Download ENERGY FLOW WITHIN THE CELL (2) LEARNING OBJECTIVES

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ENERGY FLOW
WITHIN THE
CELL
(2)
LEARNING OBJECTIVES
Justify the role of mitochondria in generation of energy (structure, enzymes
in membranes and matrix, own DNA).
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List the enzymes of electron transport chain in the increasing redox potential
order.
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Justify that electron transport chain is also termed as respiratory chain.
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Explain that oxidation is linked with phosphorylation of ATP.
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Citric acid cycle.
CITRIC ACID CYCLE:- Also known as TCA cycle or tricarboxylic acid cycle or Krebs
cycle.
It is a cyclic process.
The cycle involves a sequence of compounds interrelated by oxidation – reduction and
other reactions which finally produce CO2 and H2O.
It is a final common pathway of breakdown or catabolism of carbohydrate, fats and
proteins.
Acetyl-CoA derived mainly from oxidation of either glucose, or from fatty acids or partly
from certain amino acids combine with oxaloacetate to form citric acid or citrate (first
reaction of TCA cycle). In this reaction the acetyl-CoA transfer its acetyl group (2 C) to
oxaloacetate.
By step wise dehydrogenation and loss of two molecule of CO2, accompanied by series of
reaction the citric acid is reconverted to oxaloacetate, which again starts the cycle by taking
up another acetyl group from acetyl-CoA.
Enzymes are located in mitochondrial matrix, either free or attached to the inner surface of
the inner mitochondrial membrane,
which facilitates the transfer of
reducing equivalents to the adjacent
enzymes of the respiratory chain.
The whole process is aerobic,
requiring O2 as a final oxidant of the
reducing equivalents.
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ELECTRON TRANSPORT CHAIN
The living Cell and organism must perform work to stay alive, to grow and
reproduce. The ability to take energy and channel it to biological work is a
fundamental property of all living organism.
They convert one form of energy to another, they use chemical energy in fuel to
bring about the synthesis of complex highly ordered macro molecules from simple
pre-cursors.
Oxidation of a molecule involve the loss of electron.
The reduction of a molecule involve gain of electron.
BIOLOGICAL OXIDATION
DEFINITION :- this is the final common pathway in aerobic cells in which electron
derived from various substances are transferred to oxygen. Electron transport
chain (ETC) is a series of highly organized oxidation- reduction enzymes.
Location :- the ETC is localized in mitochondria.
MITOCHONDRIA
Mitochondria is a power house of cell, since it is within this organelles that most of
the captured energy derives from respiratory oxidation. The system that couples
respiration to the generation of high energy intermediate ATP in mitochondria is
called oxidative phosphorylation.
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The main part of the electron transport chain consist of four large protein
complexes embedded in the inner mitochondrial membrane called NADH
dehydrogenase , the cytochrome bc1 complex and cytochrome oxidase.
Electron flows from NADH to oxygen through these complexes. Each complex
contain several electron carriers, two small electron carriers are also needed to link
these large complexes, ubiquinone which is also called co enzyme Q and
cytochrome c.
Complex I NADH- COQ Reductase
Complex II or succinate dehydrogenase.
Complex III or COQ cytochrome C reductase complex.
Cytochrome bc1 complex to cytochrome C to cytochrome oxidase.
Complex IV cytochrome oxidase to oxygen.
Complex I
NAD Dehydrogenase
• Removes two electrons from NADH
• Transfers them to ubiquinone (Q).
• The reduced product is called ubiquinol (QH2) and can freely move about the
membrane.
• Moves four protons from the mitochondrial matrix to the inter-membrane space,
beginning the production of a proton gradient.
Complex II
Succinate Dehydrogenase
• Removes electrons from succinate and transfers them to ubiquinone
• Via FAD.
• Does not contribute to the proton gradient.
Complex III
Cytochrome bc1 Complex
• Removes two electrons form QH2
• Transfers them to two molecules of the electron carrier,
cytochrome c.
• Moves four protons across the inner mitochondrial
membrane, further contributing to the proton gradient.
Complex IV
Cytochrome Oxidase
• Removes two electrons from the two molecules of cytochrome c and
transfers them to molecular oxygen (O2), producing
water.
• Moves two electrons across the inner mitochondrial
membrane, adding to the proton gradient.
OXIDATIVE PHOSPHORYLATION
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Equilibrium and Metabolism
Reactions in a closed system eventually reach equilibrium and then do no work
Cells are not in equilibrium; they are open systems experiencing a constant flow of
materials
A catabolic pathway in a cell releases free energy in a series of reactions
An Analogy For Cellular Respiration – Glucose Catabolism
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Energy Coupling
Living organisms have the ability to couple exergonic and endergonic
reactions:
Energy released by exergonic reactions is captured and used to make ATP
from ADP and Pi
ATP can be broken back down to ADP and Pi, releasing energy to power the
cell’s endergonic reactions.
The Structure and Hydrolysis of ATP
ATP (adenosine triphosphate)
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Is the cell’s energy shuttle
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Provides energy for cellular functions
Energy is released from ATP when the terminal phosphate bond is broken
Energy Coupling - ATP / ADP Cycle
Releasing the third phosphate from ATP to make ADP generates energy
(exergonic):
Linking the phosphates together requires energy - so making ATP from ADP and a
third phosphate requires energy (endergonic),
Catabolic pathways drive the regeneration of ATP from ADP and phosphate
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How ATP Performs Work
ATP drives endergonic reactions by phosphorylation, transferring a phosphate
group to some other molecule, such as a reactant
The recipient molecule is now phosphorylated
The three types of cellular work (mechanical, transport, and chemical) are powered
by the hydrolysis of ATP
ATP drives endergonic reactions by phosphorylation, transferring a phosphate to
other molecules - hydrolysis of ATP
Activation Energy
All reactions, both endergonic and exergonic, require an input of energy to
get started. This energy is called activation energy
The activation energy, EA
– Is the initial amount of energy needed to start a chemical reaction
– Activation energy is needed to bring the reactants close together and
weaken existing bonds to initiate a chemical reaction.
– Is often supplied in the form of heat from the surroundings in a system.
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