Chapter 2: CELLULAR RESPIRATION 2.2 The Details 2.3 Alternate Pathways

What type of reaction is cellular respiration? The overall chemical reaction for cellular respiration is: C6H12O6(aq)+6O2(g) → 6CO2(g) + 6H2O(l) What type of reaction is cellular respiration?

REDOX Reaction Oxygen is the oxidizing agent Hydrogen atoms carry electrons away from carbon atoms in glucose to oxygen atoms (forming water). Also, carbon atoms and oxygen atoms (left in glucose) reattach forming carbon dioxide. Decrease in potential energy and increase in entropy. This aerobic oxidation of glucose moves valence electrons to a lower free energy state in water and carbon dioxide.

Main Points Breaking down glucose to carbon dioxide. Want to trap as much E as possible that is being released in the form of ATP.

An Overview The entire process occurs in four stages and in three different places within the cell. Stage 1: Glycolysis – a 10-step process occurring in the cytoplasm Stage 2: Pyruvate Oxidation – a one-step process occurring in the mitochondrial matrix 

Stage 3: The Krebs Cycle (Tricarboxylic acid cycle, TCA) – an eight-step cyclical process occurring in the mitochondrial matrix Stage 4: Electron Transport (Oxidative phosphorylation) – a multi-step process occurring in the inner mitochondrial membrane

How do we get the products from glycolysis into the mitochondria?

Transport proteins Located on the cell membrane these proteins act as channels through the lipid bilayer to allow certain molecules through

The ultimate goal of cellular respiration is to extract energy from nutrient molecules and store it in a form that the cell can use: the primary energy transfer is from glucose to ATP.

How do we make ATP? 1.) Substrate level phosphorylation 2.) Oxidative phosphorylation

Substrate Level Phosphorylation ATP is formed directly by an enzyme- catalyzed reaction

A phosphate containing compound called phosphoenolpyruvate (PEP) gives a phosphate group to ADP (which then becomes ATP) This results in the transfer of 31 kJ/mol of potential energy Actual cell is 50 kJ/mol

Oxidative Phosphorylation ATP is formed indirectly using a diffusion force similar to osmosis

NAD (nicotinamide adenine dinucleotide) is reduced to NADH, and the remaining proton dissolves into the surrounding solution Proton pumps move protons through inner mitochondrial membrane using energy from excited electrons (from food)

Proton concentration (H+) outside membrane becomes high and protons will diffuse directly back into the matrix of the mitochondrion When this occurs, via special ATP synthase protein channel, ADP joins with P to form ATP

It is a series of redox reactions Weakest attractor of electrons first, then continually stronger Each component is ultimately reduced and then oxidized Baton It is more complex than substrate-level phosphorylation, so it gives more ATP molecules/glucose

Stage 1: Glycolysis The overall reaction is glucose being split into 2 pyruvate molecules Glucose + 2 ADP + 2 Pi + 2 NAD+ → 2 pyruvate + 2 ATP + 2(NADH + H+) NADH + H+  NADH

All 10 reactions occur in the cytoplasm; each step is catalyzed by a specific enzyme This is an anaerobic process – glycolysis does not require oxygen!

Important Things to Notice! 2 ATP are used Glucose is split into 2 G3P molecules Glyceraldehyde 3 phosphate Reduction of NAD+ by a hydrogen atom gives 2 NADH Removal of high energy phosphates gives 2 ATP's for each G3P Total of 4 ATP for glycolysis Net ATP is only 2!

Efficiency Glycolysis is not a highly efficient energy- harnessing process Each ATP represents a capture of 7.3 kcal so 14.6 total kcal captured out of a possible 686 kcal from one mole of glucose only 2% of total available energy is harvested This is because most energy is still trapped in the 2 NADH molecules and the 2 pyruvate molecules

Importance of Glycolysis Believed to be the earliest of all biochemical processes to evolve Requires no oxygen, occurs in cytoplasm but very little ATP output

If gylcolysis gives very little ATP, and we have evolved other ways of getting ATP, why is gylcolysis still around?

Part of our evolutionary history Cyanobacteria Way for cells to generate ATP in the absence of low oxygen levels

Regenerating NAD+ As long as glucose is present glycolysis will continue – that is until the NAD+ runs out and no more electrons can be accepted Therefore the NADH needs to be cycled back into NAD+ so that the cycle can continue.

How do we Regenerate NAD+? 1.) Oxidative Respiration = oxygen will accept the electron (H) and will form water (aerobic metabolism) 2.) Fermentation = another organic molecule accepts H atom because oxygen is not present (anaerobic metabolism)

1.) Oxidative Respiration Carries on where glycolysis left off Oxidation of Pyruvate – Stage 2 Oxidation of Acetyl-CoA (Krebs Cycle) – Stage 3

Lactic Acid fermentation Ethanol fermentation The acetaldehyde accepts the H from the NADH, and this gives us back NAD+ to complete the cycle. Lactic Acid fermentation H is added back to the pyruvate to form lactic acid and NAD+

Why might our muscles be exceptionally sore or weak right after an extreme workout?

When we work out to a point where we are not getting enough oxygen to our cells, our body starts to use lactic acid fermentation as a way of regenerating NAD+(anaerobic) Lactic acid is a by-product, and it can build up in our muscle cells, causing pain (sore muscles)

Stage 2: Pyruvate Oxidation The following is the overall reaction for this process: 2 pyruvate + 2NAD+ + 2 CoA → 2 acetyl-CoA + 2NADH + 2H+ + 2CO2

Pyruvate is a 3-carbon molecule 1.) The first carbon is removed via a decarboxylation reaction (uses the enzyme pyruvate decarboxylase) and CO2 is released as a waste product 2.) The remaining 2-carbon portion is oxidized by NAD+ and becomes an acetic acid group (CH3COOH)

Important Things to Notice NAD+ gains 2 hydrogen atoms from organic molecules of food to become NADH Pyruvate is oxidized and NAD+ is reduced Coenzyme A (CoA) is attached to the acetate group, forming acetyl-CoA Organic non-protein cofactor

2 molecules of acetyl-CoA enter the Krebs cycle This is where energy gets decided All nutrients are converted to Acetyl-CoA and then channelled to fat production or ATP production

Stage 3: Krebs Cycle The overall reaction for this process is: Oxaloacetate + acetyl-CoA + ADP + Pi + 3NAD+ + FAD → CoA + ATP + 3NADH + 3H+ + FADH2 + 2CO2 + oxaloacetate

This process is cyclic because oxaloacetate, the product of the final step, is the reactant in the first step. REMEMBER - for each glucose we make 2 pyruvate which then make two acetyl Co-A: therefore for each glucose processed the Krebs cycle happens twice!

Important Things to Notice Acetyl Co-A (2-Carbon) reacts with an existing molecule of oxaloacetate (4- Carbon) to make a 6-carbon molecule (citrate) and the Co-A enzyme is released Over the series of reactions, 2 CO2 are given off (this regenerates our 4 carbon oxaloacetate molecule)

FADH2 (flavin adenine dinucleotide) is another electron acceptor and it harvests electrons that are not in an excited a state (like the ones that are harvested by NAD+)

7 electrons are gained (3 NADH and 1 FADH2) and 1 ATP for each Krebs cycle NAD+ is reduced to NADH, FAD+ is reduced to FADH2 Remember to multiply by 2 because these numbers are only for one acetyl-CoA!

For a single glucose molecule, the Krebs cycle will produce: __________ ATP __________ NADH __________ FADH2

For a single glucose molecule, the Krebs cycle will produce: 2 ATP 6 NADH 2 FADH2

NADH and FADH2 are reduced co- enzymes; they store a lot of energy Most of the energy stored in NADH and FADH2 will be converted to ATP in the last stage of cellular respiration (Electron Transport Chain)

What has happened to the carbons? Glucose  6 carbons Glycolysis  Two 3 carbon molecules (pyruvate) Pyruvate Oxidation  Two 2 carbon acetyl-CoA and two carbon dioxides Krebs Cycle  Four carbon dioxides So all six carbon atoms of glucose have been oxidized to CO2

Stage 4: Electron Transport Chain

Components are arranged according to electronegativity The weakest component is first (NADH dehydrogenase) and the strongest is last (cytochrome oxidase complex) Each component in the chain is alternatively reduced (by gaining electrons from the previous component in the chain) and oxidized (by losing 2 electrons to the component after it in the chain)

NADH carries electrons to NADH dehyrogenase complex which drives the pumping of H+ protons across the inner mitochondrial membrane Cytochromes (e.g. protein Q) embedded within the membrane are carrier proteins that pass the electron along and in doing so pump MORE protons across the inner membrane

REMEMBER – all of the protons that are pumped out will diffuse back into the mitochondrial matrix across ATP synthetase to form ATP!

An electron can enter at any point on the chain, but you will get the most amount of H pumped (and therefore the most ATP) if the electron starts at the beginning of the chain (NADH dehydrogenase complex)

This is why NADH gives more ATP than FADH2

So how much ATP do we get from all this? Each NADH gives 3 ATP Except for the NADH in glycolysis! (it only gives 2 ATP because it takes 1 ATP/NADH to move the NADH from the cytoplasm to the mitochondrial matrix Each FADH2 gives 2 ATP

Pyruvate Oxidation 2 NADH 6 ATP Krebs Cycle 2 ATP Glycolysis 2 ATP 2 NADH 4 ATP Pyruvate Oxidation 2 NADH 6 ATP Krebs Cycle 2 ATP 6 NADH 18 ATP 2 FADH2 4 ATP Total: 36 ATP

Efficiency = 36 ATP x 7.3 kcal = 263 kcal Therefore 263 kcal/686 kcal = 38% efficiency compared to the 2% from strictly glycolysis.