Chapter 9: Cellular Respiration

AP Biology

Oxidation and Reduction

• E is gained by the transfer of e’s• The relocation of e-’s releases the

stored E and the E ultimately makes ATP

Redox Reactions

• Oxidation: the loss of e-’s from a substance

• Reduction: the gain of e-’s by a substance

• E must be added to pull e-’s from an atom

Redox Reactions

• A redox reaction that relocates an e- from a less electronegative atom to a more electronegative atom loses potential E

Hydrogen

• In most redox reactions, it is not only the e- that is transferred but in most biological reactions the whole hydrogen atom is transferred

Carbohydrates and Fats

• Contain high levels of hydrogen and their electrons

• There is a barrier that keeps sugar from combining immediately with O2

• This barrier is reduced inside the body with the help of enzymes

Cellular Respiration

C6H12O6 + 6O2 6H2O + 6CO2 + E

Introduction

• Cellular Respiration is a catabolic pathway.–Does not directly perform

cellular work???

–Redox reactions!!

–Importance of Hydrogen??

Introduction

• Does glucose react instantaneously? Spontaneously?

• Activation barrier/ Enzymes(lots of them)– Lowers activation energy

– Allows breakdown to proceed spontaneously

Cellular Respiration• What is a potential problem with the

breakdown of glucose as a spontaneous reaction??

– Hint: TNT, Gasoline• How does the cell prevent this problem?

– Does not release energy all at once– Multi- step process catalyzed by specific enzymes

Cellular Respiration• With each step, electrons are released with a

proton (hydrogen atom)

• Each hydrogen is transferred to a coenzyme (NAD+)

• And eventually to Oxygen

Nicotinamide Adenine Dinucleotide• NAD+

• Derivative of the vitamin niacin

• Coenzyme– Oxidizing agent

• Will be reduced to NADH– (2 electrons and Hydrogen)– Dehydrogenase-removes two hydrogen from

substrate

Nicotinamide Adenine Dinucleotide

Glycolysis: • Occurs in the cytoplasm

• Net gain of 2 ATP and 2 NADH

• Start: 6 carbon glucose

• End: 2- 3 carbon pyruvate molecules

• Ten steps- two phases– Energy investment phase– Energy payoff phase

Glycolysis: • Energy investment stage:

• Glucose phosphorylated by ATP(2)– Unstable– Splits

• Each 3 carbon molecule is phosphorylated again– Inorganic phosphate comes from cytosol not ATP

Glycolysis: • Energy Payoff Phase:

• Each 3 carbon molecule reduces NAD+ to NADH

• Each 3 carbon molecule gives up its 2 phosphates to ADP to form 4 ATP.

Glycolysis: Summary

• Reactants: – Glucose

– NAD+

– ATP (2)

– ADP (2)

• Products: • Pyruvate (2)• NADH (2)• ATP (4)

Mitochondria Structure

• Double membrane organelle– Outer membrane: very permeable – Inner membrane:

• selectively permeable– pyruvate (yes)

– NADH (no)

• contains electron transport proteins

• Cristae- inner foldings– Increase surface area

• similar to plasma membrane of bacteria

Mitochondria Structure

• Double membrane organelle– Matrix: inside inner membrane

• Protein rich solution: enzymes

• In between cristae

– Krebs Cycle• Tricarboxylic Acid Cycle

– Matrix– Reactants:

• 2 molecules of pyruvate

• Each makes a circuit through the cycle– One glucose = two pyruvate = two turns of the cycle

– Products: • For each pyruvate

– 3 NAD+ 3 NADH

– 1 FAD+ 1 FADH2

– 1 ADP + P 1 ATP

Citric Acid Cycle:

Oxidation of Pyruvate:

• Pyruvate must first be converted to Acetyl CoA – Pyruvate dehydrogenase– Each pyruvate molecule loses one Carbon and two Oxygen-

Acetyl group

– Acetyl group attaches to CoA molecule forming acetyl CoA

– Reduction of NAD+ molecule to NADH• 2 total- one for each pyruvate

Cyclic Nature of Citric Acid Cycle

• CoA transfers 2 carbon molecule– Transfers 2 carbon acetyl group to 4 carbon oxaloacetate– Results = 6 carbon citrate– Start of cycle

Cyclic Nature of Citric Acid Cycle

• Citrate- goes through a series of oxidation reactions– Priming/ rearrangement stage

• Prepares the 6 carbon citrate for energy extraction

– Oxidized by NAD+ – Carbon dioxide

Cyclic Nature of Citric Acid Cycle

• Citrate also loses 2 carbon atoms (CO2) eventually returning to 4 carbon oxaloacetate again – Energy Extraction/ Acetyl group stage– More reduction of NAD+– Reduction of FAD– ATP produced- substrate level phosphorylation– Cycle starts over again

Important Features

• NAD+ and FAD+ are reduced by the oxidation of an organic compound (transfer of H atom).

• 1 ATP molecule is formed by substrate level phosphorylation during each turn of cycle (net per glucose = 2 ATP)

• For each turn of the cycle, 3 Carbon atoms are lost to Carbon Dioxide– All 6 carbons exit the system by the end of the Kreb

cycle.

Oxidative Phosphorylation

• Electron transport is coupled with ATP synthesis via chemiosmosis.

• Over all drop in ΔG as electrons are transferred from NADH to Oxygen– Releases energy in manageable amounts

• Create proton motive force – Drives the production of ATP

Electron Transport Chain

• Inner Mitochondrial membrane

• Series (I - IV) of protein complexes– Complexes one – three have increasing affinity

for electrons

• Prosthetic groups: non-protein components essential to certain enzymes

• Redox (downhill) reactions

• Does not directly make ATP- eases the fall

Prosthetic Groups:

• FMN- flavin mono nucleotide-– Gets reduced by NADH at complex I

• CoQ- Ubiquinone- – very hydrophobic– very mobile– Carries between complex I/II and complex III

Prosthetic Groups:

• Iron/Sulfur cluster- – gets reduced by FADH2 at complex II– Transfer electrons between cytochromes

• Cytochromes- transfers electrons to oxygen– Heme- Fe atom- carries electrons

Chemiosmosis

• The formation of a hydrogen ion gradient drives the cellular process of ATP synthesis– Proton motive force

Chemiosmosis

• Final protein complex = F0F1 protein

– Catalyzed by ATP synthase– Oxidative phosphorylation: synthesis of ATP

from ADP and Pi

– 3 to 4 H+ to generate 1 ATP

Animation

• http://www.science.smith.edu/departments/Biology/Bio231/etc.html

Fermentation If a cell runs out of O2, all the e- carriers

are stuck in reduced form, halting system

• Pyruvate produced by glycolysis acts as alternative acceptor of H from NADH, keeping glycolysis going to allow small ATP production

Alcoholic Fermentation

• Yeasts break down sugar into pyruvate.• Each pyruvate is dismantled into a

molecule of CO2 and a 2C compound acetaldehyde

• Acetaldehyde is reduced by accepting 2H's from NADH and H+ forming 2C alcohol ethanol (ethyl alcohol)

Lactic Acid Fermentation

• Occurs during strenuous exercise

• Pyruvate from glycolysis is reduced by accepting hydrogens from NADH and H+

• Pyruvate converted into 3C compound, lactate

Metabolic Energy Systesm:

• Phosphagen pathway- high powered activities that last around 10 secs

• Glycolytic pathway- moderately powered activities that last two minutes

• Oxidative pathway- low powered pathways that last more than several minutes

Respiration W/O O2

Anaerobic respiration: uses nitrate or sulfate as final electron acceptor

Fermentation: the anaerobic breakdown of food molecules in which the final e- acceptor is an organic molecule