7-3 Movement through the Membrane
•Identify the main functions of the cell membrane.
•Describe what happens during diffusion
•Explain the processes of osmosis, facilitated diffusion, and active transport.
Cell Membrane
• Regulates what enters and leaves the cell
• Provides protection and support
• The core is a double-layered sheet called a lipid bilayer
Diffusion
• Movement of substances from an area of higher concentration to an area of lower concentration
• Equilibrium – all areas of concentration are equal
Diffusion (cont.)
• Diffusion is passive – it does NOT require cellular energy to take place
• Molecules move along the “concentration gradient”
High
Concentration
Low
Concentration
Osmosis
• Most biological membranes are “selectively permeable”
• Only allow certain substances through
How Osmosis Works
• In a sugar-water solution: – Cell membranes are permeable to
water but impermeable to sugar– To reach equilibrium, which all cells
“want” to do…water moves toward the side of the membrane with the higher concentration of solute (sugar) molecules
– See fig. 7-17, pg. 186
Osmotic Pressure
Osmotic Pressure (cont.)
Osmotic Pressure (cont.)
Facilitated Diffusion
• Moves molecules across the membrane that can’t pass by diffusion through special protein channels
• Ions, sugars, salts
Active Transport
• Requires energy to move particles against the concentration gradient
Takes
Energy!
Active Transport (cont.)
• Sodium-Potassium Pump and Coupled Channels
Endocytosis
• Moves LARGE particles into the cell
• Phagocytosis - amoeba
Exocytosis
• Moves large particles to the outside of the cell
• “exit” – “exocytosis”
Section Review
1. What processes are made possible by the cell membrane?
2. How does diffusion affect cells?3. Describe how water moves
during osmosis.4. What is the basic structure of a
cell membrane?5. How does active transport differ
from diffusion?