Lecture 20: Cell Metabolism -REDOX and Water                                                     pdf _download pdf _download

 "lecture 20 WITHOUT voice-over" slide show... "right click" and "download"

 

Term Paper (15pts)

1. "Evolution is a man-made myth"

2. "An understanding of Genetics is fundamental to our understanding of how organisms work."

3. "Viruses are living organisms"

 

Choose one of the statements above, and provide TWO coherent arguments for me;

one in support of the chosen statement.

and one opposed to the chosen statement.

Minimally, each of your arguments should be half a page of 11pt, single-spaced writing.

Maximally, each of your arguments should be no more than one page of 11pt, single-spaced writing.

After you have provided your arguments, you must then provide in a "reference" section in which you put ALL your references on which those arguments are based...

Please submit your paper as a typed document (E-MAIL)...  by Monday, April 27th 11:59pm.

NOT 12:00am on Tuesday 28th.. or any time after that, but by Monday, April 27th 11:59pm.

When you do e-mail me your paper, please ensure that you give the Title or Subject heading: "BIO2107 Term Paper" in the subject line of the email.

        ___________________

ATP is Adenosine Triphosphate, the ultimate, "Universal Source" of useable energy in biological systems, as it can can be hydrolyzed to yield ADP an inorganic phosphate ion (Pi, short for HPO42-) and ENERGY.

 

              

ATP hydrolysis releases energy -making it available for the cell to use elsewhere.....

ATP consists of the nitrogenous base adenine bonded to ribose. Carbon 5 of the ribose has three phosphate groups attached.

ATP can be hydrolyzed to yield ADP and an inorganic phosphate ion (Pi, short for HPO42-).

 

                      ATP + H2O ----> ADP + Pi + free energy.

 

The change in free energy ( G) ~ 7.3 kcal/mol at a living cell’s typical temperature and pH.

The equilibrium for this reaction is far to the right of the equation shown above, gavouring the production of ADP ; so much so that there are approximately 1 x 107 ADP molecules to each remaining ATP.

Making ATP from ADP involves overcoming repulsive negative charges on the phosphates to be joined.

The energy that is necessary to do this in humans and many other living cells is generally stored as glucose (or condensed derivatives thereof....) or other fuel molecules and is released in the ensuing endergonic process.

aaaaa

Fats, a variety of sugars, and proteins can also be consumed for energy. However, a great many organisms (not all) preferentially use glucose as their primary fuel molecule.

As a consequence, almost all foods that are eventually used for energy are converted to glucose or to an intermediate metabolic product of its breakdown.

When burned in a flame, glucose releases heat, carbon dioxide, and water.

 

C6H12O6 + 6 O2 ---> 6 CO2 + 6 H2O + energy (heat and light).

 

The same equation applies for the biological, metabolic use of glucose. This process, however, has many steps and is carefully controlled.

 

When glucose is used up by the cell -as might be expected- about half of the energy is collected and "banked" in the cell's currency, ATP.

The delta G for the complete conversion of glucose is approximately - 686 kcal/mol.

The overall reaction is, therefore, highly exergonic, and it can consequently be used to drive the endergonic formation of a number of ATP's.

and    

Some kinds of cells metabolize glucose incompletely, and others do a much more efficient job and metabolize it completely.

Some kinds of cells metabolize glucose incompletely, and others do a much more efficient job and metabolize it completely.

The incomplete breakdown is called fermentation and is anaerobic (no oxygen involved).

The Complete capture of the energy in the hydrocarbon bonds of glucose requires oxygen -through what is termed aerobic respiration.

The two main metabolic processes for complete use of glucose are glycolysis and cellular respiration.

The entire Glycolytic process is a series of reactions that takes glucose (a six carbon sugar) to produce some usable energy and two molecules of a three-carbon sugar - pyruvate.

Cellular respiration occurs in aerobic (oxygen-containing) environments. Pyruvate can then be converted in to CO2 and H2O, with the energy resulting being used to make ATP molecules.

If cellular respiration does not occur, then fermentation ususally does.

Fermentation occurs when the environment is anaerobic (lacking gaseous oxygen).

In fermentation, lactate or ethanol and CO2 are produced, but far less energy is extracted from glucose.

How is this all brought about?    Essentially it all involves reactions called

"Redox reactions" which involve that rather important element "hydrogen" and the transfer of the energy of reactions, as they trasnitiofrom one form to another.

A gain of one or more electrons or hydrogen atoms is called reduction.

The loss of one or more electrons or hydrogen atoms is called oxidation.

Whenever one material is reduced, another is oxidized.

 

 

If a carbon compound gains a bond with hydrogen, the compound is reduced.
If a carbon compound loses a bond with hydrogen, the compound is oxidized.

 


Reaction 1 of the above diagram is oxidation because the changed carbon loses a bond with hydrogen. Reaction 2 is also oxidation because the central carbon in the reactant loses a hydrogen bond (even though the compound as a whole still has the same amount of hydrogens).

As a result of all this.....

An oxidizing agent accepts an electron OR a hydrogen atom (a proton + electron).

A reducing agent donates an electron OR a hydrogen atom (a proton + electron).

The ultimate redox reaction (in the biological world) is the formation of..... WATER

 

                                             H2 +  1/2 O2  --->   H2O

 

If this reaction were allowed to occur in one step....it would be... explosive

Moreover, the cell doesn't have any Hydrogen molecules hanging around (or else we would all float  away....). Even so, the cell is able to harness the energy of sugar metabolism to create an almost equivalent reaction.........to produce (of all molecules......) WATER!

During both the process of burning glucose and the metabolism of glucose, glucose is the reducing agent (and is oxidized), while oxygen (being the ultimate oxidation state) is the oxidizing agent (and itself is reduced).

 

                                 C6H12O6 + 6 O2 ---> 6 CO2 + 6 H2O + energy (heat and light).

 

The overall DELTA G (change in free energy) of a biologically related redox reaction should be -and is - negative. (i.e. it gives off energy!!!).

Most of the time, the "energy" of reactions does not go to ATP formation directly, but goes to an "energy production line" by being transferred in hydrogens (hydrogren ions) to another molecule -a coenzyme- named NAD, which can carry H+s (in its reduced form) and which can easily be converted to H2O -in the presence of O2

     

The molecule NAD is an essential electron carrier in cellular redox reactions.

NAD is short for nicotinamide adenine dinucleotide.

NAD exists in an oxidized form, NAD+, and a reduced form, NADH + H+.

NAD+ + 2e- + H+ ---->  NADH

 

The reduced form (NADH + H+) can be converted back into the oxidized form in the presence of oxygen:

 

NADH + H+ + 1/2 O2---->  NAD+ H2O

 

TheGo' of this oxidation reaction is -52.4 kcal/mol. (For comparisons sake, remember that the ∆Go' of the ATP to ADP reaction is ~7.3 kcal/mol.)

 

Think of NADH + H+, therefore, as a pre-packaged form of available "potential" energy source, that can eventually be turned into ATP and used to do work!!!

There is also and alternative hydrogen carrier, called FAD (Flavin Adenine Dinucleotide) that is not quite as efficient as NAD, but can also be used to carry Hydrogen in the cell.

 

  

 

FAD +  H2 ---->  FADH2

 

FADH2 + 1/2 O2----> FAD +  H2O

 

But what about the actual metabolism of Glucose:

It generally occurs in three stages

Glycolysis: From Glucose to Pyruvate

To get energy out of this overall reaction, energy first must be invested in the process.

In separate reactions, two ATP molecules are used to make chemical modifications to glucose.

Phosphates from each ATP are added to the carbon 6 and carbon 1 of the glucose molecule.

The figures below show for the details of this part of glycolysis.

              

In this part of the pathway most of the energy that is derived as ATP involves the direct synthesis of ATP directly in various reactions called substrate-level phosphorylation.

>Some cells under anaerobic conditions continue glycolysis and produce a limited amount of ATP by the process fermentation.

...There is a NET GAIN of TWO ATP's per glucose molecule -through fermentation.

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This form of anaerobic respiration produces incompletely broken down carbon skeletons such as lactic acid and alcohol

In the ABSENCE of oxygen the reduced form of NAD+ (NADH + H+) builds up and becomes a cellular toxin....

It therefore needs to be RECYCLED.....

Hence the formation of a reduced carbon intermediate : LACTIC ACID build up in muscle tissue feel the burn.

 

In the presence of oxygen, however, the reduced form of NAD+ (NADH + H+) can be converted back into the oxidized form ......

 

NADH + H+ + 1/2 O2 ---->  NAD+ H2O

 

....and just like the normal reaction this is an ENERGY yielding reaction... with almost the same energy yield.

 

H2 + 1/2 O2  ---->    H2O

 

Pyruvate Oxidation -Dehydrogenase.   In this reaction, the 3 carbon pyruvate is converted to a 2-carbon Acetyl group attached to a Coenzyme A configuration.  A molecule of NADH + H+ is generated during this reaction.

 

 

From there the acetylCoA gets involved with a "production line" system called.....

The Citric Acid Cycle / TCA cycle (Tri-Carboxylic Acid Cycle / Kreb's Cycle

  

 

The cycle begins when the two carbons from the acetate are added to oxaloacetate, a four-carbon molecule, to generate citrate, a six-carbon molecule.(2 + 4 = 6, so we're now back to 6!!)

 

Summary:

 

Glucose (6 carbon)---> Pyruvate (3 Carbon)                                            Net yield = 1 ATP and 1 NADH + H+

                                     ---> Pyruvate (3 Carbon)                                        Net yield = 1 ATP and 1 NADH + H+

Pyruvate (3 Carbon) ---> Acetyl CoA (2 Carbon) + CO2                           Net yield = 1 ATP and 1 NADH + H+          

Pyruvate (3 Carbon) ---> Acetyl CoA (2 Carbon) + CO2                           Net yield = 1 ATP and 1 NADH + H+      

 

Acetyl~ CoA (2 Carbon) + Oxaloacetate (4 Carbon)    ----->    Oxaloacetate (4 Carbon) + 2 CO2                

                                                                                                                     Net yield = 1 ATP,  1 FADH2 and  3NADH + H+

Acetyl~ CoA (2 Carbon) + Oxaloacetate (4 Carbon)    ----->    Oxaloacetate (4 Carbon) + 2 CO2                

                                                                                                                       Net yield = 1 ATP,  1 FADH2and  3NADH + H+

                                              Total Net yield = 4 ATP,   2 FADH2and  10 NADH + H+

 

The question now is, how exactly does the cell make use of all these hydrogens that are interacting with the various hydrogen carriers?  It passes them on to the....

 

 

 

 


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