HEL PLZ WRITE AT LEAST A THREE SENTANCE LOGICAL SUMMARY FOR THIS PIECE OF TEXT 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. Some prokaryotic cell walls are surrounded by a structure called a capsule. The capsule enables prokaryotes to cling to almost anything, including teeth, skin, and food. Many prokaryotes have flagella, which are long, threadlike structures that protrude from the cell’s surface and enable movement.

The first 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 specific 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 flagella for movement. Short hair like structures called cilia protrude from the surface of other eukaryotic cells. Flagella or cilia propel some cells through their environment. In other cells, cilia and flagella move substances across the cell’s surface.