Solution:
a solution contains solute (substance dissolved into a solvent to form a
solution) and solvent: liquid that dissolves solute (to form a solution).
Diffusion
is the net movement of particles from an area of high concentration to an area of
lower concentration until all particles are evenly distributed. All particles
diffuse independently of each other. Diffusion is a passive process and does
NOT require an input of energy such as ATP. All particles in liquid and gaseous
states can diffuse.
Factors
that affect the rate of diffusion in a liquid or a gas:
Concentration
gradient:
The
greater the concentration gradient, the faster the rate of diffusion.
Concentration
gradient
is the difference in concentration between an area of high concentration and
an area
of low concentration.
Temperature:
As
temperature increases so does the rate of diffusion because particles move
faster.
Diffusion
occurs more quickly in a gas than in a liquid because the particles are moving
more
quickly and there is more space between particles.
Particle
size:
The
smaller the particles the faster the rate of diffusion. Smaller particles can
fit between
other
particles more easily and they move more quickly at any given temperature
because
it takes
less energy for them to move than large particles.
Particle
shape:
Some
particles are more streamlined than others and can fit between other particles
more
easily.
Electrical
charge:
Non-polar
molecules diffuse more readily in non-polar solutions and polar molecules
diffuse
more readily in polar solutions. Polar and non-polar molecules do not mix well
so
they do
not diffuse readily in each other.
Factors
that affect the rate of diffusion across a membrane:
All of
the above factors plus:
Number of
pores/carrier proteins/channel proteins:
Most
cells can vary the number and type of pores, carrier proteins or channel
proteins that are present at any one time. The more openings that are present
for a particular ion or molecule, the more of that substance can cross the
membrane.
Pressure:
The
greater the difference in pressure the greater the rate of diffusion. For
example the greater the osmotic pressure the more water will diffuse (osmose)
across the membrane.
Pressure
gradients act the same way as concentration gradients.
Hormonal
effects:
Hormones such
as insulin can affect whether or not a substance can access a carrier protein to
cross a membrane. If insulin is present glucose can enter the cell.
Lipid
solubility:
Small
non-polar molecules can diffuse directly across the cell membrane because they
are lipid soluble.
Cyclosis:
In some
cells the cytoplasm moves in a cyclic path. This is called cyclosis or
cytoplasmic streaming. Because the movement of the cytoplasm removes particles
away from the membrane it serves to increase the concentration gradient across
the membrane.
Membrane
permeability:
One cell
may be permeable to a particular molecule and another cell may not be permeable
to that molecule. A cell may also vary its permeability to a substance
depending upon need.
Describe
osmosis and osmotic pressure.
Osmosis
is the movement of water from an area of higher pressure to an area of lower
pressure across a selectively [differentially] permeable membrane.
Or,
differently put-- it’s the diffusion of water -- into and out of cells.
Osmotic
pressure is the pressure generated by the flow of water across a semi permeable
membrane. This pressure is created by the solute of the solution.
We dunk a
cell into a solution.
The solution can be one of three types of solutions.
Isotonic
solution: the concentration of solute and water is the same both inside and
outside of the cell.
Hypotonic
solution: lower concentration of solute, higher concentration of water than the
cell.
Hypertonic
solution: this solution has a higher concentration of solute and lower
concentration of water than the cell.
The
solution can cause the following effects:
An
isotonic solution causes neither a shrinking nor swelling of the cell because
it is a balanced solution.
Hypertonic and hypotonic solutions can have
devastating effects to a cell.
Hypotonic environment causes Lysis:
Osmotic pressure builds up when the water surrounding the cell moves to the
area of least resistance, which is inside the cell. The cell explodes from the pressure.
Hypertonic environment causes Plasmolysis:
when a plant cell is placed in a hypertonic solution, the plasma membrane pulls
away from the cell wall because the large central vacuole loses water.
Plasmolysis is the shrinking of the Plant cell's cytoplasm due to osmosis.
Hypertonic environment causes Crenation:
The cell’s water rushes out to the hypotonic solution because of osmotic pressure.
The cell shrinks like a raisin.
Describe
and be able to differentiate between facilitated transport and active
transport.
Facilitated
transport: passive transfer of materials
into or out of a cell along a concentration gradient by a process that requires
a carrier.
Active
transport: Transfer of a material into or out of a cell from a region of lower
concentration to an area of higher concentration by a process that requires a
carrier and an expenditure of energy.
Both
processes require a carrier and both processes facilitate transport into or out
of the cell. Both processes move material along a concentration gradient.
However, Active transport requires an expenditure of energy and facilitated
transport does not.
Describe
endocytosis, including pinocytosis and phagocytosis, and contrast it with
exocytosis.
Endocytosis:
brings material INTO the cell. A portion of the plasma membrane invaginates to
form a vesicle around the material. Then, the vesicle pinches off inside the
cell.
Three
kinds of endocytosis:
1.
Phagocytosis: when the material coming into the cell is large- like a molecule
of food or another cell (when old red blood cells are absorbed) this is common
in unicellular organisms like amoebas that engulf their food.
2.
Pinocytosis: happens when vesicles form around liquid or very small particles.
3.
Receptor mediated endocytosis: makes use of receptor proteins in the plasma
membrane. A specific substance binds to receptors that then gather in one
location before endocytosis happens.
Exocytosis:
brings material OUT of the cell. The [intracellular] vesicle fuses with the
plasma membrane so the contents can be released outside of the cell.