4.1: What are Cells?
Cells are the smallest and basic units of life, whether it is a single cell that forms the entire organism, e.g., in a bacterium or trillions of them, e.g., in humans. No matter what organism a cell is a part of, they share specific characteristics.
Basic Characteristics of Cells
A living cell has a plasma membrane, a bilayer of lipids that separates the aqueous solution inside the cell called the cytoplasm from the outside environment.
Furthermore, a living cell possesses genetic information encoded in the form of DNA. DNA can be localized to a particular cell region, as in the nucleoid of a prokaryotic cell, or it can be contained inside another membrane, such as the nucleus of eukaryotes. The word 'eukaryote' means "true nucleus." Hence, the word prokaryote implies that these cells existed before membrane-bound nuclei appeared in the history of life.
Other special characteristics of a living cell include the ability to replicate, respond to the extracellular environment, process energy, and regulate its own functions. These characteristics are essential for the survival and functioning of the cell independently as well as a part of organized structures like tissues.
Organelles Compartmentalize Eukaryotic Cells
Prokaryotic cells lack internal membranes. In contrast, eukaryotes have membrane-bound internal compartments called organelles. Each of these organelles fulfills a specific cellular function. For example, in plant cells, chloroplasts are specialized organelles that convert light energy to sugar. Similarly, animal cells contain lysosomes, membrane-enclosed compartments that contain enzymes to break up larger molecules.