Chapter 9 and 10: Cellular Respiration and
Photosynthesis
Chapter 9. Cellular Respiration: Harvesting Chemical Energy.
The
Principles of Energy Harvest.
Cellular respiration
and fermentation are catabolic, energy-yielding pathways.
Cells
recycle the ATP they use for work.
Redox
reactions release energy when electrons move closer to electronegative atoms.
Electrons fall
from organic molecules to oxygen during cellular respiration.
The fall
is stepwise, via NAD+ and an electron transport chain.
The
Process of Cellular Respiration.
Respiration
involves glycolysis, the Krebs cycle and electron transport (overview).
Glycolysis ΰ
oxidization of glucose to pyruvate (closer look).
The Krebs
cycle completes the energy-yielding oxidation of organic molecules (closer
look).
The inner
mitochondrial membrane couples electron transport to ATP synthesis.
Cellular
respiration yields many ATP per sugar (review).
Related
Metabolic Processes.
Fermentation
enables some cells to produce ATP without the help of oxygen.
Glycolysis
and the Krebs cycle connect many other metabolic pathways.
Feedback
mechanisms control cellular respiration.
Chapter 10. Photosynthesis.
Photosynthesis
in Nature.
Plants and
other autotrophs are the producers of the biosphere.
Chloroplasts
are the sites of photosynthesis in plants.
The
Pathways of Photosynthesis.
The light
reactions and the Calvin cycle cooperate in converting light energy to the
chemical energy of food.
The light
reactions convert solar energy to chemical energy (closer look).
The Calvin
cycle uses ATP and NADPH to convert CO2 to sugar (closer look).
There are
alternative mechanisms of carbon fixation.
Photosynthesis
is the biospheres metabolic foundation (review).
Chapter 9. Cellular Respiration: Harvesting Chemical Energy.
Louis Pasteur noted three things about alcohol production.
1)
Nothing happened unless yeast were present.
2)
3)
Grew slower without fresh air and produced alcohol.
This
introduced the concept of biochemistry or the chemistry of living cells,
in this case the yeast cell.
Eventually
intermediate products and enzymes were discovered in the process of making CO2
and EtOH.
Started
the study of the metabolism of sugars and today we know why this is important: Energy.
The
Principles of Energy Harvest.
Cellular respiration and
fermentation are catabolic, energy-yielding pathways.
With the
help of enzymes, a cell can systematically degrade complex organic
compounds (catabolism) that are rich in this potential energy.
The degradation process releases energy from the
compound which can be captured (ATP) or lost as heat.
Fermentation
is one such catabolic process, it is a partial degradation of sugars that
occurs without oxygen.
The most
prevalent and efficient catabolic pathway is cellular respiration, in
which oxygen is consumed as a reactant along with the organic fuel (Fig. 9.1).
Ex: Organic compound + Oxygen ΰ Carbon dioxide + water + energy.
Ex: C6H12O6 + 6O2
ΰ 6CO2 + 6H2O
+ Energy.
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The overall reaction for
the complete oxidation of glucose (most common fuel) is a highly exergonic reaction 686 kcal/mol or
2,870 kj/mol.
The energy released from the oxidization of glucose
cannot do work we need an intermediate, ATP.
Cells
recycle the ATP they use for work (Fig 9.2).
ATP is like
a loaded spring due to the close packing of the three negatively charged
phosphates groups.
This is an unstable
arrangement that stores a large amount of energy.
The cell
utilizes this energy by hydrolyzing ATP ΰ ADP + Pi.
Enzymes are used in this process to remove the
Phosphate (Pi) from ATP and often placing it on another molecule,
this is called phosphorylation.
Redox
reactions release energy when electrons move closer to electronegative atoms.
The
relocation of electrons releases energy stored in food and this energy is used
to synthesize ATP.
A reaction
that results in the transfer of one or more electrons from one reactant to
another is a redox reaction (oxidation/reduction reaction).
Loss
of one or more electrons from one substance is oxidation (LEO).
Ex: Na + Cl ΰ Na+ + Cl-, Na is oxidized and
Cl is reduced.
Redox
reactions are always qualified by the loss or gain of electrons, however, we
may also think of this as a transfer of hydrogen atoms because they have an
electron (H = H+ + e-).
Ex: Loss of a hydrogen atom indicates a loss of an
electron or oxidation, AH2 + B ΰ BH2 + A.
Which is oxidized? Reduced?
The reduced compound (B) acts as the oxidizing
agent while the oxidized compound (A) acts as the reducing agent.
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Electron transfer must
have and electron donor (A) and acceptor (B).
Electrons
fall from organic molecules to oxygen during cellular respiration.
Oxidation of
methane (gas stove) and combustion of gasoline are both facilitated by oxygen,
but our main interest is in the oxidation of glucose (respiration).
C6H12O6 + 6O2
ΰ 6CO2 + 6H2O.
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Which is oxidized?
Reduced? Electron donor? Acceptor?
The
fall is stepwise, via NAD+ and an electron transport chain.
Cellular
respiration does not oxidize glucose in one step, glucose is broken down in a
many steps (catabolic pathway).
At each step
an enzyme strips electrons from glucose, they are not immediately added to
oxygen but instead added to an intermediate (coenzyme).
The
most common is the coenzyme nicotinamide adenine dinucleotide, NAD+ (Fig.
9.4).
Coenzyme
NAD (Nicotinamide adenine dinucleotide) is a key electron carrier in redox
reactions, exists as oxidized (NAD+) or reduced (NADH).
Each NADH formed during respiration represents stored
energy that can be tapped to make ATP.
O2
readily accepts electrons from NADH (important in energy production).
Respiration uses the electron transport chain
to break the fall of the electrons to O2 and at each step energy is
releases to generate ATP (Fig 9.5).
NADH + H+ + ½O2 ΰ NAD+ + H2O DG = -53 kcal/mol or
222 kj/mol.
The
Process of Cellular Respiration.
Overview
of glycolysis, Krebs cycle and electron transport chain.
Three
processes of glucose oxidation (Fig 9.6):
1)
2) Cellular Respiration, Krebs cycle
(citric acid) and electron transport chain (yields CO2 and
ATP).
3) Fermentation, anaerobic respiration.
Glycolysis
occurs in the cytoplasm while the Krebs cycle and electron transport chain take
place in the mitochondria.
Glycolysis
and the Krebs cycle are the pathways involved in glucose catabolism.
The electron
transport chain and oxidative phosphorylation are involved in
transferring the energy harvested from glucose catabolism (NADH mainly) to ATP.
Oxidative phosphorylation is the transfer of electrons
from food to oxygen with the energy at each step being used to add Pi
to ADP to form ATP, through an intermediate (NAD+ ΰ NADH).
A very few
reactions of glycolysis and the Krebs cycle produce ATP directly with no
intermediate, substrate-level phosphorylation (Fig 9.7).
Using these
processes a cell can take one glucose molecule and make up to 38
molecules of ATP.
Glycolysis
harvests chemical energy by oxidizing glucose to pyruvate.
Glucose a
six carbon sugar is split yielding two three carbon sugars, pyruvate(2) (Fig
9.8).
Reactants:
NAD+, ATP, ADP, Pi and glucose.
Products:
ATP, NADH, H+, H2O and pyruvate.
The energy
yield per glucose is 2 ATP and 2 NADH.
NADH holds electrons for
later energy harvest in the electron transport chain.
The 2
Pyruvate will enter the Krebs cycle as 2 Acetyl CoA molecules.
Glycolysis
is a ten reaction pathway that takes place in the cytoplasm and occurs whether
oxygen is present or not.
If no oxygen is present pyruvate cannot continue on to
the Krebs cycle instead it is shuttled to fermentation.
Energy-investing
steps of glycolysis (Reactions 1-5).
Reactions one and three use an ATP attaching the
released phosphate to the sugar (Fig 9.9).
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Raises free energy to 15
kcal/mol (62.7 kJ/mol).
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The phosphate reactions
are driven by different kinase enzymes.
All carbons are accounted for, 1 six carbon to 2 three
carbon compounds.
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No carbons are lost as
CO2 (happens in Krebs cycle).
After
reaction five we are left with two 3-carbon molecules
(glyceraldehyde-3-phosphate or G3P).
Energy
harvesting reactions 6-10 (Fig 9.9).
Reaction six is a three-step reaction resulting in a
large drop in free energy (-DG).
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This energy is not just
lost as heat some of it is stored as two molecules of NADH + H+.
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This NADH + H+
will later be used in either fermentation or in the electron transport chain to
form ATP.
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NAD+ is
present in only small amounts in the cell so it must be recycled (from NADH) if
it cannot be aerobically then it will be through fermentation (alcohol or
lactic acid).
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Finally,
the last remaining steps of glycolysis involve transfer of the two phosphates
back to ADP to form ATP (remember there are two G3P so reactions 6-10 occur
twice) forming 4 ATP and two pyruvate.
Pyruvate
oxidation: Oxidation of pyruvate to acetate is the link between glycolysis and
cellular respiration (Fig 9.10).
Pyruvate diffuses to the mitochondria where it is
oxidized (losing two H and COO-) to acetyl.
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NADH is formed and some
extra energy is stored by the acetyl CoA.
These steps occur at a huge enzyme complex attached to
the inner mitochondrial membrane (72 polypeptides).
Reactants: Pyruvate, coenzyme A and NAD+.
Products: CO2, NADH and Acetyl CoA
(acetate).
The
Krebs cycle completes the energy-yielding oxidation of organic molecules.
Acetyl CoA
is the starting point for the Krebs cycle (CAC) (Fig 9.11 and 9.12).
The energy
released is stored in NADH or FADH2 or used to make GTP/ATP.
Reactants: Acetyl CoA, NAD+, FAD, ADP and Pi.
Products: CoA, NADH, FADH2, CO2
and ATP.
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More energy harvested
here especially that held in NADH and FADH2.
The Krebs
cycle produces CO2 and reduced energy carriers.
Citric acid
(6-carbon) formation is the first reaction: 2-carbon acetyl + 4-carbon
oxaloacetate.
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The rest of the cycle is
citric acid being degraded back to a new molecule of oxaloacetate.
Reaction 5 results in GTP formation and eventually
ATP.
Reaction 6 results in FADH2 formation.
The final reaction: The
formation of oxaloacetate from malate also produces NADH + H+.
The inner mitochondrial
membrane couples electron transport to ATP synthesis.
Electron
transport chain (respiratory chain): Aerobic process that releases energy from
NADH and FADH2 in a way that it may be used to create ATP.
Consists of a series of reactions (chain) where
electrons are passed from one carrier to another (redox reactions) then finally
to O2 to produce H2O (Fig 9.13).
This passing of electrons (hydrogen) releases energy
that is used to pump protons (H+) across the inner membrane (matrix ΰ membrane space) setting up an electrochemical
gradient.
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As the protons move back
across the membrane they drive the formation of ATP by the mitochondrial ATP
synthase.
Reactants: NADH + H+, FADH2, O2,
ADP and Pi.
Products: NAD+, FAD, H2O and
ATP.
Without NAD+ and FAD the oxidative steps of
glycolysis, Krebs cycle and pyruvate oxidation would not occur.
The reduced forms (NADH + H+ and FADH2)
must have a place to donate their electrons and H+.
Three parts:
1) Electrons pass through membrane associated electron
carriers (the electron transport chain).
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Makes no ATP directly,
the steps release energy and this energy is used to eventually make ATP
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Coupling of electron
transport and energy release to ATP synthesis is called chemiosmosis.
2) This flow of electrons across the inner membrane
causes the active transport of protons (H+) creating a concentration
gradient.
3) Protons diffuse back into the matrix through a
proton channel (ATP synthase, Fig 9.14) driving the synthesis of ATP.
Electrons
are passed from complex to complex (three complexes) in the electron transport
chain.
NADH transfers its electrons to the first complex of
the ETC while electrons from FADH2 enter via complex later in the
pathway (ubiquinone).
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The many steps of this pathway after ubiquinone
usually involve a cytochrome.
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These reactions each release
a small more manageable amount of energy.
Each electron transport through one of the three
complexes, pumps protons (H+) out of the matrix into the
intermembrane space (low ΰ high) setting up an electrochemical gradient.
Active proton
transport is followed by diffusion coupled to ATP synthesis.
The movement of protons out of the matrix increase the
gradient as well as setting up a charge differential.
The force drives protons back across the membrane
(like a battery).
Protons pass through the ATP synthase complex
back into the matrix coupling this movement to ATP synthesis.
Releasing
Energy from Glucose.
With
O2 (aerobic) the pathways are: Glycolysis ΰ pyruvate oxidation ΰ Krebs cycle ΰ electron transport chain.
Without
O2 (anaerobic): Glycolysis ΰ fermentation.
Prokaryotes:
Glycolysis, fermentation and citric acid cycle enzymes are in the cytoplasm,
while enzymes for pyruvate oxidation and respiratory chain are associated with
invaginations of the PM.
Eukaryotes:
Glycolysis and fermentation occur in the cytoplasm, citric acid cycle enzymes
are found in the matrix of the mitochondria and pyruvate oxidation and
respiratory chain enzymes are associated with the inner membrane of the
mitochondria.
Glycolysis
may be followed by fermentation.
No NAD+ is available usually due to no O2
as a final electron acceptor.
The NADH + H+ oxidizes by using pyruvate as
an electron acceptor forming either lactic acid or ethanol and CO2.
Cellular
respiration generates many ATP molecules for each sugar molecule (Fig 9.16).
Total energy
yield for fermentation is 2 ATP while the total net is 36
molecules ATP per molecule of glucose for the complete process.
Why more ATP
from glycolysis + aerobic cellular respiration than just glycolysis +
fermentation?
Fermentation is an incomplete oxidation of glucose.
An organism capable of aerobic cellular respiration is
going to have an advantage over an organism capable of just fermentation.
Related
Metabolic Processes.
There
is an interaction of the processes of making energy and making or breaking down
macromolecules.
Biochemical
traffic can move in and out of the seemingly separate processes.
Glycolysis
and the Krebs cycle connect to many other metabolic pathways (Fig 9.19).
Catabolic:
Polysaccharides can be broken down to glucose
phosphate.
Lipids can be broken down to glycerol (shuttled into
glycolysis) and fatty acids.
Proteins can be broken down to amino acids which can
be shuttled into glycolysis or the Krebs cycle.
Anabolic
(biosynthesis):
Processes can be reversed to form glucose from these
intermediates.
Certain Krebs cycle intermediates are important
starting points for nucleotide biosynthesis.
All these
processes are integrated; and made depending on what your body needs!
Feedback mechanisms control
cellular respiration (Fig 9.20).
Many of the
reactions of these processes are regulated by other proteins or the products of
their own reactions.
Two main
types of regulation: positive and negative feedback.
Chapter 10. Photosynthesis.
Photosynthesis the process
(basis for life on earth) by which sunlight is turned into sugars
(autotrophic).
Photosynthesis in Nature.
Plants and other autotrophs are
the producers of the Biosphere.
Two steps:
1)
Conversion of light energy to chemical bonds in reduced energy carriers and
ATP.
2) These two
energy sources are then used to drive the formation of carbohydrates from CO2.
Chloroplasts
are the sites of photosynthesis in plants (Fig 10.2).
0.5 million
chloroplast per square millimeter of leaf surface.
The color is
from chlorophyll (green pigment), which is responsible for the absorption of
light energy to drive food synthesis.
CO2
and O2 are exchanged through the stomata of the leaves.
Chlorophyll
resides in the thylakoid membranes.
The
pathways of photosynthesis.
The
light reactions and the Calvin cycle cooperate in converting light energy to
the chemical energy of food (overview).
Reactants: Water
from the soil, CO2 from the air and light.
By 1804 the
general equation for photosynthesis was known.
6CO2 + 6H2O ΰ C6H12O6 + 6O2.
It was found
later that water is also a product.
Revised equation: 6CO2 + 12H2O ΰ C6H12O6 + 6O2
+ 6H2O.
Oxygen production is an important source of
atmospheric O2.
Two pathways
of photosynthesis (Fig 10.4).
1) First pathway is called the light reactions
and is driven by light energy producing ATP and NADPH + H+.
2) The Calvin-Benson cycle does not use light
directly it uses ATP, NADPH + H+ and CO2 to produce
sugar.
Light
Reactions: Light energy is captured by pigment molecules and used to produce
ATP from ADP and Pi.
Light
reactions are mediated by molecular assemblies called photosystems.
ATP synthesis
in this fashion is photophosphorylation.
The NADPH +
H+ and ATP produced by the light reactions are used to reduce CO2
to sugar (C6H12O6), this is the Calvin-Benson
cycle.
Photosynthesis
uses chlorophylls and accessory pigments.
Absorption of a photon by a pigment transfers energy
to that pigment (excited state).
Chlorophyll a and
b predominate in plants.
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Absorb blue and red
wavelengths and reflects green.
Carotenoids
and phycobilins are accessory pigments that absorb other wavelengths of
light.
The
light reactions convert solar energy to the chemical energy of ATP and NADPH.
A pigment
molecule absorbs a photon and enters the excited state this is a potential
energy state and does not last long.
Two things
can happen: The molecule returns to its ground state releasing the
energy or passes the energy on to another pigment molecule.
In these reactions the excitation is passed from one
pigment molecule to another to the reaction center where a special chlorophyll
a resides.
This excited
chlorophyll acts as a reducing agent.
This excited chlorophyll (Chl*) has an electron
zipping around ready to pass to an oxidizing agent.
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Chl* + A ΰ Chl+ + A -.
Electron
Flow, Photophosphorylation and Reductions.
These electrons from excited chlorophyll can then be
passed onto non-pigment acceptors which sets up electron flow.
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Similar to electron
transport of the mitochondria.
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NADP used in anabolic
reactions, while NAD+ is used in catabolic reactions.
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Two systems of electron
flow: Cyclic vs. noncyclic.
Noncyclic flow uses
two photosystems and generates ATP and NADPH + H+ (Fig 10.12).
Cyclic electron flow only produces ATP (Fig 10.14).
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The electron cycles back
to the chlorophyll after flowing through nonpigment acceptors.
Chemiosmosis
is also used in photophosphorylation.
Protons are pumped into the interior of the thylakoid
and drive ATP formation as they move into the stroma through ATP
synthase.
The Calvin cycle uses
ATP and NADPH (from light reactions) to convert CO2 to sugar.
Most of the
enzymes involved in this process are found in the stroma.
1950s Melvin
Calvin and Andrew Benson studied this cycle using 14C (radioactive
isotope) so they could follow how CO2 was fixed to become a
carbohydrate.
Composed of
three processes:
1) Carbon fixation- Fixation of CO2 to
ribulose monophosphate (RuMP) to form ribulose 1,5-bisphosphate (RuBP) by the
enzyme rubisco (most abundant protein).
2) Reduction-
Conversion of fixed CO2 to glyceraldehyde 3-phosphate (carbohydrate)
requires ATP (phosphorylation from the light reactions).
3)
The
Glyceraldehyde 3-phosphate then has two fates.
One-third is used to generate the polysaccharide
starch.
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Two-thirds is converted to the disaccharide sucrose.
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Sent to other organs of
the plant.
The glucose
generated then can be used to make amino acids, lipids or nucleotides.
Alternative
mechanisms of carbon fixation (Fig 10.18).
C3
plants have first product that has three carbons (3-phosphoglycerate).
C4
plants (corn and sugar cane) have oxaloacetate (4-carbon) as their first
product of CO2 fixation.
Are able to photosynthesize at higher temps than C3
plants.
Uses PEP carboxylase instead of rubisco for CO2
fixation in chloroplast near the leaf surface.
PEP carboxylase fixes CO2 at very low levels.
So on a hot day when the stomata of a leaf are closed
and CO2 levels are low the C4 plant can still fix CO2.
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Corn and sugar cane are
often grown in hotter climes than C3 plants like soy beans, rice,
wheat and barley.