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Background Multicellular organisms, such as plants and animals, have various levels of organization within them. Although individual cells can perform specific functions, they become dependent on one another and can work together for the good of the entire organism. The first level, cells, is the simplest level. Cells are the basic structural and functional units in living things. Examples of animal cells include blood cells, bone cells, nerve cells, and skin cells. Examples of plant cells include root cells, guard cells, and sieve tube cells. The next level, tissues, is made up of cells that are similar in structure and function. These cell groups work together to perform a specific activity. Humans, for example, have four basic tissues: connective, epithelial, muscle, and nerve. Plants have three basic tissues: dermal, vascular, and ground. Organs are made up of tissues that work together to perform a specific activity. Examples of this level for a human are the heart, brain, skin, and stomach. Organs found in plants include leaves, stems, and roots.

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Animal Plant

cell cell

tissue tissue

organ organ

6.L 14.1 Organization of Organisms

Background, continued The fourth level is the organ systems. These are groups of two or more organs that work together to perform a specific function for the organism. Examples in the human body include the circulatory, nervous, skeletal, muscular, integumentary, endocrine, digestive, reproductive, excretory, and respiratory systems. Plants only have two basic organ systems: the root system and the shoot system. The final and most complex level of organization is the organism. An organism is a living thing that carries out all basic life processes. Organisms take in materials, release energy from food, release wastes, grow, respond to the environment, and reproduce. Although an organism can be made up of only one cell such as bacteria, most organisms, including plants and animals, are usually made up of organ systems performing these functions. The levels of organization from smallest to largest are: cells → tissues → organs → organ systems → organisms.

Animal Plant

organism organism

organ system organ system

6.L 14.1 Organization of Organisms

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