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Derived copy of Bis2A 06.0 Energy in
∗
Living Systems
Erin Easlon
Based on Bis2A 06.0 Energy in Living Systems† by
OpenStax College
Mitch Singer
This work is produced by OpenStax-CNX and licensed under the
Creative Commons Attribution License 4.0
‡
Abstract
By the end of this section, you will be able to:
•
•
Discuss the importance of electrons in the transfer of energy in living systems
Explain how ATP is used by the cell as an energy source
Energy production within a cell involves many coordinated chemical pathways. Most of these pathways
are combinations of oxidation and reduction reactions.
Oxidation and reduction occur in tandem.
An
oxidation reaction strips an electron from an atom in a compound, and the addition of this electron to
another compound is a reduction reaction. Because oxidation and reduction usually occur together, these
pairs of reactions are called oxidation reduction reactions, or
redox reactions.
1 Oxidation-Reduction Reactions
The chemical reactions underlying metabolism involve the transfer of electrons from one compound to another
by processes catalyzed by enzymes. The electrons in these reactions commonly come from hydrogen atoms,
which consist of an electron and a proton. A molecule gives up a hydrogen atom, in the form of a hydrogen
+
ion (H ) and an electron, breaking the molecule into smaller parts. The loss of an electron, or
oxidation,
releases a small amount of energy; both the electron and the energy are then passed to another molecule
reduction, or the gaining of an electron. These two reactions always happen together
oxidation-reduction reaction (also called a redox reaction)when an electron is passed between
in the process of
in an
molecules, the donor is oxidized and the recipient is reduced. Oxidation-reduction reactions often happen
in a series, so that a molecule that is reduced is subsequently oxidized, passing on not only the electron it
just received but also the energy it received. As the series of reactions progresses, energy accumulates that
i
is used to combine P and ADP to form ATP, the high-energy molecule that the body uses for fuel.
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∗
†
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Oxidation-reduction reactions are catalyzed by enzymes that trigger the removal of electrons (either one
or two) from a substrate (sometimes the removal coincides with the removal of a proton) and transfers then
to a second substrate, usually a coenzyme which can temporarily maintain the electrons (and sometimes
protons) before transferring then to a second compound. There are two broad classes of coenzymes that work
in redox reactions in the cell. The rst are those coenzymesm that can carry both electrons and protons, the
nicotinamide adenine dinucleotide (NAD+ ) and avin adenine dinucleotide
(FAD). Their respective reduced coenzymes are NADH and FADH2 . A third coenzyme, nicotinamide
adenine dinucleotide phosphate (NADP+ ) is the primary reductant for anabolic reactions and has
+
similar (yet distinct) properties to its unphosphorylated counterpart NAD . The second general group
of enzymes (that contain co-factors such as heme) only carry electrons, and include cytochromes, and
Iron-Sulfur (Fe-S) proteins.
Exercise 1: Review Question
(Solution on p. 12.)
two most common are
Consider a red/ox reaction that requires NAD as a co-enzyme. As the reaction proceeds NAD+
is reduced to NADH. What would happen to the reaction rate and the substrate concentration if
the NAD+ pool is xed (a nite number of molecules in the cell)?
2 Electrons and Energy
The removal of an electron from a molecule, oxidizing it, results in a decrease in potential energy in the
oxidized compound.
The electron (sometimes as part of a hydrogen atom), does not remain unbonded,
however, in the cytoplasm of a cell.
Rather, the electron is shifted to a second compound, reducing the
second compound. The shift of an electron from one compound to another removes some potential energy
from the rst compound (the oxidized compound) and increases the potential energy of the second compound
(the reduced compound).
The transfer of electrons between molecules is important because most of the
energy stored in atoms and used to fuel cell functions is in the form of high-energy electrons. The transfer
of energy in the form of electrons allows the cell to transfer and use energy in an incremental fashionin
small packages rather than in a single, destructive burst. This chapter focuses on the extraction of energy
from food; you will see that as you track the path of the transfers, you are tracking the path of electrons
moving through metabolic pathways.
2.1 Electron Carriers
In living systems, a small class of compounds functions as electron shuttles: They bind and carry high-energy
electrons between compounds in pathways. The principal electron carriers we will consider are derived from
the B vitamin group and are derivatives of nucleotides. These compounds can be easily reduced (that is, they
accept electrons) or oxidized (they lose electrons). Nicotinamide adenine dinucleotide (NAD+) (Figure 1)
+
is derived from vitamin B3, niacin. NAD
is the oxidized form of the molecule; NADH is the reduced form
of the molecule after it has accepted two electrons and a proton (which together are the equivalent of a
hydrogen atom with an extra electron).
+
NAD
can accept electrons from an organic molecule according to the general equation:
+
RH
NAD
+
Reducing
agent
Oxidizing
agent
→
NADH
Reduced
R
+
(1)
Oxidized
When electrons are added to a compound, they are reduced. A compound that reduces another is called
a reducing agent. In the above equation, RH is a reducing agent, and NAD
+
is reduced to NADH. When
electrons are removed from compound, it oxidized. A compound that oxidizes another is called an oxidizing
agent. In the above equation, NAD
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+
is an oxidizing agent, and RH is oxidized to R.
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+
2
Similarly, avin adenine dinucleotide (FAD ) is derived from vitamin B , also called riboavin.
Its
2
+ and FAD+ are extensively used in energy extraction from sugars, and NADP plays an important role
reduced form is FADH .
A second variation of NAD, NADP, contains an extra phosphate group.
Both
NAD
in anabolic reactions and photosynthesis.
Figure 1:
+
The oxidized form of the electron carrier (NAD ) is shown on the left and the reduced form
(NADH) is shown on the right. The nitrogenous base in NADH has one more hydrogen ion and two
+
more electrons than in NAD .
3 ATP in Living Systems
A living cell cannot store signicant amounts of free energy. Excess free energy would result in an increase of
heat in the cell, which would result in excessive thermal motion that could damage and then destroy the cell.
Rather, a cell must be able to handle that energy in a way that enables the cell to store energy safely and
release it for use only as needed. Living cells accomplish this by using the compound adenosine triphosphate
(ATP). ATP is often called the energy currency of the cell, and, like currency, this versatile compound can
be used to ll any energy need of the cell. How? It functions similarly to a rechargeable battery.
When ATP is broken down, usually by the removal of its terminal phosphate group, energy is released.
The energy is used to do work by the cell, usually by the released phosphate binding to another molecule,
activating it.
For example, in the mechanical work of muscle contraction, ATP supplies the energy to
move the contractile muscle proteins. Recall the active transport work of the sodium-potassium pump in cell
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membranes. ATP alters the structure of the integral protein that functions as the pump, changing its anity
for sodium and potassium. In this way, the cell performs work, pumping ions against their electrochemical
gradients.
3.1 ATP Structure and Function
At the heart of ATP is a molecule of adenosine monophosphate (AMP), which is composed of an adenine
molecule bonded to a ribose molecule and to a single phosphate group (Figure 2). Ribose is a ve-carbon
sugar found in RNA, and AMP is one of the nucleotides in RNA. The addition of a second phosphate
group to this core molecule results in the formation of adenosine diphosphate (ADP); the addition of a third
phosphate group forms adenosine triphosphate (ATP).
Figure 2:
ATP (adenosine triphosphate) has three phosphate groups that can be removed by hydrolysis
to form ADP (adenosine diphosphate) or AMP (adenosine monophosphate).The negative charges on the
phosphate group naturally repel each other, requiring energy to bond them together and releasing energy
when these bonds are broken.
The addition of a phosphate group to a molecule requires energy.
Phosphate groups are negatively
charged and thus repel one another when they are arranged in series, as they are in ADP and ATP. This
repulsion makes the ADP and ATP molecules inherently unstable.
The release of one or two phosphate
dephosphorylation, releases energy.
Video on electron and electron/proton carriers
groups from ATP, a process called
For a 7 minute YouTube video on the role of carriers in respiration clicke here
1 https://www.youtube.com/watch?v=2vzTIYroJno
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1
.
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3.2 Energy from ATP
Hydrolysis is the process of breaking complex macromolecules apart. During hydrolysis, water is split, or
+
-
lysed, and the resulting hydrogen atom (H ) and a hydroxyl group (OH ) are added to the larger molecule.
i
The hydrolysis of ATP produces ADP, together with an inorganic phosphate ion (P ), and the release of
free energy. To carry out life processes, ATP is continuously broken down into ADP, and like a rechargeable
battery, ADP is continuously regenerated into ATP by the reattachment of a third phosphate group. Water,
which was broken down into its hydrogen atom and hydroxyl group during ATP hydrolysis, is regenerated
when a third phosphate is added to the ADP molecule, reforming ATP.
Obviously, energy must be infused into the system to regenerate ATP. Where does this energy come
from? In nearly every living thing on earth, the energy comes from the metabolism of glucose. In this way,
ATP is a direct link between the limited set of exergonic pathways of glucose catabolism and the multitude
of endergonic pathways that power living cells.
Video Link
For a more detailed explanation of ATP and how this small molecule can store so much energy, take a look
at this video (10 minutes) by clicking here
2
.
3.3 Phosphorylation
Recall that, in some chemical reactions, enzymes may bind to several substrates that react with each other
on the enzyme, forming an intermediate complex. An intermediate complex is a temporary structure, and it
allows one of the substrates (such as ATP) and reactants to more readily react with each other; in reactions
involving ATP, ATP is one of the substrates and ADP is a product. During an endergonic chemical reaction,
ATP forms an intermediate complex with the substrate and enzyme in the reaction.
This intermediate
complex allows the ATP to transfer its third phosphate group, with its energy, to the substrate, a process
called phosphorylation.
Phosphorylation refers to the addition of the phosphate (∼P). This is illustrated
by the following generic reaction:
A + enzyme + ATP
→ [A −
enzyme
− ∼ P] →
B + enzyme + ADP + phosphate ion
(2)
When the intermediate complex breaks apart, the energy is used to modify the substrate and convert it into
a product of the reaction. The ADP molecule and a free phosphate ion are released into the medium and
are available for recycling through cell metabolism.
3.4 How do cells generate ATP
All cells require energy in the form of mobile packets.
As we saw earlier in the class, these packets can
be used to couple thermodynamically unfavorable reactions to drive the formation of specic products.
Remember it costs energy to build, and the primary role of a cell is to make two cells. For what ever reason,
almost 3.25 billion years of evolution has favored
held within the
phosphoanhydride
ATP
as that mobile form of energy.
It is the energy
bonds (the terminal or gamma and beta Phosphates) that store the
energy. As shown in Figure 3, many common cellular compounds have such bonds and can also be used as
an internal energy source with specic enzymes. Such compounds include all of the NTPs as well as common
intermediates such as Phosphoenol pyruvate (PEP), which we will discuss later in glycolysis. Remember Not
all carbon-Phosphate bonds are high energy, such as glucose-6-phosphate (gure 3).
2 https://www.youtube.com/watch?v=5GMLIMIVUvo
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Figure 3:
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Table of common cellular phosphorylated molecules and there respective free energies
Because ATP is the primary source of mobile energy in the cell, a variety of mechanisms have ermerged
over the 3.25 billion years of evolution to form ATP. The majority of these mechanism are modications on
two themes: direct synthesis of ATP or indirect synthesis of ATP. However all of the known reactions fall
into two basicmechanisms:
Substrate Level Phosphorylation (SLP) and oxidative phosphorylation
and will be discussed in detail below and in the next few modules. Suce it to say both mechanisms rely on
the transfer of potential energy from one high energy compound (often called the energy source) to ADP, to
synthesize ATP. This is either done directly, as in the case of Substrate level phosphorylation, or indirectly,
as is the case for oxidative phosphorylation.
Substrate Level Phosphorylation
The simplest rout to synthesize ATP is substrate level phosphorylation. ATP molecules are generated (that
is, regenerated from ADP) as a direct result of a chemical reaction that occurs in catabolic pathways. A
phosphate group is removed from an intermediate reactant in the pathway, and the free energy of the reaction
is used to add the third phosphate to an available ADP molecule, producing ATP (Figure 4).
direct method of phosphorylation is called
substrate-level phosphorylation.
This very
It can be found in a variety
of catabolic reactions, most notably in two specic reactions in glycolysis (which we will discuss specically
later). Suce it to say what is required is a high energy intermediate whose oxidation is sucient to drive
the synthesis of ATP.
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Figure 4:
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In phosphorylation reactions, the gamma phosphate of ATP is attached to a protein.
From an evolutionary perspective consider remember that the primordial ooze was hypothesized to be
enriched with highly reduced small organic compounds. These reduced high energy compounds could have
made an excellent initial energy source for early life.
The simplest method of energy extraction being
the oxidation of these compounds coupled to ATP synthesis; i.e.
Substrate level phosphorylation.
This
simple method could theoretically have yielded large quantities of ATP for the cell along with smaller more
oxidized organic compounds. Remember, we are currently only discussing ATP synthesis. If you consider
this reaction, the coupling of the oxidation of reduced orgainic molecules to ATP synthesis, we are missing
half the reaction, the compound to be reduced; ATP is not reduced during this reaction, it is simply forming
a high energy bond. The electrons need to go somewhere, and the question where do they go?
The short answer is NAD+, those co-enzymes involved in red/ox reactions. Thus the rst step in the
synthesis of ATP is drive a Phosphate group onto the high energy compound by a red/ox reaction.
The
subsequent reduction of the energy source with the simultaneous phosphorylation produces two products,
NADH (from the reduction of NAD+)and the newly formed phosphorylated compound containing a high
energy phosphate that can now be used to synthesize ATP in a second reaction. The best example of this in
current metabolism is the reduction of glyceraldehyde-3-phosphate to 1,3-bisposphoglycerate by NAD+. The
formation of 1,3-bisphosphoglycerate and a high energy phosphoanhydride can then be used to phosphorylate
ADP to ATP and production of 3-phosphoglycerate. This is diagrammed in gure 5 below.
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Figure 5
The subsequent phosphate transfer reaction to ADP to form ATP in the reaction shown in gure 6 comes
directly from the formation of 1,3-bisphosphoglycerate (described above).
The arrow points out the high
energy phosphoanhydride that is formed in the red/ox reaction. These two enzymatic reactions demonstrate
how cells can transform one form of energy into a second, the potential energy from the substrates to
potential energy in ATP. And of course the energy in ATP can then be used to help drive thermodynamically
unfavorable reactions in the cell.
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Figure 6
There is one last consideration you need to think about in our discussion of SLP. Consider the question
posed earlier in exercise 1. Remember NAD+ is the original oxidizing agent used to form 1,3-bisphosphoglycerate
from glyceraldehyde-3-phosphate. NAD+ is a co-enzyme and its levels are nite in the cell. So, as the reactions in gures 5 and 6 occur in the cell the levels of NAD+ fall and the levels of NADH rise. If left unchecked
the cell has entered a death spiral. The answer is relatively simple, regenerate NAD+ by oxidizing NADH.
This process is called
fermentation and its role in metabolism is to reoxidize NADH to NAD+ forming a
cycle between the two forms of the same molecule; as long as there is plenty of substrate to reduce NAD+
and enough substrate to oxidize NADH back to NAD+. We will discus fermentation reactions in detail in
a dierent module. But for this discussion, simply keep in mind that the cell must reoxidize NADH back
to NAD+ in order to survive. Also keep in mind, that as with all aspects of energy metabolism, the basic
process is very simple, NAD+ to NADH to NAD+ to....., yet a variety of rich complex reactions have evolved
over time. This good news for us, because the end products of fermentation reactions include many foods
and beverages we use every day. Think how boring life would be without bread, tea, beer and chocolate.
Oxidative Phosphorylation
In addition to substrate level phosphorylation, cells can generate ATP via an
electron transport chain
membraneto both sep-
coupled to the proton dependent F0F1 ATPase. Such coupled systems require a
arate charge (electrical potential) and protons (chemical potential).
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This process is refereed, called the
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Chemiosmotic hypothesis was proposed by Peter Mitchell3
in 1961.
While we will discuss this in detail later, in essence oxidative phosphorylation works by taking a reduced
high energy compound, such as NADH+H+ or FADH2, and transferring electrons in a series of redox
reactions to other compounds. The energy that is release is then "captured" by the translocation of protons
(H+) across the membrane. The net result is an "energize membrand" that separates charge, negative on
the inside and positive on the outside (the electrical potential) and the separation of protons (chemical
potential), protons concentration greater on the outside then inside of the membrane. The overall energized
state of the membrane is then similar to a charged capacitor ready to do work. ATP generation occurs by
the F0F1 membrane bound ATPase that translocates protons across the membrane discharging the chemical
and electrical potentials with the simultaneous synthesis of ATP. For every three protons (H+) translocated,
the F0F1ATPase can synthesize 1 ATP.
Finally, oxidative phosphorylation does NOT require oxygen to function. The word oxidative in this case
is in reference to redox reactions driving the process. We are very familiar with aerobic respiration which
uses molecular oxygen as the terminal electron acceptor in some electron transport chains but this is a subset
of reactions. As we will see later, other electron transport chains can use other compounds besides molecular
oxygen as a terminal electron acceptor, such as nitrate or nitrite.
Photophosphorylation
Finally, many organisms can convert the energy from sun light to chemical energy (ATP) which is half
of the reactions used in photosynthesis; the other half being the conversion of CO2 to carbohydrates. In
photosynthetic organisms, ATP and reducing power (production of NADPH+H+)is generated via a modied
electron transport chain where the initial high energy electon(s) is derived from light energy instead of
chemical energy from NADH+H+.
While we will discuss this in detail later, in essence oxidative phosphorylation works by taking a reduced
high energy compound, such as NADH or FADH2, and transferring electrons in a series of red/ox reactions to
other compounds. The energy that is release is then "captured" by the translocation of protons (H+) across
the membrane.
The net result is an "energize membrane" that separates charge, negative on the inside
and positive on the outside (the electrical potential) and the separation of protons (chemical potential),
protons concentration greater on the outside then inside of the membrane. The overall energized state of
the membrane is then similar to a charged capacitor ready to do work.
ATP generation occurs by the
F0F1 membrane bound ATPase that translocates protons across the membrane discharging the chemical
and electrical potentials with the simultaneous synthesis of ATP. For every three protons (H+) translocated,
the F0F1ATPase can synthesize 1 ATP.
4 Section Summary
ATP functions as the energy currency for cells. It allows the cell to store energy briey and transport it
within the cell to support endergonic chemical reactions. The structure of ATP is that of an RNA nucleotide
with three phosphates attached. As ATP is used for energy, a phosphate group or two are detached, and
either ADP or AMP is produced.
Energy derived from glucose catabolism is used to convert ADP into
ATP. When ATP is used in a reaction, the third phosphate is temporarily attached to a substrate in a
process called phosphorylation. The two processes of ATP regeneration that are used in conjunction with
glucose catabolism are substrate-level phosphorylation and oxidative phosphorylation through the process
of chemiosmosis.
5 Additional Links
Chemwiki Links
• ATP4
3 http://www.life.illinois.edu/crofts/bioph354/mitchell.html
4 http://chemwiki.ucdavis.edu/Wikitexts/Sacramento_City_College/SCC%3A_Chem_309/Chapters/18._Metabolism/18.1%3A_ATP%3A_T
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Insert paragraph text here.
6 Review Questions
Exercise 2
(Solution on p. 12.)
The energy currency used by cells is ________.
a. ATP
b. ADP
c. AMP
d. adenosine
Exercise 3
(Solution on p. 12.)
A reducing chemical reaction ________.
a. reduces the compound to a simpler form
b. adds an electron to the substrate
c. removes a hydrogen atom from the substrate
d. is a catabolic reaction
7 Free Response
Exercise 4
(Solution on p. 12.)
Why is it benecial for cells to use ATP rather than energy directly from the bonds of carbohydrates? What are the greatest drawbacks to harnessing energy directly from the bonds of several
dierent compounds?
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Solutions to Exercises in this Module
Solution to Exercise (p. 2)
As the reaction proceeds, NAD+ is reduced to NADH, if the NAD+ concentration is xed, and NADH is
not recycled the reaction will stop (or sit at some equilibrium)with no net increase in NADH because the
NAD+ pool becomes so low.
to Exercise (p. 11)
A
to Exercise (p. 11)
B
to Exercise (p. 11)
ATP provides the cell with a way to handle energy in an ecient manner. The molecule can be charged,
stored, and used as needed. Moreover, the energy from hydrolyzing ATP is delivered as a consistent amount.
Harvesting energy from the bonds of several dierent compounds would result in energy deliveries of dierent
quantities.
Glossary
Denition 1: chemiosmosis
process in which there is a production of adenosine triphosphate (ATP) in cellular metabolism by
the involvement of a proton gradient across a membrane
Denition 2: dephosphorylation
removal of a phosphate group from a molecule
Denition 3: oxidative phosphorylation
production of ATP using the process of chemiosmosis and oxygen
Denition 4: phosphorylation
addition of a high-energy phosphate to a compound, usually a metabolic intermediate, a protein,
or ADP
Denition 5: redox reaction
chemical reaction that consists of the coupling of an oxidation reaction and a reduction reaction
Denition 6: substrate-level phosphorylation
production of ATP from ADP using the excess energy from a chemical reaction and a phosphate
group from a reactant
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