Role Of ATP And Its Generation In Heterotrophic Metabolism

Role of ATP in Cell

The introductory part of ATP has been already discussed in the previous note. Now we are here to discuss how ATP is helpful for living cells. Some of the vital roles of ATP have been pointed below:

• Without ATP the anabolic reactions within a cell are impossible. Synthesis of macromolecules from simple precursors in a cell require energy and ATP plays that role in providing sufficient amount of energy for formation of macromolecules. During this process, the terminal phosphate group/s from ATP are transferred enzymatically to the precursor molecule and thus becomes energetic and will have sufficient energy to get assembled to other precursors (energized) to form macromolecules.
• Every performance of muscular tissues like contraction and relaxation is possible due to the role of ATP. In involuntary muscle tissues like cardiac muscles, the movement is energized by the ultimate action of ATP.
• Some molecules required for cells need to be transported through the semi-permeable plasma membrane need ATP for transportation. Such ATP-requiring transportation is called active transportation.
• For the biosynthesis of nucleic acids, there is the involvement of ATP. During DNA replication, ATP is required for unwinding of double strands thus providing the area for replication.
• ATP is principally used in protein synthesis process called translation. Basically, it is required to activate the tRNA to make aminoacyl-tRNA.

Amino acid + tRNA + ATP → → aminoacyl tRNA+ ADP+ PPi

Heterotrophic Generation of ATP in Various Ways

We have already discussed before how heterotrophic organism get food from other organisms (autotrophs mainly). Now we are going to discuss how the obtained food materials provide us ATP. Food that heterotrophs consume usually contains carbohydrate, lipid or protein as major constituents that need to get metabolized. So for each such macromolecules, there are Different pathways for ATP generations. Here we will discuss the detail process of each of them. In every molecule, while they are metabolized they step out following two major ways of ATP synthesis in heterotrophs:

• Substrate-level phosphorylation
• Oxidative phosphorylation

In the case of the autotrophs, photophosphorylation is also discussed as an equally important pathway.

Substrate level phosphorylation involves the synthesis of ATP directly from the substrate involved in metabolism, without the help of electron transport chain, a very complicated chain reaction. For example: In carbohydrate metabolism 1 molecule of ATP is synthesized from phosphoenol pyruvate and it gets converted to pyruvate.

Pyruvate kinase

Phosphoenolpyruvate +ADP → Pyruvate+ATP

Oxidative phosphorylation, on the other hand, involves the synthesis of ATP via Electron Transport Chain (ETC). In Electron Transport Chain, the electron acceptors (oxidizing molecules) are transferred the electrons by the redox reaction. The electron rich molecules produced at substrate-level phosphorylation like NADH + H⁺, FADH₂, provide electrons for ETC. NADH provides electrons thus yielding 3 ATPs whereas that from FADH₂ yields 2 ATPs. WE will discuss oxidative phosphorylation taking the reference of electron transport chain after we complete the chain reactions of substrate level phosphorylation.

Carbohydrate metabolism:

Carbohydrates are the macromolecules, generally, hydrates of aldehydes and ketones. Carbohydrates can be monosaccharides, oligosaccharides or polysaccharides. Consumed food by heterotrophs (organisms) can be in any forms (may be mono, oligo or polymeric form). Before the entry of carbohydrates in stepwise metabolism, they should be hydrolyzed in monomeric form. While digestion different enzymes are utilized for the hydrolysis of oligosaccharides and polysaccharides to monosaccharides that may utilize ATP as an energy source. Mostly glucose yielding food sources are selected by organisms as it is the universal power source for entire lives. Glucose thus obtained is broken down and by the series of enzymatic reactions, ATPs can be generated as energy-rich molecules. The following diagram helps us to make ourselves clear about the general idea of carbohydrate metabolism.

Substrate Level Phosphorylation of Glucose

It is carried out in different phases. Glucose molecule which is eventually obtained by the hydrolysis of poly and oligosaccharides undergo glycolysis. Glycolysis is none other than the term defined by the enzymatic breakdown of glucose to yield two molecules of pyruvates from each glucose molecule. Then pyruvate thus obtained may undergo different pathways for the further yield of ATPs. Pyruvate is a very versatile molecule that can undergo aerobic as well as anaerobic pathways. As per anaerobic pathway, pyruvate can take part in fermentation giving alcohol and carbon dioxide as byproducts. Whereas TCA cycle can be chosen as per aerobic pathway.

Glycolysis:

Glycolysis is the primary metabolic pathway of every organism which deals with the breakdown of glucose molecules following different subpathways. D-glucose is selected as the major fuel molecule that occupies the central position in metabolism for the most organisms. This molecule is relatively rich in potential energy which is consumed by organisms in the form of ATP after complete oxidation of it into CO₂ and H₂o. Glucose is found stored in the form of glycogen in animals and in the form of starch in plants. At the sudden demand of energy, these polymers are hydrolyzed into monomer i.e. D-glucose and is diffused inside the cells and is broken down either aerobically or anaerobically to yield ATP.

6 carbon molecule, glucose in glycolysis is degenerated in series of enzyme-catalyzed that finally produces two pyruvates (3 carbon molecules each). During the glycolysis, there is the investment as well as yield of ATP. A reducing potential, NADH+H⁺ is also obtained which gets involved in oxidative phosphorylation and yields three molecules of ATP. Glycolysis is thus considered as the universal pathway of glucose metabolism.

Glycolysis follows different sub-pathway for the breakdown of glucose molecules depending upon the nature of organism or the condition an organism faces. EMP pathway (Embden-Meyerhof- parnas ) HMP pathway (Hexose Monophosphate ), ED pathway (Entner Doudoroff ) are the different subpathways followed of glucose during glycolysis.

(Note: Most of the students get confusion in glycolysis thinking that glycolysis and EMP pathways are same but not in fact. Remember! EMP pathway is only a subpathway of glycolysis.)

So, at first, we should be cleared about the actual meaning of glycolysis before discussing the steps involved in it. Generally, steps of glycolysis are studied on the basis of steps involved in EMP pathway as it is most common sub-pathway of glycolysis. In most of the books, it can be found that glycolysis and EMP pathway are same terms, but remember, EMP pathway is just one of the glycolytic pathway and is most common one.