Biochemical Reactions

Cellular respiration is used by all organisms. Autotrophs store carbohydrates and lipids using G3P from the light-independent reactions. Later, perhaps in response to an improvement in growing conditions, they tap into those stored energy sources. Heterotrophs similarly convert macronutrients they consume or have stored into ATP.

Just like in the light-dependent reactions, many organisms create a proton gradient that ATP synthase can use to phosphorylate ADP. Watch this process in the following animation from St. Olaf College.

ATPSynthase

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Look back at your graphic organizer from Unit 1, Activity 5 on nucleic acids, we learned about coenzymes like NAD+, FAD, and Coenzyme A.

 

Many biochemical reactions are used by cells in the steps of cellular respiration. Dozens of enzymes catalyze these reactions. For example, a cysteine amino acid in the active site of the enzyme glyceraldehyde-3-dehydrogenase is involved in both a reduction and phosphorylation reaction. Watch this animation from St. Olaf College.

ChemicalInteraction

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Interestingly, the substrate for glyceraldehyde-3-dehydrogenase is a molecule produced in photosynthesis. Likewise, using a proton gradient to produce ATP happens in both photosynthesis and cellular respiration. Even the overall chemical equation for aerobic cellular respiration reminds us of photosynthesis:

C₆H₁₂O₆  +  6 O₂  →  6 H₂O  +  6 CO₂

Energy for Movement

The primary purpose of cellular respiration is to produce ATP as needed for cells. If we think of food as fuel then how energy is stored and released from fuel is similar in many ways to energy transfer in food. When burning wood, heat is used to start the fire and it is also produced. In this video from the Genetic Science Learning Centre, we can see how ATP is also used to start cellular respiration and it is a product.

For example, a lynx burns a lot of energy when it is chasing prey. Its muscle cells demand ATP so mitochondria convert potential energy into kinetic energy. Predators like the lynx only run for short periods of time before they slow down and their energy stores are depleted: either they have caught their prey, or the prey escaped.

Similarly, we become aware of the limitation to using our muscles as they tire during strenuous or repetitive work. Perhaps our muscles shake as we try to do one more push-up. Or maybe we find we just can’t run any further so we stop to catch our breath. These are changes that we seem to do involuntarily. Our muscle cells continue to produce ATP to move, only we change the way it is made. Try this simple investigation to explore this further.

Steps in a Process

Early experiments of cellular respiration focused on the consumption of oxygen gas and the production of carbon dioxide. Many biologists believed respiration to be a variation of combustion. Heat is evidence of energy production in cells. In one experiment, the amount of oxygen used and carbon dioxide made by guinea pigs was found to be equal to the gases used and made from the burning of carbon. Yet other experiments showed that the amount of oxygen used by living organisms to burn different types of food did not follow any logical patterns. Indeed, experiments with yeast showed that they could produce carbon dioxide even in the absence of oxygen. The process of producing energy in yeast and other microbes is called fermentation. Fermenting microorganisms have the ability to change food and drinks and so are used to make a range of foods including pickles, yogurt, alcohol, bread and cheese.

The rate of fermentation is shown in the following five experiments. Interpret and analyse the following graphs to make appropriate conclusions.

Fermentation

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As we saw for photosynthesis, cellular respiration must include different reactions: some that involve oxygen, and some that do not. Enzymes play an important role.

Later research has identified that cellular respiration of glucose involves reactions that occur in the cytoplasm and some that occur in mitochondria in eukaryotic cells. This first process is called glycolysis. It occurs in the cytoplasm and produces a molecule called pyruvate.

2 ATP molecules are produced per glucose molecule by substrate-level phosphorylation. (definition: The reaction that makes ATP by moving a phosphate group from another compound to ADP.) From there, pyruvate can undergo different reactions depending on which species it’s in and also the amount of oxygen available to the cell. Glycolysis itself is composed of 10 individual reactions which you can see here: 

For this course there is no expectation for you to remember the sequence of reactions in glycolysis. For this reason, we can think of glycolysis as happening in 3 parts:

Some steps in cellular respiration are aerobic, while others are anaerobic. For most eukaryotes, oxygen is one of the requirements of life. Much more ATP is produced using aerobic processes helping to provide energy for more complex forms of life. Glycolysis doesn’t require oxygen but is the first step in glucose metabolism for both aerobic and anaerobic respiration. The fermentation done by microorganisms involves anaerobic respiration.

Smaller eukaryotic and prokaryotic organisms can survive longer in low oxygen conditions, and many have adapted to produce energy without any oxygen. Under these anaerobic conditions all the energy produced from glucose is only made in glycolysis. The 2 NADH molecules are recycled back to NAD⁺. In this way the NAD⁺ can be used again for a new round of glycolysis with a new molecule of glucose.

Cells have evolved two different methods of recycling NADH. In general, most eukaryotes and certain bacteria perform lactate fermentation whereas certain fungi, like yeast, and other bacteria perform alcoholic fermentation. These two processes are summarized in the animation below by Thomas E. Schultz of the College of Science and Engineering at Central Michigan University.

Mitochondrial Reactions

Mitochondria, like chloroplasts, have an interesting structure.

In addition to an outer phospholipid bilayer, they also have an inner bilayer membrane that is folded into cristae. The interior of the mitochondrion is a fluid-like gel called the matrix. It’s similar to the cytoplasm of a cell or stroma of a chloroplast, and contains enzymes, substrates and mitochondrial DNA. Aerobic respiration occurs in the mitochondria. Again, as in the chloroplast performing photosynthesis, the folded membrane increases the surface area and therefore helps to increase the rate of cellular respiration.

Aerobically, pyruvate can undergo further reactions. These reactions produce significantly more ATP for cells than anaerobic processes. After glycolysis there are 2 further steps in aerobic respiration: the citric acid cycle and the electron transport chain. As both these steps occur in mitochondria, pyruvate must first be transported across the mitochondrial membrane. Some biochemists view this as a separate step, between glycolysis and the citric acid cycle, called pyruvate oxidation. For our purposes in this course we don’t need to worry about this distinction.

The first reactions in the citric acid cycle include the movement of pyruvate into the mitochondrion.

In the process, pyruvate with its 3 carbons becomes a 2-carbon functional group, acetyl. Coenzyme A helps to deliver acetyl to the next enzyme in the citric acid cycle. The sulfhydryl group in coenzyme A binds to the acetyl group. For this reason we often see the chemical formula of acetyl-CoA showing this sulfide bond.

At this time, it’s worth noting that fatty acids and, under some circumstances, amino acids can also be used as sources of energy. Amino acids undergo deamination reactions in the liver. Those products are then converted into substrates for the citric acid cycle, including acetyl-CoA. 

Fatty acids are also taken into mitochondria and converted to acetyl-CoA. From an aerobic energy perspective, all roads lead to the citric acid cycle.

At this point, most of the chemical potential energy from glucose and other biological molecules has been converted to the two electron carriers: NADH and FADH2. These two molecules next move to the electron transport chain. (definition: a group of compounds in a membrane that pass an electron through a series of redox reactions coupled to moving H+ ions to produce a concentration gradient.) As the name suggests, electrons are moved, first from NADH and FADH₂ to integral membrane proteins. The terminal electron acceptor is oxygen. Finally, in the last step of aerobic respiration oxygen is used. The oxygen that we know is so essential to life plays the important role of accepting the electrons to combine with H⁺ ions making water. 

The amount of ATP produced for each glucose molecule is actually a bit less than the theoretical amounts shown here. Membranes are a bit leaky, allowing protons to pass back from the intermembrane space back into the matrix. The energy lost here is given off as heat, which is an important method of producing body heat. An easy number to remember is 30 ATP produced per glucose molecule: 2 ATP from glycolysis, 2 ATP from the citric acid cycle, and about 26 ATP from oxidative phosphorylation.

Changes to the reactions in cellular respiration sometimes have severe consequences. Mutations in the enzymes involved in aerobic and anaerobic pathways often cause serious, life-threatening diseases. Inhibitors of some of those enzymes can have toxic effects on the body.

Rate of Cellular Respiration

The reactions in cellular respiration are performed by a series of enzymes in a pathway. Enzymes in this type of pathway are often controlled using a process called feedback inhibition. As you watch this animation you will see how this form of inhibition expands our understanding of enzyme inhibition. 

In general, products from some of the steps of aerobic respiration inhibit cellular respiration. High levels of ATP, NADH, acetyl-CoA and citrate all inhibit enzymes earlier in the pathway. 

As ATP is consumed through other cellular processes inhibition stops and cellular respiration proceeds. Furthermore, certain molecules stimulate aerobic respiration, including high levels of O₂, ADP and AMP (adenosine monophosphate).

Thinking back to the muscle fatigue investigation at the beginning of this Activity, the rate of cellular respiration changes as we consume oxygen and ATP. We now have significantly more details we can use to interpret our observations. A good summary of these details is in your flowchart, but you can also see how the steps in cellular respiration relate to exercise.