4.1.1 Respiration
• state that glycolysis takes place in the cytoplasm;
• outline the process of glycolysis beginning with
the phosphorylation of glucose to hexose
bisphosphate, splitting of hexose bisphosphate
into two triose phosphate molecules and further
oxidation to pyruvate, producing a small yield of
ATP and reduced NAD;
• state that, during aerobic respiration in animals,
pyruvate is actively transported into
mitochondria;
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
Phosphorylation
2 phosphates are
added
2ADP
Hexose bisphosphate
6C
H2O
2 x triose phosphate
4ADP + 4Pi
Oxidation
(as Hydrogen
is lost)
2H
4ATP
(Hydrolysis)
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
• Overall, 2 molecules of pyruvate and a small
yield of ATP and reduced NAD are produced
What now?
• The pyruvate (pyruvic acid) is now moved by
active transport from the cell cytoplasm into
the mitochondrial matrix for the next stage of
aerobic respiration
Questions
1. Outline the role of co-enzymes in the
glycolysis pathway
2. Explain why the net gain of ATP during
glycolyisis is two and not four
3. Explain how oxidation occurs during
glycolysis, although no oxygen is involved
Questions
1. Outline the role of co-enzymes in the glycolysis
pathway the coenzyme NAD accepts hydrogen atoms
from the substrate molecules as triose phosphate is
oxidised
2. Explain why the net gain of ATP during glycolyisis is
two and not four because two molecules of ATP are
used to activate hexose sugar at the beginning of the
process, four ATP are made so the net gain is two
3. Explain how oxidation occurs during glycolysis,
although no oxygen is involved hydrogen atoms are
removed from triose phosphate and combine with
NAD which is an oxidation reaction
A
C
C
B
Task
• Now (from memory only) on A3 paper, draw
the process of glycolysis...
• Remember to leave space for the other 3
stages!!!
glucose
Number of
carbons in
the molecule
6C
2ATP
2Pi
Phosphorylation
2 phosphates are
added
2ADP
Hexose bisphosphate
6C
H2O
2 x triose phosphate
4ADP + 4Pi
Oxidation
(as Hydrogen
is lost)
2H
4ATP
(Hydrolysis)
3C
2NAD
2NADH
2 x pyruvate 3C
4.1.1 The Link Reaction
• state that the link reaction takes place in the
mitochondrial matrix;
• outline the link reaction, with reference to
decarboxylation of pyruvate to acetate and
the reduction of NAD;
• explain that acetate is combined with
coenzyme A to be carried to the next stage;
Glycolysis: True or False
a) Glycolysis produces a net gain of 4 ATP molecules
b) The co-enzyme NADH is oxidised during glycolysis
c) When coenzymes become reduced, they carry
hydrogen molecules
d) Glycolysis takes place in the matrix of the
mitochondrion
e) In glycolysis, glucose is converted in this order:
glucose, triose phosphate, hexose bisphosphate then
pyruvate
f) Hydrolysis occurs when triose phosphate is converted
to pyruvate
If you think they are false, you must write the correct version- and there are false
ones!!!
Glycolysis: True or False
a)
b)
c)
d)
e)
f)
Glycolysis produces a net gain of 4 ATP molecules- False: the net
gain is 2 ATPs as 2 are used up
The co-enzyme NADH is oxidised during glycolysis- False: NAD is
reduced during glycolysis to form NADH
When coenzymes become reduced, they carry hydrogen
molecules- False: they carry hydrogen atoms, not molecules or
ions
Glycolysis takes place in the matrix of the mitochondrion- False: it
takes place in the cytoplasm
In glycolysis, glucose is converted in this order: glucose, triose
phosphate, hexose bisphosphate then pyruvate- False: the correct
order is glucose, hexose bisphosphate, triose phosphate, pyruvate
Hydrolysis occurs when triose phosphate is converted to pyruvateFalse: hydrolysis occurs when hexose bisphosphate is converted to
triose phosphate
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
• The link reaction happens in the matrix of the
mitochondrion
Number of
carbons in
the molecule
pyruvate 3C
CO2
acetate
1C
2C
NAD
Coenzyme A
(CoA)
NADH
Acetyl CoA
2C
Decarboxylation- removal
of a carboxyl group
(which becomes CO2) by
the enzyme pyruvate
decarboxylase
Dehydrogenation- the
enzyme pyruvate
dehydrogenase removes
hydrogen atoms from
pyruvate and NAD
accepts it
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
(decarboxylation)
• Two molecules of reduced NAD are formed
and go into the electron transport chain
(dehydrogenation)
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 3carbon molecule triose phosphate (1). This is
then oxidised by removing hydrogen ions (1)
to give 2 molecules of 3-carbon 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 (decarboxylation of
pyruvate) (1). The coenzyme NAD is converted
into reduced NAD in this reaction by accepting
hydrogen ions (dehydrogenation of pyruvate)
The Challenge
• Can you draw glycolysis and the link
reaction….?