Cell MembraneDiffusion & Osmosis
Active Transport, Endocytosis, & ExocytosisSections 3.3, 3.4, & 3.5
3.3 Cell Membrane
• Key Concept:– The cell membrane is a barrier that separates a cell
from the external environment.
Cell membranes are composed of two phospholipid layers.
• The cell membrane has two major functions1. Forms a boundary between inside and outside of
the cell2. Controls passage of materials
Cell membranes are composed of two phospholipid layers.
• The cell membrane is made of a phospholipid bilayer.
• There are other molecules embedded in the membrane.
• The fluid mosaic model describes the membrane.
Phospholipid Bilayer
• Forms a double layer surrounding a cell
• Composed of a charged phosphate group, glycerol, and two fatty acid chains
• Head is polar (attracted to water) and forms hydrogen bonds with water
• Tails are nonpolar (repelled by water)
Other Molecules in the Membrane
• Cholesterol: strengthens membranes• Proteins: aid cell identification, movement of
molecules across membranes, & cell signaling• Carbohydrates: aid cell identification
Fluid Mosaic Model of the Cell Membrane
• Describes the arrangement of molecules making up a cell membrane
• The membrane is flexible like a fluid• The membrane has a variety of molecules like
the variety of tiles in a mosaic
Selective Permeability• Allows some materials to cross the membrane
but not all• Enables cell to maintain homeostasis– Homeostasis: ability to maintain internal stable
conditions• Molecules can cross in a variety of ways
• Other terms: semipermeable & selectively permeable
Receptors• Detect a signal molecule and perform an action
in response• Two Types:– Intracellular: located inside a cell; bind
to molecules that can cross the membrane
– Membrane: located in the membrane; binds to molecules that cannot cross the membrane; change in shape transmits the message to the cell interior
3.4 Diffusion & Osmosis
• Key Concept:– Materials move across membranes because of
concentration differences.
Passive transport does not require energy (ATP) input from a cell.
• Molecules can move across the cell membrane through passive transport.
• Two types of passive transport:– Diffusion: movement of molecules from high to
low concentration– Osmosis: diffusion of water
Diffusion and osmosis are types of passive transport.
• Molecules diffuse down a concentration gradient.– High to low concentration
How do different solutions affect cells?
• There are 3 types of solutions:1. Isotonic: solution has
the same concentration of solutes as the cell.
• Water moves in and out evenly
• Cell size stays constant
How do different solutions affect cells?
• There are 3 types of solutions:2. Hypertonic: solution
has more solutes than a cell
• More water exits the cell than enters
• Cell shrivels or dies
How do different solutions affect cells?
• There are 3 types of solutions:3. Hypotonic: solution
has fewer solutes than a cell
• More water enters the cell than exits
• Cell expands or bursts