Transcript Nutrient cycles - VBIOLOGY

4.2 Link Reaction and Krebs
Cycle
Learning Objectives
 Describe what the link
reaction is
 Describe what happens
during the Krebs cycle
 Explain the role of
hydrogen carrier molecules
in the Krebs cycle
Success Criteria
 Produce accurate notes on
the link reaction
 Summarise in a diagram the
key events in the Krebs cycle
 Carry out an experiment and
analyse the results to
demonstrate the role of
coenzymes in respiration
Starter
 Write down the word equation for Glycolysis
 What is used up in glycolysis?
 What else is produced in glycolysis?
Glycolysis
Glycogen
NAD+ + 2H
 NADH + H+
(oxidised form )
(reduced form)
NB Rather then write NADH, examiners often simply
refer to it as reduced NAD or reduced coenzyme
You start with glycogen.
2 ATP
4 ADP + 4Pi
Glyceraldehyde
3-phosphate
(3C x 2)
Glucose
(6C)
2 ADP
4 ATP
Pyruvate
(3C x2)
2H x 2
NAD Reduced NAD
Nicotinamide adenine dinucleotide = So it gains electrons
You finish with Pyruvate
Respiration
The process of respiration can be split
into 4 stages.
1.
2.
3.
4.
Glycolysis
Link reaction
Krebs Cycle
Electron Transport Chain
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.
Products of Glycolysis
 2 reduced NAD (NADH + H+)
 2 Pyruvate
 2 ATP
The Fate of Pyruvate?
THIS DEPENDS ON THE
AVAILABILITY OF OXYGEN!!!
Aerobic Respiration
 If O2 is present 3C pyruvate passes into mitochondria.
 Here it is completely oxidised forming CO2 and H2O.
 The second stage of aerobic respiration is the link
reaction.
The Link Reaction
Cytoplasm
Mitochondria
The Link reaction
Glycolysis
The Krebs Cycle
 Links Glycolysis to the Krebs Cycle.
 The end product of the link reaction can enter the
Krebs Cycle.
NAD+
NADH + H+
Acetate
(2C)
Pyruvate
(3C)
Coenzyme A
Acetyl coenzyme A
(2C)
CO2
Coenzyme
A now
combines
with the
The Pyruvate
is decarboxylated
(CO2) to form
Start
with
Pyruvate
produced
in Acetate
Glycolysis
This
produces
Acetate
Acetyl coenzyme
It is also dehydrogenated
(H+Aremoved)
The Acetyl coenzyme A now enters Krebs cycle
Link Reaction
 One
carbon atom is
removed from pyruvate in
the form of CO2.
 The remaining 2-carbon
molecule combines with
coenzyme A to produce
acetylcoenzyme A (acetyl
CoA).
 Another oxidation reaction
occurs when NAD+ collects
more hydrogen ions. This
forms
reduced
NAD
(NADH + H+)
 No ATP is produced in this
reaction.
Pyruvate(3C)
Decarboxylation
CO2 (1C)
Oxidation
Coenzyme
(CoA)
NAD+
Reduction
NADH + H+
Acetyl CoA (2C)
The Link Reaction occurs Twice for
every Glucose Molecule
 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.
Overall equation for one link reaction and Kreb cycle:
Pyruvate + NAD + CoA
acetyl CoA +
reduced NAD + CO2
The Products of the Link Reaction go to
the Krebs Cycle and the ETC
So for each glucose molecule:
 2 acetylcoenzyme A (go into the Krebs cycle)
 2 Carbon dioxide (released as a waste products)
 2 Reduced NAD (go to the electron transport chain)
Krebs Cycle
 Sir Hans Krebs in 1937
 AKA Citric acid cycle or the Tricarboxylic acid cycle
(TCA cycle)
 Takes place in the matrix.
Krebs Cycle
 From one glucose molecules you get 2 pyruvate
molecules.
 So Krebs cycle go around twice per glucose molecule.
The Krebs Cycle
Acetyl Coenzyme A (2C)
2C
Oxaloacetate
(4C)
FAD
(4C)
(4C)
Coenzyme A
Citrate (6C)
(6C)
NAD
CO2
Reduced
FAD
(4C)
Reduced
NAD
(5C)
(4C)
DeNa DeNa A Fa... Na
 This little tune will help you to remember the
Krebs Cycle.
 Remember, respiration is all about releasing
energy from your food.
 Oxidation releases energy.
 When a carbon compound is oxidised,
coenzymes are reduced.
 The coenzymes involved are: NAD and FAD.
 Carboxylation is the removal of CO2.
 Remember this: 665 and five 4’s.
DENA DENA A FA... NA
 DENA: Decarboxylation and production of reduced NAD
 DENA: Decarboxylation and production of reduced NAD
 A: Production of ATP
 FA... : Production of reduced FAD (The ‘...’ means a gap)
 NA: Production of reduced NAD
The Krebs Cycle: DENA DENA A FA... NA
Acetyl Coenzyme A (2C)
2C
Oxaloacetate
(4C)
FAD
(4C)
(4C)
Coenzyme A
Citrate (6C)
(6C)
NAD
CO2
DENA
Reduced
NAD
FA
Reduced
FAD
(4C)
(5C)
(4C)
Krebs Cycle Summary
Each Acetyl CoA entering the cycle results in:
1.
2 CO2 molecules
2.
1 ATP molecules (S.L.P)
3.
8 pairs of H atoms (Later used in the E.T.C to produce ATP)
Used to reduce NAD and FAD. Three reduced NAD are produced and 1 reduced
FAD per cycle.
NAD = Nicotinamide adenine dinucleotide
FAD = Flavine adenine dinucleotide
Coenzymes in Respiration
• Coenzymes are complex organic molecules that are used
by enzymes to accept or donate molecules involved in a
reaction.
• They are often referred to as ‘Helper’ molecules as they
carry chemical groups or ions about, e.g. NAD removes H+
and carries it to other molecules.
Now carry out the practical about
Coenzymes in respiration
Homework
1. Complete the questions on the practical sheet
2. Read ‘coenzymes in respiration’ on page 55 of
the textbook then answer questions 1-5