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 keep 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.
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 photosynthesise (e.g. plant cells) can get their dose of glucose simply by doing the reaction above, but if organisms can’t photosynthesise (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.
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.
Glucose + Oxygen ➡ Water + Carbon dioxide
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:
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).
Whilst photosynthesis has two different stages, aerobic cellular respiration can be split into three:
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!
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.
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.
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.
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.
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’)!
Below is a diagram summarising the three steps of aerobic cellular respiration.
Image source: modified from http://www.bio.miami.edu/~cmallery/150/makeatp/c9x6cell-respiration.jpg
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.
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.
The efficiency of cellular respiration, like all chemical reactions, can be affected by certain environmental factors.
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.