Retrieving Glucose from other Molecules:

Mainly respiration focuses on what occurs to glucose, maybe as glucose metabolism is similar in all organisms. Though, glucose is generally not abundant in cells. Respiration starts by converting storage compounds differ from one organism to another, or in plant from one organ to another. For instance, humans and other vertebrates, potatoes and bananas store starch, store polysaccharide glycogen and onions store polymers of fructose. Also, sucrose is common starter molecule for respiration in leaves, but in same plant, respiration may start with starch in non-green storage organs like roots and stems.

Retrieval from Sucrose:

Sucrose, plentiful in sugarcane stems and sugar beet roots, is common in plants and is commercially significant as man eat so much of it as table sugar. In addition being common energy source in photosynthesis cells, sucrose is carbohydrate which moves most eagerly through conducting cells to growing tissues. Transfer of phosphate is catalysed by the enzyme which binds substrate and ADP. Energy of substrate-level phosphorylation comes from phosphate having substrate. In spite of of which substrate is utilized, removal of phosphate fructose should release more energy than essential for making ATP from ADP. For instance removal of phosphate from phosphoendpyruvic acid (PEP) releases 14.8 kcal mol4 this is more than sufficient energy to phosphorylate ADP, that needs 7.3 kcal mol. The remaining energy (7.8kcal mol4) is lost as heat. Second way that ATP is prepared engages coupling energy from electron donor to electrochemical gradient which spans inner mitochondria membrane. Electron donor is generally NADH2 molecule. Electrons from NADH are passed by chain of electron carriers by the series of oxidation reduction reactions. Energy from the movement of electrons is utilized by proton pumps to make proton electrochemical gradient which fuels phosphorylation of ADP to ATP. Overall process is known as oxidative phosphorylation. Membranes play the significant role in coupling energy between electron transport chain and phosphorylation of ADP. This energy coupling is generally referred to as chemoismosis as it engages chemical reactions and transport across membranes. Chemoismosis is utilized to make ATP in chloroplasts in photosynthesis. Of two ways that ATP is prepared in respiration, substrate-level phosphorylation is simpler, but it accounts for only small percentage of ATP synthesis. Most ATP is prepared from oxidative phosphorylation.

Glycolysis:

Potential Energy of Glucose:

Glucose has 686 kcal mol4. This signifies that with addition of some heat (that is energy of activation -0 Eact) to get it going one mole of glucose will give 686,000 calories when it is entirely oxidized to CO2 and H2O. Though, all cells recover only small amount of the energy in glycolysis, rest is lost as heat. As energy is released in small increment, excess heat doesn't damage cells.

Breakdown of Glucose to Pyruvic Acid:

In glycolysis, glucose is split in two three-carbon compounds. Entire process needs ten steps, all of which happen in cytosol.

The Krebs cycle:

Pyruvic acid which is transported in mitochondrion is not utilized in Krebs cycle directly. Instead, pyruvic acid first loses molecule of carbon dioxide. Remaining acetyl group is joined to coenzyme to form acetyl coenzyme A (acetyl 2-CoA) outlines conversion of pyruvic acid to acetyl-CoA in mitochondrion when carbon dioxide is released from pyruvic acid, NAD is reduced to NADH.

Glycolysis and Krebs cycle are joined by conversion of pyruvic acid to acetyl CoA. Pyruvic acid is final product of glycolysis, and acety-CoA is compound which enters Krebs cycle.

Step in Krebs cycle:

Krebs cycle is a cycle as last segment regenerates started chemicals for first step. In all, there are eight steps, seven of which happen in mitochondria matrix. Step 6 happens, in mitochondria membrane. In first step, coenzyme A is cleaved from acetyl-CoA and acetyl group is linked to oxaloacetic acid, that has four carbons. Resulting 6 carbon compound is citric acid. Krebs cycle is also called as citric-acid cycle as it starts with citric acid; or tricarboxylic acid cycle as citric acid has 3 carboxyl groups. Citric acid is then rearranged to isocitric acid (step 2), that is starting substrate for two-oxidative steps which entail removal of two molecules of carbon dioxide. Steps also reduce two molecules of NAD+ to NADH. In first oxidative step, to form isocitric acid (step 3) the removal of carbon dioxide yields alpha-keto and glutaric acid. In second oxidative step (step 4), carbon dioxide is removed from alphaketoglutaric acid, and product is bound to the coenzyme A to make succinyl -CoA. Every step also reduces the molecule of NAD to NADH. Coenzyme A is then cleaved from succinyl -CoA, thus producing succinic acid and giving energy for substrate level phosphorylation of ADP to ATP (step 5). Following formation of succinic acid, 3 more oxidative steps happen (step 6-8) succinic acid is oxidized to fumaric acid, fumaric acid to malic acid, and malic acid to oxaloacetic acid. Oxidation of succinic acid also reduces electron carrier ubiquinone to ubiquinol oxidation of malic acid to oxaloacetic acid reduces NAD to NADH. Therefore for every acetyl-CoA which enters Krebs cycle, only one ATP is prepared by substrate level phosphorylation. Most of the energy derived from oxidative steps of Krebs cycle is accumulated in high-energy derived from oxidative steps of Krebs cycle is stored in high energy electrons of NADH and ubiquinol. Energy in the molecules is harvested in third stage of respiration, oxidative phosphorylation.

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