Could hydrogen replace oxygen in cellular respiration?

Could hydrogen replace oxygen in cellular respiration?

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I was wondering what oxygen actually does in the body. I have seen a few answers to other questions that involve the electron chain and I am really not sure what that is. So I was wondering what oxygen does and could hydrogen do the same thing as a substitute?

No, hydrogen could not replace oxygen because it has entirely different characteristics. The most important one is probably its electronegativity - oxygen 'pulls' electrons much 'stronger' than hydrogen.

Basics: Reduction potential

Oxygen is the so-called terminal electron acceptor of the electron transport chain in eukaryotes. You can see "reduction potential" as a kind of stored "energy" which molecules have, similar to the power stored in batteries (very similar actually). To make this text a bit shorter I'll call it "RP" from now on.

One maybe confusing detail is that a substance with low RP has "more energy" than a substance with high RP, so it is the opposite way of thinking.

In very generalised terms, metabolism means that molecules with a low RP (glucose) are oxidised (burned) and turn into molecules with much higher RP (CO2). Coupled with this, a different molecule with very high RP (oxygen) is reduced and becomes a molecule with slightly lower RP (H2O). (You may have heard this before - it's called a redox reaction.) *

The important part is that the RP "released" by the oxidisation (burning) is larger than the RP "taken up" by the reduction. The surplus leaves as energy - heat and light if you just burn the glucose. This is a spontaneous process, meaning it will occur just by itself - even if it takes a long time if nobody drops a match on it.

The idea of metabolism is to let that process happen - but use as much of the energy it releases as possible. This works by not just letting it burn, but intercepting that burning process at different stages so that at each step a bit of the RP can be taken off and stored in something else. This "something else" is NAD which I'm sure you've encountered before. Each step that glucose is burned down, another bit of NADH is made, which then has a respectable reduction potential.

NADH (leaving out NADPH here which is a bit different) is channeled into a process called oxidative phosphorylation which retrieves the reduction potential in an actual form of energy.

Basics: Terminal electron acceptor

Finally, the reduction potential I've been talking about all the time is really just electrons, involved in bonds which are "happy" to react. Passing down the reduction potential as I've explained is really a passing down of electrons into more and more stable, less reactive bonds. That's why it's called "electron transport chain". At the end of oxidative phosphorylation, those electrons are dropped onto O2 and make it into H2O. That's why O2 is called the "terminal electron acceptor".

Why Hydrogen can't replace Oxygen

Now to come back to why hydrogen cannot perform oxygen's function in our body. We use glucose as our source of reduction power and oxygen as our terminal electron acceptor. O has a high electronegativity (3.5) so it pulls electrons strongly towards it. H's electronegativity is only 2.1, so it's much weaker. O as a terminal electron acceptor works because it pulls them much stronger than H when they bond, so an O-H bond is almost like giving oxygen an electron. In order for hydrogen gas (H2) to perform the same function, it would need to be possible to drop electrons onto hydrogen in a bond where it pulls them much stronger than the other partner. They do exist, and such compounds are called hydrides. But the catch is: unlike H2O, these are normally strong reducing agents, meaning that hydrogen would rather not be in that bond. This not a feasible option for cell respiration, at least in humans, because it requires a lot of RP input. Making oxygen into H2O does not require a lot, it's a very cheap electron acceptor.

I hope I was able to put this in understandable terms. Let me know if I need to clarify anything.


*It works with other molecules than glucose->CO2 / O2->H2O too, many prokaryotes do that and in fact that's how batteries work

I would say that while you could not just replace oxygen with hydrogen and expect life to just go on normally, it's possible for living things to be energetically provided for by H2 if it were found in the environment in enough plenty. This is just speculation as Armatus points out metabolism does not work this way in animals, but I think that its quite possible a living organism could live off of H2 as an energy source.

Extremophiles can metabolize sulfur in underwater vents rather than oxygen.

In the soil, anoxic metabolisms reduce nitrogn gas to ammonia.

Photosynthesis uses light to reduce CO2 to glucose. Atmospheric molecular oxygen is provided entirely by photosynthesis, and at one point there was little or none of it in the atmosphere and living things were found everywhere. The glucose metabolism came after O2 appeared.

The main point is that H2 stores a reasonable amount of energy even if free oxygen is not available to drive the formation of water from another oxide, or even more exotic chemistry. There's not thermodynamic reason that it couldn't work and biology on earth has proven to be versatile in getting chemical energy wherever it needs to.

Its xenobiology and speculation, but I feel that if microorganisms evolved on a gas giant with a primarily hydrogen atmosphere, they could use the redox energy from H2, which is quite reactive even without O2 present.

Oxidative Phosphorylation

You have just read about two pathways in glucose catabolism—glycolysis and the citric acid cycle—that generate ATP. Most of the ATP generated during the aerobic catabolism of glucose, however, is not generated directly from these pathways. Rather, it is derived from a process that begins with moving electrons through a series of electron transporters that undergo redox reactions. This causes hydrogen ions to accumulate within the matrix space. Therefore, a concentration gradient forms in which hydrogen ions diffuse out of the matrix space by passing through ATP synthase. The current of hydrogen ions powers the catalytic action of ATP synthase, which phosphorylates ADP, producing ATP.

Chapter 9 – Cellular Respiration

· To perform their many tasks, living cells require energy from outside sources.

· Energy enters most ecosystems as sunlight and leaves as heat.

· Photosynthesis generates oxygen and organic molecules that the mitochondria of eukaryotes use as fuel for cellular respiration.

· Cells harvest the chemical energy stored in organic molecules and use it to regenerate ATP, the molecule that drives most cellular work.

· Respiration has three key pathways: glycolysis, the citric acid cycle, and oxidative phosphorylation.

A. The Principles of Energy Harvest

1. Cellular respiration and fermentation are catabolic, energy-yielding pathways.

· The arrangement of atoms of organic molecules represents potential energy.

· Enzymes catalyze the systematic degradation of organic molecules that are rich in energy to simpler waste products with less energy.

· Some of the released energy is used to do work the rest is dissipated as heat.

· Catabolic metabolic pathways release the energy stored in complex organic molecules.

· One type of catabolic process, fermentation, leads to the partial degradation of sugars in the absence of oxygen.

· A more efficient and widespread catabolic process, cellular respiration, consumes oxygen as a reactant to complete the breakdown of a variety of organic molecules.

° In eukaryotic cells, mitochondria are the site of most of the processes of cellular respiration.

· Cellular respiration is similar in broad principle to the combustion of gasoline in an automobile engine after oxygen is mixed with hydrocarbon fuel.

° Food is the fuel for respiration. The exhaust is carbon dioxide and water.

° organic compounds + O2 à CO2 + H2O + energy (ATP + heat).

· Carbohydrates, fats, and proteins can all be used as the fuel, but it is most useful to consider glucose.

° C6H12O6 + 6O2 à 6CO2 + 6H2O + Energy (ATP + heat)

· The catabolism of glucose is exergonic with a D G of −686 kcal per mole of glucose.

° Some of this energy is used to produce ATP, which can perform cellular work.

2. Redox reactions release energy when electrons move closer to electronegative atoms.

· Catabolic pathways transfer the electrons stored in food molecules, releasing energy that is used to synthesize ATP.

· Reactions that result in the transfer of one or more electrons from one reactant to another are oxidation-reduction reactions, or redox reactions.

° The loss of electrons is called oxidation.

° The addition of electrons is called reduction.

· The formation of table salt from sodium and chloride is a redox reaction.

° Here sodium is oxidized and chlorine is reduced (its charge drops from 0 to −1).

· More generally: Xe− + Y à X + Ye−

° X, the electron donor, is the reducing agent and reduces Y.

° Y, the electron recipient, is the oxidizing agent and oxidizes X.

· Redox reactions require both a donor and acceptor.

· Redox reactions also occur when the transfer of electrons is not complete but involves a change in the degree of electron sharing in covalent bonds.

° In the combustion of methane to form water and carbon dioxide, the nonpolar covalent bonds of methane (C—H) and oxygen (O=O) are converted to polar covalent bonds (C=O and O—H).

° When methane reacts with oxygen to form carbon dioxide, electrons end up farther away from the carbon atom and closer to their new covalent partners, the oxygen atoms, which are very electronegative.

° In effect, the carbon atom has partially “lost” its shared electrons. Thus, methane has been oxidized.

· The two atoms of the oxygen molecule share their electrons equally. When oxygen reacts with the hydrogen from methane to form water, the electrons of the covalent bonds are drawn closer to the oxygen.

° In effect, each oxygen atom has partially “gained” electrons, and so the oxygen molecule has been reduced.

° Oxygen is very electronegative, and is one of the most potent of all oxidizing agents.

· Energy must be added to pull an electron away from an atom.

· The more electronegative the atom, the more energy is required to take an electron away from it.

· An electron loses potential energy when it shifts from a less electronegative atom toward a more electronegative one.

· A redox reaction that relocates electrons closer to oxygen, such as the burning of methane, releases chemical energy that can do work.

3. The “fall” of electrons during respiration is stepwise, via NAD+ and an electron transport chain.

· Cellular respiration does not oxidize glucose in a single step that transfers all the hydrogen in the fuel to oxygen at one time.

· Rather, glucose and other fuels are broken down in a series of steps, each catalyzed by a specific enzyme.

° At key steps, electrons are stripped from the glucose.

° In many oxidation reactions, the electron is transferred with a proton, as a hydrogen atom.

· The hydrogen atoms are not transferred directly to oxygen but are passed first to a coenzyme called NAD+ (nicotinamide adenine dinucleotide).

· How does NAD+ trap electrons from glucose?

° Dehydrogenase enzymes strip two hydrogen atoms from the fuel (e.g., glucose), oxidizing it.

° The enzyme passes two electrons and one proton to NAD+.

° The other proton is released as H+ to the surrounding solution.

· By receiving two electrons and only one proton, NAD+ has its charge neutralized when it is reduced to NADH.

° NAD+ functions as the oxidizing agent in many of the redox steps during the catabolism of glucose.

· The electrons carried by NADH have lost very little of their potential energy in this process.

· Each NADH molecule formed during respiration represents stored energy. This energy is tapped to synthesize ATP as electrons “fall” from NADH to oxygen.

· How are electrons extracted from food and stored by NADH finally transferred to oxygen?

° Unlike the explosive release of heat energy that occurs when H2 and O2 are combined (with a spark for activation energy), cellular respiration uses an electron transport chain to break the fall of electrons to O2 into several steps.

· The electron transport chain consists of several molecules (primarily proteins) built into the inner membrane of a mitochondrion.

· Electrons released from food are shuttled by NADH to the “top” higher-energy end of the chain.

· At the “bottom” lower-energy end, oxygen captures the electrons along with H+ to form water.

· Electron transfer from NADH to oxygen is an exergonic reaction with a free energy change of −53 kcal/mol.

· Electrons are passed to increasingly electronegative molecules in the chain until they reduce oxygen, the most electronegative receptor.

· In summary, during cellular respiration, most electrons travel the following “downhill” route: food à NADH à electron transport chain à oxygen.

B. The Process of Cellular Respiration

1. These are the stages of cellular respiration: a preview.

· Respiration occurs in three metabolic stages: glycolysis, the citric acid cycle, and the electron transport chain and oxidative phosphorylation.

· Glycolysis occurs in the cytoplasm.

° It begins catabolism by breaking glucose into two molecules of pyruvate.

· The citric acid cycle occurs in the mitochondrial matrix.

° It completes the breakdown of glucose by oxidizing a derivative of pyruvate to carbon dioxide.

· Several steps in glycolysis and the citric acid cycle are redox reactions in which dehydrogenase enzymes transfer electrons from substrates to NAD+, forming NADH.

· NADH passes these electrons to the electron transport chain.

· In the electron transport chain, the electrons move from molecule to molecule until they combine with molecular oxygen and hydrogen ions to form water.

· As they are passed along the chain, the energy carried by these electrons is transformed in the mitochondrion into a form that can be used to synthesize ATP via oxidative phosphorylation.

· The inner membrane of the mitochondrion is the site of electron transport and chemiosmosis, processes that together constitute oxidative phosphorylation.

° Oxidative phosphorylation produces almost 90% of the ATP generated by respiration.

· Some ATP is also formed directly during glycolysis and the citric acid cycle by substrate-level phosphorylation.

° Here an enzyme transfers a phosphate group from an organic substrate to ADP, forming ATP.

· For each molecule of glucose degraded to carbon dioxide and water by respiration, the cell makes up to 38 ATP, each with 7.3 kcal/mol of free energy.

· Respiration uses the small steps in the respiratory pathway to break the large denomination of energy contained in glucose into the small change of ATP.

° The quantity of energy in ATP is more appropriate for the level of work required in the cell.

2. Glycolysis harvests chemical energy by oxidizing glucose to pyruvate.

· During glycolysis, glucose, a six carbon-sugar, is split into two three-carbon sugars.

· These smaller sugars are oxidized and rearranged to form two molecules of pyruvate, the ionized form of pyruvic acid.

· Each of the ten steps in glycolysis is catalyzed by a specific enzyme.

· These steps can be divided into two phases: an energy investment phase and an energy payoff phase.

· In the energy investment phase, the cell invests ATP to provide activation energy by phosphorylating glucose.

° This requires 2 ATP per glucose.

· In the energy payoff phase, ATP is produced by substrate-level phosphorylation and NAD+ is reduced to NADH by electrons released by the oxidation of glucose.

· The net yield from glycolysis is 2 ATP and 2 NADH per glucose.

° No CO2 is produced during glycolysis.

· Glycolysis can occur whether O2 is present or not.

3. The citric acid cycle completes the energy-yielding oxidation of organic molecules.

· More than three-quarters of the original energy in glucose is still present in the two molecules of pyruvate.

· If oxygen is present, pyruvate enters the mitochondrion where enzymes of the citric acid cycle complete the oxidation of the organic fuel to carbon dioxide.

· After pyruvate enters the mitochondrion via active transport, it is converted to a compound called acetyl coenzyme A or acetyl CoA.

· This step is accomplished by a multienzyme complex that catalyzes three reactions:

1. A carboxyl group is removed as CO2.

2. The remaining two-carbon fragment is oxidized to form acetate. An enzyme transfers the pair of electrons to NAD+ to form NADH.

3. Acetate combines with coenzyme A to form the very reactive molecule acetyl CoA.

· Acetyl CoA is now ready to feed its acetyl group into the citric acid cycle for further oxidation.

· The citric acid cycle is also called the Krebs cycle in honor of Hans Krebs, who was largely responsible for elucidating its pathways in the 1930s.

· The citric acid cycle oxidizes organic fuel derived from pyruvate.

° The citric acid cycle has eight steps, each catalyzed by a specific enzyme.

° The acetyl group of acetyl CoA joins the cycle by combining with the compound oxaloacetate, forming citrate.

° The next seven steps decompose the citrate back to oxaloacetate. It is the regeneration of oxaloacetate that makes this process a cycle.

° Three CO2 molecules are released, including the one released during the conversion of pyruvate to acetyl CoA.

· The cycle generates one ATP per turn by substrate-level phosphorylation.

° A GTP molecule is formed by substrate-level phosphorylation.

° The GTP is then used to synthesize an ATP, the only ATP generated directly by the citric acid cycle.

· Most of the chemical energy is transferred to NAD+ and FAD during the redox reactions.

· The reduced coenzymes NADH and FADH2 then transfer high-energy electrons to the electron transport chain.

· Each cycle produces one ATP by substrate-level phosphorylation, three NADH, and one FADH2 per acetyl CoA.

4. The inner mitochondrial membrane couples electron transport to ATP synthesis.

· Only 4 of 38 ATP ultimately produced by respiration of glucose are produced by substrate-level phosphorylation.

° Two are produced during glycolysis, and 2 are produced during the citric acid cycle.

· NADH and FADH2 account for the vast majority of the energy extracted from the food.

° These reduced coenzymes link glycolysis and the citric acid cycle to oxidative phosphorylation, which uses energy released by the electron transport chain to power ATP synthesis.

· The electron transport chain is a collection of molecules embedded in the cristae, the folded inner membrane of the mitochondrion.

° The folding of the cristae increases its surface area, providing space for thousands of copies of the chain in each mitochondrion.

° Most components of the chain are proteins bound to prosthetic groups, nonprotein components essential for catalysis.

· Electrons drop in free energy as they pass down the electron transport chain.

· During electron transport along the chain, electron carriers alternate between reduced and oxidized states as they accept and donate electrons.

° Each component of the chain becomes reduced when it accepts electrons from its “uphill” neighbor, which is less electronegative.

° It then returns to its oxidized form as it passes electrons to its more electronegative “downhill” neighbor.

· Electrons carried by NADH are transferred to the first molecule in the electron transport chain, a flavoprotein.

· The electrons continue along the chain that includes several cytochrome proteins and one lipid carrier.

° The prosthetic group of each cytochrome is a heme group with an iron atom that accepts and donates electrons.

· The last cytochrome of the chain, cyt a3, passes its electrons to oxygen, which is very electronegative.

° Each oxygen atom also picks up a pair of hydrogen ions from the aqueous solution to form water.

° For every two electron carriers (four electrons), one O2 molecule is reduced to two molecules of water.

· The electrons carried by FADH2 have lower free energy and are added at a lower energy level than those carried by NADH.

° The electron transport chain provides about one-third less energy for ATP synthesis when the electron donor is FADH2 rather than NADH.

· The electron transport chain generates no ATP directly.

· Its function is to break the large free energy drop from food to oxygen into a series of smaller steps that release energy in manageable amounts.

· How does the mitochondrion couple electron transport and energy release to ATP synthesis?

° The answer is a mechanism called chemiosmosis.

· A protein complex, ATP synthase, in the cristae actually makes ATP from ADP and Pi.

· ATP uses the energy of an existing proton gradient to power ATP synthesis.

° The proton gradient develops between the intermembrane space and the matrix.

· The proton gradient is produced by the movement of electrons along the electron transport chain.

· The chain is an energy converter that uses the exergonic flow of electrons to pump H+ from the matrix into the intermembrane space.

· The protons pass back to the matrix through a channel in ATP synthase, using the exergonic flow of H+ to drive the phosphorylation of ADP.

· Thus, the energy stored in a H+ gradient across a membrane couples the redox reactions of the electron transport chain to ATP synthesis.

· From studying the structure of ATP synthase, scientists have learned how the flow of H+ through this large enzyme powers ATP generation.

· ATP synthase is a multisubunit complex with four main parts, each made up of multiple polypeptides:

1. A rotor in the inner mitochondrial membrane.

2. A knob that protrudes into the mitochondrial matrix.

3. An internal rod extending from the rotor into the knob.

4. A stator, anchored next to the rotor, which holds the knob stationary.

· Protons flow down a narrow space between the stator and rotor, causing the rotor and its attached rod to rotate.

° The spinning rod causes conformational changes in the stationary knob, activating three catalytic sites in the knob where ADP and inorganic phosphate combine to make ATP.

· How does the inner mitochondrial membrane generate and maintain the H+ gradient that drives ATP synthesis in the ATP synthase protein complex?

° Creating the H+ gradient is the function of the electron transport chain.

° The ETC is an energy converter that uses the exergonic flow of electrons to pump H+ across the membrane from the mitochondrial matrix to the intermembrane space.

° The H+ has a tendency to diffuse down its gradient.

· The ATP synthase molecules are the only place that H+ can diffuse back to the matrix.

° The exergonic flow of H+ is used by the enzyme to generate ATP.

° This coupling of the redox reactions of the electron transport chain to ATP synthesis is called chemiosmosis.

· How does the electron transport chain pump protons?

° Certain members of the electron transport chain accept and release H+ along with electrons.

° At certain steps along the chain, electron transfers cause H+ to be taken up and released into the surrounding solution.

· The electron carriers are spatially arranged in the membrane in such a way that protons are accepted from the mitochondrial matrix and deposited in the intermembrane space.

° The H+ gradient that results is the proton-motive force.

° The gradient has the capacity to do work.

· Chemiosmosis is an energy-coupling mechanism that uses energy stored in the form of an H+ gradient across a membrane to drive cellular work.

· In mitochondria, the energy for proton gradient formation comes from exergonic redox reactions, and ATP synthesis is the work performed.

· Chemiosmosis in chloroplasts also generates ATP, but light drives the electron flow down an electron transport chain and H+ gradient formation.

· Prokaryotes generate H+ gradients across their plasma membrane.

° They can use this proton-motive force not only to generate ATP, but also to pump nutrients and waste products across the membrane and to rotate their flagella.

5. Here is an accounting of ATP production by cellular respiration.

· During cellular respiration, most energy flows from glucose à NADH à electron transport chain à proton-motive force à ATP.

· Let’s consider the products generated when cellular respiration oxidizes a molecule of glucose to six CO2 molecules.

· Four ATP molecules are produced by substrate-level phosphorylation during glycolysis and the citric acid cycle.

· Many more ATP molecules are generated by oxidative phosphorylation.

· Each NADH from the citric acid cycle and the conversion of pyruvate contributes enough energy to the proton-motive force to generate a maximum of 3 ATP.

° The NADH from glycolysis may also yield 3 ATP.

· Each FADH2 from the citric acid cycle can be used to generate about 2 ATP.

· Why is our accounting so inexact?

· There are three reasons that we cannot state an exact number of ATP molecules generated by one molecule of glucose.

1. Phosphorylation and the redox reactions are not directly coupled to each other, so the ratio of number of NADH to number of ATP is not a whole number.

° One NADH results in 10 H+ being transported across the inner mitochondrial membrane.

° Between 3 and 4 H+ must reenter the mitochondrial matrix via ATP synthase to generate 1 ATP.

° Therefore, 1 NADH generates enough proton-motive force for synthesis of 2.5 to 3.3 ATP.

° We round off and say that 1 NADH generates 3 ATP.

2. The ATP yield varies slightly depending on the type of shuttle used to transport electrons from the cytosol into the mitochondrion.

° The mitochondrial inner membrane is impermeable to NADH, so the two electrons of the NADH produced in glycolysis must be conveyed into the mitochondrion by one of several electron shuttle systems.

° In some shuttle systems, the electrons are passed to NAD+, which generates 3 ATP. In others, the electrons are passed to FAD, which generates only 2 ATP.

3. The proton-motive force generated by the redox reactions of respiration may drive other kinds of work, such as mitochondrial uptake of pyruvate from the cytosol.

° If all the proton-motive force generated by the electron transport chain were used to drive ATP synthesis, one glucose molecule could generate a maximum of 34 ATP by oxidative phosphorylation plus 4 ATP (net) from substrate-level phosphorylation to give a total yield of 36–38 ATP (depending on the efficiency of the shuttle).

· How efficient is respiration in generating ATP?

° Complete oxidation of glucose releases 686 kcal/mol.

° Phosphorylation of ADP to form ATP requires at least 7.3 kcal/mol.

° Efficiency of respiration is 7.3 kcal/mol times 38 ATP/glucose divided by 686 kcal/mol glucose, which equals 0.4 or 40%.

° Approximately 60% of the energy from glucose is lost as heat.

§ Some of that heat is used to maintain our high body temperature (37°C).

· Cellular respiration is remarkably efficient in energy conversion.

C. Related Metabolic Processes

1. Fermentation enables some cells to produce ATP without the help of oxygen.

· Without electronegative oxygen to pull electrons down the transport chain, oxidative phosphorylation ceases.

· However, fermentation provides a mechanism by which some cells can oxidize organic fuel and generate ATP without the use of oxygen.

° In glycolysis, glucose is oxidized to two pyruvate molecules with NAD+ as the oxidizing agent.

° Glycolysis is exergonic and produces 2 ATP (net).

° If oxygen is present, additional ATP can be generated when NADH delivers its electrons to the electron transport chain.

· Glycolysis generates 2 ATP whether oxygen is present (aerobic) or not (anaerobic).

· Anaerobic catabolism of sugars can occur by fermentation.

· Fermentation can generate ATP from glucose by substrate-level phosphorylation as long as there is a supply of NAD+ to accept electrons.

° If the NAD+ pool is exhausted, glycolysis shuts down.

° Under aerobic conditions, NADH transfers its electrons to the electron transfer chain, recycling NAD+.

· Under anaerobic conditions, various fermentation pathways generate ATP by glycolysis and recycle NAD+ by transferring electrons from NADH to pyruvate or derivatives of pyruvate.

· In alcohol fermentation, pyruvate is converted to ethanol in two steps.

° First, pyruvate is converted to a two-carbon compound, acetaldehyde, by the removal of CO2.

° Second, acetaldehyde is reduced by NADH to ethanol.

° Alcohol fermentation by yeast is used in brewing and winemaking.

· During lactic acid fermentation, pyruvate is reduced directly by NADH to form lactate (the ionized form of lactic acid) without release of CO2.

° Lactic acid fermentation by some fungi and bacteria is used to make cheese and yogurt.

° Human muscle cells switch from aerobic respiration to lactic acid fermentation to generate ATP when O2 is scarce.

§ The waste product, lactate, may cause muscle fatigue, but ultimately it is converted back to pyruvate in the liver.

· Fermentation and cellular respiration are anaerobic and aerobic alternatives, respectively, for producing ATP from sugars.

° Both use glycolysis to oxidize sugars to pyruvate with a net production of 2 ATP by substrate-level phosphorylation.

° Both use NAD+ as an oxidizing agent to accept electrons from food during glycolysis.

· The two processes differ in their mechanism for oxidizing NADH to NAD+.

° In fermentation, the electrons of NADH are passed to an organic molecule to regenerate NAD+.

° In respiration, the electrons of NADH are ultimately passed to O2, generating ATP by oxidative phosphorylation.

· More ATP is generated from the oxidation of pyruvate in the citric acid cycle.

° Without oxygen, the energy still stored in pyruvate is unavailable to the cell.

° Under aerobic respiration, a molecule of glucose yields 38 ATP, but the same molecule of glucose yields only 2 ATP under anaerobic respiration.

· Yeast and many bacteria are facultative anaerobes that can survive using either fermentation or respiration.

° At a cellular level, human muscle cells can behave as facultative anaerobes.

· For facultative anaerobes, pyruvate is a fork in the metabolic road that leads to two alternative routes.

° Under aerobic conditions, pyruvate is converted to acetyl CoA and oxidation continues in the citric acid cycle.

° Under anaerobic conditions, pyruvate serves as an electron acceptor to recycle NAD+.

· The oldest bacterial fossils are more than 3.5 billion years old, appearing long before appreciable quantities of O2 accumulated in the atmosphere.

° Therefore, the first prokaryotes may have generated ATP exclusively from glycolysis.

· The fact that glycolysis is a ubiquitous metabolic pathway and occurs in the cytosol without membrane-enclosed organelles suggests that glycolysis evolved early in the history of life.

2. Glycolysis and the citric acid cycle connect to many other metabolic pathways.

· Glycolysis can accept a wide range of carbohydrates for catabolism.

° Polysaccharides like starch or glycogen can be hydrolyzed to glucose monomers that enter glycolysis.

° Other hexose sugars, such as galactose and fructose, can also be modified to undergo glycolysis.

· The other two major fuels, proteins and fats, can also enter the respiratory pathways used by carbohydrates.

· Proteins must first be digested to individual amino acids.

° Amino acids that will be catabolized must have their amino groups removed via deamination.

° The nitrogenous waste is excreted as ammonia, urea, or another waste product.

· The carbon skeletons are modified by enzymes and enter as intermediaries into glycolysis or the citric acid cycle, depending on their structure.

· Catabolism can also harvest energy stored in fats.

· Fats must be digested to glycerol and fatty acids.

° Glycerol can be converted to glyceraldehyde phosphate, an intermediate of glycolysis.

° The rich energy of fatty acids is accessed as fatty acids are split into two-carbon fragments via beta oxidation.

° These molecules enter the citric acid cycle as acetyl CoA.

· A gram of fat oxides by respiration generates twice as much ATP as a gram of carbohydrate.

· The metabolic pathways of respiration also play a role in anabolic pathways of the cell.

· Intermediaries in glycolysis and the citric acid cycle can be diverted to anabolic pathways.

° For example, a human cell can synthesize about half the 20 different amino acids by modifying compounds from the citric acid cycle.

° Glucose can be synthesized from pyruvate fatty acids can be synthesized from acetyl CoA.

· Glycolysis and the citric acid cycle function as metabolic interchanges that enable cells to convert one kind of molecule to another as needed.

° For example, excess carbohydrates and proteins can be converted to fats through intermediaries of glycolysis and the citric acid cycle.

· Metabolism is remarkably versatile and adaptable.

3. Feedback mechanisms control cellular respiration.

· Basic principles of supply and demand regulate the metabolic economy.

° If a cell has an excess of a certain amino acid, it typically uses feedback inhibition to prevent the diversion of intermediary molecules from the citric acid cycle to the synthesis pathway of that amino acid.

· The rate of catabolism is also regulated, typically by the level of ATP in the cell.

° If ATP levels drop, catabolism speeds up to produce more ATP.

· Control of catabolism is based mainly on regulating the activity of enzymes at strategic points in the catabolic pathway.

· One strategic point occurs in the third step of glycolysis, catalyzed by phosphofructokinase.

· Allosteric regulation of phosphofructokinase sets the pace of respiration.

° This enzyme catalyzes the earliest step that irreversibly commits the substrate to glycolysis.

° Phosphofructokinase is an allosteric enzyme with receptor sites for specific inhibitors and activators.

° It is inhibited by ATP and stimulated by AMP (derived from ADP).

§ When ATP levels are high, inhibition of this enzyme slows glycolysis.

§ As ATP levels drop and ADP and AMP levels rise, the enzyme becomes active again and glycolysis speeds up.

· Citrate, the first product of the citric acid cycle, is also an inhibitor of phosphofructokinase.

° This synchronizes the rate of glycolysis and the citric acid cycle.

· If intermediaries from the citric acid cycle are diverted to other uses (e.g., amino acid synthesis), glycolysis speeds up to replace these molecules.

· Metabolic balance is augmented by the control of other enzymes at other key locations in glycolysis and the citric acid cycle.

What is Cellular Respiration?

By definition, cellular respiration is the set of catabolic pathways that break down the nutrients we consume into usable forms of chemical energy (ATP). Cellular respiration can occur both with or without the presence of oxygen, and these two main forms are referred to as aerobic and anaerobic respiration, respectively. There are a number of key differences between the two, primarily that aerobic respiration is a much more evolved process with a significantly higher yield of ATP.

Aerobic Respiration

There are three main stages of aerobic respiration &ndash glycolysis, the Krebs Cycle, and the electron transport chain &ndash each of which deserves an entire article all to itself, but when looking at the overall process of cellular respiration, we will only look at these stages at a somewhat basic level, leaving out the specific details of every chemical reaction in each stage.

This first step in the process of aerobic respiration occurs in the cytosol of the cell, and is an important starting point for the rest of the processes. In glycolysis, one molecule of glucose is converted into two molecules of pyruvate over the course of a ten-step reaction involving different enzymes at each step. Additionally, glycolysis requires two molecules of nicotinamide adenine dinucleotide (NAD+), two molecules of inorganic phosphorous, and two molecules of ADP (adenine diphosphate). The additional products from the reaction include two molecules of ATP 2 molecules of NADH(reduced nicotinamide adenine dinucleotide), 2 water molecules, 2 hydrogen molecules and heat!

(Photo Credit: YassineMrabet/Wikimedia Commons)

The heat and water are considered waste products, the ATP is an immediately usable form of cellular energy, the NADH will be usable later in the aerobic respiration process and the pyruvate acts as the primary substrate in the next step of the process.

Krebs Cycle (Citric Acid Cycle)

Similar to the process of glycolysis, there are many individual steps of the Krebs&rsquo Cycle, the details of which are beyond the scope of this article. Basically, the Krebs Cycle is a stage of cellular respiration that takes place in the mitochondria in the presence of oxygen, unlike glycolysis, which occurred in the cytosol and can occur without oxygen being present.

The final product of glycolysis, two molecules of pyruvate, will enter the Krebs&rsquo cycle in the matrix of the mitochondria, and will ultimately be converted into two molecules of ATP, 8 NADH and 2 FADH2 molecules. Those latter two molecules are high-energy electron carriers, and will be able to produce a significant amount of chemical energy in the electron transport chain.

(Photo Credit: Wikimedia Commons)

In the actual functioning of the Krebs&rsquo Cycle, however, the pyruvate from glycolysis goes on an interesting journey, albeit a bit confusing. Before the pyruvate enters the cycle, it will be converted with an enzyme into acetyl-CoA, a two-carbon molecule attached to a coenzyme. This first reaction results in the removal of an electron and a carbon group, and the production of one NADH molecule. That acetyl-CoA bonds with oxaloacetate, creating a six-carbon molecule (citric acid), and releasing the coenzyme.

As the cycle continues, additional carbon dioxide molecules are removed from the citric acid, creating an additional molecule of NADH each time. Around the halfway point of the cycle, 2 more molecules of ATP are created, and then the regenerative stage of the cycle begins. In these final reactions, the four-carbon molecule, oxaloacetate, must be re-formed to continue re-start the cycle, and that regenerative process creates two molecules of FADH2.

The NADH and FADHs molecules will move on to the final stage of cellular respiration, while the ATP will become available for use by the cell.

Electron Transport Chain

This is arguably the coolest and most unique stage of cellular respiration, and takes place near the membrane of the mitochondria, in a large protein complex that functions as an ATP factory. One of the primary functions of the membrane of the mitochondria is to prevent the flow of protons into the organelle, which establishes a strong gradient of positive charge on either side of this membrane. As some of you may know, when there is a charge gradient, there is the potential for work to be done.

In the case of the electron transport chain, there are four major proton complexes that bridge the membrane of the mitochondria, simply number 1, 2, 3 and 4. All of these protein complexes directly or indirectly pump protons out of the mitochondrial matrix into the extracellular fluid. The energy required to run those critical pumps comes from the energy released during the transfer of electrons through a waterfall series of chemical reactions.

The NADH that was produced in glycolysis and the Krebs&rsquo cycle will be the primary source of these electrons. NADH molecules drop off their electrons in protein complex 1, which are then then moved to protein complex 3 via coenzyme Q. The FADH2 molecules from the Krebs&rsquo Cycle deposit their electrons in protein complex 2. The same coenzyme Q takes those electrons to protein complex 3. Cytochrome C carries 1 electron from each coenzyme Q to protein complex 4, while the other electron can be recycled. When the electrons leave protein complex 4, oxygen functions as the final electron acceptor, and produces water.

As a result of the proton gradient that is maintained through that final step of the electron transport chain, more protons must continually be pumped into the membrane. This happens via ATP synthase, the final factory of respiration. When this protein complex is engaged, the flow of protons over the gradient will induce the creation of additional ATP.

The net product of the electron transport chain (from one molecule of glucose) is 32 molecules of ATP, as well as six molecules of water.

Combining this with the previous products of the other respiration stages, you will find that a single molecule of glucose entering the cell, in the presence of oxygen, will produce 36 ATP, 6 water molecules and 6 carbon dioxide molecules!

Anaerobic Respiration

In the absence of oxygen, there is another form of cellular respiration that is available to organisms &ndash anaerobic respiration. If there is not enough oxygen available for the energetic demands &ndash such as when you are running a marathon or undergoing intense exertion &ndash your body is still able to produce small amounts of energy without oxygen as an electron acceptor.

Without oxygen, anaerobic respiration is able to convert glucose into lactic acid, and release a small amount of energy &ndash 2 ATP. Think back to the glycolysis step of aerobic respiration the process is the same for anaerobic respiration, except the end product is not pyruvate, but lactate. However, lactic acid is actually a poisonous compound in the body, in that it will negatively impact muscle function if too much is built up (as a product of anaerobic respiration).

Lactic acid buildup is what causes cramps during intense exercise, and that discomfort can only be alleviated by re-oxygenating your body, which will allow for aerobic respiration to begin and stimulate the breakdown of lactic acid into carbon dioxide and water. This is also why your body has a limit to how far it can sprint!

Aerobic respiration is far more efficient and will generate much more energy from the same molecule of glucose anaerobic respiration produces 2 ATP versus 36 ATP in aerobic respiration, so the difference is clear.

Cellular respiration is the process of extracting energy in the form of ATP from the glucose in the food you eat. How does cellular respiration happen inside of the cell? Cellular respiration is a three step process. Briefly:

  1. In stage one, glucose is broken down in the cytoplasm of the cell in a process called glycolysis.
  2. In stage two, the pyruvate molecules are transported into the mitochondria. The mitochondria are the organelles known as the energy "powerhouses" of the cells (Figure below). In the mitochondria, the pyruvate, which have been converted into a 2-carbon molecule, enter the Krebs cycle. Notice that mitochondria have an inner membrane with many folds, called cristae. These cristae greatly increase the membrane surface area where many of the cellular respiration reactions take place.
  3. In stage three, the energy in the energy carriers enters an electron transport chain. During this step, this energy is used to produce ATP.

Oxygen is needed to help the process of turning glucose into ATP. The initial step releases just two molecules of ATP for each glucose. The later steps release much more ATP.

Figure (PageIndex<1>): Most of the reactions of cellular respiration are carried out in the mitochondria.

The Reactants

What goes into the cell? Oxygen and glucose are both reactants of cellular respiration. Oxygen enters the body when an organism breathes. Glucose enters the body when an organism eats.

The Products

What does the cell produce? The products of cellular respiration are carbon dioxide and water. Carbon dioxide is transported from your mitochondria out of your cell, to your red blood cells, and back to your lungs to be exhaled. ATP is generated in the process. When one molecule of glucose is broken down, it can be converted to a net total of 36 or 38 molecules of ATP. This only occurs in the presence of oxygen.

The Chemical Reaction

The overall chemical reaction for cellular respiration is one molecule of glucose (C6H12O6) and six molecules of oxygen (O2) yields six molecules of carbon dioxide (CO2) and six molecules of water (H2O). Using chemical symbols the equation is represented as follows:

ATP is generated during the process. Though this equation may not seem that complicated, cellular respiration is a series of chemical reactions divided into three stages: glycolysis, the Krebs cycle, and the electron transport chain.


Stage one of cellular respiration is glycolysis. Glycolysis is the splitting, or lysis of glucose. Glycolysis converts the 6-carbon glucose into two 3-carbon pyruvate molecules. This process occurs in the cytoplasm of the cell, and it occurs in the presence or absence of oxygen. During glycolysis a small amount of NADH is made as are four ATP. Two ATP are used during this process, leaving a net gain of two ATP from glycolysis. The NADH temporarily holds energy, which will be used in stage three.

The Krebs Cycle

In the presence of oxygen, under aerobic conditions, pyruvate enters the mitochondria to proceed into the Krebs cycle. The second stage of cellular respiration is the transfer of the energy in pyruvate, which is the energy initially in glucose, into two energy carriers, NADH and FADH2. A small amount of ATP is also made during this process. This process occurs in a continuous cycle, named after its discover, Hans Krebs. The Krebs cycle uses a 2-carbon molecule (acetyl-CoA) derived from pyruvate and produces carbon dioxide.

The Electron Transport Chain

Stage three of cellular respiration is the use of NADH and FADH2 to generate ATP. This occurs in two parts. First, the NADH and FADH2 enter an electron transport chain, where their energy is used to pump, by active transport, protons (H+) into the intermembrane space of mitochondria. This establishes a proton gradient across the inner membrane. These protons then flow down their concentration gradient, moving back into the matrix by facilitated diffusion. During this process, ATP is made by adding inorganic phosphate to ADP. Most of the ATP produced during cellular respiration is made during this stage.

For each glucose that starts cellular respiration, in the presence of oxygen (aerobic conditions), 36-38 ATP are generated. Without oxygen, under anaerobic conditions, much less (only two!) ATP are produced.

Aerobic Respiration

Energy is released using NAD+, FADH, and ATP Synthase.


Cells breakdown glucose molecules first during the process known as glycolysis. The glucose molecule is broken down into two pyruvate molecules and electrons are released. These electrons are picked up by NAD+. Once NAD+ has picked up these electrons, it becomes NADH. Two ATP molecules are also made (ATP transfers chemical energy between cells it is sort of like a currency in this regard).

The next step is the Krebs cycle, also known as the citric acid cycle. During this step of the process, the pyruvate molecules are converted to Acetyl CoA, these molecules are then broken down even further, releasing electrons and ATP. As in the previous step, NAD+ picks up the released electrons, becoming NADH, as does FADH, which becomes FADH2.

Lastly, we have oxidative phosphorylation, which occurs in the inner membrane of the mitochondria (or the cytoplasm of prokaryotic cells). When NAD+ and FADH picked up electrons previously, they lost hydrogen atoms.

These hydrogen atoms now pump against the concentration gradient. Proteins in the membrane undergo active transport, moving the hydrogen atoms into one concentrated area. Next, the hydrogen atoms go through ATP Synthase, which turns out a lot of ATP.

To learn more, see the following video:

Breathing involves inhale of oxygen from the atmosphere into the lungs and exhale of carbon dioxide from the lungs into the atmosphere whereas cellular respiration involves breakdown of glucose into carbon dioxide and water in living cells, releasing energy.

During breathing, termed as external respiration, air from the atmosphere enters into the lungs. Exchange of oxygen and carbon dioxide occurs between the blood present in the capillaries and the air entering the lungs.

The R.B.C. in the blood present in capillaries pick up oxygen from the air entering the lungs and the hemoglobin molecule is converted into oxy-hemoglobin. Carbon dioxide from the deoxygenated blood is released into the air. The air carrying carbon dioxide is exhaled out of the lungs.

Thus breathing involves intake of oxygen from the atmosphere into the lungs and exit of carbon dioxide from the lungs into the atmosphere.

Cellular respiration, also termed as internal respiration, occurs in living cells. The oxygenated blood is carried to all living cells in the body of an organism through blood circulatory.

Cellular respiration involves breakdown of glucose into carbon dioxide and water in presence of oxygen, releasing energy. Oxygen carried by blood is used in cellular respiration and carbon dioxide released combines with hemoglobin in RBCs.

Deoxygenated or impure blood is carried by veins to the lungs to be converted into oxygenated blood.

The energy released during cellular respiration is stored in form of ATP molecules, which are store houses of energy.

ATP molecule is converted into ADP molecule, whenever energy is needed for any metabolic reaction or activity. The energy stored in it is released to be used in metabolic reaction. ATP and ADP molecules are thus rightly termed as “ currency of energy”.

Photosynthesis, Cellular Respiration, & Fermentation

You've already learned a little bit about photosynthesis thanks to our study of plant cells. You learned that photosynthesis happens in the chloroplasts that are found only in plant cells. Let's think about what else you've already learned.

You've already learned that there are two basic types of organisms when it comes to food: producers and consumers. Producers are able to make their own food. Consumers get the food they need by eating other organisms. You learned that only plants are producers, and that they make their own food by combining water (H2O), carbon dioxide (CO2) and energy from the sun to produce sugar (C6H12O6) and oxygen (O2). This process, you learned, is called photosynthesis. In the process of making sugar, plant cells also lock some of the energy they collected from sunlight into the sugar molecule.

Okay, great. So how do cells (remember, both plant and animal cells need energy, and neither can directly use the energy provided by the sun) get the energy out of the sugar molecule? They do it with a process called cellular respiration. In cellular respiration, cells use oxygen to break the sugar molecule. That releases the energy which is then transferred to an ATP (adenosine triphosphate) molecule. ATP is the fuel that cells need for energy. And where does cellular respiration happen? As you've learned, it happens in those handy mitochondria.

So really, you already know all the basics. There are just a few details that you need to learn, and they are covered in Section 1 of Chapter 5 in your textbook and, of course, right here. Let's start with photosynthesis


If you were to look at plant cells under a microscope and compare them to animal cells, there are two things that you would notice immediately. First, you would notice the cell wall that surrounds the plant cell. You would notice it the same way that Robert Hooke noticed it. The second thing you would notice is that a plant cell is green and an animal cell is basically clear. If you were looking at a relatively large plant cell, and you were using a microscope like the ones we have at school, you would notice that not the entire plant cell was green. Instead, you would notice that there were large green objects inside of the plant cell. These large green objects, of course, are chloroplasts. And the reason that they are green is because they contain a green pigment called chlorophyll.

Have a look at this illustration from your book:

Do you notice how the chemical formula that defines photosynthesis looks a little different from the way you originally learned it? Instead of CO2 + H2O + light it shows 6CO2 + 6H2O + light. That's because chemical equations, just like math equations, have to balance. The original formula takes one carbon atom (that's how many carbon atoms are in CO2), 2 hydrogen atoms (that's how many hydrogen atoms there are in H2O), and 3 oxygen atoms (2 that are in CO2 and one that is in H2O) and turns it into glucose (which contains 6 carbon atoms, 12 hydrogen atoms, and 6 oxygen atoms) and an oxygen molecule (O2, which contains 2 oxygen atoms). That just doesn't add up! You can't magically turn 1 carbon atom from CO2 into 6 carbon atoms in C6H12O6. But if you do the math with the formula in the illustration above, you'll see that the number of atoms of carbon, oxygen, and hydrogen on both sides of the equation are correct. You will get way more practice balancing chemical equations when you study chemistry in 8th grade science.

Cellular Respiration

It is tempting to think of cellular respiration as the opposite of photosynthesis. If you look at the illustration from our book, below, you'll see why:

Do you see the way the chemical formula for cellular respiration is the reverse of the chemical formula for photosynthesis? The only real difference is that in one, the energy is sunlight and in the second, the energy is the ATP molecule. It's that reversal that makes many people think of photosynthesis and cellular respiration as being opposites. They are not! Rather, they are complementary to one another. Without photosynthesis, there would be no sugar, without which there could be no cellular respiration. On the other hand, cellular respiration produces the H2O and CO2 that are needed for photosynthesis. It's really important for you to remember that cellular respiration in eukaryotic cells takes place in the mitochondria. Both animal cells and plant cells depend on cellular respiration for their energy needs, because both animal cells and plant cells need ATP. Plant cells may be able to use the energy from the sun to make sugar, but they can't use the sun's energy as fuel. They need ATP the same way that animal cells do, and ATP can only be formed through cellular respiration. The illustration below from your book shows the way that photosynthesis and cellular respiration complement each other.

Do you see what I don't like about this illustration? Is it clear from this illustration that plant cells also have mitochondria? Not clear enough, in my opinion! So remember! Plant cells have mitochondria, too!


What happens when there is not enough oxygen to keep the cellular respiration reaction alive? Your book makes it seem like the answer is very simple. Let's start with the simple answer in your book. If there is not enough oxygen for cells to perform cellular respiration, they resort to another method of producing energy called fermentation. They still break down the sugar molecule to release the energy so that it can be transferred to an ATP molecule, but they do it without oxygen. In cellular respiration, CO2 and H2O are produced along with the energy. In fermentation, CO2 and something called lactic acid are produced. Just like your book explains, you've probably experienced fermentation yourself when you've had to run the Wednesday mile and you've really pushed yourself to get a good grade. You know that burning or stinging sensation that you feel in your muscles when you push yourself running? That's caused by a buildup of lactic acid in your muscles. No matter how hard your lungs and heart work to get oxygen to the cells in your leg muscles, they still aren't getting enough to produce all the energy they need through cellular respiration. So, they are forced to switch to fermentation, and lactic acid is produced.

There are some organisms that get all of their energy needs from fermentation. One common example is yeast. Yup. That same stuff that you drop into the bread maker. You should have noticed that there were lots of bubbles in the tubes containing the yeast and sugar water in our classroom. You've already seen live yeast cells in class that I projected from a microscope to the screen. A few classes got lucky and were able to see some yeast cells that were in the process of reproducing. I know you're going to be happy to hear this: yeast cells reproduce by budding! Just when you thought it was safe to forget all about budding and the pain it has caused you on past tests, it's back!

So how does yeast make bread rise? It's pretty simple, really. Bread is made mostly of flour. You probably already know that bread is "carbs", or carbohydrates. Do you remember what carbohydrates are? That's right, they are just long strings of sugar molecules. Yeast uses those sugar molecules to get the energy it needs, and in the process it creates CO2. That CO2 makes bubbles inside of the bread dough, and those bubbles make the dough get larger, or rise.

There is another way that fermentation caused by yeast is important. Grape juice also contains a lot of sugar. When yeast is added to grape juice, it uses the sugar for energy. Yes, it produces CO2, but it also produces alcohol. That's how grape juice is turned into wine!

The Global Warming Connection

Remember An Inconvenient Truth, the Al Gore documentary movie? One of the scenes in the movie showed the earth at night as photographed from space. Vice President Gore said that the large red areas were forests burning. There are plenty of naturally-occurring forest fires, but humans purposely set forests ablaze, too. In Brasil, for example, parts of the rainforest are burned to create more land for crops and housing. Think about what this means for global warming.

Global warming is caused by too much carbon dioxide in the atmosphere. The carbon dioxide acts as a blanket. When sunlight hits the earth, it can't radiate back into space because of the carbon dioxide and other greenhouse gases that are present in the atmosphere. So, the earth gets hotter.

Burning forests is a double-whammy. First, removing trees means that they aren't there anymore to convert carbon dioxide into sugar and oxygen. Second, when we burn the trees, we are releasing all of the carbon dioxide that they have collected. When mitochondria combine glucose with oxygen to produce energy, they are "burning" the sugar through a process called oxidation. There are many examples of oxidation in real life. When a nail gets rusty, that's oxidation. And, of course, when something burns, that's oxidation, too. The only difference between rusting, burning, and the way that mitochondria release the energy from a glucose molecule is the speed of the reaction. Rusting is very slow oxidation and burning is very fast oxidation. So burning the sugar in the trees is just a very fast version of what mitochondria do: the sugar releases carbon dioxide and energy in the form of heat. Some trees have been alive for hundreds or even thousands of years! So when we burn them, we are releasing hundreds or thousands of years worth of "captured" carbon dioxide.

That's it, folks. If you can remember the chemical formula for both photosynthesis and cellular respiration, if you can explain how the two processes complement one another, and if you can explain what happens when there is not enough oxygen for cellular respiration, then you've learned what you need to have learned.

These videos will help you to understand photosynthesis and cellular respiration. Don't be afraid of the complicated scientific vocabulary! You will understand more than you think if you just stop once in a while and try to make a connection between what is going on in the video and what you have already learned.

AP Lab 5 Sample 7

The human body has to have energy in order to perform the functions that allow life. This energy comes from the process of cellular respiration. Cellular respiration releases energy that the body can use in the form of ATP from carbohydrates by using oxygen. Cellular respiration is not just one singular reaction, it is a metabolic pathway made up of several reactions that are enzyme mediated. This process begins with glycolysis in the cytosol of the cell. In glycolysis, glucose is split into two three-carbon compounds called pyruvate, producing a small amount of ATP The final two steps of cellular respiration occur in the mitochondria. These final two steps are the electron transport system and the Krebs Cycle. The overall equation for cellular respiration is

C6H12O6 + 6O2 -> 6CO2 + 6H2O + 686 kilocalories of energy per mole of glucose oxidized.

There are three ways to measure the rate of cellular respiration. These three ways are by measuring the consumption of oxygen gas, by measuring the production of carbon dioxide, or by measuring the release of energy during cellular respiration. In order to measure the gases, the general gas law must be understood. The general gas law state: PV=nRT where P is the pressure of the gas, V is the volume of the gas, n is the number of molecules of gas, R is the gas constant, and T is the temperature of the gas (in K). The gas law also shows concepts about gases. If temperature and pressure are kept constant, then the volume of the gas is directly proportional to the number of molecules of the gas. If the temperature and volume remain constant, then the pressure of the gas changes in direct proportion to the number of molecules of gas present. If the number of gas molecules and the temperature remain constant, then the pressure is inversely proportional to the volume. If the temperature changes and the number of gas molecules is kept constant, then either pressure of volume will change in direct proportion to the temperature.

In this experiment, the rate of cellular respiration will be measured by measuring the oxygen gas consumption by using a respirometer in water. This experiment measures the consumption of oxygen by germinating and non-germinating at room temperature and at ice water temperature. The carbon dioxide produced in cellular respiration will be removed by potassium hydroxide (KOH). As a result of the carbon dioxide being removed, the change in the volume of gas in the respirometer will be directly related to the amount of oxygen consumed. The respirometer with glass beads alone will show any changes in volume due to atmospheric pressure changes or temperature changes.

The germinating peas will have a higher rate of respiration, than the beads and non-germinating peas.

This lab requires two thermometers, two water baths, beads, germinating and non-germinating peas, beads, six vials, twelve pipettes, 100 mL graduated cylinder, scotch tape, tap water, ice, KOH, absorbent and non-absorbent cotton, six washers, six rubber stoppers, scotch tape, and a one mL dropper.

Start the experiment by setting up two water baths, one at room temperature and the other at 10 degrees Celsius. Then, find the volume of twenty-five germinating peas. Next, put 50 mL of water in a graduated cylinder and put twenty-five non-germinating peas in it. Then, add beads until the volume is the same as twenty-five germinating peas. Next, pour our the peas and beads, refill the graduated cylinder with 50 mL of water, and add only beads until the volume is the same as the twenty-five germinating peas. Repeat these steps for another set of peas and beads. Also, put together the six respirometers by gluing a pipette to a stopper and taping another pipette to the pipette for all six respirometers. Then, put two absorbent cotton balls, several drops of KOH, and half of a piece of non-absorbent cotton into all six vials. Next, add the peas and beads to the appropriate respirometers. Place one set of respirometers into the room temperature water bath and the other set in the ice water bath. Elevate the respirometers by setting the pipettes onto masking tape and allow them to equilibrate for five minutes. Next, lower the respirometers into the water baths and take reading at 0, 5, 10, 15, and 20 minutes. Record the results in the table.


What is the chemical reaction for cellular respiration? Glucose + Oxygen –> Carbon Dioxide + Water + ATP (energy)
Which organisms perform cellular respiration? All carbon based organisms.
Why do all living organisms require cellular respiration to survive? In other words, what is the point, or goal, of cellular respiration? (please write a full sentence or two). The goal of cellular respiration is to harvest and amplify the energy which a cell gets from broken glucose bonds. All carbon based organisms require it to survive because without it the cell would not have energy.
What is the goal of the electron transport chain? (please write a full sentence or two) The goal of the electron transport chain is to move electrons from protein to protein while creating a hydrogen gradient. This hydrogen gradient eventually forces the ATP Synthase to amplify ATP and expel water and carbon.
In the electron transport chain, what molecules are the electron donors? In the transport chain NAD and FADH2 are electron donors.
When the electrons move protein to protein in the chain, what ion to they pull through the membrane? They pull H+ ions through the membrane.
What molecule is the final electron acceptor? Oxygen is the final electron acceptor.
When this final molecules accepts the electron, what molecule does it become? When oxygen accepts the hydrogen electron it becomes water.
The ion gradient is used to power which enzyme, and produce what molecules out of ADP and P? The hydrogen ion gradient powers the enzyme ATP Synthase, it produces ATP out of ADP and P.

What is the chemical reaction for photosynthesis? Carbon Dioxide + Water + Sun (energy) –> Glucose + Oxygen
Which organisms perform photosynthesis? All organisms with chloroplasts.
Why do all living organisms require photosynthesis to survive? In other words, what is the point, or goal, of photosynthesis? (please write a full sentence or two) All living organisms require photosynthesis to gather energy from the sun and create glucose for sustenance. Organisms which do not photosynthesize require photosynthesis because it also produces oxygen and glucose, two necessary components for carbon based organisms.
If energy is not created or destroyed (First Law of Thermodynamics), where does the energy for all living things originally come from? Is this an unlimited source of energy? The energy for all living things comes from the sun, this is an unlimited source of energy in some aspects because it will produce energy for the next 4 billion years.

Look at the diagram above and identify each labeled component of photosynthesis. Write the appropriate letter in the diagram next to the correct word in the word bank below.

Example: Thylakoid: __B__ (there should be an arrow to the green circular stacks)
Light (photons): __D___
Glucose: _K____
Light reactions: __C___
ADP+P: __M___
NADP+: __L___
Chloroplast: _A____
Calvin Cycle: __I___
ATP: __G___
H2O: __E___
NADPH: __H___
O2: __F___
The space surrounding the thylakoid membranes within the chloroplast is called the _stroma_________.
In the light reactions, what molecules are the electron donors? Water is an electron donor in light reactions.
When the electrons move protein to protein in the chain, what ion to they pull through the membrane? When electrons move from protein to protein they pull hydrogen ions thorough the membrane.
What molecule is the final electron acceptor? NADP+ is the final molecule to accept an electron.
The ion gradient is used to power which enzyme, and produce what molecules out of ADP and P? The ion gradient is used to power ATP Synthesis and produce ATP out of ADP and P.
What is the goal of the Calvin Cycle? The goal of the Clavin Cycle is to convert CO2 into glucose.
What are the reactants and products of the Calvin Cycle? The reactants of the Calvin Cycle are CO2 and the product is glucose.
From where does the cell get the energy to complete the reaction in the Calvin Cycle? The cell gets energy to complete the reaction from the light reactions.
For what does a cell use glucose? The cell uses glucose to get energy to complete cellular respiration.

MrBorden's Biology Rattler Site Room 664

Check google classroom for homework!

Oct 20 2014 Monday
Qfd: The most powerful weapon on earth is the human soul on fire.- Marshall Ferdinand Foch
Essential Question: Write he formula for photosynthesis and balance the equation
Todays Learning Objective: Students will know and understand LS1.C: Organization for Matter and Energy Flow in Organisms •The process of photosynthesis converts light energy to stored chemical energy by converting carbon dioxide plus water into sugars plus released oxygen.
•The sugar molecules thus formed contain carbon, hydrogen, and oxygen: their hydrocarbon backbones are used to make amino acids and other carbon-based molecules that can be assembled into larger molecules (such as proteins or DNA), used for example to form new cells. by writing Cornell notes and answering questions
1) concept map


36 ATP
Electron transport chain
Lactic acid
Kreb’s Cycle

Oct 21 2014 Tuesday
Qfd: Our prime purpose in this life is to help others. And if you can’t help them, at least don’t hurt them.- Tenzin Gyatso, 14th Dalai Lama
Essential Question: Describe how respiration helps to maintain homeostasis in the body
Todays Learning Objective: Students know and will be able to create a flow (thinking) map concerning how energy is transferred from one system of interacting molecules to another. Cellular respiration is a chemical process in which the bonds of food molecules and oxygen molecules are broken and new compounds are formed that can transport energy to muscles. Cellular respiration also releases the energy needed to maintain body temperature despite ongoing energy transfer to the surrounding environment.
1) CELLULAR RESPIRATION review worksheet

Oct 22 2014 Wednesday
Qfd: The purpose of life is to discover your gift. The meaning of life is to give your gift away.- David Viscott
Essential Question: How are sugar molecules formed? What energy source do they use
Todays Learning Objective: Students know and will be able to create a flow (thinking) map concerning how energy is transferred from one system of interacting molecules to another. Cellular respiration is a chemical process in which the bonds of food molecules and oxygen molecules are broken and new compounds are formed that can transport energy to muscles. Cellular respiration also releases the energy needed to maintain body temperature despite ongoing energy transfer to the surrounding environment.
1) Quiz
Oct 23 2014 Thursday
Qfd: Service is the rent we pay to be living. It is the very purpose of life and not something you do in your spare time.- Marian Wright Edelman
Essential Question:
Todays Learning Objective: LS2.B: Cycles of Matter and Energy Transfer in Ecosystems: Students will know and understand:Photosynthesis and cellular respiration (including anaerobic processes) provide most of the energy for life processes by creating a model using the biome in a bottle lab

2) hand in cellular respiration handout

3) biomes research construction

Oct 24 2014 Friday
Qfd: That is happiness to be dissolved into something completely great.- Willa Cather
Essential Question: Can you design an experiment that combines an animal and a plant?
Todays Learning Objective:LS2.B: Cycles of Matter and Energy Transfer in Ecosystems: Students will know and understand: Photosynthesis and cellular respiration (including anaerobic processes) provide most of the energy for life processes by creating a model using the biome in a bottle lab
This was found at science of a drunk in driver crash few weeks ago by my friend who is an EMT


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