The Basic Unit of Life

Cells are the building blocks of all living things. In single-celled (unicellular) organisms, each cell is a separate, individual organism. In multicellular organisms, cells can be specialized, and some independence may be lost. Human cells have diverse shapes, including the double-concave discs of blood cells and neurons with long, extended axons. Cells may have specialized structures, such as the photosensitive regions of the rod cells of the retina, or specialized secretions, such as the extracellular matrix formed by bone-making cells (osteoblasts). Multicellular organisms, with their higher levels of complexity, also have additional levels of organization, which vary greatly in size. For example, kidney cells represent the smallest and simplest units of the urinary system. Many kidney cells together form kidney tissue, and multiple layers of kidney tissue form an organ, the kidney. The kidneys, bladder, ureters, and urethra function together as the urinary system. The urinary system functions to excrete waste and control water absorption in the body of the organism. These levels of organization in multicellular organisms are called the biological hierarchy. Multicellular organisms consist of many different types of cells. A tissue consists of cells of the same or similar types working together to perform the same function or set of functions. Similarly, an organ consists of tissues working together to perform the same or similar set of functions. Likewise, a set of organs working together to accomplish the same or similar set of functions comprises an organ system.

The fundamental cellular organization is defined by the cell membrane, or plasma membrane, the phospholipid (fatty) bilayer that separates the interior of the cell from its surroundings. Phospholipids have a hydrophilic (water-loving) phosphate "head" and a hydrophobic (water-fearing) fatty acid "tail." Because the molecules of the cell membrane are primarily fat-based, interactions between the water-based cell interior and the water-based extracellular environment can be controlled by the cell. The cell membrane cannot be easily penetrated by certain types of molecules, including those that are large or carry an electrical charge. Specialized proteins embedded within the cell membrane assist with the movement of molecules into and out of the cell. In addition, membrane proteins allow each cell to maintain homeostasis (a stable internal environment) and respond to its environment. The cell membrane encloses the cytoplasm, a combination of cytosol—a jellylike fluid primarily made up of water and dissolved substances that fills the spaces around the internal cell structures—and organelles (excluding the nucleus). An organelle is a membrane-bound structure in a cell that has a specific task. Within the cytoplasm, all cells have structures for obtaining energy, growing, and reproducing.

The genetic information of all cells is carried in the molecule deoxyribonucleic acid (DNA). In all organisms this information is used as a blueprint for making proteins. DNA code is used as a template to build ribonucleic acid (RNA), and cellular components called ribosomes use the RNA code to make proteins. A ribosome is a structure composed of RNA and protein that constructs proteins based on the instructions provided by DNA. Proteins are diverse molecules that allow the cell to perform the functions of life. Some proteins are involved in cell-cell communication, some become cellular structural components, and others speed up chemical reactions taking place inside cells. Proteins are also involved in maintaining cell structure, defense, and transport. For example, the hemoglobin protein within red blood cells binds to oxygen for delivery throughout the body.

Several cellular processes are similar among diverse organisms. For example, the primary energy-storage molecule of all cells is adenosine triphosphate (ATP), and it is used for cellular reproduction. First, the cell replicates its DNA, and then it goes through a series of steps to divide. However, the process for bacteria is different from the one common to plant and animal cells, because bacterial DNA is organized differently. In particular, plant and animal cells have a membrane-bound nucleus that contains their DNA. Bacterial cells lack any membrane-bound organelles, even a nucleus, while plant and animal cells have several specialized organelles.