Essential Knowledge 2.B.1

Cell membranes are selectively permeable due to their structure.

Cell membranes separate the internal environment of the cell from the external environment.

Selective permeability is a direct consequence of membrane structure, as described by the fluid mosaic model. Cell membranes consist of a structural framework of phospholipid molecules, embedded proteins, cholesterol, glycoproteins and glycolipids. Phospholipids give the membrane both hydrophilic and hydrophobic properties. The hydrophilic phosphate portions of the phospholipids are oriented toward the aqueous external or internal environments, while the hydrophobic fatty acid portions face each other within the interior of the membrane itself. Embedded proteins can be hydrophilic, with charged and polar side groups, or hydrophobic, with nonpolar side groups. Small, uncharged polar molecules and small nonpolar molecules, such as N2, freely pass across the membrane. Hydrophilic substances such as large polar molecules and ions move across the membrane through embedded channel and transport proteins. Water moves across membranes and through channel proteins called aquaporins.

Cell walls provide a structural boundary, as well as a permeability barrier from some substances to the internal environments. Plant cell walls are made of cellulose and are external to the cell membrane. Other examples are cell walls of prokaryotes and fungi.

Students should be able to:

LO 2.10 Use representations and models to pose scientific questions about the properties of cell membranes and selectively permeability base on molecular structure.

LO 2.11 Construct models that connect the movement of molecules across membranes with membrane structure and function.