Download Lecture #9 - Suraj @ LUMS

Lecture 9
Generating Energy
Adenosine Triphosphate (ATP)
• The energy currency or coin of the cell.
• Transfers energy from chemical bonds to
endergonic (energy absorbing) reactions
within the cell.
• ATP consists of a ribose sugar, adenine
base, and 3 phosphate groups, PO4-2.
ATP
• Energy is stored in the covalent bonds between
phosphates.
• The greatest amount of energy is in the bond
between the second and third phosphate groups.
• This covalent bond is known as a pyrophosphate
bond.
• When the terminal (third) phosphate is cut loose,
ATP becomes ADP (Adenosine diphosphate), and
the stored energy is released for some biological
process to utilize.
• The input of additional energy (plus a phosphate
group) "recharges" ADP into ATP
– Shuttles Energy From Exergonic
ATP
Reactions to Endergonic Reactions
New Terms
• Dehydrogenase: Is an enzyme that removes hydrogen
atoms (with their electrons) from organic molecules
and transfers them to an electron carrier.
• Electron Carrier Molecules:
Molecules that accept and transfer H atoms and high
energy electrons released by reactions.
• E.g
(1)NADH: (Nicotinamide adenine dinucleotide).
(2)FADH2 (Flavin adenine dinucleotide): A secondary H
carrier, related to NADH.
Synthesis of ATP-1
• Two mechanisms exist that generate ATP i) substrate level
phosphorylation and ii) oxidative phosphorylation
(chemiosmosis).
• Cellular respiration – process that utilises both
mechanisms to generate ATP during its different stages.
• There are 3 stages of cellular respiration:
– 1. Glycolysis
– 2. The Kreb’s Cycle
– 3. Oxidative Phosphorylation
ATP Synthesis by:
1. Substrate-level phosphorylation
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Simple process, does not require membranes.
Phosphate group is directly transferred from an
organic molecule to ADP to make ATP.
Generates a small amount of ATP during cellular
respiration.
Occurs in first two stages of aerobic respiration:
1. Glycolysis
2. Kreb’s cycle
ATP Synthesis by:
2. Oxidative phosphorylation (Chemiosmosis)
• Complex process, requires mitochondrial membranes.
• Generates most of ATP made during cellular respiration.
• Electrons are passed from one membrane-bound enzyme
to another, losing some energy with each transfer known
as the electron transport chain.
• This "lost" energy allows for the pumping of hydrogen
ions against the concentration gradient (there are fewer
hydrogen ions outside the confined space than there are
inside the confined space).
• ATP is made by ATP synthase on mitochondrial
membranes, as H+ flow down concentration gradient.
Occurs in last stage of aerobic respiration.
• Requires the presence of OXYGEN
Two Mechanisms of ATP Synthesis:
Oxidative and Substrate Level Phosphorylation
Three Stages of Aerobic Respiration
1. Glycolysis: “Splitting sugar”
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Occurs in the cytoplasm of the cell
Does not require oxygen
9 chemical reactions
Net result: Glucose molecule (6 carbons each) is
split into two pyruvic acid molecules of 3 carbons
each.
• Pyruvic acid diffuses into mitochondrial matrix
where all subsequent reactions take place.
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2. Details of Kreb’s Cycle
3. Electron Transport Chain &
Chemiosmosis
• Most ATP is produced at this stage.
• Occurs on inner mitochondrial membrane.
• Electrons from NADH and FADH2 are
transferred to electron acceptors, which produces a
proton gradient
• Proton gradient used to drive synthesis of ATP.
• Chemiosmosis: ATP synthase allows H+ to flow
across inner mitochondrial membrane down
concentration gradient, which produces ATP.
• Ultimate acceptor of H+ and electrons is
OXYGEN, producing water.
Electron Transport & Chemiosmosis: Generates
Most ATP Produced During Cellular Respiration
Electron Transport Chain
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Fermentation Occurs When Oxygen is Unavailable
Photosynthesis-1
Is the process by which plants, some bacteria,
and some protistans use the energy from
sunlight to produce sugar, which cellular
respiration converts into ATP!!
Photosynthesis-2
6CO2 + 6H2O + ENERGY ---> C6H12O6 + 6O2
• Where does the free oxygen come from? CO2
or H2O
• Label the CO2 or H2O with radioactive O18
CO2 + 2H2O -------> CH2O + H2O + O2
CO2 + 2H2O -------> CH2O + H2O + O2.
• Plants produce oxygen by “splitting” water.
• Water is used as a source of H and electrons to
reduce CO2
Photosynthesis-3
• Light reactions:
Transform light energy into usable form of chemical
energy (ATP and NADPH).
Water is split to obtain H.
• Light independent reactions (Calvin cycle):
Use chemical energy (ATP and NADPH) to drive the
endergonic reactions of sugar synthesis..
Where does photosynthesis
occur?
• Chloroplasts are site of photosynthesis in eucaryotes
• All green parts of a plant carry out photosynthesis.
• Most chloroplasts are found in leaves, specifically in
mesophyll, green tissue in interior of leaves. Stomata:
Pores in leaf for exchange of CO2 and O2
• Green color is due to chlorophyll, a light absorbing
pigment.
• In bacteria, photosynthesis occurs on extensions of the
cell membrane.
Specific Sites for Specific
Reactions
• Thylakoids: Membrane “discs” arranged in stacks
(grana) which contain chlorophyll and other
important molecules.Site where solar energy is
trapped and converted into chemical energy (light
reactions).
• Thylakoid Membrane: Site of ATP synthesis.
• Stroma: Thick fluid outside thylakoid membranes,
surrounded by interior membrane. Site of sugar
synthesis (dark reactions).
Light reactions
• Light reactions trap energy and electrons required to
make sugar from CO2.
• Require light.
• Convert light energy to chemical energy of ATP and
reducing power of NADPH.
• Occur in the thylakoid membranes of chloroplast.
• Water is split with energy from sun into free O2, H and
electrons.
• Reduce NADP+ to NADPH
• Photophosphorylation: Light energy is used to produce
ATP from ADP + Pi ATP synthesis is driven by
chemiosmosis
• Input: ADP, NADP+, water, and light.
• Output: ATP, NADPH, and O2.
Light
Light is a Spectrum of Different Lights
Visible light spectrum - Wavelength in nanometers:
380
470
520
570
610
VIOLET BLUE GREEN YELLOW ORANGE
Higher Energy
650
RED
Lower Energy
Chlorophyll
• Pigments allow plants to absorb various wavelengths of
light, they are molecules that absorb light energy.
• Black object: All wavelengths are absorbed, White
object: All wavelengths are reflected, Green object: All
wavelengths BUT green are absorbed.
• Green light is reflected by chlorophyll
• Plants use different pigments to capture light energy,
each has its own unique absorption spectrum:
Structure of a Chlorophyll Molecule
Photosystems
• Are arrangements of chlorophyll and other pigments
packed into thylakoids.
• Many Prokaryotes have only one photosystem,
Photosystem II (so numbered because, while it was most
likely the first to evolve, it was the second one discovered).
• Eukaryotes have Photosystem II plus Photosystem I.
• Photosystem I uses chlorophyll a, in the form referred to as
P700.
• Photosystem II uses a form of chlorophyll a known as
P680.
• Both "active" forms of chlorophyll a function in
photosynthesis due to their association with proteins in the
thylakoid membrane.
Light Dependent Reactions: Light Energy Trapped by
Chlorophyll is Used to Split Water, Make NADPH & ATP
ATP Production Requires a Proton Gradient
Light Dependent Reactions: Light Energy
Trapped by Chlorophyll is Used to Split Water,
Make NADPH & ATP-1
• The P680 requires an electron, which is taken from a water
molecule, breaking the water into H+ ions and O-2 ions. These
O-2 ions combine to form the diatomic O2 that is released.
• The electron is "boosted" to a higher energy state and attached
to a primary electron acceptor, which begins a series of redox
reactions, passing the electron through a series of electron
carriers.
• Eventually attaching it to a molecule in Photosystem I.
• Light acts on a molecule of P700 in Photosystem I, causing an
electron to be "boosted" to a still higher potential.
• The electron is attached to a different primary electron
acceptor (that is a different molecule from the one associated
with Photosystem II).
Light Dependent Reactions: Light Energy
Trapped by Chlorophyll is Used to Split Water,
Make NADPH & ATP-1
• The electron is passed again through a series of redox
reactions, eventually being attached to NADP+ and H+ to form
NADPH, an energy carrier needed in the Light Independent
Reaction.
• The electron from Photosystem II replaces the excited electron
in the P700 molecule. There is thus a continuous flow of
electrons from water to NADPH.
• This energy is used in Carbon Fixation. Cyclic Electron Flow
occurs in some eukaryotes and primitive photosynthetic
bacteria. No NADPH is produced, only ATP. This occurs when
cells may require additional ATP, or when there is no NADP+
to reduce to NADPH.
• In Photosystem II, the pumping to H ions into the thylakoid
and the conversion of ADP + P into ATP is driven by electron
gradients established in the thylakoid membrane.
Light Independent (Dark) reactions (Calvin
Cycle) make sugar from CO2:
• Uses ATP and NADPH produced by light reactions to
reduce CO2 to glyceraldehyde-3-phosphate.
• Occurs in the stroma of chloroplast.
• Don’t need light directly.
• Carbon fixation: Process of gradually reducing CO2
gathered from atmosphere to organic molecules.
• NADPH provides H and electrons to reduce CO2 and
ATP provides energy.
• Input: CO2 , ATP, and NADPH.
• Output: Sugars, ADP, and NADP+.
Light and Dark Reactions of Photosynthesis
Dark Reactions (or Light Independent
Reactions)
• Also known as Carbon-Fixing Reactions.
• The Calvin Cycle occurs in the stroma of
chloroplasts (where would it occur in a
prokaryote?).
• Carbon dioxide is captured by the chemical ribulose
bisphosphate (RuBP).
• RuBP is a 5-C chemical. Six molecules of carbon
dioxide enter the Calvin Cycle, eventually
producing one molecule of glucose.
C4 Plants
• When carbon dioxide levels decline below the threshold
for RuBP carboxylase, RuBP is catalyzed with oxygen
instead of carbon dioxide. The product of that reaction
forms glycolic acid, a chemical that can be broken down
by photorespiration, producing neither NADH nor ATP, in
effect dismantling the Calvin Cycle.
• C-4 plants evolved in the tropics and are adapted to higher
temperatures than are the C-3 plants found at higher
latitudes. Common C-4 plants include crabgrass, corn, and
sugar cane.
• C-4 plants, have had to adjust to decreased levels of carbon
dioxide by artificially raising the carbon dioxide
concentration in certain cells to prevent photorespiration.
• The capture of carbon dioxide by PEP is mediated by the
enzyme PEP carboxylase, it has a stronger affinity for
carbon dioxide than does RuBP carboxylase.
C4 Photosynthesis
• Some plants have developed a preliminary step to the
Calvin Cycle - known as the C-4 pathway.
• While most C-fixation begins with RuBP, C-4 begins with
a new molecule, phosphoenolpyruvate (PEP), a 3-C
chemical that is converted into oxaloacetic acid (OAA, a 4C chemical) when carbon dioxide is combined with PEP.
• The OAA is converted to Malic Acid and then transported
from the mesophyll cell into the bundle-sheath cell, where
OAA is broken down into PEP plus carbon dioxide.
• The carbon dioxide then enters the Calvin Cycle, with PEP
returning to the mesophyll cell. The resulting sugars are
now adjacent to the leaf veins and can readily be
transported throughout the plant.
• C-4 photosynthesis involves the separation of carbon
fixation and carbohydrate synthesis in space and time
C4 Carbon Fixation Pathway
Photosynthesis Helps Counteract the
Greenhouse Effect
• The earth’s atmosphere contains about 0.03% of carbon
dioxide.
• Carbon dioxide traps solar energy in the atmosphere,
making the earth about 10oC warmer than it would
otherwise be.
• Since the mid 1800s, the atmospheric levels of carbon
dioxide have risen steadily due to the burning of fuels and
forests.
• The “Greenhouse Effect” refers to the global warming
that is caused by increased atmospheric carbon dioxide
levels.
• Global warming may cause polar ice caps to melt, which
in turn could cause massive coastal flooding and other
problems. Plants use up about half of carbon dioxide
generated by humans and other organisms.
The Carbon Cycle
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Plants may be viewed as carbon sinks, removing carbon dioxide from the
atmosphere and oceans by fixing it into organic chemicals.
Animals are carbon dioxide producers that derive their energy from
carbohydrates and other chemicals produced by plants by the process of
photosynthesis.
The balance between the plant carbon dioxide removal and animal carbon
dioxide generation is equalized also by the formation of carbonates in the
oceans. This removes excess carbon dioxide from the air and water (both of
which are in equilibrium with regard to carbon dioxide).
Fossil fuels, such as petroleum and coal, as well as more recent fuels such as
peat and wood generate carbon dioxide when burned. Fossil fuels are formed
ultimately by organic processes, and represent also a tremendous carbon sink.
Human activity has greatly increased the concentration of carbon dioxide in
air. This increase has led to global warming, an increase in temperatures
around the world, the Greenhouse Effect.
The increase in carbon dioxide and other pollutants in the air has also led to
acid rain, where water falls through polluted air and chemically combines with
carbon dioxide, nitrous oxides, and sulfur oxides, producing rainfall with pH
as low as 4. This results in fish kills and changes in soil pH which can alter the
natural vegetation and uses of the land.
All Food Molecules are Fed into The Catabolic
Pathway of Aerobic Respiration