Cells Unit Objective B-Cell membrane and transport 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 N 2, 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 for some substances to the internal environments. Plant cell walls are made of cellulose and are external to the cell membrane. Other examples are cells walls of prokaryotes and fungi.

Label the hydrophilic and hydrophobic portions of the enlarged phospholipids.

Fluid Mosaic Model

Phospholipid structures

Membrane fluidity –Held together by hydrophobic interactions –Phospholipids can move rapidly laterally –Proteins can be stationary or mobile –Fluidity of membrane affects function Affects permeability Affects ability of membrane proteins to move and function properly

What is the role of carbohydrates in the plasma membrane? Proteins?

What is the role of lipids in the plasma membrane?

Label the parts of the membrane

Cholesterol = fluidity buffer Why does cholesterol reduce fluidity at moderate temperatures? Why does cholesterol hinder solidification at low temperatures? What enhances membrane fluidity in membranes with unsaturated fatty acids? What causes membranes with saturated fatty acids to be more viscous (more resistant to flow)?

Evolution Connection: How do different membrane compositions provide evolutionary adaptations? What is the evolutionary advantage of fish that live in extreme cold temperatures having membranes with a high proportion of unsaturated fatty acids? What is the evolutionary advantage of plants that change their cell membrane composition in response to changes in temperature? For example, some plants have a seasonal change in their membranes where the percentage of unsaturated phospholipids increases in the fall and winter compared to the spring and summer.

Mosaic of different proteins within plasma membrane Integral proteinsPeripheral Proteins

Functions of membrane proteins

Glycolipids and glycoproteins are like identification tags for the cell. 4 human blood types have different glycoproteins on the surface of the red blood cells.

What is the role of each of the following molecules in the plasma membrane? Phospholipids Embedded proteins Cholesterol Glycoproteins glycolipids