Download 2-respiration

Higher Biology
Unit 2
2.2 Cellular Respiration
Respiration
Respiration is a catabolic pathway that is
controlled by different enzymes.
It releases energy from a molecule of 6
carbon glucose by breaking it down into
smaller, simpler molecules.
Respiration
Respiration consists of 3 metabolic
pathways;
1. Glycolysis
2. Citric acid cycle
3. Electron transport chain
Each pathway has a different location in
the cell.
Location of Pathways
Glycolysis occurs in the
cytoplasm.
Citric acid cycle occurs
in the central matrix of
mitochondria.
Electron transport
chain occurs in cristae
of mitochondria.
Metabolism and survival
Structure of Mitochondrion
site of electron transport chain
site of citric acid cycle
Glycolysis
• occurs in the
cytoplasm
• is anaerobic – it does
not require oxygen
• splits one molecule of
glucose (6C) into two
molecules of pyruvate
(3C)
Glycolysis
During glycolysis there is a net gain of 2ATP.
This is because;
• The first part of
glycolysis uses 2 ATP.
This is the energy
investment phase.
• The second part
produces 4 ATP. This is
the energy pay off
phase.
Glycolysis
• Hydrogen ions are
removed.
• They are removed by a
dehydrogenase enzyme.
• The H ions then combine
with a hydrogen acceptor
called NAD.
• This reduces NAD to
NADH.
Glycolysis
Glucose
6C
2ATP
2ADP + 2Pi
To electron
2NAD
transport
chain if
2NADH2
oxygen is
(reduced)
present
4ADP + 4Pi
4ATP
Energy
investment
phase
Energy
pay off
phase
Net gain of 2ATP molecules
Pyruvate
3C
Fermentation if
oxygen is absent
Aerobic Respiration
If oxygen is present aerobic respiration
takes place and;
• 2NADH carry H ions onto the electron
transport chain.
• Pyruvate progresses to the citric acid
cycle.
Aerobic Respiration
• Pyruvate is broken down to
carbon dioxide and an
acetyl group.
• The acetyl group combines
with coenzyme A to form
a 2C compound called
acetyl coenzyme A (acetyl
coA).
• Dehydrogenase removes
more H ions which reduce
NAD to form NADH.
pyruvate
CO2
NAD
NADH
acetyl Co A
To citric acid cycle
Citric Acid Cycle
• Occurs in the matrix of
mitochondria.
• 2C acetyl coA enters the
cycle and combines with a
4C compound called
oxaloacetate.
• This forms a 6C
compound called citrate.
• A series of enzymecontrolled reactions then
regenerate oxaloacetate
from citrate.
Citric Acid Cycle
• Dehydrogenase removes H
ions along with high energy
electrons.
• They combine with NAD to
form NADH and another
coenzyme called FAD to
form FADH2.
• These transport H ions and
high energy electrons to
the electron transport
chain.
• ATP and carbon dioxide
are also produced.
Questions
1. Explain why the phosphorylation of
intermediates in glycolysis is described
as an energy investment phase.
(2)
2. State the role of dehydrogenase
enzymes in glycolysis and the citric
acid cycle.
(1)
3. Describe the role of the co-enzymes
NAD and FAD.
(2)
ATP Production
In respiration so far;
• ATP has been synthesised directly
• Hydrogen ion and electron carriers
NADH and FADH2 have been
synthesised
NADH and FADH2 are now going to be
used to produce more ATP molecules,
using the enzyme ATP synthase.
ATP Synthase
• To synthesise most of its ATP, a cell
uses a source of high energy electrons.
• These are used to pump H ions across a
membrane.
• The return flow of the H ions rotates
part of the membrane protein ATP
synthase which catalyses the synthesis
of ATP.
ATP Synthase
• ATP synthase is found in
membranes of
mitochondria and
chloroplasts.
• Hydrogen ions flow from
a high concentration to a
low concentration across
ATP synthase.
• This makes part of ATP
synthase rotate and
catalyse the synthesis of
ATP from ADP and Pi
Electron Transport Chain
• The electron transport chain is a series
of proteins imbedded in the cristae of
mitochondria.
• H ions and the high energy electrons
are brought here by NAD and FAD from
glycolysis and the citric acid cycle.
• These will be used to produce many
molecules of ATP using ATP synthase.
Electron Transport Chain
• At the cristae, NADH and FADH2 pass
high-energy electrons along the
electron transport chain.
• The energy from these electrons cause
H ions to be pumped across the
membrane.
ATP synthase
Electron Transport Chain
• The hydrogen ions pass through ATP
synthase
• This causes ATP synthase to produce
ATP (from ADP and Pi).
Electron Transport Chain
• When the electrons come to the end of
the electron transport chain they
combine with oxygen.
• At the same time, the oxygen joins to a
pair of hydrogen ions to form water.
• Oxygen is the final hydrogen acceptor.
Without it, the electron transport
chain cannot not proceed and ATP is
not made here.
Electron Transport Chain
Questions
1. Name the enzyme embedded in the
inner membrane of a mitochondrion,
responsible for the regeneration of
ATP.
(1)
2. Describe the role of the high-energy
electrons transported to the electron
transport chain.
(2)
3. State the role of oxygen in the
electron transport chain.
(1)
Fermentation
• If oxygen is not available, then the
citric acid cycle and electron transport
chain cannot proceed.
• This is because there is no final
hydrogen acceptor and hydrogen cannot
pass through the electron transport
chain.
• After glycolysis, pyruvate is fermented.
• This produces different products in
different cell types.
Fermentation - Animals
• The body builds up an oxygen debt.
• Pyruvate (3C) is converted into lactate
(3C)
• If lactate builds up in tissues, it causes
muscle fatigue.
• When oxygen becomes available the
lactate can be converted back into
pyruvate.
Fermentation – Plants & Yeasts
• Pyruvate (3C) is converted into ethanol
(2C) and carbon dioxide (1C)
• Even if oxygen becomes available to the
cell, the ethanol cannot be converted
back into pyruvate (as a carbon atom is
lost when carbon dioxide is released).
Respiration Vs Fermentation
Respiration
Fermentation
Oxygen
requirements
Oxygen required
No oxygen required
Breakdown of
glucose
Complete breakdown
(efficient)
Incomplete breakdown
(inefficient)
End products
Carbon dioxide & water
Animals & some bacteria:
lactate
Plants & yeasts; ethanol
& CO2
Energy yield
38 ATP per molecule of
glucose
2 ATP per molecule of
glucose
Location
Cytoplasm
Matrix of mitochondria
Cristae of mitochondria
Cytoplasm
Alternative Substrates
Other carbohydrates;
• Starch and glycogen are
complex carbohydrates
made of glucose
molecules.
• Other sugar molecules
such as maltose and
sucrose can be converted
to the intermediates of
glycolysis.
Alternative Substrates
Fat is broken down to fatty
acids and glycerol.
• Glycerol can then be
converted to an
intermediate of glycolysis.
• Fatty acids are broken
down to intermediates of
the citric acid cycle.
Alternative Substrates
Proteins are broken
down to amino acids.
Any amino acids not used
during protein synthesis
are converted to
intermediates of
glycolysis or the citric
acid cycle.
Key Points
Glycolysis
• Occurs in the cytoplasm
• Breaks down glucose to pyruvate.
• Phosphorylation of intermediates uses 2ATP in an
energy investment phase.
• 4ATP is produced an energy pay-off stage.
• This gives a net gain of 2ATP.
• H ions are removed by dehydrogenase enzymes.
• H ions join to NAD to reduce it to NADH.
• H ions are transported to the electron transport
chain.
Key Points
• Pyruvate progresses to the citric acid
cycle if oxygen is available.
• Pyruvate is broken down to an acetyl
group.
• The acetyl group combines with coenzyme
A to form acetyl CoA.
• Acetyl CoA is transferred to the citric
acid cycle.
Key Points
Citric Acid Cycle
• Occurs in the matrix of mitochondria
• Acetyl CoA (2C) joins to oxaloacetate (4C).
• This forms citrate (6C).
• Citrate proceeds through the citric acid cycle to
regenerate oxaloacetate.
• ATP and carbon dioxide are produced during this.
• H ions and high-energy electrons are removed by
dehydrogenases.
• These reduce NAD and FAD to NADH and
FADH2.
• They are taken to the electron transport chain.
Key Points
Electron Transport Chain
• A collection of proteins attached to the
cristae of mitochondria.
• High-energy electrons release energy.
• This is used to pump H ions across the inner
mitochondrial membrane.
• The return flow of H ions rotates ATP
synthase.
• This generates ATP.
• The final hydrogen acceptor is oxygen.
• Oxygen combines with H ions to form water.
Key Points
Fermentation
Key Points
Glucose is the main substrate for
respiration. However, alternative
substrates include;
• Starch
• Glycogen
• Other sugars such as maltose or sucrose
• Fatty acids
• Glycerol
• Amino acids