What do a bacterial cell and one of your cells have in common?

There are many different types of cells, but they all have certain parts in common. As this image of human blood shows, cells come in different shapes and sizes. The shapes and sizes directly influence the function of the cell. Yet, all cells – cells from the smallest bacteria to those in the largest whale – do some similar functions, so they do have parts in common.

Common Parts of Cells

Discovery of Cells

The first time the word cell was used to refer to these tiny units of life was in 1665 by a British scientist named Robert Hooke. Hooke was one of the earliest scientists to study living things under a microscope. The microscopes of his day were not very strong, but Hooke was still able to make an important discovery. When he looked at a thin slice of cork under his microscope, he was surprised to see what looked like a honeycomb, made up of many tiny units, which Hooke called cells.

 

 

Soon after Robert Hooke discovered cells in cork, Anton van Leeuwenhoek in Holland made other important discoveries using a microscope. Leeuwenhoek made his own microscope lenses, and he was so good at it that his microscope was more powerful than other microscopes of his day. In fact, Leeuwenhoek’s microscope was almost as strong as modern light microscopes.

Using his microscope, Leeuwenhoek discovered tiny animals such as rotifers. Leeuwenhoek also discovered human blood cells. He even scraped plaque from his own teeth and observed it under the microscope. What do you think Leeuwenhoek saw in the plaque? He saw tiny living things with a single cell that he named animalcules (“tiny animals”). Today, we call Leeuwenhoek’s animalcules bacteria.

The Cell Theory

The cell theory is one of the fundamental theories of biology. For two centuries after the discovery of the microscope by Robert Hooke and Anton van Leeuwenhoek, biologists found cells everywhere. Biologists in the early part of the 19^th century suggested that all living things were made of cells, but the role of cells as the primary building block of life was not discovered until 1839 when two German scientists, Theodor Schwann, a zoologist (studies animals), and Matthias Jakob Schleiden, a botanist (studies plants), suggested that cells were the basic unit of structure and function of all life. Later, in 1858, the German doctor Rudolf Virchow observed that cells divide to produce more cells. He proposed that all cells arise only from other cells. The collective observations of all three scientists form the cell theory, which states that:

• all organisms are made up of one or more cells,
• all the life functions of an organism occur within cells,
• all cells come from preexisting cells.

Cell Diversity

Cells with different functions often have different shapes. The cells pictured in the Figure below are just a few examples of the many different shapes that cells may have. Each type of cell in the figure has a shape that helps it do its job. For example, the job of the nerve cell is to carry messages to other cells. The nerve cell has many long extensions that reach out in all directions, allowing it to pass messages to many other cells at once. Do you see the tail-like projections on the algae cells? Algae live in water, and their tails help them swim. Pollen grains have spikes that help them stick to insects such as bees. How do you think the spikes help the pollen grains do their job? (Hint: Insects pollinate flowers.)

Common Parts of a Cell

Although cells are diverse, all cells have certain parts in common. The parts include a plasma membrane, cytoplasm, ribosomes, cytoskeleton, and DNA.

1. The cell membrane (also called the plasma membrane) is a thin coat of lipids that surrounds a cell. It forms the physical boundary between the cell and its environment, so you can think of it as the ‘‘skin’’ of the cell.
2. Cytoplasm refers to all of the cellular material inside the cell membrane, other than the nucleus. Cytoplasm is made up of a watery substance called cytosol and contains other cell structures such as ribosomes.
3. Ribosomes are structures in the cytoplasm where proteins are made.
4. The cytoskeleton consists of filaments and tubules that crisscross the cytoplasm and help maintain the cell’s shape.
5. DNA is a nucleic acid found in cells. It contains the genetic instructions that cells need to make proteins.