Chapter 3 Cells 3.3 Cell Membrane

KEY CONCEPT The cell membrane is a barrier that separates a cell from the external environment.

The cell membrane has two major functions Aka Plasma Membrane Forms a boundary between inside and outside of the cell Controls import and export of materials cell membrane outside cell inside cell

Cell membranes are made of Phospholipids Molecule with a phosphate head region and two fatty acid tails Head is polar = charge (Hydrophilic) Tails are non-polar = no charge (Hydrophobic)

Phospholipid Bilayer Properties of the polar heads and nonpolar tails cause the phospholipids to arrange themselves in layers Like a sandwich Heads like bread Interact with watery environment inside and outside the cell Tails Like filling protected from water

Other Membrane Molecules Cholesterol Strengthens the cell membrane Protein Channels Allow specific molecules into the cell Cytoskeletal Protein Help cell maintain its shape Carbohydrates (Glycoproteins) Carb chain serves as an ID badge for the cell

Fluid Mosaic Model Describes the arrangement of molecules that make up a cell membrane

Fluid Mosaic Model Takes it’s name from 2 characteristics Fluid = Membrane is flexible not ridged Allows phospholipids and proteins to move side to side and slide past each other Proteins cannot flip end to end though Mosaic = because of the variety of scattered molecules

Selectively Permeable Some molecules can cross over, while others cannot Helps to maintain homeostasis – equilibrium – by monitoring what enters and exits cells. For Example: Small molecules and larger hydrophobic molecules move through easily. Examples: O2, CO2, H2O

Selectively Permeable Small non-polar molecule Pass right through the membrane Small polar molecules Transported by channel proteins Large molecules (polar and non-polar) Moved in vesicles by Endocytosis/Exocytosis

Chemical signals are transmitted across the cell membrane Receptors bind with ligands and change shape. Two types of receptors

Intracellular Receptor Found totally within cell Ligand moves across the membrane, binds with receptor inside cell, and receptor-ligand complex cause something to happen Ex. Many Hormones work this way

Membrane receptor Ligand cannot enter cell Ligand bind to external receptor Receptor changes shape Shape change causes molecules in cell to respond