Chapter 8
Learning Objectives:
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Define metabolism, and describe the fundamental differences between anabolism and catabolism.
Identify the role of ATP as an intermediate between catabolism and anabolism.
Distinguish between competitive and noncompetitive inhibition.
Explain the term oxidation-reduction.
List and provide examples of three phosphorylation reactions that generate ATP.
Describe the chemical reactions of glycolysis (what is happening).
Explain the products of the TCA cycle.
Compare and contrast anaerobic and aerobic respiration.
Describe the chemical reactions of, and list some products of, fermentation.
Compare and contrast light-dependent and light-independent reactions of photosynthesis.
Describe the electron transport chain.
Describe the importance of NAD+ and FADH+.
METABOLISM is defined as the sum of all chemical reactions occurring within a living organism. CATABOLIC reactions
are energy-releasing (exergonic) reactions which break down more complex molecules, usually by hydrolysis, into simpler
components. The chemical processes of digestion typically occur by this route. ANABOLIC reactions are energy-requiring
(endergonic) and build more complex molecules, usually by condensation, from subunit components. The energy for anabolic
reactions is provided by catabolic reactions; they are ALWAYS LINKED. Energy is ultimately stored in the form of the
energy -rich molecule, ATP.
Bacteria use three main mechanisms of PHOSPHORYLATION to produce ATP
Energy released from certain metabolic reactions can be trapped to form ATP from ADP and a phosphate group by a process
called phosphorylation. This chemical reaction is catalyzed by an enzyme called ATP synthetase.
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Substrate level phosphorylation: occurs when ATP is formed directly by the addition of a phosphate to ADP.
Occurs both in glycolysis and the Krebs cycle.
Oxidative phosphorylation: energy in the form of electrons is released stepwise from oxidized organic compounds
(e.g. glucose) to electron carriers (usually NAD+ or FADH). Electron carriers enter a membrane-associated electron
transport system (ETS). The ETS and ATP synthetase occur on the inner mitochondrial membrane of eukaryotes and
on the plasma membrane of prokaryotes. Electrons are ultimately donated to final electron acceptors to form ATP.
The electron acceptors include:
a. molecular oxygen (aerobic respiration)
b. an inorganic molecule other than oxygen (anaerobic respiration)
Photophosphorylation: occurs only in photosynthetic organisms which trap light energy by photosynthetic
pigments and convert it to the chemical energy of ATP. This process involves an electron transport system.
Photosynthetic bacteria include cyanobacteria and the green and purple sulfur bacteria . Cyanobacteria lack
chloroplasts but possess photosynthetic membranes called thylakoids. The thylakoids possess the photosynthetic
pigments chlorophyll a, carotenoids and phycobiliproteins. The green and purple sulfur bacteria possess
bacteriochlorophyll.
What is meant by OXIDATION and REDUCTION?!
In my experience, these two concepts often give students a great deal of trouble. Oxidation and reduction reactions (also
known as REDOX reactions) are always coupled in biological systems.
Put quite simply, oxidation reactions release energy. Compounds that contain the greatest amount of stored chemical
energy are hydrocarbons such as fats and lipids. In biological systems, oxidation typically involves:
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the loss of hydrogen atoms (equivalent to protons) from a substrate
loss of hydrogen atoms are known as dehydrogenation reactions
electrons are typically lost together with hydrogen atoms
the addition of oxygen is also termed oxidation
an "oxidized" molecule has given up energy. For example, the energy carrier molecule NAD+, is an energydeficient form of an electron carrier as it has given up a hydrogen and two electrons.
Reduction reactions harness chemical energy. Reduction involves:
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the gain of electrons and hydrogen atoms by a substrate. This is what confuses people; how can a gain of something
be termed "reduction?" The answer is that:
the loss of oxygen is also termed reduction
a "reduced" molecule is energy rich. For example, NADH picks up 2 energetic electrons and a hydrogen atom.
These reactions are always coupled as the electrons lost from an oxidized molecule have to be transferred to another. A
source of electrons, or electron donor, is referred to as a reducing agent, while the electron acceptor is the oxidizing agent as
it oxidizes some other molecule and becomes reduced in so doing.
GLYCOLYSIS
Glycolysis literally means "sugar splitting." It is the most common pathway for the breakdown or oxidation of glucose and is
typically the first stage in carbohydrate catabolism.
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Involves 10 distinct chemical reactions, each catalyzed by different enzyme. Glucose is split into 2 molecules of
pyruvic acid and is accompanied by ATP formation through substrate level phosphorylations.
Glycolysis (Embden-Meyerhof Pathway) is used by eukaryotic cells, many anaerobic and facultatively anaerobic
bacteria. The Archaea do not use this pathway.
4 key events occur during glycolysis:
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During a preparatory stage, glucose becomes energized by the addition of two phosphate groups. This step also
alters the conformation of glucose and renders it incapable of leaving the cell. Two molecules of ATP are used to
energize glucose. The resulting fructose 1,6 diphosphate is subsequently split into two, three-carbon compounds.
Substrate level phosphorylation yields 2 molecules of ATP directly from ADP. As two phosphorylated 3-carbon
molecules are formed from each molecule of glucose 4 molecules of ATP are actually formed at this point. There is
a net yield of 2 ATP per glucose entering glycolysis.
For each three carbon compound a molecule of NAD+ is reduced to a molecule of NADH. The energy rich
molecules of NADH can enter the electron transport system so that more ATP can be generated by oxidative
phosphorylation.
Two molecules of pyruvate result which. Pyruvate may be fermented or enter the Krebs cycle depending on the
bacterium.
Many aerobic bacteria, such as Pseudomonas spp, and the Archaea use an alternative glycolytic pathway called the EntnerDoudoroff pathway. This pathway produces pyruvate directly from glucose and lacks some of the ATP-generating steps
found in the Embden-Meyerhof pathway. Only 1 ATP is gained from each molecule of glucose.
Regardless of the pathway used, glycolysis as an energy-yielding pathway is acceptable for simple organisms such as yeast.
However, only 2% of energy is in the form of ATP; the rest is distributed between pyruvate and heat.
THE KREBS CYCLE
The purpose of the Krebs cycle is to harness useful energy in the form of electrons, carried by the reduced coenzymes,
NADH and FADH2.
Key events of the Krebs cycle:
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Each molecule of pyruvate from glycolysis is decarboxylated (loss of CO2) to become a 2 carbon compound (acetyl
group) which attaches to coenzyme A (acetyl CoA).
The 2 carbon acetyl groups enter the cycle and pass through a series of reactions whereby chemical energy is
released stepwise.
The carbons that enter the Krebs cycle are oxidized to CO2 and are eliminated as waste
For each turn of the Krebs cycle four pairs of electrons are transferred to coenzymes; 3 pairs reduce NAD+ to form
three molecules of NADH and 1 pair of electrons reduce FADH to FADH2. Energy is extracted from these
coenzymes as they enter the electron transport system.
For each turn of the Krebs cycle (two turns), substrate level phosphorylation generates the equivalent of one ATP.
ELECTRON TRANSPORT CHAIN
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The carriers of the electron transport system occur on the plasma membrane of bacteria.
Energy carrying molecules of NADH and FADH2 donate their electrons to a series of coenzymes. The electrons are
ultimately donated to a terminal electron acceptor, which in the case of aerobic respiration, is oxygen. The electrons
are then are reunited with protons to form water.
Each carrier is reduced as it picks up electrons and oxidized as it passes them on (gives up energy)
Most ATP generated by aerobic respiration is by oxidative phosphorylation:
occurs at 3 sites at ETS so ADP is phosphorylated to ATP
FAD enters lower down the ETS and only 2 sites are utilized for this coenzyme
A TOTAL OF 38 ATP IS PRODUCED BY AEROBIC RESPIRATION IN PROKARYOTES
FERMENTATION
Fermentation can have a variety of meanings, ranging from informal to more scientific definitions. The various meanings of
fermentation are nicely summarized by Tortora et al ( Microbiology, 6th Ed, 1998):
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Any spoilage of food by microbes. For example, the spoilage of wine to vinegar. This is a very general usage of
fermentation
Any process that produces alcoholic beverages or acidic dairy products (again general use)
Any large scale microbial process occurring with or without air (industrial use)
Any energy-releasing process that occurs only under anaerobic conditions (more scientific)
Any metabolic process that releases energy from a sugar or other organic molecule, does not need oxygen or
an electron transport system, and uses an organic molecule as the final electron acceptor. It is this last
definition that we will use.
Some other key points that we need to keep in mind are:
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A complete fermentation pathway begins with a substrate, includes glycolysis and results in various end-products.
The different fermentation pathways typically are named for the end products that are formed.
As far as an energy is concerned, fermentation does not generate ATP directly but recycles a limited amount of
NAD+ back into glycolysis to keep glycolysis going. Recall that each pass through glycolysis generates 2 ATP
molecules by substrate level phosphorylation.
All fermentation pathways are anaerobic.
Cells that are capable of both respiration and fermentation will typically use respiration when possible. Respiration
yields more energy from a lot less substrate.