Lewis/Life - Review of Key Concepts

Review of Key Concepts - Chapter 4

Cells, the basic units of life, are the microscopic components of all living organisms. Cells exhibit the characteristics of life.
Even the simplest cells are highly organized; complex cells may carry out very specialized functions.
Organisms may be unicellular (one cell) or multicellular (many cells). Cells in multicellular organisms specialize by expressing subsets of genes.
Cells were observed first in the late seventeenth century, when Robert Hooke viewed cork with a crude lens. Antonie van Leeuwenhoek viewed many cells under the light microscope he invented. Stains revealed subcellular details. Modern tools include the light microscope, electron microscope, and the scanning probe microscope.
The cell theory states that all life is composed of cells, that cells are the functional units of life, and that all cells come from preexisting cells. Many illnesses result from abnormalities at the subcellular level.
Prokaryotic organisms are unicellular and include bacteria and cyanobacteria. A prokaryotic cell lacks a nucleus and has DNA, ribosomes (structures that help manufacture proteins), enzymes for obtaining energy, and various other biochemicals. A cell membrane and usually a rigid cell wall enclose the cell contents. Prokaryotes are abundant and ancient.
The more complex eukaryotic cells sequester certain biochemical activities in organelles. A eukaryotic cell houses DNA in a membrane-bounded nucleus; synthesizes, stores, transports, and releases molecules along a network of organelles (endoplasmic reticulum, Golgi bodies, vesicles); degrades wastes in lysosomes; processes toxins and oxygen in peroxisomes; extracts energy from nutrients in mitochondria; and in plants, extracts solar energy in chloroplasts. A cell membrane surrounds eukaryotic cells. Cell walls protect and support cells of many organisms, except animals.
Archaea are a third form of life that share some characteristics with prokaryotes and eukaryotes but also have unique structures and biochemistry.
Simpler than cells are viruses, viroids, and prions. Viruses consist of nucleic acid cores, protein capsids, and in some cases, envelopes. Viruses require a host cell to reproduce and have varied effects on specific species. Viroids are infectious RNA, and prions are infectious glycoproteins.
The endosymbiont theory proposes that chloroplasts and mitochondria evolved from once free-living prokaryotes engulfed by larger prokaryotes---chloroplasts descend from cyanobacteria, and mitochondria descend from aerobic bacteria. Evidence for the endosymbiont theory is that mitochondria and chloroplasts resemble small aerobic bacteria in size, shape, and membrane structure and in the ways their DNA, RNA, and ribosomes interact to manufacture proteins.