Why Do We Have To Eat?

Why do we have to eat? - This simple question leads to lots of discoveries which are hidden in our body. Organisms like us (well, not just us - this covers bacteria in our guts to big oak trees in the woods) have a process called respiration which allows us to obtain a pack of energy we can use for several purposes. It has been described by the French scientist Lavoisier as a 'slow combustion of carbon and hydrogen'. How does our body burn carbon in oxygen without lighting up like a lamp? How do we control the amount of energy release from this process? We shall answers these questions soon. If we burn a bag of sugar, we see a big flame. The flame tells that there is energy in the sugar. This energy comes from the bonds that hold the carbon, hydrogen and oxygen atoms together in sugar But in our body, we couldn't light up the fire to burn all our food, could we? So, we have to slowly release the energy in the form of something else. Something which releases the optimum amount of energy, which is not too small or too inefficient,but also not too large to burn our whole body to death. We call that molecule ATP or adenosine triphosphate. It is a universal currency for energy which can be utilised in many ways. For example, we use one ATP to transport some molecules, like you pay the money at the gate to get in.. In this case, one sugar molecule could produce a maximum of 36 ATP which could be used to do things in a thousandth of a blink. But how does sugar become a pile of energy?How does our body process it? If we think about a dam which generates energy by using the waterwheel, the water falls from the cliff, hits and turns the wheel, then the turning produces the energy doesn't it? In our body, in the unit of life called the cell, there are small waterwheels in bags called mitochondria. These wheels are in the deeper parts of mitochondria (Think about your girlfriend's handbag, there is a bag inside a bag, don't ask me why). These deeper bags are called cristae and the water wheels actually stick to the bags. These water wheels turn when there is a proton gradient. Protons are particles which are responsible for acidic conditions. In the universe, there is a law that everything tries to balance,so particles move from higher concentration to the region that has a lower concentration. The same happens with protons - they move from higher concentration to lower concentration through a water wheel. Maybe I should rename this; I should call it a 'proton wheel', as the movement of protons turns it, but in this case, it's not electrical energy that's produced, guess what, it's ATP! ATP is resynthesised from this magic wheel, so I shall rename it again -as it produces ATP, this wheel is actually called ATP synthase. But to come back and think about it, how does the proton gradient actually build up? What is the missing key here? It is sugar! Sugar is the molecule that splits up and gives out hydrogen and energy to do work. How does it progress from table sugar to energy? Our body already has a special process for it, as I have said in the beginning. it's called respiration. If we think about sugar, sitting on the table, it wouldn't light itself up like magic would it? There must be something that triggers it to start the whole process. Special proteins called enzymes play an important role to speed up the reaction. In the very first step of respiration, glucose (the simplest sugar) has to be converted to a specific form that allows it to enter the cycle which produces lots of hydrogen atoms (which can split to form a proton and an electron) that could be used to produce energy. These specific reactions are all in a very precise order controlled by lots of enzyme in our body. But again, it's not as easy as it sounds -hydrogen is quite unpredictable (like a drunken man). If we think about sending your drunken friends back home from the pub, we need a taxi to make sure they will not fight with each other or lose their way. It's the same with the proton - there are two electron carriers (or taxis) called FAD+ and NAD+ which will carry all the hydrogen back to the mitochondria safely, so these molecules are crucial for life. The ext step is quite complicated, I warned you!From glucose to those specific molecules in the cycle, there are some interesting products and we shall see them. In the very beginning, we have glucose. I forgot to mention that glucose has six carbons in it, but this number is so important that we shouldn't miss it out. As I have said, glucose won't set fire to itself, neither magically change to those special molecules. We have to excite it and believe it or not, we use ATP in the very first step to produce energy- it's very weird, I know. But we use it to make the glucose become very reactive so that it can be converted further. TwoATP are used to split the glucose molecule in to two molecules which each have three carbons(6 divided by 2 is 3 -this does actually makes sense, doesn't it?). Unexpectedly, I would say surprisingly, two ATP is produced when the three carbon compound transform to that specific molecule. I think I should give thismolecule its proper name now after mentioning it for about four times! It's called pyruvate. Not only two ATP are produced, some hydrogen also falls off! And one NAD is used for each 3-carbon molecule. Let me remind you again that NAD behaves like a taxi that will take hydrogens back to the cristae (the deeper bag in the mitochondria) to generate a proton gradient and to produce ATP. After a few moments counting, we can see that from this process, two ATP are being used, two ATP are produced for each pyruvate, which makes four in total, and the net cancelled each other and made up two ATP. Don't forget two NAD that being used, the used NAD is called reduced NAD or NADH2 or NADH + H+. I will call it reduced NAD in this whole context. The story is not ending yet because pyruvate can't enter the cycle directly. There are a few more steps before the cycle starts, and that's when energy is going to be produced for real. If you recall from the last part, one molecule of glucose split in to two 3-carbon molecules called pyruvate and now, it's going to go further. Pyruvate (3 carbons) loses one carbon atom to form Acetate - aspecial 2-carbon compound which can finally, enter the cycle. Again, the carbon can't just fall off and form a lump of coal next to it. It splits as carbon dioxide, the same gas produced when you burn sugar. Also, some hydrogen does fall off so the NAD has to play its role again. If we count them carefully, don't forget that there are two of pyruvate so there are twice the amount of reduced NAD and carbon dioxide produced. Again, Acetate is another drunken man, he couldn't go straight to the cycle without a special taxi. This time the taxi is not NAD or FAD, it's a shorter journey. This taxi is called Coenzyme A or its nickname CoA. The acetate on a taxi CoA (now called Acetyl CoA) is now transferred to the next step where we could get lots of hydrogen and of course, release lots of energy from it. After the transport, CoA leaves Acetate to enter the cycle safely. The real cycle, I have to say, is quite complicated and unnecessary to be fully understood. This cycle is named after a man who discovered it, Sir Hans Krebs. Krebs cycle is the very key point of respiration. It's the place where lots of NAD is being used as the carbon compound is getting cut down (like in the link reaction, and again released in the form of carbon dioxide). To recall, acetate enters the cycle with 2 carbon, joining the party with a 4-carbon compound next to it to form a 6-carbon compound called citric acid (two plus four equals six, simple as that, nothing as complicated as someone might expect). You might have heard of citric acid when you look around the juice bottle, it's the same thing that is in lemon or some fruit. But I have to be honest sorry that it has nothing to do with this cycle, the chemical in the cycle couldn't just bump in to this specific cycle, but it could be used to do other things instead. Citric acid then got cut down to a 5-carbon compound (with a long, boring name). Again, carbon dioxide is produced and hydrogen is released, so another NAD is being reduced. Then the 5-carbon compound gets cut down to a 4-carbon compound and guess what, the same thing happens. Onecarbon dioxide is produced and NAD is being reduced. From that 4-carbon compound, it needs to be converted a bit more so that it can join the cycle again with acetate. This complicated conversion produces three things:ATP, reduced NAD and reduced FAD. Then it joins the cycle again. As you can see,the cycle produces lots of molecules from a single turn. Don't forget that one glucose molecule splits in to two in the beginning, so this cycle has actually turned twice for each glucose. This means that four carbon dioxide molecules are being produced from this cycle (two from each turn), six of reduced NAD (three from each turn), two ATP (one from each turn) and two reduced FAD (one from each turn). Now, all the molecules are ready to generate energy in the very last process, where all the protons will build up to form a gradient which will produce ATP. The last process is called the electron transport chain. As I have mentioned very early on that energy is produce from the movement of protons through a wheel called ATP synthase. This last step is very crucial as it is the location where the reduced NAD and reduced FAD will drop off their passenger, hydrogen atom. These hydrogen atoms lose their electrons to form hydrogen ions or protons. The electron is again, acting like a crazy man, but in this case the carrier for electron is quite special and is known as an electron carrier. Electrons lose their energy as they travel with the carrier, butwhere does the energy go? If not to produce ATP, the energy is used to move protons across the membranes which are against the concentration gradient. Imagine that you need to carry water to the top of the hill so that it could fall down to produce energy, then the same rule applies. Hydrogen ions then build up on the other side of the membrane called intermembrane space, which these hydrogen ion could move through ATP synthase to produce ATP. These steps sound frankly simple but they have their own mystery. What will happen if some of the carriers fail to pick up the electrons? What will happen if the gradient is not strong enough and no ATP is produced? The cell will stop, and fail to function, which leads to cell death. The last electron carrier is something that we are so familiar with, it is oxygen! Something that we breathe in and out every second, it picks up the last electron and reacts with proton to form water and that's the last product of respiration. To come and think about this carefully, people actually do count and calculate carefully how much ATP each reduced NAD and reduced FAD could produce . We couldn't end this conversation without a bit of maths. Reduced NAD could maximally pump several protons which could generate 3 ATP when FAD could only produce 2. The reason behind this is unexpectedly complicated and not yet revealed. So, by calculation, 36 ATP are produced as I have said in the beginning! Some organism can't live with oxygen. Weirdly enough, oxygen that is very useful to us behaves as a deadly poison to them. We are actually adapted to live in non-oxygen condition (with pain). Some organism like yeast (for a nice glass of beer) could live without oxygen. But what is the point of the whole process if the carrier can't produce any energy and all protons are not going to be used? Hence, the whole system stops, except glycolysis which doesn't produce many hydrogen atoms. Instead of having FAD or NAD as an electron carrier, other molecules carry out the role and produce ethanol (in plants and fungi - mushrooms or yeast) or lactic acid (in mammals). We should be familiar with lactic acid as it is the main reason that makes us get cramp. When our muscles don't gain enough oxygen, we switch back to respire anaerobically (without oxygen) to produce energy and the by-product (like lactic acid) makes you feel cramp. These sequences in the system are so well designed and as you can see that they are adapted to live in any condition. Not just us, but all organisms in the world. Should I say more about respiration? I think it is such an interesting topic to bring up on its own. The sequence has been developed in the very early life form to produce energy. Let's go back to the first question on why do we have to eat? If we don't eat, we will not have energy to do anything; our energy store will begin to run out and we'll die. There are lots of questions like this on earth which sound simple but actually have a very deep meaning behind them and that is why such a simple question, could lead to such a big discovery of the world.