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Homeostasis: Movement Across the Cell Membrane
Students know cells are enclosed within semipermeable membranes that regulate their interaction with their surroundings.
A. Cell Membrane
Cell Membranes surround ALL cells! Controls what molecules enter and leave the
cell- food & oxygen molecules must enter the cell- waste products need to be released
SEMIPERMEABLE – it allows some things through but not others
Structure of Cell Membrane Phospholipid bilayer
Polar heads “love” water (hydrophilic) Nonpolar tails “fear” water (hydrophobic)
C. Fluid Mosaic Model Describes the various molecules found in
the cell membrane Example:
-Proteins: forms channels and pumps to move materials across membranes- Carbohydrates: act like identification cards so cells can identify one another
D. Transport Across a Membrane Molecules can enter/leave the cell in 2
ways:1. Passive Transport
* Simple Diffusion* Carrier-Facilitated (“helped”) Diffusion* Osmosis
2. Active transport* Protein Pump* Endocytosis* Exocytosis
Passive Transport
Energy is not required for movement across the membrane to occur
Molecules move from area of high concentration to area of low concentration
Example: Like riding a bike downhill
Two factors determine if passive transport takes place, equilibrium and permeability- Equilibrium: when the concentrations on both sides of the membrane are equal
membrane
- Permeability: ability of a molecule to diffuse (move) across a membrane
* Impermeable: molecules that cannot pass across a membrane* semi-permeable/selective permeability: Some molecules can pass across the membrane while other molecules
cannot
higher conc. lower conc.
Cell membrane
A. A. Diffusion Molecules moving from area of high
concentration to area of low concentration No energy is required Occurs because molecules constantly move and
collide with each otherClick link
B. Carrier-Facilitated Diffusion No energy required Molecules “helped” across by carrier
proteins from high to low concentration
Video (click here)
C. Osmosis Osmosis is a special type of diffusion
The diffusion of water molecules through a selectively permeable membrane
*Click pic
ISOTONIC Concentration of
solutes (dissolved substance) is the same in and out of the cell
Equal amount of water leaving and entering cell
HYPERTONIC Solution has a
higher solute concentration than the inside of cell
More water leaves cell, causing cell to shrink
HYPOTONIC Solution has a
lower solute concentration than the inside of cell
More water enters cell, causing cell to expand (get big)
*Click pic
Active TransPort Energy required for molecules to pass
across◦ ATP – the “battery” of the cell◦ Breaking a bond in ATP releases energy◦ Can “pump” molecules from low to high
concentration (against concentration gradient) Example: Like riding a bike uphill
ATP ATP
A. Protein Pump◦Protein Pumps: pulls molecule through
◦ATP Energy is required
*Click Pic
B. EndocytosisB. Endocytosis Endocytosis: cell membrane engulfs the larger molecule bring it into the cell
Two types:◦Phagocytosis: cytoplasm surrounds a molecule and packages it in a food vacuole
◦Pinocytosis: cell membrane forms pockets filled with liquid and pinch off to form vacuoles in a cell
C. Exocytosis Exocytosis: The membrane of the vacuole
surrounding the material fuses with the cell membrane, releasing the material outside the cell
*Click Pic
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