Biological cells are the basic building blocks of all living organisms. They are the smallest units of life, capable of performing essential functions such as growth, reproduction, and response to the environment. Cells can be classified into two main types: prokaryotic cells, which lack a nucleus and are typically single-celled organisms like bacteria, and eukaryotic cells, which have a nucleus and can be found in multicellular organisms like plants and animals. Each cell is surrounded by a protective membrane and contains various structures called organelles that perform specific tasks. For example, mitochondria generate energy, while ribosomes synthesize proteins. The study of cells is fundamental to understanding biology and the processes that sustain life.