EXCHANGING MATERIALS
WITH THE ENVIRONMENT
• Consider this problem: you want to live
in a safe, comfortable environment. So
you build four thick walls and a sturdy
roof around you.
• Now you are protected from wind, rain,
and the sun, and thieves cannot reach
you.
• You cannot bring in food, water, fresh
air, or fuel. What if you wanted new
clothes and visitors? How will you throw
away your trash?
• The solution: build your room with doors and
windows to let people and air move in and
out.
• You install plumbing, gas and electric lines.
• You may consider purchasing a heater or air
conditioner and telephone or cable lines.
• And you will probably put a lock on your door,
so that you control who comes and goes.
• All these features control the world’s access
to your safe environment.
How does this relate to living things?
• Living things face similar problems. The
surface of an organism is a barrier against
destructive forces. That barrier, however,
must allow the passage of food, water, waste
and communication signals if the organism is
to survive.
Membrane Structure and Function
• The cell membrane is the boundary that
separates the living cell from its nonliving
surroundings.
• The cell membrane is so thin, you would have
to stack 8000 of them to equal the thickness
of a piece of paper.
• Its function is to control what goes in and
what comes out.
Membrane made of Phospholipids
• The cell membrane
is composed mostly
of lipids and
proteins.
• A phospholipid is a
lipid that has only
two fatty acids
instead of three.
• A phosphate group
takes the place of
the third fatty acid.
Phospholipid Characteristics
• The phosphate group is electrically charged,
which makes it hydrophilic (water-loving).
• The rest of the phospholipid consists of fat,
and fat doesn’t mix with water, so we call the
other end the hydrophobic end (waterfearing).
• The phosphate group head mixes with water,
while the fatty acid tails avoid it
• By forming a two-layered structure, or
phospholipid bilayer, the hydrophobic parts of
the molecule hide from water, while the
hydrophilic portions are wet.
Fluid Mosaic Membrane
• Most membranes have specific proteins
embedded in the bilayer.
• The phospholipids and most of the proteins
are free to drift about in the membrane.
• The model is “fluid” because of the ability of
molecules to drift freely.
• The model is “mosaic” because of the variety
of proteins that float in the phospholipid sea.
Selective Permeability
• A membrane allows some substances to cross
more easily than others and blocks the
passage of some substances altogether.
• The cell needs oxygen and nutrients to enter
for daily function, but also needs to dispose of
waste and carbon dioxide.
Membrane Proteins
• The movement of some substances can only
occur through special membranes proteins
that shuttle things in and out (the highways
of the cell).
Functions of Membrane Proteins
•
•
•
•
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Attachment to other internal cell parts
Attachment to other surrounding cells
Cell signaling
Enzymatic activity
Transport of substances
• SEE PAGE 59 in your textbook