Membrane Transport
Why are cells so small?
● Cells need to transport molecules!
○ Proteins, hormones, water, ions, wastes
● Have to pass through cell membrane
○ Phospholipid bilayer
● Phospholipid:
○ Phosphate group → POLAR
○ Glycerol
○ Fatty acid tails → NONPOLAR
● Since head is POLAR, and water is POLAR
○ Head of phospholipid is hydrophilic
■ Loves water!
● The tails are NONPOLAR,
don’t get along with water
○ Tails are hydrophobic
■ Hates water!
■ Has a phobia of water!
● Purpose of cell membrane:
○ Supports cell
○ Protects cell
○ Transport molecules in and out
■ Structure of lipids only allow certain things in!
● Some need help across!
http://www.pbslearningmedia.
org/resource/tdc02.sci.life.cell.
membraneweb/cell-membrane-just-passingthrough/
○ Cell membrane is selectively permeable
■ If it allows it to pass through → permeable
●
Oxygen, CO2, ions (SMALL)
■ Does not let it pass through → impermeable
●
●
Sugar (glucose), protein, nucleic acids (LARGE)
These need help across!
○ 2 types of transport → PASSIVE and ACTIVE
Transport
Passive
Diffusion
Osmosis
Facilitated
Diffusion
Active
Protein
Pumps
Endocytosis
Exocytosis
Passive Transport
● No energy required!
● Let’s its pass through easily
1. Diffusion
● Molecules travel from HIGH to LOW
concentration across membrane
○ “down” a concentration gradient
● Membrane is permeable to that molecule
H
L
● Molecules travel across membrane from
HIGH → LOW until there is an equal
concentration of molecules on both sides
○ This is called DYNAMIC EQUILIBRIUM
**Remember these molecules are the solute inside the solvent of water
or cytoplasm!
Move from H → L
Equal = can move in
both directions now
● Molecules travel across membrane from
HIGH → LOW until there is an equal
concentration of molecules on both sides
○ This is called DYNAMIC EQUILIBRIUM
**Remember these molecules are the solute inside the solvent of
water or cytoplasm!
Transport
NO energy
Passive
Diffusion
Osmosis
High → Low
Facilitated
Diffusion
Active
Protein
Pumps
Endocytosis
Exocytosis
Passive Transport
2. Facilitated Diffusion
● Some molecules (ions, sugars, salts)
need help to cross cell membrane
● Uses specific protein channel
○ Facilitates (helps) molecules across
● Still HIGH → LOW concentration
○ NO energy required!
HIGH concentration
of solute molecules
LOW concentration of
solute molecules
Transport
NO energy
Passive
Diffusion
Osmosis
High → Low
Facilitated
Diffusion
High → Low, with help from protein channel
Active
Protein
Pumps
Endocytosis
Exocytosis
Passive Transport
3. Osmosis
● Diffusion of WATER across cell
membrane from HIGH → LOW
● Very important in living cells!
Sugar can NOT move
across membrane →
stays where it is
Water CAN move
across because it is
small. Water will
move from H → L, so
in this picture it will
move OUT of the cell!
Outside:
15 water/5 sugar
=3
Inside:
3 water/1 sugar
=3
Gummy Bear Lab!
1.
2.
3.
4.
Get 2 gummy bears for your table
Measure their length, width, height
Obtain 2 beakers for your lab
Fill one beaker half way with tap water
4.1. Place a gummy bear inside
5. Fill the bottom of the other beaker with salt
5.1. Place enough tap water in to cover the salt → mix
5.2. Place a gummy bear inside
Hypothesize:
1. What will happen to the bear in the tap water?
-Why??
2. What will happen to the bear in the salt water?
-Why??
● Water will move to equalize the concentration
of solute to solvent
○ Equal on both sides = ISOTONIC
■ “same strength”
Equal
Equal
Cell stays the same!
● When there is less water outside of the cell
○ Water will move OUT
■ Cell shrinks
This is called HYPERTONIC
Low
High
Water leaves, cell shrinks!
● When there is more water outside of the cell
○ Water will move IN
■ Cell bursts
This is called HYPOTONIC
High
Low
Water enters, cell grows and bursts!
HYPOTONIC
ISOTONIC
HYPERTONIC
-not much water inside
-equal concentration
-not much water outside
-water moves IN
-water moves in and
-water moves OUT
-cell bursts!
and out equally
-cell shrinks!
-normal cells!
98% H2O
2% salt
90% H2O
10% salt
80% H2O
20% salt
90% H2O
10% salt
90% H2O
10% salt
90% H2O
10% salt
● Plant cells usually surrounded by fresh water
solution → lots of water on outside, so water
will rush INTO cells → not good!
● To avoid cells bursting = cell wall, vacuole!
○ Tough structure, pump water out
Water pushing into cell and filling it up is called
osmotic pressure
Gummy Bear Lab!
1.
2.
3.
4.
Obtain your beakers from yesterday
CAREFULLY get the bears out of the beakers
Measure their length, width, height
Clean-up!
Analysis:
1. What happened to the bear in tap water?
It GREW!! There was more water outside of the
bear, so water rushed in! This was a HYPOTONIC
solution.
2. What happened to the bear in salt water?
It SHRUNK!! There was more water inside the bear
than in the beaker, so water left! This was a
HYPERTONIC solution.
Transport
NO energy
Passive
Diffusion
High → Low
Osmosis
High → Low, water
Facilitated
Diffusion
High → Low, with help from protein channel
Active
Protein
Pumps
Endocytosis
Exocytosis
Active Transport
● Energy is required!
○ Energy = ATP
● Molecules go from LOW → HIGH concentration
○ move against the normal concentration gradient
○ opposite of diffusion
Active Transport
1. Protein Pumps
● Pump ions across the membrane
● Need energy to do this
Example: Sodium-Potassium Pump
● Protein pumps Na+ (sodium) ions out
● Protein pumps K+ (potassium) ions in
Transport
NO energy
Passive
Diffusion
High → Low
Osmosis
High → Low, water
Facilitated
Diffusion
High → Low, with help from protein channel
Active
Protein
Pumps
Low → High
Needs energy!
Endocytosis
Exocytosis
Active Transport
2. Endocytosis/Exocytosis
● Requires energy!
● ENDO = stuff comes into the cell
○ Phagocytosis → eating large things
○ Pinocytosis → “drinking” liquids
● EXO = stuff exits, leaves the cell
VIDEO
Endocytosis
-cell membrane folds around particle
-pinches off, forms vacuole/vesicle inside cell
Exocytosis
-getting rid of waste
-vacuole/vesicle fuses with membrane
-dumps contents outside of cell
Transport
NO energy
Passive
Diffusion
High → Low
Osmosis
High → Low, water
Facilitated
Diffusion
High → Low, with help from protein channel
Active
Needs energy!
Protein
Pumps
Endocytosis
Exocytosis
Low → High
Eating and
Excreting wastes