What you’ll learn . . .
•
Homeostasis is the property in living things which regulates its internal environment to
maintain a stable condition.
•
Diffusion is a process in which particles move from an area of high concentration to an
area of lower concentration.
•
Osmosis is the diffusion of water through a selectively permeable membrane.
•
A selectively permeable membrane permits certain molecules to pass through more
easily than others.
•
Facilitated Diffusion is the diffusion of particles with the help of carrier proteins that
does not require energy.
•
Passive Transport is the diffusion of materials without the use of energy.
•
In active transport, energy is used to pump particles across a membrane
Life means balance. To survive, an
organism must maintain a relatively stable
internal environment; no matter how much
the external environment changes. Does
that mean that life is static? Absolutely not.
An organism must respond appropriately to
an environmental stimulus, and then return
to normal when the stimulus has passed.
This ability to maintain a responsive
equilibrium is called homeostasis.
What is homeostasis?
One important aspect of homeostasis is
maintaining the perfect chemical balance
inside the living cell while taking in the
necessary food and water and eliminating
toxic wastes. Endocytosis is the uptake
(that means taking in) of materials
necessary for the cells survival;
exocytosis is the expulsion (that means
getting rid of) of materials from the cell.
Sometimes this movement occurs naturally
through a process called diffusion.
Diffusion is the movement of atoms or
particles from an area of high concentration
to an area of lower concentration. The
difference in concentration is called a
1.)
Name 2 types of passive transport that
requires none of the cells energy
2.)
What is the difference between
endocytosis and exocytosis?
concentration gradient. Diffusion is a
means of passive transport which means
that the cell does not need to expend (use)
any energy to make it happen. But not
everything needs to come into a cell and
not everything needs to leave. A free
exchange of all materials between the cell
and its environment would threaten the
cells survival. This made it necessary for
both single celled and multicellular
organisms to evolve various ways to control
the passage of materials into and out of the
cell.
Water, for example is necessary for life, but
too much or too little water is a threat to
survival. Single-celled organisms such as
the amoeba exchange water directly with
the environment. Amoebas like other cells
are surrounded by a selectively permeable
membrane, which means that certain
things (like water) are allowed to pass
through easily while other materials are
not. We call this movement of water across
a selectively permeable membrane
osmosis. It is a form of passive
transport (remember diffusion). As with
diffusion, osmosis moves water with the
concentration gradient—from an area of
high concentration to an area of lower
concentration (think of it like a stream
flowing down hill).
Cells have other ways of regulating the
exchange of materials with their
environment. Facilitated diffusion is just
one example. Imbedded in the plasma
membrane are proteins called carrier /
transport proteins. They form a water
filled channel (think of it like a pipe) some
molecules such as sugars (they are too big
to get through the membrane without some
help) to pass into or out of the cell. Like the
rest of the cell membrane, these channels
are “selective.” Each carrier protein will
What is Diffusion? (see first page)
What is a concentration Gradient?
What is osmosis?
In what direction does water move during
osmosis? (with or against the
concentration
gradient?
What type of molecule is used by the cell
to carry out facilitated diffusion?
allow only 1 type of atom or molecule to
pass through. They are also “gated”
because the are able to open and close.
Another of the processes at work is active
transport. It uses energy and transport
proteins to pump particles across the cells
membrane. Unlike passive transport, active
transport moves particles against the
concentration gradient—from an area of
low concentration to an area of higher
concentration (think of it like trying to get
water to flow up hill. It cannot get there by
itself, it must be pumped up the hill and
the pump requires energy.
What is active transport?
Explain why active transport requires
energy.