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Structure and Function of Organelles
cell (plasma) membrane
semi-permeable membrane which regulates
the passage of substances into and out of the
cell
 composition: protein molecules dispersed
throughout a double layer of lipid (fluid
mosaic)

Animal Cell
Eukaryotic Cell
Eukaryotic Cell
Plant Cell
Factors affecting permeability of cell
membrane to a particular substance:
 1. molecular size of the substance:
cell membrane generally is permeable to
small molecules such as water (H2O),
carbon dioxide (CO2), glucose (C6H12O6)
and amino acids
Factors affecting permeability of cell
membrane to a particular substance:
1. molecular size of the substance
 2. electrical charge on the atom or molecule
of the substance

+
+
+
- -
+
+
Na+
-
-
+ +
+
+
- +
-
-
+
+
Cl-
Factors affecting permeability of cell
membrane to a particular substance:
1. molecular size of the substance
 2. electrical charge on the atom or molecule
of the substance
 3. solubility of the substance in water

Factors affecting permeability of cell
membrane to a particular substance:
1. molecular size of the substance
 2. electrical charge on the atom or molecule
of the substance
 3. solubility of the substance in water
 4. solubility of the substance in lipid

What determines the direction of the
movement of molecules (diffusion) across
the cell membrane?

concentration of substances on each side of
the membrane
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Diffusion is a form of passive transport.
(no energy required)
Osmosis

diffusion of water through a semipermeable membrane from an area where
the water molecules are more concentrated
to an area where the water molecules are
less concentrated
Cells
exposed to an isotonic solution will neither
lose nor gain water
 exposed to a hypotonic solution will swell
due to the uptake of water by the cell
 exposed to a hypertonic solution will shrink
due to the loss of water from the cell

Active transport requires energy.

A transport protein actively pumps a
specific solute across a membrane against
the solute’s concentration gradient; i.e.,
away from where the solute is less
concentrated. Membrane proteins use ATP
as their energy source for active transport.
energy
energy