Thursday, May 18, 2017

Steps of cellular respiration

Cellular respiration is a metabolic pathway that breaks down glucose and produces ATP. The stages of cellular respiration include glycolysis, pyruvate oxidation, the citric acid or Krebs cycle, and oxidative phosphorylation.

Cellular respiration is one of the most elegant, majestic, and fascinating metabolic pathways on earth. At the same time, it’s also one of the most complicated. When I learned about it for the first time, I felt like I had tripped and fallen into a can of organic-chemistry-flavored alphabet soup!

Luckily, cellular respiration is not so scary once you get to know it. Let's start by looking at cellular respiration at a high level, walking through the four major stages and tracing how they connect up to one another.

Steps of cellular respiration

Overview of the steps of cellular respiration.

During cellular respiration, a glucose molecule is gradually broken down into carbon dioxide and water. Along the way, some ATP is produced directly in the reactions that transform glucose. Much more ATP, however, is produced later in a process called oxidative phosphorylation. Oxidative phosphorylation is powered by the movement of electrons through the electron transport chain, a series of proteins embedded in the inner membrane of the mitochondrion.

These electrons come originally from glucose and are shuttled to the electron transport chain by electron-carriers form:

NAD+ + 2e-  + 2H+ → NADH + H+

FAD 2e- + 2H+ → FADH2
To see how a glucose molecule is converted into carbon dioxide and how its energy is harvested as ATP and NADH/FADH2 in one of your body's cells, let’s walk step by step through the four stages of cellular respiration.

1. Glycolysis. In glycolysis, glucose—a six-carbon sugar—undergoes a series of chemical transformations. In the end, it gets converted into two molecules of pyruvate, a three-carbon organic molecule. In these reactions, ATP is made, andNAD+
 is converted to NADH.

2. Pyruvate oxidation. Each pyruvate from glycolysis goes into the mitochondrial matrix—the innermost compartment of mitochondria. There, it’s converted into a two-carbon molecule bound to Coenzyme A, known as acetyl CoA. Carbon dioxide is released and NADHN generated.

3. Citric acid cycle. The acetyl CoA made in the last step combines with a four-carbon molecule and goes through a cycle of reactions, ultimately regenerating the four-carbon starting molecule. ATP, NADHN, and FADH​2​​  are produced, and carbon dioxide is released.

4. Oxidative phosphorylation. The NADH and FADH​2 made in other steps deposit their electrons in the electron transport chain, turning back into their "empty" forms (NAD​+ and FAD). As electrons move down the chain, energy is released and used to pump protons out of the matrix, forming a gradient. Protons flow back into the matrix through an enzyme called ATP synthase, making ATP. At the end of the electron transport chain, oxygen accepts electrons and takes up protons to form water.

Glycolysis can take place without oxygen in a process called fermentation. The other three stages of cellular respiration—pyruvate oxidation, the citric acid cycle, and oxidative phosphorylation—require oxygen in order to occur. Only oxidative phosphorylation uses oxygen directly, but the other two stages can't run without oxidative phosphorylation.

Questions & Reply (Q&R)

Q: How did you get the coenzymes NADH and FADH in the cellular respiration overview.

R: The NADH resulted from the reduction (gaining of electrons) of NAD+ (nicotinamide adenine dinucleotide). NAD+ is synthesized in other metabolic pathways. The FADH2 was made from adding a hydrogen to FADH. FADH was reduced from FAD (flavin adenine dinucleotide), which was synthesized in the cytosol and mitochondria.
I hope this helps a little. I am still trying to understand this as well.

Q: What is the role of NAD+ in cellular respiration. Why is the role NAD+ plays so important in our ability to use the energy we take in?

R1:  NAD+ is an electron transport molecule inside the cristae of a cell's mitochondria. In glycolysis, the beginning process of all types of cellular respiration, two molecules of ATP are used to attach 2 phosphate groups to a glucose molecule, which is broken down into 2 separate 3-carbon PGAL molecules. PGAL releases electrons and hydrogen ions to the electron carrier molecule NADP+. Each PGAL molecule has a phosphate group added to it, forming a new 3-carbon compound. These phosphate groups and the phosphate groups from the first step are then added to adenosine diphosphate or ADP, forming 4 ATP molecules. This also produces 2 molecules of pyruvic acid.

R2: To continue on with R1 answer, the NAD+ then goes on to oxidative phosphorylation, where it will deposit the electron it gained into the cascade of proteins. The electron will jump from protein to protein, letting it pump H+ (protons) from the mitochondrial matrix out into the intermembrane space between the inner and outer membrane of the mitochondria, and will eventually reach water. The H+ floating around will want to move into the mitochondrial matrix because there is a low concentration of H+ inside and a high concentration in the intramembrane space. The only way it goes inside is through ATP synthase, which will then create ATP using the H+ movement. That ATP goes on to power other cellular processes, and that is the energy that we use every day.

Q: If glycolysis requires ATP to start how did the first glycolysis in history happen?

R: Although ATP synthesis is a process that uses a lot of energy, it can occur randomly, but it isn't very probable. You'll need a lot of ADPs and Phosphate groups in order to get a very small portion of them to bind and become ATPs.
As much as I understand, it isn't very different from the idea of "how life began?". If there is a process that occurs an infinite amount of times, every possible result will occur, even the least probable.

Q: When it states in "4. Oxidative phosphorylation" that the NADH and the FADH2 return to their "empty" forms NAD+ FADH2, the author meant FAD when referring to the "empty" forms, right?

R: I believe so, may be a typo.

Q: In the Citric Acid Cycle (Krebs Cycle), would the four-carbon molecule that combines with Acetyl CoA be Oxaloacetic acid?

R: Yes. Acetyl CoA and Oxaloacetic Acid combine to form a six-carbon molecule called Citric Acid (Citrate).

Q: After oxidative phosphorylation, the ATP created is in the mitochondrial matrix, right? If so, how does it get out of the mitochondrion to go be used as energy?
R: Just like the cell membrane, the mitochondrion membranes have transport proteins imbedded in them that bring in and push out materials.

Q: WHEN does your body go through cellular respiration? I know what happens and why it does, but when does your body do it? When you are exercising? All the time?

R1: Cellular respiration is always happening, but goes at a faster rate when exercising.

R2: Cellular Respiration is very important to the body and as you stated it is occurring all the time. Similar to what R1 said it goes at a faster rate when you are exercising but there are also many other processes going on in your body such as fermentation when undergoing strenuous activity. Hope this helps!

Q: I have a question... Which part of the body will most likely use the cellular respiration? Is it lungs?

R1: No, its your brain.

R2: Cellular Respiration happens in your cells and you entire body is made up of cells, it goes on all throughout your body including your lungs and brain.

Q: What are the inputs of Glucose Oxidation? Because in the book its very specific on the outputs but the inputs are kind of hard to identfy?

R:  Depends on your syllabus! Respiration is very complex so different textbooks shorted it in different ways. Glucose is the main starting material, and various enzymes alter its shape to turn it into pyruvate. Here is a short summary of what is happening:

Q: How many ATP'S are produced in each step of cellular respiration?

R1: Glycolysis: Net production of 2 ATPs
Krebs Cycle: Production of 2 ATPs
Electron Transport Chain: Production of 34 ATPs (theoretically at highest efficiency)
This adds up to a total maximum of 38 ATP molecules produced as a result of cellular respiration.

R2: I'm almost sure these are the correct amounts:
Glycolysis produces 2 ATP,
Preparatory Reaction produces 2 NADH and 2 Acetyl CoA (and releases 2 carbons- CO2)
Citric Acid Cycle produces 2 ATP 6 NADH and 2 FADH2 (and releases 4 carbons- CO2)
Electron Transport Chain produces 32-34 ATP.

R3: During cellular respiration the mitochondria produces 36 Adenosine triphosphate.
R3.1: Actually due to energy loss it's most likely to be around 28-30 ATP It has a 36% efficiency rate which is exceptionally high for chemical reactions.

Q: Why is FADH2 used as an electron carrier in complex II of oxidative phosphorylation? Why not NADH since it yields more ATP than FADH2?

R: Because FADH2 has a higher electronegativty and the complex's of oxidative phosphorylation are in order of increasing electronegativty therefor skips the first one.

Q: In my textbook it says glycolysis occurs in the cytoplasm of the cell. What is the difference between the cytoplasm and the cytosol?

R: The cytosol is the gel-like fluid that the nucleus and organelles are suspended in & the cytoplasm includes everything in the cell (aside from the nucleus) plus the cytosol itself. So basically the cytosol is just a portion of the cytoplasm.

Q: Is the kreb cycle another name for the citric acid cycle?
R: Yes, it is either Krebs cycle (named after the leading scientist), citrate acid cycle or even tricarboxylic acid (TCA)cycle.

Q: Water is produced during which step in cellular respiration?
R:  Oxidative Phosphorylation. Self explains where the water comes from at 2:09 in the following video:

Q: What are the capacities of the lungs?
R1:  Maximum capacities of lungs in to in/out is 5 litres
R2: The maximum volume (total lung capacity) is approximately 6 liters but you typically only breathe in/out approximately 0.5 liters per breath.

Q: What is the H2O at the end of the oxidation phosphorylation used for?
R: It is released into the organism and taken up by cells, as it is an output of cellular respiration. Excess water present in the organism, which is produced by oxidation phosphorylation, can be released via urine.

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