All organisms metabolize substances to sustain cellular processes by: breaking it down and synthesizing macromolecules from its precursors
Catabolic pathways are exergonic (release energy) and endergonic (require energy).
ATP provides the energy needed for chemical reactions that require energy input.
The catabolic process is the breakdown of complex molecules into simpler ones.
Organisms differ in the pathway that they undergo and very dependent on: functions or cells and requirements of the pathway
Carbon fixation is the conversion of carbon dioxide into organic compounds, which occurs through photosynthesis.
Anaerobes: does not require oxygen gas for all pathways they undergo
Aerobes: requires oxygen gas for all its major catabolic pathways
Respiratory Processes: maximize energy yield by oxidizing metabolites and electrontransfer
Catabolism: process of breaking down macromolecules to small precursors with a concominant release of energy
Most cells obtain energy from one two major catabolic reactions such as cellular respiration and beta-oxidation
Cellular Respiration: central catabolic pathway for all organisms
Anaerobic Stage: Glycolysis
A series of 10 enzyme catalyzed reactions that breaksdown a glucosemolecule into two pyruvatemolecules
Glycolysis is the only stage of cellular respiration that does not require O2
Oxidative or Mitochondrial Stage
The main energy producing pathways of the body
Includes intermediate phase and krebs cycle
Intermediate Phase: Pyruvate oxidized by NAD+ to form Acetyl CoA, which enters Krebs Cycle
Krebs Cycle (Citric acid cycle): A series of chemical reactions that occur within mitochondria
Beta Oxidation: The breakdown of fatty acids to produce acetyl coenzyme A
Electron Transport Chain: The final step of aerobic metabolism where electron carriers pass their electrons through a chain of proteins embedded in the inner mitochondrial membrane
Fatty acids are broken down through beta-oxidation to release their stored energy.
Breaks down glucose into smaller molecules that can pass through the mitochondrial membrane to allow for its complete oxidation or provide precursors for other pathway or processes
Also it provides energy for completely anaerobic processes
Glycolysis: a series of 10 enzyme catalyzed reactions by which glucose (6C) is oxidized to two molecules of pyruvate (3C)
Occurs in the cytosol of the cell
EnergyInvesmentPhase (Step1to5): wherein two ATP are used to convert glucose into fructose-1,6-bisphosphate and sub-sequentially cleaved into
dihydroxyacetone phosphate (DHAP) and glyceraldehyde-3-phosphate (G-3-P).
Energygenerationphase (Step6-10): wherein twoglyceraldehyde-3-phosphate (G-3-P) molecules are converted to pyruvate in a series of steps, with the formation of 4ATPs and 2NADHs.
GylcolysisGlucose-6-phosphate is the activatedform of glucose for metabolism in the cell. Without such reaction, glucosecannot be metabolized. In the liver and adiposetissues, this reaction is mediated by the hormoneinsulin which mediates the action of kinase (the enzymecatalyzing this reaction). The conversion of glucose-6-phosphatedrives the absorption of glucose into
the liver.
The formation of fructose-1,6-bisphosphate is the committedstep in glycolysis, such that once this is formed, there in no
turningback in the reaction.
Pyruvate is the end product of glycolysis, the anaerobic phase of cellular respiration.
undergoseveralalternativeroutes which include
the conversion of pyruvate to lactate in musclefibers when oxygenlevel is verylow, happens during very high muscular
activity;
conversion of pyruvate to acetaldehyde then to ethanol (or conversion to acids) during fermentationreactions in anaerobicorganisms
conversion to acetylCoA in cells containing mitochondria, or when oxygenlevel is high
LactateFermentation
anaerobicprocess, the NADHproduced is reconverted to NAD+ by convertingpyruvate to lactate, or to ethanol
this reactionensures the continuity of the glycolyticprocess because NAD+ is regenerated for the reaction which oxidizes glyceraldehyde-3-phosphate to 1,3-bisphosphoglycerate.
Cori Cycle: liver is its main maetabolicorgan
Intermediate Step
In cells containing mitochondria, or when oxygen level is high in aerobic organisms, pyruvateenters the mitochondria and is converted to acetyl CoA
Firstoxidationreaction, releasing a carbon in the form of CO2 from pyruvate, the product (an acetylgroup) is condensed with a carriermolecule, coenzyme A, producing acetyl CoA.
This reaction is the intermediate phase in cellular respiration and commitspyruvate to the complete oxidation in the aerobicphase of cellular respiration (Krebs cycle).
NADH will be oxidized back to NAD+ in the electron transport chain, with the production of 2.5 ATPmolecules.
There are 2 NADHmoleculesproduced from 2pyruvatemolecules, thus the netyield from oneglucose in the intermediatephase is 5ATPmolecules.
Citric Acid Cylce: tricarboxylic acidcycle ehich takes place in the mitochondria
The aerobic phase of cellular respiration, which oxidizes the remaining two carbons of pyruvate (in the form of acetyl CoA) and thus completes the oxidation of glucose
The fuel is the two-carbon acetyl group of acetyl CoA, with each turn of the cycle two carbons are released as CO2
The firststep is the condensation of acetylCoA with oxaloacetate to form citrate (or citric acid), after a series of 8steps in the cycle, oxaloacetate is regenerated in the process (step 8).
This reaction ensures the continuity of the aerobic phase of cellular respiration.
Reducedcofactors will be re-oxidized in the ETC.
OxidativePhosphorylation: Electron TransportChain
The electrontransportchain (ETC) facilitates the passage of energy trapped in FADH2 and NADH during oxidationsteps in glycolysis and citricacidcycle
ETC is a series of biochemicalreactions in which intermediatecarriers (protein and non-protein) aid the transfer of electrons and hydrogenionsfromNADH and FADH2 (oxidized) releasingadditionalenergy
The energy is stored in a protongradientpumped at the intermembranespace which is then used to synthesizeATP
The ultimatelyreceiver of electrons is oxygen, O2
Peter Mitchellproposed the chemiosmotictheory
Energy-releasingoxidations lead to protonpumping and create a pHgradientacross the inner mitochondrialmembrane
There is a higherconcentration of H+ in the intermembranespace than inside the mitochondria
Theproton gradient provides the drivingforce for protons to move back into the mitochondrion through a transport protein enzyme called ATP synthase
The storedenergy in the H+ gradient is used to chemicallysynthesizeATP, known as oxidativephosphorylationsince the ATPgenerated is coupled to oxidation
Approximately4 H+ ions are needed to produce1 ATPmolecule
Cytoplasmic NADH from Glycolysis
Not converted to NAD+ because there is no receiver of electrons in the cytoplasm
To regenerate NAD+, requires that the electrons of NADH must be transported into the mitochondria through electron transport chain
Mitochondrial membrane is impermeable to NADH, NAD+ not electrons thus, electrons must be passed on to the receivers and transported via shuttle systems