Cell Membrane Diffusion & Osmosis Active Transport, Endocytosis, & Exocytosis Sections 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 functions 1.Forms a boundary between inside and outside of the cell 2.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