Chapter 6
How Cells Harvest Chemical Energy
Overview
All living organisms require energy to survive, grow and reproduce. In this chapter we will examine how cells
produce ATP from carbohydrates and other nutrients. First, we will explore the relationship between cellular
respiration (and glycolysis) and photosynthesis to further understand the "web of life".
The second part of this unit will examine the stages of cellular respiration by following the path of a glucose
molecule through the cell. From glycolysis, to the Kreb's cycle and finally the electron transport chain, we will
learn the major components of each pathway. Finally, we will explore alternative to cellular respiration in
environments where oxygen is limited, such as those present in the early stages of out planet's development.
All of this can look very confusing at first, and is far more complicated than we will be covering. Take each step
slowly and individually, and don’t get scared by looking at the figures. This does make sense, but will require
some careful understanding, and yes, some rote memorization. You need to focus on the major happenings in
each pathway, including:
 Number of ATP produced and the method by which they are produced
 Cellular location of the pathway
 Inputs and outputs of the pathway (which chemical compounds go in and which come out)
 Key molecules and terms
Assigned Reading
Text, Pages 89-103
PowerPoint Presentation
Chapter Review, Page 104
Testing Your Knowledge, Page 105
Key Terms
 glycolysis
 Citric acid (Krebs) cycle
 electron transport systems
 pyruvate
 mitochondria
 cytoplasm
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NADH, FADH
ATP, ADP
fermentation
chemiosmosis
phosphorylation
oxygen
acetyl-CoA
aerobic and anaerobic
Introduction
In this unit we will examine how organisms can release this energy for use within
the cell. Energy may be released from storage molecules such as glucose by
pathways that involve oxygen (aerobic pathways) and pathways that are
independent of oxygen (anaerobic pathways). Since oxygen was almost nonexistent on the early Earth, the anaerobic pathways represent the more ancient
methods of releasing the energy in chemical bonds.
A key concept here is that oxygen is utilized within the mitochondria of eukaryotic
cells. So a good concept is to remember that aerobic pathways will occur in the
mitochondria, while anaerobic will occur in the cytoplasm.
Glycolysis
The term glycolysis means "sugar breaking". When we break chemical bonds, energy and electrons are
released. We will use the energy to create ATP from ADP and will harvest the electrons to create energy as well.
Energy reactions often require an input of energy
and glycolysis is not an exception. Glycolysis has
both an energy investment and energy releasing
phase. Examine Figure 6.7c—notice at the top that
we need to invest 2 ATP to start the reaction, while
at the end of the reaction we get 4 ATP out for a net
production of 2 ATP.
In glycolysis we break the 6 carbon glucose to form
two 3-carbon molecules called pyruvate. Like
photosynthesis, we need to have a molecule to
attract electrons.
In cellular respiration, the
molecule is NAD+, which when combined with an
electron, forms NADH.
Notice how many steps this simple process requires. Each step has enzymes which must be regulated. You do
not need to know every step, but you do need to recognize the inputs/outputs of each stage.
Formation of Acetyl-CoA
Literally for billions of years the glycolysis pathway described above was the primary method of getting energy
from biomolecules. However, after the levels of oxygen on the planet rise to significant levels, cells begin to use
the power to oxygen to "burn" biomolecules. However, this requires a specialized area of the cell, the
mitochondria.
The pyruvate leaving glycolysis must be converted to a compound called acetyl-CoA before it may be utilized in
the next pathway. This is essentially a rearrangement step - but an important one. While we will be talking
about glucose in this lecture, you already know that your body can use both proteins and fats as energy as well.
Many of these biomolecules may be converted to acetyl-CoA. So this molecule acts as a key point in the use of
biomolecules to produce ATP.
Note that since each glucose is split to form 2 pyruvates in glycolysis, 2 acetyl-CoA molecules enter the next
cycle. Figure 6.8 (preparatory steps) shows that this process releases electrons which are captured by NAD+ to
form NADH, and some carbon dioxide as well.
Citric Acid Cycle
This is also called the Krebs cycle (in honor of the man
who first described it), though the text refers to it as
the citric acid cycle. Be aware of both names, and that
they mean the same process.
Finally we are in the mitochondria and ready to "burn"
our food for energy. But as you can see by Figure
6.9B, this is a very complicated pathway. The first
thing that you should notice is that the pathway is
circular. The outputs of the pathway (oxaloacetate)
are used to start the pathway over again. With the
exception of acetyl-CoA and oxaloacetate, we will not
learn the other intermediate molecules.
This process effectively uses oxygen to break the C-C bonds found in the acetyl-CoA (which was pyruvate). As
the bonds are broken, energy may be harvested (to form ATP), but usually electrons are released and then
captured by two molecules. One of these is NAD+ (same as the previous 2 steps), but the other is FAD+. For our
purposes these molecules will be treated the same, but there are some important differences.
So what are the outputs of this pathway. Well you will notice that 2 carbons enter the cycle (one of the carbons
from pyruvate was lost in the acetyl-CoA step) and that 2 CO2 molecules depart. Thus after the Krebs cycle all of
the carbons leave the cycle as CO2. We also produce 1 ATP, 3 NADH and 1 FADH2. But for each glucose that
entered the cycle, 2 acetyl-CoA molecules are eventually produced. Thus we need to double the outputs of this
pathway. So the Krebs cycle produces 2 ATP, the same as glycolysis! Since we use oxygen to generate the ATP,
this is often called oxidative phosphorylation.
Oxidative Phosphorylation
The processes above have only given us 4 ATP for
each glucose entering the system. This is simply not
efficient. But what has been generated is 10 NADH
and 2 FADH2 molecules. These molecules have
captured electrons and we can use these electrons.
The process is much the same as above. We will
hand the electrons over to a series of proteins
located in a membrane and then generate an
electrical potential.
Yet instead of using this
electrical potential to make C-C bonds, we will use
them to generate large amounts of ATP.
Take a good look at Figure 6.10. Notice that the
NADH and FADH2 molecules enter the inner
compartment of the mitochondria and release
electrons. Once the electrons are handed off to the integral proteins, we regenerate NAD+ and FAD+, which will
then return to the pathways above (recycled).
We can then use the electrons to generate ATP. This process produces between 28 and 32 ATP (depending on
the cell). All told, from glycolysis on we generate about 36 ATP per glucose for an efficiency of about 39%. Not
bad for a biological organism!
Notice that the end of the ETS uses oxygen to accept the electrons to form water.
Anaerobic Pathways
Many prokaryotic organisms do not require oxygen to extract energy from glucose. However, in the aerobic
pathways the ETS uses oxygen to recycle NAD+ and FAD+. In anaerobic pathways, another method must be
found. Figure 6.13A and Figure 6.13B show two different methods by which this may occur.
While we may think that these pathways are not important, in actuality they are. Lactic acid fermentation may
be used to make cheese and is also used by our muscles when our circulatory system does not deliver enough
oxygen to the tissues. Alcohol fermentation is used to produce alcoholic beverages and other alcohol-related
compounds.
Lactate Formation
Ethanol Formation
Concepts
 Recognize the relationship between energy-releasing and energy-acquiring pathways
 Know the difference between aerobic and anaerobic respiration
 For glycolysis, the citric acid cycle and the electron transport chain, know the following: cellular location,
inputs to the stage (chemical compounds), outputs (chemical compounds), number of ATP produced.
 Understand how certain poisons work by affecting steps of oxidative phosphorylation.
 Understand lactic acid and alcohol fermentation.
 Understand how fats, carbohydrates, and proteins are broken down to enter the aerobic respiration
process.
Review Material
MyBiology.com—Study guides and resource for this text. Specifically look at MP3 Tutor and all Web Activities