LR247.3.1: Cell Organelles

Cells make up all living things, including your own body. The image below

shows a group of cells. Not all cells look alike. Cells can differ in shape and sizes.

And the different shapes usually mean they do different things.

A cell is the smallest structural and functional unit of an organism. Some

organisms, like bacteria, consist of only one cell. Large organisms, like humans,

consist of trillions of cells. Compare a human to a banana. On the outside, they

look very different, but if you look close enough you’ll see that their cells are

actually very similar.

Most cells are so small that you cannot see them without the help of a

microscope. It was not until 1665 that English scientist Robert Hooke invented a

basic light microscope and observed cells for the first time, by looking at a piece

of cork, which he got from a tree. You may use light microscopes in the

classroom. You can use a light microscope to see cells like the image below.

In 1858, after using microscopes much better than Hooke’s first microscope,

Rudolf Virchow developed the hypothesis that cells only come from other cells.

For example, bacteria, which are single-celled organisms, divide in half (after

they grow some) to make new bacteria. In the same way, your body makes new

cells by dividing the cells you already have. In all cases, cells only come from

cells that have existed before. This idea led to the development of one of the

most important theories in biology, the cell theory.

Cell theory states that:

1. All organisms are composed of cells.

2. Cells are alive and the basic living units of organization in all

organisms.

3. All cells come from other cells.

As with other scientific theories, many hundreds, if not thousands, of experiments

support the cell theory. Since Virchow created the theory, no evidence has ever

been identified to contradict it.

Single-celled (unicellular) organisms like bacteria are composed of just one cell,

whereas multicellular organisms can be composed of trillions of cells.

Multicellular organisms include protists (though single-celled protists also exist),

fungi, plants and animals. Most plant and animal cells are between 1 and 100 μm

(micrometer) in size and therefore can only be observed under the microscope.

The one cell of a unicellular organism must be able to perform all the functions

necessary for life. These functions include metabolism, homeostasis, and

reproduction. Specifically, these single cells must transport materials, obtain and

use energy, dispose of wastes, and continuously respond to their environment.

The cells of a multicellular organism also perform these functions, but they may

do so in collaboration with other cells.

Although cells share many of the same features and structures, they also can be

very different. Each cell in your body is designed for a specific task. In other

words, the cell's function is partly based on the cell's structure. For example:

● Red blood cells are shaped with a pocket that traps oxygen and brings

it to other body cells. These cells also have a large surface area that

aids in oxygen transfer.

● Nerve cells are long and stringy in order to form a line of

communication with other nerve cells, like a wire. Because of this

shape, they can quickly send signals, such as the feeling of touching a

hot stove, to your brain.

● Skin cells are flat and fit tightly together to protect your body.

As you can see in the images below, cells are shaped in ways that help them do

their jobs. Multicellular (many-celled) organisms have many types of specialized

cells in their bodies.

Cells have many organelles that perform specific tasks to help the cell survive. Even very different types of cells have many of the same organelles. Just like your body parts, it is important for a cell's organelles to work together. Click on the two organelles labeled in the animal cell below to learn more.

Check for Understanding:

The cell's organelles perform specific functions to help the cell survive.

1. The cell membrane separates the cell from its surroundings and gives the cell shape. It also acts as a gatekeeper, controlling what enters and leaves the cell.

2. The nucleus controls the activities in the cell and stores the cell's DNA.

3. The vacuole stores water for the cell.

4. The mitochondria turn food (sugars) and oxygen into energy for the cell.

5. The cytoplasm is a jelly-like material that fills the cell, separating the organelles and giving the cell shape.

Because plants have different needs from animals, their cells have two extra organelles. Click on the buttons below to learn more.

Check for Understanding:

Because plants have different needs from animals, their cells have two extra organelles.

1. The cell wall surrounds the cell membrane, giving plant cells extra structure and protection. Plants do not have bones, so they need stiff cell walls to hold them up.

2. The chloroplast captures sunlight like a solar panel and turns it into sugar that the plant can use to make energy. Plants are green because their chloroplasts contain a green pigment called chlorophyll.