All living organisms are made up of cells, but do you know what a cell is? In this lesson, you can learn about the three main structures that all cells contain and about the two major types of cells.
A cell is often called the basic unit of life. To explain the main characteristics of cells, scientists have developed the cell theory. Cell theory states three things:
- All organisms are made up of at least one cell.
- A cell is the smallest unit that can carry out all the functions necessary for life.
- All cells arise from pre-existing cells.
While there are many different kinds of cells, all cells have three main parts:
- Plasma membrane
The plasma membrane or cell membrane is the outer portion of a cell. It’s important for separating and protecting internal cellular structures from the surrounding environment.
This allows a cell to have internal conditions that are very different from its surroundings.This membrane is also important for allowing only certain substances to enter or leave the cell. This is called selective permeability. For example, nutrients have to be able to get into a cell so that it can survive. Waste substances must also be able to leave the cell.In some cells, the plasma membrane is not the very outermost cellular structure.
It is the cell wall instead. Cell walls are external to the plasma membrane and provide additional support and protection. They can be found in plants, fungi, and bacteria.
DNA is short for deoxyribonucleic acid. It is the molecule that makes up genes, which carry important instructions for keeping a cell alive. A gene is the unit of heredity which is transferred from parents to offspring.
Everything else inside a cell is called the cytoplasm. It’s made up of a jelly-like water-based fluid called cytosol. There are other non-fluid substances found in the cytoplasm, but they vary between types of cells.
Categories of Cells
There are two main categories of cells: prokaryotic cells and eukaryotic cells.Bacteria are very ancient and simple organisms.
One individual is composed of only a single prokaryotic cell. These cells are very small and do not have any structures separating their DNA from the cytosol. In fact, they do not have any membrane-bound structures other than the plasma membrane.
Additionally, the DNA of prokaryotic cells is arranged in a circle.Eukaryotic cells are found in animals, plants, protists, and fungi. These cells are typically larger and more complex than prokaryotic cells. Eukaryotic means ‘true nucleus.
‘ A nucleus is a membranous structure surrounding the cell’s DNA. Prokaryotes do not have a nucleus (the term prokaryotic means ‘before a nucleus’).A eukaryotic cell has many membrane-bound structures called organelles in its cytoplasm. There are many different kinds of organelles, and each one is specialized to carry out a specific and unique function. For example, the mitochondrion is an organelle that breaks down nutrients to make energy. One more difference between eukaryotic and prokaryotic cells is that the DNA in eukaryotes is linear.
Cells are the basic units of life of all organisms. They always come from pre-existing cells. All cells have a protective plasma membrane, but some cells also have an additional outer cell wall. The DNA of cells is organized into units of heredity or genes.
Between the DNA and plasma membrane of a cell is the cytoplasm.Prokaryotic cells, like bacteria, do not have a nucleus or membrane-bound organelles. Their DNA is circular.
Eukaryotic cells include those found in plants, animals, fungi, and protists. These cells have linear DNA enclosed in a nucleus. Eukaryotic cells also have many organelles to carry out important cellular functions.