Active and Passive Transport


Membrane properties:


Polar heads: hydrophilic (water loving)

Nonpolar fatty acid tails: hydrophobic (water fearing); will not allow polar molecules, ions, or large molecules to pass across the membrane.



Passive or Active Transport:



Passive Transport does not require cell energy

Active Transport Requires cell energy (ATP)

Examples: Diffusion, Facilitated diffusion and Osmosis

Examples: Carrier mediated active transport, Endocytosis and Exocytosis


Methods of Transport:

1. Diffusion: the random movement of particles of a solute from an area of higher concentration to an area of lower concentration.


·    Particles always move with (down) a concentration gradient (the difference in concentrations across a membrane).

·    Passive transport.

·    Diffusion stops at equilibrium (when the concentrations across a membrane are equal).

·    The movement of molecules continues at equilibrium but the # of molecules moving across the membrane remains the same.

·    The rate of transport is dependent on: 1) if the material is solid, liquid or gas.

         2) the size of the molecules.

         3) temperature


Examples of molecules that can diffuse through the bilayer: carbon dioxide and oxygen

2. Osmosis: the diffusion of water through a selectively permeable membrane.

·    Passive transport

·    Water molecules move from a higher concentration OF WATER to a lower concentration OF WATER.

·    Water will move to where there is a greater amount of solute because there is less water there.

Isotonic solutions: the concentration of solute inside and outside of the cell is the same.

·    Osmosis does not occur because equilibrium has already been reached.



Water in = Water out

No net movement of water.

Molecules in equilibrium.

Normal state for animal cells.

Cell in homeostasis.

Hypotonic solutions: the concentration of solute is lower outside the cell than inside the cell.

·    Have more water outside the cell so water moves into the cell

·    Causes an increase in pressure inside the cell: called turgor pressure (plants) or osmotic pressure (animals).

·    Increase in pressure in animal cells causes them to swell or even burst; gives plant cells shape and support.

Osmotic Conditions:  H = High        L = Low





Water enters cell.

Cell swells and bursts (cytolysis).

Give plant cells shape and support.

Cell hypertonic to environment.

Environment hypotonic to cell.

Hypertonic solutions: the concentration of solute is higher outside the cell than inside the cell.

·    Have more water inside the cell so water moves out of the cell

·    Causes a drop in turgor or osmotic pressure: called plasmolysis.

·    Plasmolysis causes animal cells to shrivel up and plants to wilt.


Osmotic Conditions:  H = High        L = Low





Water exits cell.

Cell shrinks (plasmolysis) due to    water loss.

Cell hypotonic to environment.

Environment hypertonic to cell.




The effects of osmotic pressure:





Plant cells have a cell wall which allows them to maintain turgor pressure: the pressure that exists inside the cell when the cell swells.




3. Facilitated Diffusion:

·    Particles always move with (down) a concentration gradient.

·    Uses transport/channel proteins.


·    Passive transport.

·    Usually for specific molecules such as glucose.

·    Facilitated diffusion stops at equilibrium.

New research has added water to the list of molecules that enter cells by facilitated diffusion.

Many cells contain huge numbers of water channel proteins, known as aquaporins, that allow water to pass right through them.




4. Active Transport: requires energy in the form of ATP.

·    Capable of moving solute particles against the conc. gradient (from low conc. to high conc.)

·    Uses transport/carrier proteins (protein pumps) embedded in the plasma membrane.


·    Carrier proteins are specific for the molecules that they allow through. The carrier protein changes shape which requires energy (ATP).


5.           Endocytosis and exocytosis:

·    also examples of active transport.


Endocytosis: a process of taking material into the cell by means of infoldings, or pockets, of the cell membrane (usually putting them into a vacuole).


Types of Endocystosis:




-“Cell Drinking

-Nonspecific molecules

-Intake of small droplets of liquid

-“Cell eating”

-Nonspecific molecules

-Intake of solids

Specific Molecules

1.   Molecule binds to receptor protein

2.   Complex migrates to “coated” pit

3.   Pit pinches off forming a vacuole


Exocytosis: a process in which the membrane of the vacuole surrounding the material fuses with the cell membrane, forcing the contents out of the cell.