Important Steps, Reactants, and Products of Cellular Respiration

Four Stages of Cellular Respiration and Where They Occur

1. Glycolysis - 10 step process within cytoplasm

2. Pyruvate Oxidization - 1 step process within mitochondrial matrix

3. Kreb’s Cycle - 8-step cyclic process within in the

   mitochondrial matrix

4. Electron Transport Chain - Multistep process in the inner

   mitochondrial membrane

Mitochondrion

• Membrane bound organelle

• Referred to as a powerhouse because it generates most of the ATP with the Kreb’s cycle and Electron Transport Chain

• Composed of an inner and outer membrane

• Intermembrane space - is space between inner and outer membrane

• Matrix - is the interior aqueous environment inside the inner membrane

What is the goal of cellular respiration? How is this accomplished? (Hint: 2 mechanisms to produce ATP)

To capture free energy in the form of ATP

1. Substrate-Level Phosphorylation

2. Oxidative Phosphorylation

Substrate-Level Phosphorylation

• Process in which ATP is formed directly, in an enzyme-catalyzed reaction:

• A phosphate group is transferred from a molecule to an ADP molecule

• Eg. A phosphate containing molecule called phosphoenolpyruvate (PEP) transfers its phosphate group to ADP, forming ATP

• Occurs in Glycolysis (step seven & ten ) and The Citric Acid Cycle (Kreb’s Cycle on step 5)

Oxidative Phosphorylation

• ATP formed indirectly through a series of enzyme-catalyzed redox reactions involving oxygen as the final electron acceptor

• REDOX Reactions: A pair of reactions where one molecule gains an electron to become REDUCED and another molecule loses an electron to become OXIDIZED

• “LEO the Lion says GER”

What yields more ATP; O.P or S.L.P? Where does O.P occur? What two enzymes does it involve?

• Yields more ATP than substrate-level phosphorylation

• *Occurs in Glycolysis, ETC

Uses two coenzymes:

1. Nicotinamide Adenine Dinucleotide (NAD+)

2. Flavin Adenine Dinucleotide (FAD)

How many hydrogen atoms does NAD+ remove from the original glucose molecule? How many electrons attach to NAD+, reducing it to NADH?

• NAD+ is an electron carrier (coenzyme)

• NAD+ removes two hydrogen atoms (two protons and two electrons) from original glucose molecule

• Two electrons and one proton attach to NAD+, reducing it to NADH

• Remaining proton dissolves in solution as H+

How is FAD reduced? Are any protons released when they bind to FAD? When does it occur?

• Also reduced by two hydrogen atoms of the original glucose molecule.

• Its’ reduced form is FADH2

• All protons and electrons of hydrogen bind directly to FAD

• Occurs as one-step reaction of the Kreb’s Cycle

Aerobic Cellular Respiration

• Process that extracts energy from food in the presence of oxygen 

• Energy used to make ATP from ADP and inorganic Pi 

• ATP used to supply energy directly to cells

ATP: Adenosine Triphosphate (ATP-ADP Cycle) What reaction occurs when cells need energy?

• Primary source of Free Energy in living cells 

• Free energy is energy that can do useful work 

• Contains nitrogenous base: adenine, ribose sugar and a chain of 3 phosphate groups 

• When cells need Free Energy: ATP undergoes hydrolysis (reaction with water), breaking the bond between the second and third phosphate groups.

• Converts ATP to ADP (ATP → ADP + Pi + Energy)

• After the energy is used, ADP can be converted back to ATP through a process called phosphorylation, which adds a phosphate group back to ADP

• ADP + Pi + Energy → ATP

• This releases 31 kJ/mol of Free Energy

• Think of a rechargable battery

Regenerating ATP

• Cells generate ATP by combining ADP with Pi (Phospahte)

• This is called a phosphorylation reaction 

• ATP synthesis requires the input of energy (endergonic process) 

• Energy needed to make ATP comes from breakdown of complex molecules which contain an abundance of energy (exergonic process) 

• These complex molecules are carbohydrates proteins and fats

Cellular Respiration

• The breakdown of glucose to make energy (ATP) using oxygen 

• Overall equation: 

  • C6H1206 +602 → 6CO2 + 6H20

    • Ultimate Goal: To extract energy from nutrients and store it as ATP 

    • Achieved by: 

      • Breaking bonds between the 6 carbon atoms in glucose, creating 6CO2’s 

      • Moving hydrogen atoms from glucose to oxygen, for 6H20’s

Glycolysis (Beginning of Process) + What does the name literally mean?

• First 10 reactions of cellular respiration 

• Greek for “sugar splitting” 

• Starts with glucose (6C sugar) and produces two 3C pyruvate (pyruvic acid) molecules 

• Occurs in cytoplasm 

• Anaerobic process: Does not require oxygen 

Glycolysis Reactants

• 1 Glucose Molecule (C6H12O6, 6-C)

• 2 ATP Molecules (1-5 Energy Investment Phase, Step 1 & 3)

• 2 NAD+ (Electron Carriers)

Glycolysis Products

• 2 Pyruvate Molecules (3-carbon molecules)

• 4 ATP Molecules (Net gain: 2 ATP, since 2 were used, 6-10 Energy Yielding Phase)

  • Two ATP molecules are invested during the energy investment phase to help prepare glucose for further breakdown (destabilize glucose, prepare it for splitting)

• 2 NADH molecules (Electron Carries)

Glycolysis Analysis

The overall goal of glycolysis is to break down glucose into smaller pyruvate molecules, producing a small amount of ATP and NADH for energy use.

Free Energy (Does it satisfy the energy requirements of multicellular organisms?)

• Glycolysis transfers only 2.2% of free energy available in 1 mol of glucose to ATP

• Not efficient at harnessing energy -does not satisfy the energy needed of most multicellular organisms 

• Thought to be earliest form of energy metabolism used by simplest anaerobic organisms 

• Most energy still stored in two pyruvate and two NADH molecules

Pyruvate Oxidation

• Following glycolysis, two pyruvate molecules are transported through the two mitochondrial membranes into the matrix 

• The main purpose of pyruvate oxidation is to convert pyruvate (from glycolysis) into acetyl-CoA, which can then enter the Krebs cycle (Citric Acid Cycle) for further energy extraction.

• This process also generates NADH and releases carbon dioxide (CO₂).

Reactants in Pyruvate Oxidation

• 2 Pyruvate Molecules (From glycolysis)

• 2 NAD+

• 2 Coenzyme A (COA)

Steps of Pyruvate Oxidation

• A multi-enzyme complex catalyzes 3 changes before Krebs cycle: 

  • #1: Decarboxylation Reaction 

    • Low energy carboxyl group (COOH) is removed as CO2

  • #2: Redox Reaction 

    • Remaining 2-C portion is oxidized (loses electrons) by NAD+ and forms an acetyl group (acetate) 

    • NAD+ reduced (gaining electrons) to NADH (plus) H+ (oxidative phosphorylation) 

  • #3 Formation of Acetyl-CoA 

    • Sulfur-containing compound called coenzyme A (CoA) is attracted to the acetate component, forming acetyl-CoA 

Overall Goal: To change pyruvate into acetyl-CoA

Products of Pyruvate Oxidation (Per Glucose Molecule)

• 2 Acetyl - Coa (One from each pyruvate)

• 2 NADH

• 2 CO2

Where do Products of Pyruvate Oxidation Go? 

• 2 molecules of Acetyl-CoA enter the Krebs cycle, where more free energy transfers occur 

• 2 molecules of NADH proceed to ETC and Chemiosmosis to produce ATP by oxidative phosphorylation 

• 2 CO2 molecules diffuse out as waste (exhalation) 

• 2 H+ stay dissolved in mitochondrial matrix 

The Citric Acid Cycle

• A.K.A Kreb’s Cycle 

• 8 enzymes catalyzed reactions 

• Result in oxidation of acetyl group to CO2 

• Completes the conversion of all carbon atoms originally in glucose to CO2 

• Synthesizes ATP, NADH and FADH2

• Its main purpose is to fully oxidize the acetyl group from acetyl-CoA into carbon dioxide (CO₂) and capture high-energy electrons in the form of NADH and FADH₂ for use in the electron transport chain.

• The cycle also generates a small amount of ATP (or GTP).

Reactants (Per cycle)

• 1 Acetyl-CoA (2-carbon molecule from pyruvate oxidation)

• 3 NAD⁺

• 1 FAD

• 1 ADP (or GDP) + Pi

Products

• 2 CO₂ (carbon dioxide)

• 3 NADH

• 1 FADH₂

• 3 H+

• 1 ATP (produced via substrate-level phosphorylation) (Step 5)

• CoA (regenerated)

For one glucose molecule (since glucose generates 2 acetyl-CoA molecules)

• 4 CO₂

• 6 NADH

• 2 FADH₂

• 2 ATP (Step 5)

Kreb’s Cycle Summary

• The Krebs cycle happens twice per glucose molecule because glycolysis splits one glucose into two pyruvate molecules, which are converted to two acetyl-CoA molecules.

• The cycle is essential for generating high-energy electron carriers (NADH and FADH₂), which provide electrons to the electron transport chain for ATP production via oxidative phosphorylation.

• Overall, the Krebs cycle produces the reducing agents (NADH and FADH₂) needed for the final and most significant ATP-producing step of cellular respiration.

• The Krebs cycle begins and ends with a molecule called oxaloacetate. It combines with a 2-carbon molecule (acetyl-CoA) to start the cycle, and after a series of reactions, it gets regenerated back to oxaloacetate so the cycle can start over again.

Electron Transport Chain

• Facilitates transfer of electrons from NADH and FADH2 to O2, ultimately producing ATP

• Series of four protein complexes built into inner mitochondrial membrane: 

• Flow of electrons facilitated by two electron shuttles:

1 UQ: Ubiquinone

• Hydrophobic, found in the core of the membrane 

• Shuttles electrons from complex I and II to complex III

2 Cyt: Cytochrome c

• On intermembrane space side of membrane 

• Transfers electrons from complex III to IV

Electron Transport Chain Continued

• Each complex lies in order of increasing electronegativity 

• Each compound is alternately reduced (gain e-) and oxidized (lose e-)

• Controlled release of energy 

• When e- reaches last protein, it becomes very stable 

• Chain starts of in NADH and FADH2, then pass electron to ETC 

  • NADH and FADH2 are oxidized 

  • H’s are removed and electrons are stripped from hydrogen atoms and become hydrogen ions

  • 2 electrons are passed from one electron (NADH Or FADH2 → O2) carrier to the next 

  • Removed from complex IV by oxygen (since oxygen has a higher electronegativity than complex IV and therefore has the ability to pull electrons without backing the ETC) 

Oxygen

• One of the most electronegative elements 

• Oxidizes complex IV by stripping last electrons from it 

• Oxygen then joins with 2 protons (hydrogen ions) and form water

  • ½ O2 + 2e- + 2Hᐩ → H2O

Free Energy

• Free energy lost by electrons at each step/complex (exergonic) is used to pump hydrogen ions from mitochondrial matrix into intermembrane space 

• This sets up a hydrogen ion concentration and electric gradient (electrochemical gradient) across the membrane, which is used to phosphorylate ADP → ATP

NADH

• Passes electrons to NADH dehydrogenase (1st protein complex) 

• NADH oxidation pumps 3 protons and are responsible for producing 3ATP

FADH2

• Passes electrons to complex II

• FADH2 oxidation pumps 2 protons and are responsible for producing 2ATP

Chemiosmosis + ATP Synthase

• Enzyme spans inner membrane of mitochondria 

• ADP + P1 → ATP 

• Only channel permeable to hydrogen ions 

• Hydrogen ions flow down concentration gradient back into the matrix and so provides energy for ATP synthesis 

• Flowing hydrogen ions causes change in the shape of ATP synthase enzyme 

• Powers bonding of P1 to ADP 

• Called “protein-motive” force 

ATP Synthase Con.

• Chemiosmosis couples ETC to ATP synthesis 

  • Use of hydrogen gradient to transfer energy from redox reactions to cellular work (ATP synthesis)

The Problem with Glycolysis

• Glycolysis produces 2 NADH molecules in the cytoplasm 

• These NADH molecules cannot add their electrons to the ETC because they are not in the mitochondria 

• The electrons in NADH can use one of two different shuttle systems to add their electrons into the ETC. 

The two shuttle systems

1. Malate - Aspartate Shuttle

2. Glycerol - Phosphate Shuttle

Malate-Aspartate Shuttle

• NADH in cytosol is oxidized to NAD+ 

• An organic compound is reduced to Malate

• The electrons are transferred into the mitochondrial matrix in malate 

• They reduce another NAD+, forming NADH in the matrix 

• NADH then adds the electrons to the ETC as normal

• The NADH in the cytosol is oxidized to NAD*, and the electrons are transferred across the membrane and used to reduce an NAD+ to NADH within the matrix.

Glycerol-Phosphate Shuttle

• NADH in cytosol is oxidized to NAD+ 

• Glycerol phosphate is reduced 

• The electrons are transferred into the mitochondrial matrix in glycerol phosphate 

• Glycerol phosphate reduces an FAD, forming FADH2 in the matrix

• FADH2 then adds the electrons in the ETC as normal

• Little less efficient than the malate-aspartate shuttle

Efficiency of ATP Production

• Many reasons why total yield may be less than max 38 ATP 

• Eg. Energy from H+ flow is lost due to other mitochondrial processes or using the glycerol phosphate shuttle 

• Only about 42% of total energy in glucose is converted to ATP 

• Rest is lost as thermal energy 

  • Cars only convert 25% of the energy from fuel into motion

Fermentation

• Two anaerobic pathways that do NOT use oxygen as the final electron acceptor 

• Glycolysis is the first step of the fermentation pathways 

• Final steps only serve to regenerate NAD+ → the coenzyme needed to run gylcolysis

Alcohol Fermentation

• Yeast (fungus) used in beer brewing and wine making, as well as some bacteria 

• 2 molecules of pyruvate are formed, along with 2 ATP and 2 NADH

• Each pyruvate formed from glycolysis rearranged into acetalehyde through a decarboxylation reaction (intermediate) 

• Acetaldehyde accepts hydrogen and electrons from NADH 

• Then converted into ethanol (As a result of being reduced)

  • Glucose → 2 ethanol + 2CO2 + 2ATP

Lactate (Lactic Acid Fermentation)

• Fungi, bacteria used in dairy industry to make cheese and yoghurt 

• Eg. Lactobacilli and streptococci ferment lactose in milk to lactic acid

• Begins with glycolysis (2 pyruvate, 2 ATP, 2 NADH)

• Pyruvate is converted into lactic acid (Reduced) 

• Human muscle cells make ATP by lactic acid fermentation when oxygen is scarce (during strenuous exercise) 

• Increase lactate causes fatigue and pain (harmful to muscles) 

• Lactate is gradually taken by blood to liver 

• Liver converts lactate back to pyruvate 

  • Glucose → 2 lactic acid + 2 ATP 

Fermentation vs. Respiration: How are they similar?

• Both use glycolysis to oxidize glucose into pyruvate 

• Both produce net yield of 2ATP (substrate level phosphorylation) 

• Use NAD+ as oxidizing agent: accepts electrons from food during glycolysis

Fermentation vs. Respiration: How are they different?

#1 NADH is oxidized back to NAD+

• Fermentation 

  • NADH passes electrons to pyruvates or some derivative (acetaldehyde) 

  • Electrons are NOT used to power ATP production 

  • NADH oxidizes because its electrons are passed to pyruvate (acetaldehyde)

• Respiration 

  • Electron transport (ETC) from NADH to oxygen drives oxidative phosphorylation and regenerates NAD+ 

  • Electron Transport Chain oxidizes NADH

#2 Final Electron Acceptor

• Fermentation: 

  • Pyruvate (latic acid fermentation) 

  • Acetaldehyde (alcohol fermentation) 

• Respiration: 

  • Oxygen

#3 Amount of Energy Harvested

• Fermentation: 

  • Energy stored in pyruvate is unavailable to cell 

• Respiration: 

  • 18 times more ATP 

  • Krebs cycle completes oxidation of glucose 

  • Taps into energy stored in pyruvate at end of glycolysis

#4 Requirements of Oxygen

• Fermentation: 

  • No need for O2 

• Respiration: 

  • Only occurs in the presence of oxygen