Cellular Respiration

cellular respiration as a redox reaction

involves transfer of electrons and hydrogen from one substance to another

oxidation reaction

loss of electrons and hydrogen atoms, release energy ex. C6H12O6 → 6CO2

reduction reaction

gain of electrons and hydrogen atoms, gain energy ex. 6O2 → 6H2O

electron carriers

NAD+ (nicotinamide adenine dinucleotide) and FAD (flavine adenine dinucleotide) gradually and over multiple steps oxidize glucose by accepting electrons and hydrogen (becoming reduced) from glucose during cell respiration

role of NAD as a carrier of Hydrogen

reduction of NAD occurs due to NAD accepting atoms of hydrogen

• glucose oxidized when hydrogen atoms removed

• each hydrogen atom consists of one electron and one proton, NAD+ accepts 2H+ and 2e- and is reduced

• NAD+ + 2H+ + 2e- → NADH + H+

role of FAD as a carrier of Hydrogen

FAD → FADH2

• FAD reduced

stages of cellular respiration

1. glycolysis

2. link reaction

3. Krebs/citric acid cycle

4. electron transport chain and chemiosmosis

glycolysis

conversion of glucose to pyruvate

• preparatory phase

• payoff phase

preparatory phase of glycolysis

phosphorylation of glucose, conversion to glyceraldehyde 3-phosphate

• makes molecule less stable, lowers activation energy for subsequent splitting to pyruvate

• prevents phosphorylated molecule from being transported through the cell membrane (not recognized by protein pumps)

• energy released in hydrolysis of ATP used for attachment of PO₄^3-

phosphorylation

addition of a PO₄^3- group

payoff phase of glycolysis

oxidative conversion of glyceraldehyde 3-phosphate to pyruvate and the coupled formation of ATP and NADH, net production of 2 ATP

uses of pyruvate

anaerobic respiration (alcoholic fermentation, lactic acid fermentation) and aerobic respiration depending on oxygen availability

anaerobic respiration

• NADH + H+ donates its H+ and e- to pyruvate during ethanol or lactate production, respectively

• NAD+ becomes available again so that e- and H+ can continue to be transferred from glucose to NAD+ when oxidized

• thus pyruvate synthesis during glycolysis continues

alcoholic fermentation

C6H12O6 → 2 pyruvate → 2 Ethanol + 2CO2 (net 2 ATP)

• in yeast (eukaryotic cell) and some bacteria (prokaryotic cell)

• cytoplasm

yeast as an example of anaerobic cell respiration

facultative anaerobe single-celled fungus, can respire aerobically or anaerobically

• breaks down starch and sugars in dough by alcoholic fermentation to make CO2 and ethanol

• CO2 released in fermentation trapped in dough, causing bread to rise

• bread baked in oven to kill yeast and trap ethanol

lactic acid fermentation

C6H12O6 → 2 Lactate (net 2 ATP)

• some bacteria (prokaryotic cell) and some mammals

• cytoplasm

aerobic respiration

C6H12O6 → 6CO2 + 6H2O (net 32-34 ATP)

before the link reaction

pyruvate/pyruvic acid enters mitochondrial matrix by facilitated diffusion to be processed further

link reaction

1. decarboxylation of pyruvate to acetate

   1. CO2 leaves cell, diffuses into the bloodstream and is expired

2. binding of enzyme CoA to acetate

3. oxidation of acetate into acetyl-coenzyme A

Krebs/citric acid cycle

oxidation and decarboxylation of acetyl groups

• acetyl groups (2C) fed into cycle by transfer from coenzme A to oxaloacetate (4C) → citrate (6C)

• citrate converted back to oxaloacetate by enzyme-catalyzed reactions, two carbons lost through decarboxylation reactions, producing waste product CO2 - each carboxylation reaction paired with reduction of NAD+

• 4 oxidation reactions release energy, mainly held in electrons removed from oxidation and transferred through reduction of NAD+ and FAD

change in free energy graph in electron transport chain

in each drop, energy is transferred to energy-storing molecules NAD+ and FAD, which later become oxidized again in electron transport chain

• energy gained from oxidation reaction used to make ATP

transfer of energy to the electron transport chain

electron carriers NAD+ and FAD bring electrons and hydrogen ions to electron transport chain in cristae of the mitochondria

electron transport chain

• reduced electron carriers NADH + H+ and FADH2 from glycolysis and Krebs cycle move to inner mitochondrial membrane

• membrane proteins accept electrons from NADH and FADH2

• each carrier in chain has slightly higher electronegativity and therefore a stronger attraction for electrons than previous carrier

• electrons passed down an energy gradient until they reach the end of the chain

• electrons “fall” from higher levels to lower ones, energy released used to pump protons from matrix into intermembrane space against the concentration gradient

• proton gradient drives chemiosmosis

membrane proteins in electron transport chain

integral carrier and channel proteins embedded within phospholipid bilayer have a high tendency to become reduced by accepting electron

chemiosmosis

• transfer of H+ sets up concentration gradient across the membrane as H+ accumulates in the intermembrane space

• protons follow natural concentration gradient by moving through the ATP synthase

• oxidative phosphorylation

• oxygen as the terminal electron acceptor

oxidative phosphorylation in between chemiosmosis and ATP synthesis

movement of protons through ATP synthase releases energy used to phosphorylate ADP to ATP

ATP synthase

complex of integral proteins located in the mitochondrial inner membrane where it catalyzes the synthesis of ATP from ADP and phosphate, driven by a flow of protons

role of oxygen as terminal electron acceptor

in the reduction of the oxygen molecule, the O2 accepts electrons and forms a covalent bond with hydrogen to produce H2O

• in the matrix, H+ combines with ½ O2 + 2e- to form water

carbohydrates as respiratory substances

simple sugars (glucose or fructose) can be used straight away in glycolysis and anaerobic respiration

lipids as respiratory substances

lipids broken down into glycerol and fatty acids, fatty acids converted to acetyl groups to be used in Krebs cycle