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Transportation and the
Cell
Transportation is the movement of materials in
and out of the cell by way of the cell
membrane, or around the cell by way of the
endoplasmic reticulum.
Most substances are transported in liquid form
therefore the ability of a substance to dissolve
in water is important. This ability is called
solubility.
The two major forms of transport are passive
and active transport.
Passive Transport
Definition: movement of molecules across
cell membranes without the expenditure
of cellular energy. It is called a downhill
process. Direction of movement is
determined by the concentration gradient
(difference between a region of high
concentration and a region of lower
concentration)and the kinetic energy of the
molecules themselves
There are 3 major forms of passive transport.
1. Diffusion
Definition: occurs only if there is a
concentration gradient. The movement of
molecules from an area of greater
concentration to an area of lower
concentration. It is the most common type of
transportation in the cell.
It resuslts in equal distribution of molecules
on the inside and outside of cell.
It occurs because the molecules are in
constant motion.
Diffusion cont`d
Factors: There are various factors that
influence diffusion through the cell
membrane. Types of factors:
A. Permeability
B. Concentartion
C. temperature
D.Pressure
(A) Permeability of the Membrane
This is the ability to allow molecules to
flow through an object. There are 3
possibilities:
(i). Permeable - all materials pass through
(ii). Impermeable - no materials pass
through
(iii). Semipermeable - some materials pass
through, while others do not
(B) Concentration of Materials: a
concentration gradient is required.The
greater the difference between area of high
and low concentration gradient the faster
diffusion occurs
(C) Temperature: higher temperature
increases the rate of diffusion
(D) Pressure: as osmotic pressure
increases diffusion will slow so the cell is
less likely to rupture
Examples of Diffusion
Digestion - movement of nutrients from
the blood stream into the body cells.
Breathing - the movement of oxygen into
the blood stream and carbon dioxide out
of the blood and into the lungs.
2. 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 because
molecules are too big to cross themselves.
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.
3. 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.
Concentration and pressure work together to
mainatin this balance.
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)
A. Isotonic Solution
B. Hypotonic Solution
C. Hypertonic Solution
(A) 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.
(B) 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)
In plant cells the central vacuole swells and
pushes the contents against the cell wall. This is
called turgor pressure.
A cell in hypotonic solution
98% H2O
2% salt
- through osmosis, water will
diffuse into the cell causing
the cell to swell and burst
100% H2O
(C) 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.
The animal cell will shrink and the vacuole of the
plant cell will collapse. The shrinking of the
cytoplasm is called plasmolysis.
A cell in hypertonic solution
98% H2O
2% salt
90% H2O
10% salt
- through osmosis, water will
diffuse out of the cell causing
the cell 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:
1. 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.
It requires energy in the form of ATP to
move the molecules against the
concentration gradient.
2. Vesicles(Bulk Transport)
Energy is used to bring about a folding of the
cell membrane to form pockets.
2 forms of this movement:
(A) Exocytosis - movement of materials out of the cell .
A vacuole or vesicle containing materials within the cell
fuses to the inside surface of the cell membrane. Its
contents are then released outside the cell.
(B) Endocytosis - movement of materials into the cell.
Materials become impocketed in the cell membrane.. The
membrane closes and pinches of to become a sac or
vesicle and the materials are released to the outside.
Two Forms of Endocytosis:
(i) Pinocytosis - liquid droplets are taken
into the cell. Contents of the vesicle are
released directly into cell. (Cell drinking)
(ii) Phagocytosis - solid particles are
taken into the cell.This requires a large
impocketing. Food vacuoles instead of
vesicles can be formed so that digestive
enzymes can be used to break down food.
Ex. Macrophages( cell eating)
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