# Wikijunior:Biology/Cells



## Cells

!Plant cells

All living things are made of cells. They are the components and
building blocks of life.

**What is a cell?** *A cell is a bag of liquid that holds in the stuff
of life.*

A cell is the smallest structural and functional unit of a living
organism. The word \"cell\" comes from the Latin word *cella*, which
means small room. If you look at living things under a microscope, you
will see that they are made of small squares or balls. **Robert Hooke**,
a biologist from England, saw these small squares in a hard material
called cork using a microscope in the year 1665. They looked like rooms,
and so he called them cells. He was also the first person to observe
dead cells.

**What types of cells are there?** There are two kinds of cells:
**eukaryotes**, which have a large ball in them called a *nucleus*, and
**prokaryotes**, which do not.

Most prokaryotes are very small. Two of the six kingdoms, *Bacteria* and
*Archea*, are made up of prokaryotes. All of the rest of the kingdoms --
*Animalia*, *Plantae*, *Fungi*, and *Protista* -- are made up of
eukaryotes.

### Endosymbiosis Theory

Every human being is a living being. But each one is made up of a large
number of tiny living things called cells. But that\'s not all! Smaller
creatures live in every cell of our body. The larger cell nourishes the
smaller cells inside, and the small cells perform important functions.

Working together for mutual benefit is called symbiosis. According to
the endosymbiont theory, single-celled organisms were taken up by the
large cells long ago. The mitochondria are derived from aerobic
bacteria. The mitochondria gain energy from food and oxygen. The
chloroplasts found in plant cells are derived from cyanobacteria. The
chloroplasts gain energy from light.

<File:Euglena> mutabilis - 400x - 1 (10388739803).jpg\|thumb\|Euglena
<File:Euglena> viridis TK-UT.svg\|thumb\|Euglena
<File:Endosymbiosis.svg%7Cthumb%7CEndosymbiosis> <File:Serial>
Endosymbiosis Theory.png\|thumb\|Endosymbiosis Theory

## What do cells look like?

Cells are surrounded by a thin layer of oil called the *cell membrane*.
It separates the inside of the cell from the outside. Some cells also
have a firm box around them called a *cell wall* that keeps it from
breaking. The water that fills a cell is called the *cytoplasm*. Inside
a cell, knowledge is stored in something called a *chromosome*. It tells
the cell how to work, like steps in a book.

Eukaryotic cells hold their chromosomes in a structure called a
**nucleus**, which has its own oily membrane around it. Cells also have
many other things with membranes called **organelles**, which means
\"little organs\". Some organelles found in eukaryotic cells are called
*ribosomes*, *vacuoles*, *mitochondria*, and *chloroplasts*. !Human
sperm
cell{width="600"}

Cells that do different things have different shapes. A plant leaf cell
takes light and uses it to make sugar. To do this, it has green
organelles called *chloroplasts*. To get the most light, it pushes
cytoplasm in circles around a hollow bubble of water in the center of
the cell called a *vacuole*.

A human sperm cell carries its chromosomes, found in the nucleus, to an
egg cell in order to make a new baby. It has a large tail called a
*flagella* that helps it to swim. It also has many organelles called
*mitochondria* that give it power, like how gasoline powers a car.

#### Vocabulary words

:   **nucleus** - A ball of membrane in the middle of the cell that
    holds the chromosomes.
:   **chromosomes** - Things that hold the knowledge of the cell.
:   **prokaryote** - A cell without a nucleus.
:   **eukaryote** - A cell with a nucleus.
:   **organelles** - Little things inside a cell.
:   **cytoplasm** - The gel-like inside of a cell.
:   **membrane** - An oil bag that holds water.
:   **vacuole** - An organelle full of water and waste inside a cell.
:   **mitochondria** - An organelle that makes power in a cell.
:   **chloroplast** - An organelle that makes sugar found in a plant or
    protist.
:   **flagella** - A tail on a cell that makes it swim.
:   **Golgi body** - An organelle which helps in secretion.
:   **ribosome** - An organelle which helps in synthesis of proteins.

