Share. By Regina Bailey. Definition: The cell membrane is a thin semi-permeable membrane that surrounds the cytoplasm of a cell, enclosing its contents. Its function is to protect the integrity of the interior of the cell by allowing certain substances into the cell, while keeping other substances out.t also serves as a base of attachment for the cytoskeleton in some organisms and the cell wall in others. Thus the cell membrane also serves to help support the cell and help maintain its shape. The cell membrane is primarily composed of a mix of proteins and lipids.
Continued from page 1. Periods and Groups. Elements in the periodic table are arranged in periods (rows) and groups (columns). Atomic number increases as you move across a row or period. Periods. Rows of elements are called periods. The period number of an element signifies the highest unexcited energy level for an electron in that element.
In general, within one row (period) the elements are metals on the left, and non-metals on the right. The rows of the table are called periods; the columns are called groups. Six groups (columns) have names as well as numbers: for example, group 17 elements are the halogens; and group 18, the noble gases. The periodic table can be used to derive relationships between the properties of the elements, and predict the properties of new elements yet to be discovered or synthesized.