Cell Transport Passive Transport:

Simple Diffusion, Osmosis, Facilitated Diffusion

Cell Transport

Homeostasis �  Necessary for an organism to maintain a constant/

stable condition �  To maintain homeostasis: organisms must respond to

stimuli �  Part of this responding requires materials to move

across the cell membrane

�  A cell membrane is semipermeable. �  Some substances can pass directly through while

others cannot

Moving Across the Membrane �  There are two ways that materials move into and

out of the cell membrane:

1.  Passive Transport: �  Does NOT use energy �  Examples: Simple diffusion, osmosis, facilitated

diffusion

2.  Active Transport: �  Requires energy in the form of ATP �  Examples: Endocytosis, exocytosis, protein pumps

Simple Diffusion �  Molecules are in CONSTANT, Random motion

�  Molecules will naturally move from areas of high concentration to low concentration �  This does NOT require energy

�  Eventually, equilibrium is reached �  The concentration is approximately equal

�  The molecules never stop moving

�  The difference between two areas of concentration is called a concentration gradient

Simple Diffusion �  Diffusion across a semipermeable membrane

Semipermeable membrane

Simple Diffusion HIGH concentration

LOW concentration

EQUILIBRIUM

EQUILIBRIUM

Example of simple diffusion: If you spray air freshener in one corner of a room, it will eventually diffuse to all parts of the room, because of the random collision of air molecules. (click on picture below for demo) Question: Will the scent spread quicker in warm air or cold air? WARM air, because the air and perfume molecules are moving faster, meaning they collide more often.

Ø  In simple diffusion, molecules move “WITH” or “DOWN” the concentration gradient, meaning they travel from high to low; (no energy used)

Ø  the ability of a molecule to diffuse through the cell

membrane depends on the SIZE and POLARITY of the molecule

Ø  Small molecules can diffuse easily across the cell membrane, such as CO2 and O2

Ø  Large molecules such as SUGARS and ENZYMES cannot easily diffuse across the membrane; they need “help” or “facilitation” to get through

Simple Diffusion, continued

What is Osmosis? �  Osmosis is a type of PASSIVE

transport; no energy is required

�  Osmosis is the movement of WATER across a membrane from an area of greater concentration of water to an area of lesser concentration of water

The Direction of Osmosis � Water will move in response to the

SOLUTE concentration inside the cell and outside the cell.

� Examples of solutes: salt, glucose, urea

� Water molecules move to where the HIGHEST solute concentration is.

Types of Solutions �  HYPERTONIC: there is a HIGHER solute concentration outside the cell

�  What is the direction of osmosis? �  Water will move OUT of the cell

�  HYPOTONIC: there is a LOWER solute concentration outside the cell. �  What is the direction of osmosis?

�  Water will move INTO the cell

�  ISOTONIC: the solute concentration outside the cell EQUALS the solute concentration inside the cell �  Water is still moving across the membrane in this case just in

equal movements

Osmosis in Animal Cells �  Animal cells contain cytoplasm, surrounded by a

SEMIPERMABLE cell membrane.

nucleus

Osmosis Scenario #1 �  What would happen to a human skin cell if it were

placed into concentrated salt water?

•  What type of solution is salt water, relative to a normal human skin cell? – HYPERTONIC

•  In which direction will the water move?

Lower NaCl concentration

Higher NaCl concentration

Higher H20 concentration Lower H20

concentration

Water moves OUT of the cell by osmosis

The cell loses water volume

� End result

The skin cell will SHRIVEL

Osmosis Scenario #2 �  What would happen to a human skin cell if it were

placed into distilled (pure) water?

•  What type of solution is distilled water, relative to a normal human skin cell? – HYPOTONIC

•  In what direction will the water move?

Water moves INTO the cell by osmosis

Lower NaCl concentration Higher NaCl

concentration

Higher H20 concentration

Lower H20 concentration

The cell gains volume and expands

The cell continues to expand…

�  End Result:

�  The skin cell will BURST/ LYSE because the cell membrane does not have the ability to resist the expansion pressure

Osmosis Overview

Discussion Question •  Why does your skin

become “pruney” after taking a bath or swimming in a pool?

Ø  There are more solutes inside your cells, and LOWER solutes in the water of the bath or pool. This means your cells are in a HYPOTONIC solution, and water will move INTO your skin cells, causing them to swell and appear wrinkled or “pruney”

Facilitated Diffusion �  Type of passive transport

�  Facilitated diffusion is used for molecules that need HELP to cross the membrane �  Use transport proteins

�  Occurs along a concentration gradient and does NOT require energy from the cell

Facilitated Diffusion by Carrier Proteins

Example of a Transport Protein/Facilitated Diffusion

�  Glucose: many cells depend on it for their energy needs, but glucose molecules are too large to cross the membrane without help from a Transport Protein

The protein changes shape when a certain molecule binds to it