Respiration (OCR A Level Biology): Flashcards

Respiration transfers chemical potential energy from nutrient molecules into a usable form through the synthesis of ATP.

Energy cannot be created or destroyed; it is only transformed from one form into another.

You should never say energy is "created" in photosynthesis or respiration.

False — energy cannot be created or destroyed, only transformed from one form into another.

False — that describes autotrophs; heterotrophs must obtain pre-made carbon compounds from their food.

A rod-shaped, double-membraned organelle (0.5-1.0 µm in diameter) that is the site of aerobic respiration in eukaryotic cells, functioning to synthesise ATP.

To synthesise ATP (the site of aerobic respiration in eukaryotic cells).

Oxidative phosphorylation, the last stage of aerobic respiration.

• Smooth

• Permeable to several small molecules

• Folded into cristae

• Less permeable than the outer membrane

• The site of the electron transport chain

• The location of ATP synthase enzymes

The folds of the inner mitochondrial membrane, which increase its surface area to hold many electron transport chain proteins and ATP synthase enzymes.

It has a high concentration of protons (H⁺ ions). This concentration gradient across the inner membrane is formed during oxidative phosphorylation and is essential for ATP synthesis.

The aqueous solution enclosed within the inner membrane of the mitochondrion, containing ribosomes, enzymes and circular mitochondrial DNA needed for the mitochondrion to function.

• Ribosomes

• Enzymes

• Circular mitochondrial DNA

Cristae give a large surface area on the inner membrane.

This means there are more electron transport chain carriers and ATP synthase enzymes.

This results in more ATP being synthesised.

The longer, more tightly packed cristae provide a larger surface area, enabling the synthesis of more ATP to meet the higher energy demand of the active cell.

Muscle cells are more active and so have a higher ATP demand; the number of mitochondria per cell varies depending on cell activity.

The inner mitochondrial membrane is folded into cristae, which increase its surface area.

Oxidative phosphorylation is the last stage of aerobic respiration, during which ATP is synthesised in the mitochondrion.

False — it is the inner membrane that is folded into cristae; the outer membrane is smooth.

True

The process of breaking down a respiratory substrate to produce ATP using oxygen.

Glucose.

1. Glycolysis\n2. Link reaction\n3. Krebs cycle\n4. Oxidative phosphorylation

In the cell cytoplasm.

In the matrix of the mitochondria.

In the matrix of the mitochondria.

At the inner membrane of the mitochondria.

Phosphorylation and splitting of glucose.

Decarboxylation and dehydrogenation of pyruvate.

A cyclical pathway made up of enzyme-controlled reactions.

The production of ATP through the oxidation of hydrogen atoms.

It is not enough to say the Krebs cycle takes place in the mitochondria; you must state that it takes place in the matrix of the mitochondria.

False — glycolysis takes place in the cell cytoplasm.

True

The first stage of respiration, taking place in the cytoplasm, in which glucose (6C) is split to form two molecules of pyruvate (3C).

In the cytoplasm of the cell.

• 2 pyruvate (3C) molecules

• A net gain of 2 ATP

• 2 reduced NAD

Glucose (6C) is phosphorylated by 2 ATP to form hexose bisphosphate (6C).

Glucose + 2ATP → Hexose bisphosphate

Hexose bisphosphate (6C) splits into two molecules of triose phosphate (3C).

Hexose bisphosphate → 2 Triose phosphate

Hydrogen is removed from each molecule of triose phosphate and transferred to the coenzyme NAD, forming 2 reduced NAD.

Phosphates are transferred from the intermediate substrate molecules to ADP, forming 4 ATP by substrate-linked phosphorylation.

The formation of ATP by the direct transfer of a phosphate group from an intermediate substrate molecule to ADP.

2 ATP are used at the start (during phosphorylation) to make glucose more reactive and lower the activation energy.

4 ATP are produced later during the process.

This gives a net gain of 2 ATP per glucose molecule.

To make glucose more reactive (glucose is normally very stable) and to lower the activation energy of the reaction.

Pyruvate (3C) is the end product of glycolysis.

During glycolysis, hexose bisphosphate is split into two molecules of triose phosphate during the lysis step.

Glycolysis is the first stage of respiration and takes place in the cytoplasm.

It is passed on to the next stage of respiration (the link reaction).

False — 4 ATP are produced but 2 are used at the start, giving a net gain of 2 ATP.

False — glycolysis does not require oxygen; it takes place in the cytoplasm.

Pyruvate

The mitochondrial matrix

Because it links glycolysis to the Krebs cycle, connecting respiration in the cytoplasm to the later stages in the mitochondria.

By active transport.

When oxygen is available.

• Pyruvate is oxidised by enzymes to produce acetate and carbon dioxide, reducing NAD to NADH\n\n- The acetate group combines with coenzyme A to form acetyl coenzyme A (acetyl CoA)

• Acetyl CoA\n\n- Carbon dioxide (CO₂)\n\n- Reduced NAD (NADH)

pyruvate + NAD + CoA → acetyl CoA + carbon dioxide + reduced NAD

• Decarboxylation – removal of carbon dioxide (loses a carbon)\n\n- Dehydrogenation (oxidation) – removal of hydrogen, which reduces NAD to NADH

A molecule that helps an enzyme carry out its function but is not used up in the reaction itself.

Coenzyme A binds to the acetyl group (2C) remaining from pyruvate to form acetyl CoA.\n\nIt then supplies the acetyl group to the Krebs cycle, where it is used to continue aerobic respiration.

For every one glucose molecule, the link reaction occurs twice because glycolysis produces two pyruvate molecules.

A transport protein and a small amount of ATP.

True

False — it produces acetyl CoA, carbon dioxide and reduced NAD, but no ATP directly.

A series of enzyme-controlled reactions (also called the citric acid cycle) that takes place in the matrix of the mitochondria, in which oxaloacetate is regenerated and reduced NAD, reduced FAD and ATP are produced.

In the matrix of the mitochondria.

Acetyl CoA, which carries a 2-carbon (2C) acetyl fragment.

Acetyl CoA can also be formed directly from:

• Fatty acids (after the breakdown of lipids)

• Amino acids

The 4C oxaloacetate accepts the 2C acetyl fragment from acetyl CoA to form 6C citrate.

Coenzyme A (CoA) is released in this reaction.

Because the acceptor molecule oxaloacetate is regenerated during the reactions, so it can combine with another acetyl CoA and start the cycle again.

A series of oxidation-reduction (redox) reactions.

The removal of a carboxyl group, releasing carbon dioxide. In the Krebs cycle, decarboxylation of citrate releases 2 CO₂ as a waste gas.

Hydrogen atoms are released and used to reduce the coenzymes NAD and FAD, forming reduced NAD and reduced FAD.

By substrate-level phosphorylation: a phosphate is transferred from one of the intermediates to ADP, forming 1 ATP.

• 3 reduced NAD (NADH + H⁺)

• 1 reduced FAD (FADH₂)

• 1 ATP (via substrate-level phosphorylation)

• 2 CO₂ (via decarboxylation)

In the Krebs cycle, the release of CO₂ from citrate is an example of decarboxylation.

Oxaloacetate is the 4C molecule that accepts the acetyl fragment from acetyl CoA to form citrate.

True

False — one turn produces 3 reduced NAD (along with 1 reduced FAD).

A molecule that helps an enzyme carry out its function but is not used up in the reaction itself.

A nucleoside (ribose and adenine) and a vitamin.

CoA binds to the remainder of the pyruvate molecule (the acetyl group, 2C) to form acetyl CoA.

It supplies the acetyl group to the Krebs cycle, where it is used to continue aerobic respiration.

This links the initial stage of respiration in the cytoplasm to the later stages in the mitochondria.

They act as hydrogen carriers. When hydrogen atoms become available at different points during respiration, NAD and FAD accept these hydrogen atoms.

A hydrogen ion and an electron.

When the coenzymes NAD and FAD gain a hydrogen atom, they are said to be reduced.

Oxidation Is Loss, Reduction Is Gain (of electrons/hydrogen).

To the electron transport chain on the inner mitochondrial membrane.

The hydrogen atoms are removed from the coenzymes, so the coenzymes are oxidised.

• Electrons are given to the electron transport chain

• Hydrogen ions are released when the electrons are lost

• The electron transport chain drives hydrogen ions across the inner mitochondrial membrane into the intermembrane space, creating a proton gradient

• Movement of hydrogen ions back down the gradient into the matrix provides the energy for ATP synthesis

• Glycolysis: 2

• Link reaction: 2

• Krebs cycle: 6

2 reduced FAD, produced in the Krebs cycle.

Because one glucose molecule is split in two during glycolysis, so these reactions occur twice per single molecule of glucose.

False — a coenzyme helps an enzyme carry out its function but is not used up.

True

The last stage of aerobic respiration, which takes place at the inner mitochondrial membrane and produces many molecules of ATP and water from oxygen.

The current model for oxidative phosphorylation: energy from electrons passing through the electron transport chain is used to pump protons into the intermembrane space; the protons then flow back through ATP synthase, driving the phosphorylation of ADP into ATP.

At the inner mitochondrial membrane.

• Many molecules of ATP

• Water (produced from oxygen)

Reduced NAD (NADH) and reduced FAD (FADH₂).

They split into protons (H⁺ ions) and electrons.

• High-energy electrons enter the electron transport chain and release energy as they pass from carrier to carrier

• This energy is used to pump protons across the inner mitochondrial membrane from the matrix into the intermembrane space

• As the membrane is impermeable to hydrogen ions, a concentration gradient of protons builds up between the intermembrane space and the matrix

• Protons return to the matrix by facilitated diffusion through the channel protein ATP synthase

• The movement of protons down their concentration gradient provides the energy for the phosphorylation of ADP into ATP

Oxygen acts as the final electron acceptor, combining with protons and electrons at the end of the electron transport chain to form water.

Without oxygen to accept the electrons and hydrogens at the end of the chain:

• The electron transport chain cannot continue, as the electrons have nowhere to go

• NADH and FADH₂ cannot be oxidised back to NAD and FAD, so they can no longer be used in further hydrogen transport

A series of membrane proteins (electron carriers) positioned close together so electrons can pass from carrier to carrier.

Protons return to the matrix by facilitated diffusion through the channel protein ATP synthase.

Oxygen acts as the final electron acceptor at the end of the electron transport chain, combining with protons and electrons to form water.

Mainly from the Krebs cycle.

Because the inner mitochondrial membrane is impermeable to hydrogen ions, the carriers are needed to pump protons across the membrane and establish the concentration gradient.

False — the electron transport chain carriers pump protons into the intermembrane space; ATP synthase lets protons flow back into the matrix, driving ATP synthesis.

False — it is impermeable to hydrogen ions, which is why electron carriers must pump protons across it to build the concentration gradient.

Oxygen is the final electron acceptor from the electron transport chain.

Without it, the chain stops and no more ATP is produced via oxidative phosphorylation.

Reduced NAD and FAD are not oxidised by an electron carrier (as the electron transport chain has stopped).

So no oxidised NAD and FAD are available for dehydrogenation reactions, and the Krebs cycle stops.

Some cells can oxidise the reduced NAD produced during glycolysis so it can be reused for hydrogen transport.

This means glycolysis can continue and small amounts of ATP are still produced.

• Yeast and microorganisms use ethanol fermentation

• Other microorganisms and mammalian muscle cells use lactate fermentation

1. Pyruvate is decarboxylated to ethanal, producing CO₂.

1. Ethanal is reduced to ethanol by alcohol dehydrogenase, with ethanal accepting hydrogens from reduced NAD.

Ethanal is the hydrogen acceptor.

Ethanol cannot be further metabolised; it is a waste product.

Reduced NAD transfers its hydrogens to pyruvate to form lactate.

Pyruvate is reduced to lactate by the enzyme lactate dehydrogenase, with pyruvate acting as the hydrogen acceptor.

Lactate can be further metabolised. Either:

• It is oxidised back to pyruvate, which is channelled into the Krebs cycle for ATP production

• It is converted into glycogen for storage in the liver

Oxygen debt is the extra oxygen required to oxidise lactate back to pyruvate after anaerobic respiration.

Lactate produced during anaerobic respiration must be oxidised back to pyruvate, which needs extra oxygen.

This is why animals breathe deeper and faster after exercise.

The enzyme that catalyses the reduction of ethanal to ethanol in the final step of ethanol fermentation.

The enzyme that catalyses the reduction of pyruvate to lactate in lactate fermentation.

In ethanol fermentation, pyruvate is first decarboxylated to ethanal, releasing carbon dioxide.

Lactate fermentation is the one-step anaerobic pathway used by mammalian muscle cells.

True

False — CO₂ is released during ethanol fermentation, not lactate fermentation.

Aerobic respiration gives a much greater energy yield than anaerobic respiration.

In anaerobic respiration glucose is only partially oxidised, so only some of its chemical potential energy is released and transferred to ATP.

Glycolysis is the only ATP-producing reaction that continues (yielding ~2 ATP).

There is no oxygen to act as the final electron acceptor, so the reactions within the mitochondria cannot proceed.

The stages that take place inside the mitochondria produce much more ATP than glycolysis alone, and these can only occur when oxygen is present as the final electron acceptor.

• Anaerobic: ~2 ATP (from glycolysis only)

• Aerobic: ~36 ATP (glycolysis plus the mitochondrial stages)

In anaerobic respiration, glucose is only partially oxidised, so only some of its chemical potential energy is transferred to ATP.

The stages of respiration that take place inside the mitochondria produce much more ATP than glycolysis alone.

The incomplete breakdown of glucose in which only some of its chemical potential energy is released, as occurs in anaerobic respiration, giving a low ATP yield.

Oxygen acts as the final electron acceptor, allowing the mitochondrial stages of respiration to continue and produce far more ATP.

True

False — with no oxygen as the final electron acceptor, the mitochondrial reactions stop, so only glycolysis (~2 ATP) continues.

A substance that changes colour when it is reduced or oxidised.

• DCPIP

• Methylene blue

Because these dyes do not damage the cells.

Blue → colourless

Dehydrogenation occurs throughout respiration, with the enzyme dehydrogenase removing hydrogens.

When the redox indicator is present, it takes up these hydrogens from the organic compounds and is reduced instead of NAD.

The enzyme that catalyses the production of reduced NAD in glycolysis, by removing hydrogens from organic compounds.

A faster rate of respiration means a faster rate of hydrogen release, so the dye is reduced and changes colour more quickly. The rate of colour change therefore corresponds to the rate of respiration.

The rate of respiration is inversely proportional to the time taken for the solution to lose its colour.

Rate of respiration (s⁻¹) = 1 / time (s)

• Add a set volume of yeast suspension to a test tube containing a set concentration of glucose

• Place the tube in a temperature-controlled water bath and leave for 5 minutes so the temperature stabilises

• Add a set volume of DCPIP and start the stopwatch immediately

• Stop the stopwatch when the solution becomes colourless and record the time

• Repeat across a range of temperatures (e.g. 30°C, 35°C, 40°C, 45°C)

Because deciding when the colour change is complete is subjective, so using the same person helps to control this and make results more comparable.

Any three of:

• Volume of dye added

• Volume of yeast suspension

• Type of respiratory substrate

• Concentration of substrate

• pH (using a buffer solution)

Add different concentrations of a substrate (e.g. 0.1%, 0.5%, 1.0% glucose) to the yeast suspension and record the time taken for the colour change once the dye is added.

A respirometer measures the rate of oxygen consumption in organisms such as seeds or invertebrates.

Volume of oxygen consumed = πr²h

Aerobic respiration.

False — they change from blue to colourless when reduced.

True

A biological molecule that is broken down (oxidised) during respiration to release energy for the production of ATP.

Glucose.

• Other carbohydrates

• Lipids

• Proteins (amino acids)

Because amino acids often have essential functions elsewhere in the cell.

They are required to make proteins with structural roles (e.g. in the cytoskeleton) and functional roles (e.g. as enzymes).

• Lipids = 39.4 kJ g⁻¹ (highest)

• Proteins = 17.0 kJ g⁻¹

• Carbohydrates = 15.8 kJ g⁻¹ (lowest)

The number of hydrogen atoms that become available when the substrate molecule is broken down.

A higher hydrogen content gives a greater energy value.

• The substrate is broken down and hydrogen atoms become available

• Hydrogen carrier molecules (NAD and FAD) pick up the hydrogen atoms (become reduced) and transfer them to the inner mitochondrial membrane

• Reduced NAD and FAD release the hydrogen atoms, which split into protons and electrons

• Protons are pumped across the inner mitochondrial membrane into the intermembrane space, forming a proton (chemiosmotic) gradient

• This gradient is used in chemiosmosis to produce ATP as protons flow back through ATP synthase

A higher hydrogen content produces a greater proton gradient across the inner mitochondrial membrane.

This allows the formation of more ATP via chemiosmosis.

Fatty acids in lipids are made up of long hydrocarbon chains containing many hydrogen atoms.

When the lipid is broken down, these many hydrogen atoms are released, producing a greater proton gradient and therefore more ATP.

They are oxidised (combined with oxygen and electrons) to form water.

Hydrogen carrier molecules that pick up hydrogen atoms (becoming reduced) during respiration and transfer them to the inner mitochondrial membrane.

Reduced NAD and FAD release hydrogen atoms, which split into electrons and protons.

Protons are pumped across the inner mitochondrial membrane into the intermembrane space, forming a proton or chemiosmotic gradient.

True

False — lipids have the highest energy value (39.4 kJ g⁻¹), greater than carbohydrates (15.8 kJ g⁻¹).

The ratio of carbon dioxide produced to oxygen taken in during respiration.

When glucose is respired aerobically, equal amounts of carbon dioxide are produced to oxygen taken in, giving it an RQ value of 1.

• Carbohydrates: 1.0

• Lipids: 0.7

• Proteins: 0.9

Lipids have more carbon-hydrogen bonds than carbohydrates.

More hydrogen atoms are available to create the proton gradient, so more ATP can be produced.

More oxygen is therefore required to break down the molecule (in the final step of oxidative phosphorylation to form water), lowering the RQ.

In a balanced equation, the number in front of each chemical formula gives the number of molecules (moles) of that compound.

Because equal numbers of gas molecules occupy equal volumes, you can put the number of CO₂ molecules over the number of O₂ molecules:

During lactate fermentation no oxygen is used and no carbon dioxide is produced, so the RQ cannot be calculated.

No oxygen is used while carbon dioxide is still being produced, so dividing CO₂ produced by O₂ consumed (zero) gives a value tending towards infinity.

The rate of oxygen consumption during respiration in organisms (such as seeds or invertebrates).

It can also be used to calculate respiratory quotients.

With soda-lime present, CO₂ is absorbed, so the manometer reading reflects only oxygen consumed (x).

With soda-lime removed, the reading reflects both oxygen consumed and carbon dioxide produced (y).

The difference between the two readings gives the carbon dioxide produced, allowing the RQ to be calculated.

That the substrate being respired has changed.

An intermediate value (e.g. 0.85) suggests a mixture of substrates is being used, since carbohydrate has an RQ of 1 and lipid has an RQ of 0.7.

• An RQ of more than 1 suggests overfeeding (excessive carbohydrate/calorie intake).

• An RQ of less than 0.7 suggests underfeeding.

False — RQ is the ratio of carbon dioxide produced to oxygen consumed.

True

A piece of apparatus used to measure the rate of oxygen consumption during aerobic respiration in organisms.

The rate of oxygen consumption during aerobic respiration in organisms (e.g. germinating seeds or invertebrates).

By placing the apparatus in a thermostatically controlled water bath, so the respiration rate can be measured at different, controlled temperatures.

Because they are actively respiring and therefore consuming oxygen, producing a measurable change in gas volume.

To absorb the carbon dioxide produced during respiration, so any change in gas volume is due only to oxygen consumption.

Soda-lime pellets are placed in the respirometer to absorb the carbon dioxide produced during respiration.

It acts as a control tube, containing an equivalent volume of non-respiring material so that changes due to temperature or pressure can be accounted for.

• Set up the respirometer with both tubes in a controlled-temperature water bath\n\n- Close the screw clip to begin\n\n- Use the manometer reading to calculate the change in gas volume in a given time (cm³ min⁻¹)

The distance moved by the manometer fluid along the capillary tube.

πr²h\n\nwhere r is the radius of the capillary tube (cm) and h is the distance moved by the manometer fluid (cm) in a minute. This gives the volume of oxygen consumed in cm³ min⁻¹.

The rate of oxygen consumption (cm³ min⁻¹).

Because no oxygen is consumed during anaerobic respiration, so there is no decrease in gas volume for the manometer to detect. The respirometer only measures aerobic respiration.

At extremely high temperatures the enzymes controlling respiration become denatured and can no longer catalyse the reactions, so the rate falls.

• Reset the apparatus: let air back in via the screw cap and reset the manometer fluid using the syringe

• Change the temperature of the water bath and allow the tubes to acclimatise, then close the screw clip

• Run the experiment again and calculate the new change in gas volume

• Repeat at several different temperatures

It is used to calculate the change in gas volume, which equals the volume of oxygen consumed in a given time.

False — no oxygen is consumed in anaerobic respiration, so there is no change in gas volume for the manometer to detect.

True

A device attached to a respirometer in which the movement of fluid measures the change in gas volume as oxygen is consumed.