Glycolysis, Krebs Cycle, and other Energy-Releasing Pathways

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

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

Note: this reaction is the opposite of photosynthesis

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

• Glycolysis
• The Kreb's 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 (3C)
• 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

Krebs's Cycle (citric acid cycle, TCA cycle)

• Goal: take pyruvate and put it into the Krebs's cycle, producing NADH and FADH₂
• Where: the mitochondria
• There are two steps
  • The Conversion of Pyruvate to Acetyl CoA
  • The Kreb's Cycle proper
• In the Krebs's cycle, all of Carbons, Hydrogens, and Oxygeng in pyruvate end up as CO₂ and H₂O
• The Krebs's cycle produces 2 ATP's, 8 NADH's, and 2FADH₂'s per glucose molecule

• 2 NADH's are generated
• 2 CO₂ are released

• 6 NADH's are generated
• 2 FADH₂ is generated
• 2 ATP are generated
• 4 CO₂'s are released

• Therfore, for each glucose molecule that enters into the Kreb's cycle (including the prepatory conversion to Acetyl CoA), the net production of products are:
  • 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
• Electron Transport Phosphorylation typically produces 32 ATP's

• Glycolysis: 2 ATP
• Kreb's 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
  • Each NADH produced in the conversion of pyruvate to acetyl COA and Kreb's 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!

Anaerobic Respiration

• Goal: to reduce pyruvate, thus generating NAD⁺
• Where: the cytoplasm
• Why: in the absence of oxygen, it is the only way to generate NAD⁺ and ADP
• Alcoholic 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 - fermentation - 2 ATP's produced, aerobic respiration - 36 ATP's produced
• 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

• 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 Oxydation

• 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 bad 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 reactions