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