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Cell Respiration
Glycloysis and the Krebs Cycle
First - an Overview of Life
Introduction to Cell Respiration and Fermentation
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Cellular Respiration- is a metabolic process used to obtain
energy from organic compounds or food, to replace ATP’s
This process can run under aerobic and anaerobic conditions.
Aerobic conditions yield more ATP.
Both cellular respiration and fermentation are catabolic
pathways, breaking down organic molecules like glucose.
Cell respiration occurs in the mitochondria of your cells for the
most part.
4 main steps: Glycolysis, the Link Reaction, the Krebs
Cycle, and the ETC (electron transport chain)
Goal of Cell Respiration is ATP. Why???
The 3 Steps
Another Brief Introduction
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Glycolysis occurs in the cytoplasm it breaks glucose into two
molecules of pyruvate.
Krebs cycle occurs in the mitochondrial matrix it degrades
pyruvate to carbon dioxide.
Both processes transfer electrons from substrates to NAD+,
forming NADH. NADH passes these electrons to the electron
transport chain.
The ETC electrons move from molecule to molecule until they
combine with oxygen and hydrogen ions to form water.
Introduction continued….
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As they move along the energy in these electrons is stored in
the mitochondria to be used in the formation of ATP via
oxidative phosphorylation, which produces 90% of all ATP.
Some ATP is made during glycolysis and the Krebs Cycle by
substrate-level phosphorylation, enzyme transfers a
phosphate group from an organic molecule (the substrate) to
ADP, forming ATP.
Ultimately 38 ATP are produced per mole of glucose
Substrate-Level Phosphorylation
Step 1 Glycolysis
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During glycolysis, glucose,6 carbon-sugar, is split into 2, 3carbon sugars.
Glucose which is formed when carbohydrates break down in
digestion is the primary source of fuel for cellular respiration.
Other substances can also be used like protein, fats, and
nucleic acids, but most of these are used to build parts of the
cell. They are only used when energy from carbs. Is not
present.
All ten steps of glycolysis is catalyzed by a specific enzyme.
2 Phases of Glycolysis
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energy investment
phase, ATP provides
activation energy by
phosphorylating glucose,
2 ATP per molecule
energy investment
phase, ATP provides the
activation energy by
phosphorylating glucose
4 ATP (net) and
2 NADH are produced
per glucose.
End of Glycolysis
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net yield of glycolysis is 2 ATP and 2 NADH/glucose.
Glycolysis occurs whether O2 is there or not. So it can
run under aerobic or anaerobic conditions.
The Krebs Cycle
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Krebs cycle is named after Hans Krebs who was mainly
responsible for discovering its pathways in the 1930’s.
Entering the Krebs Cycle 75% of the original energy in glucose
is still present in the 2 molecules of pyruvate
With oxygen present the pyruvate enter the in the
mitochondrion where enzymes of the Krebs cycle oxidize the
organic fuel to carbon dioxide.
Step 1: When pyruvate enters the mitochondria sveral enzymes
modify it into acetyl CoA, carboxyl group is removed as CO2
NADH is also made using that are given off by the reaction.
Link reaction
Krebs Cycle
Krebs Cycle
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Each cycle produces
one ATP by
substrate-level
phosphorylation,
three NADH + H+,
and one FADH2
Krebs cycle produces
large quantities of
electron carriers
ETC
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So far only 4 of the 38 ATP that will be produced have been,
all by substrate level phosphorylation.
The remaining will be produced by the ETC.
The majority of the ATP produced comes from the energy
carried in the electrons of NADH (and FADH2) that were
produced by the Krebs Cycle. 6 NADH and 2 FADH2
The energy in these electrons is used in the ETC to power the
synthesis of ATP.
There are thousands of ETC’s found in each mitochondria,
which can number in the 100’s depending on the cell type.
Electrons drop in energy as they pass down the ETC
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Electrons carried by
NADH are transferred
to the first molecule of
the ETC
The electrons carried
by FADH2 have lower
free energy and are
added later in the
chain.
ETC Continued……
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For every 2 electron carriers (4 electrons), one O2 molecule is
reduced to 2 molecules of water.
ETC makes no ATP directly.
Its function is to break the large drop in energy from food
molecules down to oxygen into small manageable steps.
The actual production of ATP comes from a protein complex,
ATP synthase, that adds a phosphate onto ADP making ATP.
The ETC used the energy from the moving electrons to pump
H protons into the inter-membrane of the mitochondria to
create a concentration gradient.
ETC Continued....
Fermentation Overview
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Produces ATP without oxygen.
No ETC is present since there is no oxygen
NAD+ gets recycled by use of an organic hydrogen acceptor
like lactate or ethanol.
Common in prokaryotes and very useful to humans.
Two type lactic acid and alcohol fermentation.
A build up of lactate in your muscles from over exerting
yourself and not taking in enough oxygen causes soreness.
Alcohol fermentation has a by product of CO2 and ethanol
which is used to make alcoholic beverages. Yeast and fungus
go through alcohol fermentation.
The release of CO2 by yeast is what causes bread to rise.
Alcohol Fermentation
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Pyruvate is
converted to
ethanol in two
steps.
Alcohol
fermentation
by yeast is used
in
brewing and
winemaking.
Lactic Acid Fermentation
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Pyruvate is reduced directly
by NADH to form lactate
Lactic acid fermentation by
some fungi and bacteria is
used to make cheese and
yogurt
The waste product, lactate,
may cause muscle fatigue,
but ultimately it is converted
back to pyruvate in the liver.
Glycolysis
Can Use
Many
Substances
Metabolism is
extraordinarily
versatile and
adaptable
Control
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Regulated by
supply and demand
If ATP levels drop,
metabolism speeds
up to produce more
ATP.
If there is an excess
it slows