Glycolysis, Krebs Cycle, and other Energy-Releasing Pathways

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 three major reaction pathways

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

The First Stage of Glycolysis

The Second Stage of Glycolysis

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
In the Krebs cycle, all of Carbons, Hydrogens, and Oxygeng in pyruvate end up as CO₂ and H₂O
The Krebs cycle produces 2 ATP's, 8 NADH's, and 2FADH₂'s per glucose molecule

The Conversion of Pyruvate to Acetyl CoA for Entry Into the Krebs Cycle

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 Krebs 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
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
- 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!

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
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 - 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

Energy Yields:

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 reactions

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.