Biology: Respiration

Question 1

Respiration

Answer 1

Use of oxygen by an organism. Intake of oxygen from environment, the transport of oxygen in the blood, and the ultimate oxidation of fuel molecules in the cell.

Question 2

External Respiration

Answer 2

The entrance of air into the lungs and the gas exchange between the alveoli and the blood.

Question 3

Internal Respiration

Answer 3

The exchange of gas between the blood and the cells and the intracellular processes of respiration.

Question 4

Fuel

Answer 4

Carbohydrates and fats are the favored fuel molecules in living cells. As hydrogen is removed, bond energy is made available. The C-H bond is energy rich; in fact, compared with other bonds, it is capable of releasing the largest amount of energy per mole.

In contrast, carbon dioxide contains little usable energy. It is the stable, energy-exhausted end product of respiration.

Question 5

Dehydrogenation

Answer 5

During respiration, high-energy hydrogen atoms are removed from organic molecules. This is called dehydrogenation and is an oxidation reaction.

The subsequent acceptance of hydrogen by a hydrogen acceptor (oxygen in the final step) is the reduction component of the redox reaction. Energy released by this reduction is used to form a high-energy phosphate bond in ATP.

Although the initial oxidation step requires an energy input, the net result of the redox reaction is energy production. If all this energy was released in a single step, little could be harnessed. Instead, the reductions occur in a series of steps called the electron transport chain.

Question 6

Glucose Catabolism

Answer 6

The degradative oxidation of glucose occurs in two stages: glycolysis and cellular respiration.

Question 7

Glycolysis

Answer 7

The first stage of glucose catabolism is glycolysis. Glycolysis is a series of reactions that lead to the oxidative breakdown of glucose into two molecules of pyruvate (the ionized from of pyruvic acid), the production of ATP, and the reduction of NAD+ into NADH.

All of these reactions occur in the cytoplasm and are mediated by specific enzymes.

For example, the process of glycolysis is defined as the sequence of reactions that converts glucose into pyruvate with the concomitant production of ATP.

Question 8

Fermentation

Answer 8

NAD⁺ must be regenerated for glycolysis to continue in the absence of O₂. This is accomplished by reducing pyruvate into athanol or lactic acid.

Fermentation refers to all of the reactions involved in this process (i.e., glycolysis and the additional steps leading to the formation of ethanol or lactic acid).

Fermentation produces only two ATP per glucose molecule.

Question 9

Alcohol Fermentation

Answer 9

Often occurs only in yeast and some bacteria. The pyruvate produced in glycolysis is converted to athanol. In this way, NAD+ is regenerated and glycolysis is regenerated when pyruvate is reduced.

Question 10

Lactic Acid Fermentation

Answer 10

Occurs in certain fungi and bacteria and in human muscle cells during strenuous activity. Whent he oxygen supply to muscle cells lags behind the rate of glucose catabolism, the pyruvate generated is reduced to lactic acid. As in alcohol fermentation, the NAD+ used in step 5 of glycolysis is regenerated when pyruvate is reduced.

Question 11

Cellular Respiration

Answer 11

Most efficient catabolic pathway used by organisms to harvest the energy stored in glucose. Whereas glycolysis yields only 2 ATP per molecule of glucose, cellular respiration can yield 36-38 ATP.

Cellular respiration is an aerobic process; oxygen acts as the final acceptor of electrons that are passed from carrier to carrier during the final stage of glucose oxidation.

The metabolic reactions of cell respiration occur in the eukaryotic mitochondrion and are catalyzed by reaction-specific enzymes.

Can be divided into 3 stages: pyruvate decarboxylation, the citric acid cycle, and the electron transport chain.

Question 12

Pyruvate Decarboxylation

Answer 12

The pyruvate formed during glycolysis is transported from the cytoplasm into the mitochondrial matrix where it is decarboxylated (i.e., it loses a CO2), and the acetyl group that remains is transferred to coenzyme A to form acetyl-CoA. Int he process, NAD+ is reduced to NADH.

Question 13

Citric Acid Cycle

Answer 13

Also known as Krebs cycle. Begins when the two-carbon acetyl group from acetyl-CoA combines with oxaloacetate, a four-carbon molecule, to form the six-carbon citrate.

Through a complicated series of reactions, two CO₂ are released, and oxaloacetate is regenerated for use in another turn of the cycle. For each turn of the citric acid cycle one ATP is produced by substrate-level phosphorylation via a GTP intermediate.

In addition, electrons are transferred to NAD⁺ and FADH, generating NADH and FADH₂, respectively. These coenzymes then transport the electrons to the electron transport chain, where more ATP is produced via oxidative phosphorylation.

Question 14

Electron Transport Chain

Answer 14

A complex carrier mechanism located on the inside of the inner mitochondrial membrane. During oxidative phosphorylation, ATP is produced when high energy potential electrons are transferred from NADH and FADH₂ to oxygen by a series of carrier molecules located int he inner mitochondrial membrane. As the electrons are transferred from carrier to carrier, free energy is released, which is then used to form ATP.

Most of the molecules of the ETC are cytochromes, electron carriers that resemble hemoglobin in the structure of their active site.

The functional unit contains a central iron atom, which is capable of undergoing a reversible redox reaction (i.e., it can be alternatively reduced and oxidized). Sequential redox reactions continue to occur as the electrons are transferred from one carrier to the next; each carrier is reduced as it accepts an electron and is then oxidized when it passes it on to the next carrier.

The last carrier of the ETC passes its electron to the final electron acceptor, O₂. In addition to the electrons, O₂ picks up a pair of hydrogen ions from the surrounding medium, forming water.

Question 15

Carbohydrates as Alternate Energy Source

Answer 15

Disaccharides are hydrolyzed into monosaccharides, most of which can be converted into glucose or glycolytic intermediates. Glycogen stored int he liver can be converted, when needed, into a glycolytic intermediate.

Question 16

Fats as Alternative Energy Source

Answer 16

Fat molecules are stored in adipose tissue in the form of triglycerides. When needed, they are hydrolyzed by lipases to fatty acids and glycerol and are carried by the blood to other tissues for oxidation. Glycerol can be converted into PGAL, a glycolytic intermediate.

A fatty acid must fire to be “activated” in the cytoplasm; this process requires two ATP. Once activated, the fatty acid is transported into the mitochondrion and taken through a series of beta-oxidation cycles that convert it into two-carbon fragments, which are then converted into acetyl CoA. Acetyl-CoA then enters the citric acid cycle. With each round of beta-oxifation of a saturated fatty acid, one NADH and one FADH2 are generated.

Of all the high-energy compounds used in cellular respiration, fats yield the greatest number of ATP per gram. This makes them extremely efficient energy storage molecules. Thus, whereas the amount of glycogen stored in humans is enough to meet the short-term energy needs of about a day, the stored fat reserves can meet the long-term energy needs for about a month.

Question 17

Proteins as Alternative Energy Source

Answer 17

The body degrades proteins only when not enough carbohydrate or fat is available.

Most amino acids undergo a transamination reaction in which they lose an amino group to form an alpha-keto acid. The carbon atoms of most amino acids are converted into acetyl-CoA, pyruvate, or one of the intermediates of the citric acid cycle. These intermediates enter their respective metabolic pathways, allowing cells to produce fatty acids, glucose, or energy in the form of ATP.

Oxidative deamination removes an ammonia molecule directly from the amino acid. Ammonia is a toxic substance in vertebrates. Fish can excrete ammonia, whereas insects and birds convert it to uric acid, and mammals convert it to urea for excretion.

Question 18

Respiration In Humans

Answer 18

In the human respiratory system, air enters the lungs after traveling through a series of respiratory airways. The air passages consist of the nose, pharynx (throat), larynx, trachea, bronchi, broncioles, and the alveoli. Gas exchange between the lungs and the circulatory system occurs across the very thin walls of the alveoli, which are air-filled sacs at the terminals of the airway branches. 300 million alveoli provide approximately 100m^2 of moist respiratory surface for gas exchange. After gas exchange, air rushes back through the respiratory pathway and is exhaled. The primary functions of the respiratory system in humans are to provide the necessary energy for growth, maintenance of homeostasis, defense mechanisms, repair, and reproduction of cells in the body. The respiratory system also provides a very large area for gas exchange, as well as continually moving oxygenated air over this area, protecting the respiratory surface from infection, dehydration, and temperature changes. It moves air over the vocal cords for the production of sound and assists int he regulation of body pH by regulating the rate of carbon dioxide removal from the blood.

Question 19

Ventilation

Answer 19

Ventilation of the lungs (breathing) is the process by which air is inhaled and exhaled. The purpose of ventilation is to take in oxygen from the atmosphere and eliminate carbon dioxide from the body. During inhalation, the diaphragm contracts and flattens, and the external intercostal muscles contract, pushing the rib cage and chest wall up and out. This causes the thoracic cavity to increase in volume. This volume increase, in turn, reduces the pressure, causing the lungs to expand and fill with air. The phrenic nerve innervates the diaphragm and causes it to contract and flatten. This produces inhalation. Exhalation is generally a passive process. The lungs and chest wall are highly elastic and tend to recoil to their original positions after inhalation. The diaphragm and external intercostal muscles relax, and the chest wall pushes inward. The consequent decrease in thoracic cavity volume causes the pressure to increase. This forces air out of the alveoli, causing the lungs to deflate.

Question 20

Respiratory Centers

Answer 20

Ventilation is regulated by neurons (referred to as respiratory centers) located in the medulla oblongata, whose rhythmic discharges stimulate the intercostal muscles or the diaphragm to contract.

When the partial pressure of CO2 rises, the medulla oblongata stimulates an increase int eh rate of ventilation. The primary goal of respiration is to maintain proper concentrations of oxygen, carbon dioxide, and hydrogen ions in tissues. Hence, respiratory activity is highly responsive to changes in the blood levels of these compounds. Excessive carbon dioxide and hydrogen ions are the primary stimulus for respiration.

When carbon dioxide and hydrogen ion levels are increased, the respiratory center stimulates both the inspiratory and expiratory muscles of the lungs.

Oxygen blood levels do not have a significant effect on the respiratory center. However, oxygen blood levels are monitored by peripheral chemoreceptors, which indirectly stimulate the respiratory center.

Question 21

Pulmonary Capillaries

Answer 21

A dense network of minute blood vessels that surrounds the alveolil.

Gas exchange occurs by diffusion across these capillary walls and those of the alveoli; gases move from regions or higher partial pressure to regions of lower partial pressure.

Oxygen diffuses from the alveolar air into the blood while carbon dioxide diffuses from the blood into the lungs to be exhaled.