`star` Photosynthesis in Plants
`star` Glycolysis


● The `color{Brown}"complete combustion of glucose"`, which produces `CO_2` and `H_2O` as `color{violet}"end products"`, yields `color{violet}"energy"` most of which is given out as `color{violet}"heat"`.

● If this `color{violet}"energy is to be useful"` to the cell, it should be able to `color{violet}"utilise it to synthesise"` other molecules that the `color{violet}"cell requires"`.

● The strategy that the`color{violet}" plant cell uses"` is to `color{Brown}"catabolise the glucose"` molecule in such a way that `color{violet}"not all the liberated energy"` goes out as `color{violet}"heat"`.

● The key is to `color{violet}"oxidise glucose"` not in one step but in `color{violet}"several small steps"` enabling `color{violet}"some steps"` to be `color{violet}"just large enough"` such that the energy released can be `color{violet}"coupled to ATP"` synthesis.

● How this is done is, essentially, the `color{violet}"story of respiration"`.

● During the `color{violet}"process of respiration"`, `color{violet}"oxygen"` is utilised, and `color{violet}"carbon dioxide"`, water and energy are released as products.

● The `color{violet}"combustion reaction"` requires oxygen.

● But some `color{violet}"cells live"` where `color{violet}"oxygen"` may or may not be `color{violet}"available"`.

● There are `color{violet}"sufficient reasons"` to believe that the `color{violet}"first cells on this planet"` lived in an atmosphere that `color{violet}"lacked oxygen"`.

● Even among `color{violet}"present-day living organisms,"` we know of several that are `color{Brown}"adapted to anaerobic"` conditions.

● Some of these organisms are `color{Brown}"facultative anaerobes"`, while in others the requirement for an`color{violet}"aerobic condition"` is `color{Brown}"obligate"`.

● In any case, `color{violet}"all living organisms"` retain the enzymatic machinery to `color{violet}"partially oxidise glucose"` without the help of oxygen.

● This breakdown of `color{violet}"glucose to pyruvic acid"` is called `color{brown}"glycolysis."`


● The term `color{Brown}"glycolysis"` has originated from the `color{violet}"Greek words, glycos"` for `color{violet}"sugar,"` and `color{violet}"lysis for splitting"`.

● The scheme of glycolysis was given by `color{Brown}"Gustav Embden"`, `color{brown}"Otto Meyerhof"`, and `color{brown}"J. Parnas"`, and is often referred to as the `color{violet}"EMP pathway"`.

● In an`color{violet}"aerobic organisms"`, it is the `color{violet}"only process"` in respiration.

● `color{violet}"Glycolysis"` occurs in the `color{violet}"cytoplasm"` of the cell and is present in all `color{violet}"living organisms"`.

● In this process, `color{violet}"glucose undergoes partial oxidation"` to form `color{violet}"two molecules"` of `color{Brown}"pyruvic acid"`.

● In plants, this `color{violet}"glucose is derived from sucrose"`, which is the `color{violet}"end product of photosynthesis"`, or from `color{violet}"storage carbohydrates"`.

● `color{Brown}"Sucrose"` is converted into `color{violet}"glucose and fructose"` by the enzyme, `color{brown}"invertase"`, and these `color{violet}"two monosaccharides"` readily enter the `color{violet}"glycolytic pathway"`.

● `color{violet}"Glucose and fructose"` are `color{violet}"phosphorylated"` to give rise to `color{violet}"glucose-6- phosphate"` by the activity of the enzyme `color{violet}"hexokinase"`.

● This `color{violet}"phosphorylated form"` of glucose then `color{violet}"isomerises"` to produce `color{violet}"fructose-6-phosphate"`.

● Subsequent `color{violet}"steps of metabolism"` of glucose and fructose are `color{violet}"same"`.

● The `color{violet}"various steps of glycolysis"` are depicted in Figure.

● In glycolysis, a `color{violet}"chain of ten reactions"`, under the control of `color{violet}"different enzymes"`, takes place to produce `color{violet}"pyruvate from glucose"`.

● While studying the `color{violet}"steps of glycolysis"`, please note the steps at which `color{violet}"utilisation (ATP energy)"` or `color{violet}"synthesis of ATP"` or (in this case of) `NADH` + `H^+` take place.

● ATP is `color{violet}"utilised at two steps"`: first in the conversion of `color{violet}"glucose into glucose 6-phosphate"` and second in the conversion of `color{violet}"fructose 6-phosphate"` to `color{violet}"fructose 1, 6-diphosphate"`.

● The `color{violet}"fructose 1, 6-diphosphate"` is split into `color{violet}"dihydroxyacetone phosphate"` and `color{violet}"3-phosphoglyceraldehyde (PGAL)"`.

● We find that there is one step where `NADH` + `H^+` is formed from `NAD^+;` this is when `color{violet}"3-phosphoglyceraldehyde "`(PGAL) is converted to `color{violet}"1, 3-bisphosphoglycerate"` (DPGA).

● `color{violet}"Two redox-equivalents"` are removed (in the form of two hydrogen atoms) from `color{violet}"PGAL"` and transferred to a `color{violet}"molecule"` of `NAD^+`.

● `color{violet}"PGAL is oxidised"` and with `color{violet}"inorganic phosphate"` to get converted into `color{violet}"DPGA"`.

● The conversion of `color{violet}"DPGA to 3-phosphoglyceric acid"` (PGA), is also an energy yielding process; this energy is trapped by the `color{violet}"formation of ATP."`

● Another `color{violet}"ATP is synthesized"` during the conversion of `color{violet}"PEP to pyruvic acid."`

● `color{violet}"Pyruvic acid"` is then the `color{Brown}"key product of glycolysis"`.

● The `color{violet}"metabolic fate of pyruvate"` depends on the `color{violet}"cellular need."`

● There are `color{violet}"three major ways"` in which different `color{violet}"cells handle pyruvic acid"` produced by `color{violet}"glycolysis"`.

● These are `color{Brown}"lactic acid fermentation"`, `color{Brown}"alcoholic fermentation"` and `color{Brown}"aerobic respiration"`.

● `color{Brown}"Fermentation"` takes place under `color{violet}"anaerobic conditions"` in many prokaryotes and unicellular eukaryotes.

● For the `color{violet}"complete oxidation of glucose"` to `CO_2` and `H_2O`, however, organisms adopt `color{violet}"Krebs’ cycle"` which is also called as `color{Brown}"aerobic respiration"`.

● This `color{violet}"requires"` `O_2` `color{violet}"supply."`