The Same Glucose Gives Very Different Energy With and Without Oxygen
Compare aerobic and anaerobic respiration by their products and their energy yield, follow air from the nostrils to the alveoli and see how gases cross there, and explain why a fish breathes faster than you do.
Why does the same glucose release different amounts of energy?
Every living cell starts by doing the same thing to glucose. In the cytoplasm, a six-carbon glucose molecule is broken into two three-carbon molecules of pyruvate, releasing a little energy. That step happens whether oxygen is present or not.
What happens next depends entirely on the oxygen supply, and the three possible routes give three different sets of products:
- With oxygen, in the mitochondria: pyruvate is broken all the way down to carbon dioxide and water, releasing a large amount of energy
- Without oxygen, in yeast: pyruvate becomes ethanol and carbon dioxide, releasing less energy
- Without enough oxygen, in our own muscles: pyruvate becomes lactic acid, again releasing less energy
So the fuel is the same and only the finishing is different. Breaking a molecule down completely releases more energy than stopping halfway, which is why aerobic respiration yields so much more than anaerobic — and why an organism that can use oxygen almost always does.
The energy released is not used directly. It is first stored in a molecule called ATP, which the cell then spends wherever energy is needed. ATP is the energy currency of the cell, and respiration is the process that earns it.
This page covers the second part of the CBSE Class 10 Science chapter on life processes: aerobic and anaerobic respiration, the human respiratory system and the alveoli, and how different organisms manage gas exchange.
What happens next depends entirely on the oxygen supply, and the three possible routes give three different sets of products:
- With oxygen, in the mitochondria: pyruvate is broken all the way down to carbon dioxide and water, releasing a large amount of energy
- Without oxygen, in yeast: pyruvate becomes ethanol and carbon dioxide, releasing less energy
- Without enough oxygen, in our own muscles: pyruvate becomes lactic acid, again releasing less energy
So the fuel is the same and only the finishing is different. Breaking a molecule down completely releases more energy than stopping halfway, which is why aerobic respiration yields so much more than anaerobic — and why an organism that can use oxygen almost always does.
The energy released is not used directly. It is first stored in a molecule called ATP, which the cell then spends wherever energy is needed. ATP is the energy currency of the cell, and respiration is the process that earns it.
This page covers the second part of the CBSE Class 10 Science chapter on life processes: aerobic and anaerobic respiration, the human respiratory system and the alveoli, and how different organisms manage gas exchange.
How do aerobic and anaerobic respiration compare?
Same starting point, different end points, and a large difference in the energy released.
The common first step, in the cytoplasm of every cell:
Route one — aerobic, in the mitochondria:
Route two — anaerobic in yeast, called fermentation:
Route three — anaerobic in our muscles, during vigorous exercise:
The comparison in full.
- Oxygen: needed for the aerobic route, not for the other two
- Site: the first step is in the cytoplasm for all three; the aerobic breakdown continues in the mitochondria, while the anaerobic routes stay in the cytoplasm
- Products: carbon dioxide and water for aerobic; ethanol and carbon dioxide for yeast; lactic acid for muscle
- Energy: much more from the aerobic route, because the glucose has been broken down completely
The muscle case is the one you have felt. During heavy exercise the muscles need energy faster than the blood can deliver oxygen, so some cells switch to the anaerobic route. The lactic acid that builds up causes cramps.
And the remedy follows from the explanation. A hot water bath or a massage improves the blood circulation to the muscle, which brings more oxygen, which lets the accumulated lactic acid be broken down. The cure works on the cause, which is why it is asked for as a reason and not only as a fact.
Why yeast's version is useful. The ethanol and carbon dioxide it produces are exactly what brewing and baking need — the gas raises the dough and the ethanol is the product of fermentation. The same limitation that makes anaerobic respiration inefficient for the organism makes it valuable to us.
The misconception to clear. Anaerobic respiration is not the same as breathing less. Breathing is the mechanical movement of air; respiration is the chemical release of energy inside cells. An organism can breathe perfectly well and still respire anaerobically in a muscle that is outrunning its oxygen supply — which is exactly what happens when you sprint.
The common first step, in the cytoplasm of every cell:
Route one — aerobic, in the mitochondria:
Route two — anaerobic in yeast, called fermentation:
Route three — anaerobic in our muscles, during vigorous exercise:
The comparison in full.
- Oxygen: needed for the aerobic route, not for the other two
- Site: the first step is in the cytoplasm for all three; the aerobic breakdown continues in the mitochondria, while the anaerobic routes stay in the cytoplasm
- Products: carbon dioxide and water for aerobic; ethanol and carbon dioxide for yeast; lactic acid for muscle
- Energy: much more from the aerobic route, because the glucose has been broken down completely
The muscle case is the one you have felt. During heavy exercise the muscles need energy faster than the blood can deliver oxygen, so some cells switch to the anaerobic route. The lactic acid that builds up causes cramps.
And the remedy follows from the explanation. A hot water bath or a massage improves the blood circulation to the muscle, which brings more oxygen, which lets the accumulated lactic acid be broken down. The cure works on the cause, which is why it is asked for as a reason and not only as a fact.
Why yeast's version is useful. The ethanol and carbon dioxide it produces are exactly what brewing and baking need — the gas raises the dough and the ethanol is the product of fermentation. The same limitation that makes anaerobic respiration inefficient for the organism makes it valuable to us.
The misconception to clear. Anaerobic respiration is not the same as breathing less. Breathing is the mechanical movement of air; respiration is the chemical release of energy inside cells. An organism can breathe perfectly well and still respire anaerobically in a muscle that is outrunning its oxygen supply — which is exactly what happens when you sprint.
How does air travel from the nostrils to the blood?
Down a branching set of tubes to millions of tiny air sacs, where the gases cross by diffusion into a dense net of capillaries.
The path, in order.
- Nostrils — air enters and passes through the nasal passage, where fine hairs and mucus trap dust and moisten the air
- Pharynx and larynx — the throat region
- Trachea — the windpipe, held open by rings of cartilage so that it cannot collapse when there is less air in it
- Bronchi — one to each lung, dividing again and again into finer bronchioles
- Alveoli — balloon-like air sacs at the ends of the finest tubes
Why the alveoli are shaped as they are. Their walls are extremely thin, so gases cross easily; they are present in enormous numbers, giving a very large surface area for exchange; and each is wrapped in a dense network of blood capillaries. Thin walls, large area and a rich blood supply are the three features of every good exchange surface, and a question asking why the alveoli are efficient wants all three.
What crosses, and in which direction.
- Oxygen diffuses from the air in the alveoli into the blood, because the blood arriving there is low in oxygen
- Carbon dioxide diffuses the other way, from the blood into the air, and is breathed out
How each gas is carried in the blood. Oxygen is carried by haemoglobin in the red blood cells, a pigment with a high affinity for it. Carbon dioxide is more soluble in water than oxygen is, so it is carried mostly dissolved in the blood plasma rather than on a pigment. That asymmetry is examined directly — the two gases are transported in two different ways for a reason that is simply about solubility.
The breathing mechanism. Breathing in and breathing out are muscular actions:
- Breathing in — the ribs lift up and outward and the diaphragm flattens, so the chest cavity becomes larger and air rushes in to fill it
- Breathing out — the ribs fall and the diaphragm returns to its domed shape, so the cavity becomes smaller and air is pushed out
Why the lungs never empty completely. Some air always remains in them — the residual volume — so that oxygen can keep being absorbed between breaths. Exchange is continuous while breathing is rhythmic, and the leftover air is what bridges the gap.
One consequence of the design worth noticing. The exchange surface is thin and wet, and both of those make it fragile and easily dried out. That is why lungs are kept inside the body rather than on the surface, and why the air is moistened on its way in. Every land animal pays for its oxygen with water loss, which is the trade-off the next section explores.
The path, in order.
- Nostrils — air enters and passes through the nasal passage, where fine hairs and mucus trap dust and moisten the air
- Pharynx and larynx — the throat region
- Trachea — the windpipe, held open by rings of cartilage so that it cannot collapse when there is less air in it
- Bronchi — one to each lung, dividing again and again into finer bronchioles
- Alveoli — balloon-like air sacs at the ends of the finest tubes
Why the alveoli are shaped as they are. Their walls are extremely thin, so gases cross easily; they are present in enormous numbers, giving a very large surface area for exchange; and each is wrapped in a dense network of blood capillaries. Thin walls, large area and a rich blood supply are the three features of every good exchange surface, and a question asking why the alveoli are efficient wants all three.
What crosses, and in which direction.
- Oxygen diffuses from the air in the alveoli into the blood, because the blood arriving there is low in oxygen
- Carbon dioxide diffuses the other way, from the blood into the air, and is breathed out
How each gas is carried in the blood. Oxygen is carried by haemoglobin in the red blood cells, a pigment with a high affinity for it. Carbon dioxide is more soluble in water than oxygen is, so it is carried mostly dissolved in the blood plasma rather than on a pigment. That asymmetry is examined directly — the two gases are transported in two different ways for a reason that is simply about solubility.
The breathing mechanism. Breathing in and breathing out are muscular actions:
- Breathing in — the ribs lift up and outward and the diaphragm flattens, so the chest cavity becomes larger and air rushes in to fill it
- Breathing out — the ribs fall and the diaphragm returns to its domed shape, so the cavity becomes smaller and air is pushed out
Why the lungs never empty completely. Some air always remains in them — the residual volume — so that oxygen can keep being absorbed between breaths. Exchange is continuous while breathing is rhythmic, and the leftover air is what bridges the gap.
One consequence of the design worth noticing. The exchange surface is thin and wet, and both of those make it fragile and easily dried out. That is why lungs are kept inside the body rather than on the surface, and why the air is moistened on its way in. Every land animal pays for its oxygen with water loss, which is the trade-off the next section explores.
Why does a fish breathe faster than a land animal?
Because water contains far less dissolved oxygen than air does, so a fish must move much more medium across its exchange surface to get the same amount.
In water — fish and gills. A fish takes in water through its mouth and forces it over its gills, where the dissolved oxygen is taken up by the blood. Because the oxygen concentration in water is low, the rate of breathing has to be much faster than in a land animal. Watch a fish's gill covers and they move constantly, while a resting human breathes slowly.
On land — lungs. Air is rich in oxygen, so a much slower rate suffices. The cost is different: the exchange surface must be kept moist, so it is folded away inside the body where it does not dry out, and the air must be brought to it and taken away again. Terrestrial animals have more oxygen available and a harder delivery problem; aquatic animals have the reverse.
In plants — diffusion, and no transport system at all. Plants exchange gases through stomata in the leaves and lenticels in the stems, and the movement is by simple diffusion.
Three features make that enough for a plant:
- The needs are low, because a plant uses far less energy than an animal of similar size
- Each part looks after itself: roots, stems and leaves exchange gases independently, so nothing has to be carried far
- Every cell is close to the surface of its own organ, so diffusion covers the distance
And the direction of the exchange changes with the time of day. At night only respiration is happening, so carbon dioxide is released. During the day photosynthesis is also happening and is usually faster, so the net effect is that oxygen is released — the carbon dioxide made by respiration is consumed by photosynthesis inside the same leaf.
That explains an observation that puzzles students. A plant respires all the time, day and night, but you only see it releasing carbon dioxide at night. Photosynthesis has not switched respiration off — it has simply overtaken it, and a question asking do plants respire at night only is testing precisely that.
The general principle behind all three cases. A larger, more active organism needs a more efficient respiratory surface and a transport system to go with it. Size and activity, not the medium alone, decide how elaborate the arrangement has to be — which is why a plant manages with pores, a fish with gills, and a mammal with lungs plus blood plus a pigment to carry the oxygen.
In water — fish and gills. A fish takes in water through its mouth and forces it over its gills, where the dissolved oxygen is taken up by the blood. Because the oxygen concentration in water is low, the rate of breathing has to be much faster than in a land animal. Watch a fish's gill covers and they move constantly, while a resting human breathes slowly.
On land — lungs. Air is rich in oxygen, so a much slower rate suffices. The cost is different: the exchange surface must be kept moist, so it is folded away inside the body where it does not dry out, and the air must be brought to it and taken away again. Terrestrial animals have more oxygen available and a harder delivery problem; aquatic animals have the reverse.
In plants — diffusion, and no transport system at all. Plants exchange gases through stomata in the leaves and lenticels in the stems, and the movement is by simple diffusion.
Three features make that enough for a plant:
- The needs are low, because a plant uses far less energy than an animal of similar size
- Each part looks after itself: roots, stems and leaves exchange gases independently, so nothing has to be carried far
- Every cell is close to the surface of its own organ, so diffusion covers the distance
And the direction of the exchange changes with the time of day. At night only respiration is happening, so carbon dioxide is released. During the day photosynthesis is also happening and is usually faster, so the net effect is that oxygen is released — the carbon dioxide made by respiration is consumed by photosynthesis inside the same leaf.
That explains an observation that puzzles students. A plant respires all the time, day and night, but you only see it releasing carbon dioxide at night. Photosynthesis has not switched respiration off — it has simply overtaken it, and a question asking do plants respire at night only is testing precisely that.
The general principle behind all three cases. A larger, more active organism needs a more efficient respiratory surface and a transport system to go with it. Size and activity, not the medium alone, decide how elaborate the arrangement has to be — which is why a plant manages with pores, a fish with gills, and a mammal with lungs plus blood plus a pigment to carry the oxygen.
Exam tip
What layout keeps a respiration answer complete?
Name the site, the products and the energy comparison — three items for every respiration question. Most lost marks here are omissions rather than mistakes.
- Say where each step happens: glucose to pyruvate in the cytoplasm, the aerobic breakdown in the mitochondria
- Give the products exactly: ethanol and carbon dioxide in yeast, lactic acid in muscle. Swapping the two is the classic error
- Compare the energy qualitatively: much more from aerobic respiration. Do not invent a figure
- List all three features of a good exchange surface — thin, large area, rich blood supply — when explaining the alveoli
- Say that oxygen travels on haemoglobin and carbon dioxide mostly dissolved in plasma, with solubility as the reason
- Describe breathing as ribs and diaphragm, and say which way each moves
- Explain the fast breathing of a fish by the low oxygen content of water, not by the size of the fish
- For plants, say diffusion, stomata and lenticels, and note that each part manages its own exchange
The distinction to state carefully. Breathing is the mechanical intake and output of air; respiration is the chemical breakdown of glucose inside cells that releases energy. Breathing supplies respiration, and one can continue while the other is inadequate. A question asking for the difference between breathing and respiration is asking for exactly that sentence — and it is asked often.
- Say where each step happens: glucose to pyruvate in the cytoplasm, the aerobic breakdown in the mitochondria
- Give the products exactly: ethanol and carbon dioxide in yeast, lactic acid in muscle. Swapping the two is the classic error
- Compare the energy qualitatively: much more from aerobic respiration. Do not invent a figure
- List all three features of a good exchange surface — thin, large area, rich blood supply — when explaining the alveoli
- Say that oxygen travels on haemoglobin and carbon dioxide mostly dissolved in plasma, with solubility as the reason
- Describe breathing as ribs and diaphragm, and say which way each moves
- Explain the fast breathing of a fish by the low oxygen content of water, not by the size of the fish
- For plants, say diffusion, stomata and lenticels, and note that each part manages its own exchange
The distinction to state carefully. Breathing is the mechanical intake and output of air; respiration is the chemical breakdown of glucose inside cells that releases energy. Breathing supplies respiration, and one can continue while the other is inadequate. A question asking for the difference between breathing and respiration is asking for exactly that sentence — and it is asked often.
Did you know
Why does a fish die in air, which has far more oxygen than water?
Air contains many times more oxygen than the same volume of water. A fish taken out of water is surrounded by a far richer supply than it has ever known — and it suffocates.
The reason is not the amount of oxygen but the surface that collects it. A gill is built from many fine, feathery filaments, and in water each filament floats separately, so the total area exposed to the medium is enormous. Water holds the filaments apart.
Lift the fish out and there is nothing to support them. The filaments collapse and stick together into a wet clump, and the area that was doing the absorbing almost disappears. The gills have not stopped working — they have folded up, and a folded exchange surface cannot exchange.
So the design that makes gills excellent in water makes them useless in air, and the reverse is true of lungs: a lung is a set of internal sacs that would be crushed and waterlogged in water but works perfectly when air can be pumped in and out of it.
Two more features of gills follow from the same reasoning.
- They are thin and richly supplied with blood, exactly like alveoli, because every exchange surface needs the same three properties
- They are protected under gill covers, because a delicate feathery structure exposed on the outside of the body would be damaged constantly
And one clever case shows what the limits really are. A few fish can survive out of water for a time by keeping their gill chambers full of water, or by absorbing oxygen through moist skin — they carry their medium with them rather than adapting the surface. That is the same solution land animals arrived at by moving the surface inside the body and keeping it wet.
The general lesson is the one this chapter keeps making. An exchange surface must be thin, extensive and well supplied with blood, and it must be kept in the medium it was built for. Change the medium and the same surface can fail completely, however much oxygen is available.
The reason is not the amount of oxygen but the surface that collects it. A gill is built from many fine, feathery filaments, and in water each filament floats separately, so the total area exposed to the medium is enormous. Water holds the filaments apart.
Lift the fish out and there is nothing to support them. The filaments collapse and stick together into a wet clump, and the area that was doing the absorbing almost disappears. The gills have not stopped working — they have folded up, and a folded exchange surface cannot exchange.
So the design that makes gills excellent in water makes them useless in air, and the reverse is true of lungs: a lung is a set of internal sacs that would be crushed and waterlogged in water but works perfectly when air can be pumped in and out of it.
Two more features of gills follow from the same reasoning.
- They are thin and richly supplied with blood, exactly like alveoli, because every exchange surface needs the same three properties
- They are protected under gill covers, because a delicate feathery structure exposed on the outside of the body would be damaged constantly
And one clever case shows what the limits really are. A few fish can survive out of water for a time by keeping their gill chambers full of water, or by absorbing oxygen through moist skin — they carry their medium with them rather than adapting the surface. That is the same solution land animals arrived at by moving the surface inside the body and keeping it wet.
The general lesson is the one this chapter keeps making. An exchange surface must be thin, extensive and well supplied with blood, and it must be kept in the medium it was built for. Change the medium and the same surface can fail completely, however much oxygen is available.
Exam relevance
Why does NEET keep returning to respiration and the alveoli?
This is foundation work for two Class 11 Biology chapters and for one Class 11 Physics idea, all examined in NEET.
Where the pathways lead. Class 11 Respiration in Plants expands the three routes into glycolysis, the Krebs cycle and the electron transport chain, with each step's location, its enzymes and its ATP yield named. The split you learn here — common glycolysis, then aerobic or anaerobic — is exactly the structure of that chapter, and the fermentation products remain ethanol for yeast and lactic acid for muscle.
Where the alveoli lead. Class 11 Breathing and Exchange of Gases treats the same system with partial pressures, the oxygen-haemoglobin dissociation curve, and the mechanisms of carbon dioxide transport — as bicarbonate, on haemoglobin and dissolved in plasma. The fact that carbon dioxide is mainly carried in the plasma because it is more soluble is the Class 10 version of that discussion, and NEET asks about the proportions directly.
Where the exchange-surface rule leads. Thin, extensive, well supplied with blood is used to compare gills, lungs, skin and tracheae in Class 11, and the same reasoning explains the villi of the digestive chapter and the nephron of the next part. One rule, four organs — and NEET sets comparison questions across them.
Where it touches Physics. Diffusion across a membrane is driven by a concentration difference, and the same idea appears in Class 11 Kinetic Theory and in Class 12 transport phenomena. The reason a gas moves at all is physics, not biology.
Question types to expect. At this level: compare aerobic with anaerobic, describe the path of air, explain the fish's breathing rate. In competitive papers: match-the-column for site against product against energy, assertion-reason items on carbon dioxide transport, and diagram-based questions on the respiratory system.
The single trap that costs marks. Giving lactic acid as the product of anaerobic respiration in yeast. Yeast gives ethanol and carbon dioxide; lactic acid is the muscle route. In NEET this is a standard distractor and the two are never interchangeable.
A second trap. Saying that plants respire only at night. They respire all the time; at night photosynthesis has stopped, so the carbon dioxide release becomes visible. **The word only is what makes the statement wrong.
Board versus competitive emphasis. The CBSE paper marks the comparison table, the labelled diagram and the stated reason; a competitive paper marks a matched pair or a single fact. The transferable asset is the exchange-surface rule** — three properties that explain alveoli, gills, villi and root hairs alike.
Where the pathways lead. Class 11 Respiration in Plants expands the three routes into glycolysis, the Krebs cycle and the electron transport chain, with each step's location, its enzymes and its ATP yield named. The split you learn here — common glycolysis, then aerobic or anaerobic — is exactly the structure of that chapter, and the fermentation products remain ethanol for yeast and lactic acid for muscle.
Where the alveoli lead. Class 11 Breathing and Exchange of Gases treats the same system with partial pressures, the oxygen-haemoglobin dissociation curve, and the mechanisms of carbon dioxide transport — as bicarbonate, on haemoglobin and dissolved in plasma. The fact that carbon dioxide is mainly carried in the plasma because it is more soluble is the Class 10 version of that discussion, and NEET asks about the proportions directly.
Where the exchange-surface rule leads. Thin, extensive, well supplied with blood is used to compare gills, lungs, skin and tracheae in Class 11, and the same reasoning explains the villi of the digestive chapter and the nephron of the next part. One rule, four organs — and NEET sets comparison questions across them.
Where it touches Physics. Diffusion across a membrane is driven by a concentration difference, and the same idea appears in Class 11 Kinetic Theory and in Class 12 transport phenomena. The reason a gas moves at all is physics, not biology.
Question types to expect. At this level: compare aerobic with anaerobic, describe the path of air, explain the fish's breathing rate. In competitive papers: match-the-column for site against product against energy, assertion-reason items on carbon dioxide transport, and diagram-based questions on the respiratory system.
The single trap that costs marks. Giving lactic acid as the product of anaerobic respiration in yeast. Yeast gives ethanol and carbon dioxide; lactic acid is the muscle route. In NEET this is a standard distractor and the two are never interchangeable.
A second trap. Saying that plants respire only at night. They respire all the time; at night photosynthesis has stopped, so the carbon dioxide release becomes visible. **The word only is what makes the statement wrong.
Board versus competitive emphasis. The CBSE paper marks the comparison table, the labelled diagram and the stated reason; a competitive paper marks a matched pair or a single fact. The transferable asset is the exchange-surface rule** — three properties that explain alveoli, gills, villi and root hairs alike.
Key takeaways
What should you know about respiration before transport?
One shared first step, three finishes, and one rule for every exchange surface.
- Glucose becomes pyruvate in the cytoplasm whether oxygen is present or not
- Aerobic, in the mitochondria: carbon dioxide and water, with much more energy
- Anaerobic in yeast: ethanol and carbon dioxide — fermentation
- Anaerobic in muscle: lactic acid, which causes cramps; a hot bath or massage brings more oxygen and clears it
- ATP is the energy currency in which the released energy is stored
- The path of air: nostrils, nasal passage, pharynx, larynx, trachea with cartilage rings, bronchi, bronchioles, alveoli
- An exchange surface must be thin, extensive and richly supplied with blood — true of alveoli, gills and villi alike
- Oxygen is carried by haemoglobin; carbon dioxide mostly dissolved in plasma, because it is more soluble
- Breathing in: ribs lift and the diaphragm flattens. A residual volume keeps absorption continuous between breaths
- Fish breathe faster because water holds far less dissolved oxygen than air
- Plants use diffusion through stomata and lenticels, each part independently, and they respire day and night
- Breathing is mechanical; respiration is chemical — the two are not synonyms
The sharpest self-test is the three-route diagram. Draw glucose at the top and the three endings below it, label the site of each, name the products, and mark which route releases the most energy.
- Glucose becomes pyruvate in the cytoplasm whether oxygen is present or not
- Aerobic, in the mitochondria: carbon dioxide and water, with much more energy
- Anaerobic in yeast: ethanol and carbon dioxide — fermentation
- Anaerobic in muscle: lactic acid, which causes cramps; a hot bath or massage brings more oxygen and clears it
- ATP is the energy currency in which the released energy is stored
- The path of air: nostrils, nasal passage, pharynx, larynx, trachea with cartilage rings, bronchi, bronchioles, alveoli
- An exchange surface must be thin, extensive and richly supplied with blood — true of alveoli, gills and villi alike
- Oxygen is carried by haemoglobin; carbon dioxide mostly dissolved in plasma, because it is more soluble
- Breathing in: ribs lift and the diaphragm flattens. A residual volume keeps absorption continuous between breaths
- Fish breathe faster because water holds far less dissolved oxygen than air
- Plants use diffusion through stomata and lenticels, each part independently, and they respire day and night
- Breathing is mechanical; respiration is chemical — the two are not synonyms
The sharpest self-test is the three-route diagram. Draw glucose at the top and the three endings below it, label the site of each, name the products, and mark which route releases the most energy.