The Cell

What Are Cells?

E. coli Bacterium
This is a colored transmission electron micrograph (TEM) of an Escherichia coli bacterium in the early stages of binary fission, the process by which the bacterium divides. Credit: CNRI/Getty Images

Life is both wonderful and majestic. Yet for all of its majesty, all organisms are composed of the fundamental unit of life, the cell. The cell is the simplest unit of matter that is alive. From the unicellular bacteria to multicellular animals, the cell is one of the basic organizational principles of biology. Let's look at some of the components of this basic organizer of living organisms.

Eukaryotic Cells and Prokaryotic Cells

There are two primary types of cells: eukaryotic cells and prokaryotic cells. Eukaryotic cells are called so because they have a true nucleus. The nucleus, which houses DNA, is contained within a membrane and separated from other cellular structures. Prokaryotic cells, however, have no true nucleus. DNA in a prokaryotic cell is not separated from the rest of the cell but coiled up in a region called the nucleoid.

Classification

As organized in the Three Domain System, prokaryotes include archaeans and bacteria. Eukaryotes include animals, plants, fungi and protists (ex. algae). Typically, eukaryotic cells are more complex and much larger than prokaryotic cells. On average, prokaryotic cells are about 10 times smaller in diameter than eukaryotic cells.

Cell Reproduction

Eukaryotes grow and reproduce through a process called mitosis. In organisms that also reproduce sexually, the reproductive cells are produced by a type of cell division called meiosis. Most prokaryotes reproduce asexually and some through a process called binary fission. During binary fission, the single DNA molecule replicates and the original cell is divided into two identical daughter cells. Some eukaryotic organisms also reproduce asexually through processes such as budding, regeneration, and parthenogenesis.

Cellular Respiration

Both eukaryotic and prokaryotic organisms get the energy they need to grow and maintain normal cellular function through cellular respiration. Cellular respiration has three main stages: glycolysis, the citric acid cycle, and electron transport. In eukaryotes, most cellular respiration reactions take place within the mitochondria. In prokaryotes, they occur in the cytoplasm and/or within the cell membrane.

Comparing Eukaryotic and Prokaryotic Cells

There are also many distinctions between eukaryotic and prokaryotic cell structures. The following table compares the cell organelles and structures found in a typical prokaryotic cell to those found in a typical animal eukaryotic cell.

Cell Structure Prokaryotic Cell Typical Animal Eukaryotic Cell
Cell Membrane Yes Yes
Cell Wall Yes No
Centrioles No Yes
Chromosomes One long DNA strand Many
Cilia or Flagella Yes, simple Yes, complex
Endoplasmic Reticulum No Yes (some exceptions)
Golgi Complex No Yes
Lysosomes No Common
Mitochondria No Yes
Nucleus No Yes
Peroxisomes No Common
Ribosomes Yes Yes
Eukaryotic and Prokaryotic Cell Structures