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Cell Membranes and Osmosis
What separates a cell from its
surroundings?
- Cell membrane
- Controls what enters and leaves cell
Cell membrane is made out of two layers
of fats. These fats are called
“phospholipids”
Phospholipid
Lipid
Bilayer
Since your cell membrane is made out of two
layers of fats, the cell membrane is often
called a “phospholipid bilayer”
The outside of the cell membrane likes to be
near water
Hydrophilic – loves water
Hydrophobic – hates or fears water
The inside of the cell membrane hates to be
near water
Fluid mosaic model
In addition to phospholipids, the cell membrane also has
proteins in it.
These proteins help a cell to control what comes in and
out of the cell.
These proteins can float around on the cell membrane
like icebergs in the ocean and decorate the cell like a
mosaic.
Cell membranes are “selectively permeable”
Permeable – EVERYTHING can pass through membrane
Non-Permeable – NOTHING can pass through membrane
Selectively-Permeable –
SOME THINGS can pass
through membrane
As long as it is small
enough, it will DIFFUSE
across cell membrane from
high to low concentration
Not all molecules can diffuse through a cell
membrane. However, one molecule can always
diffuse WATER!
Osmosis (diffusion of water) – water flows from an area of
high concentration to an area of low concentration
Try osmosis with an animal cell. Assume starch – a really BIG sugar
molecule – cannot move across an animal cell membrane.
Start by comparing the percent water inside and outside the
cell.
Then, draw an arrow pointing in the direction that water will
flow (high to low).
Finally, draw the result. (NOTE: animal cells can swell and shrink
like a balloon – to a certain extent. Your own red blood cells can
explode if they swell too much).
100%
water
80% water
20% starch
Cell will lose water and shrivel
Try osmosis with an animal cell. Assume starch – a really BIG sugar
molecule – cannot move across an animal cell membrane.
Start by comparing the percent water inside and outside the
cell.
Then, draw an arrow pointing in the direction that water will
flow (high to low).
Finally, draw the result. (NOTE: animal cells can swell and shrink
like a balloon – to a certain extent. Your own red blood cells can
explode if they swell too much).
80%
water
100% water
20%
starch
Cell will expand or even burst!
Use mouse to click here for an Osmosis Video
Turn on speakers
PLASMOLYSIS
Plasmolysis – a special case involving osmosis and plants
Plasmolysis – water moving from high to low concentrations
in plant cells
Why would osmosis be
different in plant cells?
(Think: what makes plant
cells different from animal
cells?)
Plant cells have a cell
membrane AND a cell wall.
WHAT DO YOU THINK THIS CELL WILL
LOOK LIKE?
100% water
80% water
20% starch
PLASMOLYSIS
100% water
80% water
20% starch
Use mouse to click here for plasmolysis Video
Which one is in salt water?
Red blood cells in salt water
Red blood cells in pure water
PASSIVE AND ACTIVE TRANSPORT
How can cells transport nutrients and molecules that are
too big to pass through their membranes?
Proteins –
1. Channels open up
and allow certain
molecules into the
cell.
2. Receptors “grab”
molecules and bring
them into the cell.
PASSIVE TRANSPORT
Proteins can work by “passive” or “active” transport
Passive transport (also
known as “facilitated
diffusion”) – proteins help
molecules move across cell
membrane from high to low;
NO ENERGY is required
Animation Link
Animation Link 2
ACTIVE TRANSPORT
Active transport – proteins
help move molecules across
cell membrane from LOW
to HIGH;
ENERGY IS required
Animation Link
ENDOCYTOSIS AND EXOCYTOSIS
Not all molecules can move
through cell membrane.
Some are just too big!
Different methods are
needed.
Endocytosis – cell wraps
membrane around object
and pulls it in
ENDOCYTOSIS
There are three specific
types of endocytosis
1. Phagocytosis – white
blood cells “eat” bacteria
and digest them
Animation Link
ENDOCYTOSIS
There are three specific
types of endocytosis
2. Pinocytosis – cell is
“drinking” in liquids
ENDOCYTOSIS
There are three specific
types of endocytosis
3. Receptor-aided
endocytosis– protein
receptors work to pull
molecules into cell
EXOCYTOSIS
There are three specific
types of endocytosis
Exocytosis – molecules are
released from cell