Biology ReviewCellular Respiration

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Much of the text material is from, “Essential Biology with Physiology” by Neil A. Campbell, Jane B. Reece, and Eric J. Simon

(2004 and 2008). I don’t claim authorship. Other sources were also used and are noted.

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Outline

• Background• ATP and ADP• Components of cellular respiration

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Background

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Cellular Respiration

• Cellular respiration is a part of metabolism, the sum of all chemical processes in cells of the body.

• Much, but not all, of cellular respiration occurs in the mitochondria.

• The potential energy in food is converted to chemical energy for use by cells.

• More than two dozen chemical reactions are involved in cellular res-piration.

• A specific enzyme catalyzes the chemical reaction in each metabolic pathway.

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Solar Energy and Food

• Food molecules represent the storage of solar energy in indirect form, involving photosynthesis in plants.

• Animals rely on plants to convert energy from sunlight to the potential energy of sugars and other organic molecules.

• Humans also depend on plant life for cotton, lumber, paper, and many other products.

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Autotrophs and Heterotrophs

• Plants are autotrophs, or self-feeders, that synthesize organic matter from inorganic molecules such as carbon dioxide, water, and minerals from the soil.

• Animals are heterotrophs, or other-feeders, that are unable to synthe-size organic matter from inorganic molecules—they must obtain nutri-ents from food.

• Heterotrophs ultimately depend on autotrophs for organic materials needed for tissue growth and repair.

Autotrophs = also known as producers.

Heterotrophs = consumers.

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A Food Web and Its Dependencies

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Autotrophs

Heterotrophs

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Sunlight

PhotosynthesisChloroplasts in plants

Cellular respirationMitochondria in animals and plants

CO2 (carbon dioxide)+ H2O (water)

C6H12O6 (glucose) + O2 (oxygen)

ATP

Cellular work

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Chemical Cycle in Ecosystems

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ATP and ADP

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Cellular Respiration

• The chemical equation for aerobic cellular respiration is shown on the next slide.

• A key product of cellular respiration is adenosine triphosphate (ATP).

• The left- and right-hand sides of the equation are shown in a previous slide, “chemical cycle in ecosystems.”

• The chemical equation represents what is known as a redox reaction.

Aerobic = requires oxygen.

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Chemical Equation

C6H12O6

(glucose)+ 6O2 6CO2 + 6H2O ATP

(chemical energy)

+Cellular

Respiration

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Glucosemolecule

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ATP molecule

Up to 38 ATP molecules are

produced for each glucose molecule.

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Redox Reaction

• The transfer of electrons from one molecule to another molecule is an oxidation-reduction reaction.

• It is also called, more simply, a redox reaction.

• The loss of electrons is known as oxidation—glucose is oxidized, losing electrons to oxygen.

• Oxygen is reduced by accepting electrons and hydrogen atoms from glucose.

• Energy is released when electrons and hydrogen atoms change part-ners from sugar to oxygen.

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Adenosine Triphosphate

• The tail of adenosine triphosphate (ATP) contains energy for cellular work.

• The three phosphate groups tend to repel each other because each has a negative charge—they are held together by covalent bonds.

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ATP and ADP

• The crowding of negative charges in the molecular tail of ATP is similar to the storing of energy in a compressed spring.

• When released, a spring can perform useful work.

• The release of the third phosphate group from its molecular tail makes the energy available for cellular work.

• The molecule, which now has two remaining phosphate groups, is called ADP (adenosine diphosphate).

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Phosphate Transfer

• The third phosphate group released from ATP is transferred to other molecules.

• The transfer enables cells to perform work—mechanical, chemical, or transport.

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Examples of Cellular Work

• Mechanical—phosphate groups from ATP molecules are transferred to motor proteins to enable muscle fibers to contract.

• Chemical—ATP provides energy for dehydration synthesis of macro-molecules such as starches and proteins.

• Transport—ATP enables certain ions to be pumped across the plasma membranes of neurons and other cells.

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ATP Cycle

• ATP is restored by adding a phosphate group to ADP using the chem-ical energy cellular respiration harvests from food molecules (such as fats and carbohydrates).

• The process is called the ATP cycle.

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ATP Cycle (continued)

The circle turns

clockwise

ATP

ADP +

Potential energy from food molecules

Chemical energy forcellular work

P

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Components of Cellular Respiration

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Components

Electron micrograph of a human lymphocyte cell—a number of

mitochondria are visible.

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Glycolysis

Krebs Cycle

Electron Transport Chain

The three processes involved in cellular respiration.

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Glycolysis

• The enzymes for glycolysis are in the cytosol of eukaryotic and pro-karyotic cells.

• Glycolysis is anaerobic—it does not consume oxygen.

• The process breaks glucose molecules consisting of six carbons into two, three-carbon molecules of pyruvic acid.

• For each molecule of glucose, four molecules of ATP are produced.

• Two electrons are also transferred to the molecule, NAD+ to produce NADH for the electron transport chain.

NAD+ = an electron acceptor known as nicotine adenine dinucleotide.

NADH = nicotine adenine dinucleotide, reduced.

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Biochemistry of Glycolysis

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Glycolysis (continued)

• Pyruvic acid retains much of the energy of glucose that will be harves-ted in the Krebs cycle.

• Pyruvic acid is converted to a two-carbon compound called acetic acid.

• Acetic acid enters the Krebs cycle attached to a carrier molecule known as coenzyme A (CoA) to form acetyl-CoA.

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ATP Output

• Glycolysis is not an especially efficient process since only four ATP molecules are produced for every glucose molecule, along with two electrons.

• In comparison, 36 ATP molecules (and many more electrons) are pro-duced by the Krebs cycle.

• To sustain energy output in glycolysis, cells compensate by consuming more glucose molecules if an adequate supply of carbohydrates is avail-able.

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Anaerobic Effort

• Cells can function for brief periods of time without oxygen through the anaerobic conversion of glucose to pyruvic acid and ATP.

• Skeletal muscle fibers have sufficient amount of ATP molecules to support anaerobic activity for about five seconds.

• These muscle fibers also have a secondary supply of the molecule creatine phosphate to provide an additional ten seconds of energy reserve.

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Lactic Acid

• Lactic acid is a metabolic byproduct of pyruvic acid from the process of glycolysis.

• During strenuous exercise, lactic acid accumulates in skeletal muscles, which can produce muscle burning sensations and soreness.

• Skeletal muscles may temporarily shut down if lactic acid accumulates in high concentrations.

• This is sometimes called, “hitting the wall.”

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Hitting the Wall

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Endurance runners must learn to stay within their physiological limits until the final dash to the finish line.

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Lactic Acid (continued)

• Lactic acid is transported to the liver in the blood, where it is inacti-vated.

• The inactivation requires oxygen, which is one reason why a person continues to breathe fast and heavy after vigorous exercise.

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Krebs Cycle

• The Krebs cycle occurs in mitochondria of eukaryotic (plant, animal, and fungus) cells.

• It is also known by other names, and especially the citric acid cycle.

• The process is not found in prokaryotic cells because they lack mito-chondria.

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Mitochondria

An electron micrograph of a mitochondrion.

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Krebs Cycle (continued)

• The Krebs cycle extracts chemical energy until CO2 is formed as a by-product of aerobic cellular respiration.

• Each turn of the cycle produces two ATP molecules.

• Six electrons are donated to NAD+ molecules to produce NADH for the electron transport chain.

• Two electrons are also donated to the molecule FADH2, for the electron transport chain.

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Biochemistry of the Krebs Cycle

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Electron Transport Chain

• The molecules of the electron transport chain are found in the inner membrane of the mitochondria.

• Hydrogen ions (H+) “fall” toward oxygen molecules that entered the mitochondria by passive diffusion along their concentration gradient.

• The process is aerobic—it requires a constant supply of oxygen mole-cules.

• The electron transport chain uses the electrons in NADH and FADH2 to pump hydrogen ions against their concentration gradient across the mitochondrial membrane.

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Electron Transport Chain (continued)

• The hydrogen ions diffuse along their concentration gradient back into the mitochondria.

• H+ inflow turns turbines of protein molecules, known as ATP synthases, in the mitochondrial membrane.

ATP synthasehttp://www.sparknotes.com

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Hoover Dam

Powerhouse turbineshttp://www.bossanova.com

Hoover Dam, Nevada and Arizonahttp://www.mcnarybergeron.com

Turbines connected to generators produce electrical energy from the

downhill flow of water.

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ATP Regeneration

• Energy from the spinning of an ATP synthase attaches a phosphate group to an ADP molecule to regenerate an ATP molecule.

• Up to 34 ATP molecules are produced by a ATP synthase—compare this number with the much smaller ATP output from glycolysis.

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Versatility of Cellular Respiration

• So far, we have focused on glucose as a fuel source for cellular respi-ration.

• Cellular respiration also uses other carbohydrates, fats, and proteins.

• The digestive process hydrolyzes large food molecules into monomers that can be absorbed by the small intestine for glycolysis and the Krebs cycle.

http://www.borderfoodsinc.com

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Carbon Monoxide and Cyanide

• Carbon monoxide (CO) and cyanide block the transfer of electrons to oxygen in the electron transport chain.

• The mitochondria cannot harvest food energy to convert ADP to ATP.

• The cells stop working and the organism can die, usually very rapidly.ht

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