Transcript Energy and Respiration

Energy and
Respiration
The need for energy in living organisms
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continuous supply of energy for:
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Synthesis of complex substances from simpler
ones (anabolic reactions)
Active transport
Mechanical work – movement
Maintenance of internal body temperature
ATP
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Adenosine triphosphate
Energy released is not then directly used,
it is passed on to ATP.
ATP is made of:
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Adenine
Ribose
3 phosphate molecules
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When a phosphate group is removed from
ATP, ADP is formed and energy is
released.
ATP + H2O = ADP + H3PO4 ± 30.5kJ
ATP is the universal intermediary
molecule. It is known as the energy
currency.
Synthesis of ATP
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Two ways: (see page 199)
1. energy released by reorganising chemical
bonds.
2. using electrical potential energy when
electrons are transferred by electron
carriers. This is called chemiosmosis.
Respiration
Organic molecules are broken down to release
energy to make ATP.
 Two types:
A) Aerobic respiration – in the presence of oxygen.
B) Anaerobic respiration – in the absence of
oxygen.
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Both start with glycolysis.
Glycolysis
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Phosphorylation (adding phosphate) of glucose
using ATP
Occurs in the cytoplasm.
Splitting hexose phosphate (6C) into two triose
phosphate molecules (3C)
These are then oxidised, releasing ATP and
reducing NAD
Nicotinamide
Adenine
Diphosphate
NAD
Consists of two
nucleotides joined
by their phosphate
groups
Transfers electrons
during respiration
reactions
Glucose (hexose) (6C)
Hexose phosphate (6C) produced by phosphorylation using ATP
Hexose bisphosphate (6C) adding another phosphate using ATP
This splits into two
2 molecules of triose phosphate (3C)
A sequence of Intermediate molecules are formed, by reducing
NAD and losing phosphates to produce 4 molecules of ATP
2 x Pyruvate (3C)
GLUCOSE (6C)
2ATP
2 ATP USED
2ADP
HEXOSE BIPHOSPHATE
TRIOSE PHOSPHATE (3C)
2ADP
2ATP
TRIOSE PHOSPHATE (3C)
NAD+
2ADP
NAD+
NADH
2ATP
NADH
PYRUVATE
4 ATP PRODUCED
PYRUVATE
Link reaction
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Occurs when oxygen available
Pyruvate enters the mitochondrion by
active transport.
It is decarboxylated (carbon removed)
Dehydrogenated (hydrogen removed)
As a result of this, CO2 is formed and NAD
is reduced
Krebs cycle
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Closed pathway of enzyme-controlled reactions
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Occurs in matrix of mitochondria
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Acetyl CoA (2C) enters the cycle and joins with a
4 carbon compound to make a 6 carbon
compound.
A series of steps now transfer the 6C (citrate)
back to the 4C (oxaloacetate)
These steps include more decarboxylation and
dehydrogenation
Pg 203
LINK REACTION. Pyruvate molecules (3-carbon)
from glycolysis are converted into another type
of molecule called Acetyl-CoA in a process
known as pyruvic oxidation. This conversion
occurs when the pyruvate is broken down by a
complex of 3 enzymes called pyruvate
dehydrogenase, releasing a carbon atom which
goes on to form carbon dioxide (CO2). The 2
remaining carbon molecules bond with
coenzyme A forming Acetyl-CoA. During this
process, electrons and a hydrogen ion are
passed to NAD+, thus oxidizing the pyruvate,
hence the name of the process.
Step 1. The Acetyl-CoA then enters the
Krebs cycle. It initially combines with a
4-carbon molecule called oxoaloacetic
acid, forming a 6-carbon molecule of
citric acid (citrate). This reaction is
catalyzed by the enzyme citrate
synthase. Upon this formation, the
coenzyme A is released, returning to the
link reaction.
Step 2. The citrate molecule is then
dehydrated (H20 molecule is removed)
and then rehydrated by the enzyme
aconitase. The resulting molecule is just
a rearranged form of citrate known as
isocitrate.
Step 3. Next, isocitrate undergoes what is
known as a oxidative carboxylation, which
simply means that a carbon and hydrogen
are given off. The result of this is a 5-carbon
molecule called alpha-ketoglutarate. This
process is catalyzed by the enzyme
isocitrate dehydrogenase. Additionally, the
carbon that broke off forms CO2, while the
hydrogen reduces NAD+ to form NADH.
Step 4. In the next reaction, alphaketoglutarate has yet another carbon
molecule removed and is then
transferred to a CoA molecule by the
enzyme alpha-ketoglutarate
dehydrogenase. The resulting product is
a 4-carbon molecule of Succinyl-CoA.
Additionally, CO2 and NADH is formed.
Step 5. After succinyl-CoA is formed, the
molecule then undergoes the removal of the
CoA carrier, resulting in the production of
succinate. Additionally, the enzyme succinylCoA synthetase that removes the CoA also
produces GTP (Guanosine Triphosphate)
through substrate level phosphorylation
(phosphate molecule directly added to another
molecule). (GTP is a high energy molecule
similar to ATP, and later an ADP molecule
takes the phosphate from GTP and makes
ATP)
Step 6. Next, succinate is dehydrated by the
enzyme succinate dehydrogenase. The
resulting product is furmate.
Step 7. Furmate is then hydrated (water
added) by enzyme furmase to form malate
Step 8. Lastly, the malate is dehydrogenated
by the enzyme malate dehydrogenase,
forming the original molecule oxaloacetate.
From this reaction, NADH and H+ are also
produced.
SUMMARY
Every pyruvate molecule that enters the Krebs cycle generates 3
molecules of CO2, one molecule of ATP, one molecule of FADH and 4
molecules of NADH
ADP+P
ATP
Pyruvate
3CO2
4NAD+
4NADH
FAD+
FADH
The reduced NAD and FAD molecules enter the electron transfer
chain, and result in a large number of ATP molecules being
produced.
Electron Transport Chain
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NADH and FADH2 oxidised - electron and proton
released
electron picked up by an electron carrier on the
inner membrane
It is passed from one acceptor to another along
a chain.
electron has a high potential energy at
beginning of chain but as it is passed along the
electron falls to a lower energy state.
energy released actively pumps the hydrogen
ion (proton) into the intermembrane space.
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electron reaches the end of the chain it
rejoins to the hydrogen ion to make a
hydrogen atom.
These hydrogen atoms then join to
oxygen to form water.
Chemiosmosis
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hydrogen ions actively transported into the
intermembrane space.
Chemiosmosis is the movement of ions across
a selectively-permeable membrane, down their
electrochemical gradient.
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concentration of hydrogen ions in the
intermembrane space builds up so diffusion
occurs.
The hydrogen ions move through a protein
channel and as they move they provide energy
for ATP synthase to join ADP and P to make
ATP.
- If there is no oxygen there is no where for
the hydrogen to go
- which then blocks the electron transport
chain
- which stops the NAD from being
regenerated
- so the krebs cycle is blocked
- so the link reaction is blocked
- and only glycolysis can occur – anaerobic
respiration.
Anaerobic Respiration
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To regenerate NAD to be able to continue
glycolysis, pyruvate becomes the hydrogen
acceptor.
This either forms lactic acid or ethanol.
In animals end product is lactic acid
C6H12O6 → 2CH3CH(OH)COOH + 2 ATP
In plants and yeast end product is ethanol and
carbon dioxide
C6H12O6 → 2CH3CH2OH + CO2 + 2ATP
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Lactic acid is produced just by adding 2
hydrogen molecules to pyruvate.
Ethanol is produced by first removing a
carbon molecule (releasing carbon
dioxide) and then adding the 2 hydrogen
molecules. That is why alcoholic
fermentation is accompanied by evolution
of carbon dioxide.
What happens to the products of
anaerobic respiration?
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Both lactic acid and ethanol contain a lot of
energy.
In animals this energy can be released by
changing lactic acid back to pyruvate and then
pyruvate continuing on the rest of the aerobic
respiration pathways.
This requires oxygen to unblock the ETC and
Krebs cycle
The amount of oxygen required to do this is
called the oxygen debt.
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Plants cannot use the ethanol.
It cannot be converted back into pyruvate
and it cannot be oxidised
The ethanol is toxic and if anaerobic
respiration continues for too long the plant
will be poisoned and die.
Seeds and plants growing in waterlogged
conditions can respire anaerobically for a
short time.
Respiratory Quotient
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It is a unitless number used in calculations
of basal metabolic rate (BMR)
It is the ratio of the volume of carbon
dioxide released to the volume of oxygen
consumed by a body tissue or an
organism in a given period.
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The respiratory quotient (RQ) is
calculated from the ratio:
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RQ = CO2 eliminated / O2 consumed
The range of respiratory coefficients for
organisms in metabolic balance usually
ranges from 1.0 (representing the value
expected for pure carbohydrate oxidation)
to ~0.7 (the value expected for pure fat
oxidation).
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Carbohydrates
The value of RQ is equal to 1 if carbohydrates are the respiratory
substrates in aerobic respiration.
Fats
When the respiratory substrate is fat, the RQ is about 0.7.
Example: Tripalmitin
Fats contain less oxygen than carbohydrates and so they require
more oxygen for oxidation.
Anaerobic respiration
The value of RQ is infinity during anaerobic respiration because
CO2 is produced but O2 is not utilised.
Measuring RQ
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This is done by measuring the change in the volume of
gas surrounding the material as it respires –
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first as carbon dioxide is absorbed (to measure the rate of
oxygen consumption)
and then without absorbing the carbon dioxide (from which you
can calculate the rate of production of carbon dioxide by
comparison with the first measurment).
The apparatus consists of two vessels. One vessel
contains the organisms and the other acts as
a thermobarometer – small changes in temperature or
pressure cause air in this vessel to expand or contract,
compensating for similar changes in the first vessel.
Changes in the manometer level are thus due only to the
activities of the respiring material.