1 Cells to Organ System

CELLS TO ORGAN SYSTEMS

WHAT ARE CELLS? Hi there! I'm a cell! A cell is the basic unit of life. In fact, you might call cells the building blocks of life! All living things are made up of cells. Some living things are made up of only one cell. Like this bacterium! But some plants and animals are made up of many cells. Humans are animals, too!

The human body has more than 10 trillion cells! Most cells are so tiny you can't see them without a microscope. SEE?! It takes millions of cells to make up the skin on the palm of your hand! Just like you, a cell "breathes," takes in food, and gets rid of wastes. Cells also grow and reproduce, or create their own kind. And, just like other organisms... ...cells die!

GROWTH Do you know one of the most amazing things about you? At one time, you were a single cell. When a human egg cell... ...was fertilized by a human sperm cell... ...the resulting cell had all the instructions to make all of you. That cell divided and grew, and each resulting cell had its own complete set of instructions. Over time, some cells changed and took on different jobs.

Some cells became your eyes... Some cells became your nose... Some cells became your fingers... After about nine months, you were born—a baby human! Your cells will continue to divide throughout your life. That's how your body grows and repairs itself.

KINDS OF CELLS All animal cells share certain characteristics. But different cells do different jobs. As you read these words, nerve cells in your eyes are carrying messages of what you are reading to your brain cells. Muscle cells attached to your eyeballs are moving your eyes across the page. SEE?!