Respiration
The Four Stages
Respiration: The 4 Parts
Respiration consists of 4 parts:
• Glycolysis
• Link Reaction
• Krebs Cycle
• Oxidative Phosphorylation (the electron
transport chain)
Glycolysis
• Glycolysis is the first stage of respiration
• Glycolysis splits one molecule of glucose
into two smaller molecules of pyruvate
• Glucose is a hexose (6-carbon) molecule
• Pyruvate is a triose (3-carbon) molecule.
• Pyruvate is also known as pyruvic acid.
• Glycolysis takes place in the cytoplasm of cells
• It’s the first stage of both aerobic and anaerobic
respiration, and doesn’t need oxygen to take
place. It is therefore an anaerobic process.
glucose
Number of
carbons in
the molecule
6C
2ATP
2Pi
2ADP
Hexose bisphosphate
6C
H2O
2 x triose phosphate
4ADP + 4Pi
2H
4ATP
3C
2NAD
2NADH
2 x pyruvate 3C
Stage 1: Phosphorylation
• Glucose is phosphorylated by adding 2
phosphates from 2 molecules of ATP to
give a hexose bisphosphate.
• The hexose bisphosphate is split using
water (hydrolysis)
• 2 molecules of triose phosphate and 2
molecules of ADP are created
Stage 2: Oxidation
• The triose phosphates are oxidised (lose
hydrogen) forming 2 molecules of pyruvate
• Coenzyme NAD+ (a co-enzyme is a helper
molecule that carries chemical groups or
ions around) collects the hydrogen ions
forming 2 reduced NAD (NADH + H+)
• 4ATP are produced, but 2 were used up at
the beginning so there is a net gain of
2ATP
Respiration: The 4 Parts
Respiration consists of 4 parts:
• Glycolysis
• Link Reaction
• Krebs Cycle
• Oxidative Phosphorylation (the electron
transport chain)
The Link Reaction
• The link reaction happens when oxygen is
available…
• For each glucose molecule used in glycolysis,
two pyruvate molecules are made
• But the link reaction uses only one pyruvate
molecule, so the link reaction and the krebs
cycle happen twice for every glucose molecule
which goes through glycolysis
Number of
carbons in
the molecule
pyruvate 3C
CO2
acetate
1C
2C
NAD
Coenzyme A
(CoA)
NADH
Acetyl CoA
2C
The Link Reaction Converts
Pyruvate to Acetyl Coenzyme A
• One carbon atom is removed from
pyruvate in the form of CO2
• The remaining 2-carbon molecule
(acetate) combines with coenzyme A to
produce acetyl coenzyme A (acetyl CoA)
• Another oxidation reaction happens when
NAD collects more hydrogen ions. This
forms reduced NAD (NADH)
• No ATP is produced in this reaction
The Products of the Link Reaction
go to the Krebs Cycle and the ETC
• So for each glucose molecule:
• Two molecules of acetyl co enzyme A go
into the Krebs cycle
• Two carbon dioxide molecules are
released as a waste product of respiration
• Two molecules of reduced NAD are
formed and go into the electron transport
chain
Exam Questions
1. Describe simply how a 6-carbon
molecule of glucose can be changed to
pyruvate (5)
2. Describe what happens in the link
reaction (4)
Answers
1. The 6 –carbon glucose molecule is
phosphorylated using phosphate from 2
molecules of ATP (1) and hydrolysed/ split
using water (1), to give 2 molecules of the
3-carbon molecule triose phosphate (1).
This is then oxidised by removing
hydrogen ions (1) to give 2 molecules of 3carbon pyruvate (1)
Answers
2. The 3-carbon pyruvate is combined with
coenzyme A (1) to form a 2-carbon
molecule, acetyl coenzyme A (1). The
extra carbon is released as carbon dioxide
(1). The coenzyme NAD is converted into
reduced NAD in this reaction by accepting
hydrogen ions
The Challenge
• Can you draw glycolysis and the link
reaction….?
Number of
carbons in
the molecule
pyruvate 3C
CO2
acetate
1C
2C
NAD
Coenzyme A
(CoA)
NADH
Acetyl CoA
2C
glucose
Number of
carbons in
the molecule
6C
2ATP
2Pi
2ADP
Hexose bisphosphate
6C
H2O
2 x triose phosphate
4ADP + 4Pi
2H
4ATP
3C
2NAD
2NADH
2 x pyruvate 3C
Respiration: The 4 Parts
Respiration consists of 4 parts:
• Glycolysis
• Link Reaction
• Krebs Cycle
• Oxidative Phosphorylation (the electron
transport chain)
The Krebs Cycle is the Third Stage
of Aerobic Respiration
• The krebs cycle takes place in the matrix
of the mitochondria.
• It happens once for each pyruvate
molecule made in glycolysis, and so it
goes round twice for every glucose
molecule that enters the respiration
pathway
Acetyl CoA
ATP
4- carbon
compound
ADP + Pi
oxaloacetate
NADH
4C
2C
CoA
6- carbon compound
NAD
citrate
FADH
6C
FAD
CO2
5- carbon
compound
NADH
NAD
5C
CO2
1C
NAD
NADH
1C
The Krebs Cycle
• Acetyl co enzyme A from the link reaction
combines with oxaloacetate to form citrate
• Coenzyme A is released back to the link reaction
to be used again
• The 6-carbon citrate molecule is decarboxylated
(loses CO2) to give a 5-carbon molecule
• Both citrate and the 5-carbon molecule formed
from it are also dehydrogenated (lose hydrogen)
in the cycle, to reduce the coenzymes NAD and
FAD (flavin adenine dinucleotide)
The Krebs Cycle
• Overall, 3 reduced NAD+ and 1 reduced
FAD are produced. These coenzymes can
now be used to carry the hydrogen to the
electron transport chain
• The 5 carbon compound is
decarboxylated, bringing you back to 4carbon oxaloacetate. ATP and CO2 are
released
Products of the Krebs Cycle enter the
final Stage of Aerobic Respiration
• Some products are reused, some are
released and others are used in the final
stage, oxidative phosphorylation: the
electron transport chain…
Products of the Krebs Cycle enter the
final Stage of Aerobic Respiration
• One coA is reused in the next link reaction
• Oxaloacetate is regenerated so it can be reused in the next krebs
cycle
• Two carbon dioxide molecules are released as a waste product of
respiration
• One molecule of ATP is made per turn of the cycle- by substrate
level phosphorylation
• Three reduced NAD and one reduced FAD co-enzymes are made
and enter the electron transport chain
Oxidative Phosphorylation happens
via the electron transport chain
• All the products from the previous stages are used in this final stage.
Its purpose is to transfer the energy from molecules made in
glycolysis, the link reaction and the Kreb’s cycle to ADP. This forms
ATP, which can then deliver the energy to parts of the cell that need
it. The synthesis of ATP as a result of the energy released by the
electron transport chain is called oxidative phosphorylation
• The electron transport chain is where most of the ATP from
respiration is produced. In the whole process of aerobic respiration,
32 ATP molecules are produced from one molecule of glucose: 2
ATP in glycolysis, 2 ATP in the Krebs cycle and 28 ATP in the
electron transport chain
• The electron transport chain also reoxidises NAD and FAD so they
can be reused in previous steps
Respiration: The 4 Parts
Respiration consists of 4 parts:
• Glycolysis
• Link Reaction
• Krebs Cycle
• Oxidative Phosphorylation (the electron
transport chain)
Oxidative Phosphorylation
produces lots of ATP
• The energy needed for ATP synthesis is
provided by the electron transport chain. It
uses the reduced NAD and FAD from the
previous 3 stages to produce 28
molecules of ATP for every molecule of
glucose
Outer membrane of
mitochondrion
Intermembrane space
H+
H+
H+
H+
H+
H+
H+
Inner membrane of
mitochondrion
Stalked
particle
Carrier 1
Carrier 2
Carrier 3
2eNADH + H+
ATPsynthase
ADP
+ Pi
2e-
2H
2H+
H+
H+
H2O
NAD+
Matrix of
mitochondrion
½ O2
+ 2H+
ATP
H+
Oxidative Phosphorylation
produces lots of ATP
1. Hydrogen atoms are released from NADH + H+
and FADH2 (as they are oxidised to NAD+ and
FAD). The H atoms split to produce protons (H+)
and electrons (e-) for the chain.
The electrons move along the electron chain
(made up of three electron carriers) losing
energy at each level. This energy is used to
pump the protons (H+) into the space between
the inner and outer mitochondrial membranes
(the intermembrane space)
Oxidative Phosphorylation
produces lots of ATP
• The concentration of protons is higher in the
intermembrane space than in the mitochondrial matrix,
so an electrochemical gradient exists.
• The protons then move back through the inner
membrane down the electrochemical gradient, through
specific channels on the stalked particles of the cristaethis drives the enzyme ATPsynthase. By spinning like a
motor, this enzyme supplies electrical potential energy to
make ATP from ADP and inorganic phosphate
• The protons and electrons recombine to form hydrogen,
and this combines with molecular oxygen (from the
blood) at the end of the transport chain to form water.
Oxygen is said to be the final electron acceptor