Download Overview of Cellular Respiration

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Overview of Cellular Respiration
∗
Robert Bear
David Rintoul
This work is produced by OpenStax-CNX and licensed under the
Creative Commons Attribution License 4.0†
Abstract
This is a general overview of cellular respiration.
Introduction
Surely the mitochondrion that rst entered another cell was not thinking about the future benets
of cooperation and integration; it was merely trying to make its own living in a tough Darwinian
world.
Stephen Jay Gould, in Wonderful Life: the Burgess Shale and the Nature of History, (1990)
All living organisms require energy, and for all organisms this energy comes from the chemical energy
found in compounds that they acquire from their environment. The mitochondrion, a descendent of an
aerobically-respiring bacteria, is the site of energy generation in eukaryotes. As we learned previously, the
process of photosynthesis uses solar energy (sunlight) and converts this energy into chemical energy in the
form of carbohydrates. In order for the chemical energy in the carbohydrates to be made available to do
cellular work, the energy must be converted into a useable form known as ATP. Adenosine Triphosphate is
the energy currency of the cell, and everything you do from walking down the street to reading this book
requires energy in the form of ATP. Organisms need a constant supply of ATP, and the potential energy
stored in food is the source of energy to meet this need. By connecting all this together, you should realize
that your daily activities are fueled by the energy from the sun and that even on the cellular level nutrients
cycle and energy ows (Figure 1).
∗ Version
1.7: Jul 9, 2014 9:47 am -0500
† http://creativecommons.org/licenses/by/4.0/
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Figure 1: This image illustrates the relationship between photosynthesis and cellular respiration. (Image
by Eva Horne and Robert Bear)
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All organisms need ATP, but not all organisms use the same pathways to generate ATP from the food
that is consumed. Aerobic cellular respiration, the main subject of this chapter, uses oxygen (O2 ) and
glucose to generate ATP. Organisms (plants, animals, fungi and microbes) that live in an oxygen (O2 ) rich
environment use this process to generate ATP. The overall equation for aerobic cellular respiration is the
reverse of photosynthesis, is an exergonic reaction, and supplies the ATP for cellular functions (Figure 2).
Figure 2: This image illustrates the overall equation for aerobic cellular respiration and how the amounts
of free energy diers between the reactants and the products. (Image by Robert Bear)
As the aerobic cellular respiration equation shows (Figure 2), an organism needs to acquire the O2 from
its surroundings and to get rid of the CO2 that is produced. The acquisition of O2 and the release of CO2 is
accomplished in a variety of ways. In single celled organisms, the movement of O2 and CO2 (gas exchange)
is done by simple diusion. However, in complex organisms there are specialized organs that allow for gas
exchange; for example, gills in aquatic organisms and lungs in terrestrial animals.
A common misconception is that plants do not undergo cellular respiration because they make their
own energy by photosynthesis. Plants do perform cellular respiration using the carbohydrates produced via
photosynthesis; this occurs in tissues that are not photosynthetically active (e.g., roots), as well as in leaves
and stems. Approximately half of the glucose produced by photosynthesis is consumed by the plant, mostly
to generate ATP during aerobic cellular respiration. Other uses of glucose in the plant include synthesis of
cell walls, starch, and other plant carbohydrates. So, plants harvest light energy via photosynthesis, making
carbohydrates, and then they use the energy stored in those carbohydrates to perform various cellular
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functions. This is the reason why they are called autotrophs, or self feeders.
Another pathway that organisms can use to extract energy from carbohydrates is anaerobic cellular
respiration; this process occurs in the absence of oxygen. In this chapter, we will explore one type of
anaerobic cellular respiration called fermentation. You may already be familiar with a one type of fermentation, lactic acid fermentation, especially if you have recently over-exerted your muscles. Anaerobic cellular
respiration is used by many organisms to produce ATP when oxygen is not available and thus pathways
which require oxygen cannot be used. The amount of ATP produced by fermentation is much less then that
produced by aerobic cellular respiration, so there is a cost and benet associated with organisms utilizing
fermentation.
1 Summary of Aerobic Cellular Respiration
Aerobic cellular respiration (Figure 3) is series of linked chemical reactions that can be best understood if
it is separated into four stages. These are glycolysis, pyruvate oxidation, the Krebs Cycle, and oxidative phosphorylation. Similar to photosynthesis, cellular respiration uses a series of oxidation-reduction
reactions. During these reactions, electrons are stripped from the chemical bonds of the original glucose
molecule and eventually added to oxygen, via a series of intermediate steps. This series of reactions releases
small amounts of energy at each step; this energy is used to drive the formation of ATP. This section is a
brief introduction to the stages of aerobic respiration with more detail to follow in the chapter.
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Figure 3: This image illustrates aerobic cellular respiration. (Image by Eva Horne and Robert Bear)
The rst stage of cellular respiration is called Glycolysis and occurs in the cytoplasm of the cell. During
glycolysis, 1 glucose molecule (with 6 carbon atoms) is broken down into 2 pyruvate molecules (with three
carbon atoms each). This is accompanied by the production of a few ATP molecules and the storage of some
high-energy electrons on the electron carrier NADH. Note that no O2 is needed for this set of reactions,
which means that glycolysis can proceed in the absence of oxygen.
The second stage is a short series of reactions called the oxidation of pyruvate during which pyruvate (3
carbon atoms) is converted to Acetyl-CoA (two carbon atoms), accompanied by the production of CO2 (one
carbon atom). This process occurs on the mitochondrial inner membrane, and as a result the Acetyl-CoA is
formed inside the mitochondria. Pyruvate is made in the cytoplasm, and this step moves the next compound
in the pathway into the mitochodria. This is critical, since all subsequent steps in the pathway occur within
the mitochondria. The other important event of this stage is the addition of high-energy electrons to NAD+ ,
generating another molecule of the electron carrier NADH.
Acetyl-CoA enters into the Krebs cycle, a series of mitochondrial reactions that completes the breakdown of the original glucose, thereby releasing CO2 . In this third stage of the process, energy is harvested
in the form of high-energy electrons being used to generate NADH as well as another high-energy electron
carrier, FADH2 . The reactions of the Krebs cycle also produce a small amount of ATP.
So far, a minimal amount of ATP has been produced, but a lot of energy has been stored in the electron
carriers NADH and FADH2 . In the nal stage of aerobic cellular respiration, Oxidative Phosphorylation,
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a series of enzymes known as the electron transport chain uses those high-energy electrons to produce a
large amount of ATP. The high energy electrons harvested in the rst three stages, and ferried by electron
carriers (NADH and FADH2 ) to the electron transport chain, are used to produce large amounts of ATP
vir the mitochondrial membrane protein known as the ATP synthase. During this nal stage is also when
atmospheric oxygen is used as the nal electron and hydrogen ion acceptor, in a reaction which produces
water. The need for O2 in this nal step means that these reactions are part of aerobic cellular respiration.
2 Location and Structures of Aerobic Cellular Respiration
All eukaryotic cells (protists, fungi, plants and animals) have mitochondria, and mitochondria are often called
the power plants of the cell because these organelles produce a large amount of ATP. As you may remember
from a previous module, the mitochondrion is an organelle that is hypothesized to have originated as an
endosymbiotic aerobic bacteria. Some of the evidence for this hypothesis comes from the relationship of the
functional parts of the mitochondria (Figure 4) to the structure of a typical aerobic bacteria. There is an
outer membrane which denes the organelle and represents the membrane which enveloped the bacteria
when it was taken into the cell via endocytosis. The inner membrane represents the plasma membrane
of the bacteria; the inner and outer membranes together form the intermembrane space. The inner
membrane is highly folded; these folds are called cristae. The extensive folding increases the surface area
for the numerous electron transport chain enzymes and the ATP synthases that are used to make ATP. In
bacteria all of these enzymes are packed into the plasma membrane, as one would expect if the endosymbiotic
hypothesis is correct. The production of ATP is driven by a concentration gradient between the outer and
inner compartment; in aerobic bacteria this concentration gradient is between the inside and the outside of
the cell. The innermost compartment, derived from the cytoplasm of the ancestral bacteria, is called the
matrix, and this compartment (just like the cytoplasm of today's bacteria) contains ribosomes and DNA;
It is also the location of the Krebs Cycle reactions.
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Figure 4: This image illustrates the structures within the mitochondria. (Image by Eva Horne and
Robert Bear)
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