HiSET Science: Hierarchy of Multicellular Organisms

Organizational Hierarchy within Multicellular Organisms
Cells are the smallest living units of organisms. Tissues are groups of cells that work together to perform a specific function. Organs are groups of tissues that work together to perform a specific function. Organ systems are groups of organs that work together to perform a specific function. An organism is an individual that contains several body systems.

Cells
Cells are the basic structural units of all living things. Cells are composed of various molecules including proteins, carbohydrates, lipids, and nucleic acids. All animal cells are eukaryotic and have a nucleus, cytoplasm, and a cell membrane. Organelles include mitochondria, ribosomes, endoplasmic reticulum, Golgi apparatuses, and vacuoles. Specialized cells are numerous, including but not limited to, various muscle cells, nerve cells, epithelial cells, bone cells, blood cells, and cartilage cells. Cells are grouped to together in tissues to perform specific functions.

Tissues
Tissues are groups of cells that work together to perform a specific function. Tissues can be grouped into four broad categories: muscle tissue, connective tissue, nerve tissue, and epithelial tissue. Muscle tissue is involved in body movement. Muscle tissues can be composed of skeletal muscle cells, cardiac muscle cells, or smooth muscle cells. Skeletal muscles include the muscles commonly called biceps, triceps, hamstrings, and quadriceps. Cardiac muscle tissue is found only in the heart. Smooth muscle tissue provides tension in the blood vessels, controls pupil dilation, and aids in peristalsis. Connective tissues include bone tissue, cartilage, tendons, ligaments, fat, blood, and lymph. Nerve tissue is located in the brain, spinal cord, and nerves. Epithelial tissue makes up the layers of the skin and various membranes. Tissues are grouped together as organs to perform specific functions.

Organs and Organ Systems
Organs are groups of tissues that work together to perform specific functions. Organ systems are groups of organs that work together to perform specific functions. Complex animals have several organs that are grouped together in multiple systems. In mammals, there are 11 major organ systems: integumentary system, respiratory system, cardiovascular system, endocrine system, nervous system, immune system, digestive system, excretory system, muscular system, skeletal system, and reproductive system.