Download 3.1 - Introduction to Metabolism (Taken from Biology 12, MHR, 2011

3.1 - Introduction to Metabolism
(Taken from Biology 12, MHR, 2011)
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Metabolism is the sum of all the biochemical reactions in a cell.
Catabolic reactions break down molecules and release energy while anabolic
reactions require energy to build up molecules.
Cellular respiration is an example of a catabolic pathway while photosynthesis is an
example of an anabolic pathway.
Metabolic pathways are sequences of reactions that use the products of one reaction as
the substrate for the next. Each reaction is catalyzed by an enzyme.
The downhill reactions of catabolism can be used to drive the uphill reactions of
anabolism.
This transfer of energy is known as “energy coupling”.
How is Energy Converted From One Form to Another?
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Energy is the ability to do work, and it can be classified as kinetic or potential.
Chemical energy can be tapped when chemical reactions rearrange the atoms of
molecules in such a way that potential energy (P.E.) stored in the molecules is
converted to kinetic energy (K.E.).
Cellular respiration releases energy stored in sugars and make that energy available to
do cellular work.
The chemical energy stored in these fuel molecules had themselves been derived from
light energy by plants during photosynthesis.
Bond energy is the energy required to break (or form) a chemical bond.
The amount of chemical potential energy possessed by compounds is less than the
amount of chemical potential energy possessed by the atoms they contain.
Laws of Thermodynamics
1st Law of Thermodynamics
Energy cannot be created nor destroyed, but can be transformed from one type into another
and transferred from one object to another.
2nd Law of Thermodynamics
Every energy transfer or transformation increases the entropy (disorder or randomness) of the
universe.
Endergonic (Anabolic) Reactions
 Endergonic (anabolic) reactions require energy.
 In an endergonic reaction, the products of the reaction contain more energy than the
reactants, and energy must be supplied for the reaction to proceed
Exergonic (Catabolic) Reactions
 Exergonic (catabolic) reactions release energy.
 In an exergonic reaction, the products contain less energy than the reactants, and excess
energy is released.
Thermodynamics and Metabolism
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In cells, energy from catabolic reactions is used to power anabolic reactions.
The source of energy that links these sets of reactions is ATP – adenosine
triphosphate (nitrogenous base adenine with a chain of three phosphates groups
attached to the ribose).
ATP hydrolysis releases energy to drive endergonic reactions, and it is synthesized with
energy from exergonic reactions.
Electron Carriers
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A redox reaction is a chemical reaction involving the transfer of one or more electrons
from one atom to another.
When a compound accepts electrons, it becomes reduced, and when it loses electrons, it
becomes oxidized.
Electron carriers donate electrons from energy-rich to low-energy compounds.
Two important electron carriers in metabolic reactions are NAD+ (nicotinamide adenine
dinucleotide) and FAD (flavin adenine dinucleotide).
NAD+ and FAD are the oxidized forms, and NADH and FADH2 are the reduced forms.
HOMEWORK:
pg 121 #1-14
3.2 – CELLULAR RESPIRATION: AN OVERVIEW
(Taken from Biology 12, MHR, 2011)
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It is important to remember that metabolism is the sum of all the chemical reactions
carried out by an organism.
It involves both catabolic reactions (breakdown of complex molecules into smaller ones)
and anabolic reactions (building of more complex molecules from simpler ones).
Catabolic reactions break down molecules and release energy while anabolic
reactions require energy to build up molecules.
Cellular respiration is an example of a catabolic pathway while photosynthesis is an
example of an anabolic pathway.
Oxidation and Reduction
 A redox reaction is a chemical reaction involving the transfer of one or more
electrons from one atom to another.
 oxidation: chemical reaction in which an atom loses one or more electrons.
 reduction: chemical reaction in which an atom gains one or more electrons.
 When a compound accepts electrons, it becomes reduced, and when it loses
electrons, it becomes oxidized.
 When redox reactions occur, the reduced form of a molecule always has more
potential energy than the oxidized form of the molecule.
 Redox reductions play a key role in the flow of energy through living things because
the electrons that are flowing from one molecule to the next are carrying energy with
them.
(Carter-Edwards, Gerards, Gibbons, McCallum, Noble, Parrington, Ramlochan, Ramlochan,
2011), (Damon, McGonegal, Tosto, Ward, 2007, pg 218)
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One way to remember the general meaning of oxidation and reduction is to think of the
words OIL RIG.
OIL = Oxidation Is Loss (of electrons)
RIG = Reduction Is Gain (of electrons)
(Damon, McGonegal, Tosto, Ward, 2007, pg 218)
 The sequence of chemical reactions and energy changes that occurs in aerobic
respiration may seem complex, but keep in mind the overall equation:
C6H12O6(aq) + 6O2(g)  6CO2(g) + 6H2O(l)
glucose + oxygen  carbon dioxide + water
 It provides you with a reminder of the three overall goals of the process:
1. to break the bonds between the six carbon atoms of glucose, resulting in six carbon
dioxide molecules
2. to move hydrogen atom electrons from glucose to oxygen, forming six water molecules
3. to trap as much of the free energy released in the process as possible in the form of ATP
 The entire process occurs in four stages and in three different places within the cell:
 Stage 1: Glycolysis - a 10-step process occurring in the cytoplasm
 Stage 2: Pyruvate oxidation - a one-step process occurring in the mitochondrial
matrix
 Stage 3: The Krebs cycle (or the citric acid cycle) - an eight-step cyclical process
occurring in the mitochondrial matrix
 Stage 4: Electron transport and chemiosmosis (oxidative phosphorylation) - a
multistep process occurring in the inner mitochondrial membrane
http://courtneystanifer.edublogs.org/
This diagram summarizes the metabolism of glucose by aerobic respiration. The maximum total yield
of ATP from cellular respiration in eukaryotes is 36 (2 + 2 + 4 + 6 + 22) and in prokaryotes is 38 (2 + 2 +
6 + 6 + 22). These numbers reflect the fact that the yield of ATP from total glycolytic NADH is 4 ATP in
eukaryotes and 6 ATP in prokaryotes.
Biology 12 Blackline Masters
BLM 3-11 Metabolism of Glucose by Aerobic Respiration
Copyright © 2011 McGraw-Hill Ryerson Limited
978-0-07-106045-5
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The goal of capturing as much of the available energy as possible in the form of ATP is
accomplished through two distinctly different energy-transfer mechanisms called:
substrate-level phosphorylation and oxidative phosphorylation.
Substrate-Level Phosphorylation
 ATP is formed directly in an enzyme-catalyzed reaction. (ADP + P  ATP)
 A phosphate-containing compound (phosphoenolpyruvate – PEP) transfers a phosphate
group directly to ADP, forming ATP.
 For every glucose molecule processed, six ATP are made this way – 4 in Glycolysis and 2
in Krebs Cycle.
http://yushang-sbi4u.blogspot.ca/
Oxidative Phosphorylation
 ATP is formed indirectly, it involves a series of redox reactions, with oxygen as the
final electron acceptor.
 Nicotinamide adenine dinucleotide (NAD+) or flavin adenine dinucleotide
(FAD) remove two hydrogen atoms (two protons and two electrons) from a portion of
the original glucose molecule.
 For NAD+, both electrons and only one of the protons attach to the NAD+ to produce
NADH + H+ - the other proton just dissolves in the surrounding solution as H+(aq).
 NAD+ reduction occurs in one reaction of glycolysis (Stage 1), during pyruvate oxidation
step (Step 2), and in three reactions of Krebs cycle (Stage 3).
 For FAD, both protons and both electrons bind directly onto the FAD to produce FADH2.
 FAD is reduced to FADH2 in one of the reactions of Krebs cycle (Stage 3).
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Limited amount of ATP is generated in the first three stages; their main function is to
produce electrons for the electron transport chain (Stage 4) which produces most of the
ATP during respiration.
3.2 – CELLULAR RESPIRATION
Stage 1: Glycolysis
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Glycolysis consists of 10 reactions through which glucose (6-carbon sugar) is broken into
2 pyruvate (pyruvic acid) molecules (3-carbon).
It is accompanied by the net production of 2 ATP and the transfer of H to NAD+.
Glycolysis takes place in the cytoplasm (cytosol) of the cell and it uses no oxygen
therefore, can occur in aerobic or anaerobic environments.
Glycolysis occurs in both prokaryotic and eukaryotic cells.
Glycolysis involves a series of steps but can be explained in 3 main stages.
Phosphorylation
Adding a phosphate group is called phosphorylation.
Two molecules of ATP are used to provide phosphate groups to glucose (hexose) to form
fructose-1, 6-bisphosphate (hexose bisphosphate).
By adding the two phosphate groups to glucose, the energy level of the compound is
increased which makes the next reactions possible.
(Allott, 2007, pg 73), (Damon, McGonegal, Tosto, Ward, 2007, pg 219)
(Damon, McGonegal, Tosto, Ward, 2007, pg 219)
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Lysis
Splitting molecules is called lysis.
The fructose-1, 6-bisphosphate is split into two 3-carbon sugars called glyceraldehyde-3phosphate (G3P).
(Damon, McGonegal, Tosto, Ward, 2007, pg 219)
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Oxidation and ATP Formation
Recall – oxidation is the removal or loss of hydrogen atoms from a substance.
Each G3P (triose phosphate) molecule undergoes oxidation.
Two hydrogen atoms are removed from each G3P molecule to form a reduced molecule
of NAD+, which is NADH.
The energy released by the oxidation is used to add an inorganic phosphate to the
remaining 3-carbon compound. This results in a compound with 2 phosphate groups.
The two phosphate groups are removed by enzymes so that they can be added to ADP
to form ATP.
The remaining compound is pyruvate.
The end result is the formation of 4 ATP molecules, 2 NADH molecules, and 2 pyruvate
molecules.
(Allott, 2007, pg 73), (Damon, McGonegal, Tosto, Ward, 2007, pg 220)
(Damon, McGonegal, Tosto, Ward, 2007, pg 220)
Summary of Glycolysis
 2 ATP are used to begin the process.
 4 ATP are produced  net gain of 2 ATP.
 2 NADH are produced.
 2 pyruvate molecules are present at the end of the process.
 Involves substrate-level phosphorylation, lysis, oxidation and ATP formation.
 Occurs in the cytoplasm of the cell.
 This metabolic pathway is controlled by enzymes. When ATP levels in cell are high,
feedback inhibition will the first enzyme of the pathway. This will slow down or stop the
process.
 Once pyruvate is obtained, the next pathway is determined by the presence of oxygen.
 If oxygen is present, pyruvate enters the mitochondria and aerobic respiration occurs.
 If oxygen is not present, anaerobic respiration occurs in the cytoplasm. Pyruvate is
converted to lactate in animals and ethanol and carbon dioxide in plants.
(Damon, McGonegal, Tosto, Ward, 2007, pg 220)
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Overall chemical equation for glycolysis is:
Glucose + 2ADP + 2Pi + 2NAD+  2pyruvate + 2ATP + 2(NADH + H+) + 2H20
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The following is the energy yield for glycolysis:
4 ATP produced
2 ATP used
2 ATP produced net
2 NADH + H+ produced
(may be used by the cell immediately)
(may be further processed by some cells to obtain
more ATP)
Mitochondria
 rod-shaped organelles known as the ‘powerhouse’ of the cell
 organelle with a double membrane  outer membrane is smooth however the inner
membrane is folded into cristae
 composed of a fluid-filled matrix
 site of aerobic cellular respiration (produces ATP)
 contain their own DNA  a circular chromosome similar to that of a bacterial cell
 only present in eukaryotic cells
 produces and contains its own ribosomes (70S type  similar to prokaryotic cells)
(Damon, McGonegal, Tosto, Ward, 2007, pg 23)
Stage 2: Pyruvate Oxidation (The Link Reaction)
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If oxygen is present, the 2 pyruvate molecules formed in glycolysis are transported
through the two mitochondrial membranes (via active transport) into the matrix, where
the following three reactions occur:
A CO2 is removed (decarboxylation reaction) forming a 2-carbon acetyl group.
NAD+ oxidizes the remaining 2-carbon portion. In the process, NAD+ gains 2H atoms to
become NADH + H+ (reduction).
A sulfur-containing compound called coenzyme A (CoA) attaches to the acetyl group,
forming acetyl-CoA.
 Overall equation for pyruvate oxidation is:
2 pyruvate + 2 NAD+ + 2 CoA  2 acetyl-CoA + 2 NADH + 2 H+ + 2 CO2
(Carter-Edwards, Gerards, Gibbons, McCallum, Noble, Parrington, Ramlochan, Ramlochan,
2011)
Stage 3: Krebs Cycle (Tricarboxylic Acid Cycle)
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The Krebs cycle has 8 steps and each step is catalyzed by a specific enzyme. (See pg
102)
It occurs in the matrix of the mitochondrion.
For each turn of the Krebs cycle, 2 carbons enter in the relatively reduced form of the
acetyl group and 2 different carbons leave in the completely oxidized form of CO2.
Acetyl-CoA (2C) enters the cycle at step 1 and reacts with oxaloacetate (OAA) (4C) to
form citrate (6C), a 6-carbon molecule; CoA is then released (can be used to process
another pyruvate molecule in the pyruvate oxidation step).
Citrate (6C) is oxidized to form a 5C compound. In the process, decarboxylation takes
place (a carbon dioxide molecule is released) and NAD+ is reduced to form NADH + H+.
The 5C compound is then oxidized and decarboxylated to form a 4C compound. A
carbon dioxide molecule is released and NAD+ is reduced to form NADH + H+.
The 4C compound undergoes a series of changes to become oxaloacetate again. In the
process, another NADH is produced, coenzyme FAD is reduced to form FADH2 and there
is a reduction of an ADP to form ATP.
Throughout the cycle, 2 CO2 molecules are released; 3 NADH, 1 ATP and 1 FADH2
are produced. The oxaloacetate (OAA) is free to bind to the next acetyl-CoA.
Overall chemical equation for one Krebs cycle is:
oxaloacetate + acetyl-CoA + ADP + Pi + 3 NAD+ + FAD -> CoA + ATP + 3 NADH
+ 3 H+ + FADH2 + 2 CO2 + oxaloacetate
(Oxaloacetate – product & reactant to show that process is cyclic)
(Allott, 2007, pg 74)
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Krebs cycle occurs twice for each molecule of glucose processed. (Figure 17, pg 103)
Therefore, for each glucose metabolized the following products will result: 4 CO2
molecules released, 6 NADH molecules (allow energy storage and transfer), 2 ATP
molecules, and 2 FADH2 molecules.
 By the end of Krebs cycle, all six carbon atoms of glucose have been oxidized to CO2 and
released from the cell as waste.
 All that is left of the original glucose molecule is most of its energy, which is stored in
the form of four ATP (2 from glycolysis, 2 from Krebs cycle) and 12 reduced
coenzymes (2 NADH from glycolysis, 2 NADH from pyruvate oxidation, 6 NADH from
Krebs cycle, and 2 FADH2 from Krebs cycle).
 Most of the energy stored in NADH and FADH2 will eventually be transferred to ATP in
the last stage of cellular respiration (electron transport and chemiosmosis).
(Carter-Edwards, Gerards, Gibbons, McCallum, Noble, Parrington, Ramlochan, Ramlochan,
2011), (Damon, McGonegal, Tosto, Ward, 2007, pg 222)
Stage 4: Electron Transport Chain and Chemiosmosis
(Oxidative Phosphorylation)
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The electron transport chain occurs on the inner mitochondrial membrane and on the
membranes of the cristae.
The NADH and FADH2 that have been made so far, transfer the H atom electrons (e-‘s)
they carry to a series of compounds, mainly proteins, which are embedded in the inner
mitochondrial membrane (cristae), called the electron transport chain (ETC).
The components of the ETC are arranged in order of increasing electronegativity. From
weakest to strongest, they are: NADH dehydrogenase, Ubiquinone (Q), Cytochrome b-c1
complex, Cytochrome c, and Cytochrome oxidase complex.
NADH passes its electrons on to the first protein complex, NADH dehydrogenase, and
FADH2 transfers its electrons to Q (Ubiquinone), the second component of the chain.
The electrons are passed down this chain of proteins. As they move down, they lose
potential energy. This energy is used to pump H+ ions from the matrix into the fluidfilled intermembrane space.
The enzyme cytochrome oxidase catalyzes the reaction between the electrons, protons
(H+ ions), and oxygen (final electron acceptor) to form water.
Chemiosmosis
 For every H+ ion pumped out of the matrix, one ATP is made; the result is that 2 ATP
are formed per FADH2 and 3 ATP are formed per NADH.
 As protons (H+ ions) accumulate in the intermembrane space, there are two gradients
that are established: the increase # of protons establishes a concentration gradient
(chemical gradient) and the increased intensity of +ve charge establishes an electrical
gradient.
 Basically, the increased electrochemical gradient across the inner mitochondrial
membrane creates a “battery” effect where the potential for H+ ions to move back into
the matrix is high – the free energy stored in the electrochemical gradient is referred to
as a proton-motive force (PMF).
 The intermembrane space becomes a H+ reservoir since the inner mitochondrial
membrane is impermeable to H+ ions.
 Since the protons (H+ ions) cannot diffuse through the lipid bilayer of the inner
membrane, they are forced to move through a special protein channel linked to ATP
synthase (ATPase).
 The PMF drives the hydrogen ions to move back into the matrix, via ATPase, releasing its
free energy to the enzyme, causing ADP to pick up a free phosphate in the matrix and
become ATP.
 Finally, the formation of water occurs by using electrons in the ETC and protons (H+
ions) that are now in the matrix and oxygen (the final acceptors of the electrons).
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A distinction must be made between the NADH produced during glycolysis and the NADH
produced during pyruvate oxidation and Krebs cycle.
The inner mitochondrial membrane is impermeable the NADH produced by glycolysis in
the cytoplasm (cytosolic NADH). It may pass through the outer mitochondrial membrane
into the intermembrane space, but not through the inner membrane into the matrix.
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There is a shuttle system that passes electrons from cytosolic NADH in the
intermembrane space to the matrix. This shuttle is called the glycerol-phosphate shuttle
where electrons are transferred from the cytosolic NADH to FAD to produce FADH2. Like
FADH2 produced in Krebs cycle, it transfers its electrons to Q, resulting in the synthesis of
two ATP molecules by chemiosmosis.
(Carter-Edwards, Gerards, Gibbons, McCallum, Noble, Parrington, Ramlochan, Ramlochan,
2011)
ETC 32 ATP (Oxidative phosphorylation)
8 NADH x 3 ATP = 24 ATP
4 FADH2 x 2 ATP = 8 ATP
The electron carriers continuously cycle between their reduced form and their oxidized form
while passing electrons from one to the next and finally to oxygen.
Biology 12 Blackline Masters
BLM 3-15 Oxidative Phosphorylation
Copyright © 2011 McGraw-Hill Ryerson Limited
978-0-07-106045-5
The Role of Oxygen
 Oxygen is the final acceptor of electrons in the electron transport chain.
 The enzyme cytochrome oxidase catalyzes the reaction between the electrons, protons
(H+ ions), and oxygen (final electron acceptor) to form water.
 If O2 is not available, electron flow along the electron transport chain stops and NADH +
H+ cannot be reconverted to NAD+.
 Supplies of NAD+ in the mitochondrion run out and the link reaction and Krebs cycle
cannot continue.
 Glycolysis still takes place because conversion of pyruvate into lactate or ethanol and
carbon dioxide produces as much NAD+ as is used in glycolysis.
 Without oxygen (anaerobic respiration), only 2 ATP are produced.
 Aerobic cellular respiration produces a total of 36 ATP 2 ATP during glycolysis, 2 ATP
during Krebs cycle, and 32 ATP during the electron transport chain (ETC).
 Oxygen increases the ATP yield.
(Allott, 2007, pg75)
Summary of Cellular Respiration
• Glycolysis 2 ATP (Substrate-level phosphorylation) (2H2O, 2 NADH  2 FADH2)
• Pyruvate Oxidation  0 ATP (2 CO2, 2 NADH)
• Krebs Cycle  2 ATP (Substrate-level phosphorylation) (2 CO2, 3 NADH, 1 FADH2
/cycle) (total  4 CO2, 6 NADH, 2 FADH2)
• ETC  32 ATP (Oxidative phosphorylation)
8 NADH x 3 ATP = 24 ATP
4 FADH2 x 2 ATP = 8 ATP
Total ATP produced per glucose molecule = 36 ATP
Relationship Between Structure of Mitochondrion and its Function
Structure
Function
Outer mitochondrial membrane
Separates the contents of the mitochondrion from the rest of
the cell
Matrix
Internal cytosol-like area that contains the enzymes for the
link reaction (pyruvate oxidation) and the Krebs cycle
Cristae
Tubular regions surrounded by membranes increasing surface
area for oxidative phosphorylation (electron transport chain)
Inner mitochondrial membrane
Contains the carriers for the electron transport chain and ATP
synthase for chemiosmosis
Space between inner and outer
membranes
Reservoir for hydrogen ions (protons), the high concentration
of hydrogen ions is necessary for chemiosmosis
(Damon, McGonegal, Tosto, Ward, 2007, pg 226)
Sources
Allott, A. (2007) IB Study Guide – Biology For the IB Diploma. Oxford: Oxford University Press.
Allott, A., & Mindorff, D. (2010). Biology Course Companion. Oxford: Oxford University Press.
(Burrell, J. G. (2002-11) Click4Biology (version 0820.2011). Thailand: Bangkok; URL
http://click4biology.info)
Carter-Edwards, T., Gerards, S., Gibbons, K., McCallum, S., Noble, R., Parrington, J.,
Ramlochan, C., Ramlochan, S. (2011) Biology 12. Toronto: McGraw-Hill Ryerson.
Damon, A., McGonegal, R., Tosto, P., Ward, W. (2007) HL Biology: Developed Specifically for
the IB Diploma. Essex: Pearson Education Limited.
3.2 – Related Pathways
(Taken from Biology 12, MHR, 2011)
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Carbohydrates are the first nutrients most organisms catabolize for energy.
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Sometimes lipids, proteins, and nucleic acids are catabolized for their energy content in
times of starvation.
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They are first digested into their component monomers, which the cell may reassemble
into macromolecules for its own use.
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Otherwise, they may be metabolized for energy by feeding into various parts of glycolysis
or Krebs cycle.
Protein Catabolism
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Under normal conditions, proteins are first digested into individual amino acids, which are
absorbed and used to produce the cell’s own proteins.
Proteins undergo deamination, a process in which the amino group is removed from
amino acids, converted into ammonia, and excreted.
Other chemical reactions convert the remaining portions of the amino acids into various
components of glycolysis or Krebs cycle.
The point of entry into these metabolic pathways depends on the type of amino acid.
Lipid Catabolism
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Lipids are first digested into glycerol and fatty acids.
The glycerol portion may be converted into glucose in a process called
gluconeogenesis, or it may be changed into dihydroxyacetone phosphate (DHAP) and
then 3-phosphoglycerate (G3P) and fed into the glycolytic pathway for energy
production.
The fatty acids are transported into the matrix of the mitochondria where they undergo a
process called β-oxidation.
In β-oxidation, fatty acids are sequentially degraded into two-carbon acetyl portions that
are converted into acetyl-CoA and respired through Krebs cycle, electron transport chain,
and chemiosmosis.
Carbohydrates supply approximately 16 kJ/g whereas fats provide approximately 38 kJ/g
(more than twice the energy).
See Figure 3.14, pg 131
3.3 - Anaerobic Respiration
(Without oxygen)
(Taken from Biology 12, MHR, 2011)
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All biochemical pathways start with glycolysis.
Some organisms get their ATP without the use of oxygen. This is known as
anaerobic respiration.
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When oxygen is absent, NADH builds up, resulting in a decrease in available NAD+.
Since the cells are limited in the number of NAD+ molecules, and none are being “freed
up” by the ETC, glycolysis stops. Respiration comes to a halt!!
When NADH cannot “dump” its electrons onto the NADH dehydrogenase of the ETC, it
goes to “plan B” – it transfers hydrogen atoms to certain molecules via a process called
fermentation.
Two types of fermentation occur in eukaryotic cells: ethanol fermentation and lactate
(lactic acid) fermentation.
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Ethanol (Alcohol) Fermentation (Figure 3.16, pg 135)
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occurs in yeast cells
Yeast cells take in glucose to produce a net gain of 2 ATP and 2 pyruvate molecules
through glycolysis.
Yeast then converts the two pyruvate molecules into 2 molecules of ethanol.
Accumulating NADH passes its hydrogen atoms onto acetaldehyde (formed when CO2 is
removed from pyruvate).
This forms ethanol (alcohol used in alcoholic beverages).
As pyruvate is being consumed, NAD+ is being recycled which allows glycolysis to
continue.
Since ethanol is a 2-carbon molecule, carbon dioxide is given off in the process.
Both the ethanol and carbon dioxide are waste products to the yeast and are released
into the environment.
Yeast is used in baking since the carbon dioxide helps the dough to rise. Yeast is also
used to make alcoholic beverages (production of ethanol).
(Damon, McGonegal, Tosto, Ward, 2007, pg 71)
(Damon, McGonegal, Tosto, Ward, 2007, pg 71)
Lactic Acid Fermentation (Figure 3.16, pg 135)
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occurs in humans
During strenuous exercise the ATP demands are high.
Oxygen cannot be delivered to the cells fast enough to facilitate the oxidative
phosphorylation process – ETC stops, NADH accumulates and glycolysis stops.
“Plan B” – the accumulating NADH transfers its hydrogen atoms to pyruvate, making
lactate.
Transferring hydrogen to pyruvate does two things: it produces NAD+ so that glycolysis
can continue and ATP can be made.
The disadvantage is that lactate accumulates in the area of need (muscle tissue), causing
stiffness and fatigue.
After vigorous activity ceases, and O2 reaches the mitochondria , the lactate is
transported to the liver to be oxidized back into pyruvate.
The extra oxygen required to oxidize the lactate into pyruvate (oxygen removes the two
hydrogen from lactate) is called oxygen debt.
Panting after strenuous exercise is how the body pays back this debt.
Sources
Allott, A. (2007) IB Study Guide – Biology For the IB Diploma. Oxford: Oxford University Press.
Allott, A., & Mindorff, D. (2010). Biology Course Companion. Oxford: Oxford University Press.
(Burrell, J. G. (2002-11) Click4Biology (version 0820.2011). Thailand: Bangkok; URL
http://click4biology.info)
Carter-Edwards, T., Gerards, S., Gibbons, K., McCallum, S., Noble, R., Parrington, J.,
Ramlochan, C., Ramlochan, S. (2011) Biology 12. Toronto: McGraw-Hill Ryerson.
Damon, A., McGonegal, R., Tosto, P., Ward, W. (2007) HL Biology: Developed Specifically for
the IB Diploma. Essex: Pearson Education Limited.
(Damon, McGonegal, Tosto, Ward, 2007, pg 72)