chapter 18 p2

Question 1

Oxidative phosphorylation:
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Answer 1

• The hydrogen atoms that have been collected by the coenzymes NAD and FAD are delivered to electron transport chains present in the membranes of the cristae of the mitochondria.
• The hydrogen atoms dissociate into hydrogen ions and electrons.
• The high energy electrons are used in the synthesis of ATP by chemiosmosis.
• Energy is released during redox reactions as the electrons reduce and oxidise electron carriers as they flow along the electron transport chain.

Question 2

Oxidative phosphorylation:
p2

Answer 2

• This energy is used to create a proton gradient leading to the diffusion of protons through ATP synthase resulting in the synthesis of ATP.
• At the end of the electron transport chain the electrons combine with hydrogen ions and oxygen to form water.
• Oxygen is the final electron acceptor and the electron chain cannot operate unless oxygen is present.
• Respiration which involves the complete breakdown of glucose is therefore an aerobic process.

Question 3

Oxidative phosphorylation:
p3

Answer 3

• The phosphorylation of ADP to form ATP is dependent on electrons moving along electron transport chains.
• This requires the presence of oxygen and is known as oxidative phosphorylation.
• The hydrogens released from NAD and FAD could combine directly with oxygen, releasing energy from the formation of bonds during the production of water.
• However, this energy could not be used to synthesise ATP.
• Heat released in the exothermic reaction would simply raise the temperature of the cell.

Question 4

diagram of electron transport chain in respiration

Answer 4

Question 5

Substrate level phosphorylation:

Answer 5

is the production of ATP involving the transfer of a phosphate group from a short-lived, highly reactive intermediate such as creatine phosphate.

This is different from oxidative phosphorylation which couples the flow of protons down the electrochemical gradient through ATP synthase to the phosphorylation of ADP to produce ATP.

Question 6

Aerobic respiration produces

Answer 6

around 38 molecules of ATP per glucose molecule whereas fermentation (a form of anaerobic respiration) only produces two molecules of ATP (net).

Question 7

Anaerobic respiration in eukaryotic organisms:

Answer 7

Eukaryotic cells respire aerobically if enough oxygen is available.
Anaerobic respiration, resulting in the synthesis of smaller quantities of ATP, occurs in the absence of oxygen and is also used when oxygen cannot be supplied fast enough to respiring cells.
The use of this less efficient process to produce ATP is a temporary ‘emergency’ measure to keep vital processes functioning.

Question 8

Organisms fall into different categories determined by their dependence on oxygen or not:

Answer 8

obligate anaerobes
facultative anaerobes
obligate aerobes

Question 9

obligate anaerobes

Answer 9

cannot survive in the presence of oxygen.
Almost all obligate anaerobes are prokaryotes, for example, Clostridium (bacteria that cause food poisoning), although there are some fungi as well.

Question 10

facultative anaerobes

Answer 10

• synthesise ATP by aerobic respiration if oxygen is present, but can switch to anaerobic respiration in the absence of oxygen, for example, yeast.

Question 11

obligate aerobes

Answer 11

can only synthesise ATP in the presence of oxygen, for example, mammals.
The individual cells of some organisms, such as muscle cells in mammals, can be described as facultative anaerobes because they can supplement ATP supplies by employing anaerobic respiration in addition to aerobic respiration when the oxygen concentration is low.
However, this is only for short periods and oxygen is eventually required.
The shortfall of oxygen during the period of anaerobic respiration produces compounds that have to be broken down when oxygen becomes available again, so the organism as a whole is an obligate aerobe.

Question 12

Fermentation:

Answer 12

(a form of anaerobic respiration) is the process by which complex organic compounds are broken down into simpler inorganic compounds without the use of oxygen or the involvement of an electron transport chain.

The organic compounds, such as glucose, are not fully broken down so fermentation produces much less ATP than aerobic respiration.

The small quantity of ATP produced is synthesised by substrate-level phosphorylation alone.
The end products of fermentation differ depending on the organism.

Question 13

Alcoholic fermentation

Answer 13

occurs in yeast and some plant root cells.
Here the end products are ethanol (an alcohol) and carbon dioxide.

Question 14

Lactate fermentation results in

Answer 14

the production of lactate and is carried out in animal cells.
When there is no oxygen to act as the final electron acceptor at the end of the electron transport chain in oxidative phosphorylation, the flow of electrons stops.
This means the synthesis of ATP by chemiosmosis also stops.

Question 15

As the flow of electrons along the electron transport chain has stopped..

Answer 15

, the reduced NAD and reduced FAD are no longer able to be oxidised because there is nowhere for the electrons to go.
This means NAD and FAD cannot be regenerated and so the decarboxylation and oxidation of pyruvate and the Krebs cycle comes to a halt as there are no coenzymes available to accept the hydrogens being removed.
Glycolysis would also come to halt due to the lack of NAD if it were not for the process of fermentation.

Question 16

Lactate fermentation in mammals:
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Answer 16

In mammals, pyruvate can act as a hydrogen acceptor taking the hydrogen from reduced NAD, catalysed by the enzyme lactate dehydrogenase.
The pyruvate is converted to lactate (lactic acid) and NAD is regenerated.
This can be used to keep glycolysis going so a small quantity of ATP is still synthesised.

Question 17

Lactate fermentation in mammals:
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Answer 17

In mammals in particular, anaerobic respiration in the muscles is often supported by ATP from aerobic respiration, which is still being produced as fast as oxygen can be delivered in other parts of the body.
Lactic acid is converted back to glucose in the liver but oxygen is needed to complete this process.
This is the reason for the oxygen debt (and the need to breathe heavily) after exercise.

Question 18

Lactate fermentation cannot occur indefinitely for two main reasons:

Answer 18

the reduced quantity of ATP produced would not be enough to maintain vital processes for a long period of time

the accumulation of lactic acid causes a fall in pH leading to proteins denaturing. Respiratory enzymes and muscle filaments are made from proteins and will cease to function at low pH.

Question 19

Lactic acid is removed from

Answer 19

muscles and taken to the liver in the bloodstream.
One of the main aims when improving physical fitness is to increase the blood supply and flow, through muscles.
This increases the rate of lactic acid removal allowing the intensity and duration of exercise to be increased.

Question 20

diagram of Lactate fermentation in mammals

Answer 20

Question 21

Alcoholic fermentation in yeast (and many plants):
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Answer 21

Alcoholic fermentation is not a reversible process like lactate fermentation.
Pyruvate is first converted to ethanal, catalysed by the enzyme pyruvate decarboxylase.
Ethanal can then accept a hydrogen atom from reduced NAD, becoming ethanol.
The regenerated NAD can then continue to act as a coenzyme and glycolysis can continue.

Question 22

Alcoholic fermentation in yeast (and many plants):
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Answer 22

This is not a short-term process and can continue indefinitely in the absence of oxygen.
Ethanol is a toxic waste product to yeast cells and they are unable survive if the ethanol accumulates above approximately 15%.
This is allowed to happen during the production of alcohol in brewing or wine making.

Question 23

Alcoholic fermentation in yeast (and many plants):
diagram

Answer 23

Question 24

Investigation into respiration rates in yeast:
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Answer 24

The apparatus shown could be used to measure the rate of carbon dioxide production of a yeast suspension.
This will be equivalent to the rate of anaerobic respiration or alcoholic fermentation of the yeast cells.
The glucose in solution provides a respiratory substrate.
The flask is sealed during the experiment to ensure anaerobic conditions.

Question 25

Investigation into respiration rates in yeast:
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Answer 25

As the yeast respires carbon dioxide is released increasing the volume of gas in the flask.
As the volume of gas in the tube increases the pressure will increase causing the coloured liquid to move along the capillary tube.
The distance moved by the liquid together with diameter of the tube can be used to calculate the increase in volume of gas (carbon dioxide) in the flask over a certain time. This is a measure of the rate of respiration.

Question 26

Data logging:

Answer 26

Respiration is not 100% efficient and energy is lost as heat when organisms respire.
When yeast respires it produces heat which will increase the temperature of a solution containing yeast.
Sensors can be used to measure changes in temperature.
A student carried out an investigation into respiration in yeast using a data logger to measure the changes in carbon dioxide concentration as a measure of the rate of respiration.
The student placed a solution containing yeast and glucose in the flask and inserted a carbon dioxide sensor.
The solution was covered with a layer of liquid paraffin.
The software was set up to record readings every 50 seconds for 1 600 seconds.