MCB 150 Lecture 10: Introduction to Cellular Respiration

Announcements

• Exam 1 starts tomorrow. Be sure to review all materials and come prepared.

• Observation about "Save & Grade" vs "Save only." Use "Save & Grade" to finalize your submission; use "Save only" to keep your work without submitting.

• Student hours today from 4:00–5:30 in 124 Burrill Hall. Take advantage of this opportunity for additional help.

• No class activities on Friday. Use this time to catch up on readings or assignments.

Cellular Respiration

• We eat food to give us energy, but how does the energy from food get to ATP? Cellular respiration is the process through which this occurs.

• Cellular Respiration: The breakdown of glucose to CO2 and H2O. This process releases energy that is then used to synthesize ATP.

• Multiple reactions in 3 distinct pathways or “phases”:

  • Glycolysis: The initial breakdown of glucose in the cytoplasm.

  • Pyruvate oxidation and Krebs cycle: Further processing of pyruvate in the mitochondria.

  • Oxidative phosphorylation (electron transport and chemiosmosis): The primary ATP-generating process using the electron transport chain and ATP synthase.

Glycolysis

• Phase 1 in the path of making ATP from glucose: Glycolysis- “Glyco” (sugar) + “lysis” (splitting). The breakdown of glucose.

  • Starts with a 6-carbon sugar (glucose), ends with two 3-carbon molecules (pyruvate). Each glucose molecule is split into two pyruvate molecules.

  • Pathway is actually endergonic up to production of first 3-carbon molecules (uses cell’s store of ATP). The initial steps require an investment of ATP.

  • Occurs in the cytoplasm of all living cells. This universal process occurs in both prokaryotic and eukaryotic cells.

• Glycolysis:

  • 2 steps are endergonic: These steps require energy input, using ATP.

  • 3 steps are exergonic: These steps release energy, producing ATP and NADH.

  • This energy is harnessed and saved for later: The energy released is captured in the form of ATP and NADH.

  • 2 SLP reactions: Substrate-level phosphorylation directly generates ATP.

Problems at the End of Glycolysis

1. Molecules still are not at their lowest energy state: Pyruvate still contains significant potential energy.

2. Some of our energy is being held in NADH: NADH needs to be recycled to NAD^+ to sustain glycolysis.

3. NAD^+ is being used up and not replaced: Regeneration of NAD^+ is essential for glycolysis to continue.

Questions Arising from Glycolysis Problems

1. How do we get more energy out of pyruvate? Further oxidation is needed to extract more energy.

2. How do we transfer the energy in NADH to ATP? The electron transport chain is used to convert NADH's energy to ATP.

3. How do we regenerate NAD^+? Fermentation or aerobic respiration can regenerate NAD^+.

Answer to Previous Questions

• It depends on the presence or absence of oxygen (O_2) or other terminal electron acceptor:

  • If oxygen (O_2) is present, cells will undergo aerobic respiration: Efficient ATP production using oxygen as the final electron acceptor.

  • If oxygen (O_2) is absent but an alternative terminal electron acceptor exists, cells will undergo anaerobic respiration: Less efficient ATP production using other molecules as final electron acceptors.

  • If oxygen (O_2) is absent and no terminal electron acceptor exists, cells might be able to undergo fermentation: Inefficient ATP production via glycolysis alone, with regeneration of NAD^+.

Aerobic Respiration

• Carbon source (2 molecules of pyruvate) completely converted to carbon dioxide:

  • Pyruvate molecules first converted to acetyl-CoA, which then enters the Krebs (or Citric Acid) Cycle: This is the link between glycolysis and the Krebs cycle.

  • All C-H bonds converted to C-O bonds (6 CO_2 released): Complete oxidation of the carbon source.

• More energy transferred to NAD^+ and FAD (makes more NADH & FADH_2): These electron carriers will supply electrons to the electron transport chain.

• Another SLP reaction in Krebs cycle (GTP is ATP analog): GTP can be readily converted to ATP.

• Occurs in mitochondria of eukaryotes; cytoplasm and plasma membrane of prokaryotes: The location of aerobic respiration varies by cell type.

Mitochondrion Organization

• Inner Membrane:

  • Principal site of ATP generation: Location of the electron transport chain and ATP synthase.

  • >70% protein (no porins):

  • Impenetrable to ions & small molecules except by transporters: Highly selective permeability to maintain the electrochemical gradient.

• Outer Membrane:

  • Typical protein %: Contains porins for permeability.

  • Porins: Allows free passage of small molecules and ions.

• Intermembrane Space (IMS):

  • Composition of ions and small molecules is the same as the cytoplasm: Due to the presence of porins in the outer membrane.

• Matrix:

  • Krebs enzymes: Enzymes required for the Krebs cycle.

  • DNA & ribosomes: Contains its own genetic material and protein synthesis machinery.