Animal Cells

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What Are Cells?

Cells are the building blocks of all living things.

Plants and animals alike have cells. Yet, because of the way members of the animal kingdom differ from plants — especially when it comes to eating, movement, and reproduction — the cell structure differs between the two.

Where plant cells are typically uniform across the plant kingdom, animal cells can vary greatly within a single animal in terms of size and shape.

Much like the animals that the cells join together to form, each animal cell has common characteristics that are essential to overall function.

Think of the animal like a city or town, and each cell a house or building within that city or town. Each individual building is comprised of multiple rooms, all with their individual purpose. Some rooms are meant for sleeping, while others are configured for tasks like preparing meals.

Specific activities take place in each room, and the cell functions as it should. If this occurs with all structures or cells within the city, then an organ system is formed.

But before we jump ahead to how animal cells function in groups, let's first understand how animal cells work independently.

How Do Cells Work?

All animals are multicellular, meaning they are made up of many different types of cells.

Each individual cell is also made up of many different parts called organelles. Animal cells are surrounded by plasma membrane, a protective sack filled with fluid, and within that protective all the organelles — including the nucleus — are bound together and work as an independent entity.

Animal cells come in a variety of shapes and sizes, depending on their function. Most of the cells range in size between 1 and 100 micrometers and are visible only with help of microscope. Trillions of cells are found in the human body.

• Can you believe that there are approximately 210 distinct cell types in an adult human body?

Although they differ in shape, size, and function within the body of an animal, most cells have the same basic structure.

The first thing we want to remember is that, like plant cells, animal cells are eukaryotic, which means they have a true nucleus. The nucleus is the cell's control center, and contains all of the genetic information. The nucleus is contained in its own room of the cell.

Much like the tissues, organs, organ systems, and organisms they form, the tiny cells themselves are made of organelles, complex structures within the cells. Each of these organelles is contained within its own room or area of the cell and carries out its own unique function, just as each organ system in the body resides in a specific area and serves a specific purpose.

Take the digestive system, for example. It begins with the mouth and includes various organs: the stomach, liver, pancreas, gull bladder, and large and small intestines. The function of the digestive system it to take in food, break down that food into useful materials that the body needs — such as proteins and lipids — store useful materials, and excrete waste.

Although we have not yet identified these individual structures and their unique functions, take a look at this example of a very basic animal cell:

• What Do All Those Tiny Things in the Cell Do?

Cell Membrane: The Border Guard

Before we hit the inside of the cell, let's take note of the plasma, or cell membrane.

This is a thin, semi-permeable layer that surrounds and protects the cell and helps it to keep its shape.

• Notice the word semi-permeable?

Take a minute to think about what that means and why it may be important to the function of a cell that is part of a bigger picture.

• Any ideas?

Semi-permeable means that the membrane allows only certain materials in and out of the cell. It lets in good things like oxygen molecules, and bad things, like excess carbon molecules, out. Just as animals rely on their skin for protection from the environment, cells rely on the cell membrane for the same reason.

The cell membrane holds the contents of a cell together, gives the cell shape, and protects the contents of the cell from its surrounding environment. Much like skin, this thin, flexible covering helps the cell by allowing only certain substances to enter and leave the cell.

Nucleus: The Control Center

The first — and most important — organelle to address is the nucleus. As mentioned above, the nucleus is like the brain of the cell. It contains, or remembers, the genetic information, and also controls all other parts of the cell. We can also think of a cell as a tiny animal.

• Have you ever heard of the word chromosomes?

Chromosomes are the threadlike structures of nucleic acids and protein found in the nucleus of most living cells, carrying genetic information in the form of genes. In humans, our chromosomes carry our DNA and RNA, that make us who we are. Each of us inherits a set of chromosomes from each parent.

In general, chromosomes are inside the nucleus and store directions that the cell uses to carry on activities.

• They control processes, such as how fast a cell grows or when it reproduces.
• Not all organisms have the same number of chromosomes in their cells.
• Humans have a total of 24 chromosomes (12 from each parent).

If you take a look at the diagram above, you will notice that the nucleus is nestled pretty much right in the center of the cell.

• Wondering about the nucleolus?

Don't worry too much about that structure right now. Just know that it's the structure within the nucleus that houses the RNA, and its function comes into play during reproduction.

You may be thinking that all those other organelles you see labeled on the diagram above are simply floating around, bumping into each other, but thanks to a cell component called cytoplasm, all of the organelles — especially the nucleus — are well-protected.

Cytoplasm fills the space inside the cell membrane and the nucleus.

• Cytoplasm is a clear, jellylike material, similar to the white in a raw egg.
• It is made mostly of water.

Remember, since animal cells are what make up animals, the have many of the same characteristics as animals. Cells depend on water for survival — without enough water, the cell will die.

Mitochondria: The Nuclear Power Plant

Now, let's talk about how the cell produces, transports, and expels energy, starting with the mitochondria.

The mitochondria take in nutrients, break them down, and create energy-rich molecules for the cell. The biochemical processes of the cell are known as cellular respiration.

While the nucleus is the control center of the cell, many refer to the mitochondria as the power house or power plant of the cell. Many of the reactions involved in cellular respiration happen in the mitochondria. Mitochondria are the working organelles that keep the cell full of energy.

A few interesting points to keep in mind about mitochondria are:

• They consist of an inner and outer membrane.
• They are spherical or rod-shaped.
• They are an independent organelle with unique genetic material.

Lysosomes: The Tummy

On to Lysosomes, the digestive system of the cell. Lysosomes are also membrane-bound, which is extremely important because they contain digestive enzymes that help break down waste materials.

Vacuoles: The Storage Tanks

Let's move on to vacuoles. Vacuoles are storage bubbles found in cells. Vacuoles might store food or any variety of nutrients a cell might need to survive. They can even store waste products so the rest of the cell is protected from contamination. Eventually, those waste products will be sent out of the cell.

The structure of vacuoles is fairly simple. There is a membrane that surrounds a mass of fluid. In that fluid are nutrients or waste products.

Endoplasmic Reticulum (ER): The Superhighway

Next is the endoplasmic reticulum (ER) that is an organelle that transports materials inside the cell.

Think of the endoplasmic reticulum as roadway. They are a series of channels used to transport materials from one part of the cell to another, or even from one cell to another.

As you may have noticed, there are two types of ER: smooth and rough. Rough ER has ribosomes, which synthesize proteins to make RNA. Ribosomes are also found floating freely in other parts of the cell, but they most often attach to the ER.

You may have guessed that smooth ER does not have ribosomes attached to it!

The Golgi Complex: The Warehouse

Earlier, when discussing the human digestive system, we mentioned that the human body stores excess useful material for later use. Individual cells do this as well within the Golgi Complex. The Golgi Complex is responsible for the packaging and storage of proteins and other useful materials.

Centrosome: The Multiplying Divider

Finally, you may have noticed a small organelle near the nucleus called the centrosome. This organelle will play an important role during cellular reproduction. During mitosis, the centrosomes are responsible for distributing genetic material to both sides of the cell.

Review All That!

Although these are not all of the organelles that are contained in animal cells, they are the most important, and the ones you will need to focus on first in order to understand how cells function independently to become part of a bigger system.

For an overview of what you just learned, watch Smart Learning for All's Animal Cell (below). Follow along carefully and take notes on each organelle as needed:

Now, continue on to the Got It? section to listen to a cell song!