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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
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
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. 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