Archaea have unique biochemistry and molecular structure, including different types of cell membranes, cell wall compositions, and the ability to use alternative metabolic pathways.

They include two classes of bacteria: eubacteria (including photosynthetic organisms, or cyanobacteria), which are common bacteria that inhabit soil, water, and larger organisms and archaebacteria, which are also called archaea. Prokaryotic cells contain no nucleus, are relatively small (1–10 μm in diameter), and have a simple internal structure ( Fig. They comprise protists (single-celled organisms), fungi, plants, and animals, and they are generally 5–100 micrometers (μm) in linear dimension. Cells can be separated into prokaryotic and eukaryotic categories. The microscopic functional and structural unit of all living organisms. Many, but not all, plant cells contain characteristic features found only in plants, including a rigid outer cell wall, internal vacuoles for storage, and chloroplasts for energy production. In addition to a nucleus, the organelles of eukaryotic cells include the endoplasmic reticulum, Golgi apparatus, lysosomes, mitochondria, peroxisomes, and vesicles.

Prokaryotic cells, which are relatively smaller than eukaryotic cells, lack a nucleus or organelles and have a single circular molecule of DNA (chromosome) plus optional, smaller, circular molecules of DNA called plasmids.Įukaryotic cells have a nucleus, membrane-bound organelles, and multiple, compartmentalized molecules of linear DNA. All cells have a plasma membrane, made of lipids and proteins, to regulate substance exchange with the external environment. A cell is the microscopic functional and structural unit of all living organisms.

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