Animal Cells

A tiny city with four big jobs

Animal cells are soft and roundish, with no stiff walls. Inside, every part has a job, just like the buildings in a city:

• Cell membrane = the city wall and gate. A thin, squishy skin around the cell that decides what comes in and goes out.
• Nucleus = city hall. The boss. It holds the instruction book called DNA.
• Mitochondria = the power plants. They turn food into energy for the cell.
• Cytoplasm = the jelly. The goo that fills the cell. Everything floats in it.

The instruction book

Inside the nucleus is DNA, the cell's instruction book. It's written in a secret code that uses only four letters: A, T, C, and G. The order of those letters tells your body how to build everything about you.

A part of the DNA that has the instructions for one thing is called a gene. You have genes for your eye color, your hair, and thousands of other things.

Here's a puzzle: almost every cell in your body has the same instruction book. So why is a nerve cell different from a muscle cell? Because each kind of cell reads different pages of the book!

The gate and the power plants

The cell membrane is a picky gatekeeper. It lets in the good stuff the cell needs, like water, food, and oxygen. It keeps out things that don't belong. It also sends out the cell's trash.

Once food and oxygen are inside, the mitochondria (my-toe-KON-dree-uh) get to work. They turn them into energy the cell can use, like a power plant making electricity for a city.

• Cells that work hard, like muscle cells, have lots of mitochondria.
• Mitochondria make a waste gas called carbon dioxide. Your blood carries it to your lungs, and you breathe it out!

One cell becomes two

Cells can make copies of themselves. Scientists call this cell division (the fancy name is mitosis, my-TOE-sis). Here's how it goes:

• 1. Grow. The cell gets bigger and stores up energy.
• 2. Copy. It makes a full copy of its DNA instruction book.
• 3. Pull apart. The two copies move to opposite sides, and the middle pinches in.
• 4. Split! Now there are two cells, each with its own copy of the DNA.

Cell division is how you grow taller, how a scraped knee heals, and how your body replaces old, worn-out cells.

Animal cell quiz