Cell Transport Passive Transport: Simple Diffusion, Osmosis, Facilitated Diffusion
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