Movement of substances in and out of the cell
Passive Transport
Definition: movement of molecules across cell membranes without the
expenditure of cellular energy. Direction of movement is determined by the
concentration gradient (difference between a region of high concentration
and a region of lower concentration).
Diffusion - The movement of molecules from an area of higher
concentration to an area of lower concentration. It is the most common type
of transportation in the cell.
Facilitated Diffusion
Definition: movement of materials from an area of high concentration to one
of lower concentration by a protein carrier present in the cell membrane.
This form of diffusion is more rapid than normal diffusion.
Example of facilitated diffusion: Transport of glucose from your blood into
the cells of your body.
Osmosis
Definition: a special type of diffusion. It is a movement of water molecules
from an area of greater concentration to an area of lesser concentration
through a semipermeable membrane.
Osmosis will continue until the rate of water diffusion across the cell
membrane in both directions is equal. This is called osmotic balance.
The pressure generated from this movement of water is called osmotic
pressure.
Effects of Osmosis
The effects of osmosis can be seen by placing animal cells into solutions
containing different concentrations of water. (page 55; figure 2.26)
Isotonic Solution
Has the same concentration of dissolved substances as the cell placed in it.
The concentration of water molecules in the cell and in an isotonic solution
is the same. Since the concentration gradient is zero, there is no net gain or
loss of water in the cell.
Hypotonic Solution
Contains a lower concentration of dissolved substances than the cell put in it.
(i.e. the solution is mostly H2O)
The concentration of water molecules is higher in the hypotonic solution
than it is in the cell. Since the concentration of water molecules is higher
outside the cell than inside, there is a net movement of water into the cell.
Osmotic pressure produced by water entering the cell causes it to swell and
burst. (e.g. blood cell in water will burst)
Hypertonic Solution
Contains a higher concentration of dissolved substances than the cell placed
in it. The concentration of water molecules is lower in the hypertonic
solution than it is in the cell. Since the concentration of water molecules is
higher inside the cell than outside, there is a net movement of water out of
the cell. Water leaving the cell causes it to shrink.
Active Transport
Definition: movement of molecules across a cell membrane requiring the
expenditure of cellular energy. It is called an uphill process. Movement is
from an area of low concentration to an area of high concentration.
There are 2 mechanisms of active transport:
Carrier Proteins
Carrier proteins located in the cell membrane receive an energy boost. The
energized proteins capture specific solute molecules and move them either
into or out of the cell against the concentration gradient.
Vesicles
Energy is used to bring about a folding of the cell membrane to form
pockets. 2 forms of this movement Endocytosis (movement in) and
Exocytosis (movement out).
Two Forms of Endocytosis:
(i) Pinocytosis - liquid droplets are taken into the cell.
(ii) Phagocytosis - solid particles are taken into the cell.
Exocytosis – when a vesicle makes contact with the cell membrane,
releasing the contents to the outside of the cell.