Animal Cell

• Cell (biology), basic unit of life. Cells are the smallest structures capable of basic life processes, such as taking in nutrients, expelling waste, and reproducing. All living things are composed of cells. Some microscopic organisms, such as bacteria and protozoa, are unicellular, meaning they consist of a single cell.

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Prokaryotic cells are among the tiniest of all cells, ranging in size from 0.0001 to 0.003 mm (0.000004 to 0.0001 in) in diameter. About a hundred typical prokaryotic cells lined up in a row would match the thickness of a book page. These cells, which can be rodlike, spherical, or spiral in shape, are surrounded by a protective cell wall

• Eukaryotic cells are typically about ten times larger than prokaryotic cells. In animal cells, the plasma membrane, rather than a cell wall, forms the cell’s outer boundary.

• CYTOCHROME darkly colored protein that plays a vital role in transporting chemical energy in all living cells. Animal cells derive energy from food through a process called cellular respiration and plants trap energy from sunlight in another process called photosynthesis. CYTOCHROMES are intimately involved in both of these processes.

Endoplasmic Reticulum (ER), an extensive network of tubes that manufacture, process, and transport materials within nucleated cells. The ER consists of a continuous membrane in the form of branching tubules and flattened sacs that extend throughout the cytoplasm (the cell’s contents outside of the nucleus) and connect to the double membrane that surrounds the nucleus. There are two types of ER: rough and smooth.

• Nucleolus, structure within the nucleus of cells, involved in the manufacture of RIBOSOMES (cell structures where protein synthesis occurs). Each cell nucleus typically contains one or more nucleoli, which appear as irregularly shaped fibers and granules embedded in the nucleus. There is no membrane separating the nucleolus from the rest of the nucleus

• Nucleus (biology), membranebound structure of a cell that plays two crucial roles. The nucleus carries the cell’s genetic information that determines if the organism will develop, for instance, into a tree or a human; and it directs most cell activities including growth, metabolism, and reproduction by regulating protein synthesis (the manufacture of long chains of amino acids).

• Plasmid, small, usually ringshaped molecule of deoxyribonucleic acid (DNA), which is the hereditary material in all living cells. Plasmids are present in almost all bacteria and may also be found in some yeasts and other fungi, protozoa, and even some plants and animals. They are separate from chromosomes, the primary structures that contain DNA in cells.

• Protein, any of a large number of organic compounds that make up living organisms and are essential to their functioning. First discovered in 1838, proteins are now recognized as the predominant ingredients of cells, making up more than 50 percent of the dry weight of animals. The word protein is coined from the Greek proteins, or “primary.”

Ribosome, cell structure that uses genetic instructions transported in ribonucleic acid (RNA) to link a specific sequence of amino acids into chains to form proteins. RIBOSOMES , which measure about 0.00025 mm (0.00001 in), are dispersed in the cytoplasm (the cell contents outside the nucleus) of all prokaryotic cells— ARCHAEBACTERIA and bacteria.

• The first observations of cells were made in 1665 by English scientist Robert Hook, who used a crude microscope of his own invention to examine a variety of objects, including a thin piece of cork

• Brain, portion of the central nervous system contained within the skull. The brain is the control center for movement, sleep, hunger, thirst, and virtually every other vital activity necessary to survival. All human emotions— including love, hate, fear, anger, elation, and sadness —are controlled by the brain.