Introduction

In this essay, I will demonstrate my understanding of the generation of ATP, metabolic pathways and oxidative phosphorylation. The structure I will write is as it follows, and I will include a labelled diagram to support each of these explanations:

•         The generation of ATP in metabolism

•         The term metabolic pathway and how these pathways are regulated.

•         The significance of the tricarboxylic acid cycle in generating ATP

Adenosine Triphosphate or ATP is known as the energy currency of a cell due to their vital role in metabolism, particularly in energy transfer within cells.  Multiple processes are metabolic reactions which use ATP to transport chemical energy, including:

•         Aerobic respiration

•         Fermentation

•         Cellular division

•         Photophosphorylation

•         Protein synthesis

•         Photosynthesis

(Helmenstine, Anne Marie, Ph.D., 2019).

Adenosine triphosphate (ATP) consists of three phosphate groups connected to adenine which is attached to 9-nitrogen atoms to 1’ carbon of sugar (ribose) (Bonora, M., Patergnani, S., Rimessi, A. et al., 2012), (Helmenstine, Anne Marie, Ph.D., 2019). In metabolic reactions, the available energy contains the bonds between the phosphates that releases when they are broken.  This normally occurs in the process known as hydrolysis.  The adenine and sugar groups are unchanged; however, the outer phosphate is removed from ATP produce energy. So, the triphosphate is converted to diphosphate and monophosphate also known as their derivatives ADP and AMP (Helmenstine, Anne Marie, Ph.D., 2019). For cellular processes, the ATP is continuously broken down by cells to obtain energy, and it is constantly synthesised from ADP and phosphate in the processes known as cellular respiration. The enzyme ATP synthase located in the mitochondria of cells and also found in chloroplasts in plant cells; produces most of the ATP in cells, by converting ADP and phosphate to ATP (Helmenstine, Anne Marie, Ph.D., 2019).

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There are two main types of metabolic pathways, characterised by its ability to either synthesise molecules in order to utilise energy (anabolic pathways) or breaking down molecules to release energy (catabolic pathway). Both pathways complement one another in that the energy released from one is used up by the other. These pathways are vital for the maintenance of homeostasis of an organism and often regulated by feedback inhibition (Encyclopedia Britannica, 2018), (Helmenstine, Anne Marie, Ph.D. 2018). 

The catabolic pathway also is known as (catabolism) is a complex series of reactions that produce a net which releases energy in the form of a high energy phosphate bonded formed with the energy carriers adenosine diphosphate and guanosine diphosphate to produce ATP AND GTP. A catabolic pathway is an exergonic system that provides energy in the form of ATP GTP, NADH etc from sources such as carbohydrates, fats and proteins (Encyclopedia Britannica, 2018), (Helmenstine, Anne Marie, Ph.D. 2018).

Within catabolism, there are three phases;

•    Phase 1: The large molecules of food are broken down into small constituent units for the energy-releasing processes.

•    Phase 2- the small molecules which are produced in phase 1 is incompletely oxidised. The end products are Co2, water, and one of these possible substances: two carbon-compound acetates, the four carbon-compound oxaloacetates and the five-carbon compound a-oxoglutarate (Encyclopedia Britannica, 2018). (Encyclopedia Britannica, 2018).

•    Phase 3- In this phase, the products from phase 2 go through with complete oxidation in a cyclic sequence of chemical reactions known as either the tricarboxylic acid (TCA) cycle or Krebs cycle discovered by Sir Hans Krebs (Encyclopedia Britannica, 2018). The cycle is initiated by the formation of citrate with six carbon compounds. It transfers a two-carbon acetyl group to the four carbon-compound oxaloacetates to form the six-carbon compound citrate. The available energy made by the oxidative steps of the cycle is transported as energy-rich electors to NAD+, forming NADH. Additionally, the electors from the succinate oxidation step are also transported to FAD cofactor of succinate dehydrogenase first, reducing it to FADH2. As a result, every NADH and FADH2 produced in the cycle, ATP molecules are generated in oxidative phosphorylation. The TCA cycle is a vital role in the catabolism of organic molecules.

The NADH and FADH2 that is formed in the TCA cycle, glycolysis and fatty acid oxidation are rich in energy molecules containing pairs of electrons which have a high transfer potential. A high amount of energy is released when the electrons are used to reduced molecular oxygen to water and used to generate ATP. This process is known as oxidative phosphorylation; the process when ATP is formed using the transfer of electrons from NADH or FADH2 to O2.  This process happens in the mitochondria where a flow of electrons from NADH or FADH2 via protein complexes leads to protons been pumped out of the mitochondrial matrix. This leads to an uneven distribution of protons which generates A pH gradient. ATP is synthesised when protons are pumped back to the mitochondrial matrix through an enzyme complex.   

(Encyclopedia Britannica, 2018).

In contrast to catabolism, anabolism is a biosynthetic pathway; it is the combination of smaller molecules to form larger and more complex ones. Anabolic reactions are endergonic; meaning it does not happen spontaneously and an input of energy is required. The coupling of catabolic and anabolic reactions is essential, where catabolism provides the energy for anabolism (Helmenstine, Anne Marie, Ph.D. 2018).  The reduced agents such as NADH, NADPH and FADH2 as well as metal ions act as cofactors in many steps in anabolic pathways.

Another primary metabolic process is glycolysis. It is a pathway that converts glucose into pyruvate. The free energy released during the breakdown of glucose and other organic molecules is used to form or stored as ATP (Khan Academy), (Encyclopedia Britannica, 2018). The compound NAD+ is converted to NADH.  Glycolysis can be broken down into phases.

•         The energy-requiring phase: When starting glucose molecule gets rearranged, and two phosphate groups are attached to it. A phosphate group is transferred from ATP to fructose-6-phosphate which produces fructose-1, 6-biphosphate. It then splits to form two three-carbon sugars (DHAP) and glyceraldehyde-3-phosphate. Two ATP is used in this phase (Khan Academy).

•         The energy releasing phase: Each 3-carbon sugar is converted into a three-carbon molecule, pyruvate via a series of reactions. Four ATP molecules and two NADH molecules are the exclusive products of this process. Since a glucose molecule splits into two three-carbon molecules, therefore these reactions happen twice through the pathway. One crucial enzyme in the regulation of glycolysis is phosphofructokinase. Phosphofructokinase speeds up or slows down glycolysis depending on the needs of the cell. (Khan Academy).

(Encyclopedia Britannica, 2018).

Bibliography

•         Bonora, M., Patergnani, S., Rimessi, A. et al. Purinergic Signalling (2012) 8: 343. https://doi.org/10.1007/s11302-012-9305-8

•         Helmenstine, Anne Marie, Ph.D. “What You Need To Know About Adenosine Triphosphate or ATP.” ThoughtCo, Mar. 21, 2019, thoughtco.com/atp-important-molecule-in-metabolism-4050962.

•         Encyclopedia Britannica. (2018). Metabolism – The carrier of chemical energy. [online] Available at: https://www.britannica.com/science/metabolism/The-carrier-of-chemical-energy [Accessed 27 Apr. 2019].

•         Encyclopedia Britannica. (2018). Metabolism – Incomplete oxidation. [online] Available at: https://www.britannica.com/science/metabolism/Incomplete-oxidation [Accessed 27 Apr. 2019].

•         Encyclopedia Britannica. (2018). Metabolism – The study of metabolic pathways. [online] Available at: https://www.britannica.com/science/metabolism/The-study-of-metabolic-pathways#ref52084 [Accessed 27 Apr. 2019].

•         Encyclopedia Britannica. (2018). cellular respiration | Process & Products. [online] Available at: https://www.britannica.com/science/cellular-respiration#ref338913 [Accessed 27 Apr. 2019].

•         Helmenstine, Anne Marie, Ph.D. “Anabolism and Catabolism: Definition and Examples.” ThoughtCo, Nov. 12, 2018, thoughtco.com/anabolism-catabolism-definition-examples-4178390.

•         Khan Academy. (n.d.). Glycolysis. [online] Available at: https://www.khanacademy.org/science/biology/cellular-respiration-and-fermentation/glycolysis/a/glycolysis [Accessed 27 Apr. 2019].