Cellular Respiration First and Second Steps

Important Overview

When talking about cellular respirationThe process of gas exchange, including ventilation, external and internal respiration. it’s really important to have a map, whether that’s a visual map or whether it’s a map in your headRounded proximal end that fits into the acetabulum of the hip bone. thinking about locations of where things are happening.  So many different maps are out there online and many different books use different maps.  I like this map that’s being used here from our book.  Before we even dig into like what’s going on here in this map, there are some things that we can recognize.  Whenever you look at a complicated diagram like this, identify things that are familiar to you.  If I were to give you a map of America you probably zone in on whatever place it is that you’re living right now.  Looking at this map there are a few things that we can recognize. 

In the background of this diagram, we can see a mitochondrion.  Some steps are occurring outside it, in the cytoplasmThe gel-like substance within a cell that contains organelles and cytosol. of a cell, and some steps are happening inside it.  Location is important. 

At the bottom of this diagram, we can see that there are the star bursts of ATPThe energy currency of cells used for muscle contraction. and it also gives you how many are being created and by what process.  We’ll get to defining those processes.  We have carbon dioxide being created as a product and that makes sense to us because we know that we breathe out carbon dioxide as a waste product.  We have oxygen being used and converted into waterThe universal solvent essential for life..  And then the last thing that really seems familiar at this point is that on the top of this diagram there seems to be some of those electron taxis that were talked about in a previous PowerPoint called cellular respiration background.  If you look carefully, the electron taxis are being created in the first few steps and they’re all being funneled into this last step here.   The third step of cellular respiration makes a whole bunch of ATP, and also seems to be using the oxygen. 

Glycolysis

Glycolysis is the first step of cellular respiration.  The word glycolysisprocess of breaking glucose into two three-carbon molecules with the production of ATP and NADH means the splitting of glucoseA simple sugar that is the main source of energy for cells..  The suffix lysis means to split.  I think of this process of as the mugging of glucose where the mugger makes off with electronsNegatively charged subatomic particles found in atoms. inside of a wallet or a purse.  There is an enzyme in the cytoplasm that comes up to glucose, strips electrons off of it, and turns it into a completely new substance.  This enzyme’s name is dehydrogenase.  We know that the -ase suffix means that this is an enzyme.  What it does is strip hydrogen atomsThe smallest units of matter that retain the properties of an element., one electron and one proton, off the glucose.  De-hydrogen the glucose. 

Whenever we look at a step in cellular respiration or photosynthesis, it’s important to note the reactants and the products.  For glycolysis, we are starting with glucose and ending with two, 3-carbon chains called pyruvatethree-carbon sugar that can be decarboxylated and oxidized to make acetyl CoA, which enters the citr.  But, during this process we also turn two NAD+s into NADHs and we turn two ADPA molecule produced when ATP releases energy. into ATP. 

You see what some of what I’m saying here in this diagram, which is a very simplified overview of glycolysis.  We’re starting out with glucose which is a 6 carbon chain.  The carbons are noted as silver spheres in these diagrams.  We can see on the right hand side that were stripping 2 electrons off and loading an electron taxi.  We can see here on the left hand side that were creating an ATP moleculesGroups of atoms bonded together..  And what we’re left with after stripping the electrons in creating little ATP is two 3-carbon chains called pyruvate.  Considering we started with one 6 carbon chain it just looks like we broke glucose in half.  Yes.  The process of making ATP here is done via an enzyme, and so, this would be substrate phosphorylationaddition of a high-energy phosphate to a compound, usually a metabolic intermediate, a protein, or A.  I reviewed this topic in the Cellular Respiration Background Video.  You should stop here and view it, if you have not yet done so.

Glycolysis

Unfortunately, your textbook makes glycolysis overly complicated with ten or so steps.  You will find that different books, or teaching materials will have a different number of steps.  In a biochemistry class, you might have more than ten steps.  What I do like about your textbook is that is divides glycolysis up into two phases: an energy-requiring steps, along the top of this picture, and energy-releasing steps, along the bottom.  Yes, to get glycolysis started, we need a little bit of an energyThe capacity to do work or cause change. investment.  But, we will replace that energy, and more, when glycolysis is done.  Getting back to this diagram.  Don’t be overwhelmed.  Let’s look at what goes in: glucose, and what comes out: pyruvate.  And, over here on the right, we can see that an NAD+ goes in, picks up some electrons, and that NADH comes out.  There is a bit of a finer detail in this diagram that you might have missed.  Look at these three carbons over here that are green,  You can track them through the first five, energy-requiring steps.  This diagram then goes on to show you how one of these three carbon molecules then goes through the energy-releasing steps.  Note this little “2X” here reminding you that these steps down here happen twice.

The first part of the glycolysis pathway traps the glucose molecule in the cell.  In order to modify the glucose so that its six carbons can be split evenly into the two three-carbon molecules, a small investment of 2 ATP molecules is needed. The second part of glycolysis extracts energy from breaking the bones between the carbons in the glucose.  This energy is stored in the form of ATP and NADH.  First of all, we yield four ATP when we do this.  Yeah, but we invested two ATP in the first part, so we’re actually only yielding two ATP overall.  We know this, ATP molecules hold energy that cellsThe basic structural and functional units of life. can use.  But, NADH also holds energy that the cell can use.  Remember that NADH is an electron taxi and it is going to shuttle those electrons to the mitochondria and drop them off into the electron transport chain.  We’ll get to that.  I also want to point out that NADH is the reduced form of NAD+.  What does that mean?  Remember that reduction means that you have become more negative, or that you have taken on some electrons.  If this stuff about electron taxis and reduction is confusing, you should go back and review the Background for Cellular Respiration Mini-Lecture. 

Grooming Pyruvate

Unfortunately pyruvate is not exactly the substance that can enter into the official second step a cellular respiration called the citric acid cycleseries of enzyme-catalyzed chemical reactions of central importance in all living cells for extracti.  It’s like that big comforter that you just can’t fit into your own washing machine.  So, we have this step here called the grooming of pyruvate and this is like step 1b.  If you’ve ever seen the movie Fight Club when they are holding the night-store clerk at gunpoint, this would be similar to a basement apartment that gets the letter, like Apartment 1A.  It’s not an official step of cellular respiration, because no ATP is created, but it is a necessary step of cellular respiration. 

What we’re doing here is making pyruvate into a molecule that can be accepted by the first enzyme in the citric acidA substance that releases hydrogen ions (H⁺) in solution. cycle, the second step of cellular respiration. 

Let’s start with what we know.  The top of this diagram shows the stripping of two electrons and loading them into the electron taxis creating NADH.  This taxi will take these electrons to the third step of cellular respiration.  On the bottom of the diagram, it appears that we are creating a carbon dioxide molecule, which we know is a general waste product of cellular respiration.  This grooming of pyruvate requires an enzyme called Coenzyme A.  This enzyme will replace one of the carbons on the pyruvate, which causes this release of a carbon in the form of carbon dioxide.  What we end up with is a molecule called Acetyl CoA, which is the Coenzyme A stuck onto the carbon chain. 

Note that our carbon chain, which started as a 6-carbon chain, then converted to two 3-carbon chains, is now only two 2-carbon chains.  Take a step back and realize that all this is still taking place in the cytoplasm with enzymesProteins that speed up chemical reactions in the body..  We’ve not yet entered the mitochondria, although we’ve been sending electron taxis to the mitochondria.  Also, it’s important to realize that we didn’t make and ATP in this step of grooming the pyruvate. 

I do not at all like the diagram in your textbook about this process.  I have it pictured here on the top right of this slide.  I don’t like the visual representation, but I do like the text in the three boxes along the bottom of the diagram.  These three sentences are a very nice overview of the process.  Say each sentence out loud to yourself and connect it to what you see on the bottom diagram here.

Do you know what

My gosh what is going on in this picture?!?!?  Yes, this is quite complicated.  If we break it down into a couple things that we want to follow, then it becomes a little bit easier.  Your textbook, like all textbook, likes to makes steps of this process.  I’m OK with that, but remember that if you are consulting different references, you will find that they may have a different number of steps. Instead of going through the steps of the citric acid cycle that you see here labeled with the little red bubbles I’m going to lead you through realizing some of the things that are produced and just some of the processes that happen along the way.

Let’s remember where we are.  The citric acid cycle takes place in the juicy center of a mitochondrion that’s called the matrix.  Our carbon chain, now called acetylcholinealso know as ACh A neurotransmitter that stimulates muscle contraction. A, has moved from the cytoplasm into the mitochondrial matrix.

First thing we realize is that we are loading whole bunch electron taxis.  In factA statement based on direct observation that is repeatedly confirmed., we see that for every turn of the cycle we’re loading four electron taxis into NADH removing 8 electrons from our dwindling carbon chain.  There’s a new electron taxi that we can see here which is called FAD when it picks up its 2 electrons it turns into FADH 2.  I’m going to save discussion of this particular electron taxi for when we talk about metabolic pathways which I’m covering another PowerPoint fermentationprocess of regenerating NAD with either an inorganic or organic compound serving as the final elect and metabolic pathways. 

Secondly, we see that carbon dioxide is again being created as a byproduct of this process.  Referring back to the grooming of pyruvate we know that removing carbon dioxide actually takes one of the carbons from your dwindling carbon chain and removes it as a by product.  Because this gas is nonpolar, it will slip through the mitochondrial membranes into the cytoplasm, through the cell membrane, into your interstitial fluids, into your bloodstream, and finally make its way to your lungs where it will be exhaled as waste. 

ATP is also leaving this citric acid cycle.  Much like the very first step of cellular respiration this ATP is made by substrate phosphorylation, using an enzyme and creating only a small amount of ATP. 

The last thing we want to focus on in the citric acid cycle is the manipulation of our carbon chain which is also the most difficult to understand so we’re gonna keep it pretty simple.  At the top of this diagram you can see acetyl CoAcombination of an acetyl group derived from pyruvic acid and coenzyme A, which is made from pantothe entering into the citric acid cycle.  This is the molecule that’s able to enter because it fits with the specific enzyme that gets the cycle going.  Immediately the coenzyme A is stripped from our 2 carbon chain.  The coenzyme A can circle back to your cytoplasm to be used again.   And this is where it gets kind of confusing because then you look and you see that we’ve created a molecule called citrate which actually has 6 carbons did we put a couple of the 2 carbon chains together or what happened here?  You are going to love this.  The citric acid cycle already has a 4 carbon chain in it that exists as the last cycle.  On the top left hand corner of the cycle there is a molecule that’s a 4 carbon chain.  It starts the cycle.  This this should make you question how did that four carbon chain get in the cycle to begin with?  How can you start a cycle if you don’t have the final product of the cycle from a previous cycle? These are questions that we answer in the next part of this class we talked about evolution and the beginnings of life.  This newly created six carbon chain called citrate goes through the citric acid cycle breaking down into a 5 carbon chain and then into a 4 carbon chain and then it gets starts the cycle again.  This is how our citric acid cycle continues to go breaking down are carbons, excusing carbon dioxide, creating electron taxis, and making just a little bit of ATP.