Biology, Answering the Big Questions of Life/Metabolism/Metabolism3

=Glucose Catabolism=

How many ATPs are generated by Aerobic respiration?
(Table modified from [] Oct 2007).

What is the purpose of anaerobic and aerobic respiration?
The sugar glucose is the major food molecule in the cell, but it is too energetic to use directly in most chemical reactions. To be useful, glucose is broken down into an energy storing molecule (ATP) that can be used throughout the cell.

Why do cells need to ferment if they already get 2 ATP from glycolysis?
Glycolysis yields 2 net ATPs and 2NADHs. NADH is another high energy molecule. (NAD has low energy, NADH has higher energy). NADH has many fewer uses in the cell than ATP. It is normally converted into ATP in the mitochondrial electron transport chain if oxygen is present.

If no oxygen is present, then NADH builds up and the cell can run completely out of NAD. Without NAD glycolysis stops. NAD becomes a "limiting reagent" The chemical whose concentration determines whether the reaction will happen or not.

In the absence of Oxygen, the cell runs out of NAD and glycolysis is stopped until it can be regenerated. To regenerate NAD the cell uses a process called Fermentation.

In Fermentation, Pyruvate is transformed into another molecule using the energy provided by NADH. NADH gets converted to NAD so that it can be used again in glycolysis, and pyruvate becomes Lactic Acid in animal cells, or Ethanol + Carbon Dioxide in plants, yeast, and bacterial cells. (Lactic acid builds up in muscle cells and causes cramps. When oxygen is present again, lactic acid is converted back into pyruvate and broken down by aerobic respiration).

The anaerobic pathway is glycolysis + fermentation. This pathway recycles the NADH generated, so the only energy molecules made from the breakdown of sugar by this pathway is 2ATP for every glucose molecule. The molecules made by anaerobic fermentation still contain lots of energy in the form of chemical bonds. Anaerobic fermentation in not a very efficient pathway to yield energy from glucose.

What pathways make up aerobic respiration?
Aerobic respiration starts by breaking Glucose down into Pyruvate by glycolysis. Next in a preparatory step for the Krebs cycle, Coenzyme A joins to pyruvate causing a loss of one carbon and the generation of NADH. The acetyl-CoA formed enters the Krebs cycle and the acetyl group is transferred to a molecule of oxaloacetic acid making a molecule of citric acid. The Krebs cycle releases CO2 and the high energy molecules NADH, and FADH2 which are converted into ATP by the mitochondrial electron transport chain.

Why do we need oxygen to break down glucose completely by aerobic respiration?
Aerobic respiration requires oxygen because oxygen is the final electron acceptor of the electron transport chain in the mitochondria. If there is no oxygen to accept electrons, then the electron transport chain stops working and the high energy molecules NADH and FADH2 cannot be converted back into NAD and FAD. Without these molecules, the glucose biochemical pathway stops. These molecules become the limiting reagents needed for glucose break down to continue, and when they run out, the pathway stops.

How does the electron transport chain convert NADH and FADH2 into ATP?
In order to get the full amount of energy released by the breakdown of sugar, you must convert the high energy molecules NADH and FADH2 into ATP. This occurs in the inner membrane of the mitochondria.

Energy in the form of electrons is taken from these molecules and passed from molecule to molecule in a molecular game of "hot potato" until it is ultimately dumped on oxygen. The extra pair of electrons allows oxygen to make another chemical bond, and it makes a water molecule. This frees up the electron transport chain to take another pair of electrons from an FADH2 or a NADH.



The electron gradient
The mitochondria contains two compartments, the matrix which is the area inside of the inner membrane of the mitochondria, and the intermembrane space which lies between the inner and outer membranes of the mitochondria.

The Kreb's cycle occurs in the matrix of the mitochondria. This is where NADH and FADH2 are produced. They travel to the inner membrane and dump their electrons onto the membrane. This loss of electrons is a redox reaction and converts NADH back into NAD while FADH2 changes back into FAD.

The membrane proteins in the Electron Transport Chain are protein pumps. The passage of electrons across them makes them change shape and pump protons across the inner membrane from the matrix to the intermembrane space. Each NADH pumps three protons whereas each FADH2 pumps two protons.

NADH = 3ATP

FADH2 = 2ATP

This pumping of electrons across the inner membrane causes a concentration gradient of Hydrogen atoms across the membrane. By diffusion, the hydrogen ions will want to travel back into the matrix to reach equilibrium. They can do so by traveling through a special channel found in the membrane called ATP synthase. This channel uses the energy of the passage of the Hydrogen ions to make ATP. For each proton that passes, one ATP is made. This is why each NADH makes three ATP and each FADH2 makes 2 ATP.

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