09/12/2017
Energy, metabolism, mitochondria, Krebs cycle and how does the food we eat makes it possible.
If we were to summarise what metabolism is, we would say that it is the total sum of all reactions in our bodies. Things are broken down, built, converted into other things, transported in or out of cells. All this requires energy. Within this, I will talk specifically about the catabolic pathway called respiration where complex molecules such as carbohydrates, fats and protein are broken down in a process that releases energy that is then used for all the reactions I mentioned. Breaking down of complex molecules into simple building blocks also means that the body can now use those to build whatever it needs. This is called anabolism.
You probably heard many times that ATP is the energy currency in our bodies. Indeed, almost every process requires ATP hydrolsysis that releases energy that then powers various reactions. ATP stands for adenosine triphosphate. This means one ATP molecule contains three phosphate atoms and in a hydrolysis reaction, it loses one phosphate molecule and this is what releases energy.
Respiration, just like combustion of petrol, is a redox reaction. This means in a reaction, one molecule loses electrons (is oxidized) and another is reduced (gains electrons). The breakdown of glucose involves transfer of electrons that in turn allows ATP to be made. This reaction can be summed up as glucose + oxygen → carbon dioxide (which we breathe out) + water + energy. The foods we eat, especially carbohydrates and fats are great reservoirs of electrons (due to their atomic structure) that is why they are the preferred sources of energy.
Before I continue, I will remind you that all carbohydrates we eat end up as glucose molecules required for this reaction (other macronutrients are converted into intermediates that can be fed into particular steps of the reactions). Therefore for this purpose it is irrelevant whether you ate a sweet potato or a bag of sweets; all will be glucose in the end. The differences in the food choices we make of course are to do with all the nutrients a sweet potato will bring you versus simple sugar as well as time it will take to process and its effect on your blood sugar levels, but that’s a different topic.
There are four stages to respiration and I will describe them briefly. First we have glycolysis where our glucose molecule is split into two pyruvate molecules. Secondly we have the link reaction where pyruvate moves from the cell cytosol into the mitochondria where it is oxidised into acetate. Acetate in turn is picked up by coenzyme A, making acetyl coenzyme A – the molecule that can now enter the Krebs cycle. This cycle is a series of reactions whose main purpose is to provide a supply of electrons for the electron transport chain where most of the ATP will be made. The electrons will arrive on two molecules NADH and FADH2.
Now we finally get to the electron transport chain, which is located in the inner membrane of mitochondria. NADH and FADH2 arrive and here, in a different series of steps (hence the name of the chain), will transfer their electrons. Various molecules, proton pumps and carriers are located within this chain and make all this possible. The transfer of electrons through the chain causes an H proton gradient to accumulate in the intermembrane space of the mitochondria but due to the proton motive force mechanism (a gradient that essentially makes H+ want to come back down) H+ do come back into the mitochondrial matrix. Their only way to do so is via an enzyme called ATP synthase, which as the name suggests makes ATP. Every time an H+ passes, the enzyme adds a phosphate onto ADP - adenosine diphosphate. Now, we have made energy!
The end of this process, and reason why we need oxygen, is that an oxygen molecule is the final electron acceptor. It mops up the hydrogen atoms to make water. This is why if we have no oxygen, we cannot make ATP, all bodily reactions stop (since they have no energy) and we die. The importance of the chain is also demonstrated by the use of poisons such as cyanide. It blocks one of the ETC steps thus inhibiting ATP production.
So now we know how energy to live is made, we can see why mitochondria are crucial organelles to this process and anything that affects their working, will have a tremendous impact on energy production and your life. Mitochondria are also unique from the rest of the organelles within cells because they contain their own DNA (all the DNA for our genes in contained in the cell nucleus). That mitochondrial DNA codes for the enzymes necessary to make the ATP like the proton pumps and carriers (and other necessary molecules) within the electron transport chain. Anything that damages the mitochondrial DNA, will mean those enzymes are malfunctioning and cannot produce ATP properly resulting in disease. This includes inherited mutations as well as acquired ones. Mitochondrial DNA is especially vulnerable and so any reactive species such as toxic compounds from our environment (pesticides, pollution, components of daily care products, plastics) can attack it. Due to the importance of mitochondria in crucial to life energy production, anything that causes its dysfunction has multi organ consequences especially where energy requirement is most prominent such as the brain, heart and muscle function.
Image source: hithigherhighs.com.au/peak-performance/mighty-mitochondria
