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.
A horizontal row is called a period. The atomic number of the elements in a period increases from left to right along the row. The modern periodic table. Before the development of the modern periodic table, there were other attempts to arrange the elements in a useful way.For example, Doebereiner dÃ¶bereiner arranged groups of three elements with similar properties âinto'. Triads newlands And mendeleev arranged the elements in order of increasing relative atomic. massll substances are made from one or more of these elements. The modern periodic table is a way of showing how these elements can be ordered. Before the development of the modern periodic table, there were other attempts to arrange the elements in a useful way, by scientists such as Doebereiner, DÃ¶bereiner newlands And. mendeleev