Brains are complex assemblies of cells arranged to form processing centers in animals. When we get to smaller and simpler animals, such as worms, the number of cells in the brain is proportionally smaller.While human brains have many billions of cells, a worm may have millions, and the simplest multcelled life, such as the nearly microscopic worms C. elegens, have only dozens of brain cells. But even then, the brain is a group of specialized cells, such as neurons.

In an individual cell, there is no brain. But single cells must still"make decisions". They must react to the changing environment around them; engage in growth and cell division, and many of processes. On the cellular scale, these processes are mostly mediated by feedback loops-- that is, chains of chemical events that either reinforce that chain of events happening again, or that die off when a critical threshold is released.

For example, if a lake water bacteria has a protein on its surface that detects food (sugar), then it might set of a range of chemical reactions in the cell that cause the cell to move in the direction of the food. Once it reaches the food, that sensor protein won't sense food in that direction anymore, and will stop the chain of chemical reactions, so the cell will stop and can eat the food it found. That reaction is very simple, but as you add up the thousands of proteins and chains of signaling events within a cell, quite complex behavior can start to appear. This is a phenomenon called "emergent behavior", and is also seen in the patterns that emerge in traffic, or the complexities that can emerge from a simple set of rules (like all the billions of possible games of chess, even though the rules are very simple).

Within each cell this complex set of interacting chains of simple chemical reactions controls cell behavior. When we form multicellular creatures, like humans, the cells send signals between each other and some cells specialize in signaling, becoming nerve cells. This makes the map of chemical interactions incredibly more complex and intricate, and allows the formation of higher-order emergent behavior, such as brain activity and thought.

First we have to define the words cell and brain. Scientists consider a cell to be the most basic structure of all living organisms. All living organisms are composed of one or more cells, and cells are capable of sustaining themselves: cells can take in nutrients, convert nutrients to energy and reproduce themselves. Some organisms, such as bacteria, are composed of only one cell, while other organisms, such as humans, are composed of millions of cells.

Scientists define a brain as the organ of the body that is enclosed within the skull and contains all the higher nervous centers. The brain receives information from other parts of the body and tells those parts how to operate. So technically, cells do not have brains because a brain is defined by scientists as an organ composed of many cells. Plus, brains are enclosed within skulls, and only vertebrate animals have skulls. Individual cells, such as bacteria, do not have skulls or brains.

However, individual cells may seem to have brains because they can perform complicated activities on their own. Like brains, they can receive information and respond to it.

The short answer is that cells do not have brains. Brains are made of many nerve cells (neurons).

Here's the longer, more interesting, answer. Some living things are made of only one cell. We call some of them "protozoa" (Pro-toe-zoh-uh). Protozoa can have behaviors. They can go toward or away from light, go around obstacles, and go toward food. So they don't have brains, but have simple systems that work like very basic brains. They can't think,"I'll go over there," but they can have chemical reactions that make them move toward the side of a dish that has more light.

Even some animals, the sponges, don't have nerve cells, but the different parts of the animal communicate with chemicals. Sponge adults don't move much but their young are a lot like protozoa and they swim without any brains.

Some plants actually move their flowers to face the light. Is this a behavior? Is it different than a fish swimming toward the light?

What a great question!We don't generally think of a cell in our body as having a brain in the way we think of the human brain. I bet you have heard about or studied organisms that don't have true brains but rather "nerve networks" that regulate and coordinate all the processes of the organism's body. It seems that our definition of"brain" is what determines the answer to a question such as yours. So if we define "brain" as the ability to receive information, process it and then respond, then you know what? Cells DO have that ability and I guess we can say they have "brains." Each cell has the ability to detect signals from other cells or the environment and respond to those signals. That's true for a single celled organism such as an amoeba or paramecium as well as for an individual cell living in your body right now.

No - cells (eukaryotic cells, anyway, which include animals and plants) have organization centers that function similarly to brains, however. A cell's information that allows it to make proteins that do the things that the cell does is contained in the nucleus. These proteins are then organized and released in a cluster of membrane-bound vesicles called the Golgi apparatus. Individual other organelles within the cell, such as eye spots and flagella, can communicate directly, just as our muscles can communicate with our spinal cord and don't need direct input from the brain. So, if you will, the nucleus is like the outer cerebrum (where at least some memory is stored), and the Golgi apparatus is like the cerebellum (which coordinates muscle movement).

No, but our brains are made of many cells!

There is a part of the cell called the nucleus, and it contains the instructions (genes) that the cell needs to do its job. But this is not like a brain. It's more like a recipe or a building plan.

Cells do have something like a brain; it is called the nucleus. The nucleus is the part of the cell that holds the entire DNA. I don't know how much you've learned about DNA, but it is like a set of blueprints or instructions for the cell. The DNA has the information the cell needs to make all of the proteins that it uses to do its job in the body. Your body can also use chemicals called hormones to send instructions to the nucleus of a cell. When a hormone reaches the nucleus, it gives the nucleus a signal to have the cell use different parts of the DNA to make different proteins, which can change the function of the cell in the body.

There is one major way that the nucleus is not like a brain, though. A cell's nucleus can't think for itself - it just follows the instructions in the DNA. It can get signals from your body or the outside environment that tell it to follow those instructions in a slightly different way. But a cell cannot plan for the future or make decisions the same way you can with your brain.

I hope this helps answer your question.