In this topic you will learn about the small parts that make up the large parts of living things.

A cell is a tiny, box-like part that is the basic building block of living things. Plant and animal cells have many of the same parts. Both plant and animal cells are filled with cytoplasm. Cytoplasm is a clear, jelly-like material that fills plant and animal cells. Both types of cells have a cell membrane. A cell membrane is a thin outer covering of plant and animal cells. Plant and animal cells also have a nucleus. The nucleus is the main control center found in plant and animal cells.

Plant cells and animal cells have some differences. Plant cells are usually larger than animal cells. Plant cells often have a box-like shape. Animal cells come in a wide variety of shapes. Plant cells have a stiff cell wall and a thin cell membrane. Animal cells only have a cell membrane.

Many plant cells have green chloroplasts. Chloroplasts make food. Animal cells do not have chloroplasts.

A tissue is a group of cells that are alike. Heart tissue is made of heart cells. Brain tissue is made of brain cells. Heart tissue is different from brain tissue.

An organ is a group of tissues that work together to do a job. The heart, liver, brain, and stomach are organs. Some organs work together in body systems. Body systems include the digestive system and the nervous system. Each body system does a special job. A plant's organs include leaves, stems, and roots.