Biological Membranes Sections 5.1-5.3

Learning Objectives Explain the significance of the selectively permeable membrane to the transport of molecules Describe the structure of the cell membrane using the fluid-mosaic model and relate it to the semi-permeable membrane Relate the properties of the lipid bilayer to the properties & functions of cell membranes Describe ways that membrane proteins associate with the lipid bilayer & their functions & give examples

Plasma Membrane Plasma membrane = cell membrane Forms boundary between inside & outside cell Controls movement of molecules into & out of the cell Takes up substances the cell needs Disposes of the cell’s wastes Is selectively permeable = allows some substances to cross more easily than others

Why Selectively Permeable? By controlling what crosses the plasma membrane, a cell controls its volume and its internal composition Inside the cell can be very different from the surrounding environment

Phospholipids Bilayers Phospholipids are the main structural component of the cell membrane Made up of a glycerol, phosphate group, and two fatty acids Amphipathic = one side is hydrophobic and one side is hydrophilic Head is polar & hydrophilic (water loving) Tails are nonpolar & hydrophobic (water fearing)

Phospholipids form a two-layer sheet called a phospholipid bilayer Hydrophobic tails point inward (are protected from the watery environment) Hydrophilic heads point outward, toward the environment & cytoplasm

Hydrophobic middle of the cell membrane is why it’s selectively permeable Nonpolar, hydrophobic molecules Soluble in lipids Can easily pass through the membrane Polar, hydrophilic molecules & ions Not soluble in lipids Need a transport protein to pass through the membrane Ex. Aquaporins = channel transport proteins that allow water to cross the cell membrane

Fluid Mosaic Model Fluid mosaic model = states that the cell membrane is made up of many parts that are in constant motion Mosaic = picture made up of many smaller tiles The plasma membrane has many different proteins embedded in the phospholipid bilayer Fluid = not solid, can flow & move Most proteins & phospholipids can move laterally, & phospholipids can also flip to the opposite layer

Membrane Fluidity The plasma membrane needs to stay fluid & flexible to function properly Unsaturated fatty acid tails of phospholipids Kinks stop phospholipids from packing too tightly together Cholesterol is embedded in the membrane of animal cells High temps: Holds phospholipids together Low temps: Holds phospholipids apart to keep the membrane fluid

Membrane Composition Mostly made of phospholipids Different types of proteins as well Integral membrane proteins = proteins that are embedded (stuck in) the membrane Transmembrane proteins = integral proteins that extend completely through the membrane (stick out both sides)

Peripheral membrane proteins = proteins attached to the surface of the plasma membrane Not stuck in the bilayer Can be easily removed without changing the structure of the bilayer

Membrane Proteins Proteins perform most of the functions of a membrane Activities of the membrane are varied Each type of cell has a different set of proteins Membranes within a cell also have different sets of proteins

Membrane also contains molecules used for cell identification Glycoprotein = protein with sugars attached Glycolipid = lipid with sugars attached