Biology: Energy System

Chapter 5

Energy system 

Cellular respiration:  The process of oxidative breakdown of food materials within the cell to release energy and trap it in the form of ATP is called cellular respiration.

ATP as energy currency of cell: ATP is a renewable source of energy for the cell. It remains stored in the cell and powers all sort of cellular works- chemical, mechanical and transport as and when required. When ATP breaks down, large amount of energy is released. This energy is used in all cellular activities or reactions.

An ATP molecule has three components: Adenine, Ribose sugar and a chain of three phosphate groups. Most of the free energy is present in the triphosphate groups, especially in the last two covalent bonds. These bonds are known as high energy bonds. Energy stored in the high energy bonds is called biologically useful energy because it is readily available to the cell for its need.

Floating and protoplasmic respirations: respiration which uses carbohydrates or fats as respiratory substrate is termed as floating respiration, whereas that which uses proteins is called protoplasmic respiration.

Aerobic and anaerobic respirations:

Respiration is the process whereby organic products such as carbohydrates, lipids and proteins are oxidised to produce the energy carrier molecule, ATP. It can be done in the presence of O₂ where the process is then called AEROBIC RESPIRATION and in the absence of O₂ where the process is called ANAEROBIC RESPIRATION.

Glucose and the other organic products that we obtain from our diet, or that are plant manufactured are rich in chemical energy in the form of their C-C and C-H bonds. 

Therefore respiration is the extraction of this chemical energy from these bonds and conversion of it into an easily usable form,which is ATP. This is done in four main stages from the point that glucose is absorbed into the cell, whether it is obtained from the diet or storage (glycogen/starch), they are:-

1.      The conversion of glucose (6C) to 2 molecules of pyruvate or pyruvic acid (3C) via glycolysis, in the cytoplasm of the cell.

2.      The pyruvate is modified by losing CO₂ and being oxidised in a link reaction to produce acetyl-Coenzyme A.

3.      Then acetyl-CoA is fed into a cycle of reactions called Kreb’s cycle or the Tri-Carboxyllic Acid cycle (TCA cycle), in the mitochondrion of the cell, to produce reduced electron carriers (which cages the energy to be converted to ATP until it is passed to the next stage).

4.      The reduced electron carriers that were produced from glycolysis and the TCA cycle are passed along the electron transport chain where ATP is produced by a process called oxidative phosphorylation.

GLYCOLYSIS

·         Takes place in the cytoplasm of the cells of living organisms.

·         The net products of glycolysis are 2ATP and a reduced NAD (Nicotinamide Adenine Dinucleotide) or NADH.

·         ATP is used as the energy carrier molecule and NADH is used to produce more ATP by oxidative phosphorylation.

·         Firstly glucose enters the cell via facilitated diffusion and is activated to glucose-6-phosphate, by ATP. (This phosphorylation event increases the instability of the glucose molecule and keeps the glucose in the cell by preventing it from diffusing back out)

·         Then the atoms on this glucose-6-phosohate intermediate are rearranged to form fructose-6-phosphate by isomerisation process.

·         The fructose-6-phosphate molecule is then further phosphorylated to fructose-1,6-bisphosphate, again by ATP, which increases the instability of the intermediate. (phosphorylation events makes the C-C & C-H bonds in glucose more susceptible to breaking and releasing energy which would be captured and converted to ATP)

·         Fructose-1,6-bisphosphate is split into DHAP (DiHydroxyAcetonePhosphate) and glyceraldehyde-3-phosphate, only glyceraldehyde-3-phosphate is usable to continue because it is an aldehyde and only aldehydes can be further oxidised.

·         Glyceraldehyde-3-phosphate is then reduced by NAD to give NADH, glycerate-3-phosphate and ATP as the energy evolved during this stage is used to manufacture the ATP.

·         Glycerate-3-phosphate is then further converted to pyruvate upon the formation of another ATP molecule. Pyruvate is ready for use in the next stage, only in the presence of oxygen. Therefore glycolysis is referred to as the anaerobic stage of respiration.

·         As it is able to generate a net supply of 2ATP and the oxidised NAD is regenerated in the conversion of pyruvate to lactate in the presence of LDH (lactate dehydrogenase) so that glycolysis can continue.

THE LINK REACTION

·         Takes place in the matrix of the mitochondrion and pyruvate is converted to acetyl-CoA in the presence of oxygen, NAD and a Pyruvate DeHydrogenase (PDH) enzyme complex.

·         Firstly pyruvate is decarboxylated (CO₂ is lost)

·         Then it is oxidised and joined to Co-A acetyl-CoA.

·         The final overall reaction is called an oxidative decarboxylation reaction.

THE TCA CYCLE OR KREB’S CYCLE:

·         Occurs in the matrix of the mitochondrion and is linked directly to the machinery required for oxidative phosphorylation or the electron transport chain.

·         Acetyl-CoA enters the cycle and combines with a 4C compound called oxaloacetate to give a 6C derivative called Citrate.

·         Citrate is decarboxylated (removal of CO2) and oxidised (breaking of a C-H bond in acetyl-CoA) in the presence of NAD+ to generate a 5C derivative called α-ketoglutarate, NADH and CO2.

·         α-Ketoglutarate is again decarboxylated and reduced by FAD (Flavin Adenine Dinucleotide) and NAD+ to produce maleate, a 4C derivative, CO2, NADH and FADH2.

·         Maleate is the 4C derivative at the end of STEP 7, which means that the other intermediate 4C derivatives are SUCCINYL-CoA, SUCCINATE AND FUMARATE.

·         Maleate is then further oxidised in the presence of NAD+ back to oxaloacetate, such that the starting 4C compound is regenerated along with another NADH molecule

·         The net products per acetyl-CoA, per turn of the cycle = 1ATP, 1FADH₂, 2CO₂, 3NADH

The net products per glucose (2 acetyl-CoA), per turn of the cycle   = 2ATP, 2FADH, 4CO₂, 6NADH

RESPIRATORY CHAIN/ELECTRON TRANSPORT CHAIN MECHANISM OF OXIDATIVE PHOPHORYLATION.

Oxidative Phosphorylation in Mitochondria

Oxidative phosphorylation is the process by which NADH and FADH2 are oxidized and ATP is formed.  Respiratory Electron-transport Chain (ETC) is a series of enzyme complexes embedded in the inner mitochondrial membrane, which oxidize NADH and QH2. Oxidation energy is used to transport protons across the inner mitochondrial membrane, creating a proton gradient ATP synthase is an enzyme that uses the proton gradient energy to produce ATP

·         On the cristae are embedded enzymes (ATP synthase), electron/hydrogen carriers, and proton channels (H⁺ channels). This is so situated such that the reduced hydrogen carriers (NADH and FADH) which are produced in the matrix are made conveniently accessible to the ATP generating machinery on the cristae.

·         FADH and NADH provide the high energy electrons that were extracted from the chemical bonds of glucose, which are coupled to the production of ATP.

·         NADH enters the chain of electron carriers at a higher energy level than would FADH that is why it produces approximately 1 ATP more. Here NADH is oxidised firstly to NAD⁺ and an H⁺ ion is released to combine with O₂ to produce water, where in the process 1 ATP is generated. However the high energy electrons from NADH are passed on to the next electron carrier in the chain.

·         The next carrier is FAD such that it accepts the electron to become reduced as FADH or it enters already reduced as FADH, as the electrons are passed from one carrier to the next they constantly loose energy that is coupled to the production of ATP. The electrons from here are passed to the other carrier which is a cytochrome protein with an iron center. The Fe³⁺ becomes reduced then to Fe²⁺ which passes its electrons to cytochrome oxidase with a copper center again draining the energy from the electron that is coupled to the production of ATP.

·         The Cu²⁺ center of the cytochrome oxidase is reduced on the acceptation of the electron to a Cu¹⁺ again draining the energy of the electron to couple it to the formation of ATP. The Cu¹⁺ center is again oxidised when O₂ the final electron acceptor in the chain takes the electrons and combines it with H⁺ ions to produce water.

ATP PRODUCTION

Firstly ATP is generated by the ATPase (ATP synthase) enzyme complex ,found embedded on the inner mitochondrial membrane. Secondly H⁺ ions accumulate in the intermembrane space setting up an electrochemical gradient (high concentration of H⁺ in the intermembrane space compared to that on the other side of the inner membrane) .  The H⁺ ions accumulate because the NADH and FADH₂ are oxidised to (NAD⁺, H⁺, FAD⁺) in the ETC. These H⁺ ions pass across the inner membrane via proton channels and so provide the electrical potential that fires up the ATP synthase molecule (“a cellular motor engine that produces ATP (combining ADP + P_i) by rotating to the force of H⁺ ions diffusing back into the matrix”).

It consists of two major components:

F₀ or Base piece: forms the channel through which proton crosses the inner membrane to enter F₁ piece.

F₁ or Head piece: contains site for the synthesis of ATP

ALCOHOLIC FERMENTATION (YEAST)

• Itis a form of anaerobic respiration.
• Pyruvate is converted to ethanal  and CO₂ and the ethanal is further reduced to ethanol such that the NADH produced in glycolysis is oxidised back to NAD⁺ to go another round in glycolysis.

• This replenishes the NAD⁺ stores (which is limited) such that glycolysis is ensured to continue and the net production of 2 ATP.This ensures the net production of 2 ATP.
• This concept is used commercially and industrially in the manufacture of alcoholic beverages.

LACTATE FERMENTATION (ANIMALS) 

• It is a form of anaerobic respiration.
• Pyruvate in animals is converted to lactate by lactate dehydrogenase such that the NADH produced by glycolysis is oxidised back to NAD so that it can go another round in glycolysis.

The significance of Krebs cycle:-

1.      It brings about complete and final oxidation of 2-c acetyl Co-A in the aerobic respiration.

2.      It produces simple and harmless end products such as CO₂ and H₂O.

3.      It is responsible for the major share of energy release and supply during aerobic respiration (out of the total 38 ATP formed during aerobic respiration of glucose, kreb’s cycle accounts for 24 ATP).

4.      TCA cycle occurs in mitochondrial matrix in presence of oxygen. Thus, it takes place only in the aerobes.

5.      TCA cycle produces many important 4-c, 5-c and 6-c organic acids as the intermediates

a.       Acetyl CoA is used as a raw material for the synthesis of fatty acids, aromatic compounds, carotenoids etc.

b.      Succinyl CoA takes part in the synthesis of cytochrome, phytochrome and pyrrole ring of chromosome.

c.       Oxalo acetic acid is a raw material for the amino acid, aspartate.