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Diffusion/Osmosis
I.
II.
Cellular Transport – movement of molecules within the cell and in and out of the cell
A. 2 Types
1. Passive Transport – does not require energy
2. Active Transport – requires energy (ATP)
B. Cell membrane
1. Semipermeable – controls what enters and exits the cell
2. Phospholipid – composition
a. Molecules must have the proper chemical composition to cross through lipid
bilayer (must dissolve in lipids)
b. Molecules must be the correct size (too large they can’t fit through pores in
membrane)
Passive Transport – movement of molecules through cell membrane that requires no
energy. There are several types: diffusion, osmosis and facilitated diffusion.
A. Diffusion – movement of molecules from an area of high concentration to an area of low
concentration.
1. Simplest passive transport
2. Requires no energy
3. Driven by kinetic energy of molecules (molecules vibrate and collide, eventually
spreading evenly throughout an area)
4. Eventually – state of equilibrium (molecules are evenly spread)
B. Osmosis – diffusion of water (movement of water across the cell membrane from an
area of high concentration to an area of low concentration)
1. 3 types of osmosis
a. Hypotonic – when a cell has more solute (solid particles) than the region outside
the cell (water moves into the cell until equilibrium reached)
b. Hypertonic – cell has less solute than outside region (water moves out of the
cell) example: soaking a swollen finger in Epson salts
c. Isotonic – concentration of solutes equal on both sides of the cell
2. How cells deal with osmosis
a. Hypotonic environment – occurs in fresh water with single-celled organisms.
They often have contractile vacuoles to push out the excess water
b. Hypotonic environment for plants – The cell wall prevents the cell from
exploding
c. Turgor pressure – pressure of the cell membrane against the cell wall
d. Plasmolysis – lost turgor pressure (plant wilts)
e. Hypotonic environment for human cells – Many human cells cannot cope and
will swell and burst
III.
C. Facilitated Diffusion – needed when molecules too large to pass through pores or not
soluble in liquids
1. Still passive transport
a. No energy needed
b. Requires carrier protein
c. Carrier proteins – proteins in membrane that assist in diffusion
2. Method – protein binds to molecule and changes shape to transport molecule
3. Specific – each protein is specific for one type of molecule
Active Transport – movement of molecules from area of low concentration to high
***REQUIRES ENERGY (several types include: pumps, Na-K pump, endocytosis, and
exocytosis)
A. Cell membrane pumps
1. Carrier proteins that move molecules from low to high concentration (against the
gradient)
B. Na-K pump (Na – sodium, K- potassium)
1. Special pump for Na and K
2. Needed for animal cells
3. Especially important for muscle and nerve cells
4. Movement of Na and K ions creates a charge in cells
5. A charge is necessary for muscle and nerve function
C. Endocytosis and Exocytosis – requires energy. Process where cells ingest external fluid,
macromolecules and large proteins
1. Cell encloses material, folds into itself and forms a pouch with its cell membrane.
2. Pouch pinches off from cell membrane and becomes a membrane-bound organelle
called a vesicle.
3. Endocytosis – Large particles taken IN by cell.
4. Exocytosis – Large particles EXIT cell.
5. Pinocytosis – Large particles of fluids wrapped in a membrane.
6. Phagocytosis – Large particles of solids.