陆怡璇老公戴向宇:V20110815 THE KREBS CYCLE

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: Glycolysis, Krebs Cycle, and other Energy-Releasing Pathways

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Glycolysis, Krebs Cycle, and other Energy-Releasing Pathways

: [ 12-17-2009 - 10:18 PM ]

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 four major reaction pathways

Glycolysis
Conversion of Pyruvate to Acetyl CoA (Oxidation of Pyruvate)
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 The First Stage of Glycolysis

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

The Second Stage of Glycolysis

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

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

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

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

The Krebs Cycle

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

Electron Transport Phosphorylation (Chemiosmosis)

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

Net Engergy Production from Aerobic Respiration

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!
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

: Glycolysis, Krebs Cycle, and other Energy-Releasing Pathways -||- : -||- :

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