Some Definitions
TERM [solute] greater: net flow of water cell shape change
hypotonic inside cell into cell cell expands
isotonic equal [solute] in and outside cell no net flow no shape change
hypertonic outside cell out of cell cell shrinks

A brief demonstration - What happens to bags of molasses when you put them into a hypertonic, hypotonic, or isotonic solution?

Passive Transport - three types

Diffusion - the movement of a material from an area of high concentration to an area of low concentration. Diffusion is dependent upon

Diffusion is NOT the same as convection or other processes involving mechanical mixing, it is usually much slower. And especially note that for any given molecule, motion is random. That means that some molecules actually move the "wrong" way (i.e. up the concentration gradient), even though the movement of the bulk of the molecules is down the concentration gradient.

An understanding of diffusion is important for understanding the movement of gases, water, and ions in biological systems. It is a phenomenon that occurs on the level of molecules, but these molecular movements are important for living systems at the subcellular, cellular and organismic levels.

Osmosis - a special case of diffusion, describing the diffusion of water across a semi-permeable membrane. Since water is necessary for life, we have created a special definition to describe the movement of water via diffusion. But all of the things that affect diffusion of other molecules (e.g. temperature, concentration etc) also affect osmosis.

 The selectively permeable membrane allows some molecules to exchange, and prohibits others. Note again that for any given molecule capable of crossing the barrier,  motion is random. Some molecules will end up on one side of the barrier, and other molecules will end up on the other side.


Passive Transport (also known as "facilitated diffusion", which is- passage of charged or bulky molecules across a cell membrane, mediated by reversible association of those molecules with a protein (carrier or transport protein) embedded in the membrane. This type of transport is specific (i.e. a glucose transporter will move glucose, but it will not move mannose), reversible (if the concentration gradient is reversed, the direction of transport will be reversed), and does not require active expenditure of energy (no ATP is used to move the molecules across the membrane.
Active Transport
Active Transport - passage of charged or bulky molecules across a cell membrane, mediated by reversible association of these molecules with a protein (carrier or transport protein) which is embedded in the membrane AND which hydrolyzes ATP (or some other high energy compound) during the transport process. This type of transport is specific, usually not reversible (molecules accumulate on one side of the membrane or the other), and requires active expenditure of energy in the form of high energy phosphate-containing compounds. The classic example of this type of transport is the sodium-potassium pump, which is found in every cell in your body and which maintains the sodium and potassium gradients which are essential for general cellular metabolism and for specialized processes such as nerve and muscle signal conduction. Here is a figure (5.8) from your textbook; an animated version of this figure is available on the CD-ROM that came with your textbook.


 Summary Table for Cellular Transport


Type of cellular transport
Molecule transported
Requires specific membrane protein?
Requires energy expenditure?
Can move molecules against a gradient?
Example
diffusion
gases, small molecules, lipophilic molecules
no
no
no
oxygen
osmosis
water
YES
no
no
water
passive transport
sugars, amino acids, ions, etc
yes
no
no
glucose
active transport
ions, sugars
yes
yes
yes
sodium, potassium