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Mitochondria - The Powerhouse of the Cell

What are mitochondria? 

You may have heard mitochondria associated with the moniker, “the powerhouse of the cell”. How did they earn that nickname? 

Mitochondria are the miniature power stations or factories in each and every cell in your body. A typical living human cell contains anywhere from hundreds to thousands of mitochondria. 

Much in the same way as your digestive system, mitochondria are like small digestive systems in your cell, turning food into energy. Sugars, fats, and amino acids from proteins that we eat are converted into energy through the mitochondria. They are so effective at this that they generate an estimated 90% of the energy that our cells need.

What do mitochondria look like?

Mitochondria look like little beans in your cell. They are made of two membranes: the outer membrane and the inner membrane. 

The outer membrane acts as a wall, covering the entirety of the organelle. 

The inner membrane looks like a series of folds, consisting of several compartments. This layered shape is intended to maximize the mitochondria’s surface area, supporting a higher efficiency in its function. 

Within the inner membrane is a fluid called the matrix; this is where the magic happens.

Where did mitochondria come from?

Before the mitochondria became invaluable to human cells, they existed completely outside of them as single-celled, independent organisms. They looked a lot like bacteria. However, some time in ancient biological history, over two billion years ago, they merged with a simple cell to form a symbiotic relationship. 

At first, the plan wasn’t just to merge. The mitochondria, as bacteria, only wanted to rob the host cells of their energy and then leave them to die. But the bacteria soon realized the benefit of working together with simple cells.

The simple cells provide them with antioxidants to protect them from free radicals and toxic reactive oxygen species that the mitochondria generate as a byproduct of energy production. In return, the mitochondria produced the energy the simple cells needed. It’s a pretty sweet deal. It’s like the mitochondria are paying rent in return for housing and utilities.

Mitochondria are essentially aliens in your body. Our mitochondria even possess their own DNA, called mtDNA, giving them an independent genome. Furthermore, mitochondrial DNA is only passed down from mother to child, making you more genetically similar to your mother than your father. In fact, modern ancestry testing companies lean on your maternal ancestry line by using mitochondrial DNA.

The mitochondria’s purpose.

The mitochondria have one primary purpose: to produce energy. In order to create energy, they create a much-needed molecule known as adenosine triphosphate or ATP. 

What is ATP?

Our bodies don’t just create and harness energy straight away. It actually stores the energy we produce from our food in a molecule. ATP, or adenosine triphosphate, is the primary energy storage solution for our cells. They are like tiny batteries floating around, waiting to be used. “Tri”, meaning three, denotes that there are three phosphates in the molecular structure. 

When cells need energy, ATP is broken down through a process called hydrolysis. This is actually pretty easy to do because ATP is such an unstable molecule. The three phosphates in ATP are like three roommates sharing a room. They don’t like each other and are just waiting to be split up.

When the split happens, the molecular bond between the phosphates in ATP’s tri-phosphate group is snapped off, removing one of the phosphates in the ATP molecule. The trio becomes a duo, thus turning ATP into ADP or adenosine di-phosphate.

This breakage releases immense energy and our cells use the energy to power important cellular activity. 

Our mitochondria work hard to make sure our cells have enough of these ready-to-use “batteries”, or ATP, floating around.

How do mitochondria create ATP?

In order to create more ATP, our mitochondria go through a series of chemical reactions to break down our food, particularly glucose, amino acids, and fatty acids. Glucose is really the primary molecule that our food is broken down into so let’s focus on glucose to understand how our mitochondria convert food into energy.

Our mitochondria take our glucose molecules through a process called cellular respiration which is essentially just a process of breaking down and converting glucose by combining oxygen with a glucose molecule. The oxygen is derived from the air we breathe. 

This process of adding oxygen to glucose produces a string of molecules. At its most rudimentary form, the process looks like the following formula:

Glucose + Oxygen = Carbon Dioxide, Water, and ATP. 

Carbon Dioxide and water are byproducts of the process. This is cellular respiration, simplified.

However, our mitochondria do not take glucose in its raw form. It’s not usable in its regular state so our cells break glucose down even more before passing it to our mitochondria. This process is called glycolysis.

The broken-down form of glucose is what is really combined with oxygen to produce a net of carbon dioxide, NADH, FADH2, and ATP. This process is what’s called the Krebs Cycle. Let’s break the products of this process down:

Carbon dioxide: One of our byproducts. You breathe this out. 

NADH and FADH2: Nicotinamide adenine dinucleotide (NAD+) and flavin adenine dinucleotide (FAD) are coenzymes that help generate more ATP. NADH and FADH2 are their electron charged forms. Ignore this for now. We’ll talk about these important players later.

ATP: Energy!

So the Krebs Cycle creates energy but the Krebs Cycle alone does not produce enough of the ATP our cells require. The real prizes are the NADH and FADH2 that are produced in the process. They are what really produce us the majority of our ATP through what’s called the electron transport chain

The electron transport chain is essentially a process where our mitochondria constantly “steal” from its guests. NADH and FADH2 are electron-charged molecules and our mitochondria “steal” these electrons from NADH and FADH2, turning them into NAD+ and FAD as a result. 

In turn, our mitochondria take these charged electrons and produce a ton of ATP, turning lemons into lemonade. This process is so efficient in producing ATP, the electron transport chain produces the majority of our ATP energy. Fortunately, the mitochondria’s willing friends, NAD+ and FAD, continue to come back bearing gifts of charged electrons to sustain the process. It's a perfect supply chain and the only byproduct in this process is water, thus completing our formula:

Glucose + Oxygen = Carbon Dioxide, Water, and ATP.

Mitochondria and aging.

Research from the School of Kinesiology and Health Science from York University shows we make fewer mitochondria as we age. Your mitochondria also gradually deteriorate as you grow older, making the few mitochondria you do have left work that much harder. In fact, mitochondrial dysfunction is considered a hallmark of aging.

The same researchers from York University believe this is a result of an imbalance between our number of free radicals and our cell’s ability to remove them. But most of the scientific community agrees that the mitochondria grow less effective over time because of their decreased ability to make ATP.

Mitochondria respond to your energy needs.

In most cases, the number of mitochondria we create correlates to the amount of energy we need. This means, in large part, our daily activity dictates the number of mitochondria we create and sustain. Whenever there is a significant change in our lifestyle or habits, our mitochondria adjust their numbers. 

David A. Hood, from York University, believes there is a connection between exercise routines and mitochondrial biogenesis. Mitochondrial biogenesis is a series of complex chemical reactions within the body that signal the need for more ATP and therefore more mitochondria. Our mitochondria essentially clone themselves through a self-replication process in order to meet the new energy demand.

However, the opposite is also true. A sedentary lifestyle can signal the body that we don’t need as much ATP and inhibit the mitochondria from replicating. As a result, your mitochondria produce less cellular energy overall, leading to more general metabolic dysfunction.

Mitochondria and NAD+.

As crucial as mitochondria are for creating energy, it’s not as simple as one organelle. A bunch of different chemical reactions and coenzymes are at play, namely one critical molecule known as nicotinamide adenine dinucleotide or NAD+.

As mentioned before, two coenzymes are created in cellular respiration, FAD and NAD+. However, between the two, we produce far more NAD+ than we do FAD. If the mitochondria were factories, the NAD+ molecules are the fleet of delivery trucks and the FAD molecules are the temp drivers that only work part-time.

NAD+ is like the mitochondrion’s most reliable friend, constantly delivering charged electrons to produce bountiful ATP in the electron transport chain. 

Unfortunately, the amount of NAD+ we produce naturally declines with age. Like the mitochondria, the number of NAD+ we have in our cells is also largely affected by our lifestyle and habits. A study published in Physiological Reports shows that exercise training can naturally increase NAD+ levels. Conversely, things like age, metabolic stress, immune stress, drinking, overeating can all contribute to NAD+ depletion. 

In pursuit of understanding the science of aging and how to best manage it, the scientific community has put a large focus on NAD+ research and its relationship to mitochondrial dysfunction. It’s widely accepted that mitochondrial health plays a huge part in our overall human health and NAD+ is part of that story. Luckily, maintaining healthy mitochondrial function is possible with a few lifestyle changes. Here are some tips on how to support mitochondrial health.

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