Cell membrane outline
I. Controls homeostasis
II. Selectively permeable
III. Structure of membrane
1. lipid bilayer
2. transport proteins
3. cholesterol
drawing
IV. Diffusion
V. Osmosis
1. isotonic
2. hypotonic
plant cells
animal cells
3. hypertonic
• A cell’s survival depends on the cell’s
ability to maintain necessary conditions
inside itself
• Maintaining these constant internal
conditions is called--homeostasis
What controls homeostasis in a cell?
•
Plasma (Cell) membrane:
the flexible boundary
between the cell and its
environment.
–
It allows-- water and
nutrients to enter the cell
and wastes to leave the
cell
•
The cell membrane is selectively permeable.
–
Selective permeability--when a membrane
allows some materials to pass through,
while keeping others out.
Examples:
Spaghetti strainer—
allows water, but not
spaghetti to pass
through
Summary Section 2 – pages 175-178
mosquito netting—
allows air molecules to
pass through but not
mosquitoes
Plasma Selective Permeability
Membrane
Water
Structure of the Plasma Membrane
1. Lipid Bilayer--The cell membrane is 2
layers thick; made of phosopholipids
Lipid Bilayer
• Polar heads face out
and are attracted to water
• Nonpolar tails face in
and repel water
This arrangement makes the
membrane selectively permeable.
2. Transport proteins:
embedded in the
membrane, they
monitor which
molecules enter and
exit the cell
3. Cholesterol- stabilizes the cell
membrane by preventing lipids from
sticking together
Cholesterol
Draw and label the parts of the cell
membrane below:
Diffusion and Osmosis
• What needs to cross a cell membrane?
water, nutrients, oxygen
• How do materials cross the cell
membrane?
Diffusion or osmosis
Diffusion
Random movement of particles to establish
dynamic equilibrium (balanced movement
and concentration)
• Occurs from high to low concentration
gradient (moves from area where there is a
lot to an area where there is little)
• ex. Oxygen diffuses into bloodstream
because higher conc. in lungs
Osmosis
Osmosis--Diffusion of water across a selectively
permeable membrane
Review--Diffusion occurs from an area of
HIGH concentration to an area of LOW
concentration
Osmosis Diagram
Example of osmosis?
After
Osmosis
Before
Osmosis
Selectively
permeable
membrane
Water molecule
Sugar molecule
3 Types of Osmotic Solutions
H2O
H2O
Water Molecule
Dissolved Molecule
• Isotonic Solution-Equal concentrations
of water, inside and
outside of the cell.
Water moves in and
out of cell equally
Cells in an isotonic solution
Shape of isotonic cell-Normal
http://www.visualsunlimited.com/images/watermarked/195/195901.jpg
Cells in a hypotonic solution
• Hypotonic
solution—More
water outside the cell
than inside the cell;
water diffuses into the
cell
H2O
H2O
Water Molecule
Dissolved Molecule
Cells in a hypotonic solution
• Shape of hypotonic cell—
swells
http://www.visualsunlimited.com/images/watermarked/195/195902.jpg
• Plant cells have cell walls that allow a lot
of water to enter without bursting; they
stay plump and crisp (this is why grocers
mist produce)
• Animal cells do NOT have cell walls and
could burst
Cells in a hypertonic solution
• Hypertonic cell-More water on inside
of cell than outside of
cell. water moves out
of cell
H2O
H2O
Water Molecule
Dissolved Molecule
Cells in a hypertonic solution
• Shape of hypertonic
cell—Shriveled; this is
why plants wilt
http://www.visualsunlimited.com/images/watermarked/899/899126.jpg