Topic 12: Energy and Respiration

Topic 12: Energy and respiration

12.1 Energy

Students should be able to:

1) outline the need for energy in living organisms, as illustrated by active transport, movement and anabolic reactions, such as those occurring in DNA replication and protein synthesis

2) describe the features of ATP that make it suitable as the universal energy currency

3)state that ATP is synthesised by:

• transfer of phosphate in substrate-linked reactions

• chemiosmosis in membranes of mitochondria and chloroplasts

4) explain the relative energy values of carbohydrates, lipids and proteins as respiratory substrates

5) state that the respiratory quotient (RQ) is the ratio of the number of molecules of carbon dioxide produced to the number of molecules of oxygen taken in, as a result of respiration

6) calculate RQ values of different respiratory substrates from equations for respiration

7) describe and carry out investigations, using simple respirometers, to determine the RQ of germinating seeds or small invertebrates (e.g. blowfly larvae)

(1) Outline the need for energy in living organisms, as illustrated by active transport, movement and anabolic reactions, such as those occurring in DNA replication and protein synthesis

Living organisms require energy continuously because cells carry out many processes that do not proceed spontaneously or that must occur under controlled conditions. Energy is required to drive metabolic work that would otherwise not occur, to maintain organisation (against entropy), and to fuel mechanical, transport and biosynthetic activities.

Active transport: Cells often move ions or molecules against their concentration gradient (for example, the Na⁺/K⁺-ATPase pump in animal cells). This requires energy because moving substances from low to high concentration is thermodynamically unfavourable.
Movement: In both unicellular and multicellular organisms, movement (e.g., cilia, flagella, muscle contraction) requires energy to power motor proteins (like myosin, dynein) and to change cytoskeletal or membrane structure.
Anabolic reactions: Building complex molecules from simpler ones is endergonic (requires input of energy). For example:
- DNA replication: synthesis of new DNA strands involves polymerising nucleotides, unwinding the helix, proofreading, etc., which all demand energy input (in the form of nucleoside triphosphates, ATP, and enzyme activity).
- Protein synthesis: translation of mRNA into polypeptides, folding of proteins, post-translational modifications all require energy (ATP and GTP hydrolysis) to make the process efficient and accurate.

In sum, energy is essential for maintaining cell structure and function, for transporting substances, for movement, and for biosynthesis of the macromolecules required for growth, repair, reproduction and metabolic regulation.

(2) Describe the features of ATP that make it suitable as the universal energy currency

The molecule Adenosine triphosphate (ATP) is widely used in living cells as the “energy currency” for several reasons. The following features (structure and function) make it especially suited to this role:

Structure: ATP is composed of the nitrogenous base adenine, a ribose sugar (so adenosine) and three phosphate groups linked in series (alpha, beta, gamma).
High-energy phosphate bonds: The bonds between phosphate groups (especially between the β- and γ-phosphates) are often referred to as “high-energy” (technically the hydrolysis releases a large free-energy change). Hydrolysis of ATP to ADP + Pi (inorganic phosphate) releases energy that can be harnessed to do cellular work.
Readily recycled: After ATP has donated a phosphate (and energy) to a reaction, it becomes ADP (adenosine diphosphate) (or even AMP). The cell has mechanisms (in respiration, photosynthesis) to re-phosphorylate ADP to ATP, so ATP is a re-usable carrier.
Small, soluble and mobile: ATP is relatively small, water-soluble, and can diffuse within the cell to where energy is needed (cytosol, mitochondria, etc.).
Controlled energy release: Unlike direct oxidation of e.g. glucose which would release a huge uncontrolled burst of energy, ATP hydrolysis releases a manageable, controlled amount of energy – suitable for coupling to biochemical reactions without damage.
Universal coupling role: ATP links exergonic (energy-releasing) reactions (such as breakdown of substrates) with endergonic (energy-requiring) reactions (e.g., biosynthesis, active transport) — useful because cells need a common intermediate energy currency to coordinate many pathways.

Thus, ATP’s chemical structure, ability to store and release energy, recyclability, and versatility in coupling reactions are all features that make it a universal energy currency in living systems.

(3) State that ATP is synthesised by:

transfer of phosphate in substrate-linked reactions
chemiosmosis in membranes of mitochondria and chloroplasts
Cells synthesise ATP via two major mechanisms:
Substrate-level phosphorylation (transfer of phosphate in substrate-linked reactions): This is the process by which a phosphate group is directly transferred from a phosphorylated substrate (metabolic intermediate) to ADP to form ATP. For example, during glycolysis and the Krebs cycle, certain enzyme-mediated reactions generate ATP by substrate-level phosphorylation.
Chemiosmosis (in membranes of mitochondria and chloroplasts): In oxidative phosphorylation (in mitochondria) and photophosphorylation (in chloroplasts in plants), ATP is synthesised using the energy of a proton (H⁺) gradient across a membrane. The flow of protons back through ATP synthase drives phosphorylation of ADP to ATP. It’s important to state that in mitochondria the inner mitochondrial membrane is where the chemiosmotic process occurs.

The specification point is thus satisfied by stating that ATP is synthesised by those two mechanisms.

(4) Explain the relative energy values of carbohydrates, lipids and proteins as respiratory substrates

Different classes of macromolecules (carbohydrates, lipids, proteins) can act as respiratory substrates (i.e., be broken down in respiration to release energy). Their energy yields differ because of their chemical structure and how completely they can be oxidised.

Carbohydrates: Typically the primary respiratory substrates (e.g., glucose). They have moderate energy yield per gram, are relatively easily mobilised and oxidised. Because they contain several oxygen atoms in their structure relative to fatty acids, their oxidation yields less energy per unit mass than lipids.
Lipids (fats and oils): These provide the highest energy yield per gram among the three classes. This is because their hydrocarbon chains are highly reduced (many C–H bonds) and have relatively fewer oxygen atoms built in; when oxidised they release more energy (i.e., more electrons for the electron transport chain) and more ATP. Also lipids are stored in anhydrous form (less water associated) so more energy per mass.
Proteins: These are used less often as primary respiratory substrates but can be oxidised when necessary. Proteins need to be deaminated (removal of amino group) and then the remaining keto acid enters metabolic pathways (e.g., Krebs cycle). Because of the cost of deamination (removal of nitrogen, excretion of ammonia or urea) and the fact that amino acids may be used for other purposes (e.g., synthesis of proteins, enzymes), the net energy yield is somewhat lower and less efficient compared to lipids. Also the yield varies according to the amino acid.

To explain “relative” energy values, one might say: lipids provide more energy per gram than carbohydrates, which in turn generally provide more energy than proteins (when proteins are used purely as substrates). The cell tends preferentially to oxidise carbohydrates and lipids; proteins are more of a last resort or used for specific metabolic intermediates, not primarily for maximal energy yield.

(5) State that the respiratory quotient (RQ) is the ratio of the number of molecules of carbon dioxide produced to the number of molecules of oxygen taken in, as a result of respiration

The respiratory quotient (RQ) is a concept used to quantify the relation between CO₂ produced and O₂ consumed during respiration.

For example, if a substrate when fully oxidised produces 6 CO₂ and consumes 6 O₂, then RQ = 1. If it uses more O₂ than CO₂ produced, the RQ is < 1, etc.

So one needs to state exactly that definition: the ratio of the number of molecules (or moles) of carbon dioxide released to the number of molecules (or moles) of oxygen taken in during respiration.

(7) Describe and carry out investigations, using simple respirometers, to determine the RQ of germinating seeds or small invertebrates (e.g. blowfly larvae)

Students must understand and be able to describe (and practically carry out) investigations that determine RQ using respirometers, for example:

Description: Set-up a respirometer where seeds (e.g., germinating peas) or small invertebrates respire in a sealed chamber, measuring either O₂ uptake or CO₂ production (or both) over time. A common method may involve using potassium hydroxide to absorb CO₂ (so that only O₂ uptake is measured) or using soda lime to absorb CO₂, tracking volume change, etc. Then repeat with a second chamber absorbing O₂ or measure CO₂ production, and from these values derive RQ (CO₂ / O₂).
Variables: Use germinating seeds (alive) as they will respire more than non-germinating (control). Maintain temperature, etc.
Calculation: From measured change in O₂ and/or CO₂ over a known time, compute moles or volumes, then RQ.
Interpretation: If RQ ≈ 1, then carbohydrate is main substrate; if RQ ≈ 0.7, lipid may be predominant; if RQ > 1 (rare) might indicate anaerobic respiration or biosynthesis dominating.
Practical considerations: Ensuring seeds are germinating, avoiding leaks, assuring stable temperature, calibrating respirometer, ensuring no significant gas exchange other than respiration.

Thus students must be familiar with the apparatus (respirometer), know how to perform the experiment, and know how to calculate and interpret RQ from their data.

12.2 Respiration

Students should be able to:

1) State where each of the four stages in aerobic respiration occurs in eukaryotic cells:

• glycolysis in the cytoplasm

• link reaction in the mitochondrial matrix

• Krebs cycle in the mitochondrial matrix

• oxidative phosphorylation on the inner membrane of mitochondria

2) outline glycolysis as phosphorylation of glucose and the subsequent splitting of fructose 1,6-bisphosphate (6C) into two triose phosphate molecules (3C), which are then further oxidised to pyruvate (3C), with the production of ATP and reduced NAD

3) explain that, when oxygen is available, pyruvate enters mitochondria to take part in the link reaction

4) describe the link reaction, including the role of coenzyme A in the transfer of acetyl (2C) groups

5) outline the Krebs cycle, explaining that oxaloacetate (4C) acts as an acceptor of the 2C fragment from acetyl coenzyme A to form citrate (6C), which is converted back to oxaloacetate in a series of small steps

6) explain that reactions in the Krebs cycle involve decarboxylation and dehydrogenation and the reduction of the coenzymes NAD and FAD

7) describe the role of NAD and FAD in transferring hydrogen to carriers in the inner mitochondrial membrane

8) explain that during oxidative phosphorylation:

• hydrogen atoms split into protons and energetic electrons

• energetic electrons release energy as they pass through the electron transport chain

• the released energy is used to transfer protons across the inner mitochondrial membrane

• protons return to the mitochondrial matrix by facilitated diffusion through ATP synthase, providing energy for ATP synthesis

• oxygen acts as the final electron acceptor to form water

9) describe the relationship between the structure and function of mitochondria using diagrams and electron micrographs

10) outline respiration in anaerobic conditions in mammals (lactate fermentation) and in yeast cells (ethanol fermentation)

11) explain why the energy yield from respiration in aerobic conditions is much greater than the energy yield from respiration in anaerobic conditions

12) explain how rice is adapted to grow with its roots submerged in water, limited to the development of aerenchyma in roots, ethanol fermentation in roots and faster growth of stems

13) describe and carry out investigations using redox indicators, including DCPIP and methylene blue, to determine the effects of temperature and substrate concentration on the rate of respiration of yeast

14) describe and carry out investigations using simple respirometers to determine the effect of temperature on the rate of respiration

(1) State where each of the four stages in aerobic respiration occurs in eukaryotic cells:

glycolysis in the cytoplasm
link reaction in the mitochondrial matrix
Krebs cycle in the mitochondrial matrix
oxidative phosphorylation on the inner membrane of mitochondria
It’s important to state accurately the location for each stage:
Glycolysis: occurs in the cell cytoplasm (cytosol) of eukaryotic cells.
Link reaction (also called pyruvate oxidation or “transition reaction”): occurs in the mitochondrial matrix (in eukaryotes).
Krebs cycle (citric acid cycle, tricarboxylic acid cycle): also in the mitochondrial matrix.
Oxidative phosphorylation: takes place on/at the inner mitochondrial membrane (in eukaryotes) — the electron transport chain and ATP synthase are embedded in the inner membrane; protons accumulate in the inter-membrane space and flow back into matrix.

(2) Outline glycolysis as phosphorylation of glucose and the subsequent splitting of fructose 1,6-bisphosphate (6C) into two triose phosphate molecules (3C), which are then further oxidised to pyruvate (3C), with the production of ATP and reduced NAD

Here we outline the process:

Phosphorylation of glucose: Glycolysis begins with glucose (6-carbon sugar) being phosphorylated (usually first to glucose-6-phosphate, then to fructose-6-phosphate, then to fructose-1,6-bisphosphate) by using ATP. This phosphorylation helps trap the sugar in the cell and destabilises it, making it more reactive.
Splitting (lysis) of fructose 1,6-bisphosphate: The 6-carbon fructose-1,6-bisphosphate is split (via aldolase enzyme) into two triose phosphate molecules, which are 3-carbon compounds (glyceraldehyde-3-phosphate or dihydroxyacetone phosphate which converts to glyceraldehyde-3-phosphate).
Further oxidation to pyruvate: Each triose phosphate is then oxidised: hydrogen atoms (electrons) are removed and transferred to NAD⁺ to form reduced NAD (NADH). Also substrate-level phosphorylation occurs, generating ATP from ADP + Pi. Ultimately each triose phosphate is converted to pyruvate (3-carbon).
Production of ATP and reduced NAD: The net result per glucose molecule (in typical simplified scheme) is: 2 molecules of NADH produced, a net gain of 2 ATP (since 2 ATP were used in the investment phase, 4 ATP produced in payoff = net 2). Pyruvate (3C) is the end-product of glycolysis. (Exact numbers may vary in texts).
Anaerobic possibility: Although glycolysis does not require oxygen, the pyruvate can proceed to aerobic or anaerobic pathways. But for this outline the steps above hold.

Thus, the description captures phosphorylation → splitting → oxidation → ATP + NADH + pyruvate.

(3) Explain that, when oxygen is available, pyruvate enters mitochondria to take part in the link reaction

Once glycolysis has produced pyruvate in the cytoplasm, if oxygen is available (i.e., conditions are aerobic), then the pyruvate is transported into the mitochondrion (specifically into the mitochondrial matrix) via specific transporters across the mitochondrial membranes.

In the mitochondrial matrix, pyruvate undergoes the link reaction (or pyruvate oxidation) to prepare it for the Krebs cycle: decarboxylation, dehydrogenation, and combination with coenzyme A to form acetyl-CoA.
The presence of oxygen is necessary because the subsequent chain of reactions (Krebs cycle & oxidative phosphorylation) rely on final electron acceptor O₂ in oxidative phosphorylation. If oxygen is unavailable, pyruvate cannot enter that aerobic pathway and must be diverted (anaerobic respiration/fermentation).
Hence the explanation: under aerobic conditions, pyruvate enters the mitochondrion and takes part in the link reaction, enabling further oxidative breakdown of substrates and maximising ATP yield.

(4) Describe the link reaction, including the role of coenzyme A in the transfer of acetyl (2C) groups

In the link reaction (also often called the pyruvate oxidation or transition reaction), the 3-carbon pyruvate from glycolysis is converted into a 2-carbon acetyl group (acetyl-CoA), releasing CO₂ and generating reduced NAD. The steps are:

Pyruvate (3C) in the mitochondrial matrix is decarboxylated: one carbon is removed and released as carbon dioxide (CO₂). This is decarboxylation.
The remaining 2-carbon fragment (acetate) is oxidised: hydrogen atoms are removed (dehydrogenation) and transferred to NAD⁺ to form NADH + H⁺.
The acetate is then combined with coenzyme A (CoA) to form acetyl-CoA (acetyl coenzyme A). CoA thus carries the 2-carbon acetyl group into the Krebs cycle. The presence of the coenzyme is essential for the “transfer of acetyl (2C) groups”.
Because each glucose gives 2 pyruvate molecules, the link reaction happens twice per glucose.
The acetyl-CoA produced then enters the Krebs cycle for complete oxidation.

Thus the description includes decarboxylation, dehydrogenation, formation of acetyl-CoA via coenzyme A transfer of the 2-carbon group.

(5) Outline the Krebs cycle, explaining that oxaloacetate (4C) acts as an acceptor of the 2C fragment from acetyl coenzyme A to form citrate (6C), which is converted back to oxaloacetate in a series of small steps

The Krebs cycle (or citric acid cycle / TCA cycle) is a cyclic metabolic pathway in the mitochondrial matrix in which the acetyl group from acetyl-CoA is completely oxidised, generating reduced coenzymes and ATP (or GTP) and releasing CO₂.

Outline of key steps:

Acetyl-CoA (2-carbon) from the link reaction combines with the 4-carbon molecule oxaloacetate (OAA) to form citrate (6-carbon) — this is the initial condensation reaction.
Citrate (6C) then undergoes a series of enzymatic reactions: isomerisations, decarboxylation (removal of CO₂), dehydrogenations (removal of H⁺/e⁻ to NAD⁺ and FAD), regenerations of OAA, etc. Through this cycle the original 4-carbon oxaloacetate is regenerated at the end, so the cycle can continue.
During the cycle:
- Two CO₂ molecules are released per turn (since the acetyl group’s two carbons become CO₂).
- Reduced NAD (NADH) and FADH₂ (from FAD) are produced, which carry electrons to oxidative phosphorylation.
- A small yield of ATP (or GTP) may be produced via substrate-level phosphorylation in the cycle.
The oxaloacetate acting as the acceptor for the acetyl (2-carbon) fragment is a key point: without this initial 4-carbon acceptor, the acetyl fragment could not enter the cycle.
The series of small steps returning citrate through intermediates (e.g., isocitrate, α-ketoglutarate, succinate, fumarate, malate) back to oxaloacetate is important to emphasise.

(6) Explain that reactions in the Krebs cycle involve decarboxylation and dehydrogenation and the reduction of the coenzymes NAD and FAD

Within the Krebs cycle, the important types of chemical reactions that take place include:

Decarboxylation: removal of carbon dioxide (CO₂) from intermediate molecules (for example, isocitrate → α-ketoglutarate, and α-ketoglutarate → succinyl-CoA). This results in the release of CO₂.
Dehydrogenation: removal of hydrogen atoms (H⁺ + e⁻) from molecules (e.g., succinate → fumarate, malate → oxaloacetate). These hydrogens are transferred to coenzymes (NAD⁺ or FAD) to form their reduced forms (NADH or FADH₂).
Reduction of coenzymes NAD and FAD: As the dehydrogenation occurs, NAD⁺ is reduced to NADH (plus H⁺) and FAD is reduced to FADH₂. These reduced coenzymes carry electrons (and protons) to the electron transport chain (oxidative phosphorylation) where their energy will be harvested.
These reactions are central to the cycle’s function of harvesting high-energy electrons from the acetyl group. Without decarboxylation and dehydrogenation, the cycle would not generate the reduced coenzymes necessary for the next stage.

(7) Describe the role of NAD and FAD in transferring hydrogen to carriers in the inner mitochondrial membrane

The coenzymes Nicotinamide adenine dinucleotide (NAD⁺) and Flavin adenine dinucleotide (FAD) play key roles as hydrogen (and associated electrons) carriers in respiration:

In glycolysis, link reaction, and the Krebs cycle, substrate molecules are oxidised (hydrogen atoms are removed) and the hydrogen (H⁺ + e⁻) is transferred to NAD⁺ or FAD to form NADH (plus H⁺) or FADH₂ respectively.
These reduced coenzymes then carry the hydrogen/electron pairs to the inner mitochondrial membrane (specifically to the electron transport chain complexes embedded in the inner membrane).
At the inner membrane, NADH and FADH₂ donate their electrons (and associated protons) to the electron transport chain. The electrons pass through a series of carriers, the protons are pumped across the membrane generating a proton gradient.
Thus NAD and FAD act as intermediate shuttles: they pick up hydrogen/electrons in metabolic pathways, transport them to the membrane, and deliver them for oxidative phosphorylation. Without these carriers, the electrons derived from substrate oxidation could not be transferred efficiently to the electron transport chain, and ATP yield would be reduced.

Therefore the description emphasises the role of NAD and FAD in transferring hydrogen (i.e., electrons and protons) to the carriers in the mitochondrial inner membrane.

(8) Explain that during oxidative phosphorylation:

hydrogen atoms split into protons and energetic electrons
energetic electrons release energy as they pass through the electron transport chain
the released energy is used to transfer protons across the inner mitochondrial membrane
protons return to the mitochondrial matrix by facilitated diffusion through ATP synthase, providing energy for ATP synthesis
oxygen acts as the final electron acceptor to form water

In the process of oxidative phosphorylation, which occurs at the inner mitochondrial membrane, the following sequence of events occurs:

Hydrogen atoms split into protons and energetic electrons: Reduced coenzymes (NADH, FADH₂) deliver hydrogen atoms to the electron transport chain (ETC). The hydrogen atoms split so that the electrons (high-energy) travel through the chain, while protons (H⁺) either remain or are pumped across the membrane.
Energetic electrons release energy as they pass through the electron transport chain: As electrons pass through a series of protein complexes (Complex I, II, III, IV) in the inner mitochondrial membrane, the energy difference between their redox states is used to drive conformational changes and proton pumping.
Released energy is used to transfer protons across the inner mitochondrial membrane: The energy from electron flow is used to pump protons from the mitochondrial matrix across the inner membrane into the inter-membrane space (or into the membrane’s proton-pool side), creating an electrochemical proton gradient (proton motive force).
Protons return to the mitochondrial matrix by facilitated diffusion through ATP synthase, providing energy for ATP synthesis: The proton gradient is maintained until protons flow back down their gradient through ATP synthase (a protein channel/complex). The flow of protons through ATP synthase drives the phosphorylation of ADP + Pi → ATP (chemiosmotic theory).
Oxygen acts as the final electron acceptor to form water: At the end of the electron transport chain, electrons combine with protons and oxygen (O₂) to form water (H₂O). Without oxygen, electron flow would stop, the gradient collapse, and ATP production would cease (or drop dramatically).

Thus the explanation covers all five bullet points of the specification for oxidative phosphorylation.

(9) Describe the relationship between the structure and function of mitochondria using diagrams and electron micrographs

Students are expected to understand how the ultrastructure of the mitochondrion relates to its function in respiration.

Key structural features and their functional significance:

Double membrane: The mitochondrion has an outer mitochondrial membrane and a much folded inner mitochondrial membrane (cristae). The outer membrane is permeable to small molecules, the inner membrane is selectively permeable and houses the electron transport chain, ATP synthase complexes, etc.
Cristae (folds of the inner membrane): These increase the surface area for embedding the electron transport chain and ATP synthase complexes, thus enhancing capacity for ATP production.
Inter-membrane space: The space between inner and outer membranes is where protons are pumped during oxidative phosphorylation, enabling the proton gradient to build.
Matrix: The inner compartment (mitochondrial matrix) contains the enzymes of the Krebs cycle and link reaction, as well as mitochondrial DNA, ribosomes and other features. It is here that substrates enter and reduced coenzymes are produced.
Compartmentalisation: The spatial segregation of different stages (glycolysis in cytosol; link reaction + Krebs in matrix; oxidative phosphorylation on inner membrane) ensures efficient control and minimises unwanted interactions.
Transport proteins: There are carrier proteins in the inner membrane that transport pyruvate, ADP/ATP, phosphate, and protons across membranes, aiding function.

Thus, the relationship is that structure (membranes, cristae, compartments) serves the function of efficient ATP generation, proton gradient establishment, and metabolic regulation.

Students should be able to refer to diagrams and electron micrographs (ultrastructure) showing mitochondria cristae, matrix, etc., and explain how that structural detail underpins function in respiration.

(10) Outline respiration in anaerobic conditions in mammals (lactate fermentation) and in yeast cells (ethanol fermentation)

When oxygen is not available (anaerobic conditions), cells cannot rely on oxidative phosphorylation as the final electron acceptor is missing. Instead they switch to anaerobic pathways to regenerate NAD⁺ (so glycolysis can continue) and produce ATP (though much less than aerobic respiration).

In mammals (lactate fermentation):

Glycolysis continues to produce 2 ATP per glucose and 2 NADH.
Because there's no oxidative phosphorylation, pyruvate is reduced to lactate (lactic acid) by lactate dehydrogenase, using NADH (oxidising it back to NAD⁺) so glycolysis can proceed.
No further CO₂ is produced in this pathway (for the lactate fermentation step).
The lactate can build up in muscles and be transported in the blood to the liver (Cori cycle) where it may be converted back to glucose.

In yeast (ethanol fermentation):

After glycolysis: pyruvate is decarboxylated (CO₂ removed) to acetaldehyde; then acetaldehyde is reduced by NADH to ethanol, regenerating NAD⁺.
The overall reaction: glucose → 2 ethanol + 2 CO₂ + 2 ATP (net from glycolysis).
This allows glycolysis to proceed in the absence of oxygen.

Thus the outline covers the two major types of anaerobic respiration: lactate in mammals and ethanol fermentation in yeast.

(11) Explain why the energy yield from respiration in aerobic conditions is much greater than the energy yield from respiration in anaerobic conditions

The energy yield from aerobic respiration is much greater primarily because the full oxidation of substrates occurs (via glycolysis → link reaction → Krebs cycle → oxidative phosphorylation) and oxygen serves as the final electron acceptor allowing the electron transport chain and chemiosmosis to generate large amounts of ATP. In contrast, anaerobic pathways stop at glycolysis (and subsequent conversion to lactate or ethanol) and cannot harness the proton gradient or electron transport chain.

Key points of explanation:

In aerobic respiration, each molecule of glucose can generate approximately 30-38 molecules of ATP (depending on cell type/organism) because of the many reactions of reduced coenzymes delivering electrons to the ETC and large proton gradient driving ATP synthase. Osmosis+1
In anaerobic respiration (either lactate or ethanol fermentation), only the ATP from glycolysis (~2 ATP per glucose) is produced, because no further oxidative metabolism occurs (no ETC, no proton gradient, no large-scale ATP synthesis).
In aerobic respiration, substrates are fully oxidised to CO₂ and H₂O; in anaerobic conditions, substrates are only partially oxidised (e.g., to lactate or ethanol), meaning much potential free energy remains unused.
Also, in aerobic conditions the regenerated NAD⁺ via ETC allows more flux through the metabolic pathways and allows complete oxidation, whereas anaerobic regeneration of NAD⁺ through fermentation is less efficient and limits ATP yield.
Therefore, because of the difference in completeness of oxidation and the presence/absence of the ETC + chemiosmotic ATP synthesis, aerobic yield vastly exceeds anaerobic yield.

(12) Explain how rice is adapted to grow with its roots submerged in water, limited to the development of aerenchyma in roots, ethanol fermentation in roots and faster growth of stems

This specification point focuses on a specific adaptation: how rice (a wetland plant) copes with submerged roots (low oxygen conditions) and the particular adaptations enabling growth.

Aerenchyma in roots: In flooded soils, oxygen diffusion into the soil is limited, so the roots receive very little oxygen. Plants such as rice (Oryza spp.) develop aerenchyma (air spaces) in the root and stem tissues which facilitate internal diffusion of oxygen from aerial parts down to the submerged roots. This internal aeration allows some aerobic respiration in roots despite external hypoxia.
Ethanol fermentation in roots: Because the roots are submerged and oxygen limited, they may rely partly on anaerobic respiration (ethanol fermentation) to continue ATP production under low-oxygen conditions. The accumulation of ethanol may diffuse away or be further metabolised when oxygen becomes available.
Faster growth of stems: With roots partially under anaerobic stress, the plant allocates more resources into elongating stems and leaves to reach the water’s surface (or above) where oxygen is more plentiful, thereby improving gaseous exchange. The ability to elongate quickly allows the plant to maintain access to atmospheric oxygen and light, thus sustaining growth.
The combination of structural adaptation (aerenchyma) plus metabolic flexibility (ethanol fermentation) plus growth strategy (stem elongation) allows rice to thrive in submerged conditions. By having roots that can function under low oxygen and by rapidly producing above-water structures, rice is adapted to flooded fields.

(13) Describe and carry out investigations using redox indicators, including DCPIP and methylene blue, to determine the effects of temperature and substrate concentration on the rate of respiration of yeast

Students should be able to describe experiments that use redox indicators (such as 2,6‑dichlorophenolindophenol, DCPIP, and Methylene blue) with yeast to investigate how variables like temperature and substrate concentration affect respiration rate.

Redox indicators: These chemicals change colour when they accept or donate electrons/hydrogen. In respiration, reduced coenzymes donate hydrogen to/through the system; an indicator such as DCPIP is blue when oxidised, colourless when reduced, so the rate at which it changes indicates rate of hydrogen transfer (hence respiration).
Yeast respiration investigations: One can set up yeast suspensions with known concentrations of substrate (e.g., glucose) and add a small amount of DCPIP (or methylene blue). As yeast respire, hydrogen (via reduced NAD etc) will reduce the indicator, causing a colour change. By varying temperature or substrate concentration and timing how long it takes for the indicator to decolourise (or measuring absorbance change), one can infer how respiration rate varies.
Effects of temperature: At higher temperature (within optimal range) enzyme activity increases and respiration rate is faster, so indicator decolourises faster; at too high temperature enzymes denature and rate decreases.
Effects of substrate concentration: As substrate concentration increases (up to a point) more respiration occurs (more substrate to oxidise), so faster indicator reduction; beyond saturation the effect plateaus.
Practical issues: Ensuring yeast is active, controlling volume, mixing, maintaining same volume and indicator concentration, using identical apparatus, controlling for non-respiration reduction of indicator, recording temperature etc.
Data and interpretation: Graphs of time to decolourisation versus temperature or substrate concentration; deducing rate changes and indicating optimum temperature or substrate saturation.

(14) Describe and carry out investigations using simple respirometers to determine the effect of temperature on the rate of respiration

This point requires describing how to set up and carry out a respirometer experiment to determine how respiration rate varies with temperature.

Description of apparatus: A simple respirometer might consist of a sealed container holding respiring organisms (e.g., small invertebrates, germinating seeds, yeast) connected to a manometer or gas syringe which measures the change in gas volume (O₂ consumption or CO₂ production). Alternatively, CO₂ may be absorbed (e.g., with KOH) so the volume change corresponds to O₂ uptake.
Procedure: Place the respiring organisms in the respirometer, allow them to acclimatise, then measure the rate of gas volume change over a fixed period. Repeat at different temperatures (e.g., 10 °C, 20 °C, 30 °C, 40 °C) to see how respiration rate changes. All other factors (organism mass, number of individuals, substrate availability, water availability) must be kept constant.
Effect of temperature: Typically, as temperature increases (within biological limits) enzyme activity increases, so respiration rate increases (steeper gas uptake). However, above an optimum, enzymes may denature, membranes may lose integrity and rate may drop. Also, very low temperatures slow metabolic rate.
Control experiment: Include non-respiring control (e.g., dead seeds or boiled organisms) to measure changes due to temperature alone or apparatus effect.
Data interpretation: Plot rate (e.g., ml O₂ per minute per gram) versus temperature, identify optimum temperature, Q₁₀ value (factor by which rate increases for a 10 °C rise) maybe calculated.
Practical considerations: Ensure no leaks, ensure the temperature is accurate and constant, ensure the organisms are healthy and acclimatised, ensure that O₂ diffusion from the surroundings is minimised.

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