Cell Membranes
Osmosis and Diffusion
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Functions of Membranes
1. Protect cell
2. Control incoming and outgoing substances
3. Maintain ion concentrations of various
substances
4. Selectively permeable - allows some
molecules in, others are kept out
5. ALL THIS MAINTAINS HOMEOSTASIS
(internal balance)
Phospholipid Bilayer
Fluid Mosaic Model
Methods of Transport Across
Membranes
1. Diffusion
2. Osmosis
3. Facilitated Diffusion
4. Active Transport
Polar heads Fluid Mosaic
love water
Model of the
& dissolve. cell membrane
Non-polar
tails hide
from water.
Carbohydrate cell
markers
Proteins
Membrane
movement
animation
•Animations of Active
Transport & Passive
Transport
Types of Cellular Transport
•
Passive Transport
cell doesn’t use energy
1. Diffusion
2. Facilitated Diffusion
3. Osmosis
•
Weeee!!
!
high
low
Active Transport
cell does use energy
1. Protein Pumps
2. Endocytosis
3. Exocytosis
This is
gonna
be hard
work!!
high
low
Diffusion
• Movement of molecules from an area of
high concentration to an area of low
concentration.
• Movement from one side of a membrane
to another, un-facilitated
Diffusion
Passive Transport:
Diffusion
Simple Diffusion
Animation
Diffusion: random movement of
particles from an area of high
concentration to an area of low
concentration.
(High to Low)
•
Diffusion continues until all
molecules are evenly spaced
(equilibrium is reached)-Note:
molecules will still move around but
stay spread out.
http://bio.winona.edu/berg/Free.htm
Passive Transport:
Osmosis
Osmosis
animation
• Osmosis: diffusion of
water through a
selectively permeable
membrane
• Water moves from high to
low concentrations
•Water moves freely
through pores.
•Solute (green) too
large to move across.
Osmosis
Tonicity is a relative term
• Hypotonic Solution - One solution has a
lower concentration of solute than
another.
• Hypertonic Solution - one solution has a
higher concentration of solute than
another.
• Isotonic Solution - both solutions have
same concentrations of solute.
Plant and Animal Cells put into
various solutions
•
Hypotonic Solution
Osmosis Animations
for isotonic,
hypertonic, and
hypotonic solutions
Hypotonic: The solution has a lower concentration of
solutes and a higher concentration of water than
inside the cell. (Low solute; High water)
Result: Water moves from the solution to inside the
cell): Cell Swells and bursts open (cytolysis)!
•
Hypertonic Solution
Osmosis Animations
for isotonic,
hypertonic, and
hypotonic solutions
Hypertonic: The solution has a higher concentration
of solutes and a lower concentration of water than
inside the cell. (High solute; Low water)
shrinks
Result: Water moves from inside the cell into the
solution: Cell shrinks (Plasmolysis)!
•
Isotonic Solution
Osmosis Animations
for isotonic,
hypertonic, and
hypotonic solutions
Isotonic: The concentration of solutes in the solution
is equal to the concentration of solutes inside the cell.
Result: Water moves equally in both directions and
the cell remains same size! (Dynamic Equilibrium)
Types of Transport
Passive Transport:
Facilitated Diffusion
A
Facilitated diffusion: diffusion
of specific particles through
transport proteins found in
the membrane
a. Transport Proteins are
specific – they “select” only Facilitated
certain molecules to cross
diffusion
(Channel
the membrane
Protein)
b. Transports larger or
charged molecules
Carrier Protein
B
Diffusion
(Lipid
Bilayer)
Passive Transport: Facilitated Diffusion
Glucose
molecules
Cellular Transport From aHigh Concentration
High
• Channel Proteins
animations
Cell Membrane
Low Concentration
Through a 
Go to
Section:
Transport
Protein
Protein
channel
Low
Types of Active Transport
•
Endocytosis: taking bulky
material into a cell
• Uses energy
• Cell membrane in-folds
around food particle
• “cell eating”
• forms food vacuole &
digests food
• This is how white blood
cells eat bacteria!
Types of Active Transport
Exocytosis: Forces material
out of cell in bulk
• membrane surrounding the
material fuses with cell
membrane
• Cell changes shape –
requires energy
• EX: Hormones or wastes
released from cell