How do organisms keep things
How is the plasma membrane like
the border between two countries?
Just like countries
regulate who get in and
who goes out, our cells
must do the same thing?
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The plasma membrane
helps the cell maintain
regulating what goes in
and out of the cell
You can think of the
plasma membrane like a
“plastic bag with tiny
wholes” because . . .
The bag holds
all of the cell
pieces and fluids
inside the cell and keeps any nasty
things outside the cell. The holes
are there to let some things move in
and out of the cell.
The cell membrane is
not a solid structure.
It is made of millions
of smaller molecules
that create a flexible
and porous container.
Proteins and phospholipids make up most of the
The fluid mosaic model
shows you that phospholipid
molecules are shaped with a
head and a tail region.
Click the membrane
to view a YouTube video
of the fluid mosaic model
The head section of the molecule likes water
(hydrophilic) while the tail does not
Because the tails want to avoid water, they
tend to stick to each other and let the heads
face the watery (aqueous) areas inside and
outside of the cell. The two surfaces of
molecules create the lipid bilayer.
You will find thousands (millions?) of proteins
throughout the cell membrane. Some are just on the
inside of the cell and some on the outside. A special
few cross the cell membrane. Each type of protein has
a specific purpose. There are also embedded proteins in
the other membranes for cell organelles.
There are two types of proteins in the cell membrane - peripheral proteins and integral proteins.
Integral membrane proteins are permanently
connected to the cell membrane. They have
large sections embedded in the hydrophobic
(middle) layer of the membrane.
Peripheral proteins are not bonded as strongly to the
membrane. They may just sit on the surface of the
membrane, anchored with a few hydrogen (H) bonds.
Integral proteins are the hard workers of the
cell membrane. Some integral proteins cross
the membrane and act as pathways for ions
Some of the ion movement may not require work
(passive transport), but other processes require
lot of energy and pumping action
When you look at the
whole membrane, there
are very few integral
compared to the
Substances such as oxygen, carbon
dioxide and steroids easily diffuse
across the cells membrane . . .
. . . because the cell membrane is mostly
lipids, only lipid soluble molecules (things that
can dissolve in lipids) are able to easily pass
Glucose, amino acids, ions and water
need the help of various proteins
for transport across the membrane.
Because they are soluble (dissolve) in
water, not lipids.