Cells: the Tiniest Living Things

A cell is the smallest unit of life. Some organisms are made of a single cell; others are made of billions!

Each and every cell, whether it is independent or part of a larger organism, is always interacting with its environment. In multicellular organisms like human beings, cells communicate with each other and with the “extracellular matrix” around them.

Cells that perform similar functions are organized into tissues.

The life of a cell isn’t all that different from ours, really. A cell is born, lives its busy little life, interacts with its environment and with other cells, reproduces (OK, we don’t all do that part!), and eventually, it dies. Even if the cell doesn’t reproduce, it contains instructions for how to do so. These are the genes in the cell’s DNA.

Types

Did you know there are more different types of cells than there are different types of living things? Fundamentally, though, there are two main types of cells: prokaryotic and eukaryotic.

Prokaryotic cells, such as bacteria, are pretty simple and have no nuclei. Scientists currently think prokaryotes evolved first, and then absorbed other prokaryotes to become eukaryotes – complex cells that have nuclei and structures like the energy-producing mitochondria.

Eukaryotic organisms, like mushrooms, pine trees, and your best friend, are all made of cells with a nucleus and surrounding membrane. They also contain organelles, which are just like they sound—little organs. One kind of organelle is the mitochondria.

Just like in the body, where you can compare different organs to machines, and see how they all work together, the different parts of the cell are the same way! The mitochondria are like little energy factories, the cell membrane, like the skin, is a protective wall, and the nucleus is like the brain, directing the cell’s activity.

Other important parts of the cell include the endoplasmic reticulum and Golgi apparatus. These are where the cell makes proteins from DNA, which contains our genes.

Genes: Our Blueprint

All cells (human, bacterial and everything in between) hold some form of genetic material. This genetic material regulates everything about the cell - what it looks like and what it does. The genetic material is made of combinations of special molecules, called nucleic acids, which are expressed into different proteins.

These proteins either become part of the surface of the cell - its membrane - or they’re released into the environment.

Each species has a different set of genetic material, and within each species, each organism has a unique genetic signature – in humans we can easily detect some of the variations of DNA by noting how no two humans look alike (except sometimes twins – and they actually share the same genetic material!)

Within one organism, though, all cells share the same genetic material. But, each cell makes different proteins according to its type (whether it’s a bone or a brain cell, for example) and it also makes different proteins in different circumstances: if it’s healthy, if it’s about to divide, if it's dealing with a lack of an essential nutrient, or if it’s unhealthy!

So, the proteins a cell produces are a pretty good indicator of how the cell is doing. For this reason, our immune system often uses proteins to keep track of what’s going on inside each cell.

Cells Working Together

Scientists think that life began as single cells floating in the ancient oceans on our planet. Then, sometime in the last billion years, cells started sticking to each other (or swallowing each other)!

As cells began to live together, they developed the ability to cooperate. They developed the ability to start focusing on different things:

1. Some swallowing cells took over the process of eating for their neighbors, so their neighbors could have time and energy to do other things.

2. Some cells on the outside of the group became really tough, like the skin on a sponge, to protect their neighbors (like the eating cells).

3. Some cells became really good at moving around in a group of cells and sniffing out bad or dead cells to remove them before they harmed their neighbors.

In time, cells working together to perform the same function turned into various organs. Organs are different groups of cells working together to perform specific functions particularly well. Such functions include protecting the body from the outside environment, or having muscles and legs to allow organisms to be able to move and look for food instead of just waiting for food to come to them.

This kind of cell cooperation happens throughout our bodies, like in our heart, muscles, and nerves.

For example, in your immune system, you have special cells called B and T cells that travel through the body. When they come across a new particle, they talk to each other to determine whether it is good or bad.
For more, see immunology >>

Another example of cell cooperation is that nerve cells, called neurons, send signals to one another. This is how messages from your brain get all the way to your toes and the rest of your body!
For more, see nervous system >>

Cells are constantly interacting with the environment. They are always talking to each other and the world around them!

Limited Supply

Our bodies can make new cells to replace damaged ones. Most of the cells in the adult human body will be replaced during their lifetime. So your body is prepared to do some level of self-regenerating and healing. This healing ability is dependant on the body being able to make more new cells. If we could make new cells indefinitely, we would never get scars and we would never age!

But the body can only make so many new cells to replace damaged ones. Also, most cells in your body will accumulate some damage over the years so that their ability to self-renew is very debilitated – this is one of the reasons we do age. And sometimes in the case of a big injury, the body just can’t make enough new cells and it takes too long! So, regenerative medicine doctors give the body more cells with stem cells.

Proteins

Proteins are big molecules that are the stars of the action inside the cell. They catalyze chemical reactions, build the extracellular matrix, and control the cell cycle.

Proteins are essential to cell communication: Cells release them into the environment and stick them on their surface. The proteins they release indicate the state of the cell, whether it’s healthy or not, and whether it’s hungry or about to divide.

One type of protein essential to regenerative medicine is the growth factor.

Growth Factors: What Do You Want To Be When You Grow Up?

How do you tell a stem cell whether to become bone, skin, or muscle?
Surrounding cells do this by sending out chemical signals called growth factors. The human body naturally produces growth factors to tell its cells what to turn into. Stem cells pick up these growth cues and follow them, becoming differentiated.