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How do the structures of the cell
membrane help it to function?
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Found in every cell
Flexible boundary between the cell and its
environment
Controls what enters and exits the cell
Allows the cell to maintain homeostasis – the
balanced internal environment needed for life
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The cell membrane is made from TWO layers
of phospholipids with proteins and other
molecules embedded in them
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Main component of the cell membrane
Polar phosphate “head”
◦ Hydrophilic (water loving)
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Two nonpolar lipid “tails”
◦ Hydrophobic (water fearing)
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Polar molecules do not share electrons
equally, so they have a positive side and a
negative side
Water is polar
Polar molecules are hydrophilic (water-loving)
because they are attracted to (and can easily
dissolve in) water
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Non-polar molecules do not have a charge
Oil and most other lipids are non-polar
Non-polar molecules are hydrophobic
(water fearing) because they are repelled by
(and won’t easily dissolve in) water
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The structure of the phospholipid molecules
is the key…
◦ Polar heads are attracted to the water inside and
outside the cell
◦ Non-polar tails are repelled by the water and
form a layer in between!
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Embedded in the lipid bilayer
Functions:
◦ TO CONTROL WHAT GOES THROUGH
 (CHANNELS OR PUMPS)
◦ TO COMMUNICATE WITH OTHER CELLS
 (SIGNAL OR RECEPTOR PROTEINS)
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Some proteins also have carbohydrate chains attached that
are involved in the signaling process
Different types of cells have different membrane
proteins, depending on their function!
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describes the structure of the plasma
membrane
the lipids and proteins are not locked in place
within the membrane
The membrane is fluid, and molecules move
around within the bilayer
Fluid Mosaic Model
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Cell/plasma membrane is Selectively
Permeable
◦ Some molecules can pass through easily
◦ Some molecules can only pass through with help
◦ Some molecules can’t pass through at all
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Three factors that will determine if a molecule
can cross the plasma membrane:
◦ Its SIZE
◦ Its SHAPE
◦ Its ELECTRICAL CHARGE
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Size matters
◦ Small molecules like CO2 and O2 can easily pass
directly through the bilayer
◦ Large molecules like glucose can’t pass directly
through and need the help of membrane proteins
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A molecule’s shape must match up with a
transport protein
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Ions like Na+ and polar molecules like water
can’t easily pass through the bilayer, so they
need the help of membrane proteins
http://www.teachersdomain.org/resource/tdc02.sci.life.
cell.membraneweb/
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The plasma membrane separates the inside of the
cell from the outside or the cell, but
Many organelles in the cell have their own membranes:
◦ Nucleus
◦ Golgi
◦ Endoplasmic reticulum
◦ Vesicles
◦ Chloroplasts and Mitochondria have DOUBLE membranes!