Learning Unit 1: Introduction to Metabolism
Important Terms
Metabolism: The entire network of reactions an organism carries out.
Catabolism: Comprises reactions that degrade or break down complex substances to simpler molecules
to provide energy.
Anabolism: Comprises all processes that involve synthesis or building of complex substances from
simple molecules and it uses energy.
Metabolic Pathway: The process whereby a product of one reaction becomes a substrate of the next.
Intermediary Metabolism: All reactions whose primary goal is to produce or store metabolic energy,
and the use of energy to synthesise low molecular weight compounds and energy storage compounds.
Central Metabolism: Pathways that are substantially the same in many organisms.
Autotrophs: Organisms that synthesise glucose (photosynthesis) and other organic compounds from
carbon dioxide (CO2). Plants use CO2 and sunlight to synthesise their carbohydrates. Plants are
therefore autotrophs and the source of their fuel is sunlight.
Heterotrophs: Organisms that depend on organic compounds from other organisms and use them to
synthesise their own organic metabolites. Animals consume carbohydrates and other compounds
nitrogen-containing compounds from plants to use for energy generation and synthesis of organic
metabolites.
Describe roles of different metabolic pathways.
Glycolysis (glycol – sugar; lysis – breaking)
Glycolysis is the first pathway used in the breakdown of glucose to extract energy. One glucose molecule (a six-carbon
sugar) is broken into two pyruvate molecules (a 3-carbon sugar)
Carbohydrates (monosaccharides) are degraded in a series of reactions to result in a net gain of two pyruvate molecules,
two ATP molecules, and two NADH molecules for the cell to use for energy. This is a catabolic process.
1
,The Citric Acid Cycle (Krebs Cycle)
The pyruvate molecules produced in glycolysis can be oxidised in the presence of oxygen to two metabolically active
carbon fragments of acetyl-coenzyme-A. Acetyl-coenzyme A enters the citric acid cycle, producing CO2 and reduced
electron carriers, reduced nicotinamide adenine dinucleotide (NADH) and reduced flavin adenine dinucleotide (FADH2).
Acetyl-coenzyme A is also derived from the β-oxidation of fatty acids. The primary function of the citric acid cycle is to
provide energy (ATP). It is also an amphibolic process – plays a role in both catabolic and anabolic processes.
Electron Transport and Oxidative Phosphorylation
Electron carriers resulting from the citric acid cycle are then oxidised in the mitochondria by the electron transport,
creating a proton gradient. The resulting gradient provides enough energy for the phosphorylation of ADP to ATP.
2
,Gluconeogenesis
When blood glucose levels drop, glucose is synthesised from non-carbohydrate precursors such as glycerol and amino
acids. Gluconeogenesis shares some common reactions with glycolysis but there are two processes/ reactions that are
not the reverse of each other. This is an anabolic process.
Discuss the general mechanisms that regul ate metabolic flux.
Enzyme levels:
The rate of synthesis of an enzyme and its concentration directly control the rate of reaction it catalyses. The
concentration of an enzyme is affected by the rate at which its gene is copied into mRNA and protein (gene expression).
When the product of its reaction is needed, the enzyme levels are increased by inducing gene expression. Repression
refers to the switching off genes when their products are not needed.
Control of enzyme activity:
Enzyme activity can be controlled through a variety of mechanisms. Covalent modification refers to addition of a chemical
group to an enzyme, resulting in its activation or inhibition, depending on the nature of the enzyme.
Compartmentation:
Different enzymes are localised in different regions/organelles of the cell. This ensures that processes of the cell take
place in an ordered environment.
3
, Hormonal regulation:
Hormones convey their specific messages to the cells by binding to receptors. Receptors relay the message to the cells,
resulting in the desired metabolic action through a series of reactions (involving second messengers) that may involve
covalent modification of the enzyme.
Describe bioenergetics in relation to metabolic rate.
Bioenergetics is the study of the transformation of energy in living organisms.
The amount of energy expended by an animal over a specific time is called its metabolic rate.
4
Important Terms
Metabolism: The entire network of reactions an organism carries out.
Catabolism: Comprises reactions that degrade or break down complex substances to simpler molecules
to provide energy.
Anabolism: Comprises all processes that involve synthesis or building of complex substances from
simple molecules and it uses energy.
Metabolic Pathway: The process whereby a product of one reaction becomes a substrate of the next.
Intermediary Metabolism: All reactions whose primary goal is to produce or store metabolic energy,
and the use of energy to synthesise low molecular weight compounds and energy storage compounds.
Central Metabolism: Pathways that are substantially the same in many organisms.
Autotrophs: Organisms that synthesise glucose (photosynthesis) and other organic compounds from
carbon dioxide (CO2). Plants use CO2 and sunlight to synthesise their carbohydrates. Plants are
therefore autotrophs and the source of their fuel is sunlight.
Heterotrophs: Organisms that depend on organic compounds from other organisms and use them to
synthesise their own organic metabolites. Animals consume carbohydrates and other compounds
nitrogen-containing compounds from plants to use for energy generation and synthesis of organic
metabolites.
Describe roles of different metabolic pathways.
Glycolysis (glycol – sugar; lysis – breaking)
Glycolysis is the first pathway used in the breakdown of glucose to extract energy. One glucose molecule (a six-carbon
sugar) is broken into two pyruvate molecules (a 3-carbon sugar)
Carbohydrates (monosaccharides) are degraded in a series of reactions to result in a net gain of two pyruvate molecules,
two ATP molecules, and two NADH molecules for the cell to use for energy. This is a catabolic process.
1
,The Citric Acid Cycle (Krebs Cycle)
The pyruvate molecules produced in glycolysis can be oxidised in the presence of oxygen to two metabolically active
carbon fragments of acetyl-coenzyme-A. Acetyl-coenzyme A enters the citric acid cycle, producing CO2 and reduced
electron carriers, reduced nicotinamide adenine dinucleotide (NADH) and reduced flavin adenine dinucleotide (FADH2).
Acetyl-coenzyme A is also derived from the β-oxidation of fatty acids. The primary function of the citric acid cycle is to
provide energy (ATP). It is also an amphibolic process – plays a role in both catabolic and anabolic processes.
Electron Transport and Oxidative Phosphorylation
Electron carriers resulting from the citric acid cycle are then oxidised in the mitochondria by the electron transport,
creating a proton gradient. The resulting gradient provides enough energy for the phosphorylation of ADP to ATP.
2
,Gluconeogenesis
When blood glucose levels drop, glucose is synthesised from non-carbohydrate precursors such as glycerol and amino
acids. Gluconeogenesis shares some common reactions with glycolysis but there are two processes/ reactions that are
not the reverse of each other. This is an anabolic process.
Discuss the general mechanisms that regul ate metabolic flux.
Enzyme levels:
The rate of synthesis of an enzyme and its concentration directly control the rate of reaction it catalyses. The
concentration of an enzyme is affected by the rate at which its gene is copied into mRNA and protein (gene expression).
When the product of its reaction is needed, the enzyme levels are increased by inducing gene expression. Repression
refers to the switching off genes when their products are not needed.
Control of enzyme activity:
Enzyme activity can be controlled through a variety of mechanisms. Covalent modification refers to addition of a chemical
group to an enzyme, resulting in its activation or inhibition, depending on the nature of the enzyme.
Compartmentation:
Different enzymes are localised in different regions/organelles of the cell. This ensures that processes of the cell take
place in an ordered environment.
3
, Hormonal regulation:
Hormones convey their specific messages to the cells by binding to receptors. Receptors relay the message to the cells,
resulting in the desired metabolic action through a series of reactions (involving second messengers) that may involve
covalent modification of the enzyme.
Describe bioenergetics in relation to metabolic rate.
Bioenergetics is the study of the transformation of energy in living organisms.
The amount of energy expended by an animal over a specific time is called its metabolic rate.
4
