Cells are the smallest units displaying the properties of life. Cells normally are measured in micrometers because they are so small. Their small size ensures a sufficient amount of plasma membrane to serve the cytoplasm.

All organisms are composed of cells. The two major kinds of cells are prokaryotic and eukaryotic. They differ by the organization of chromosomal DNA and the presence of organelles in the cytoplasm. Prokaryotic cells lack a nucleus and other membranous organelles.

The nucleus of eukaryotic cells (plant and animal) is defined by a nuclear envelope which separates the nucleoplasm from the cytoplasm. The chromosomal material exists as chromatin until the cell divides. The nucleolus in the nucleus contains ribosomal RNA and the proteins of ribosomal subunits.

The eukaryotic cell contains a variety of structures in the cytoplasm. Ribosomes are the site of protein synthesis. They may exist freely or be attached to the endoplasmic reticulum. Several structures are part of the endomembrane system in the cell. The endoplasmic reticulum provides channels that transport substances through the cell. Substances are processed and packaged by the Golgi apparatus. Lysosomes contain enzymes that promote the breakdown of cell substances.

Some organelles are specialized to handle energy in the cell. Chloroplasts are the site of photosynthesis, whereas the mitochondria are regions involved in cellular respiration. These organelles may be remnants of prokaryotes that inhabited eukaryotic cells over evolutionary time.

The cytoskeleton contains microtubules, intermediate filaments, and microfilaments. They maintain cell shape and assist movement of cell parts.

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