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Tuesday, May 23, 2017
Unit 4: Respiration
Title: Link reaction and Krebs cycle
Keywords:
• the link reaction
Learning Objectives:
• Krebs cycle
We are learning….
• acetylcoenzyme A
• What is the link reaction?
• FAD
• What happens during the Krebs cycle?
• What are hydrogen carrier molecules and what is
their role in the Krebs cycle?
Starter: Quick recap
Describe what happens to a molecule of glucose during
glycolysis.
Glycolysis
Glucose (6C)
2 ATP
phosphorylation
Initial
energy
investment
2 ADP
Fructose diphosphate (6C)
Glyceraldehyde
3-phosphate (3C)
Glyceraldehyde
3-phosphate (3C)
Glyceraldehyde
3-phosphate (3C)
Glyceraldehyde
3-phosphate (3C)
G3P is
oxidised
and NAD
is reduced
Energy
Production
NAD
NAD
Reduced NAD
Reduced NAD
2 ADP
2 ADP
2 ATP
2 ATP
Pyruvate (3C)
Pyruvate (3C)
What Happens Where?
 Glycolysis = Cytoplasm of the
cell.
 Link reaction = Matrix of the
mitochondria.
 Krebs cycle = Also in the
matrix.
 Electron transfer chain
Utilises proteins found in the
membrane of the christa.
Linking glycolysis and the Krebs cycle
The pyruvate molecules produced during glycolysis
possess potential energy that can only be released using
oxygen in a process called the Krebs cycle.
Before pyruvate molecules can enter the Krebs cycle, they
must first be oxidised.
This oxidation happens during the link reaction.
The Krebs cycle and the link reaction take place in the
mitochondria in eukaryotic cells.
In prokaryotes, these reactions take place in the cytoplasm
and plasma membrane.
What is a coenzyme?
Coenzymes are molecules that some enzymes require in order to
function.
Coenzymes play a major role in photosynthesis and respiration
where they carry hydrogen atoms from one molecule to another.
Examples of coenzymes include:
NAD – accepts hydrogen ions to become NADH2 (reduced NAD)
FAD – accepts hydrogen ions to become FADH2 (reduced FAD) in
the Krebs cycle
NADP – accepts hydrogen ions to become NADPH2 (reduced
NADP) during photosynthesis.
Methylene blue is a blue dye which accepts hydrogen ions and
becomes reduced (colourless). Scientists use it to model how
coenzymes work.
Blue to colourless = hydrogen ions produced so respiration
happening.
The link reaction
Before pyruvate enters the Krebs cycle it combines with a
compound called coenzyme A to form acetyl coenzyme A.
First of all pyruvate is oxidised by removing hydrogen (loss
of electrons).
This hydrogen is accepted by NAD to form reduced NAD
(NADH2).The reduced NAD is later used to produce ATP.
Carbon dioxide is also given off (2 x CO2).
The 2-carbon molecule, called an acetyl group, then
combines with coenzyme A to form acetylcoenzyme A and a
molecule of CO2 is released.
The 2-carbon acetyl coenzyme A now enters the Krebs
cycle.
To summarise:
• Pyruvate is decarboxylated: CO2 is removed.
• It is added to CoA to form Acetyl CoA
• Acetyl CoA is then ready for use in the Krebs Cycle.
The Link reaction is important as acetyl-CoA is needed for
the Krebs cycle to happen.
The Krebs cycle
The idea of the Krebs cycle is to ‘unlock’ the remainder of
the energy from the glucose molecule.
Some of this energy (2 ATP) was released during
glycolysis, but most remains in pyruvate.
During the Krebs cycle, pyruvate molecules enter the
mitochondria and, in the presence of oxygen, are broken
down to carbon dioxide and hydrogen atoms.
The Krebs cycle is also knows as the citric acid cycle and
tricarboxylic acid cycle.
The Krebs Cycle
Acetyl Coenzyme A
(2C)
Oxaloacetate (4C)
NAD
Citrate (6C)
Reduced NAD
FAD
Reduced FAD
NAD
Reduced
NAD
ADP
ATP
(4C)
CO2
(5C)
Reduced
NAD
NAD
CO2
What happens during the Krebs cycle?
The 2-carbon acetyl coenzyme A enters the Krebs cycle.
It combines with the 4-carbon oxaloacetate (oxaloacetic
acid) to give the 6-carbon citrate (citric acid).
Coenzyme A is reformed and may be used to combine
with a further pyruvate molecule.
The citrate is degraded to a 5-carbon α-ketogluterate (αketoglutaric acid) and then the 4-carbon oxaloacetate by
the loss of two carbon molecules.
This completes the cycle.
What happens during the Krebs cycle?
For each turn of the cycle, a total of four pairs of
hydrogen atoms are also formed.
Of these, three pairs are combined with the hydrogen
carrier nicotinamide adenine dinucleotide (NAD) and yield
2.5 ATP’s for each pair of hydrogen atoms.
The remaining pair combines with a different hydrogen
carrier, flavine adenine dinucleotide (FAD) and yield only
1.5 ATP’s.
Each cycle produces enough energy to yield a single
molecule of ATP.
Energy yields
Remember that all of the products of the Krebs cycle (NAD,
FAD, ATP, etc.) formed from a single pyruvate molecule.
There are two pyruvate molecules produced from a single
glucose molecule.
The total yields from a single glucose molecule are therefore
double those stated.
So for example……
Energy yields continued….
1 pyruvate produces….
(Link reaction – ATP released later on)
Pyruvate to Acetylcoenzyme A = 1 x NADH2 = 1 x 2.5 = (2.5 ATP)
Krebs cycle (ATP produced during electron transport chain)
Citrate to α-ketogluterate = 1 x NADH2 = 1 x 2.5 = (2.5 ATP)
α-ketogluterate to oxaloacetate = 2 x NADH2 = 2 x 2.5 = (5 ATP)
1 x FADH2 = 1 X 1.5 = (1.5 ATP)
ATP produced directly (no carriers) = 1 ATP
You must double these values because glucose
produces two pyruvate molecules.
So 12.5 x 2 = (25 ATP) from Link and Krebs
Add this to (7 ATP) from glycolysis = 32 ATP
Key Questions
1. Where does the link reaction and krebs cycle take place?
2. What is pyruvate converted to during the link reaction?
3. What is the final 4 carbon molecule called in the krebs
cycle?
4. How many molecules of reduced NAD are produced per
molecule of glucose in the link reaction and Krebs cycle?
5. How many molecules of FAD are produced per molecule
of glucose in the Krebs cycle?
6. How many molecules of ATP are produced per molecule
of glucose in the Krebs cycle?
7. What is the waste product of the two chemical pathways?
Summary of Link Reaction and Krebs Cycle
Per molecule of glucose:
• 8 molecules of reduced NAD are produced
(2 x 4).
• 2 molecules of reduced FAD are produced
(2 x 1).
• 2 molecules of ATP are produced.
• 6 molecules of CO2 are produced as waste
products (2 x 3).
Main Activity:
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Plenary: How Science Works
Plenary: How Science Works
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Learning Objective
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