Lecture 10

Textbook Notes - Chapter 9: Cellular Respiration and Fermentation

9.1 Overview of Cellular Respiration

• glucose is oxidized through a series of redox reactions (aka cellular respiration)

• cellular respiration completely oxidizes glucose, while fermentation doesn’t

  • cellular respiration harvests more energy (more efficient)

• process of cellular respiration:

  1. glycolysis — glucose → 2 pyruvate, ATP is produced, NAD+ → NADH

     • occurs in cytosol of eukaryotes and prokaryotes

  2. pyruvate processing — pyruvate → CO2 + CoA, NAD+ → NADH

     • occurs in matrix of mitochondria or cytosol of prokaryotes

  3. citric acid cycle — CoA → 2 CO2, ATP and NADH produced, FAD → FADH2

     • occurs in matrix of mitochondria or cytosol of prokaryotes

  4. electron transport chain and oxidative phosphorylation — electrons from NADH and FADH2 go through a series of redox reactions, creating a proton gradient across the inner membrane of mitochondria, used to make ATP

     • occurs in inner membrane of mitochondria or plasma membrane of prokaryotes

9.2 Glycolysis: Oxidizing Glucose to Pyruvate

• glycolysis — process of turning glucose into pyruvate

• glycolysis is a sequence of 10 reactions in cytosol

  • starts by using 2 ATP molecules, termed the energy-investment phase

  • steps 6-10 are the energy payoff phase

  • for each molecule of glucose, the net yield is 2 NADH, 2 ATP, and 2 pyruvate

• substrate-level phosphorylation — enzymes catalyze transfer of phosphate from phosphorylated substrate to ADP, creating ATP

How is Glycolysis Regulated?

• high levels of ATP inhibit phosphofructokinase which catalyzes synthesis of fructose-6-phosphate to fructose-1,6-biphosphate

  • it has 2 binding sites for ATP, one active site and one regulatory site

Lecture Slides

cellular respiration

• the breakdown of glucose to CO2 and H2O

• transforms the energy from food to ATP

• can’t extract anymore biologically usable energy from CO2 and H2O

• multiple reactions in 3 distinct pathways or “phases”

  • glycolysis

  • pyruvate oxidation and Krebs cycle

  • oxidative phosphorylation (electron transport and chemiosmosis)

glycolysis

• phase 1 in the path of making ATP from glucose

• “glyco” (sugar) + “lysis” (splitting)

• starts with a 6-carbon sugar (glucose), ends with two 3-carbon molecules (pyruvate)

• this pathway is actually endergonic up to production of first 3-carbon molecules (uses cell’s store of ATP)

  • the beginning of glycolysis is endergonic

• occurs in the cytoplasm of all living cells

• 2 steps are endergonic (coupled with ATP → ADP + Pi)

  • steps 1, 3

    • ATP “pays for” these steps by being hydrolyzed into ADP and attaching a phosphate group to the molecules

• 3 steps are exergonic

  • steps 6, 7, 10

    • step 6 (so exergonic) — coupled with the reduction (endergonic) of 2 NAD+ → 2 NADH

    • step 7 — coupled with 2 ADP + 2 Pi → 2 ATP

    • step 10 — coupled with 2 ADP + 2 Pi → 2 ATP

• glycolysis: glucose + 2 ATP → 2 pyruvate + 2 ATP + 2 NADH + 2 H2O + 2 H+

• problems at the end of glycolysis

  1. molecules still are not at their lowest energy state

  2. some of our energy is being held in NADH

  3. NAD+ is being used up and not replaced

After glycolysis

• it depends on the presence or absence of oxygen (O2) or other terminal electron acceptor

• if oxygen is present, cells will undergo aerobic respiration

• if oxygen is absent but an alternative terminal electron acceptor exists, cells will undergo anaerobic respiration

  • only difference is the last electron acceptor

• if oxygen is absent and no terminal electron acceptor exists, cells might be able to undergo fermentation

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

  • all C-H bonds converted to C-O bonds (releasing 6 CO2)

• more energy transferred to NAD+ and FAD (making more NADH and FADH2)

• another substrate-level phosphorylation (SLP) reaction in Krebs cycle (GTP is ATP analog)

• occurs in mitochondria of eukaryotes; cytoplasm and plasma membrane of prokaryotes

mitochondrion

• matrix

  • where krebs cycle occurs

  • Krebs enzymes

  • DNA and ribosomes

• inner membrane

  • folds/tubes

  • principle site of ATP generation

  • >70% protein (no porins aka channels)

  • impenetrable to ions and small molecules except by transporters

  • cristae

    • sacs of inner membrane joined to the rest of the inner membrane by short tubes

    • the more cristae, more electron transport chains

• intermembrane space (IMS)

  • composition of ions and small molecules is the same as the cytoplasm (same pH) because of porins

• outer membrane

  • typical protein 50%, lipid 50%

  • has porins

    • like a straw

    • open channel, completely unregulated except by diameter