Notes

The smallest and simplest cells are prokaryotes, which are bacteria. A prokaryote is a
single-celled organism that lacks a nucleus and other internal compartments. Prokaryotic
cells depend on a strong cell wall to give the cell shape. A prokaryotic cell wall is made of
strands of polysaccharides connected by short chains of amino acids. Some prokaryotic
cell walls are surrounded by a structure called a capsule, which is also composed of
polysaccharides. The capsule enables prokaryotes to cling to almost anything, including
teeth, skin, and food. Many prokaryotes have fl agella, which are long, threadlike structures
that protrude from the cell’s surface and enable movement.
The fi rst cells with internal compartments were primitive eukaryotic cells, which evolved
about 2.5 billion years ago. A eukaryote, such as a plant or an animal, is an organism
whose cells have a nucleus. The nucleus is an internal compartment that houses the cell’s
DNA. Other internal compartments, or organelles, enable eukaryotic cells to function in
ways different from prokaryotes. An organelle is a structure that carries out specifi c
activities in the cell.
Many organelles—such as the endoplasmic reticulum, vesicles, Golgi apparatus, lysosomes,
and mitochondria—are found in both animal cells and plant cells. However, plant cells
have three additional structures that are not found in animal cells:
• The cell wall—composed of proteins and carbohydrates, including the polysaccharide cellulose—surrounds the cell membrane of plant cells.
• Chloroplasts are organelles that use light energy to make carbohydrates from
carbon dioxide and water.
• The central vacuole is a large membrane-bound space that stores water and may
contain many substances, including ions, nutrients, and wastes; when full, it
makes the cell rigid.
Many single-celled eukaryotes use fl agella for movement. Short hairlike structures called
cillia protrude from the surface of other eukaryotic cells. Flagella or cilia propel some cells
through their environment. In other cells, cilia and fl agella move substances across the
cell’s surface.