The Cell

The Structure of Cells

Hooke saw hundreds of cells when he looked at a piece of cork under a microscope.

In the mid-1600s, Robert Hooke was using a microscope to look at a piece of cork. He noticed that the cork seemed to be made of “little rooms,” which he called cells. It was not long before scientists realized that all living things were made of cells. Now we know that cells are the building blocks of life.


Cells are basic to life, but they are not solid bits of matter. Inside each cell is a jelly like substance known as cytoplasm. Some scientists call this the “stuff of life.”

Cytoplasm is hard to describe because it is constantly changing.

However, it always contains some of the same ingredients:

  • About 70 percent of the cell’s interior is made of water.
  • Cytoplasm contains about 20 different amino acids. These acids combine and recombine to form thousands of different kinds of proteins. The proteins are used to build and repair the cell.
  • Carbohydrates provide the cell with energy. Fats are the cell’s storage tanks. Fats contain all the energy the organism does not use immediately.
  • Nucleic acids control the cell’s activities.

Cell Structures

Cytoplasm contains a variety of structures that help the cell to function, or work, properly. As you read about cell structures, refer to the diagrams of plant and animal cells below.


Cell Membrane

Beginning at the outer edge of the cell, notice the cell membrane. This structure is soft and flexible, but it holds the cell together. It also serves as a gate. Substances needed by the cell can pass through the membrane and into the cell. Other substances are shut out. Waste products pass out of the cell through this membrane.


The nucleus is one of the larger structures of the cell. It controls the activities of the cell. Like the cell, the nucleus is made of a substance much like cytoplasm. It is also surrounded by a membrane. Long, thin strands of genetic material float in the nucleus. Inside the nucleus is the nucleolus, a dense, round body that makes specialized cell structures called ribosomes.

Specialized Cell Structures

Cells may differ in small ways, but all cells contain a membrane, cytoplasm, and nuclear material. In addition, most also contain some specialized structures that help the cell to function. Just a few of these structures are described below.

Most cells contain mitochondria, which are shown near the top of both diagrams on the previous page. These sausage-shaped structures trap the energy from food and release it to the cell.

Throughout the cell is a network of canal-like structures known as the endoplasmic reticulum (ER). The ER leads from the cell membrane to the nucleus. It transports and stores substances needed by the cell. Ribosomes are the tiny dots seen on the ER. They combine amino acids into proteins. The vacuoles have a variety of functions. Some digest food. Others store or dispose of waste. In plant cells, the vacuole may be filled with water, which helps to support the stem and leaves.

Specialized Cell Structures in Plants

Examine the diagrams again. Notice that plant cells have several structures that are not found in animal cells. The chloroplasts contain the chlorophyll that plants use to trap energy from the Sun for the process of photosynthesis.

The outermost structure just outside the plant’s cell membrane is the cell wall This rigid wall provides support for the cell. When Robert Hooke found cells in a slice of cork, he was looking at the cork’s cell walls.

How a Cell Works

To stay alive, most cells need to take in food, water, and oxygen and to eliminate waste. This means cells must let materials pass through the cell membrane without losing their cytoplasm. Transporting materials across the cell membrane takes place one molecule at a time. Like a filter, the cell membrane has tiny holes that allow some small molecules to pass through it. Larger molecules are filtered out.


Material, such as water or simple carbohydrates, tends to seek a balance on either side of the cell membrane. If more of a specific kind of molecule, such as water, is on the outside of a cell, the tendency is for those molecules to spread evenly into the cell. Scientists describe this process of spreading molecules evenly through an area as diffusion. Diffusion moves molecules in and out of cells without using any of the cell’s energy.

Active Transport

Cells can also move molecules from less crowded areas outside the cell to more crowded areas inside the cell. This movement against the normal flow of diffusion is called active transport. The process uses energy and works something like a revolving door. Special molecules in the membrane pick up materials outside the cell. They then turn to the inside of the cell and release the material.