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(The Biology Project, University of Arizona) Anaerobic Breakdown of Pyruvate
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Metabolism refers to all the chemical reactions taking place in a cell. There are
thousands of these in a typical cell, and to make them easier to understand,
biochemists arrange them into metabolic pathways. The intermediates in
these metabolic pathways are called metabolites.
and respiration are the reverse of each other, and you couldnít have one
without the other. The net result of all the photosynthesis and respiration by
living organisms is the conversion of light energy to heat energy.
All living cells require energy, and this energy is provided by respiration.
carbon dioxide + water
form is this energy in? Itís in the form of chemical energy stored in a
compound called ATP (adenosine triphosphate). So all respiration really
does is convert chemical energy stored in glucose into chemical energy stored in
ATP. ATP is a nucleotide, one of the four found in DNA (see module 2), but it
also has this other function as an energy storage molecule. ATP is built up from
ADP and phosphate (PO43-)
, abbreviated to Pi):
All the processes in a cell that require energy (such as muscle contraction, active transport and biosynthesis) use ATP to provide that energy. So these processes all involve ATPase enzymes, which catalyse the breakdown of ATP to ADP + Pi, and make use of the energy released. So the ATP molecules in a cell are constantly being cycled between ATP and ADP + Pi.
is essentially the reverse of respiration. It is usually simplified to:
dioxide + water (+ light
glucose + oxygen
again this simplification hides numerous separate steps. To understand
photosynthesis in detail we can break it up into 2 stages:
shall see that there are many similarities between photosynthesis and
respiration, and even the same enzymes are used in some steps.
Photosynthesis takes place entirely within chloroplasts. Like
mitochondria, chloroplasts have a double membrane, but in addition chloroplasts
have a third membrane called the thylakoid membrane. This is folded into
thin vesicles (the thylakoids), enclosing small spaces called the thylakoid
lumen. The thylakoid vesicles are often layered in stacks called grana.
The thylakoid membrane contains the same ATP synthase particles found in
mitochondria. Chloroplasts also contain DNA, tRNA and ribososomes, and they
often store the products of photosynthesis as starch grains and lipid droplets.
Chloroplasts contain two different kinds of chlorophyll, called
chlorophyll a and b, together with a number of other light-absorbing accessory
pigments, such as the carotenoids and luteins (or xanthophylls).
These different pigments absorb light at different wavelengths, so having
several different pigments allows more of the visible spectrum to be used. The absorption
spectra of pure samples of some of these pigments are shown in the graph on
the left. A low absorption means that those wavelengths are not absorbed and
used, but instead are reflected or transmitted. Different species of plant have
different combinations of photosynthetic pigments, giving rise to different
coloured leaves. In addition, plants adapted to shady conditions tend to have a
higher concentration of chlorophyll and so have dark green leaves, while those
adapted to bright conditions need less chlorophyll and have pale green leaves.
measuring the rate of photosynthesis using different wavelengths of light, an action
spectrum is obtained. The action spectrum can be well explained by the
absorption spectra above, showing that these pigments are responsible for
is a fairly small molecule (not a protein) with a structure similar to haem, but
with a magnesium atom instead of iron. Chlorophyll and the other pigments are
arranged in complexes with proteins, called photosystems. Each
photosystem contains some 200 chlorophyll molecules and 50 molecules of
accessory pigments, together with several protein molecules (including enzymes)
and lipids. These photosystems are located in the thylakoid membranes and they
hold the light-absorbing pigments in the best position to maximise the
absorbance of photons of light. The chloroplasts of green plants have two kinds
of photosystem called photosystem I (PSI) and photosystem II (PSII).
These absorb light at different wavelengths and have slightly different jobs in
the light dependent reactions of photosynthesis.
light-dependent reactions take place on the thylakoid membranes using four
membrane-bound protein complexes called photosystem I (PSI), photosystem II (PSII),
cytochrome complex (C) and ferredoxin complex (FD). In these reactions light
energy is used to split water, oxygen is given off, and ATP and hydrogen are
Chlorophyll molecules in PSII absorb photons of light, exciting
chlorophyll electrons to a higher energy level and causing a charge
separation within PSII. This charge separation drives the splitting (or photolysis)
of water molecules to make oxygen (O2), protons (H+) and
O2 + 4H+ + 4e-
Water is a very stable molecule and it requires the
energy from 4 photons of light to split 1 water molecule. The oxygen produced
diffuses out of the chloroplast and eventually into the air; the protons build
up in the thylakoid lumen causing a proton gradient; and the electrons from
water replace the excited electrons that have been ejected from chlorophyll.
The excited, high-energy electrons are passed along a chain of protein
complexes in the membrane, similar to the respiratory chain. They are passed
from PSII to C, where the energy is used to pump 4 protons from stroma to lumen;
then to PSI, where more light energy is absorbed by the chlorophyll molecules
and the electrons are given more energy; and finally to FD.
In the ferredoxin complex each electron is recombined with a proton to
form a hydrogen atom, which is taken up by the hydrogen carrier NADP.
Note that while respiration uses NAD to carry hydrogen, photosynthesis always
uses its close relative, NADP.
The combination of the water splitting and the proton pumping by the
cytochrome complex cause protons to build up inside the thylakoid lumen. This
generates a proton gradient across the thylakoid membrane. This gradient is used
to make ATP using the ATP synthase enzyme in exactly the same way as
respiration. This synthesis of ATP is called photophosphorylation because
it uses light energy to phosphorylate ADP.
light-independent, or carbon-fixing reactions, of photosynthesis take
place in the stroma of the chloroplasts and comprise another cyclic
pathway, called the Calvin Cycle, after the American scientist who
Carbon dioxide binds to the 5-carbon sugar ribulose bisphosphate (RuBP)
to form 2 molecules of the 3-carbon compound glycerate phosphate. This
carbon-fixing reaction is catalysed by the enzyme ribulose bisphosphate
carboxylase, always known as rubisco. It is a very slow and inefficient
enzyme, so large amounts of it are needed (recall that increasing enzyme
concentration increases reaction rate), and it comprises about 50% of the mass
of chloroplasts, making the most abundant protein in nature. Rubisco is
synthesised in chloroplasts, using chloroplast (not nuclear) DNA.
Glycerate phosphate is an acid, not a carbohydrate, so it is reduced and
activated to form triose phosphate, the same 3-carbon sugar as that found
in glycolysis. The ATP and NADPH from the light-dependent reactions provide the
energy for this step. The ADP and NADP return to the thylakoid membrane for
Triose phosphate is a branching point. Most of the triose phosphate
continues through a complex series of reactions to regenerate the RuBP and
complete the cycle. 5 triose phosphate molecules (15 carbons) combine to form 3
RuBP molecules (15 carbons).
Every 3 turns of the Calvin Cycle 3 CO2 molecules are fixed to
make 1 new triose phosphate molecule. This leaves the cycle, and two of these
triose phosphate molecules combine to form one glucose molecule using the
glycolysis enzymes in reverse. The glucose can then be used to make other
material that the plant needs.
The equation for cellular respiration is usually simplified to:
in fact respiration is a complex metabolic pathway, comprising at least 30
separate steps. To understand respiration in detail we can break it up into 3
we look at these stages in detail, there are a few points from this summary.
Much of respiration takes place in the mitochondria. Mitochondria have a
double membrane: the outer membrane contains many protein channels called
porins, which let almost any small molecule through; while the inner
membrane is more normal and is impermeable to most materials. The inner
membrane is highly folded into folds called christae, giving a larger
surface area. The electron microscope reveals blobs on the inner membrane, which
were originally called stalked particles. These have now been identified
as the enzyme complex that synthesises ATP, are is more correctly called ATP
synthase (more later). The space inside the inner membrane is called the matrix,
and is where the Krebs cycle takes place. The matrix also contains DNA, tRNA and
ribosomes, and some genes are replicated and expressed here.
is not a single reaction, but consists of about 30 individual reaction steps.
For now we can usefully break respiration into just two parts: anaerobic and
The first part of respiration is simply the breakdown of glucose to a compound called pyruvate. This doesnít require oxygen, so is described as anaerobic respiration (without air). It is also called glycolysis and it takes place in the cytoplasm of cells. It only produces 2 molecules of ATP per molecule of glucose.
Normally pyruvate goes straight on to the aerobic part, but if there is no oxygen it is converted to lactate (or lactic acid) instead. Lactate stores a lot of energy, but it isnít wasted: when oxygen is available it is converted back to pyruvate, which is then used in the aerobic part of respiration.
The second part of respiration is the complete oxidation of pyruvate to carbon dioxide and water. Oxygen is needed for this, so it is described as aerobic respiration (with air). It takes place in the mitochondria of cells and produces far more ATP: 34 molecules of ATP per molecule of glucose.
Fats (mainly triglycerides) can also be used in aerobic respiration (but not anaerobic) to produce ATP.
Details of Respiration [back to top]
Glucose enters cells from the tissue fluid by passive transport using a
specific glucose carrier. This carrier can be controlled (gated) by hormones
such as insulin, so that uptake of glucose can be regulated.
The first step is the phosphorylation of glucose to form glucose
phosphate, using phosphate from ATP. Glucose phosphate no longer fits the
membrane carrier, so it canít leave the cell. This ensures that pure glucose
is kept at a very low concentration inside the cell, so it will always diffuse
down its concentration gradient from the tissue fluid into the cell. Glucose
phosphate is also the starting material for the synthesis of pentose sugars (and
therefore nucleotides) and for glycogen.
Glucose is phosphorylated again (using another ATP) and split into two triose
phosphate (3 carbon) sugars. From now on everything happens twice per
original glucose molecule.
triose sugar is changed over several steps to form pyruvate, a 3-carbon
compound. In these steps some energy is released to form ATP (the only ATP
formed in glycolysis), and a hydrogen atom is also released. This hydrogen atom
is very important as it stores energy, which is later used by the respiratory
chain to make more ATP. The hydrogen atom is taken up and carried to the
respiratory chain by the coenzyme NAD, which becomes reduced in the
+ 2H ŗ
NADH + H+
(oxidised form ) (reduced form)
NB Rather then write NADH, examiners often simple refer to it as reduced NAD or reduced coenzyme
Pyruvate marks the end of glycolysis, the first
stage of respiration. In the presence of oxygen pyruvate enters the
mitochondrial matrix to proceed with aerobic respiration, but in the
absence of oxygen it is converted into lactate (in animals and bacteria) or
ethanol (in plants and fungi). These are both examples of anaerobic
respiration. Pyruvate can also be turned back into glucose by reversing
glycolysis, and this is called gluconeogenesis.
Once pyruvate has entered the inside of the mitochondria (the matrix), it
is converted to a compound called acetyl coA. Since this step is between
glycolysis and the Krebs Cycle, it is referred to as the link reaction.
In this reaction pyruvate loses a CO2 and a hydrogen to form a
2‑carbon acetyl compound, which is temporarily attached to another
coenzyme called coenzyme A (or just coA), so the product is called acetyl coA.
The CO2 diffuses through the mitochondrial and cell membranes by
lipid diffusion, out into the tissue fluid and into the blood, where it is
carried to the lungs for removal. The hydrogen is taken up by NAD again.
acetyl CoA then enters the Krebs Cycle, named after Sir Hans Krebs, who
discovered it in the 1940s at Leeds University. It is one of several cyclic
metabolic pathways, and is also known as the citric acid cycle or the
tricarboxylic acid cycle. The 2-carbon acetyl is transferred from acetyl coA to
the 4-carbon oxaloacetate to form the 6-carbon citrate. Citrate is then
gradually broken down in several steps to re-form oxaloacetate, producing carbon
dioxide and hydrogen in the process. As before, the CO2 diffuses out
the cell and the hydrogen is taken up by NAD, or by an alternative hydrogen
carrier called FAD. These hydrogens are carried to the inner mitochondrial
membrane for the final part of respiration.
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respiratory chain (or electron transport chain) is an unusual metabolic
pathway in that it takes place within the inner mitochondrial membrane,
using integral membrane proteins. These proteins form four huge trans-membrane
complexes called complexes I, II, II and IV. The complexes each contain up to 40
individual polypeptide chains, which perform many different functions including
enzymes and trans-membrane pumps. In the respiratory chain the hydrogen atoms
from NADH gradually release all their energy to form ATP, and are finally
combined with oxygen to form water.
NADH molecules bind to Complex I and release their hydrogen atoms as
protons (H+) and electrons (e-). The NAD molecules then
returns to the Krebs Cycle to collect more hydrogen. FADH binds to complex II
rather than complex I to release its hydrogen.
The electrons are passed down the chain of proteins complexes from I to
IV, each complex binding electrons more tightly than the previous one. In
complexes I, II and IV the electrons give up some of their energy, which is then
used to pump protons across the inner mitochondrial membrane by active transport
through the complexes. Altogether 10 protons are pumped across the membrane for
every hydrogen from NADH (or 6 protons for FADH).
In complex IV the electrons are combined with protons and molecular
oxygen to form water, the final end-product of respiration. The oxygen diffused
in from the tissue fluid, crossing the cell and mitochondrial membranes by lipid
diffusion. Oxygen is only involved at the very last stage of respiration as the
final electron acceptor, but without the whole respiratory chain stops.
The energy of the electrons is now stored in the form of a proton
gradient across the inner mitochondrial membrane. Itís a bit like using
energy to pump water uphill into a high reservoir, where it is stored as
potential energy. And just as the potential energy in the water can be used to
generate electricity in a hydroelectric power station, so the energy in the
proton gradient can be used to generate ATP in the ATP synthase enzyme. The ATP
synthase enzyme has a proton channel through it, and as the protons ďfall
downĒ this channel their energy is used to make ATP, spinning the globular
head as they go. It takes 4 protons to synthesise 1 ATP molecule.
method of storing energy by creating a protons gradient across a membrane is
called chemiosmosis. Some poisons act by making proton channels in
mitochondrial membranes, so giving an alternative route for protons and stopping
the synthesis of ATP. This also happens naturally in the brown fat tissue of
new-born babies and hibernating mammals: respiration takes place, but no ATP is
made, with the energy being turned into heat instead.
can now summarise respiration and see how much ATP is made from each glucose
molecule. ATP is made in two different ways:
Some ATP molecules are
made directly by the enzymes in glycolysis or the Krebs cycle. This is called substrate
level phosphorylation (since ADP is being phosphorylated to form
Most of the ATP molecules
are made by the ATP synthase enzyme in the respiratory chain. Since this
requires oxygen it is called oxidative phosphorylation. Scientists
donít yet know exactly how many protons are pumped in the respiratory chain,
but the current estimates are: 10 protons are pumped by NADH; 6 by FADH; and 4
protons are needed by ATP synthase to make one ATP molecule. This means that
each NADH can make 2.5 ATPs (10/4) and each FADH can make 1.5 ATPs (6/4).
Previous estimates were 3 ATPs for NADH and 2 ATPs for FADH, and these numbers
still appear in most textbooks, although they are now know to be wrong. (You donít need to know these
numbers, so donít worry)
Two ATP molecules are
used at the start of glycolysis to phosphorylate the glucose, and these must be
subtracted from the total.
table below is an ďATP accountĒ for aerobic respiration, and shows that 32
molecules of ATP are made for each molecule of glucose used in aerobic
respiration. This is the maximum possible yield; often less ATP is made,
depending on the circumstances. Note that anaerobic respiration (glycolysis)
only produces 2 molecules of ATP.
produced per glucose
4 ATP produced (2 per triose phosphate)
2 NADH produced (1 per triose phosphate)
2 NADH produced (1 per pyruvate)
2 ATP produced (1 per acetyl coA)
6 NADH produced (3 per acetyl coA)
2 FADH produced (1 per acetyl coA)
substances can also be used to make ATP. Triglycerides are broken down to fatty
acids and glycerol, both of which enter the Krebs Cycle. A typical triglyceride
might make 50 acetyl CoA molecules, yielding 500 ATP molecules. Fats are a very
good energy store, yielding 2.5 times as much ATP per g dry mass as
carbohydrates. Proteins are not normally used to make ATP, but in times of
starvation they can be broken down and used in respiration. They are first
broken down to amino acids, which are converted into pyruvate and Krebs Cycle
metabolites and then used to make ATP.
It is sometimes useful to deduce which substrate is being used in a personís metabolism at a specific time. This can be done is the volume of oxygen taken in, and the volume of carbon dioxide given out are measured. From this data the respiratory quotient (RQ) can be calculated:
Volume of carbon dioxide given off
Volume of oxygen taken in
The values of RQ to be expected vary depending of which substances are broken down by respiration.
It is interesting to know which substrate is being metabolised. Under normal conditions the human RQ is in the range of 0.8-0.9, indicating that some fats and proteins, as well as carbohydrates, are used for respiration. Values greater than 1.0 are obtained when anaerobic respiration is in progress.
Measuring respiratory rate can be done by using a respirometer.
The potassium hydroxide solution acts to remove carbon dioxide from the surrounding air. This means that any carbon dioxide, which is produced by respiration, is immediately absorbed so that it does not affect the volume of air remaining. Therefore, any changes in volume, which do take place, must be due to the uptake of oxygen. A manmeter and the calibrated scale measure these changes. Tube B acts as a control.
energy is used for muscle contraction in animals than for any other process. The
proteins in muscle use ATP to provide the energy for contraction, but the exact
way in which the ATP is made varies depending on the length of the contraction.
There are five sources of ATP:
ATP stored in muscles
A muscle cell stores only enough ATP for a few seconds of contraction. This ATP was made by respiration while the muscle was relaxed, and is available for immediate use.
ATP from creatine phosphate
Creatine phosphate is a short-term energy store in muscle cells, and there is about ten times more creatine phosphate than ATP. It is made from ATP while the muscle is relaxed and can very quickly be used to make ATP when the muscle is contracting. This allows about 30 seconds of muscle contraction, enough for short bursts of intense activity such as a 100 metre sprint.
ATP from anaerobic respiration of glucose
respiration doesnít provide much ATP (2 ATP molecules for each glucose
molecule), but it is quick, since it doesnít require oxygen to be provided by
the blood. It is used for muscle activities lasting a few minutes. There is not
much glucose as such in muscle cells, but there is plenty of glycogen,
which can be broken down quickly to make quite large amounts of glucose.
end product of anaerobic respiration is lactate, which gradually diffuses out of
muscle cells into the blood and is carried to the liver. Here it is converted
back to pyruvate.
Some muscles are specially
adapted for anaerobic respiration and can therefore only sustain short bursts of
activity. These are the white (or fast twitch) muscles (such as birdsí
breast muscle and frogs legs) and they are white because they contain few
mitochondria and little myoglobin. Mitochondria are not needed for anaerobic
ATP from aerobic respiration of glucose
For longer periods of exercise
muscle cells need oxygen supplied by the blood for aerobic respiration. This
provides far more energy (36 molecules of ATP from each molecule of glucose),
but the rate at which it can be produced is limited by how quickly oxygen can be
provided. This is why you canít run a marathon at the same speed as a sprint.
Muscles that are specially
adapted for aerobic respiration are called red (or slow twitch) muscles
(such as heart, leg and back muscles). They are red because they contain many
mitochondria (which are red) and a lot of the red protein myoglobin,
which is similar to haemoglobin, but is used as an oxygen store in these
muscles. Myoglobin helps to provide the oxygen needed for aerobic respiration.
ATP from aerobic respiration of fats
The biggest energy store in the body is in the
form of triglycerides, which store more energy per gram than glucose or
glycogen. They are first broken down to fatty acids and glycerol, and then fully
oxidised to carbon dioxide and water by aerobic respiration. Since fats are
insoluble it takes time to ďmobiliseĒ them (i.e. dissolve and digest them),
so fats are mainly used for extended periods of exercise, and for the countless
small contractions that are constantly needed to maintain muscle tone and
Most muscles canít keep contracting for ever, but need to have a rest. This is called muscle fatigue. It starts after 30s to 5 mins of continuous contraction (depending on muscle type) and can be quite painful. It is caused by the build-up of two chemicals inside muscle cells.
Phosphate from ATP splitting. This tends to drive the muscle ATPase reaction backwards and so reduces muscle force.
from anaerobic respiration. This lowers the pH and so slows the enzymes involved
in muscle contraction.
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Last updated 07/11/2005