Download 5. Respiration Booklet [A2]

SACKVILLE SCIENCE DEPARTMENT
A2 BIOLOGY
Respiration
Learning objectives:
 Outline why plants, animals and micro-organisms need to respire, with reference to active
transport and metabolic reactions;
 Describe, with the aid of diagrams, the structure of ATP;
 State that ATP provides the immediate source of energy for biological processes;
 Explain the importance of coenzymes in respiration, with reference to NAD and coenzyme A;
 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;
Key definitions:
Compile a glossary by writing your own definitions for the following key terms related to the
learning objectives above.
Key term
energy
ATP
metabolism
anabolic
catabolic
photoautotrophs
oxidation
reduction
coenzyme
Definition
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Key term
A2 BIOLOGY
Definition
NAD
glycolysis
hexose
hydrolysis
triose
hexose 1,6bisphosphate
substrate-level
phosphorylation
Energy in cells and the role of ATP
Respiration is the process whereby energy stored in complex organic molecules is used to make
ATP. It occurs in living cells.
Energy exists as potential (stored) energy and kinetic energy (movement). Moving molecules have
kinetic energy that allows them to diffuse down a concentration gradient. Large organic molecules
contain chemical potential energy.
All living organisms need energy to drive their biological processes. All the reactions that take place
within organisms are known collectively as metabolism. Metabolic reactions that build large
molecules are described as anabolic and those that break large molecules into smaller ones are
catabolic.
Some of the energy from catabolic reactions is released in the form of heat. This is useful as
metabolic reactions are controlled by enzymes, so organisms need to maintain a suitable
temperature that allows enzyme action to proceed at a speed that will sustain life.
Plants, some protoctists and some bacteria are photoautotrophs. They use sunlight energy in
photosynthesis to make large, organic molecules that contain chemical potential energy, which they
and consumers and decomposers can then use. Respiration releases the energy, which is used to
phosphorylate (add inorganic phosphate to) ADP, making ATP. This phosphorylation also transfers
energy to the ATP molecule.
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The molecule ATP (adenosine triphosphate) is the universal energy carrier for the cell. ATP can
release its energy quickly; only one chemical reaction (hydrolysis of the terminal phosphate) is
required. This reaction is catalysed by the enzyme ATPase. Once ATP has released its energy, it
becomes ADP (adenosine diphosphate), a low energy molecule that can be recharged by adding a
phosphate. This requires energy, which is supplied by the controlled breakdown of respiratory
substrates in cellular respiration. The most common respiratory substrate is glucose, but other
molecules e.g. fats or proteins may also be used.
In the presence of the enzyme ATPase, the ATP molecule loses a phosphate. The energy released
from the loss of a phosphate (30.7kJ) is available for immediate work inside the cell i.e. powering
chemical reactions. A free phosphate is released from the ATP (this may be reused later to
regenerate ADP into ATP again).
Respiration generates ATP by two methods:
substrate-level phosphorylation where an enzyme transfers a phosphate group from a
substrate to ADP;
oxidative phosphorylation where glucose is oxidised in a series of redox reactions that
provide the energy for the formation of ATP;
SACKVILLE SCIENCE DEPARTMENT
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Cellular respiration
Cellular respiration is the process by which organisms break down energy rich molecules e.g.
glucose to release energy in a useable form (ATP). All living cells respire in order to exist, although
the substrates they use may vary. Aerobic respiration requires oxygen. Forms of cellular respiration
that do not require oxygen are said to be anaerobic. Some plants and animals can generate ATP
anaerobically for short periods of time. Other organisms use only anaerobic respiration and live in
oxygen-free environments. For these organisms, there is some other final electron acceptor other
than oxygen e.g. nitrate or Fe2+.
Respiration involves three metabolic stages, summarised below:
The first two stages are the catabolic pathways that decompose glucose and other organic fuels. In
the third stage, the electron transport chain accepts electrons from the first two stages and passes
these from one electron acceptor to another. The energy released at each stepwise transfer is used
to make ATP. The final electron acceptor in this process is molecular oxygen.
1. Glycolysis. This occurs in the cytoplasm and involves the breakdown of glucose into two
molecules of pyruvate.
2. The Krebs cycle. This occurs in the mitochondrial matrix, and decomposes a derivative of
pyruvate to carbon dioxide.
3. Electron transport and oxidative phosphorylation. This occurs in the inner membranes of
the mitochondrion and accounts for almost 90% of the ATP generated by respiration.
SACKVILLE SCIENCE DEPARTMENT
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Glycolysis
Glycolysis is the first part of respiration that involves the breakdown of glucose in the cytoplasm.
Glucose (a 6-carbon sugar) is broken into two molecules of pyruvate (also called pyruvic acid), a 3carbon acid. A total of 2 ATP and 2NADH + 2H+ are generated from this stage. No oxygen is required
(the process is anaerobic).
This pathway involves a sequence of ten reactions, each catalysed by a different enzyme. The
coenzyme NAD is also involved. The pathway can be considered to involve four stages.
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Stage 1: Phosphorylation
Glucose is a hexose (6C) sugar and as its molecules are stable, they need to be activated before they
can be split into two.
 One ATP molecule is hydrolysed and the phosphate group released is attached to the
glucose molecule at carbon 6.
 Glucose 6-phosphate is changed to fructose 6-phosphate.
 Another ATP is hydrolysed and the phosphate group released is attached to fructose 6phosphate at carbon 1. This activated hexose sugar is now called fructose 1, 6-bisphosphate.
 The energy from the hydrolysed ATP molecules activates the hexose sugar and prevents it
from being transported out of the cell. This activated, phosphorylated sugar is referred to as
hexose 1, 6-bisphosphate.
This stage has used two molecules of ATP for each molecule of glucose.
Stage 2: Splitting of hexose 1, 6-bisphosphate
Each molecule of hexose bisphosphate is split into two molecules of triose phosphate (3 carbon
sugar molecules each with one phosphate group attached).
Stage 3: Oxidation of triose phosphate
Although this process is anaerobic, it involves oxidation.
 Two hydrogen atoms (with their electrons) are removed from each triose phosphate
molecule (the substrate). This involves dehydrogenase enzymes.
 These are aided by the coenzyme NAD (nicotinamide adenine dinucleotide), which is a
hydrogen acceptor. NAD combines with the hydrogen, becoming reduced NAD.
 At this stage of glycolysis, two molecules of NAD are reduced per molecule of glucose. Also
at this stage, two molecules of ATP are formed. This is called substrate-level
phosphorylation.
Stage 4: Conversion of triose phosphate to pyruvate
Four enzyme-catalysed reactions convert each triose phosphate molecule to a molecule of
pyruvate. Pyruvate is also a 3-carbon compound.
In the process another two molecules of ADP are phosphorylated (an inorganic phosphate group, P i
is added) to two molecules of ATP (by substrate-level phosphorylation).
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1. Aerobic respiration may be summarised by the following equation:
C6H12O6 + 6O2 → 6CO2 + 6H2O
Although carbon dioxide and water are products of aerobic respiration, the
equation is an over-simplification of the process.
State and explain one way in which this equation is an over-simplification.
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2. (a) Fig. 1.1 represents a molecule of ATP.
(i) Name the parts of the ATP molecule labelled X, Y and Z.
X ___________________________________________________________
Y ___________________________________________________________
Z ___________________________________________________________
[3]
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(ii) With reference to Fig. 1.1, describe and explain the role of ATP in the cell.
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3. Organisms require energy in order to carry out essential metabolism.
Organisms are able to release energy by carrying out both aerobic and
anaerobic respiration.
(a) Complete the table below to compare anaerobic respiration in mammals
and yeast.
(b) Suggest one benefit of anaerobic respiration to an organism.
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4. Adenosine triphosphate (ATP) is an important product of respiration. The ATP
molecule is made up of five sub-units, as shown in Fig. 5.1.
(i)
In the space below, indicate how these sub-units are joined in a molecule
of ATP.
[2]
(ii) Suggest the type of reaction that removes a phosphate group from an ATP
molecule.
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5. Glycolysis is the initial stage of cellular respiration.
(a) State precisely where in the cell glycolysis occurs.
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SACKVILLE SCIENCE DEPARTMENT
A2 BIOLOGY
(b) Outline the process of glycolysis.
In your answer, you should use appropriate technical terms, spelled correctly
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(c) Yeast cells can carry out anaerobic respiration.
Fig. 5.1 outlines the process of anaerobic respiration in yeast.
Identify the compounds W to Z.
W ___________________________________________________________
X ___________________________________________________________
Y ___________________________________________________________
Z ___________________________________________________________
[4]
SACKVILLE SCIENCE DEPARTMENT
A2 BIOLOGY
6. (a) Fig. 2.1 represents the first stage of respiration.
(i)
Name the stage represented by Fig. 2.1.
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(ii)
State precisely where in the cell this stage takes place.
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(iii)
Identify the compounds D, E and F.
D
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E
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F
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[3]
SACKVILLE SCIENCE DEPARTMENT
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(b) In anaerobic conditions, compound F does not proceed to the link reaction.
Describe the fate of compound F during anaerobic respiration in an animal
cell and explain the importance of this reaction.
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SACKVILLE SCIENCE DEPARTMENT
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The link reaction, Krebs cycle and electron transport chain
Learning objectives:
 State that, during aerobic respiration in animals, pyruvate is actively transported into
mitochondria;
 Explain with the aid of diagrams and electron micrographs, how the structure of
mitochondria enables them to carry out their functions;
 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;
 State that the Krebs cycle takes place in the mitochondrial matrix;
 Outline the Krebs cycle, with reference to the formation of citrate from acetate and
oxaloacetate and the reconversion of citrate to oxaloacetate (names of intermediate
compounds are not required);
 Explain that during the Krebs cycle, decarboxylation and dehydrogenation occur, NAD and
FAD are reduced and substrate level phosphorylation occurs;
 Outline the process of oxidative phosphorylation, with reference to the roles of electron
carriers, oxygen and the mitochondrial cristae;
 Outline the process of chemiosmosis, with reference to the electron transport chain, proton
gradients and ATP synthase;
 State that oxygen is the final electron acceptor in aerobic respiration;
 Evaluate the experimental evidence for the theory of chemiosmosis;
 Explain why the theoretical maximum yield of ATP per molecule of glucose is rarely, if ever,
achieved in aerobic respiration;
 Explain why anaerobic respiration produces a much lower yield of ATP than aerobic
respiration;
 Compare and contrast anaerobic respiration in mammals and yeast;
 Define the term respiratory substrate;
 Explain the difference in relative energy values of carbohydrate, lipid and protein respiratory
substrates;
Key definitions:
Compile a glossary by writing your own definitions for the following key terms related to the
learning objectives above.
Key term
link reaction
pyruvate
dehydrogenase
Definition
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Key term
A2 BIOLOGY
Definition
pyruvate decarboxylase
acetyl coenzyme A
Krebs cycle
substrate-level
phosphorylation
mitochondrial matrix
cristae
oxidative
phosphorylation
electron transport chain
chemiosmosis
anaerobic respiration
respiratory substrate
mole
The link reaction
Pyruvate produced during glycolysis is transported across the inner and outer mitochondrial
membranes to the matrix. The pyruvate that enters the mitochondrion has carbon dioxide
removed. Coenzyme A (CoA) picks up the remaining 2-carbon fragment of the pyruvate to form
acetyl coenzyme A.
The following equation summarises the link reaction:
2 pyruvate + 2NAD+ + 2CoA → 2CO2 + 2 reduced NAD + 2 acetyl CoA
SACKVILLE SCIENCE DEPARTMENT
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Decarboxylation and dehydrogenation of pyruvate to acetate are enzyme-catalysed reactions:
 pyruvate dehydrogenase removes hydrogen atoms from pyruvate;
 pyruvate decarboxylase removes a carboxyl group, which eventually becomes carbon
dioxide, from pyruvate;
 the coenzyme NAD accepts the hydrogen atoms;
 Coenzyme A (CoA) accepts acetate, to become acetyl coenzyme A. The function of CoA is to
carry acetate to the Krebs cycle.
Krebs cycle
The Krebs cycle also takes place in the mitochondrial matrix. It is a series of enzyme-catalysed
reactions that oxidise the acetyl group of acetyl CoA to two molecules of carbon dioxide. It also
produces one molecule of ATP by substrate-level phosphorylation, and reduces three molecules of
NAD and one molecule of FAD. NAD and FAD are hydrogen acceptors, transporting hydrogens to the
electron transport chain.
The acetyl group passes into a cyclic reaction and combines with a 4-carbon molecule to form a 6carbon molecule. The CoA is released for reuse. Successive steps in the cycle remove carbon as
carbon dioxide.
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The acetate is offloaded from coenzyme A (which is then free to collect more acetate) and
joins with a 4-carbon compound, called oxaloacetate, to form a 6-carbon compound, called
citrate.
Citrate is decarboxylated (one molecule of carbon dioxide removed) and dehydrogenated (a
pair of hydrogen atoms removed) to form a 5-carbon compound. The pair of hydrogen
atoms is accepted by a molecule of NAD, which becomes reduced.
The 5-carbon compound is decarboxylated and dehydrogenated to form a 4-carbon
compound and another molecule of reduced NAD.
The 4-carbon compound is changed into another 4-carbon compound. During this reaction a
molecule of ADP is phosphorylated to produce a molecule of ATP. This is substrate-level
phosphorylation.
The second 4-carbon compound is changed into another 4-carbon compound. A pair of
hydrogen atoms is removed and accepted by the coenzyme FAD, which is reduced.
The third 4-carbon compound is further dehydrogenated and regenerates oxaloacetate.
Another molecule of NAD is reduced.
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There is one turn of the cycle for each molecule of acetate, which was made from one molecule of
pyruvate. Therefore there are two turns of the cycle for each molecule glucose.
Electron transport chain
The final stage of aerobic respiration involves electron carriers embedded in the inner
mitochondrial membranes. These membranes are folded into cristae, increasing the surface area
for electron carriers and ATP synthase enzymes.
Hydrogen pairs are transferred to the electron transport chain. The hydrogens or electrons are
passed from one carrier to the next, losing energy as they go. The energy released in this stepwise
process is used to produce ATP. Oxygen is the final electron acceptor and is reduced to water.
*FAD enters the electron transport chain at a lower energy level than NAD, and only 2ATP are
generated per FAD.H2.
SACKVILLE SCIENCE DEPARTMENT
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A2 BIOLOGY
Reduced NAD and reduced FAD are reoxidised when they donate hydrogen atoms, which are
split into protons and electrons, to the electron carriers.
The first electron carrier to accept electrons from reduced NAD is a protein complex I, called
NADH-coenzyme Q reductase (also known as NADH dehydrogenase).
The protons (H+) go into solution in the matrix.
The electrons are passed along a chain of electron carriers and then donated to molecular
oxygen, the final electron acceptor.
As electrons flow along the electron transport chain, energy is released and used, by
coenzymes associated with some of the electron carriers (complexes I, III and IV), to pump
the protons across to the intermembrane space.
This builds up a proton gradient which is also a pH gradient and an electrochemical gradient.
Thus, potential energy builds up in the intermembrane space.
The hydrogen ions cannot diffuse through the lipid part of the inner membrane but can
diffuse through ion channels in it. These channels are associated with the enzyme ATP
synthase. This flow of hydrogen ions (protons) is chemiosmosis.
Oxidative phosphorylation is the formation of ATP by the addition of inorganic phosphate to
ADP in the presence of oxygen. As protons flow through an ATP synthase enzyme, they drive
the rotation of part of the enzyme and join ADP and P i (inorganic phosphate) to form ATP.
The electrons are passed from the last electron carrier in the chain to molecular oxygen,
which is the final electron acceptor.
Hydrogen ions also join so that oxygen is reduced to water.
4H+ + 4e- + O2 → 2H2O
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Chemiosmosis
Chemiosmosis is the process whereby the synthesis of ATP is coupled to electron transport and the
movement of protons (H+ ions). Electron chain carriers are arranged over the inner membrane of
the mitochondrion and oxidise NADH + H+ and FADH2. Energy from this process forces protons to
move, against their concentration gradient, from the mitochondrial matrix into the space between
the two membranes. Eventually the protons flow back into the matrix via ATP synthase molecules in
the membrane. As the protons flow down their concentration gradient, energy is released and ATP
is synthesised. Chemiosmotic theory also explains the generation of ATP in the light dependent
phase of photosynthesis.
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Evaluating the evidence for chemiosmosis
By the early 1960s, research teams were extracting mitochondria from cells and examining them,
using electron microscopes and special staining techniques. They could identify an outer and inner
membrane with a space between them, and could see that the inner membrane was folded into
cristae covered on the inner surface with many small (9nm in diameter) mushroom-shaped
particles.
In 1961, Peter Mitchell realised that the build-up of hydrogen ions on one side of a membrane
would be a source of potential energy and that the movement of ions across the membrane, down
an electrochemical gradient, could provide the energy needed to power the formation of ATP from
ADP and Pi. he called this chemiosmosis theory.
The inner mitochondrial membrane is therefore an energy-transducing membrane. He postulated
that the energy released from the transfer of electrons along the electron transport chain was used
to pump hydrogen ions from the matrix to the intermembrane space and that these protons then
flowed through protein channels, attached to enzymes. The kinetic energy or the force of this flow,
the proton motive force, drove the formation of ATP.
At first his theory was greeted with great scepticism as it was radically different from the idea of a
high-energy intermediate compound. However, by 1978 there was much evidence supporting the
theory and Mitchell was awarded the Nobel Prize for chemistry. Since then scientists have
established that the stalked particles are ATP synthase enzymes and have discovered how they
function. It is also now known that some of the complexes in the electron transport chain have
coenzymes that can use the energy released from electron transport to pump hydrogen ions across
the membrane, into the intermembrane space, where a proton or electrochemical gradient builds
up.
SACKVILLE SCIENCE DEPARTMENT
A2 BIOLOGY
1. Fig. 1.2 is an electron micrograph of a mitochondrion from an animal cell.
(i)
Name the structure labelled A.
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(ii)
Name the specific process that is carried out by structure A in the
mitochondrion.
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2. Fig. 3.1 (on the next page) represents some of the reactions that take place in a
leaf cell of a flowering plant.
(a) Name the reaction pathways indicated by the letters W, X and Y.
W ___________________________________________________________
X ___________________________________________________________
Y ___________________________________________________________
[3]
SACKVILLE SCIENCE DEPARTMENT
A2 BIOLOGY
(b) Triose phosphate is a compound that is central to the metabolism of this
cell. Explain how the three reaction pathways (W, X and Y) are able to work
independently of each other in the same leaf cell.
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SACKVILLE SCIENCE DEPARTMENT
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(c) Identify which of these three reaction pathways (W, X and Y) are associated
with:
photosynthesis _________________________________________________
aerobic respiration ___________________________________________ [2]
(d) Fig. 3.1 shows that compounds from two of the three pathways are used in
oxidative phosphorylation. State the products of oxidative phosphorylation.
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(e) Explain the role of coenzymes in this leaf cell, with respect to the metabolic
reactions outlined in Fig. 3.1.
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3. Fatigue is a symptom of some medical conditions. One feature of fatigue is
extreme tiredness, due to a lack of energy.
Medical conditions that have fatigue as a characteristic symptom include Type
2 diabetes, certain heart conditions, chronic fatigue syndrome (CFS) and
emphysema.
(a) Explain how emphysema could result in fatigue.
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SACKVILLE SCIENCE DEPARTMENT
A2 BIOLOGY
(b) In Type 2 diabetes, the target cells do not respond correctly to the insulin
produced when there is an increase in blood glucose concentration.
Suggest why fatigue may occur in a person with Type 2 diabetes who is not
taking medication.
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(c) Certain heart conditions result in a weak and irregular heart beat.
Suggest how a weak and irregular heart beat could result in fatigue.
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(d) Chronic fatigue syndrome (CFS) is a condition in which symptoms vary from
individual to individual.
It is thought that a number of malfunctioning processes can contribute to
this condition in an individual.
CFS can affect every system in the body and is identified by symptoms that
include fatigue, muscle weakness and aching muscles.
(i) It has been suggested that, in the cells of people with CFS, pyruvate
might not be transferred into mitochondria efficiently.
Outline the consequences of an inefficient transfer of pyruvate into
mitochondria and link this to the symptoms of CFS stated above.
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(ii) Some people with CFS overproduce T lymphocytes and associated
cytokines. Despite this, the specific immune response is poor in these
people, resulting in an increased susceptibility to infection.
Suggest a reason for the poor specific immune response in people with
CFS.
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4. The formation of ATP is now widely accepted as being achieved by the process
of chemiosmosis.
Various pieces of evidence have been documented to support this theory.
Three of these are described below:
SACKVILLE SCIENCE DEPARTMENT
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Identify the piece of evidence above, 1, 2 or 3, that supports each of the
following statements about the theory of chemiosmosis.
Write ‘none’ if a statement is not supported by any of the pieces of evidence
above.
(i)
Electron transfer occurs on the inner membrane
of the mitochondrion.
_____________ [1]
(ii)
(iii)
Protons are actively pumped across the inner
mitochondrial membrane into the intermembrane space.
_____________ [1]
Protons accumulate in the inter-membrane space. _____________ [1]
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Measuring respiration
In small animals or germinating seeds, the rate of cellular respiration can be measured using a
simple respirometer: a sealed unit where the carbon dioxide produced by the respiring tissues is
absorbed by soda lime and the volume of oxygen consumed is detected by fluid displacement in a
manometer. Germinating seeds are also often used to calculate the respiratory quotient (RQ): the
ratio of the amount of carbon dioxide produced during cellular respiration to the amount of oxygen
consumed. RQ provides a useful indication of the respiratory substrate being used.
The respiratory quotient (RQ) can be expressed simply as:
RQ = CO2 produced
O2 consumed
When pure carbohydrate is oxidised in cellular respiration, the RQ is 1.0; more oxygen is required to
oxidise fatty acids (RQ = 0.7). the RQ for protein is about 0.9. Organisms usually respire a mix of
substrates, giving RQ values of between 0.8 and 0.9.
Anaerobic pathways
All organisms can metabolise glucose anaerobically (without oxygen) using glycolysis in the
cytoplasm, but the energy yield from this process is low and few organisms can obtain sufficient
energy for their needs in this way. In the absence of oxygen, glycolysis soon stops unless there is an
alternative acceptor for the electrons produced from the glycolytic pathway. In yeasts and the root
cells of higher plants this acceptor is ethanal, and the pathway is called alcoholic fermentation. In
the skeletal muscle of mammals, the acceptor is pyruvate itself and the end product is lactic acid. In
both cases, the duration of the fermentation is limited by the toxic effects of the organic compound
produced. Although fermentation is often used synonymously with anaerobic respiration, they are
not the same. Respiration always involves hydrogen ions passing down a chain of carriers to the
terminal acceptor, and this does not occur in fermentation. In anaerobic respiration, the terminal H +
acceptor is a molecule other than oxygen e.g. Fe2+ or nitrate.
Alcoholic fermentation
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In alcoholic fermentation, the H+ acceptor is ethanal (acetaldehyde) which is reduced to ethanol
with the release of CO2. Yeasts respire aerobically when oxygen is available but can use alcoholic
fermentation when it is not. At levels above 12-15%, the ethanol produced by alcoholic
fermentation is toxic to the yeast cells and this limits their ability to use this pathway indefinitely.
The root cells of plants also use fermentation as a pathway when oxygen is unavailable but the
ethanol must be converted back to respiratory intermediates and respired aerobically.
Lactic acid fermentation
In the absence of oxygen, the skeletal muscle cells of mammals are able to continue using glycolysis
for ATP production by reducing pyruvate to lactic acid (the H+ acceptor is pyruvate itself). This
process is called lactic acid fermentation. Lactic acid is toxic and this pathway cannot continue
indefinitely. The lactic acid must be removed from the muscle and transported to the liver, where it
is converted back to respiratory intermediates and respired aerobically.
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1. One way of calculating the rate of respiration is to measure the volume of
oxygen taken up over a period of time.
A student carried out an experiment to investigate the effect of temperature
on the rate of respiration in soaked (germinating) pea seeds and dry (dormant)
pea seeds.
A simple piece of apparatus called a respirometer was used, as shown in Fig.
4.1.
The potassium hydroxide solution in this apparatus absorbs carbon dioxide. If
the apparatus is kept at a constant temperature, any changes in the volume of
air in the respirometer will be due to oxygen uptake.
(a) State the stage or stages of aerobic respiration during which:
(i) carbon dioxide is produced
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SACKVILLE SCIENCE DEPARTMENT
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(ii) oxygen is used
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(b) The student set up three respirometers, A, B and C, in water baths at two
different temperatures. The respirometers were left for 10 minutes in order
to equilibrate. The contents of each respirometer are shown in Table 4.1.
At each temperature, respirometer C, which contained only glass beads,
was a control. Respirometer B, at each temperature, also contained some
glass beads.
(i) Suggest why, at each temperature, respirometer B contained some glass
beads.
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(ii) Suggest how the student determined the quantity of glass beads to place
in respirometer B at each temperature.
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(c) After the student had left each respirometer to equilibrate, a small volume
of coloured fluid was introduced into each graduated tube.
The respirometers were then left in the appropriate water baths for 20
minutes and maintained at the correct temperature. During this time, the
coloured fluid in the graduated tube moved. The level of the coloured fluid
in each respirometer was recorded at the start of the experiment and after
20 minutes.
The results are summarised in table 4.2.
SACKVILLE SCIENCE DEPARTMENT
(i)
A2 BIOLOGY
Table 4.2 is incomplete.
Calculate the missing value for the rate of oxygen uptake for soaked pea
seeds (A) at 25°C.
Show your working.
Answer = _______________ cm3min-1
[2]
(ii)
Explain why there is an increased rate of respiration in soaked seeds at
25°C compared with soaked seeds at 15°C.
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(iii) Suggest a reason for the difference in the rate of respiration between
soaked and dry pea seeds.
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SACKVILLE SCIENCE DEPARTMENT
A2 BIOLOGY
2. The compound 2, 3, 5-triphenyl-tetrazolium chloride (TTC) is an electron
carrier. TTC will diffuse into actively respiring cells and accept electrons from
the electron transport chain.
When TTC accepts electrons and becomes reduced, it changes from colourless
to pink. The tissues in which this reaction takes place will be stained a pink
colour.
(a) State the precise location of the electron transport chain in the cell.
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(b) A student carried out an investigation into the respiratory activity of plant
tissue. She used three groups of germinating bean seeds. These were first
treated as shown in Table 3.1.
The groups of seeds were then sliced longitudinally and placed cut surface
down, in a shallow dish containing a small volume of TTC solution. The cut
surfaces remained in contact with the solution for 10 minutes.
The seeds were then removed from the dish. The excess TTC solution was
wiped off and the cut surfaces of the seeds in each group were observed.
The appearance of the seeds in each group is shown in Fig. 3.1. the shaded
areas are the regions where the tissues have stained a pink colour.
SACKVILLE SCIENCE DEPARTMENT
(i)
A2 BIOLOGY
Describe the differences observed in the seeds in groups A, B and C.
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(ii) Suggest reasons for the results observed in the seeds in group A.
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(iii) Suggest reasons for the difference in the amount of staining
observed in the groups B and C when compared to those in group A.
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SACKVILLE SCIENCE DEPARTMENT
A2 BIOLOGY
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3. In South-East Asia the main source of commercial sugar is the palm, Borassus
flabellifer. Sap of this species has a high sugar content. Yeasts and bacteria,
however, can contaminate the sap as it is collected and ferment the sugar,
producing ethanol. This contamination makes it less suitable as a source of
sugar.
A study was carried out to investigate the effect of three treatments
traditionally used to reduce fermentation during the collection of sap. The sap
is treated in one of the following ways:
 with a weak alkaline solution (treatment A)
 with bark from the tree Vateria copallifera (treatment V)
 with bark from the tree Careya arborea (treatment C)
the sap was collected from the palm trees over a 60-hour period. Samples of
the collected sap were taken at 15 hour intervals. In each sample, the
concentration of alcohol and the number of bacteria were recorded.
The results are shown in Table 5.1 (see next page).
(a) With reference to Table 5.1, describe the effect of the different treatments
on the alcohol concentration of the treated samples compared with the
control samples.
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SACKVILLE SCIENCE DEPARTMENT
A2 BIOLOGY
(b) Suggest a reason for the difference in alcohol concentration at 60 hours
between the two bark treatments V and C.
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SACKVILLE SCIENCE DEPARTMENT
A2 BIOLOGY
(c) To be used as a source of commercial sugar, the sap needs to be as
uncontaminated as possible.
Suggest, with a reason, which of the treatments shown in Table 5.1 would
be the best for use with sap so that it is suitable as a source of commercial
sugar.
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4. Humans harvest a wide range of fruits and vegetables as food. Cellular
respiration supplies energy and forms part of the natural ripening process in
fruits and vegetables. This ripening process may continue after the fruits and
vegetables are harvested, as the cells continue to respire.
The rate of cellular respiration after harvesting affects the shelf-life of fruits
and vegetables as it can lead to changes in food quality. After harvesting, some
fruits and vegetables enter a dormant (inactive) state while others remain
active during storage.
Table 5.1 (on the next page) contains data that show the respiration rate of a
selection of fruits and vegetables stored at different temperatures after
harvesting. The respiration rate is measured by the rate of carbon dioxide
produced.
(i)
Describe the pattern of respiration shown by cauliflower at increasing
storage temperatures of 0°C to 20°C.
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SACKVILLE SCIENCE DEPARTMENT
(ii)
A2 BIOLOGY
Discuss what the data in Table 5.1 indicate about the best conditions for
storage of fruits and vegetables.
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(iii)
Identify, with reasons, which fruit or vegetable listed in Table 5.1 is least
likely to spoil during storage.
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SACKVILLE SCIENCE DEPARTMENT
A2 BIOLOGY
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(iv)
Which fruit or vegetable listed in Table 5.1 is likely to be the most
difficult to keep fresh during storage? Give a reason for your answer.
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5. Respiration can be aerobic or anaerobic.
(a) Certain parasites live in the blood of mammals.
Suggest why, even though blood carries oxygen, these parasites are adapted
to respire anaerobically.
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(b) The anaerobic respiration pathway in animal cells can be reversed but the
anaerobic respiration pathway in yeast cells cannot be reversed.
Explain why, using your knowledge of the differences between the two
pathways.
SACKVILLE SCIENCE DEPARTMENT
A2 BIOLOGY
In your answer, you should use appropriate technical terms, spelled correctly
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6. Some animals conserve energy by entering a state of torpor (a short period of
dormancy), in which they allow their body temperature to fall below normal for
a number of hours.
In an investigation into torpor in the Siberian hamster, Phodopus sungorus, the
animal’s respiratory quotient (RQ) was measured before and during the period
of torpor.
The respiratory quotient is determined by the following equation:
RQ = volume of carbon dioxide produced
volume of oxygen consumed in same time
SACKVILLE SCIENCE DEPARTMENT
A2 BIOLOGY
RQ values for different respiratory substrates have been determined and are
shown in Table 1.1.
Initially, the RQ value determined for the hamster was 0.95 but as the period of
torpor progressed, its RQ value decreased to 0.75.
What do these values suggest about the substrates being respired by the
hamster during the period of the investigation?
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7. The liver is an organ that is metabolically active, carrying out over 500 different
functions. Some of its important functions include converting chemicals
including toxins, into other compounds.
Fig. 2.1 (on the next page) outlines some of the reaction pathways that take
place in the liver cells.
The underlined words represent toxic compounds.
SACKVILLE SCIENCE DEPARTMENT
A2 BIOLOGY
(a) The lactate that enters pathway S is produced by cells, such as muscle cells,
undergoing anaerobic respiration.
Suggest why this lactate is converted into pyruvate by the hepatocytes (liver
cells) rather than by the respiring cells in which it is produced.
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Alcohol (ethanol) is oxidised in the liver by Pathway Q. if a person has a high
alcohol intake, it will result in the production of excess reduced NAD.
(b) Excess reduced NAD in the liver cells will influence some metabolic
pathways by:
 inhibiting the conversion of lactate to pyruvate
 inhibiting fatty acid oxidation
 promoting fatty acid synthesis
Using this information and the information in Fig. 2.1, suggest the
consequences for liver metabolism if a person has a regular high alcohol
intake.
SACKVILLE SCIENCE DEPARTMENT
A2 BIOLOGY
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(c) State precisely where in the liver cell the excess reduced NAD can be reoxidised.
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8. If oxygen is not present or is in short supply, respiration can take an anaerobic
pathway after glycolysis. In plant cells, this pathway is the same as the one
used in yeast cells.
(i)
Name the hydrogen acceptor in this pathway.
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(ii)
Name the intermediate compound in this pathway.
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(iii)
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Name the products of this pathway.
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(iv)
[1]
[1]
Explain why this pathway is important for the plant cell.
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