Similarities and Differences Among Cells

All cells have structural and functional similarities. Structures shared by all cells include a cell membrane, an aqueous cytosol, ribosomes, and genetic material (DNA). All cells are composed of the same four types of organic molecules: carbohydrates, lipids, nucleic acids, and proteins. All cells perform the essential functions of life. They control what materials enter and exit through their cell membranes and the concentrations of materials maintained within the cell interior. In addition, all cells interact with their environments. Every cell breaks down nutrients to obtain energy and synthesizes some necessary molecules from raw materials. All cells also use ribosomes to make proteins, translating instructions stored on DNA and transcribed on RNA. All can reproduce by replicating their DNA and dividing to form new cells. However, the means by which different cells accomplish the tasks of life vary widely.

There are also many structural differences between cells. For example, nearly all prokaryotic cells have cell walls, while only some eukaryotic cells do. The interior of a prokaryotic cell is a single compartment, allowing any interior molecules to come into contact with each other. The interior of a eukaryotic cell, on the other hand, contains many separate compartments, the organelles, which perform specialized functions. The genetic material (DNA) of prokaryotic cells is circular, while that of eukaryotic cells is linear. Prokaryotic cells have a single molecule of chromosomal DNA, but eukaryotes have several chromosomes (structures that carry genetic material). The structure of the prokaryotic ribosome is smaller than the eukaryotic ribosome and less complex.

In multicellular organisms, there are structural differences between the specialized cells of different organs and tissues. The shapes of cells that form the different tissues are adapted to perform the particular functions of that tissue. For example, types of animal tissues include nervous tissue, epithelial tissue, connective tissue, and muscle tissue. Cells in epithelial tissue have connections called tight junctions that allow the cells to form an impermeable layer. In contrast, nerve cells have projections called dendrites and axons that allow the cells to interact as a network and pass nerve impulses. All eukaryotic cells share common characteristics but also have differences in their cell structures. Plants and animals are multicellular organisms, while a fungus (plural, fungi) can be either single-celled or multicellular. Animal cells do not have cell walls. Plants and some fungi do have cell walls, but they are composed of different polysaccharides (chains of carbohydrate molecules). The cell walls of plants are made of cellulose, while those of fungi are made of chitin. Only plant cells contain chloroplasts, the organelles that perform photosynthesis. The vacuoles, membrane-bound storage compartments, of plant and fungal cells include a large central vacuole used to maintain turgor pressure, the outward-pushing force that helps to maintain cell shape.

Animal cells, unlike the cells of a plant or a fungus, have a structure called the centrosome that aids in organization of chromosomes during mitosis. Some animal cells can move independently, unlike the cells of plants and some fungi. They move by means of cilia or flagella, structures that are anchored in the cell membrane. Cilia and flagella push fluids along the surfaces of cells. Flagella are thin, whiplike tails that assist cells in movement. A sperm cell, for example, has a flagellum that enables it to move to find and fertilize an egg.