Respiration

RESPIRATION Respiratory System: • • •

Primary function is to obtain oxygen for use by body's cells & eliminate carbon dioxide that cells produce Includes respiratory airways leading into (& out of) lungs plus the lungs themselves Pathway of air: nasal cavities (or oral cavity) > pharynx > trachea > primary bronchi (right & left) > secondary bronchi > tertiary bronchi > bronchioles > alveoli (site of gas exchange)

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The exchange of gases (O2 & CO2) between the alveoli & the blood occurs by simple diffusion: O2 diffusing from the alveoli into the blood & CO 2 from the blood into the alveoli. Diffusion requires a concentration gradient. So, the concentration (or pressure) of O2 in the alveoli must be kept at a higher level than in the blood & the concentration (or pressure) of CO2 in the alveoli must be kept at a lower lever than in the blood. We do this, of course, by breathing - continuously bringing fresh air (with lots of O2 & little CO2) into the lungs & the alveoli. Breathing is an active process - requiring the contraction of skeletal muscles. The primary muscles of respiration include the external intercostal muscles (located between the ribs) and the diaphragm (a sheet of muscle located between the thoracic & abdominal cavities). The external intercostals plus the diaphragm contract to bring about inspiration: •

Contraction of external intercostal muscles > elevation of ribs & sternum > increased front- to-back dimension of thoracic cavity > lowers air pressure in lungs > air moves into lungs

•

Contraction of diaphragm > diaphragm moves downward > increases vertical dimension of thoracic cavity > lowers air pressure in lungs > air moves into lungs:

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To exhale: •

relaxation of external intercostal muscles & diaphragm > return of diaphragm, ribs, & sternum to resting position > restores thoracic cavity to preinspiratory volume > increases pressure in lungs > air is exhaled

Intra-alveolar pressure during inspiration & expiration As the external intercostals & diaphragm contract, the lungs expand. The expansion of the lungs causes the pressure in the lungs (and alveoli) to become slightly negative relative to atmospheric pressure. As a result, air moves from an area of higher pressure (the air) to an area of lower pressure (our lungs & alveoli). During expiration, the respiration muscles relax & lung volume descreases. This causes pressure in the lungs (and alveoli) to become slight positive relative to atmospheric pressure. As a result, air leaves the lungs. The walls of alveoli are coated with a thin film of water & this creates a potential problem. Water molecules, including those on the alveolar walls, are more attracted to each other than to air, and this attraction creates a force called surface tension. This surface tension increases as water molecules come closer together, which is what happens when we exhale & our alveoli become smaller (like air leaving a balloon). Potentially, surface tension could cause alveoli to collapse and, in addition, would make it more difficult to 're-expand' the alveoli (when you inhaled). Both of these would represent serious

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problems: if alveoli collapsed they'd contain no air & no oxygen to diffuse into the blood &, if 're-expansion' was more difficult, inhalation would be very, very difficult if not impossible. Fortunately, our alveoli do not collapse & inhalation is relatively easy because the lungs produce a substance called surfactant that reduces surface tension. Role of Pulmonary Surfactant •

Surfactant decreases surface tension which: o increases pulmonary compliance (reducing the effort needed to expand the lungs) o reduces tendency for alveoli to collapse

Lung cells that produce surfactant

Exchange of gases: 45

•

External respiration:

• •

exchange of O2 & CO2 between external environment & the cells of the body o efficient because alveoli and capillaries have very thin walls & are very abundant (your lungs have about 300 million alveoli with a total surface area of about 75 square meters) Internal respiration - intracellular use of O2 to make ATP occurs by simple diffusion along partial pressure gradients o

What is Partial Pressure?: •

it's the individual pressure exerted independently by a particular gas within a mixture of gasses. The air we breath is a mixture of gasses: primarily nitrogen, oxygen, & carbon dioxide. So, the air you blow into a balloon creates pressure that causes the balloon to expand (& this pressure is generated as all the molecules of nitrogen, oxygen, & carbon dioxide move about & collide with the walls of the balloon). However, the total pressure generated by the air is due in part to nitrogen, in part to oxygen, & in part to carbon dioxide. That part of the total pressure generated by oxygen is the 'partial pressure' of oxygen, while that generated by carbon dioxide is the 'partial pressure' of carbon dioxide. A gas's partial pressure, therefore, is a measure of how much of that gas is present (e.g., in the blood or alveoli).

•

the partial pressure exerted by each gas in a mixture equals the total pressure times the fractional composition of the gas in the mixture. So, given that total atmospheric pressure (at sea level) is about 760 mm Hg and, further, that air is about 21% oxygen, then the partial pressure of oxygen in the air is 0.21 times 760 mm Hg or 160 mm Hg.

Partial Pressures of O2 and CO2 in the body (normal, resting conditions): •

•

Alveoli o PO2 = 100 mm Hg o PCO2 = 40 mm Hg Alveolar capillaries o Entering the alveolar capillaries  PO2 = 40 mm Hg (relatively low because this blood has just returned from the systemic circulation & has lost much of its oxygen)

PCO2 = 45 mm Hg (relatively high because the blood returning from the systemic circulation has picked up carbon dioxide).

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While in the alveolar capillaries, the diffusion of gasses occurs: oxygen diffuses from the alveoli into the blood & carbon dioxide from the blood into the alveoli. o

Leaving the alveolar capillaries  PO2 = 100 mm Hg  PCO2 = 40 mm Hg

Blood leaving the alveolar capillaries returns to the left atrium & is pumped by the left ventricle into the systemic circulation. This blood travels through arteries & arterioles and into the systemic, or body, capillaries. As blood travels through arteries & arterioles, no gas exchange occurs. o

o

Entering the systemic capillaries  PO2 = 100 mm Hg  PCO2 = 40 mm Hg Body cells (resting conditions)  PO2 = 40 mm Hg  PCO2 = 45 mm Hg

Because of the differences in partial pressures of oxygen & carbon dioxide in the systemic capillaries & the body cells, oxygen diffuses from the blood & into the cells, while carbon dioxide diffuses from the cells into the blood. o

Leaving the systemic capillaries  PO2 = 40 mm Hg  PCO2 = 45 mm Hg

Blood leaving the systemic capillaries returns to the heart (right atrium) via venules & veins (and no gas exchange occurs while blood is in venules & veins). This blood is then pumped to the lungs (and the alveolar capillaries) by the right ventricle.

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How are oxygen & carbon dioxide transported in the blood? •

Oxygen is carried in blood: 1 - bound to hemoglobin (98.5% of all oxygen in the blood) 2 - dissolved in the plasma (1.5%)

Because almost all oxygen in the blood is transported by hemoglobin, the relationship between the concentration (partial pressure) of oxygen and hemoglobin saturation (the % of hemoglobin molecules carrying oxygen) is an important one. Hemoglobin saturation: • •

extent to which the hemoglobin in blood is combined with O2 depends on PO2 of the blood:

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The relationship between oxygen levels and hemoglobin saturation is indicated by the oxygen-hemoglobin dissociation (saturation) curve (in the graph above). You can see that at high partial pressures of O2 (above about 40 mm Hg), hemoglobin saturation remains rather high (typically about 75 - 80%). This rather flat section of the oxygenhemoglobin dissociation curve is called the 'plateau.' Recall that 40 mm Hg is the typical partial pressure of oxygen in the cells of the body. Examination of the oxygen-hemoglobin dissociation curve reveals that, under resting conditions, only about 20 - 25% of hemoglobin molecules give up oxygen in the systemic

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capillaries. This is significant (in other words, the 'plateau' is significant) because it means that you have a substantial reserve of oxygen. In other words, if you become more active, & your cells need more oxygen, the blood (hemoglobin molecules) has lots of oxygen to provide When you do become more active, partial pressures of oxygen in your (active) cells may drop well below 40 mm Hg. A look at the oxygen-hemoglobin dissociation curve reveals that as oxygen levels decline, hemoglobin saturation also declines - and declines precipitously. This means that the blood (hemoglobin) 'unloads' lots of oxygen to active cells - cells that, of course, need more oxygen.

Factors that affect the Oxygen-Hemoglobin Dissociation Curve: The oxygen-hemoglobin dissociation curve 'shifts' under certain conditions. These factors can cause such a shift: • • • •

lower pH increased temperature more 2,3-diphosphoglycerate increased levels of CO2

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These factors change when tissues become more active. For example, when a skeletal muscle starts contracting, the cells in that muscle use more oxygen, make more ATP, & produce more waste products (CO2). Making more ATP means releasing more heat; so the temperature in active tissues increases. More CO2 translates into a lower pH. That is so because this reaction occurs when CO2 is released: CO2 + H20 -----> H2CO3 -----> HCO3- + H

+

& more hydrogen ions = a lower (more acidic) pH. So, in active tissues, there are higher levels of CO2, a lower pH, and higher temperatures. In addition, at lower PO2 levels, red blood cells increase production of a substance called 2,3-diphosphoglycerate. These changing conditions (more CO2, lower pH, higher temperature, & more 2,3diphosphoglycerate) in active tissues cause an alteration in the structure of hemoglobin, which, in turn, causes hemoglobin to give up its oxygen. In other words, in active tissues, more hemoglobin molecules give up their oxygen. Another way of saying this is that the oxygen-hemoglobin dissociation curve 'shifts to the right' (as shown with the light blue curve in the graph below). This means that at a given partial pressure of oxygen, the percent saturation for hemoglobin with be lower. For example, in the graph below, extrapolate up to the 'normal' curve (green curve) from a PO2 of 40, then over, & the hemoglobin saturation is about 75%. Then, extrapolate up to the 'right-shifted' (light blue) curve from a PO2 of 40, then over, & the hemoglobin saturation is about 60%. So, a 'shift to the right' in the oxygen-hemoglobin dissociation curve (shown above) means that more oxygen is being released by hemoglobin - just what's needed by the cells in an active tissue!

Carbon dioxide - transported from the body cells back to the lungs as: 1 - bicarbonate (HCO3) - 60% o

formed when CO2 (released by cells making ATP) combines with H2O (due to the enzyme in red blood cells called carbonic anhydrase) as shown in the diagram below

2 - carbaminohemoglobin - 30% o

formed when CO2 combines with hemoglobin (hemoglobin molecules that have given up their oxygen)

3 - dissolved in the plasma - 10%

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Control of Respiration Your respiratory rate changes. When active, for example, your respiratory rate goes up; when less active, or sleeping, the rate goes down. Also, even though the respiratory muscles are voluntary, you can't consciously control them when you're sleeping. So, how is respiratory rate altered & how is respiration controlled when you're not consciously thinking about respiration? The rhythmicity center of the medulla: • •

controls automatic breathing consists of interacting neurons that fire either during inspiration (I neurons) or expiration (E neurons) o I neurons - stimulate neurons that innervate respiratory muscles (to bring about inspiration) o E neurons - inhibit I neurons (to 'shut down' the I neurons & bring about expiration)

Apneustic center (located in the pons) - stimulate I neurons (to promote inspiration) Pneumotaxic center (also located in the pons) - inhibits apneustic center & inhibits inspiration Factors involved in increasing respiratory rate • •

Chemoreceptors - located in aorta & carotid arteries (peripheral chemoreceptors) & in the medulla (central chemoreceptors) Chemoreceptors (stimulated more by increased CO2 levels than by decreased O2 levels) > stimulate Rhythmicity Area > Result = increased rate of respiration

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Heavy exercise ==> greatly increases respiratory rate Mechanism? • •

•

NOT increased CO2 Possible factors: o reflexes originating from body movements (proprioceptors) o increase in body temperature o epinephrine release (during exercise) impulses from the cerebral cortex (may simultaneously stimulate rhythmicity area & motor neurons)

` Oxygen Delivery System

The primary function of the respiratory system is to supply the blood with oxygen in order for the blood to deliver oxygen to all parts of the body. The respiratory system does 54

this through breathing. When we breathe, we inhale oxygen and exhale carbon dioxide. This exchange of gases is the respiratory system's means of getting oxygen to the blood. Respiration is achieved through the mouth, nose, trachea, lungs, and diaphragm. Oxygen enters the respiratory system through the mouth and the nose. The oxygen then passes through the larynx (where speech sounds are produced) and the trachea which is a tube that enters the chest cavity. In the chest cavity, the trachea splits into two smaller tubes called the bronchi. Each bronchus then divides again forming the bronchial tubes. The bronchial tubes lead directly into the lungs where they divide into many smaller tubes which connect to tiny sacs called alveoli. The average adult's lungs contain about 600 million of these spongy, air-filled sacs that are surrounded by capillaries. The inhaled oxygen passes into the alveoli and then diffuses through the capillaries into the arterial blood. Meanwhile, the waste-rich blood from the veins releases its carbon dioxide into the alveoli. The carbon dioxide follows the same path out of the lungs when you exhale. The diaphragm's job is to help pump the carbon dioxide out of the lungs and pull the oxygen into the lungs. The diaphragm is a sheet of muscles that lies across the bottom of the chest cavity. As the diaphragm contracts and relaxes, breathing takes place. When the diaphragm contracts, oxygen is pulled into the lungs. When the diaphragm relaxes, carbon dioxide is pumped out of the lungs.

Cellular Respiration Cellular respiration is the process of oxidizing food molecules, like glucose, to carbon dioxide and water The equation for the oxidation of glucose is: C6H12O6 + 6O2 → 6CO2 + 6H2O + Energy released (2830 kJ mol−1) . The energy released is trapped in the form of ATP for use by all the energy-consuming activities of the cell. ATP is known as the universal currency because when the phosphoanhydride bonds in ATP are hydrolysed in an exergonic reaction, the energy yield is 30kJ per mole under standard conditions. Waste products (CO2 + H2O) are released through exhaled air, sweat and urine. The process occurs in two phases: • •

glycolysis, the breakdown of glucose to pyruvic acid the complete oxidation of pyruvic acid to carbon dioxide and water

In eukaryotes, glycolysis occurs in the cytosol. (Link to a discussion of glycolysis). The remaining processes take place in mitochondria.

Mitochondria Mitochondria are membrane-enclosed organelles distributed through the cytosol of most eukaryotic cells. Their number within the cell ranges from a few hundred to, in very

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active cells, thousands. Their main function is the conversion of the potential energy of food molecules into ATP. Mitochondria have: • • • • •

an outer membrane that encloses the entire structure an inner membrane that encloses a fluid-filled matrix between the two is the intermembrane space the inner membrane is elaborately folded with shelflike cristae projecting into the matrix. a small number (some 5–10) circular molecules of DNA

The number of mitochondria in a cell can • •

increase by their fission (e.g. following mitosis); decrease by their fusing together.

The Outer Membrane The outer membrane contains many complexes of integral membrane proteins that form channels through which a variety of molecules and ions move in and out of the mitochondrion.

The Inner Membrane The inner membrane contains 5 complexes of integral membrane proteins: • • • • •

NADH dehydrogenase (Complex I) succinate dehydrogenase (Complex II) cytochrome c reductase (Complex III; also known as the cytochrome b-c1 complex) cytochrome c oxidase (Complex IV) ATP synthase (Complex V)

The Matrix The matrix contains a complex mixture of soluble enzymes that catalyze the respiration of pyruvic acid and other small organic molecules. Here pyruvic acid is • •

oxidized by NAD+ producing NADH + H+ decarboxylated producing a molecule of •

carbon dioxide (CO2) and

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o

a 2-carbon fragment of acetate bound to coenzyme A forming acetyl-CoA

The Citric Acid Cycle • • •

This 2-carbon fragment is donated to a molecule of oxaloacetic acid. The resulting molecule of citric acid (which gives its name to the process) undergoes the series of enzymatic steps shown in the diagram. The final step regenerates a molecule of oxaloacetic acid and the cycle is ready to turn again.

Summary: •

• •

Each of the 3 carbon atoms present in the pyruvate that entered the mitochondrion leaves as a molecule of carbon dioxide (CO2). At 4 steps, a pair of electrons (2e-) is removed and transferred to NAD+ reducing it to NADH + H+. At one step, a pair of electrons is removed from succinic acid and reduces FAD to FADH2.

The electrons of NADH and FADH2 are transferred to the electron transport chain.

The Electron Transport Chain o

• • •

The electron transport chain consists of 3 complexes of integral membrane proteins

the NADH dehydrogenase complex (I) the cytochrome c reductase complex (III) the cytochrome c oxidase complex (IV)

and two freely-diffusible molecules • •

ubiquinone cytochrome c

that shuttle electrons from one complex to the next. The electron transport chain accomplishes: •

the stepwise transfer of electrons from NADH (and FADH2) to oxygen molecules to form (with the aid of protons) water molecules (H2O); (Cytochrome c can only transfer one electron at a time, so cytochrome c oxidase must wait until it has accumulated 4 of them before it can react with oxygen.) 57

• • • •

harnessing the energy released by this transfer to the pumping of protons (H+) from the matrix to the intermembrane space. Approximately 20 protons are pumped into the intermembrane space as the 4 electrons needed to reduce oxygen to water pass through the respiratory chain. The gradient of protons formed across the inner membrane by this process of active transport forms a miniature battery. The protons can flow back down this gradient, reentering the matrix, only through another complex of integral proteins in the inner membrane, the ATP synthase complex .

Chemiosmosis in mitochondria The energy released as electrons pass down the gradient from NADH to oxygen is harnessed by three enzyme complexes of the respiratory chain (I, III, and IV) to pump protons (H+) against their concentration gradient from the matrix of the mitochondrion into the intermembrane space (an example of active transport).

As their concentration increases there (which is the same as saying that the pH decreases), a strong diffusion gradient is set up. The only exit for these protons is through the ATP synthase complex. As in chloroplasts, the energy released as these protons flow down their gradient is harnessed to the synthesis of ATP. The process is called chemiosmosis and is an example of facilitated diffusion. One-half of the 1997 Nobel Prize in Chemistry was awarded to Paul D. Boyer and John E. Walker for their discovery of how ATP synthase works.

How many ATPs? It is tempting to try to view the synthesis of ATP as a simple matter of stoichiometry (the fixed ratios of reactants to products in a chemical reaction). But (with 3 exceptions) it is not.

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Most of the ATP is generated by the proton gradient that develops across the inner mitochondrial membrane. The number of protons pumped out as electrons drop from NADH through the respiratory chain to oxygen is theoretically large enough to generate, as they return through ATP synthase, 3 ATPs per electron pair (but only 2 ATPs for each pair donated by FADH2). With 12 pairs of electrons removed from each glucose molecule, • •

10 by NAD+ (so 10x3=30); and 2 by FADH2 (so 2x2=4),

this could generate 34 ATPs. Add to this the 4 ATPs that are generated by the 3 exceptions and one arrives at 38. But the energy stored in the proton gradient is also used for the active transport of several molecules and ions through the inner mitochondrial membrane into the matrix. •

NADH is also used as reducing agent for many cellular reactions.

So the actual yield of ATP as mitochondria respire varies with conditions. It probably seldom exceeds 30.

The three exceptions A stoichiometric production of ATP does occur at: • •

one step in the citric acid cycle yielding 2 ATPs for each glucose molecule. This step is the conversion of alpha-ketoglutaric acid to succinic acid. at two steps in glycolysis yielding 2 ATPs for each glucose molecule.

Mitochondrial DNA (mtDNA) The human mitochondrion contains 5–10 identical, circular molecules of DNA. Each consists of 16,569 base pairs carrying the information for 37 genes which encode: • • •

2 different molecules of ribosomal RNA (rRNA) 22 different molecules of transfer RNA (tRNA) (at least one for each amino acid) 13 polypeptides

The rRNA and tRNA molecules are used in the machinery that synthesizes the 13 polypeptides. The 13 polypeptides participate in building several protein complexes embedded in the inner mitochondrial membrane.

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• • • •

7 subunits that make up the mitochondrial NADH dehydrogenase 3 subunits of cytochrome c oxidase 2 subunits of ATP synthase cytochrome b

Each of these protein complexes also requires subunits that are encoded by nuclear genes, synthesized in the cytosol, and imported from the cytosol into the mitochondrion. Nuclear genes also encode ~900 other proteins that must be imported into the mitochondrion. [More]

Mutations in mtDNA cause human diseases. A number of human diseases are caused by mutations in genes in our mitochondria: • • • • •

cytochrome b 12S rRNA ATP synthase subunits of NADH dehydrogenase several tRNA genes

Although many different organs may be affected, disorders of the brain and muscles are the most common. Perhaps this reflects the great demand for energy of both these organs. Some of these disorders are inherited in the germline. In every case, the mutant gene is received from the mother because none of the mitochondria in sperm survives in the fertilized egg. Other disorders are somatic; that is, the mutation occurs in the somatic tissues of the individual.

Example: exercise intolerance A number of humans who suffer from easily-fatigued muscles turn out to have a mutations in their cytochrome b gene. Curiously, only the mitochondria in their muscles have the mutation; the mtDNA of their other tissues is normal. Presumably, very early in their embryonic development, a mutation occurred in a cytochrome b gene in the mitochondrion of a cell destined to produce their muscles. The severity of mitochondrial diseases varies greatly. The reason for this is probably the extensive mixing of mutant DNA and normal DNA in the mitochondria as they fuse with one another. A mixture of both is called heteroplasmy. The higher the ratio of mutant to normal, the greater the severity of the disease. In fact by chance alone, cells can on occasion end up with all their mitochondria carrying all-mutant genomes — a condition called homoplasmy (a phenomenon resembling genetic drift).

Why do mitochondria have their own genome? Many of the features of the mitochondrial genetic system resemble those found in bacteria. This has strengthened the theory that mitochondria are the evolutionary 60

descendants of a bacterium that established an endosymbiotic relationship with the ancestors of eukaryotic cells early in the history of life on earth. However, many of the genes needed for mitochondrial function have since moved to the nuclear genome. The recent sequencing of the complete genome of Rickettsia prowazekii has revealed a number of genes closely related to those found in mitochondria. Perhaps rickettsias are the closest living descendants of the endosymbionts that became the mitochondria of eukaryotes.

There are two types of respiration.1.Aerobic 2.Anaerobic.

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Aerobic respiration

Aerobic respiration requires oxygen in order to generate energy. It is the preferred method of pyruvate breakdown from glycolysis and requires that pyruvate enter the mitochondrion to be fully oxidized by the Krebs cycle. The product of this process is energy in the form of ATP (Adenosine Triphosphate), by substrate-level phosphorylation, NADH and FADH2. The reducing potential of NADH and FADH2 is converted to more ATP via an electron transport chain with oxygen as the "terminal electron acceptor". Most of the ATP produced by cellular respiration is by oxidative phosphorylation, ATP molecules are made due to the chemiosmotic potential driving ATP synthase. Respiration is the process by which cells obtain energy when oxygen is present in the cell. Theoretically, 36 ATP molecules can be made per glucose during cellular respiration, however, such conditions are generally not realized due to such losses as the cost of moving pyruvate into mitochondria. Aerobic metabolism is more efficient than anaerobic metabolism. They share the initial pathway of glycolysis but aerobic metabolism continues with the Krebs cycle and oxidative phosphorylation. The post glycolytic reactions take place in the mitochondria in eukaryotic cells, and at the cell membrane in prokaryotic cells.

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Overview of the cellular respiration processes. Glycolysis Glycolysis is a metabolic pathway that is found in the cytoplasm of cells in all living organisms and does not require oxygen. The process converts one molecule of glucose into two molecules of pyruvate, and makes energy in the form of two net molecules of ATP. Four molecules of ATP per glucose are actually produced but two are consumed for the preparatory phase. The initial phosphorylation of glucose is required to destabilize the molecule for cleavage into two triose sugars. During the pay-off phase of glycolysis four phosphate groups are transferred to ADP by substrate-level phosphorylation to make four ATP and two NADH are produced when the triose sugars are oxidized. Glycolysis takes place in the cytoplasm of the cell. The overall reaction can be expressed this way: Glucose + 2 ATP + 2 NAD+ + 2 Pi + 4 ADP → 2 pyruvate + 2 ADP + 2 NADH + 4 ATP + 2 H2O + 4 H+

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Graphic summary of the glycolysis process. Oxidative decarboxylation of pyruvate Produces acetyl-CoA from pyruvate inside the mitochondrial matrix. This oxidation reaction also releases carbon dioxide as a product. In the process one molecule of NADH is formed per pyruvate oxidized.

Krebs cycle/Citric Acid cycle When oxygen is present, acetyl-CoA enters the citric acid cycle inside the mitochondrial matrix, and gets oxidised to CO2 while at the same time reducing NAD to NADH. NADH can be used by the electron transport chain to create further ATP as part of oxidative phosphorylation. To fully oxidise the equivalent of one glucose molecule two acetyl-CoA must be metabolised by the Krebs cycle. Two waste products, H2O and CO2 are created during this cycle.

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Summary of the Krebs' (or citric acid) cycle.

Oxidative phosphorylation In eukaryotes, oxidative phosphorylation occurs in the mitochondrial cristae. It comprises of the electron transport chain that establishes a proton gradient (chemiosmotic potential) across the inner membrane by oxidising the NADH produced from the Krebs cycle. ATP is synthesised by the ATP synthase enzyme when the chemiosmotic gradient is used to drive the phosphorylation of ADP.

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Electron transport system.

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Theoretical yields The yields in the table below are for one glucose and molecule being fully oxidised to carbon dioxide. It is assumed that all the reduced coenzymes are oxidised by the electron transport chain and used for oxidative phosphorylation. coenzyme ATP Source of ATP yield yield

Step Glycolysis preparatory phase

-2 4

Glycolysis off phase

pay-

Oxidative carboxylation Krebs cycle

Total yield

2 NADH

4

2 NADH

6

6 NADH 2 FADH2

2 18 4

Phosphorylation of glucose and fructose 6-phosphate uses two ATP from the cytoplasm. Substrate-level phosphorylation Oxidative phosphorylation. Only 2 ATP per NADH since the coenzyme must feed into the electron transport chain from the cytoplasm rather than the mitochondrial matrix. Oxidative phosphorylation

Substrate-level phosphorylation Oxidative phosphorylation Oxidative phosphorylation From the complete oxidation of one glucose 36 molecule to carbon dioxide and oxidation of all the ATP reduced coenzymes.

Although there is a theoretical yield of 36 ATP molecules per glucose during cellular respiration, such conditions are generally not realized due to losses such as the cost of moving pyruvate (from glycolysis), phosphate and ADP (substrates for ATP syhthesis) into the mitochondria. All are actively transported using carriers that utilise the stored energy in the proton electrochemical gradient. •

•

•

The pyruvate carrier is a symporter and the driving force for moving pyruvate into the mitochondria is the movement of protons from the intermembrane space to the matrix. The phosphate carrier is an antiporter and the driving force for moving phosphate ions into the mitochondria is the movement of hydroxyls ions from the matrix to the intermembrane space. The adenine nucleotide carrier is an antiporter and exchanges ADP and ATP across the inner membrane. The driving force is due to the ATP (-4) having a more negative charge than the ADP (-3) and thus it dissipates some of the electrical component of the proton electrochemical gradient.

The outcome of these transport processes using the proton electrochemical gradient is that more than 3 H+ are needed to make 1 ATP. Obviously this reduces the theoretical efficiency of the whole process. Other factors may also dissipate the proton gradient 67

creating an apparently leaky mitochondria. An uncoupling protein known as thermogenin is expressed in some cell types and is a channel that can transport protons. When this protein is active in the inner membrane it short circuits the coupling between the electron transport chain and ATP synthesis. The potential energy from the proton gradient is not used to make ATP but generates heat. This is particularly important in a babies brown fat, for thermogenesis, and hibernating animals.

Anaerobic respiration In the absence of oxygen, pyruvate is not metabolized by cellular respiration but undergoes a process of fermentation. The pyruvate is not transported into the mitochondrion, but remains in the cytoplasm, where it is converted to waste products that may be removed from the cell. This waste product can vary depending on the organism. In muscles, the waste product is lactate, or lactic acid. In yeast, the waste product is ethanol and carbon dioxide. 2 ATP are produced during anaerobic respiration per glucose, compared to the 36 ATP per glucose produced by aerobic respiration.

Under anaerobic conditions, the absence of oxygen, pyruvic acid can be routed by the organism into one of three pathways: lactic acid fermentation, alcohol fermentation, or cellular (anaerobic) respiration. Humans cannot ferment alcohol in their own bodies, we lack the genetic information to do so. These biochemical pathways, with their myriad reactions catalyzed by reactionspecific enzymes all under genetic control, are extremely complex. We will only skim the surface at this time and in this course. Alcohol fermentation is the formation of alcohol from sugar. Yeast, when under anaerobic conditions, convert glucose to pyruvic acid via the glycolysis pathways, then go one step farther, converting pyruvic acid into ethanol, a C-2 compound.

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Fermentation of ethanol Many organisms will also ferment pyruvic acid into, other chemicals, such as lactic acid. Humans ferment lactic acid in muscles where oxygen becomes depleted, resulting in localized anaerobic conditions. This lactic acid causes the muscle stiffness couch-potatoes feel after beginning exercise programs. The stiffness goes away after a few days since the cessation of strenuous activity allows aerobic conditions to return to the muscle, and the lactic acid can be converted into ATP via the normal aerobic respiration pathways.

Fermentation of lactate (lactic acid). 69

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