Chapter 4: Cellular Metabolism
Cellular metabolism: Refers to the collective chemical processes that occur within living cells to accomplish these activities.
Potential energy: It is stored energy.
Energy that is not doing work but has the capacity to do so.
Kinetic energy: Energy of motion.
Energy which can do work.
First law of thermodynamics: States that energy cannot be created or destroyed.
It can change from one form to another, but the total amount of energy remains the same.
Second law of thermodynamics: States that a closed system moves toward increasing disorder, or entropy, as energy is dissipated from the system.
Free energy: The energy in a system available for doing work.
Exergonic: Reactions in cells release free energy.
Endergonic: Reactions in cells that require the addition of free energy.
For any reaction to occur, even exergonic ones that tend to proceed spontaneously, chemical bonds first must be destabilized.
Activation energy: This must be supplied before the bond is stressed enough to break.
If exergonic, reaction products will develop and free energy will be lost only then.
Raising the temperature increases molecule collisions and breaks chemical bonds, providing activation energy to chemical reactants.
Activating a reaction requires heat.
Metabolic processes must occur at biologically acceptable temperatures, which are too low for life.
Catalysts: Chemical substances that accelerate reaction rates without affecting the products of the reaction and without being altered or destroyed by the reaction.
Enzymes: They reduce the amount of activation energy required for a reaction.
They steer the reaction through one or more intermediate steps, each of which requires much less activation energy than that required for a single-step reaction.
Cofactors: Small nonprotein groups which perform their enzymatic functions.
These can be metallic ions.
Carbonic anhydrase contains zinc.
Cytochromes contain iron.
Troponin requires calcium to perform its function.
Coenzymes: Contain groups derived from vitamins, most of which must be supplied in the diet.
B-complex vitamins
Nicotinamide adenine dinucleotide (NAD) contains the vitamin **nicotinic acid (**niacin).
Coenzyme A contains the vitamin pantothenic acid.
Flavin adenine dinucleotide (FAD) contains riboflavin (vitamin B2).
Ribosomal RNA: A major component of ribosomes, provides the activation energy that enables amino acids to assemble into polypeptide chains during the process of translation.
Substrate: The molecule whose reaction it catalyzes.
Enzyme-substrate complex (ES complex): Formed during the binding of enzyme to substrate, in which the substrate is secured by covalent bonds to one or more points in the active site of the enzyme.
Hydrolysis: Breaking with water.
Hydrolysis reaction: A molecule is cleaved by the addition of water at the cleavage site.
A hydrogen atom is attached to one subunit and a hydroxyl (—OH) unit is attached to another.
This breaks the covalent bond between subunits.
Condensation: Subunits of molecules are linked together by removal of water.
This is where macromolecules are built.
Although some enzymes appear to function automatically, the activity of others is rigidly controlled.
If an enzyme acts reversibly, either synthesis or degradation may result.
Mechanisms exist for critically regulating enzymes in both quantity and activity.
Mechanisms that alter activity of enzymes can quickly and finely adjust metabolic pathways to changing conditions in a cell.
Feedback inhibition: The final end product of a particular metabolic pathway inhibits the first enzyme in the pathway.
Oxidation-reduction “redox” reaction: Involves a transfer of electrons from an electron donor (the reducing agent) to an electron acceptor (the oxidizing agent).
As soon as the electron donor loses its electrons, it becomes oxidized.
As soon as the electron acceptor accepts electrons, it becomes reduced.
In an oxidation-reduction reaction the electron donor and electron acceptor form a redox pair:
When electrons are accepted by the oxidizing agent, energy is liberated because the electrons move to a more stable position.
The nature of this final electron acceptor is the key that determines the overall efficiency of cellular metabolism.
Heterotrophs: Organisms that cannot synthesize their own food but must obtain nutrients from the environment, including animals, fungi, and many single-celled organisms.
Aerobes: Those that use molecular oxygen as the final electron acceptor.
Anaerobes: Those that employ another molecule as the final electron acceptor.
Cellular respiration: The oxidation of fuel molecules to produce energy with molecular oxygen as the final electron acceptor.
Oxidation of fuel molecules: describes the removal of electrons from fuel molecules and not the direct combination of molecular oxygen with fuel molecules.
Aerobic cellular respiration: Uses oxygen as the final electron acceptor and releases carbon dioxide and water from the complete oxidation of fuels.
Hans Krebs: A British biochemist who described three stages in the complete oxidation of fuel molecules to carbon dioxide and water.
Stage I: Food passing through the intestinal tract is digested into small molecules that can be absorbed into the circulation.
Stage II: Also called glycolysis, most of the glucose is converted into two 3-carbon units (pyruvic acid) in the cell cytoplasm.
The pyruvic acid molecules then enter mitochondria, where in another reaction they join with a coenzyme to form acetyl coenzyme A.
Stage III: The final oxidation of fuel molecules occurs, with a large yield of ATP. This stage occurs entirely in mitochondria.
Acetyl-CoA is channeled into the Krebs cycle, where the acetyl group is completely oxidized to carbon dioxide.
Krebs cycle: Also known as citric acid cycle and tricarboxylic acid cycle.
A cyclic sequence where oxidation of the 2-carbon acetyl group of acetyl-CoA occurs within the mitochondrial matrix.
The central purpose of carbohydrate and fat metabolism is to provide energy, much of which is needed to construct and maintain cellular structure and metabolic processes.
Triglycerides (neutral fats): Are especially rich depots of metabolic energy because the fatty acids of which they are composed are highly reduced and free of water.
Fatty acids: Are degraded by sequential removal of 2-carbon units, which enter the Krebs cycle through acetyl-CoA.
When animals eat proteins, most are digested in the digestive tract, releasing their constituent amino acids, which are then absorbed.
Tissue proteins also are hydrolyzed during normal growth, repair, and tissue restructuring; their amino acids join those derived from protein found in food to enter the amino acid pool.
A portion of the amino acid pool is used to rebuild tissue proteins, but most animals ingest a surplus of protein.
Before an amino acid molecule may enter the fuel depot, nitrogen must be removed by deamination or by transamination.
Deamination: the amino group splits to form ammonia and a keto acid
Transamination: the amino group is transferred to a keto acid to yield a new amino acid.
Thus amino acid degradation yields two main products, carbon skeletons and ammonia, which are handled in different ways.
Once nitrogen atoms are removed, the carbon skeletons of amino acids can be completely oxidized, usually by way of pyruvic acid or acetic acid.
Another product of amino acid degradation is ammonia.
It is highly toxic because it inhibits respiration by reacting with α-ketoglutaric acid to form glutamic acid, and effectively removes α-ketoglutarate from the Krebs cycle
Terrestrial animals cannot get rid of ammonia so conveniently and must detoxify it by converting it to a relatively nontoxic compound; urea and uric acid.
Among vertebrates, amphibians and especially mammals produce mainly urea.
Reptiles and birds, as well as many terrestrial invertebrates, produce mainly uric acid.
Cellular metabolism: Refers to the collective chemical processes that occur within living cells to accomplish these activities.
Potential energy: It is stored energy.
Energy that is not doing work but has the capacity to do so.
Kinetic energy: Energy of motion.
Energy which can do work.
First law of thermodynamics: States that energy cannot be created or destroyed.
It can change from one form to another, but the total amount of energy remains the same.
Second law of thermodynamics: States that a closed system moves toward increasing disorder, or entropy, as energy is dissipated from the system.
Free energy: The energy in a system available for doing work.
Exergonic: Reactions in cells release free energy.
Endergonic: Reactions in cells that require the addition of free energy.
For any reaction to occur, even exergonic ones that tend to proceed spontaneously, chemical bonds first must be destabilized.
Activation energy: This must be supplied before the bond is stressed enough to break.
If exergonic, reaction products will develop and free energy will be lost only then.
Raising the temperature increases molecule collisions and breaks chemical bonds, providing activation energy to chemical reactants.
Activating a reaction requires heat.
Metabolic processes must occur at biologically acceptable temperatures, which are too low for life.
Catalysts: Chemical substances that accelerate reaction rates without affecting the products of the reaction and without being altered or destroyed by the reaction.
Enzymes: They reduce the amount of activation energy required for a reaction.
They steer the reaction through one or more intermediate steps, each of which requires much less activation energy than that required for a single-step reaction.
Cofactors: Small nonprotein groups which perform their enzymatic functions.
These can be metallic ions.
Carbonic anhydrase contains zinc.
Cytochromes contain iron.
Troponin requires calcium to perform its function.
Coenzymes: Contain groups derived from vitamins, most of which must be supplied in the diet.
B-complex vitamins
Nicotinamide adenine dinucleotide (NAD) contains the vitamin **nicotinic acid (**niacin).
Coenzyme A contains the vitamin pantothenic acid.
Flavin adenine dinucleotide (FAD) contains riboflavin (vitamin B2).
Ribosomal RNA: A major component of ribosomes, provides the activation energy that enables amino acids to assemble into polypeptide chains during the process of translation.
Substrate: The molecule whose reaction it catalyzes.
Enzyme-substrate complex (ES complex): Formed during the binding of enzyme to substrate, in which the substrate is secured by covalent bonds to one or more points in the active site of the enzyme.
Hydrolysis: Breaking with water.
Hydrolysis reaction: A molecule is cleaved by the addition of water at the cleavage site.
A hydrogen atom is attached to one subunit and a hydroxyl (—OH) unit is attached to another.
This breaks the covalent bond between subunits.
Condensation: Subunits of molecules are linked together by removal of water.
This is where macromolecules are built.
Although some enzymes appear to function automatically, the activity of others is rigidly controlled.
If an enzyme acts reversibly, either synthesis or degradation may result.
Mechanisms exist for critically regulating enzymes in both quantity and activity.
Mechanisms that alter activity of enzymes can quickly and finely adjust metabolic pathways to changing conditions in a cell.
Feedback inhibition: The final end product of a particular metabolic pathway inhibits the first enzyme in the pathway.
Oxidation-reduction “redox” reaction: Involves a transfer of electrons from an electron donor (the reducing agent) to an electron acceptor (the oxidizing agent).
As soon as the electron donor loses its electrons, it becomes oxidized.
As soon as the electron acceptor accepts electrons, it becomes reduced.
In an oxidation-reduction reaction the electron donor and electron acceptor form a redox pair:
When electrons are accepted by the oxidizing agent, energy is liberated because the electrons move to a more stable position.
The nature of this final electron acceptor is the key that determines the overall efficiency of cellular metabolism.
Heterotrophs: Organisms that cannot synthesize their own food but must obtain nutrients from the environment, including animals, fungi, and many single-celled organisms.
Aerobes: Those that use molecular oxygen as the final electron acceptor.
Anaerobes: Those that employ another molecule as the final electron acceptor.
Cellular respiration: The oxidation of fuel molecules to produce energy with molecular oxygen as the final electron acceptor.
Oxidation of fuel molecules: describes the removal of electrons from fuel molecules and not the direct combination of molecular oxygen with fuel molecules.
Aerobic cellular respiration: Uses oxygen as the final electron acceptor and releases carbon dioxide and water from the complete oxidation of fuels.
Hans Krebs: A British biochemist who described three stages in the complete oxidation of fuel molecules to carbon dioxide and water.
Stage I: Food passing through the intestinal tract is digested into small molecules that can be absorbed into the circulation.
Stage II: Also called glycolysis, most of the glucose is converted into two 3-carbon units (pyruvic acid) in the cell cytoplasm.
The pyruvic acid molecules then enter mitochondria, where in another reaction they join with a coenzyme to form acetyl coenzyme A.
Stage III: The final oxidation of fuel molecules occurs, with a large yield of ATP. This stage occurs entirely in mitochondria.
Acetyl-CoA is channeled into the Krebs cycle, where the acetyl group is completely oxidized to carbon dioxide.
Krebs cycle: Also known as citric acid cycle and tricarboxylic acid cycle.
A cyclic sequence where oxidation of the 2-carbon acetyl group of acetyl-CoA occurs within the mitochondrial matrix.
The central purpose of carbohydrate and fat metabolism is to provide energy, much of which is needed to construct and maintain cellular structure and metabolic processes.
Triglycerides (neutral fats): Are especially rich depots of metabolic energy because the fatty acids of which they are composed are highly reduced and free of water.
Fatty acids: Are degraded by sequential removal of 2-carbon units, which enter the Krebs cycle through acetyl-CoA.
When animals eat proteins, most are digested in the digestive tract, releasing their constituent amino acids, which are then absorbed.
Tissue proteins also are hydrolyzed during normal growth, repair, and tissue restructuring; their amino acids join those derived from protein found in food to enter the amino acid pool.
A portion of the amino acid pool is used to rebuild tissue proteins, but most animals ingest a surplus of protein.
Before an amino acid molecule may enter the fuel depot, nitrogen must be removed by deamination or by transamination.
Deamination: the amino group splits to form ammonia and a keto acid
Transamination: the amino group is transferred to a keto acid to yield a new amino acid.
Thus amino acid degradation yields two main products, carbon skeletons and ammonia, which are handled in different ways.
Once nitrogen atoms are removed, the carbon skeletons of amino acids can be completely oxidized, usually by way of pyruvic acid or acetic acid.
Another product of amino acid degradation is ammonia.
It is highly toxic because it inhibits respiration by reacting with α-ketoglutaric acid to form glutamic acid, and effectively removes α-ketoglutarate from the Krebs cycle
Terrestrial animals cannot get rid of ammonia so conveniently and must detoxify it by converting it to a relatively nontoxic compound; urea and uric acid.
Among vertebrates, amphibians and especially mammals produce mainly urea.
Reptiles and birds, as well as many terrestrial invertebrates, produce mainly uric acid.
