Download The Cell Membrane

Movement Across the Cell
Membrane
Cell Biology standard 1a
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
* 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
Cell membrane
higher conc.
lower conc.
B. Carrier-Facilitated Diffusion


No energy required
Molecules “helped” across by carrier proteins
from high to low concentration
C. Osmosis


Osmosis is a special type of diffusion
The diffusion of water molecules
through a selectively permeable
membrane


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)
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: cell membrane
changes shape and pulls molecule
through
◦ ATP Energy is required
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