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Metabolism and the Immune System

Life: Eating and Being

Xray Millions of years ago, Life consisted mostly of one-celled organisms

Life forms can be found that are made up of only one cell or millions of cells. Millions of years ago, life consisted mostly of one-celled organisms, but over time a myriad of bigger and more complex organisms emerged.

Single-cell organisms like bacteria are very efficient at surviving in very specific environments and sometimes even very harsh environments! More complex organisms need more energy but are also more able to move and interact with their environment and seek food and comfort, with more specialized ways of protecting themselves or muscles and legs that allow them to move greater distances with greater control.

Life has evolved over millions of years, taking a myriad of shapes and forms, from one-cell organisms like bacteria to organisms with millions and millions of cells, like human beings! But in every size, shape and form, Life is always in a constant balancing act of getting food for energy and protecting ourselves from things that might disrupt and infect our bodies.   These basic needs give rise to the most fundamental processes in our bodies: metabolism and the immune system! 


All organisms, no matter how big or small, and all the cells that compose them, need energy to survive. Metabolism is the process through which a life form gets energy from its environment and uses it to live. Metabolism includes eating food, breaking it down into nutrients, getting the nutrients to each cell of the body and each cell’s processes that utilizes this food energy.

Immune System

The immune system is not a simple organ or a simple function. It is a constellation of cells and tissues that work together in the challenging task of identifying what belongs to the body (the self) from those that may be harmful- like old, dead cells or outsider particles like some kinds of bacteria, viruses. Our immune systems have to be very intelligent to deal with the magnificent variety of unknowns in the world.

Metabolism and the immune system share a common purpose, somehow they must swallow other particles, one for energy, one to stop a harmful invader. Discover the role of the swallowing cells in both…

Swallowing Cells: The Basic Unit

Early on, all organisms were single-celled. Then, some cells appeared that had the ability to swallow other smaller cells, the swallowing cells.

All cells, even today, are able to absorb small particles from their environment and digest them for energy. Early on, all organisms were single-celled. These cells could get all their energy from the ocean and the Sun. They absorbed and digested what they could, individually.

Xray Swallowing cells have the ability to eat other smaller cells

Then, some cells appeared that had the ability to swallow other smaller cells, the swallowing cells. These cells were able to get energy a lot faster! These swallowing cells became central to the development of more complex digestion and immune systems.

Cell Cooperation & Specialization
Millions of years ago, cells started to be able to work together. This cooperation allowed the cells that make up these tiny animals to start specializing, and carry out different roles. Some cells, on the outside of the animal become tougher to protect the animal like a sort of skin. Some swallowing cells became really good at absorbing food and took over eating. In very small animals, this works very well, as all cells were very close to eating cells, so they can easily share energy with their neighboring cells.

Cell cooperating involves cells being able to communicate with each other. This communication is at the basis of almost all processes in your body! For example, your immune cells use tiny chemicals to work together, and your neurons and muscle cells send electrical signals from one to another.

Xray The sponge has a cavity lined with eating cells.

Some swallowing cells became really good at moving around and sniffing out harmful cells or particles, like in the sponge to the right. This is the beginning of a simple immune system!

Even in modern animals, the ancient ability to swallow up certain cells and small debris has been retained by some special cells. So, in addition to digesting food, swallowing cells can destroy things that may be harmful to an animal, like some kinds of bacteria, viruses, or old dead cells.  So what was once a form of nutrition became a form of protection too!

These swallowing cells are found in the immune system of human beings too! They are called macrophages and B cells.

As animals got larger, these mechanisms were not enough, find out what came about>>

Evolution of Metabolism

In the obelia above, digestion of food starts in the digestive cavity, outside of each individual cell!

Over time more complex organisms, with more and more cells, and a greater need for energy, appeared.

Extra Cellular Digestion
Some cells formed stomach-like pockets, which became special areas devoted to eating, like a kind of stomach.

In the obelia, to the right, we can see a special pocket or cavity. This pockets is lined with special cells including swallowing cells that release tiny chemicals to the pocket. These chemicals breakdown food as itcomes in.

These different cells cooperate to breakdown and absorb the food, and then share the food energy with their neighboring cells. So, digestion started to occur more outside each cell, in the digestive pocket - or, extra-cellular digestion.

The Obelia shows a stomach-like pocket lined in eating cells. These cells digest the food and then easily share the energy with their neighbors.

Distribution System
As animals got larger, some cells became further and further away from the eating cells—and wouldn’t be able to get food in the simple way. So in larger animals, cells are found to have formed passageways, like human blood vessels, that spanned their whole bodies, to distribute energy and oxygen to all their cells.

In the polychaete above, passageways like blood vessels transport food and oxygen.

The circulatory system of blood vessels is not the only distribution system in the human body! Our immune cells also travel in the lymphatic system, a super immune system of vessels and nodes that span the body.

The more an animal can eat, the faster and the more efficiently he can digest its food, the more energy an animal has to move around, and seek food and a comfortable environment.

In order to eat faster and get even more energy, some animals’ stomach pocket elongated to become a passageway going through the center of their entire bodies—the gut or digestive tube!  This digestive tube allows the outside environment—the ocean and all its nutrients—to pass through the animal without being in contact with most of its cells

Other special pockets evolved around the digestive tube in humans, forming organs like the liver and the pancreas. The pancreas helps by making chemicals that help break down food in the intestine. Also, it makes insulin that helps cells absorb sugars really well.

The digestive tube, being very good at absorbing broken down food, is also a big point of entry for outsiders! So, lots of immune cells are found very close to digestive tubes.

As organisms became more complex and specialized, with many different types of cells, the harder the immune system's job became. Think of the human body, how many different cells and particles can you think of that belong to you body? Skin cells, hair cells, eye cells, calcium in your teeth, bone cells!.. So, the immune system needed to become more complex! New types of cells, like B cells and T cells, and tissues, like the lymphatic system, evolved to work together and be more efficient and specific. Once the immune system has discovered a particle or dead cell in your body, it calls a team of special swallowing cells that set to work breaking it down, like food!

Metabolism, Sugar and Diabetes

Your body needs a great deal of energy to complete your day-to-day activities, such as doing homework, playing soccer, or even reading a book! For any of these activities, even when you’re just sleeping, all of your cells need energy to continue being healthy.


Therefore, your body is constantly busy getting energy, storing storing the food energy, and then spending it, maintaining itself and doing fun things! All of the processes in your body that involve getting or spending energy are known as your metabolism. A high metabolism occurs when your body is both getting a lot of energy and using a lot of energy, whereas a low metabolism occurs when the body neither has nor is able to use a lot of energy.

We get energy through digestion, and insulin is very necessary!

So where does your body get all of this energy from? Well, eating a healthy diet helps to provide your body with a great deal of its energy. The food you eat is broken-down into smaller parts like sugars, fats and proteins and other nutrients in a process called digestion. The broken-down nutrients are absorbed by the body, mostly in your intestine, and are then transported to every cell and tissue in your body with the help of your blood vessels. A special chemical helps cells absorb sugar from the blood stream – it is called insulin. Sugar is one of the main sources of energy for the body so without insulin, most of the cells in your body would starve!

Our body has a very careful regulating mechanism to deal with the amount of sugar in our body. Too much and too little can be harmful! Our body constantly decides how much sugar each cell needs, how much sugar to have on our blood stream for quick use and how much sugar to store for later.

The trouble with Diabetes: not enough insulin

Learn more about Diabetes here >>

In some cases, like in the case of Diabetes, the body does not have enough insulin. In this case, the cells can’t absorb sugars as well and the sugar level in the blood start to rise. The body gets tired, dehydrated and blood pressure lowers, and lots of other health problems can occur.

So what does all this information about sugar, metabolism, and its effects on health mean? Maybe you think that you should then just eat tons of sugar! But, if you gave a human being that much sugar their metabolism level would not be sustained and would quickly drop! But why? The human body has a very delicate mechanism that controls the use and storage of its sugars. Too much sugar can over-whelm the body’s ability to store and use sugar appropriately!

Getting too much or too little sugar both cause problems. But getting too much sugar is a more common problem, especially because sugars pack a lot of calories into a small bite so it is easy to over-eat them!

Some sugars are better than others

Contrary to what you may think, there are “good sugars” out there! Natural sugars are kinds of “good sugars” your body can easily store and use. They even have other health benefits like boosting your immune system, strengthening the power of your pancreas, and ridding your body of toxins that may make you feel sluggish and not healthy. These kinds of “healthy sugars” can be found in some fruits like apples, pears, and blueberries, vegetables and legumes like yams and beans, nuts, and foods containing whole grains.

So, Sugar and Sugar aren't the same thing?

While there are many “good sugars” out there, you have to watch out for the sugars that aren't so good for you! The main sugar to watch out for is called “refined sugar”. It is also known as “white sugar” and most people use it everyday! It is present not only in obvious foods like candy and soda, but also in other foods like breads, lunch meats, ketchup, canned vegetables and fruits!

So what does this bad sugar do?!

When a bar of candy is digested, white sugar is broken down by the body into very simple sugars unusually quickly. This causes a person’s sugar levels in their blood to rise drastically (hyperglycemia), and causes an equally quick response of big insulin-production from the pancreas. This uncontrolled production of insulin can lead to removing too much sugar from the blood! In this situation, the body gives order to release sugar that is in storage in the liver to replenish the sugar in the bloodstream. If these unbalances happen a lot, they can really wear out our sugar control mechanism! This can lead to health problems like diabetes and hypoglycemia, as well as a lot of trauma to the pancreas!

People with diabetes are prone to experience the sharp highs and lows described above, since the lack of insulin leaves the body in a state of stress usually with too much sugar in their blood stream. So, when a diabetic eats, their body will immediately have a harder time dealing with the added sugars.

The balance of sugar in your body is directly related to the kinds of foods you eat. A healthy, balanced diet lets your body have an easy time digesting and using its energy.

As you read above, eating too much sugar (especially refined sugar) can have serious effects on your health and general well-being.

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