Apical

Apical may refer to:

See also

Trigonal bipyramidal molecular geometry

In chemistry a trigonal bipyramid formation is a molecular geometry with one atom at the center and 5 more atoms at the corners of a triangular dipyramid. This is one geometry for which the bond angles surrounding the central atom atom are not identical (see also pentagonal dipyramid), because there is no geometrical arrangement with five terminal atoms in equivalent positions. Examples of this molecular geometry are phosphorus pentafluoride (PF₅), and phosphorus pentachloride (PCl₅) in the gas phase.

Axial and equatorial positions

The five atoms bonded to the central atom are not all equivalent, and two different types of position are defined. For phosphorus pentachloride as an example, the phosphorus atom shares a plane with three chlorine atoms at 120 degree angles to each other in equatorial positions, and two more chlorine atoms above and below the plane (axial or apical positions).

According to the VSEPR theory of molecular geometry, an axial position is more crowded because an axial atom has three neighboring equatorial atoms (on the same central atom) at a 90° bond angle, whereas an equatorial atom has only two neighboring axial atoms at a 90° bond angle. For molecules with five identical ligands, the axial bond lengths tend to be longer because the ligand atom cannot approach the central atom as closely. As examples, in PF₅ the axial P−F bond length is 158 pm and the equatorial is 152 pm, and in PCl₅ the axial and equatorial are 214 and 202 pm respectively.

Cell membrane

The cell membrane (also known as the plasma membrane or cytoplasmic membrane) is a biological membrane that separates the interior of all cells from the outside environment. The cell membrane is selectively permeable to ions and organic molecules and controls the movement of substances in and out of cells. The basic function of the cell membrane is to protect the cell from its surroundings. It consists of the phospholipid bilayer with embedded proteins. Cell membranes are involved in a variety of cellular processes such as cell adhesion, ion conductivity and cell signalling and serve as the attachment surface for several extracellular structures, including the cell wall, glycocalyx, and intracellular cytoskeleton. Cell membranes can be artificially reassembled.

Function

The cell membrane (or plasma membrane or plasmalemma) surrounds the cytoplasm of living cells, physically separating the intracellular components from the extracellular environment. Fungi, bacteria and plants also have a cell wall in addition, which provides a mechanical support to the cell and precludes the passage of larger molecules. The cell membrane also plays a role in anchoring the cytoskeleton to provide shape to the cell, and in attaching to the extracellular matrix and other cells to help group cells together to form tissues.