Page 118 - The Miracle in the Cell Membrane

SIMPLE DIFFUSION
Molecules in a high-density
High-concentra- area collide at random more
tion area frequently.
Cell membrane

Low-concentra- Collisions direct molecules
tion area towards low-concentration
areas.
This is employed by small molecules
such as H 2 0, C 2 , CO 2 , and ethanol.
FACILITATED
DIFFUSION
Transport molecules in
Cell membrane
the pore-like protein chan-
nels in the cell membrane
Some soluble substances cannot permit some molecules
pass directly through the double through the membrane,
layer of fat. Their passage is depending on the concen-
made possible by means of chan- tration.
nels. The cell's ATP is not used
to facilitate this process.
Soluble
molecules 1. Soluble mole-
ACTIVE Transport protein cules attach to the
TRANS- Binding region binding point of the
PORT opened transport
protein in the cell
membrane.

2. ATP transmits
phosphate to the
transport protein.

3. The phosphorized
transport protein opens
and changes shape so as
to deposit the soluble
molecules inside the cell.

4. The phosphate sep-
arates from the protein
by assuming its origi-
nal form. The stage is
now set for another
soluble molecule to be
transported.

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