MET - Cellular Respiration [LESSON]

Cellular Respiration

Cells use cellular respiration to “recharge” their ATP batteries and produce as much ATP as possible from sugar and oxygen. In fact, this pathway (including the enzymes and DNA codes used to produce those enzymes) is almost identical in all living things, providing strong evidence for the interrelatedness of all species. Let’s look at each of the four big steps of cellular respiration in this lesson.

Redox Reactions

First, let’s have a quick overview of oxidation-reduction (redox) reactions. You may have learned some of this in your previous chemistry course. In AP Biology, you simply need to know what redox reactions are and which reactants and products in these major pathways are oxidized or reduced. Redox reactions occur when there is a transfer of electrons from one molecule to another during a chemical reaction. One molecule is oxidized (loses electrons) while the other is reduced (gains electrons). We’ll revisit this concept often in the next two lessons.

Click here to reveal a great mnemonic device used to remember which is which!

OIL RIG – oxidation is LOSS of electrons and reduction is GAIN of electrons

Electron Carriers

Electron carriers, also called electron taxis, are small organic molecules that play key roles in cellular respiration and other metabolic pathways. The name is a good description of their job: they pick up electrons from one molecule (becoming reduced) and drop them off with another (becoming oxidized). There are two types of electron carriers that are particularly important in cellular respiration: NAD⁺ (nicotinamide adenine dinucleotide, shown below) and FAD (flavin adenine dinucleotide). 

 

Remember that the empty electron taxi (NAD⁺) is the oxidized form and when it “picks up electrons” from something else during a redox reaction, it becomes reduced (NADH). Then, it can “drop off electrons” with a different molecule and become oxidized again.

Cellular Respiration

Cellular respiration is the central metabolic pathway to ALL living things and the purpose is to break down glucose to make lots of ATP. In order to do this, cells must harvest the high energy electrons in glucose and transport them via an electron taxi to a complex system of enzymes. Let’s look at the overall reaction below, split into two since they are coupled together.

Overall, glucose is oxidized to carbon dioxide in the first two steps. Oxygen is reduced to water in the third step. ATP is produced in a small amount in the first and second steps, but the vast majority is made in the last step. Let’s look now at each of the four steps.

Glycolysis

Glycolysis occurs in the cytoplasm of both prokaryotes and eukaryotes. During this process, enzymes in the cytoplasm chop glucose in half to produce pyruvate (a 3-carbon sugar). To begin, two molecules of ATP are required to phosphorylate glucose which makes it very unstable and willing to split. Once the glucose has been phosphorylated, it is split into two 3-carbon compounds called pyruvate. Finally, energy is generated through the production of four ATP and the reduction of NADH. This ATP is created through substrate level phosphorylation, which means that the phosphate is taken from an intermediate in the pathway to create the ATP. There is a net production of 2 ATP and 2 NADH during glycolysis.

 

You can see how complex glycolysis is from the image above. It’s actually 10 different steps and therefore, 10 different enzymes. However, you just need to know the overall concept here which is that glucose is cut in half to make a couple of ATP and to load up a couple of NAD⁺ electron taxis.

 Citric Acid (Kreb’s) Cycle

Pyruvate will be shuttled into the mitochondrial matrix (the very inner part of the mitochondrion) as long as oxygen is available. Here, it will be converted into a molecule called acetyl coenzyme A (acetyl co-A) which releases carbon dioxide and loads up an electron taxi (NAD⁺). This acetyl co-A molecule is what will enter the citric acid cycle. The citric acid cycle is (again) extremely complex and driven by enzymatic reactions. The overall purpose is to completely break down (oxidize) the remainder of the sugar molecule and load up as many electron taxis (NAD⁺ and technically a couple FAD) as possible. You can see from the image below how complex the cycle is. It is called a cycle because the final product is also the first reactant.

 

Watch the Cellular Respiration video to learn more about the first two steps of cellular respiration.

Electron Transport Chain

The full electron taxis (NADH) from both glycolysis and the citric acid cycle shuttle electrons to the next step called the electron transport chain (ETC).  There are four protein complexes embedded in the inner mitochondrial membrane which move electrons in a series of redox reactions. These reactions are exergonic and release energy which is used to move protons (H⁺)  from the mitochondrial matrix to the intermembrane space. This build up of hydrogen ions in the intermembrane space creates an electrochemical proton gradient. At the end, complex IV reduces oxygen (picks up electrons and protons) to water. Oxygen is considered the final electron acceptor. This is the reason why you breathe oxygen. Some anaerobic organisms will use a different molecule (often sulfur) to pick up electrons (called anaerobic respiration), but most organisms use oxygen (aerobic respiration) because it is the most efficient.

 

Chemiosmosis

Now that there is a large proton (H⁺) gradient, the protons want to move passively back into the matrix. The only way through is an enzyme called ATP synthase. The passage of protons through this enzyme physically turns the six subunits on one side and allows a phosphate group to be added to ADP to form ATP.  Recall that ATP is extremely unstable and so creating it is very endergonic. This is why it is coupled with the exergonic breakdown of glucose. The process of making ATP this way (using the electron transport chain and then ATP synthase) is called oxidative phosphorylation and it is MUCH more efficient than substrate-level phosphorylation.

Fermentation

If oxygen is not present in the cell, glycolysis is followed by a different pathway called fermentation. Fermentation is an anaerobic process and does not produce much energy because it does not use oxygen or an electron transport pathway. There are several forms of fermentation. Two of these forms are lactic acid fermentation and alcoholic fermentation. Both turn pyruvate into something other than acetyl co-A in order to oxidize (unload) the NADH electron taxis formed in glycolysis. This allows glycolysis to continue running which produces a little bit of ATP so the cell can temporarily survive.

Watch the Respiration Part 2 video below to learn more about the last two steps of aerobic cellular respiration and fermentation.

Feedback Inhibition

Recall that feedback inhibition is when the end product of a pathway can serve as an allosteric inhibitor of the pathway (see Lesson 2 for review if necessary). The main site of feedback inhibition for cellular respiration is an enzyme in glycolysis called phosphofructokinase (PFK). ATP binds allosterically to PFK and turns it off since the cell has plenty. ADP, on the other hand, activates PFK and enhances respiration since the cell needs to make more ATP. This is a great example of feedback inhibition which regulates energy levels even in a large metabolic pathway like respiration.

Take a look at the Respiration Model below by clicking through the learning object below. There are 2 slides in the activity, so use the right arrow of each slide to advance to the next.

[CC BY 4.0] UNLESS OTHERWISE NOTED | IMAGES: LICENSED AND USED ACCORDING TO TERMS OF SUBSCRIPTION