BIS 1A Handout 9 - Respiration

Life Is Work

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.

Catabolic pathways yield energy by oxidizing organic fuels

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.

The overall process is:

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

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.

More generally: Xe" + Y ( 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).

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 is released in the process.

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 and protons of hydrogen are stripped from the glucose.

The electrons and protons of hydrogen 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.

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.

In summary, during cellular respiration, most electrons travel the following "downhill" route: food ( NADH ( electron transport chain ( oxygen.

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.

As electrons 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.

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.

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.

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

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, releasing one CO2 and reducing NAD+ to NADH.

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

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.

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

During oxidative phosphorylation, chemiosmosis couples electron transport to ATP synthesis

The inner mitochondrial membrane couples electron transport to ATP synthesis.

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.

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

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

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

For every two electron carriers (four electrons), one O2 molecule is reduced with four hydrogen ions 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.

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.

The electron transport 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, driving 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 - "chemiosmosis".

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.

Here is an accounting of ATP production by cellular respiration.

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

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.

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 38 ATP.

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.

Fermentation enables some cells to produce ATP without the use 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).

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 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.

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.

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.

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.

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.

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.

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