INTRODUCTORY METABOLISM MODULE

WHY DO CELLS METABOLISE FUELS?

It is suggested that the first time you study this material, you work progressively through the pages below. When you subsequently review the material, use the links below to navigate to particular topics in this section.

Work within cells
Biological systems perform various kinds of work. What are they?
  • Mechanical work - a change of location or posture of an organism, cell or cellular structure.
  • Osmotic or electrical work - compounds or ions are often moved against a concentration gradient.
  • Synthetic work - a change in chemical bonds required to generate complex molecules from simple precursors.
The energy required to carry out this work can only come from chemical bond energy. This is achieved by coupling energetically favourable reactions to those that require a nett energy input.

 

Adenosine triphosphate (ATP)
Adenosine triphosphate (ATP) is the common medium of exchange between energy producing and energy consuming reactions. About 2 kg of ATP is consumed every day in humans. ATP is referred to as the "energy currency of cells".
One aspect of metabolism is to supply this very large requirement for ATP.
A second aspect is to supply the precursors (building blocks) for the synthesis of biological polymers such as proteins and polysaccharides.

Metabolic pathways
Chemical reactions are needed to extract the chemical bond energy from energy supplying compounds and to synthesise different biological molecules.
Chemical reactions in biological systems are organised into sequences - these are called metabolic pathways.
Organising the reactions into pathways :
  • permits control of the rate and direction of the cellular activity

Control of pathways
Control is achieved via the enzymes which specifically catalyse each of the steps in a pathway. Individual reactions can be stimulated or inhibited by changing the concentration of key compounds or by chemical modification of the enzyme catalysing the reaction. It is often the role of intercellular messengers called hormones to carry out these functions.
Organising the reactions into pathways also :
  • prevents very large chemical bond energy releases which would be damaging to cells
  • permits branch points
  • allows pathways to be directed (under different circumstances) to different end products

Metabolism
Metabolism is the sum of all the chemical reactions within a biological system related to the management of the supply of energy to power cellular activity and the generation of molecules for cellular syntheses.
Metabolism is divided into :
  • catabolism

    This is the oxidation of biological fuels to produce energy in the form of ATP.

    Oxidation of biological fuels (which are largely hydrocarbons) ultimately to carbon dioxide and water is the mechanism for energy release. Some of this energy is lost as heat, but much is captured in the chemical bond energy of ATP.

  • anabolism

    This is the synthesis of molecules using energy supplied in the form of ATP.

    The molecules which are the precursors of biological polymers are either supplied in the diet or synthesised within cells. Both processes require energy.

Oxidation/reduction reactions
Oxidation reactions are always paired with reduction reactions. The two reaction types involve electron transfer between molecules.
  • Oxidation is a loss of electrons from a molecule.

    It is often accompanied by a loss of one or more hydrogen atoms from the molecule.

  • Reduction is the gain of electrons by a molecule.

    It is often accompanied by a gain of one or more hydrogen atoms by the molecule.

Dehydrogenase enzymes and their cofactors
Metabolic oxidation/reduction reactions are catalysed by a class of enzyme designated as dehydrogenases. These enzymes use one of a limited number of cofactors as electron acceptors or electron donors to complete the reaction as the metabolic substrate is reduced or oxidised. The commonly encountered cofactors are shown in the table below.

 

 
Two examples of biological oxidation reduction reaction pairs are shown below. Both reactions are components of metabolic pathways to be discussed later.

 

Biological fuels
Three groups of biological molecules are considered to be "fuels" for the body. They are :-
  • carbohydrates

    Glucose is the most abundant monomer in this group and animals (including humans) store carbohydrate as glycogen.

  • proteins

    The monomers of proteins (amino acids) are a significant fuel for carnivores (meat eaters), but are a less significant component of our omnivorous diet. Proteins provide an important reserve of fuel molecules but are only used to a large extent in circumstances of very prolonged starvation.

  • fats

    These are stored primarily as triglycerides in adipose tissue and make up our major fuel store. The fatty acid components of triglycerides are readily oxidised to produce ATP.

Subsequent parts of this module discuss the role of each of these in metabolism. Ultimately they are oxidised to carbon dioxide and water. The metabolic pathways are directed towards fuel oxidation with simultaneous ATP synthesis when energy is required. At times of rest the metabolic pathways are essentially reversed to facilitate storage of fuels for use in times of need.   (This often requires a different and unique set of reactions to return to the starting material.)

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