Download CHAPTER 12 – RESPIRATION

Chapter 12 - Respiration
CHAPTER 12 – RESPIRATION
Respiration is a process which makes ATP using energy in organic molecules such
as glucose. It has three main stages - glycolysis, the Krebs cycle and oxidative
phosphorylation
12.1 An outline of respiration
Glucose molecules contain energy. The energy in glucose originally came from
sunlight which was captured by chlorophyll molecules and used to make glucose in
photosynthesis.
If glucose is placed in oxygen and set alight, it burns vigorously (Fig. 12.1).
Large amounts of heat energy are released as the glucose molecules combine with
oxygen to form carbon dioxide and water, and the energy from the glucose is rapidly
transferred to heat energy. This is an oxidation reaction:
In a living cell, a similar process takes place, but in a much more controlled
way. The glucose molecule is dismantled steadily, in a series of reactions catalysed
by enzymes. The energy in the glucose molecule is released in small stages, and
some of this energy is used to make ATP. The whole dismantling sequence has about
25 individual steps. Such a sequence is called a metabolic pathway; you can think
of it as being rather like a production line in a factory with each enzyme receiving its
substrate, carrying out its own small part of the overall task by catalysing the
conversion of the substrate to a product, and allowing the product to pass on to the
next enzyme.
The metabolic pathway of respiration is divided into three main stages (Fig.12.2).
 Firstly, in the cytoplasm of the cell, glucose is converted to pyruvate. This stage is
called glycolysis.
 Next, inside mitochondria, pyruvate is fed into a cycle of reactions called the
Krebs cycle.
 Finally, still inside mitochondria, electrons produced in the Krebs cycle are passed
along an electron transport chain, producing ATP in a process called oxidative
phosphorylation (Fig 12.5).
Fig 12.1 – A simple calorimeter with
which the amount of energy released by
the oxidation of glucose can be
measured.
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Fig 12.2 – the stages of
respiration
12.2 Adenosine triphosphate (ATP)
Adenosine triphosphate (ATP) is the short-term energy store of all cells. It is
easily transported and is therefore the universal energy carrier.
12.2.1 Structure of ATP
ATP is formed from the nucleotide adenosine monophosphate by the addition of two
further phosphate molecules. Its structure is shown in Fig. 12.3
12.2.2 Importance of ATP
The hydrolysis of ATP to ADP is catalyzed by the enzyme ATPase, and the
removal of the terminal phosphate yields 30.6 kJ molˉ¹ of free energy. Further
hydrolysis of ADP to AMP yields a similar quantity of energy, but the removal of the
last phosphate to give adenosine produces less than half this quantity of energy. For
this reason the first two phosphate bonds are often termed high energy bonds on
account of the relatively large quantity of energy they yield on hydrolysis, This is
misleading in that it implies that all the energy is stored in these bonds, The energy
is in fact stored in the molecule as a whole, although the breaking of the bonds
initiates its release.
AMP and ADP may be reconverted to ATP by the addition of phosphate
molecules in a process called phosphorylation, of which there are two main forms.
1. Photosynthetic phosphorylation – occurs during photosynthesis in chlorophyllcontaining cells.
2. Oxidative phosphorylation-occurs during cellular respiration in all aerobic
cells.
The addition of each phosphate molecule requires 30.6kJ of energy. If the energy
released from any reaction is less than this, it cannot be stored as ATP and is lost
as heat. The importance of ATP is therefore as a means of transferring free
energy from energy-rich compounds to cellular reactions requiring it. While not
the only substance to transfer energy in this way, it is by far the most abundant
and hence the most important.
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Fig 12.3 structure of ATP
12.2.3 Uses of ATP
A metabolically active cell may require up to two million ATP molecules every
second. ATP is the source of energy for:
1. Anabolic processes-It provides the energy needed to build up macromolecules
from their component units, e.g.
-polysaccharide synthesis from monosaccharides
-protein synthesis from amino acids
-DNA replication
2. movement – It provides the energy for many forms of cellular movement
including
-muscle contraction
-ciliary action
-spindle action in cell division
3. Active transport – it provides the energy necessary to move materials against
a Concentration gradient, e.g., ion pumps.
4. Secretion – it is needed to form the vesicles necessary in the secretion of cell
products.
5. Activation of chemicals – it makes chemicals more reactive, enabling them to
react more readily, e.g. the phosphorylation of glucose at the start of
glycolysis.
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Chapter 12 - Respiration
THE STAGES OF AEROBIC RESPIRATION
Fig 12.4
Glycolysis
Glycolysis (Glyco – ‘sugar’; lyso – ‘breakdown’) is the
breakdown of a hexose sugar, usually glucose, into two
molecules of the three-carbon compound pyruvate
(pyruvic acid). It occurs in all cells; in anaerobic
organisms it is the only stage of respiration. Although
they contain quite large amounts of energy, glucose
molecules are relatively unreactive. They must be
activated before glycolysis can proceed. This is done by
the addition of a phosphate group to the glucose
molecule, forming glucose phosphate (Fig. 12.4). The
atoms in this molecule are then rearranged to form
fructose phosphate, and another phosphate group
added to form fructose bisphosphate.
Each of these additions of a phosphate group is done by
transferring a phosphate group to the sugar from ATP.
The ATP is converted into ADP in the process. So far
then, far from making any ATP, glycolysis has actually
used two ATP molecules! However, as you will see, more
ATP molecules are made later, resulting in a net gain at
the end.
Next, the six-carbon fructose bisphosphate molecule is
split into two three-carbon molecules, triose phosphate.
Each of these is then converted to glycerate 3phosphate (GP) and then to pyruvate in a series of
small steps. These steps release enough energy from the
GP molecules to make some ATP.
Four molecules of ATP (two for each of the two
triose phosphate molecules) are made directly, there and then in the cytoplasm,
using energy released as the triose phosphate molecules are gradually changed
to pyruvate. So, although two molecules of ATP were put into the process at the
beginning, four have been made at the end.
However, this is not all the ATP which can be made in this process. The
conversion of triose phosphate into GP also releases hydrogen ions (H+) and
electrons (e-) which are transferred to the coenzyme NAD (nicotinamide adenine
dinucleotide) to form reduced NAD. These hydrogen ions and high energy
electrons are passed into a mitochondrion where they can be used to produce
up to 5 ATP molecules in oxidative phosphorylation. This, however, can only
happen if oxygen is available (Fig 12.6).
The overall balance sheet for one molecule of glucose undergoing glycolysis is
therefore:
ATP molecules
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produced
4
ATP molecules
used
2
net gain of
ATP molecules
2
In addition, the reduced NAD produced in glycolysis can yield up to 5
additional ATPs when oxygen is available.
Fig 12.5 - The four stages of aerobic respiration- the first stage, glycolysis,
occur in the cytosol. Pyruvate, the product of glycolysis, enters a mitochondrion,
where cellular respiration continues with the formation of acetyl coenzyme A, the
citric acid cycle and electron transport/chemiosmosis. Most ATP is synthesized by
chemiosmosis.
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Fig 12.6 -Overview of glycolysis. The energy investment phase of glycolysis
(left column) leads to the splitting od sugar; ATP and NADH are produced during
the energy capture phase (right colum). During glycolysis, each glucose molecule
is converted to two pyruvates, with a net yeid of two ATP molecules and two
NADH molecules.
Krebs (tricarboxylic acid) cycle
Although glycolysis releases a little of the energy from the glucose molecule, the
majority still remains ‘locked-up’ in the pyruvate. These pyruvate molecules enter
the mitochondria and, in the presence of oxygen, are broken down to carbon dioxide
and hydrogen atoms. The process is called the Krebs cycle, after its discoverer Hans
Krebs. There are a number of alternative names, notably the tricarboxylic acid cycle
(TCA cycle) and citric acid cycle (Fig 12.9). While the carbon dioxide produced is
removed as a waste product, the hydrogen atoms are oxidized to water in order to
yield a substantial amount of free energy. The 2-carbon acetyl coenzyme A now
enters the Krebs cycle (Fig.12.5) by combing 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-ketoglutarage (α-ketoglutaric acid) and then the 4-carbon
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oxaloacetate by the progressive loss of two carbon dioxide molecules, thus
completing the cycle.
At two stages in the Krebs cycle, carbon dioxide is removed from the compounds
involved. This process is called decarboxylation. This carbon dioxide, plus that
which was produced when pyruvate was converted to acetylcoenzyme A, diffuses out
of the mitochondrion, out of the cell, and eventually out of the organism.
Other important products of the Krebs cycle are electrons and hydrogen ions which
are both picked up by the oxidised form of the coenzyme NAD, and some by oxidised
FAD (flavin adenine dinucleotide):
NAD+ + H+ + 2e-  NADH
Oxidised NAD
reduced NAD
These coenzymes can hold electrons which will then be fed into the electron
transport chain to make ATP.
When one glucose molecule is respired, two pyruvates are produced and they
result in the production of six reduced NAD and two reduced FAD in the Krebs cycle.
Two more reduced NAD molecules are produced in the conversion of pyruvate to
acetylcoenzyme A.
In addition, one step in the Krebs cycle makes ATP directly. Two ATP molecules are
produced in this way per original glucose molecule.
Fig 12.8 formation of acetyle coenzyme
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Fig 12.9 - the formationof aectylcoenzyme A from pyruvate, and the
krebs cycle.
Fig 12.10
12.3.1 Importance of Krebs cycle
The Krebs cycle plays an important role in the biochemistry of a cell for three
main reasons:
1. It brings about the degradation of macromolecules – The 3-carbon for
pyruvate is broken down to carbon dioxide
2. It provides the reducing power for the electron (hydrogen) transport systemIt produces pairs of hydrogen atoms which are ultimately the source of
metabolic energy for the cell.
3. It is an interconversion center – It is valuable source of intermediate
compounds used in the manufacture of other substances, e.g. fatty acids,
amino acids, chlorophyll.
Oxidative phosphorylation
An important result of respiration is the formation of ATP. ATP is made by the
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addition of inorganic phosphate, Pi, to ADP, a phosphorylation reaction. In
respiration, this process requires oxygen and so is known as oxidative
phosphorylation (Fig. 12.14). (Compare this with the way in which ATP is made in
the light-dependent stages of photosynthesis, where light is required for the process
of photophosphorylation.)
The Krebs cycle takes place in the matrix of a mitochondrion. The next stage, in
which ATP is made, happens in the inner mitochondrial membrane. Within this
membrane lie the components of the electron transport chain.
Reduced NAD and FAD provide electrons for the synthesis of ATP. The electrons are
used to provide energy for ATP synthesis as they are passed along the electron
transport chain.
The electrons pass from the reduced NAD to the first member of this chain, and then
from one carrier to the next. As the electrons are passed along, ATP is made. It is
not certain exactly how many ATP molecules are made using the energy of four
electrons from every two reduced NAD. It was originally thought to be six, but is
now thought to be five or a little less.
You may have noticed that there has been no oxygen involved in any of these
processes so far. It is only now, right at the end of the electron transport chain, that
oxygen is needed. Its role is to combine with the electrons, as they come off the end
of the chain, and with hydrogen ions to form water. If there is not enough oxygen
available to do this, then the electron transport chain cannot work. This produces a
complete traffic jam, stopping the Krebs cycle completely.
Now that we have arrived at the end of all three stages of respiration, we can draw
up a complete balance sheet to show how many ATP molecules are made from one
glucose molecule when oxygen is available.
Fig 12.11
Electron
transport
system
The
electron
transport
system is the means by which the energy from the Krebs cycle, in the form of
hydrogen atoms, is converted to ATP. The hydrogen atoms attached to the hydrogen
carriers NAD and FAD are transferred to a chain of other carriers at progressively
lower energy levels (fig). as the hydrogens pass from one carrier to the next, the
energy released is harnessed to produce ATP. The series of carriers is termed the
respiratory chain. The carriers in the chain include NAD, flavoprotein, coenzyme Q
and iron- containing proteins called cytochromes. Initially hydrogen atoms are
passed along the chain, but these later split into their protons and electrons, and
only the electrons pass from carrier to carrier. For this reason, the pathway can be
called the electron, or hydrogen, transport system. At the end of the chain the
protons and electrons recombine, and the hydrogen atom created link with oxygen to
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form water. This formation of ATP through the oxidation of the hydrogen atoms is
called oxidative phosphorylation. It occurs in the mitochondria.
The role of oxygen is to act as the final acceptor of the hydrogen atoms.
While it only performs this function at the end of the many stages in respiration,
oxygen is nevertheless vital as it drives the whole process. In its absence, only the
anaerobic glycolysis stage can continue. The transfer of hydrogen atoms to oxygen is
catalysed by the enzyme cytochrome oxidase. This enzyme is inhibited by cyanide,
so preventing the removal of hydrogen atoms at the end of the respiratory chain. In
these circumstances the hydrogen atoms accumulate and aerobic respiration ceases,
making cyanide a most effective respiratory inhibitor.
Fig 12-17
F
ig
12.
12
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Fig 12.13
Fig 12.14
ANAEROBIC RESPIRATION
Respiration without oxygen
Oxygen is used as a final acceptor of electrons. As this oxygen comes from the air,
the process is called aerobic respiration.
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When oxygen is not available, however, oxidative phosphorylation and the Krebs
cycle cannot take place. This is also true in organisms or cells which do not contain
the machinery to carry these out; human red blood cells, for example, have no
mitochondria, and so can only perform glycolysis. In these circumstances, respiration
takes place without oxygen. It is anaerobic.
In anaerobic respiration, glycolysis takes place as usual, producing pyruvate and a
small yield of ATP. If pyruvate were allowed to build up it would inhibit glycolysis, so
it is converted to something else. More importantly, the reduced NAD which is
produced in glycolysis must be oxidised back to NAD again or the cell would soon run
out of it, bringing ATP production to a halt.
There are two different solutions to the problem, both of which get rid of the
pyruvate, and regenerate NAD. These are alcoholic fermentation, which is used by
fungi and plants, and lactic fermentation, which is used by animals.
• Alcoholic fermentation
Yeast converts pyruvate to ethanol. First, carbon dioxide is removed from pyruvate
to produce ethanal (Fig. 12.15). Next, the enzyme alcohol dehydrogenase (working
in the reverse direction to the one its name suggests) converts the ethanal to
ethanol. This step requires hydrogen which is taken from reduced NAD.
This process has been used by humans for thousands of years. If yeast is provided
with a supply of carbohydrate, it will carry out glycolysis and alcoholic fermentation.
In bread making we provide it with starch in flour, and make use of the carbon
dioxide it releases to make the bread rise. In the making of alcoholic drinks it is the
ethanol which is required.
• Lactic fermentation
This process contributes to the discomfort you may experience in exhausted
muscles. If muscle is exercising hard, it may run out of oxygen and have to respire
anaerobically for a short period. The pyruvate produced by glycolysis is converted, in
a single step, to lactate. The enzyme responsible for this conversion is lactate
dehydrogenase, and the process requires hydrogen from reduced NAD.
It is the build-up of lactate, which forms lactic acid, in muscles which causes the
pain. The lactate must be broken down, and for this to take place it is transported in
the blood to the liver. Here, some is converted back into glucose. This process
requires oxygen, which is why you go on breathing deeply even when your
strenuous exercise is over. You are supplying extra oxygen to the liver, to help it to
deal with the lactate produced because of a shortage of oxygen earlier on. You are
paying off an 'oxygen debt’.
Although lactic acid in muscles is unpleasant, we do enjoy lactic acid in other ways!
Yoghurt, soured cream and cheese all contain lactic acid, which helps to give them
their distinctive flavours. The lactic acid is produced during anaerobic respiration by
bacteria such as Lactobacillus bulgaricus. The bacteria are added to milk where they
use the sugar lactose as a respiratory substrate. The lactic acid moves out of the
bacteria into the surrounding milk. As well as providing flavour, the lactic acid lowers
the pH of the milk, causing the proteins in it to coagulate.
Neither alcoholic fermentation nor lactic fermentation produce any additional ATP.
So anaerobic respiration provides only two molecules of ATP for every glucose
molecule. This is much less than the 31 molecules of ATP which can be produced if
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aerobic respiration, involving the Krebs cycle and oxidative phosphorylation.
Fig 12.15 – Fermentation
Fig 12-16
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