Glycolysis, Krebs Cycle, and other Energy-Releasing Pathways

Learning Goals for Respiration - After these lectures, students will be able to:

• Understand the various forms of potential engery utilized in the photosynthetic and respiration processes;
• Describe the similarities and differences between the electron transport chains utilized in photosynthesis and aerobic respiration and how the oxidation and reduction of molecules and the flow of electrons through these electron transport chains is use to generate stable forms of energy in plants and animals
• Explain the general flow of electrons from glucose to NAD + and FAD+ through a series of chemical intermediates;
• Explain how ATP is produced via substrate-level phosphorylation and by the ATP synthase
• Trace the flow of an electron from a water molecule to NADPH to glucose to NADH back to a water molecule during photosynthesis and respiration

All organisms produce ATP by releasing energy stored in glucose and other sugars.

• Plants make ATP during photosynthesis.
• All other organisms, including plants, must produce ATP by breaking down molecules such as glucose

Aerobic respiration - the process by which a cell uses O₂ to "burn" molecules and release energy

The reaction: C₆H₁₂O₆ + 6O₂ >> 6CO₂ + 6H₂O

Note: this reaction is the opposite of photosynthesis

This reaction takes place over the course of four major reaction pathways

• Glycolysis
• Conversion of Pyruvate to Acetyl CoA (Oxidation of Pyruvate or Pyruvate Processing)
• The Krebs Cycle
• Electron Transport Phosphorylation (chemiosmosis)

Glycolysis (glyco = sugar; lysis = breaking)

• Goal: break glucose down to form two pyruvates
• Who: all life on earth performs glyclolysis
• Where: the cytoplasm
• Glycolysis produces 4 ATP's and 2 NADH, but uses 2 ATP's in the process for a net of 2 ATP and 2 NADH

NOTE: this process does not require O₂ and does not yield much energy

• Glucose (6C) is broken down into 2 PGAL's (Phosphoglyceraldehyde - 3Carbon molecules)
• This requires two ATP's

• 2 PGAL's (3C) are converted to 2 pyruvates
• This creates 4 ATP's and 2 NADH's
• The net ATP production of Glycolysis is 2 ATP's

• ATP is produced through substrate-level phosphorylation
  • A phosphate is transfered from a molecule to ADP producing ATP

Oxidation of Pyruvate and the Krebs Cycle (citric acid cycle, TCA cycle)

• Goal: take pyruvate and put it into the Krebs cycle, producing NADH and FADH₂
• Where: the mitochondria
• There are two steps
  • The Conversion of Pyruvate to Acetyl CoA
  • The Krebs Cycle proper
• The Krebs cycle and the conversion of pyruvate to Acetyl CoA produce 2 ATP's, 8 NADH's, and 2FADH₂'s per glucose molecule

• 2 NADH's are generated (1 per pyruvate)
• 2 CO₂ are released (1 per pyruvate)

• 6 NADH's are generated (3 per Acetyl CoA that enters)
• 2 FADH₂ is generated (1 per Acetyl CoA that enters)
• 2 ATP are generated (1 per Acetyl CoA that enters)
• 4 CO₂'s are released (2 per Acetyl CoA that enters)

  

   

• Therefore, the total numbers of molecules generated in the Oxidation of Pyruvate and the Krebs Cycle is:
  • 8 NADH
  • 2 FADH₂
  • 2 ATP
  • 6 CO₂

• Goal: to break down NADH and FADH₂, pumping H⁺ into the outer compartment of the mitochondria
• Where: the mitochondria
• In this reaction, the ETS creates a gradient which is used to produce ATP, quite like in the chloroplast
• Again, electrons move down an energy gradient until the meet the ultimate electron acceptor, oxygen gas (O₂)
• Electron Transport Phosphorylation typically produces 32 ATP's
• ATP is generated as H+ moves down its concentration gradient through a special enzyme called ATP synthase

• Glycolysis: 2 ATP
• Krebs Cycle: 2 ATP
• Electron Transport Phosphorylation: 32 ATP
  • Each NADH produced in Glycolysis is worth 2 ATP (2 x 2 = 4) - the NADH is worth 3 ATP, but it costs an ATP to transport the NADH into the mitochondria, so there is a net gain of 2 ATP for each NADH produced in gylcolysis
    • Plants are a bit more efficient and they don't have to spend 2 ATP to transport NADH into their mitochondria
  • Each NADH produced in the conversion of pyruvate to acetyl CoA and Krebs Cycle is worth 3 ATP (8 x 3 = 24)
  • Each FADH₂ is worth 2 ATP (2 x 2 = 4)
  • 4 + 24 + 4 = 32
• Net Energy Production: 36 ATP (38 ATP for Plants)!
  • Obviously, nothing is perfect, and the actual yeild of ATP is less than 36, but this is theoretical max
• Respiration animation

Fermentation

• Goal: to reduce pyruvate, thus generating NAD⁺ in the absense of O₂
• Where: the cytoplasm
• Why: in the absence of oxygen, it is the only way to generate NAD⁺

• Alcohol Fermentation - occurs in yeasts in many bacteria
  • The product of fermentation, alcohol, is toxic to the organism

• Lactic Acid Fermentation - occurs in humans and other mammals
  • The product of Lactic Acid fermentation, lactic acid, is toxic to mammals
  • This is the "burn" felt when undergoing strenuous activity

• The only goal of fermentation reactions is to convert NADH to NAD⁺ (to use in glycolysis).
• No energy is gained
• Note differences - anaerobic respiration - 2 ATP's produced (from glycolysis), aerobic respiration - 36 ATP's produced (from glycolysis, Krebs cycle, and Oxidative Phosphorylation)
• Thus, the evolution of an oxygen-rich atmosphere, which facilitated the evolution of aerobic respiration, was crucial in the diversification of life

Photosynthesis: 6 CO₂ + 6 H₂O >> C₆H₁₂O₆ + 6 O₂

Respiration: C₆H₁₂O₆ + 6 O₂ >> 6 CO₂ + 6 H₂O

Notice that these reactions are opposites - this is important since the earth is a closed system

All life has a set amount of natural materials to work with, so it is important that they all be cycled through effectively and evenly

Energy Yields:

• Glucose: 686 kcal/mol
• ATP: 7.5 kcal/mol
• 7.5 x 36 = 270 kcal/mol for all ATP's produced
• 270 / 686 = 39% energy recovered from aerobic respiration

Related Catabolic Processes - Beta Oxidation

• Fats consist of a glycerol backbone with two or three fatty acids connected to it
• The body absorbs fats and then breaks off the fatty acids from the glycerol
• Glycerol is converted to glyceraldehyde phosphate, an intermediate of glycolysis
• The fatty acids are broken down into two-carbon units which are then converted to acetyl CoA.
  • An eight-carbon fatty acid can produce 4 acetyl CoA's
  • Each acetyl CoA is worth 12 ATP's (3 NADP, 1 FADH₂, 1 ATP)
  • Therefore, this short fatty acid is worth 48 ATP's, a fat with three chains of this length would be worth 144 ATP's!
  • This is why fats are such a good source of energy, and are hard to lose if you want to lose weight

A comparison between Plants and Animals

• Animal cells and Plant cells contain mitochondria!
  • However, animal cells contain many more mitochondria than plant cells
• Animal cells get most of their ATP from mitochondria
• Plant cells get most of their ATP from the chloroplast
  • The ATP generated from the mitochondria is only used when the plant cannot generate ATP directly from the light-dependent reaction

Other Uses for Molecules used in Glycolysis and the Krebs Cycle

• Not all of the molecules that enter Glycolysis and the Krebs Cycle are used for energy
• Some are used to synthesize fats, nucleotides, amino acids, and other biologically important molecules.

Regulation of Glycolysis and the Krebs Cycle

• Step 3 of Glycolysis - The conversion of Fructose 6-phosphate to Fructose 1,6-bisphosphate
  • Enzyme catalyzing this reaction =  Phosphofructokinase
  • "Committing Step"  - Fructose 6-phosphate can be used by the cell for lots of things, but fructose 1,6-bisphosphate has limited  use except in glycolysis
  • Phosphofructokinase inhibited by high levels of ATP
    • ATP is also a substrate - odd, eh? 
    • Enzyme has two ATP binding sites, one in the active site and one in an allosteric site
    • Low to mid levels of ATP cause ATP to bind to the active site
    • High levels of ATP also enable ATP to bind to allosteric site, causing a conformation change and shutting down the enzyme
• Conversion of Pyruvate to Acetyl CoA
  • Enzyme involved in catalyzing this reaction = pyruvate dehydrogenase
  • High levels of ATP slow down this reaction by phosporylating the enzyme, changing its shape and shutting it down
    • High levels of NADH and Acetyl CoA also inhibit this enzyme
  • NAD+, CoA, or AMP (an indicator of low ATP) can speed up the reactio