● `color{violet}"All living organisms"`, be it a simple bacterial cell, a protozoan, a plant or an animal, contain `color{violet}"thousands of organic compounds"`.

● These `color{violet}"compounds or biomolecules"` are present in `color{violet}"certain concentrations"` (expressed as `color{Brown}"mols/cell or mols/litre"` etc.).

● One of the `color{violet}"greatest discoveries"` ever made was the observation that all these biomolecules have a `color{Brown}"turn over"`.

● This means that they are `color{violet}"constantly being changed"` into some `color{violet}"other biomolecules"` and also made from some other biomolecules.

● This `color{violet}"breaking and making"` is through `color{violet}"chemical reactions"` constantly occurring in living organisms.

● `color{violet}"Together"` all these chemical reactions are called `color{Brown}"metabolism"`.

● Each of the `color{violet}"metabolic reactions"` results in the `color{violet}"transformation of biomolecules"`.

● A few examples for such metabolic transformations are:

`star` `color{violet}"Removal"` of `CO_2` from `color{violet}"amino acids"` making an amino acid into an amine,
`star` `color{violet}"Removal"` of amino group in a `color{violet}"nucleotide base"`
`star` `color{violet}"Hydrolysis"` of a glycosidic bond in a `color{violet}"disaccharide"`, etc.

● `color{violet}"Majority"` of these metabolic reactions `color{violet}"do not occur in isolation"` but are always linked to some other reactions.

● In other words, `color{violet}"metabolites are converted"` into each other in a series of linked reactions called `color{Brown}"metabolic
pathways"`.

● These `color{violet}"metabolic pathways"` are similar to the `color{violet}"automobile traffic"` in a city.

● These pathways are either `color{violet}"linear or circular"`.

● These pathways `color{violet}"crisscross each other"`, i.e., there are `color{violet}"traffic junctions"`.

● `color{violet}"Flow of metabolites"` through metabolic pathway has a `color{violet}"definite rate and direction"` like automobile traffic.

● This `color{violet}"metabolite flow"` is called the `color{Brown}"dynamic state"` of body constituents.

● What is `color{violet}"most important"` is that this interlinked metabolic traffic is `color{violet}"very smooth"` and without a `color{violet}"single reported mishap"` for healthy conditions.

● `color{violet}"Another feature"` of these metabolic reactions is that every chemical reaction is a `color{Brown}"catalyzed reaction"`.

● There is `color{violet}"no uncatalysed metabolic"` conversion in living systems.

● Even `CO_2` dissolving in water, a `color{violet}"physical process"`, is a `color{violet}"catalysed reaction"` in living systems.

● The `color{violet}"catalysts"` which `color{violet}"hasten the rate"` of a given metabolic conversation are also `color{violet}"proteins"`.

● These proteins with `color{violet}"catalytic power"` are named `color{violet}"enzymes"`.

● `color{violet}"Metabolic pathways"` can lead to a `color{violet}"more complex structure"` from a simpler structure (for example, acetic acid becomes cholesterol) or lead to a `color{violet}"simpler structure"` from a complex structure (for example, glucose becomes lactic acid in our skeletal muscle).

● The former cases are called `color{violet}"biosynthetic pathways"` or`color{Brown}"anabolic pathways"`.

● The latter constitute `color{violet}"degradation"` and hence are called `color{Brown}"catabolic pathways"`.

● `color{violet}"Anabolic pathways"`, as expected, consume `color{violet}"energy"`.

● `color{violet}"Assembly of a protein"` from amino acids requires `color{violet}"energy input"`.

● On the other hand, `color{violet}"catabolic pathways"` lead to the `color{violet}"release of energy"`.

● For example, when `color{violet}"glucose is degraded"` to l`color{violet}"actic acid"` in our skeletal muscle, `color{violet}"energy is liberated"`.

● This metabolic pathway from `color{violet}"glucose to lactic acid"` which occurs in `color{violet}"10 metabolic steps"` is called `color{Brown}"glycolysis"`.

● `color{violet}"Living organisms"` have learnt to `color{violet}"trap this energy"` liberated during degradation and store it in the form of `color{violet}"chemical bonds"`.

● As and when needed, this `color{violet}"bond energy"` is utilized for `color{violet}"biosynthetic, osmotic"` and `color{violet}"mechanical work"` that we perform.

● The `color{violet}"most important form"` of energy currency in living systems is the bond energy in a chemical called `color{Brown}"adenosine triphosphate (ATP)"`.