Metabolism 101: Food and Anaerobic Respiration

I’m not sure how humans began using microbes (bacteria and fungi) in food production but it was well before we even knew what microbes are. Nevertheless, microbes are useful because they can undergo a process called fermentation. This process is similar to human anaerobic respiration but produces different end-products.

See, microbes are different from us and can actually thrive in anaerobic or oxygen lacking environments. Bacteria are quite simple and unlike our cells lack key components necessary for aerobic respiration. Therefore, they undergo fermentation which results in the production of byproducts such as ethanol (alcohol), carbon dioxide, or lactic acid. Although simply byproducts for the bacteria, humans taken advantage of them to make food.

So what are some of these commonly used microbes?

Saccharomyces cereviceae is a common strain of yeast. These little guys have been used everywhere from my old lab to wine, beer, and bread making. They convert sugars present in foods into ethanol and carbon dioxide. These sugars range from maltose in barley for beer, glucose and fructose in grapes for wine, or starch in wheat for bread. The alcohol gives beverages its distinct properties while the carbon dioxide helps develop taste and fizziness.  In terms of bread, the carbon dioxide makes the dough rise while ethanol evaporates during the baking process.

In the milk industry the well known probioticsLactobacillus bulgaricus and Streprococcus thermophilus, are commonly used. These little guys convert the milk sugar, lactose, into lactic acid, thus giving the final product that acidic taste. At the same time the acidity will alter the milk protein structure which gives yogurt its thick texture. The beauty of Lactobacillus is that it’s not necessary to use milk in order to culture them. Anything that’s got a sugar will do the job! That is the reason why you can make all of these non-dairy yogurts. All the benefits of probiotics without all the nasty saturated fat 😉

Lastly I wanted to mentioned kombucha which is fermented tea. In this case the fermentation involves SCOBY or symbiotic culture of bacteria and yeast. The exact composition of the culture varies but generally consists of Acetobacter sp. along with various yeasts such as Bretanomyces sp. and Saccharomyces sp. All in all, the sugar in the tea will feed the culture which will give off useful bi-products such as a bit of alcohol, acetic acid and gluconic acid. The gluconic acid is key for kombucha and is believed (although not well researched) to have liver cleansing properties.

La voila! Here are some of the common microbes we use in our food.

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Metabolism 101: Fate of Lactic Acid

Recall the last time you’ve had an all out sprint. Remember that build up of pain in your legs that made you stop moving but then subsided? Well that’s lactic acid for you! As I’ve explained in the previous post, when we perform high intensity exercise we do not have enough oxygen to drive aerobic respiration. Therefore, our bodies are forced to switch to the inefficient process of anaerobic respiration and in turn lactic acid production.

That darn lactic acid! Well not quite… Lactic acid is more than just a waste product and isn’t necessarily bad. You must recall that shortly after high intensity exercise the lactate-associated pain subsides. Why? Our bodies have designed mechanisms to remove lactic acid from the muscle and actually convert it back into energy. How this occurs depends on the location of the lactic acid- actively contracting muscle, relaxed muscles, or the liver.

When oxygen becomes available, lactic acid within actively contracting muscles can be further oxidized to enter the Kreb’s Cycle and yield energy. After exercise, the lactic acid remaining in that muscle can be resynthesized into glycogen.

Alternatively, lactate can leave contracting muscles and enter the bloodstream through something called the monocarboxylate transport (MCT) protein. Using the bloodstream, lactic acid can travel to other muscles in the body or the liver.

If you think of biking, your leg muscles will be contracting while the arm muscles will be relatively relaxed. Therefore, lactic acid accumulated in the legs can travel to the arms where it can also be used for energy or synthesized into glycogen.

The liver though is where all of the magic happens. Here lactate can be converted back into glucose through a pathway called “The Cori Cycle”. This mechanism is somewhat wasteful however it does relieve muscles from lactic acid accumulation and pain. That can be quite useful if you’re being chased and need to run for as long as possible.

The Cori Cycle is the exact opposite of anaerobic respiration and involves lactic acid being turned into pyruvate and then into glucose through gluconeogenesis. I shall not bother you with details but it is important to note that the cycle consumes 6 molecules of ATP while anaerobic respiration only generates 2 ATP! Wasteful indeed! However, it’s still useful to make glucose as it has potential to yield much more ATP through aerobic respiration.

Et la voila! That is how lactic acid is removed from our bodies. Stay tuned on how we take advantage of anaerobic respiration in food production 😉 Yes… I’m talking beer 😛

Metabolism 101: Why Anaerobic Respiration?

Oxygen, the essence of life. Why? Well that’s what this post is all about 😉

Our bodies can break down glucose through two ways, with or without oxygen. What determines how it’s broken down? Well remember that reducing agent, NAD+. Yeah… that little guy that accepts electrons from others to become NADH+ H+. The balance between NAD+ and NADH+ H+ is the key, let me explain to you why.

Typically glucose breakdown consists of 3 distinct parts- glycolysis, Kreb’s cycle (aka citric acid cycle aka tricarboxylic acid cycle), and electron transport chain (ETC). I will focus on the big picture as I have memorized these pathways in detail multiple times for school just to forget them.

In glycolysis, the 6 carbon glucose is broken down into two 2 carbon pyruvate molecules. Through the process there’s a net production of 2 ATP molecules and 2 NAD+ molecules get reduced to 2 NADH+ H+. Subsequently, each pyruvate molecule loses a carbon as carbon dioxide, generates another NADH+ H+, and eventually becomes acetyl-CoA.

Then each acetyl-CoA will enter the Kreb’s cycle to be fully broken down into 2 carbon dioxide molecules, 3 NADH+ H+, 1 FADH2, and 1 GTP (like ATP). At this point the glucose has been fully broken down.

Now the big money (ATP) maker is the electron transport chain reaction. Here NADH+ H+ and FADH2 will unload their gained electrons to produce ATP. The process is driven by oxygen which will react with NADH+ H+’s electrons and hydrogens to form water. At the same time NAD+ and FAD will be reconstituted. In total, aerobic respiration will yield 38 ATPs.

Without oxygen, the electron transport chain will not occur and NAD+ with FAD will not be regenerated. With no NAD+, glycolysis or the first step of glucose metabolism cannot occur and absolutely no ATP can be generate. That’s when the body goes to plan b! Anaerobic respiration! By converting glucose into lactate we can regenerate NAD+. Unfortunately, this process is highly inefficient and only produces 2 ATP molecules. That’s why you can only sustain anaerobic activity for such short bursts. Think about it you can only make 2 ATPs instead of 38 ATPs through aerobic respiration. Better then nothing during the fight or flight response though 😉

This is the reason why we undergo anaerobic respiration. We can only sustain anaerobic respiration for a short time and hence will die without oxygen. In the next post I’ll talk about what happens to that lactose and how anaerobic respiration has been taken advantage off in food production.

Metabolism 101: Enzymes

People talk about enzymes all the time, especially raw vegans. It’s all about making sure that we don’t denature the enzymes in our food or using enzymes to help digest our food. Well, enzymes are about a lot more than that!

Basically, enzymes are biological catalysts. A catalyst is something that will speed up the rate of a reaction. In biology, catalysts are a bit different because they don’t just speed up reactions but actually allow them to happen. Furthermore, enzymes are proteins so our bodies make them based on our genetic blueprint.

Biological catalysts, what does that mean? It means that they will lower the initial energy requirements (activation energy) necessary for a reaction to occur. Remember the post about exothermic and endothermic reactions? Looking at the energy diagrams again you can see that in-between products and reactants there is always a high energy “bump”.  It is the energy investment needed for that reaction. This investment is usually very high and prevents the reaction from occurring. It’s sort of like having to pay a 95% down-payment for a house; it’ll detract anyone from buying it. That’s where enzymes come in because they lower this bump, thus making the reaction possible.

Enzyme activation energy

Enzyme activation energy (Photo credit: Wikipedia)

How do enzymes make reactions happen? It is believed that enzymes provide a space where reactants can come together in a specific orientation. You can think of  them as a conference organizing committee. They will invite important individuals to a specific location, thus facilitating them to come up with ideas. Furthermore, enzymes will have an active site pocket within them where the substrate can enter. That’s where the reaction will occur.

Enzymes are also picky little brats and need very specific conditions to function. A deviation of a few degrees from its optimal temperature could not only stop the reactions but destroy the enzyme. Therefore, yes raw vegans are right in that cooking food will destroy the enzymes in the food. At the same time, enzymes work only at a certain pH with highly basic or acidic conditions destroying the enzyme. For this reason, eating raw food isn’t necessarily better than cooked food. Our stomach is highly acidic, about 10,000 times more acidic then our blood. Hence, most enzymes in raw food will get destroyed in our stomach anyways. It’s important to note that enzymes are not only picky about their working conditions but also their substrate. In general, one enzyme will catalyze one specific reaction. For this reason most enzymes found in raw plants will probably not do anything to us, because they’re meant for catalyzing plant reactions not human ones. Hopefully that dispels some myths 😉

Apart from being able to make reactions occur, they can also regulate them. Think of it, enzymes are proteins which are controlled at the genetic level. Certain stimuli can turn genes on’n’off,  in turn modulating the production of that enzyme. For example, when you sit down to eat a bowl of pasta, your body will increase production of salivary amylase to break down the starch in the pasta. Alternatively, absence of food will do the opposite. 

Most of all, enzymes are crucial for metabolism! They are its driving force and its gatekeepers. This will make a lot more sense in subsequent posts. Stay tuned for aerobic vs anaerobic respiration 😉