Download KREBS CYCLE - hutchhighIBbiology

IB HL #7
Cell respiration involves the production of
ATP using energy released by the
oxidation of glucose, fat or
other substrates.
If
glucose is
the substrate, the first
stage of cell respiration is a
metabolic pathway called glycolysis.
GLYCOLYSIS SUMMARY:
• The pathway is catalyzed by enzymes in the
cytoplasm.
• Glucose is partially oxidized & a small amount
of ATP is produced.
• This partial oxidation is achieved without the
use of oxygen, so glycolysis can form part of
both aerobic and anaerobic respiration.
COMPARISON OF OXIDATION &
REDUCTION
Oxidation reaction
Reduction reaction
• Addition of oxygen
• Removal of oxygen
atoms to a substance
from a substance
• Removal of hydrogen
• Addition of hydrogen
atoms from a substance
atoms to a substance
• Loss of electrons from
• Addition of electrons
a substance
to a substance
OXIDATION & REDUCTION IN A CELL
• Hydrogen carriers accept atoms removed
from substrates in cell respiration.
• The most commonly used hydrogen carrier is NAD.
(nicotinamide adenine dinucleotide)
• Hydrogen atoms consist of one proton & one electron.
• When two hydrogen atoms are removed from a
respiratory substrate, NAD+ accepts the electrons
from both atoms & the proton from one of them.
NAD+
+ 2H
NADH + H+
Examples of oxidation & reduction in cell respiration
Fe3+ + electron
Fe2+
Fe2+
Fe3+ + electron
succinate + FAD
malate + NAD+
fumarate + FADH2
oxaloacetate + NADH + H+
pyruvate + NADH + H+
lactate + NAD+
CONVERTING GLUCOSE TO PYRUVATE
IN GLYCOLYSIS
THERE ARE FOUR MAIN STAGES
STAGE #1
• Two phosphate groups are added to a molecule of
glucose to form hexose biphosphate
• Adding a phosphate group is called
phosphorylation .
• Two molecules of ATP provide the phosphate groups.
• The energy level of the hexose is raised by
phosphorylation making the subsequent reactions
possible.
STAGE #2
• The hexose biphosphate is split to form two molecules
of triose phosphate.
• Splitting molecules is called lysis.
STAGE #3:
• Two atoms of hydrogen are removed from each triose
phosphate molecule.
• This is an oxidation.
• The energy released by this oxidation is used to link
on another phosphate group, producing a 3-carbon
compound.
• NAD+ is the hydrogen carrier that accepts the
hydrogen atoms.
STAGE #4:
• Pyruvate is formed by removing the two phosphate
groups and passing them on to ADP.
• This results in ATP formation.
SUMMARY OF GLYCOLYSIS
• ONE GLUCSOE IS CONVERTED INTO TWO
PYRUVATES.
• TWO ATP MOLECULES ARE USED PER
GLUCOSE BUT FOUR ARE PRODUCED SO
THERE IS A NET YIELD OF TWO ATP’S.
• TWO NAD+ ARE CONVERTED INTO TWO
NADH + H+.
Enzymes in the matrix of the mitochondria
catalyze a cycle of reactions called
the Krebs Cycle.
These
reactions can
only occur if oxygen is
available. (aerobic cell respiration)
One turn of the Krebs cycle yields:
• 2 CO2
• 3 x NADP + H+
• 1 x FADH2
• 1 ATP
C2 + C4 = C6
C5
CO2
C4
CO2
Acetyl groups (CH3CO) are the substrate used in the
Krebs Cycle.
A carrier called CoA (Coenzyme A) accepts acetyl
groups produced in metabolism & brings them for
use in the cycle.
acetyl group + CoA
acetyl CoA
Acetyl CoA is formed from the metabolism of
carboydrates and fats.
• Carbohydrates are converted into pyruvate & the
pyruvate is converted into acetyl CoA by a
reaction that is called the the link
reaction.
•Fats are broken down into fatty acids &
glycerol and then oxidized to form acetyl CoA.
• It’s called the link reaction because it links
glycolysis & the Krebs Cycle.
THE LINKS REACTION
•Pyruvate from glycolysis is absorbed by the
mitochondrion.
• Enzymes in the matrix of the mitochondria remove
hydrogen & carbon dioxide from the pyruvate.
• The hydrogen is accepted by NAD+ . Removal of
hydrogen is called oxidation.
• Removal of carbon dioxide is decarboxylation.
• Therefore the whole conversion is called:
oxidative decarboxylation.
• The product of o. d. is acetyl group which is
accepted by CoA.
Summary of the Link Reaction
NAD+
NAD + H+
Pyruvate
acetyl CoA
CoA
CO2
KREBS CYCLE:
• An acetyl group is transferred from acetyl CoA to
a four-carbon compound (oxaloacetate) to form
a six-carbon compound (citrate)
• Citrate is converted back into oxaloacetate at the
end of the cycle.
• Carbon dioxide is removed in two of the reactions.
These reactions are called decarboxylations.
CO2 is a waste product.
• Hydrogen is removed in four of the reactions.
These reactions are oxidations.
In three of oxidations H is accepted by NAD+.
In the other oxidation FAD accept it.
These oxidative reactions release energy, which is
stored by the carriers when they accept hydrogen.
This energy is later released by the electron
transport chain and used to make ATP.
• ATP is produced directly in one of the reactions.
This reaction is substrate-level phosphorylation.
fats
Summary of metabolic
Pathways involving
Acetyl CoA
carbohydrates
glucose
glycerol
pyruvate
fatty acids
acetyl CoA
Krebs Cycle
The Electron Transport Chain:
The electron transport chain is a series of electron
carriers, located in the inner membrane of
the mitochondria.
• NADH supplies two electrons to the first carrier in
the chain. The two electrons come from oxidative
reactions in the early stages of cell resp.
• The two electrons pass along the chain of carriers &
give up energy each time they pass from one carrier
to the next.
• At three points along the chain enough energy is
given up for ATP to be made by ATP synthase.
• ATP synthase is located in the inner mitochondria
membrane.
• This ATP production relies on energy released by
oxidation so it is called oxidative phosphorylation.
• FADH2 also feeds electrons into the transport chain.
THE ROLE OF OXYGEN
• At the end of the electron transport chain the
electrons are given to oxygen.
• At the same time oxygen accepts hydrogen ions
to form water.
• This happens on the matrix
• This is the only stage at which oxygen is used in
cell respiration.
• If oxygen is not available, electron flow stops &
NADH + H+ can’t be converted to NAD+ .
Supplies of NAD+ run out & the link reaction
& Krebs Cycle can’t continue.
• Glycolysis can continue.
• Aerobic cell resp. gives a yield of about 36 ATP’s
per glucose mol.
• Glycolysis produces only two ATP’s.
The coupling of electron transport to ATP synthesis
• Going down the energy gradient, the electrons release
energy.
• The energy released from the electrons is used to
pump protons (H+), from the matrix to the
intermembranal space.
Due to the small volume of this space, it becomes
highly concentrated with protons very quickly.
• This creates two compartments in the mitochondria
with different proton conc. The matrix with low conc.
& the intermembranal space with high conc.
• This results in the protons moving down their .
gradient from high conc. to low.
However the only path they can pass through is
channels provided by the enzyme ATPsynthase
embedded in the inner membrane of the mitochondria.
• Protons diffuse quickly through the ATPsynthase
channels activating the enzyme & releasing lots of
energy.
• The active enzyme catalyzes the phosphorylation
of ADP into ATP. Carbon from the glucose are
released as a waste (CO2).
The coupling of ATP synthesis to electron
tranport via a conc. gradient of protons
is called chemiosmosis.
MITOCHONDRIA
Relationship between structure & function.
• Cristae form a large surface area for oxidative
phosphorylation
• The small space between inner & outer
membrane allows for accumulation of protons.
• The fluid matrix contains enzymes for the
Krebs cycle.
• The inner membrane contains electron transport
chains & ATP synthase to carry out oxidative
phosphorylation.