism stops the cell
from wasting chemical resources by making more of a
substance than it needs.
In some metabolic reactions,
several steps are required for the synthesis of a particular
chemical compound, called the end-product.
The process is
similar to an assembly line, with each step catalyzed by a
separzyme .
In many anabolic pathways,
the product can allosterically inhibit the activity
of one enzymes earlier in the pathway. This phenomenon
is feed back inhibition.
Feedback inhibition generally acts on the first
enzym in a metabolic pathway (similar to shutting down
an as sembly line by stopping the first worker).
Because the enzyme is inhibited, the product of the first
enzymatic reaction in the pathway is not synthesized.
Because that unsynthesized product would normally be the
substrate for the second enzyme in the pathway, the second
reaction stops immediately as well.
Thus, even though only the
first enzyme in the pathway is inhibited, the entire
pathway shut down and no new end-product is formed.
By
inhibitingthe first enzyme in the
pathway, the cell also keeps metabolic intermediates from accumulating.
As the existing
endduct is used up by the cell, the first enzyme's
allosteric prodite will more often remain unbound, and the
pathway will resume activity.
The bacterium E. coli can be used to demonstrate
feed back inhibition in the synthesis of the amino
acid isoleucine, which is required for the cell's growth.
In this
metabolic five steps are taken to enzymatically convert
the ino acid threonine to isoleucine.
If isoleucine
is added to growth medium for E. coi, it inhibits the first
enzyme in the pathway, and the bacteria stop synthesizing
isoleucine.
This condition is maintained until the supply of
isoleucine is epleted.
This type of feedback inhibition is
also involved in regulating the cells' production of other amino
acids, as well as vitamins, purines, and pyrimidine
RIBOZYMES
Prior to 1982, it was believed that only protein
molecules had enzymatic activity.
Researchers working on
microbes discovered a unique type of RNA called a
ribozyme.
Like protein enzymes, ribozymes function as
catalysts, have active sites that bind to substrates, and are not
used up in a chemical reaction.
Ribozymes specifically act on
strands of RNA by removing sections and splicing
together the remaining pieces.
In this respect, ribozymes are
more restricted than protein enzymes in terms of the
diversity of substrates with which they interact.
ENERGY PRODUCTION
Explain what is meant by oxidation-reduction. List and provide examples of three types of
phosphorylation reactions that generate ATP.
Nutrient molecules, like all molecules, have
energy associated with the electrons that form bonds between
their atoms.
When it is spread throughout the
molecule, this energy is difficult for the cell to use.
Various
reactions in catabolic pathways, however, concentrate the energy
into the bonds of ATP, which serves as a convenient
energy carrier.
ATP is generally referred to as having
"high-energy"bonds.
Actually, a better term is probably
unstable bonds.
Although the amount of energy in these bonds is
not exceptionally large, it can be released quickly
and easily.
In a sense, ATP is similar to a highly flammable
liquid such as erosene.
Although a large log might eventually
burn to produce more heat than a cup of kerosene, the
kerosene is easier to ignite and provides heat more quickly
and conveniently.
In a similar way, the
"high-energy" unstable bonds How do oxidation and
reduction differt of ATP provide the cell with readily available
energy for anabolic reactions.
Before discussing the catabolic pathways, we
will con sider two general aspects of energy production:
the concept of oxidation-reduction and the mechanisms
of ATP generation.
OXIDATION-REDUCTION REACTIONS
Oxidation is the removal of electrons (e) from
an atom or molecule, a reaction that often produces
energy.
an example of an oxidation in
which molecule A loses an electron to molecule B.
Molecule A has undergone oxidation (meaning that it has
lost one or more electrons), whereas molecule B has
undergone reduction (meaning that it has gained one or more
electrons).
*Oxidation and reduction reactions are
always coupled; in other words, each time one substance
is oxidized, another is simultaneously reduced.The
pairing of these reactions is called oxidation-reduction or
a redox reaction.
In many cellular oxidations, electrons and
protons (hydrogen ions, H") are removed at the same
time; this is equivalent to the removal of hydrogen atoms,
because a hydrogen atom is made up of one proton and one
electron Because most
biological oxidations involve the loss of hydrogen atoms, they
are also called dehydrogenation reactions.
Figure 5.10
shows an example of a biological oxidation.
An organic
molecule is oxidized by the loss of two hydrogen atoms, and
a molecule of NAD+ is reduced.
Recall from our earlier
discussion of coenzymes that NAD+ assists enzymes by accepting
hydrogen atoms removed from the substrate, in this
case the organic molecule NAD
accepts two electrons and one proton.
One proton (H) is
left over and is released into the surrounding medium.
The
reduced coenzyme, NADH, contains more energy than NAD This energy can be used to generate ATP in later
reactions.
An important point to remember about biological oxidation reduction reactions is that cells use
them in catabolism to extract energy from nutrient
molecules.
Cells take nutrients, some of which serve as
energy sources, and degrade them from highly reduced compounds
(with many hydrogen atoms) to highly oxidized
compounds.
Forexample, when a cell oxidizes a molecule of
glucose (CsH120) to CO2 and H20, the energy in the
glucose molecule is removed in a stepwise manner and
ultimately is trapped by ATP, which can then serve as an
energy source for energy-requiring reactions. Compounds such
as glucose that have many hydrogen atoms are highly reduced
comounds, containing a large amount of potential
energy.Thus, glucose is a valuable nutrient for
organisms.
THE GENERATION OF ATP
Much of the energy released during
oxidation-reduction
reactions is trapped within the cell by the
formation of ATP.
Specifically, a phosphate group, », is
added to ADP
with the input of energy to form ATP:
ADP
Adenosine-+ Energy
Adenosine-O~O~()
ATP
The symbol - designates a
"high-energy" bond-that is, one that
can readily The high-energy bond that attaches the third in
a sense contains the energy stored in this
reaction.
When this B is removed, usable energy is released.
The addition
of ® to a chemical compound is called
phosphorylation.
Organisms use three mechanisms of
phosphorylation to
generate ATP from ADP.
SUBSTRATE-LEVEL PHOSPHORYLATION
In substrate-level phosphorylation, ATP is
usually generated when a high-energy is directly
transferred from a phosphorylated compound (a substrate) to ADP.
Generally, the has acquired its energy during an
earlier reaction in which the substrate itself was oxidized.
The following example shows only the carbon skeleton
and the
*P of a tvpical substrate:
OXIDATIVE PHOSPHORYLATION
In oxidative phosphorylation, electrons are
transferred from organic compounds to one group of electron
carriers usually to NAD and
FAD).
Then, the electrons are passed through a series of different electron carriers
to molecules of oxygen (O2) or other oxidized inorganic and
organic moleles.
This process occurs in the plasma membrane
of prokaryotes and in the inner mitochondrial
membrane of eukaryotes.
The sequence of electron carriers used in oxidative phosphorylation is called an electron
transport chain (system)
The transfer of
electrons from the next releases energy, some of which is used to generation ATP from ADP through a process called chemiosmosis.
PHOTOPHOSPHORYLATION
The third mechanism of phosphorylation,
photophos phorylation, occurs only in photosynthetic
cells, which contain light-trapping pigments such as
chlorophylls.
In photosynthesis, organic molecules, especially
sugars, are synthesized with the energy of light from the
energy-poor
building blocks carbon dioxide and water.
Photophosphor-ylation starts this process by converting light
energy to the chemical energy of ATP and NADPH, which, in
turn, are used to synthesize organic molecules.
As in
oxidative phosphorylation, an electron transport chain is
involved.
METABOLIC PATHWAY OF ENERGY PRODUCTION
Organisms release and store energy írom organic
molecules
by a series of controlled reactions rather than
in a single burst.
If the energy were released all at once,
as a large amount of heat, it could not be readily used to
drive chemical reactions and would, in fact, damage the
cell.
To extractenergy from organic compounds and store it in
chemical form, organisms pass electrons from one compound
to an other through a series of oxidation-reduction
reactions.
As noted earlier, a sequence of enzymatically
catalyzed chemical reactions occurring in a cell is called
a metabolic pathway.
The first step is
the conversion of molecule A to molecule B The curved arrow indicates that the reduction of
coenzyme NAD* to NADH is coupled to that reaction; the
electrons and protons come from molecule A.
Similarly, the
two arrows in 3 show a coupling of two reactions. As C
is converted to D, ADP is converted to ATP; the energy
needed comes from C as it transforms into D.
The
reaction converting D to E is readily reversible, as indicated
by the double arrow. In the fifth step, the curved arrow leading
from O2 indicates that O2 is a reactant in the reaction.
The curved arrows leading to CO2 and H2O indicate that
these substances are secondary products produced in the
reaction, in
addition to F, the end-product that (presumably)
interests us the most.
Secondary products such as CO2 and H,O
shown here are sometimes called
"by-products" or "waucts." Keep in mind that almost every
reaction in a metabolic pathway is catalyzed by a specific enzyme;
sometimes the name of the enzyme is printed near the arrow.
CARBOHYDRATE CATABOLISM
Most microorganisms oxidize carbohydrates as
their primary source of cellular energy.
Carbohydrate
catabolism, the breakdown of carbohydrate molecules to
produce energy, is therefore of great importance in cell
metabolism Glucose is the most common carbohydrate energy
source used by cells.
Microorganisms can also
catabolize various lipids and proteins for energy production
To produce energy from glucose, microorganisms
use two general processes: cellular respiration and
fermentation (In discussing cellular respiration, we
frequently refer to the process simply as respiration, but it should
not be confused with breathing.) Both processes usually
start with the same first step, glycolysis, but follow
different subsequent. Before examining the details of pathways glycolysis, respiration, and fermentation, we
will first look at a general overview of the processes.
The respiration of
glucose typically occurs in three principal stages:
glycolysis, the Krebs cycle, and the electron transport chain (system).
1. Glycolysis is the oxidation of glucose to
pyruvic acid with the production of some ATP and energy containing NADH
2.The Krebs cycle is the oxidation of acetyl CoA
(a derivative of pyruvic acid) to carbon dioxide.
GLYCOLYSIS
Glycolysis, the oxidation of glucose to pyruvic
acid, is ly the first stage in carbohydrate catabolism.
Most this pathway; in fact, it occurs in most croorganisms use
living cells.
Glycolysis is also called the Embden-Meyerhof
pathway The word glycolysis means splitting of sugar,
and this is exactly what happens.
The enzymes of glycolysis
catalyze the splitting of glucose, a six-carbon sugar, into
two three carbon sugars.
These sugars are then oxidized,
releasing energy, and their atoms are rearranged to form
two molecules of pyruvic acid. During glycolysis NAD is
reduced to NADH, and there is a net production of two
ATP molecules by substrate-level phosphorylation.
Glycolysis does not require oxygen; it can occur whether oxygen
is present not. This pathway is a series of ten chemical
reactions each catalyzed by a different enzyme.
a
more detailed representation of glycolysis To summarize the process, glycolysis consists of
two basic stages, a preparatory stage and an
energy-conserving or
stage:
1. First, in the preparatory stage two molecules of ATP are used as a
six-carbon glucose molecule is phosphorylated,
restructured,and split into two three-carbon compounds:
glyceraldehyde 3-phosphate (GP) and dihydroxyacetone phosphate (DHAP).DHAP is readily converted toGP. (The reverse reaction may also occur.) The
conversion of DHAP into GP means that from this
point on in glycolysis, two molecules of GP are fed
into the remaining chemical reactions.
2. In the
energy-conserving stage the two three-carbon molecules are
oxidized in several steps to two molecules of pyruvic
acid.
In these reactions, two molecules of NAD+ are
reduced to NADH, and four molecules of ATP are formed by substrate-level phosphorylation.
Because two molecules of ATP were needed to get
glycolysis started and four molecules of ATP are
generated by the process, there is a net gain of two molecules of
ATP for each
molecule of glucose that is oxidized.
ALTERNATIVES TO GLYCOLYSIS
Many bacteria have another pathway in addition
to glycolysis for the oxidation of glucose. The most common
alternative is the pentose phosphate pathway;
another alternative is the Entner-Doudoroff pathway.
THE PENTOSE PHOSPHATE PATHWAY
The pentose phosphate pathway (or hexose
monophosphate shunt) operates simultaneously with
glycolysis and provides a means for the breakdown of
five-carbon sugars (pentoses) as well as glucose.A key feature of this pathway is that
it produces important intermediate pentoses used in the
synthesis of
(1) nucleic acids,
(2) glucose from carbon
dioxide in photosynthesis
(3) certain amino acids.
The
pathway is an important producer of the reduced coenzyme
NADPH from NADP*.
The pentose phosphate pathway yields
anet gain of only one molecule of ATP for each
molecule of glucose oxidized.
Bacteria that use the pentose
phosphate pathway include Bacillus subtilis (sub'til-us),
E. coli, Leuconostoc mesenteroides (lü-kõ-nos'tok
mes-en-ter-oi'dēz),and Enterococcus faecalis (fe-kāl'is)
THE ENTNER-DOUDOROFF PATHWAY
From each molecule of glucose, the
Entner-Doudoroff pathway produces two molecules of NADPH and one molecule of ATP for use in cellular biosynthetic
reactions.
Bacteria that have the enzymes for the Entner Doudoroff pathway can metabolize glucose without either
glycolysis or the pentose phosphate pathway.
The
Entner-Doudoroff pathway is found in some gram-negative bacteria,
including Rhizobium, Pseudomonas (sü-do-mo'nas), and
Agrobacterium (ag-ro-bak-ti're-um); t is generally not
found among gram-positive bacteria.
Tests for the
ability to oxidize glucose by this pathway are sometimes used
CELLULAR RESPIRATION
Cellular respiration, or simply
respiration, is defined as an ATP-generating process in which
molecules are oxidized and the final electron acceptor is
(almost always) an inorganic molecule. An essential
feature of respiration is the operation of an electron
transport chain.
There are two types of respiration, depending on whether an organism is an aerobe, which uses
oxygen,on anaerobe, which does not use oxygen and may
even be killed by it. In aerobic respiration, the final
electron acceptor is O2; in anaerobic respiration, the
final electrorn acceptor is an inorganic molecule other than O2
or, rarely, an organic molecule.
AEROBIC RESPIRATION
The Krebs Cycle The Krebs cycle, also called the
tricarboxylic acid (TCA) cycle or citric acid
cycle, is a series of biochemical reactions in which tential chemical energy stored in acetyl CoA is released step by step . In this cycle, a
series of oxidations and reductions transfer that potential
energy, in the form of electrons, to electron carrier
coenzymes,chiefly NADt. The pyruvic acid derivatives are
oxidized; the large amount of coenzymes are reduced.
Pyruvic acid, the product of glycolysis, cannot
enter the Krebs cycle directly. In a preparatory step,
it must lose one molecule of CO2 and become a two-carbon
compound . This process is called
decarboxylation. The two-carbon compound, called an acetyl
group,attaches to coenzyme A through a high-energy
bond; the resulting complex is known as acetyl coenzyme A
(acetyl CoA). During this reaction, pyruvic acid is also
oxidizedand NAD+ is reduced to NADIH.
Remember that the oxidation of one glucose
molecule produces two molecules of pyruvic acid, so for
each molecule of glucose, two molecules of CO2 are
released in this preparatory step, two molecules of NADH are
produced,and two molecules of acetyl CoA are formed. Once
the pyruvic acid has undergone decarboxylation and
its derivative (the acetyl group) has attached to CoA,
the resulting acetyl CoA is ready to enter the Krebs cycle.
As acetyl CoA enters the Krebs cycle, CoA
detaches from the acetyl group.The two-carbon acetyl
group combines with a
four-carbon compound called oxaloacetic acid to form the six-carbon citric acid
This
synthesis reaction requires energy, which is provided by the
cleavage of the high-energy bond between the acetyl group and
CoA.
The formation of citric acid is thus the first step
in the Krebs
cycle.
The chemical reactions of the Krebs cycle fall
into several general categories; one of these is
decarboxylation.
For example, in step 3 isocitric acid, a six-carbon
compound is decarboxylated to the five-carbon compound
called a-ketoglutaric acid. Another decarboxylation
takes place in step 4 4 Because one decarboxylation has taken
place in the preparatory step and two in the Krebs cycle,
all three carbon atoms in pyruvic acid are eventually
released as C. Keep in mind that Krebs cycle. This represents the conversion to CO2 of all six carbon atoms contained in the
original glucose molecule.
Another general category of Krebs cycle chemical
reactions is oxidation-reduction. For example, in
step 3,two hydrogen atoms are lost during the conversion
of the six-carbon isocitric acid to a five-carbon
compound. Inother words, the six-carbon compound is oxidizedgen atoms are also released in the Krebs cycle
in steps , 6 and 6 and are picked up by the coenzymes NAD and Becausee picks up two electrons but only on<e additional proton, its reduced form is
represented as NADH; however, FAD picks up two complete
hydrogen atoms and is reduced to FADH2 If we look at the Krebs cycle as a whole, we see
that for every two molecules of acetyl CoA that enter the
cycle four molecules of CO2 are liberated by
decarboxylation six molecules of NADH and two molecules of FADH2
are produced by oxidation-reduction reactions, and
two molecules of ATP are generated by substrate-level
phosphorylation. Many of the intermediates in the Krebs
cycle also play a role in other pathways, especially in
amino acid biosynthesis.
The CO2 produced in the Krebs cycle is
ultimately liberated into the atmosphere as a gaseous
by-product of aerobic respiration. (Humans produce CO2 from the
Krebs cycle in most cells of the body and discharge it
through the lungs during exhalation.) The reduced coenzymes
NADH and FADH2 are the most important products of the
Krebs cycle because they contain most of the energy
originally stored in glucose. During the next phase of
respiration, a series of reductions indirectly transfers the
energy stored in those coenzymes to ATP. These reactions are
collectively called the electron transport chain
Q what are the
products of the Krebs cycle?
The Electron
Transport Chain (System) An electron transport chain (system) consists of a sequence
of carrier molecules that are capable of oxidation and
reduction. As electrons are passed through the chain, there is
a stepwise release of energy, which is used to drive the
chemiosmotic generation of ATP, to be described shortly. The
final oxidation is irreversible. In eukaryotic cells, the electron
transport chain is contained in the inner membrane of
mitochondria; in prokaryotic cells, it is found in the plasma
membrane three
are three classes of carrier molecules in electron transport chains. The first are flavoproteins.
these proteins flavin, a coenzyme derived from riboflavin and are capable of performing alternating oxidations and reductions. One important flavin coenzyme is
flavin mononucleotide.
(FMN). The second class of carrier molecules are
cytochromes, proteins with an iron containing group are (heme) capable of
existing alternately as a reduced form and an oxidized form.
The What are the functions
of the electrotransport chain?
The electron transport chains of bacteria are
somewhat
diverse, in that the particular carriers used by
a bacterium
and the order in which they function may differ
from those
of other bacteria and from those of eukaryotic
mitochon
drial systems. Even a single bacterium may have
several
types of electron transport chains. However,
keep in mind
that all electron transport chains achieve the
same basic
goal, that of releasing energy as electrons are
transferred
from higher-energy compounds to lower-energy
compounds
Much is known about the electron transport chain
in the
mitochondria of eukaryotic cells, so this is the
chain we will
describe
The first step in the mitochondrial electron
transport
chain involves the transfer of high-energy
electrons
from NADH to FMN, the first carrier in the chain
This transfer actually involves the
passage of a
hydrogen atom with two electrons to FMN, which
then
picks up an additional H from the surrounding
aqueous
medium. As a result of the first transfer, NADH
is oxidized
to NAD+, and FMN is reduced to FMNH2. In the
second
step in the electron transport chain, FMNH2
passes 2H
to the other side of the mitochondrial membrane
(see
Figure 5.16) and passes two electrons to Q. As a
result
FMNH2 is oxidized to
FMN. Q also picks up an additional
2H+ from the surrounding aqueous medium and
releases it
on the other side of the membrane.
The next part of the electron transport chain
involves
the cytochromes. Electrons are passed successively
from Q
to cyt b, cyt C1, Cyt C, cyt a, and cyt a3. Each
cytochrome
in the chain is reduced as it picks up electrons
and is oxi-
dized as it gives up electrons. The last
cytochrome, cyt az,
passes its electrons to molecular oxygen (O2),
which be-
comes negatively charged and then picks up
protons from
the surrounding medium to form H2O
Notice that Figure 5.14 shows FADH2, which is
de-
rived from the Krebs cycle, as another source of
electrons.
However, FADH2 adds its electrons to the
electron trans-
port chain at a lower level than NADH. Because
of this,
the electron transport chain produces about
one-third less
energy for ATP generation when FADH2 donates
elec
trons than when NADH is involved
An important feature of the electron transport
chain
is the presence of some carriers, such as FMN
and Q, that
accept and release protons as well as electrons,
and other
carriers, such as cytochromes, that transfer
electrons
only. Electron flow down the chain is
accompanied at
several points by the active transport (pumping)
of
protons from the matrix side of the inner
mitochondrial
membrane to the opposite side of the membrane.
The result
is a buildup of protons on one side of the
membrane. Just
as water behind a dam stores energy that can be
used to
The mechanism of ATP
synthesis using the electron transport
chain is called chemiosmosis. To understand
chemios-
osis, we need to recall several concepts that
were intro-
duced in as part of the section on the
movement
of materials across membranes . Recall
that sub-
stances diffuse passively across membranes from
areas of
high concentration to areas of low
concentration; this dif-
fusion yields energy. Recall also that the
movement of sub-
stances against such a concentration gradient
requires
energy and that, in such an active transport of
molecules
or ions across biological membranes, the
required energy is
usually provided by ATP. In chemiosmosis, the
energy re
leased when a substance moves along a gradient
is used to
synthesize ATP. The "substance" in
this case refers to pro-
tons. In respiration, chemiosmosis is
responsible for most
of the ATP that is generated. The steps of
chemiosmosis
are as follows
1.As energetic electrons from NADH (or
chlorophyll)
pass down the electron transport chain, some of
the
carriers in the chain pump-actively transport
protons across the
membrane. Such carrier molecul
are called proton pumts
A Summary of Aerobic
Respiration The electron trans-
port chain regenerates NAD+ and FAD*, which can
be
used again in glycolysis and the Krebs cycle.
The various
electron transfers in the electron transport
chain generate
about 34 molecules of ATP from each molecule of
glucose
oxidized: approximately three from each of the
ten mole-
cules of NADH (a total of 30), and approximately
two
from each of the two molecules of FADH2 (a total
of four)
To arrive at the total number of ATP molecules
generated
for each molecule of glucose, the 34 from
chemiosmosis are
added to those generated by oxidation in
glycolysis and the Krebs cycle. In aerobic respiration among prokaryotes, :a
total of 38 molecules of ATP can be generated
from one
molecule of glucose. Note that four of those
ATPs come
from substrate-level phosphorylation in
glycolysis and the
rebs cycle. Table 5.3 provides a detailed
accounting of
the ATP yield during prokaryotic aerobic
respiration
Aerobic respiration among eukaryotes produces a
total
of only 36 molecules of ATP. There are fewer
ATPs than in
prokaryotes because some energy is lost when
electrons are
shuttled across the mitochondrial membranes that
sepa-
rate glycolysis (in the cytoplasm) from the
electron trans-
port chain. No such separation exists in
prokaryotes. We
can now summarize the overall reaction for
aerobic respi-
ration in prokaryotes as follows:
Glucose Oxygen
6 co, + 6 H2O + 38 ATP
Carbon
Water
dioxide
A summary of the various stages of aerobic
respiration in
prokaryotes is presented in Figure 5.17
ANAEROBIC RESPIRATION
In anaerobic respiration, the
inorganic substance other than oxygen (O2). Some
bacte-
ria, such as Pseudomonas and Bacillus, can use a
nitrate ion
(NO; ) as a final electron acceptor; the nitrate
ion is re-
duced to a nitrite ion (NO2), nitrous oxide
(N2O), or ni-
trogen gas (N2). Other bacteria, such as
Desulfovibrio
(de-sul-fo-vib're-ö), use sulfate (SO42
final electron acceptor is an
z-) as the final electronacceptor to form
hydrogen sulfide (H2S). Still other bacte
ria use carbonate (CO,27)
Anaerobic respiration by bacteria using nitrate
and sulfate
as final acceptors is essential for the nitrogen
and sulfu
cles that occur in nature. The amount of ATP
generated
anaerobic respiration varies with the organism
and th
pathway. Because only part of the Krebs cycle
operates un
der anaerobic conditions, and since not all the
carriers i
the electron transport chain participate in
anaerobic respi
ration, the ATP yield is never as high as in
aerobic respira
tion. Accordingly, anaerobes tend to grow more
slowly than
aerobes. k Animations: Go to The Microbiology
Place website
to form methane (CH
r c
in
or CD-ROM and click "Animations" to
view Electron Transport
Chains and Chemiosmosis, and Krebs Cycle
FERMENTATION
LEARNING OBJECTIVE
After glucose has been broken down into pyruvic
acid, the
pyruvic acid can be completely broken down in
respiration,
as previously described, or it can be converted
to an organic
roduct in fermentation, whereupon NAD+ and NADP
are regenerated and can enter another round of
glycolysis
(see Figure 5.11). Fermentation can be defined
in several
ways (see the box, page 137), but we define it
here as a
process that
1. releases energy from sugars or other organic
molecules
such as amino acids, organic acids, purines, and
pyrimidines;
2. does not require
oxygen (but sometimes can occur in
its presence);
3. does not require the use of the Krebs cycle
or an
electron transport chain;
4. uses an organic molecule as the final
electron acceptor;
5. produces only small amounts of ATP (only one
or two
ATP molecules for each molecule of starting
material)
because much of the original energy in glucose remains in the chemical bonds of the organic end-
products, such as lactic acid or ethanol
During fermentation, electrons are transferred
(along
with protons) from reduced coenzymes (NADH,
NADPH)
to pyruvic acid or its derivatives (Figure
5.18a). Those
final electron acceptors are reduced to the
end-products
shown in Figure 5.18b. An essential function of
the sec-
ond stage of fermentation is to ensure a steady
supply of NAD and NADP+ so that
glycolysis can continue. In
ntation, ATP is generated only during
glycolysis.
Microorganisms can ferment various substrates;
the
roducts depend on the particular microorganism,
the
substrate, and the enzymes that are present and
active.
Chemical analyses of these end-products are
useful in
identifying microorganisms. We next consider two
of the
more important processes: lactic acid
fermentation and al-
cohol fermentation.
LACTIC ACID FERMENTATION
During glycolysis, which is the first phase of
lactic acid fer
mentation, a molecule of glucose is oxidized to
two mole-
cules of pyruvic acid (Figure 5.19; see also
Figure 5.10). This
oxidation generates the energy that is used to form
the two
molecules of ATP. In the next step, the two
molecules of
pyruvic acid are reduced by two molecules of
NADH to form
two molecules of lactic acid Because lactic
acid is the end-product of the reaction, it
undergoes no
further oxidation, and most of the energy
produced by the
reaction remains stored in the lactic acid.
Thus, this fermen-
tation yields only a small amount of energy.
Two important genera of lactic acid bacteria are
Strep-
tococcus and Lactobacillus (lak-to-bä-sil'lus).
Because these
microbes produce only lactic acid, they are
referred to as
homolactic (or homofermentative). Lactic acid
fermenta-
tion can result in food spoilage. However, the
process can
also produce yogurt from milk, sauerkraut from
fresh cab
bage, and pickles from cucumbers.
ALCOHOL FERMENTATION
Alcohol fermentation also begins with the
glycolysis
of a molecule of glucose to yield two molecules
of pyruvic acid and two molecules of ATP. In the next reaction, the
two molecules of pyruvic acid are converted to
two mol
ecules of acetaldehyde and two molecules of CO2
The two molecules of
acetaldehyde are
next reduced by two molecules of NADH to form
two
molecules of ethanol. Again, alcohol
fermentation is a
low-energy-yield process because most of the
energy
contained in the initial glucose molecule
remains in the
ethanol, the end-product
Alcohol fermentation is carried out by a number
of
bacteria and yeasts. The ethanol and carbon
dioxide pro-
duced by the yeast Saccharomyces (sak-ä-ro-mĩ'sēs)
are
waste products for yeast cells but are useful to
humans.
Ethanol made by yeasts is the alcohol in
alcoholic bever-
ages, and carbon dioxide made by yeasts causes
bread
dough to rise
Organisms that produce lactic acid as well as
other
acids or alcohols are known as heterolactic (or
heterofer-
mentative) and often use the pentose phosphate
pathway.
Table 5.4 lists some of the various microbial
fermenta-
tions used by industry to convert inexpensive
raw materi-
als into useful end-products. A summary
comparison of
aerobic respiration, anaerobic respiration.
PHOTOSYNTHESIS
In all of the metabolic pathways just discussed,
organisms
obtain energy for cellular work by oxidizing
organic com
pounds. But where do organisms obtain these
organic com
pounds? Some, including animals and many
microbes, feed
on matter produced by other organisms. For
example, bac-
teria may catabolize compounds from dead plants
and ani-
mals or may obtain nourishment from a living
host
Other organisms synthesize complex organic com
pounds from simple inorganic substances. The
major
mechanism for such synthesis is a process called
photo-
synthesis, which is used by plants and many
microbes. Es
sentially, photosynthesis is the conversion of
light energy
from the sun into chemical energy. The chemical
energy is
then used to convert CO2 from the atmosphere to
more
reduced carbon compounds, primarily sugars. The
word
photosynthesis summarizes the process: photo
means light,
and synthesis refers to the assembly of organic
compounds.
This synthesis of sugars by using carbon atoms
from CO2
gas is also called carbon fixation. Continuation
of life
as we know it on Earth depends on the recycling
of carbon
in this way Cyanobacteria,
algae, and
green plants all contribute to this vital
recycling with
photosynthesis.
Photosynthesis can be summarized with the
following
equations:
1. Plants, algae, and cyanobacteria use water as
a hydro-
gen donor, releasing O2-
6 CO2 + 12 H2O + Light energy
2. Purple sulfur and green sulfur bacteria use
H2S as a
hydrogen donor, producing sulfur granules.
6 CO2 + 12 H,S + Light energy- →
C6H1206 + 6H20 12S
In the course of photosynthesis, electrons are
taken from
hydrogen atoms, an energy-poor molecule, and
incorpo-
rated into sugar, an energy-rich molecule. The
energy
boost is supplied by light energy, although
indirectly.
Photosynthesis takes place in two stages. In the
first
stage, called the light-dependent (light)
reactions, light energy is used to convert ADP and ⓟ to ATP. In addition,
in the predominant form of the light-dependent
reactions
the electron carrier NADP+ is reduced to NADPH.
The
coenzyme NADPH, like NADH, is an energy-rich
carrier
of electrons. In the second stage, the
light-independent
(dark) reactions, these electrons are used along
with
energy from ATP to reduce CO2 to sugar.
THE LIGHT-DEPENDENT REACTIONS:
PHOTOPHOSPHORYLATION
Photophosphorylation is one of the three ways
ATP is
formed, and it occurs only in photosynthetic
cells. In this
mechanism, light energy is absorbed by
chlorophyll mole-
cules in the photosynthetic cell, exciting some
of the mol-
ecules' electrons. The chlorophyll principally
used by
green plants, algae, and cyanobacteria is
chlorophyll a. It is
located in the membranous thylakoids of
chloroplasts in
algae and green plants and in the thylakoids found in the photosynthetic structures of
cyanobacteria. Other bacteria use
bacteriochlorophylls
The excited electrons jump from the chlorophyll
to the
first of a series of carrier molecules, an
electron transport
chain similar to that used in respiration. As
electrons are
passed along the series of carriers, protons are
pumpe
across the membrane, and ADP is converted to ATP
by
chemiosmosis. In cyclic photophosphorylation the
electrons eventually return to chlorophyll
In noncyclic photophosphorylation, which is the
more
common process, the electrons released from
chlorophyll
do not return to chlorophyll but become
incorporated into
NADPH The electrons lost from
chloro-
phyll are replaced by electrons from HO. To
summarize:
the products of noncyclic photophosphorylation
are ATP
(formed by chemiosmosis using energy released in
an elec
tron transport chain), O2 (from water
molecules), and
NADPH (in which the hydrogen electrons and
protons
were derived ultimately from water).
THE
LIGHT-INDEPENDENT REACTIONS:
THE CALVIN-BENSON CYCLE
light-independent (dark) reactions are so
named be-
cause no light is directly required for them to
occur. They
include a complex cyclic pathway called the
Calvin-
Benson cycle, in which CO2 is
"fixed"-that is, used to syn
thesize sugars.
A SUMMARY OF ENERGY
PRODUCTION MECHANISM:S
LEARNING O BJECTIVE
Write a sentence to summarize energy production
in cells
In the living world, energy passes from one
organism to
other in the form of the potential energy
contained in the
bonds of chemical compounds. Organisms obtain
the en-
ergy from oxidation reactions. To obtain energy
in a usable
form, a cell must have an electron (or hydrogen)
donor,
which serves as an initial energy source within
the cell.
Electron donors can
be as diverse as photosynthetic pig-
ments, glucose or other organic compounds,
elemental
sulfur, ammonia, or hydrogen gas Next,
electrons removed from the chemical energy
sources are
transferred to electron carriers, such as the
coenzymes
NAD*, NADP+, and FAD. This transfer is an
oxidation-
reduction reaction; the initial energy source is
oxidized as
this first electron carrier is reduced. During
this phase,
some ATP is produced. In the third stage,
electrons are
transferred from electron carriers to their
final electron ac
ceptors in further oxidation-reduction
reactions, produc-
ing more ATP
In aerobic respiration, oxygen (O2) serves as
the final
electron acceptor. In anaerobic respiration,
inorganic sub-
stances other than oxygen, such as nitrate ions
(NO37) or
sulfate ions (SO4), serve as the final electron
acceptors.
METABOLIC DIVERSITY
AMONG ORGANISMS
We have looked in detail at some of the ener
metabolic pathways that are used by animals and
plant
well as by many microbes. Microbes are
distinguished by
their great metabolic diversity, however, and
some can sus-
tain themselves on inorganic substances by using
pathwa
that are unavailable to either plants or
animals.All organ-
isms, including microbes, can be classified
metabolicall
according to their nutritional pattern-their
source of en
ergy and their source of carbon.
First considering the energy source, we can
generally
classify organisms as phototrophs or
chemotrophs. Photo
trophs use light as their primary energy source,
whereas
chemotrophs depend on oxidation-reduction
reactions
of inorganic or organic compounds for energy.
For their
principal carbon source, autotrophs
(self-feeders) use car-
bon dioxide, and heterotrophs (feeders on
others) require
an organic carbon source. Autotrophs are also
referred to
as lithotrophs (rock eating), and heterotrophs
are also re-
ferred to as organotrophs
If we combine the energy and carbon sources, we
de-
rive the following nutritional classifications
for organisms:
photoautotrophs, photoheterotrophs,
chemoautotrophs, and
chemoheterotrophs Almost all of
the med
ically important microorganisms discussed in
this book are
chemoheterotrophs. Typically, infectious
organisms catab-
olize substances obtained from the host.
PHOTOAUTOTROPHS
Photoautotrophs use light as a source of energy
and car-
bon dioxide as their chief source of carbon.
They include
photosynthetic bacteria (green and purple
bacteria an
cyanobacteria), algae, and green plants. In the
photosyn
thetic reactions of cyanobacteria, algae, and
green plants,
the hydrogen atoms of water are used to reduce
carbon
dioxide, and oxygen gas is given off. Because
this photo-
synthetic process produces O2, it is sometimes
calle
oxygenic
In addition to the cyanobacteria
there are several other families of
photosyn
thetic prokaryotes. Each is classified according
to the
way it reduces CO2 These bacteria cannot use H2O
to
reduce CO2 and cannot carry on photosynthesis
when
oxygen is present (they must have an anaerobic
environ-
ment). Consequently, their photosynthetic
process does
not produce O2 and is called anoxygenic. The
anoxy-
genic photoautotrophs are the green and purple
bacteria
The green bacteria, such as Chlorobium
(klô-ro'be-um),
use sulfur (S), sulfur compounds (such as
hydrogen sul-
fide, H2S), or hydrogen gas (H2) to reduce carbon
diox-
ide and form organic compounds. Applying the
energy from
light and the appropriate enzymes, these bacteria
oxidize sulfide (S2) or sulfur (S) to sulfate
(SO47) or
oxidize hydrogen gas to water (H2O). The purple
bac-
teria, such as Chromatium (krõ-ma'te-um), also
use sul-
fur, sulfur compounds, or hydrogen gas to reduce
carbon
dioxide. They are distinguished from the green
bacteria
by their type of chlorophyll, location of stored
sulfur,
and ribosomal RNA
The chlorophylls used by these photosynthetic
bacte-
ria are called bacteriochlorophylls, and they
absorb light at
longer wavelengths than that absorbed by
chlorophyll a
Bacteriochlorophylls of green sulfur bacteria
are found in
vesicles called chlorosomes (or chlorobium
vesicles) underly-
ing and attached to the plasma membrane. In the
purple
sulfur bacteria, the bacteriochlorophylls are
located in
invaginations of the plasma membrane (intracytoplasmic
membranes)
Several characteristics that distinguish
eukaryotic pho-
tosynthesis from prokaryotic photosynthesis are
presented
. See the box on the facing page for
a discus
ion of an exceptional photosynthetic system that
exists in
Halobacterium. The system does not use
chlorophyll
PHOTOHETEROTROPHS
Photoheterotrophs use light as a source of energy
but
cannot convert carbon dioxide to sugar; rather,
they use
organic compounds, such as alcohols, fatty
acids, other or
ganic acids, and carbohydrates, as sources of
carbon. They
are anoxygenic. The green nonsulfur bacteria,
such as
Chloroflexus (klô-rö-flex'us), and purple
nonsulfur bac-
teria, such as Rhodopseudomonas
(ro-do-su-do-mo'nas), are
photoheterotrophs.
CHEMOAUTOTROPHS
Chemoautotrophs use the electrons from reduced
inor-
ganic compounds as a source of energy, and they
use CO2 as
their principal source of carbon
Inorganic sources of energy for these
organisms in
clude hydrogen sulfide (H2S) for Beggiatoa
(bej-jē-ä-to'ä);
elemental sulfur (S) for Thiobacillus
thiooxidans; ammonia
(NH,) for Nitrosomonas (nī-trö-sö-mõ'näs);
nitrite ons
NO2-) for Nitrobacter (ni-trơ bak tér): hydrogen
gas (H2)
or Hydrogenomonas (hi-dro-je-no-mo, nas): ferrous
iron
Fe2+) for Thiobacillus ferrooxidans; and carbon
monoxide
(CO) for Pseudomonas carboxydohydrogena. The
energy
derived from the oxidation of these inorganic
compounds is
eventually stored in ATP.
CHEMOHETEROTROPHS
When we discuss
photoautotrophs, photoheterotrophs,
and chemoautotrophs, it is easy to categorize
the energy
source and carbon source because they occur as
separate
entities. However, in chemoheterotrophs, the
distinction
is not as clear because the energy source and
carbon source
are usually the same organic compound-glucose,
for ex-
ample. Chemoheterotrophs specifically use the
electro
from hydrogen atoms in organic compounds as
their en-
ergy source
Heterotrophs are further classified according to
their
source of organic molecules. Saprophytes live on
dead or-
ganic matter, and parasites derive nutrients
from a living
host. Most bacteria, and all fungi, protozoa,
and animals,
are chemoheterotrophs.
Bacteria and fungi can use a wide variety of
organic
compounds for carbon and energy sources. This is
why
they can live in diverse environments.
Understanding mi-
crobial diversity is scientifically interesting
and economi-
cally important. In some situations microbial
growth is
undesirable, such as when rubber-degrading
bacteria de-
stroy a gasket or shoe sole. However, these same
bacteria
might be beneficial if they decomposed discarded
rubber
products such as tires, Rhodococcus erythropolis
(rơdỡ
kokkus er-i-throp,o-lis) is widely distributed
in so
can cause disease in humans and other animals.
This sam
species is able to replace sulfur atoms in
petroleum with
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