Link Search Menu Expand Document

Cellular Respiration

Fall Biology

Table of contents
  1. Video - Cellular Respiration Introduction
    1. Glycolysis
    2. Krebs Cycle
    3. Electron Transport Chain
    4. Lactic Acid and Fermentation
  2. Cellular Respiration Lab Background
    1. ATP and Energy
    2. Aerobic Cellular Respiration
    3. Glycolysis
    4. Acetyl-Coenzyme A Synthesis
    5. Krebs Cycle
    6. Electron Transport Chain
      1. ATP Synthetase
      2. Oxygen
    7. Using a Respirometer
    8. Summary in Tabular Form
  3. Determing the Rate of Cellular Respiration

Video - Cellular Respiration Introduction

By Khan Academy, access here

  • Cellular respiration is a very important chemical reaction.
    • This is how we derive energy from glucose (most food ends up as glucose).
  • Chemical reaction:
    • Glucose: C6 H12 O6.
    • Add 6 molecular oxygen: 6O2.
    • Produces 6CO2 + 6H2O + energy.
    • Complete: C6 H12 O6 + 6O2 ➡️ 6CO2 + 6H2O + energy
  • ATP is the energy currency.
    • Energy is used to produce ATP.
    • Some of the energy released is heat; also produces 38 ATPs that can be used by cells.
      • Large variation; can also have 28-29 ATPs.


  • Means “breaking up glycose” (glycol = glycose, ysis = break up)
    • glucose = gluc (sweet) + ose (sugar)
      • e.g. sucrose, lactose, etc.
  • Breaks up the glucose from a 6-carbon molecule (e.g. C-C-C-C-C-C) into two (C-C_C and C-C-C)
    • Glucose is a ring, portrayed here as the backbone. Also has hydrogens and oxygens not included.
  • Glycolysis needs 2 ATPs and generates 4 ATPs.
    • Net-wise generates 2 ATPs.
    • ! Can occur in the absence (or presence) of oxygen. (anaerobic process)

Krebs Cycle

  • The Krebs Cycle generates an additional 2 ATP.
  • ! Requires oxygen; is aerobic.

Electron Transport Chain

  • Produces the bulk of the ATPs (34 of them).
  • Is an aerobic process (required oxygen).
  • Glycolysis and Krebs cycle are taking NAD+ and adding hydrogens to from NADH.
    • For one molecule of glucose, converts NAD+s into 10 NADHs, which drive the electron transport chain.
    • Durns FAD into FADH2.
  • Cellular respiration is repackaging the energy in glucose to 38 ATPs and produces heat.
    • Done through three stages: glucose (generating NADHs), Krebs Cycle, Electron Transport Chain.

Lactic Acid and Fermentation

  • You can produce 2 ATPs without oxygen; nowhere near enough that can be produced when you have oxygen.
  • When you run out of oxygen, byproducts of glycolysis go into a side product called fermentation (because cannot proceed to Krebs cycle).
    • Muscles produce lactic acid fermentation; yeast will do alcohol fermentation.

Cellular Respiration Lab Background

Pages 3-5 of cellular respiration lab manual.

  • Plants and algae are the only organisms that can use sun energy through photosynthesis.
    • Sugars produced are transported and stored in roots and seeds.
  • How do plants acquire necessary energy?
    • Answer: metabolize stored sugars through cellular respiration.

ATP and Energy

  • All cells need energy.
  • Energy is contained in the chemical bonds of organic compounds.
    • e.g. carbohydrates, proteins, fats.
  • When sugar molecule chemical bonds are broken down, energy is used to synthesize a molecule called adenosine triphosphate (ATP).
  • ATP is the main form of energy used.
    • Energy stored in the three-phosphate tail.
    • When one of the bonds is broken, ATP becomes ADP (adenosine diphosphate) and releases a large amount of energy.
  • ATP and ADP are constantly cycled in cells.
    • Energy used for respiration, fermentation, & other metabolic processes.
    • Energy is required to turn ADP ➡️ ATP (adding a phosphate group).
    • Energy is released when ATP ➡️ ADP (removing a phosphate group).

Aerobic Cellular Respiration

  • Four processes: glycolysis, acetyl-coenzyme A (acetyl-CoA) synthesis, Krebs cycle, and electron-transport chain.
  • Complete metabolism of one sugar molecule during cellular respiration:
    C6 H12 O6 + 6O2 ➡️ 6CO2 + 6H2O + 36 ATP


  • A series of 10 reactions that converts a 6-carbon glucose into two 3-carbon molecules called pyruvate.
  • Occurs in the cell cytoplasm.
  • Four ATP molecules and two NADH molecules are produced.
    • Two ATP are required to start the reactions.
  • If there is oxygen, cells will undergo aerobic cellular respiration.

Acetyl-Coenzyme A Synthesis

  • In the presence of oxygen, pyruvate enters the mitochondria.
  • Is converted into a molecule called acetyl-CoA.
    • One molecule of CO2 and one molecule of NADH are produced during this conversion.

Krebs Cycle

  • Also known as the citric acid cycle.
  • Each molecule of acetyl-CoA is metabolized to produce 2 molecules of CO2.
    • Some released energy is used to produce ATP.
    • Some energy is released via electrons.
      • Transferred to carrier molecules NAD+ and FAD to yield reduced forms NADH and FADH2.

Electron Transport Chain

  • Electrons in these electron carriers can be used to synthesize ATP in the electron-transport chain.
  • NADH and FADH2 molecules are electron carriers because they transport electrons and hydrogen atoms to several proteins.
    • These are called the electron-transport chain.
  • Hydrogens and electrons are removed; NADH is oxidized to become NAD+ and FADH2 is oxidized to become FADH.
    • These electron carrier molecules cycle between oxidized and reduced forms.
  • When electrons are removed from NADH and FADH2, are transported from one protein to the next.
    • Release energy used to move hydrogen ions around the mitochondria.
    • Ions accumulate and form an ionic gradient.
  • Ionic gradient creates an electrochemical imbalance used to produce ATP.
    • Hydrogen ions pass through a membrane channel formed by ATP synthetase.

ATP Synthetase

  • Acts like a windmill.
  • Flow of hydrogen ions throuhg the channel causes one part of the ATP synthetase to spin.
    • This drives production of ATP.
  • Catalyzes the phosphorylation of ADP to add another phosphate, producing ATP.
    • Eukaryotic cells: electron donated from each NADH yields 3 ATP molecules, one electron donated from each FADH2 yields 2 ATP molecules.
    • Electron-transport chain produces 16 ATP molecules for each pyruvate.
    • Glucose molecule produces two pyruvate molecules; therefore aerobic respiration produces about 36 ATP.


  • Oxygen enters equations at last step of the electron-transport chain.
  • Electrons are transferred to oxygen when they reach the last protein.
    • Results in the formation of water (H2O).
  • Oxygen is the terminal electron carrier.
    • Transfer of electrons to oxygen enables more electrons to enter the start of the chain.
  • If no oxygen is available, no more electrons can be transferred.
    • In the absence of oxygen, the electron-transport chain and Krebs cycle cannot function.
    • Cell is starved of ATP for energy.

Using a Respirometer

  • This lab uses a respirometer to measure the respiration rate of germinating and dormant pea seeds.
    • Beads are used as a controls ample.
  • Respirometer is composed of:
    • Vial containing peas
    • Volume of air
    • Mouth of vial sealed with 1-hole rubber stopper.
  • Experiment:
    1. Enter respirometer is submerged underwater.
    2. If the peas respire, they will use O2 and release CO2.
    3. There will be no change in volume of gas (1 mole of oxygen for 1 mole of carbon dioxide).
    4. Experiment with changing this equilibrium by placing a potassium hydroxide (KOH)-saturated cotton ball at the bottom of the vial.
    5. KOH reacts w/ CO2 to form potassium carbonate (K2CO3) (a solid).
      • Reaction: CO2 + 2KOH ➡️ K2CO3 + H2O
      • Inside the respirometer, K2CO3 dissolves and the volume of gas is reduced. 6 As the volume of gas decreases, water moves from the bath into a submerged pipet.
    6. Gas volume decrease is measured, which will be used to calculate the rate of respiration.
  • Avogadro’s Law.

    At constant temperature and pressure, 1 mole of gas has the same volume as 1 mole of any other gas.

Summary in Tabular Form

ProcessStarting MaterialNet Energy Output
Glycolysis1 Glucose2 NADH, 2 ATP
Acetyl-CoA Synthesis and the Krebs Cycle2 Pyruvate8 NADH, 2 FADH2, 2 ATP
Electron-Transport Chain10 NADH, 2 FADH232 ATP

Determing the Rate of Cellular Respiration

  • Looking at the equation for cellular respiration:
    C6 H12 O6 + 6O2 ➡️ 6CO2 + 6H2O + 36 ATP
  • Rate of respiration can be measured by production of CO2, consumption of O2, or release of energy (int he from of heat).
  • We will measure oxygen production in the lab.
  • Consider the ideal gas law:
    • pV = nRT.
    • p = pressure of gas
    • V = volume of gas
    • n = number of molecules of the gas
    • R = the gas constant
    • T = temperature of the gas
  • Can be rewritten as V = nRT / P.
    • Changes in volume are due to temperature and pressure fluctuations.
    • Water baths minimize this change.
    • Volume changes in control group (w/ beads, not peas) will be used to correct volume changes.
  • KOH will be used to convert CO2 into a solid such that the gas quantity depletes over time.