Energy & Respiration simpliefied

7/27/2019 Energy & Respiration simpliefied

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

Note: this reaction is the opposite of photosynthesisThis reaction takes place over the course ofthree major reaction pathways

Glycolysis

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 4ATP's and 2NADH, but uses 2 ATP's in the process for a net of 2 ATP

and 2NADH

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
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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

o The Conversion of Pyruvate to Acetyl CoA

o 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

Krebs Cycle Animation

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)
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Therefore, the total numbers of molecules generated in the oxidation of pyruvate and the Krebs

Cycle is:

o 8 NADH

o 2 FADH2

o 2 ATP

o 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

Electron Transport Phosphorylation typically produces 32 ATP's

ATP is generated as H+ moves down its concentration gradient through a special enzyme calledATP synthase
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Net Engergy Production from Aerobic Respiration

Glycolysis: 2 ATP Krebs Cycle: 2 ATP

Electron Transport Phosphorylation: 32 ATP

o 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

o Each NADH produced in the conversion of pyruvate to acetyl COA and Krebs Cycle is

worth 3 ATP (8 x 3 = 24)

o Each FADH2 is worth 2 ATP (2 x 2 = 4)

o 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+

Alcohol Fermentation - occurs in yeasts in many bacteria

o The product of fermentation, alcohol, is toxic to the organism
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Lactic Acid Fermentation - occurs in humans and other mammals

o The product of Lactic Acid fermentation, lactic acid, is toxic to mammals

o 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 O2Respiration: 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 cycledthrough 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.o An eight-carbon fatty acid can produce 4 acetyl CoA's

o Each acetyl CoA is worth 12 ATP's (3 NADP, 1 FADH2, 1 ATP)

o Therefore, this short fatty acid is worth 48 ATP's, a fat with three chains of this length

would be worth 144 ATP's!

o 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!
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o 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

o The ATP generated from the mitochondria is only used when the plant cannot generate

ATP directly from the light-dependent reactions

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

o Enzyme catalyzing this reaction = Phosphofructokinase

o "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

o 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 aconformation change and shutting down the enzyme

Conversion of Pyruvate to Acetyl CoA

o Enzyme involved in catalyzing this reaction = pyruvate dehydrogenase

o 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

o NAD+, CoA, or AMP (an indicator of low ATP) can speed up the reaction
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Energy & Respiration simpliefied