All living things are made up of cells. The simplest organisms consist of only one cell, but such complex living things as dogs and human beings have many billions of cells.
The simplest organisms, including bacteria and many kinds of algae and protozoans, live their entire lives as individual cells. Some other types of algae and protozoans are colonial organisms. Such an organism consists of a loosely organized group of similar cells. Most species of animals and plants are multicellular organisms. They are made up of many kinds of cells, each having its own special functions.
The simplest multicellular animals include sponges and jellyfish. Sponges have some specialized cells but resemble colonial organisms. If the cells of a sponge are separated, they can readily rejoin and form a new individual. Jellyfish are somewhat more highly organized and have well-defined tissues. Higher animals have organs and organ systems.
Cells. A thin covering called the plasma membrane or cell membrane encloses every cell and separates it from its surroundings. All cells except those of bacteria have two main parts--the nucleus and the cytoplasm. A nuclear membrane surrounds the nucleus and separates it from the cytoplasm. The nucleus contains the chromosomes, the hereditary material that carries the instructions for nearly all cellular activities. The cytoplasm contains many kinds of specialized structures called organelles. Each type of organelle has a specific task, such as manufacturing proteins or converting the energy of food molecules into usable forms.
Organelles and a well-defined nucleus are lacking only in the cells of bacteria. Such cells are called prokaryotic, which means before the nucleus. Biologists refer to all other cells as eukaryotic, which means having a true nucleus. For a detailed discussion of cells, see CELL (Inside a living cell).
Tissues, organs, and organ systems. Higher organisms have many extremely specialized kinds of cells, which are grouped together and form larger, more complex structures. A tissue consists of a collection of cells that are similar in structure and function. Animal tissues include muscle tissue and nerve tissue. Plants also have several kinds of tissues. For example, tissue called xylem carries water and minerals from the roots to the rest of the plant.
The basic structural and functional units of higher organisms are organs. An organ consists of several types of tissues. The human heart, for example, consists of muscle tissue, nerve tissue, and connective tissue. Other animal organs include the brain, liver, and kidneys. The principal organs of flowering plants are roots, stems, leaves, and flowers. In higher animals, each important life function is carried out by a group of organs working together. Such a group is called an organ system. Major organ systems include the circulatory system, digestive system, and reproductive system.
The simplest organisms, including bacteria and many kinds of algae and protozoans, live their entire lives as individual cells. Some other types of algae and protozoans are colonial organisms. Such an organism consists of a loosely organized group of similar cells. Most species of animals and plants are multicellular organisms. They are made up of many kinds of cells, each having its own special functions.
The simplest multicellular animals include sponges and jellyfish. Sponges have some specialized cells but resemble colonial organisms. If the cells of a sponge are separated, they can readily rejoin and form a new individual. Jellyfish are somewhat more highly organized and have well-defined tissues. Higher animals have organs and organ systems.
Cells. A thin covering called the plasma membrane or cell membrane encloses every cell and separates it from its surroundings. All cells except those of bacteria have two main parts--the nucleus and the cytoplasm. A nuclear membrane surrounds the nucleus and separates it from the cytoplasm. The nucleus contains the chromosomes, the hereditary material that carries the instructions for nearly all cellular activities. The cytoplasm contains many kinds of specialized structures called organelles. Each type of organelle has a specific task, such as manufacturing proteins or converting the energy of food molecules into usable forms.
Organelles and a well-defined nucleus are lacking only in the cells of bacteria. Such cells are called prokaryotic, which means before the nucleus. Biologists refer to all other cells as eukaryotic, which means having a true nucleus. For a detailed discussion of cells, see CELL (Inside a living cell).
Tissues, organs, and organ systems. Higher organisms have many extremely specialized kinds of cells, which are grouped together and form larger, more complex structures. A tissue consists of a collection of cells that are similar in structure and function. Animal tissues include muscle tissue and nerve tissue. Plants also have several kinds of tissues. For example, tissue called xylem carries water and minerals from the roots to the rest of the plant.
The basic structural and functional units of higher organisms are organs. An organ consists of several types of tissues. The human heart, for example, consists of muscle tissue, nerve tissue, and connective tissue. Other animal organs include the brain, liver, and kidneys. The principal organs of flowering plants are roots, stems, leaves, and flowers. In higher animals, each important life function is carried out by a group of organs working together. Such a group is called an organ system. Major organ systems include the circulatory system, digestive system, and reproductive system.
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