Transcript File

Homeostasis: Movement Across the
Cell Membrane
Students know cells are enclosed
within semipermeable
membranes that regulate their
interaction with their
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
-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
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
 Molecules move from area of high
concentration to area of low
 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
- 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
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 other
*Click link
Cell membrane
higher conc.
lower conc.
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
*Click pic
Concentration of
solutes (dissolved
substance) is the
same in and out
of the cell
Equal amount of
water leaving and
entering cell
Solution has a
higher solute
than the inside of
More water leaves
cell, causing cell
to shrink
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
A. Protein Pump
◦ Protein Pumps: pulls molecule
◦ ATP Energy is required
*Click Pic
B. Endocytosis
Endocytosis: cell membrane engulfs
the larger molecule bring it into the
 Two types:
◦ Phagocytosis: cytoplasm surrounds a
molecule and packages it in a food
◦ 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
*Click Pic