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The Cell Membrane
The barrier to the cell
Mrs. Dignan’s Science Class
Cell Membrane
Physical Description: thin, flexible barrier
made of phospholipids around the cell
(just inside the cell wall OR the outside
border of animal cells)
 Function: controls what enters and leaves
the cell; support and protection
 Type of Cell: found in all cells
 Analogy: it is like the security guard of a
factory because it controls what enters
and leaves the cell

Plant Cell
Animal Cell
Bacterial Cell
Why Does The Cell Membrane
Matter?
What would happen at an airport if there were
no security guards and airline checkers?
 Answer: Dangerous materials could enter, there
could be way too many people there at one time
and flights would become overcrowded and
unable to fly.
 What if, instead of security guards, we had giant
cement walls to keep people out of airports?
 Answer: No one would be able to fly and the
world would come to an end!

Selectively Permeable
Permeable: something
that allows other
substances to pass
through
 Selective: choosing or
“selecting”
 Like a security guard,
the cell membrane
controls what enters and
what leaves the cell.
 They are sometimes also
described as “semipermeable”.

Permeability has everything to do
with the cell membrane’s
structure

https://www.youtube.
com/watch?v=Rl5Em
UQdkuI
The phospholipid bilayer
“Phospho” – the head of the phospholipid
is composed of a phosphorus atom,
surrounded by other atoms (called a
phosphate group).
 “Lipid” – the tail of the phospholipid is
composed of lipids, which are fatty acids
like oils, waxes, and fats.

Hydrophillic – “hydro”
means water, “phillic”
means love.
Hydrophillic molecules
love water and are
attracted to it.
 Hydrophobic –
“hydro” means water,
“phobic” means fear.
Hydrophobic
molecules hate water
and repel it.
 Phospholipids have a
hydrophillic head and
a hydrophobic tail.

The cell membrane is
composed of a bilayer
(two layers) of
phospholipids.
 In both layers, the
head is attracted to
the water inside and
outside of the cell.
The tail is repelled
and ends up facing
it’s counterpart in the
other layer.

Some shall pass
In order for
molecules to
enter or exit
the cell,
they must
pass
through the
phospholipid
bilayer.
 Some have
an easier
time than
others….

Fat soluble molecules: Molecules that dissolve
easily in fats. They can usually squeeze through
the membrane easily because the fatty tails of
the phospholipids like them. (Exps – O2, CO2,
steroids, Vitamins A, D, E, and K.)
 Water soluble molecules: Molecules that dissolve
easily in water. They are usually repelled by the
hydrophobic tails of the phospholipids. (Exps –
sugars, sodium, calcium)
 Water: Surprisingly, water can pass through the
phospholipid bilayer relatively easily because it is
a very small molecule.

Types of Transport

Passive Transport: molecules move
randomly according to their concentration
gradient.




Does NOT require energy
Happens spontaneously
Example: a bad smell diffusing across a room
Active Transport: molecules are actively
moved by cell organelles.



Does require energy
Something has to make it happen
Example: some movers carrying boxes into the house.
Passive transport

Diffusion: molecules spread out through a liquid
or gas.


Molecules always move from an area of higher
concentration to less concentration
Concentration=the amount of molecules per unit of
area
 New York City has a high concentration of people,
Montana has a low concentration of people
Diffusion of dye through water
Passive transport
Osmosis: The diffusion of water
molecules across a semi-permeable
membrane.
 A specific type of diffusion.

 Semi
permeable = some things get through,
others don’t

The cell membrane has small pores.
Small molecules like H2O and O2 get
through, but bigger molecules like
C6H12O6 cannot.
Osmosis


water is the only thing that moves in osmosis.
Solution: a liquid formed when a solute dissolves in a
solvent.
 Solvent: the liquid that the solute is dissolved in
(typically water)
 Solute: the thing that is dissolved in the solvent
(sugar, salt, kool-aid, etc.)
Passive transport

Types of solutions

Hypotonic- Low solute concentration compared to the
cell
 (example- pure water)
Isotonic- The same solute concentration compared to
the cell
 (example- slightly salty water)
hypertonic- High solute concentration compared to the
cell
 (example- very salty water)


Osmosis
Active transport

Active Transport: ATP is used to open and close
special proteins in the membrane to let bigger
molecules into or out of the cell.
Active transport
Cytokinesis: ATP is used to actually move the cell
membrane around.





Cyto=cell
Kinesis=movement
Endocytosis- the cell membrane engulfs food particles to take
them inside the cell
Exocytosis-the cell membrane “spits out” waste particles to get
rid of them
Cytokinesis is also important when the cell reproduces by
splitting into two cells.
Cell membranes
Task: Make a concept diagram that contains
the following terms.
Hypertonic
Transport
Solute
Endocytosis
Osmosis
Cell division
Passive Transport
Cytokinesis
Hydrophobic tail
Isotonic
Solution
Cell membrane
Solvent
Hydrophilic head
Phospholipid
Proteins
hypotonic
Active Transport
Diffusion
Exocytosis
Active transport