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Decipher the ins and outs of cellular respiration

Jen Huang

While some think that money makes the world go round, the argument that photosynthesis and cellular respiration make the world go round has more scientific merit. One or both of these processes are carried out by all cellular organisms. They’re crucial because they help cells get the energy they need to survive, and even keeps Earth’s atmospheric composition in check.

However, one or both of these processes – especially cellular respiration – are also extremely confusing from a VCE student’s standpoint. What exactly goes in and out of each step, and what’s the easiest way to understand this? Why is the Krebs cycle said to be aerobic? What factors affect the efficiency of cellular respiration? In this article, Jen Huang, one of our amazing SuperTutors, sheds some light on this challenging topic, and gives you some of the tools SuperClasses students have mastered to understand it.

The aerobic cellular respiration reaction

Glucose is the major product of photosynthesis, and in fact it’s the very chemical that’s ‘burnt’ in cellular respiration to produce energy for the cell. So it’s a pretty important molecule. Cells that can photosynthesis (e.g. plant cells) can get their dose of glucose simply by doing the reaction above, but if organisms can’t photosynthesis (e.g. us), they have to obtain glucose in the food they eat.

For cells to actually access and use the energy stored in the bonds of glucose, it must be converted to ATP. The process of this conversion is called cellular respiration.

There are two broad types of cellular respiration: anaerobic cellular respiration and aerobic cellular respiration.

  • Since the prefix ‘an’ means ‘without’ or ‘in absence of’, it follows that anaerobic cellular respiration occurs without oxygen as a reactant.
  • The word ‘aerobic’ by itself means ‘requiring or involving free oxygen’. Therefore, it makes sense that aerobic cellular respiration has oxygen as one of its reactants.

When we talk about how these two types of respiration work in our next tutorial, we’ll discover how much difference a little oxygen can make to the efficiency of cellular respiration. For now, let’s focus on the aerobic side of things.

Word equation:

Glucose + Oxygen ➡ Water + Carbon dioxide

Chemical equation:

C6H12O6 + 6O2 + 36-38 ADP + 34-36 Pi ➡ 6H2O + 6CO2 + 36-38 ATP

A few things to be aware of about these cellular respiration equations:

  • It’s common practice to include ADP-ATP conversions in the chemical equation. Doing so shows that you acknowledge the whole point of cellular respiration, which is to pump out ATP.
  • Can you see how, apart from the ADP-ATP conversions, the word and chemical equations for aerobic cellular respiration are the reverse of those for photosynthesis? That is, the reactants for aerobic cellular respiration are the products for photosynthesis, and vice versa.
  • Since we know that the opposite of all endergonic, anabolic reactions are exergonic and catabolic, we now know that aerobic cellular respiration must be an exergonic and catabolic process. We can also tell this by the fact that ATP, an energy source, is produced by the reaction (i.e. we get a net energy output), and the fact that glucose (C6H12O6); in addition, our reactant (glucose) is a lot more complex and bigger than our product, CO2 (meaning the reaction is catabolic).

Note: it’s super important to remember both the word and chemical forms of the cellular equation, as it’s a commonly tested topic in exams! But don’t stress: if you remember one of the photosynthesis or aerobic cellular respiration equations, you’ve automatically remembered the other one (since they’re almost exact mirror images).

The stages of cellular respiration

Whilst photosynthesis has two different stages, aerobic cellular respiration can be split into three:

  • Glycolysis: this occurs in both aerobic and anaerobic cellular respiration. The process essentially splits the glucose molecule, yielding a bit of ATP.
  • Krebs cycle (citric acid cycle): in aerobic respiration, the product of glycolysis can undergo further reactions to produce more ATP and ‘load up’ lots of proton and electron carriers.
  • Electron transport chain: finally, in aerobic respiration, all the proton/electron carriers that stocked up on H+ and electrons during the Krebs cycle get to dump their electrons and hydrogen ions in this ‘transport chain’, which provides energy to produce even more ATP.

Tips for understanding each stage

  1. The diagrams accompanying each of the in-depth analyses below are a great way to understand and memorise the inputs and outputs of each step. The straight arrows show the conversion of the most important substrates into the most important products, so if you’re finding this topic challenging, remembering the things on either side of that arrow is your best bet. If you’re looking to gain a fuller picture of what’s happening in each step, look at the substrates and products on either side of the curly arrow.
  2. In addition, a fantastic way to keep things in perspective is to become a kind of carbon accountant. Carbons are represented by blue circles labelled ‘C’ in each diagram. If you recall that the amount of carbons going into the reaction should be equal to the amount of carbons coming out (even if the molecule the carbon is present in changes), then the identity and quantity of inputs and outputs start making a whole lot of sense.

Note: The VCAA study design makes it very clear that you don’t need to know the details of what happens in each stage; you only need to know the chemicals that go in and come out of each stage, and where in the cell each stage happens.

However, in addition to discussing this essential information, we’ll also be delving a little into the mechanics of how aerobic cellular respiration works to make things more interesting. Just remember that the text under the ‘Process’ heading isn’t as important as the rest of the information!

Glycolysis: glucose ➡ pyruvate, plus NADH

Inputs: one glucose (C6H12O6), 2 NAD+, 2 ATP, 4 ADP, 4 Pi
Outputs: two pyruvate molecules, 2 NADH, 4 ATP (Net output of 2 ATP)
Location: enzyme rich cell cytosol (i.e. NOT the mitochondria)
Process:
  • In an anaerobic process (one that doesn’t need oxygen), glucose is broken down into two pyruvate molecules. This catabolic reaction is also exergonic, and the energy it releases is used to make a net amount of 2 ATP.
  • The breakdown process also releases some highly energetic electrons and H+ ions, which are scooped up by NAD+ to make 2 NADH molecules.

Note: In an exam question asking you how many ATP are produced in glycolysis, simply writing ‘two ATP’ won’t cut it. You MUST say ‘NET two ATP’, since in actual fact there are 10 reactions in the process of glycolysis, and during this a total of four ATP are made. However, since two ATP are also used up as reactants, we produce only two ATP overall.

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Note that in the diagram above, the structures of glucose and pyruvate are simplified for clarity, and don’t represent the actual chemical makeup of these molecules.

Krebs cycle: pyruvate ➡ carbon dioxide, plus lots of NADH and FADH2

Inputs: two pyruvate molecules, 8 NAD+, 2 FAD, 2 ADP, 2 Pi
Outputs: 6 CO2, 8 NADH, 2 FADH2, 2 ATP
Location: mitochondrial matrix
Process:
    • Inside the enzyme-rich mitochondrial matrix, all of the carbon and oxygen atoms are shaved off our two pyruvate molecules (which kind of look like a glucose molecule cut in half), and expelled as 6 CO2 molecules. Can you see how the number of carbons removed as CO2 (six) is the same as the number of carbons originally present in glucose?
    • In this respect, the Krebs cycle is kind of like the opposite to carbon fixation in light independent photosynthesis: instead of adding carbons and oxygens to a molecule, we take them away.
    • High energy electrons and hydrogen ions (H+) released in this process are loaded onto NAD+ and FAD to produce NADH and FADH2. The process also yields 2 ATP.

Note: the Krebs cycle, which is also called the citric acid cycle and the tricarboxylic cycle, is considered aerobic because it requires heaps of NAD+ and FAD as reactants to work. As you’ll see soon, these ‘unloaded’ electron carriers can only be made as products of the electron transport chain (which is absolutely aerobic). Therefore, without the aerobic electron transport chain, we simply can’t have the Krebs cycle.

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Note that in the diagram above, the structure of pyruvate is simplified for clarity, and doesn’t represent the actual chemical makeup of the molecule.

Electron transport chain: ADP ➡ ATP

Inputs: 10 NADH and 2 FADH2 (from glycolysis and Krebs cycle), 6 O2, 32-34 ADP, 32-34 Pi
Outputs: 6 H2O, 10 NAD+, 2 FAD, 32-34 ATP
Location: cristae of mitochondria
  • Process:
    • Remember all the NADH and FADH2 we made during glycolysis and the Krebs cycle? They dump all the high energy electrons and H+ ions they’ve picked up so far into a series of proteins embedded in the mitochondrial cristae, collectively called the electron transport chain (ETC). In doing so, they become NAD+ and FAD again.
    • Just like in the light dependent stage of photosynthesis, the ETC involves the electrons getting passed from protein to protein embedded in the mitochondrial cristae (a bit like a hot potato). At each protein, a bit of each electron’s energy gets sucked out of it, until it has no available energy left.
    • This harvested energy is then used to plug a Pi group onto ADP to make 32 to 34 ATP molecules.
    • The oxygen comes in right at the end of the aerobic cellular respiration process. It helps mop up all the de-energised electrons and hydrogen ions that are left over from the ETC, thus acting as a terminal acceptor molecule for these particles. When O2 combines with some H+ and electrons in the right ratio, some H2O is made.

Note: ‘ETC’ isn’t a universally accepted abbreviation for ‘electron transport chain’, so it’s a good idea to write a little key when using it (i.e. ‘ETC = electron transport chain’)!

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Below is a diagram summarising the three steps of aerobic cellular respiration.

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 Image source: modified from http://www.bio.miami.edu/~cmallery/150/makeatp/c9x6cell-respiration.jpg

Aerobic vs anaerobic cellular respiration

What if there isn’t enough oxygen around to mop up spent electrons and hydrogen ions in the ETC?

The lack of oxygen shuts down both the Krebs cycle and the electron transport chain, since many of the Krebs cycle’s reactants are the products of the ETC (a directly aerobic process).

However, since glycolysis doesn’t need oxygen, it can still go ahead without the Krebs cycle and the ETC. In fact, it becomes the only ATP-yielding process in anaerobic cellular respiration. This means anaerobic respiration can give us a maximum of net 2 ATP per glucose molecule: a tiny amount compared to the 36 to 38 ATP we get per glucose if we use aerobic cellular respiration.

The lack of efficiency is not the only problem with anaerobic respiration. You see, in aerobic respiration, oxygen molecules mop electrons and hydrogen ions at the end of the ETC, thereby converting FADH2 and NADH back to FAD and NAD+ so that they can be used again to make more ATP. If there’s no oxygen around, the cell needs an alternative way to free up the electron/proton carriers. Therefore, anaerobic respiration results in the creation of toxic by-products.

  • Alcohol fermentation is the anaerobic respiration pathway in most plant, bacterial and fungal cells. During the process, each glucose (C6H12O6) is converted to two ethanol molecules and two carbon dioxide molecules, with a net yield of 2 ATP.
  • Lactic acid fermentation is the anaerobic respiration pathway in animal cells. During the process, one glucose (C6H12O6) is converted to two lactic acid molecules, with a net yield of 2 ATP.

Note: Both alcohol and lactic acid are cytotoxic (in other words, toxic to cells in sufficient concentrations). In fact, lactic acid is what makes your muscles feel sore when you’re exercising.

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Factors affecting cellular respiration

The efficiency of cellular respiration, like all chemical reactions, can be affected by certain environmental factors.

  • Factors affecting enzyme activity: Since both aerobic and anaerobic cellular respiration are catalysed by enzymes, temperatures, pH levels, cofactor and inhibitor concentrations, and substrate (e.g. glucose) concentrations must be kept optimal to ensure efficient photosynthesis.
  • Oxygen availability: Despite being a substrate and therefore a factor affecting enzyme activity, oxygen deserves a dot point all to itself. If abundant oxygen is present in the mitochondria, the aerobic processes of the Krebs cycle and electron transport chain can go ahead, enabling higher rates of ATP production. In times of oxygen deprivation, only anaerobic respiration can occur, leading to much lower ATP yields, not to mention cytotoxicity.

In summary

As you can see, understanding cellular respiration involves more than just remembering the word equation and calling it a day. However, with the handy tricks and SuperClasses-level explanations to help you connect the dots, it’s also not the immense challenge that you thought it was.

Use the same thought processes, diagrams and analytical techniques you applied here to photosynthesis, and you’ll find yourself on the way to mastering both topics.

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