Cells In Their Environment
Review Facts
• Solution: a uniform mixture of 2 or more
substances.
• Solute: the dissolved substance (salt)
• Solvent: the dissolving substance (water)
• When placed in water, NaCl will dissolve
into sodium and chloride ions.
• The mixture would be considered a
solution.
What Does This Have To Do
With Cells???
• All cells live at least partly in touch with a
water solution.
• To survive, cells must take in nutrients and
eliminate waste materials.
• To do this, nutrients and waste materials
must cross the cell membrane between the
solution outside of the cell, and the
solution inside the cell.
What Kinds of Membranes Are
There?
• Permeable membrane: membrane that
allows for almost any substance to
penetrate and pass through.
• Semi permeable membrane: allows certain
molecules to pass through and prevents
others from passing.
• Non permeable membranes: membranes
that allow no passage.
Semi Permeable Cell Membrane
• Small molecules like water can enter and leave
the cell freely.
• However, small particles with strong electrical
charge such as ions cannot pass easily through
the membrane.
• The electrical charge prevents the ion from
moving through the bilayer.
• Larger molecules such as proteins and
carbohydrates cannot enter and leave the cell
freely due to their size.
Transportation Processes (2)
• Passive Transport: movement of a
substance across a cell membrane without
the input of the cell’s energy.
• Active Transport: uses cellular energy to
move substances across the cell membrane.
Passive Transport (3 types)
• Simple Diffusion
• Facilitated Diffusion
• Osmosis
All three involve the movement of a
substance across a cell membrane without the
input of the cell’s energy.
Factors To Consider
• The rate of diffusion depends on the size
and temperature of the molecule involved.
• Molecules diffuse faster at a higher
temperature than at a lower temperature
• Smaller molecules are diffused more easily
than larger ones.
Factors To Consider (2)
• Diffusion always occurs down a concentration
gradient.
• A concentration gradient is the difference
between the concentration of a particular
molecule in one area and its concentration in an
adjacent area.
• When molecules have been dispersed evenly,
there is no concentration gradient, an equilibrium
has been reached, and diffusion stops.
Simple Diffusion
• Most common form of passive transport.
• The random movement of molecules from
an area of higher concentration (more
molecules) to an area of lower
concentration (fewer molecules).
• Enables oxygen and carbon dioxide to
cross the membranes.
Simple Diffusion
Initially, the concentration
of a substance outside the
cell is higher than the
inside.
Later, the substance has
diffused into the cell so
that the concentration is
the same on both sides of
the membrane.
If a lump of sugar is dropped into a beaker of
water, the molecules dissolve (A) and diffuse (B
and C).
Eventually, diffusion results in an even distribution
of sugar molecules throughout the water (D).
Facilitated Diffusion
• Molecules diffuse across a cell membrane with
the help of carrier proteins in the membrane.
• Always move down the concentration gradient
from a higher level of concentration to a lower
level.
• Increases the rate at which larger molecules such
as glucose can cross the cell membrane.
• Allows the glucose molecule into the blood cells.
Facilitated Diffusion
During facilitated
diffusion, carrier
proteins shuttle
molecules across
the lipid bilayer of
the cell
membrane.
Facilitated Diffusion
Osmosis
• The third form of passive transport.
• The diffusion of water across a semi permeable
membrane
• Occurs when the concentrations of solutions on
the 2 sides of a semi permeable membrane are
different.
• Again water moves down its concentration
gradient.
• Moves from solutions with higher water
concentration (higher solute) to solutions with
lower water concentration (lower solute).
Osmosis
Types of Solutions
• Hypertonic Solution: concentration of
solutes outside is higher than it is inside the
cell in the cytoplasm.
• Water diffuses out.
• Ex: lettuce in a bowl of salt water.
Types of Solutions (2)
• Isotonic Solution: concentration of solutes
outside the cell is equal to that found inside
the cell.
• Osmosis does not occur because solution is
said to be in equilibrium.
Types of Solutions (3)
• Hypotonic Solution: Solution in which the
concentration of solutes outside the cell is
lower than it is inside the cell.
• Water moves into the cell causing the cell
to swell.
Very Hypotonic Solution
• If the cell swells too much, it will burst due
the the large amount of water entering it.
• This is referred to as being a very
hypotonic solution.
• Distilled water is hypotonic because it
contains no solutes.
• When animal cells are placed in distilled
water, the cell bursts due to osmosis.
Solution Types
Active Transport
• Sodium Potassium Pump
Uses cellular energy to move substances
across the cell membrane.
Active Transport Factors
• Cells must use energy to move molecules across
a membrane against a concentration gradient
from areas of low concentration to areas of high
concentration.
• Involves carrier proteins as seen in facilitative
diffusion.
• Carrier proteins use energy to pump ions and
molecules across the membrane.
• The energy comes from ATP (adenosine
triphosphate)
The Sodium Potassium Pump
• Active transport is especially important in
maintaining the ion concentration in cells.
• Ex: Animal cells pump sodium ions out and
potassium ions into the cell.
• Results in high concentration of potassium ions
inside of the cell.
• High concentration of sodium ions outside of the
cell.
• Uses 1/3 of the cells available energy (active).
Sodium Potassium Pump
Sodium Potassium Pump
• Important for the contraction of muscles,
transmission of nerve impulses, and the
absorption of nutrients.
• Ex: plants use the pump in its roots to absorb
nutrients from the soil
• Nutrients are more concentrated in the roots than
in the surrounding soil itself.
• Without active transport, the nutrients would
diffuse out of the roots and back into the soil.
Bulk Movement
• The use of several processes to transport
large molecules such as polysaccharides or
proteins across its membrane.
• Large molecules move across the cell
membrane by being packaged in
membrane bound sacs.
Bulk Movement (2 types)
• Exocytosis
• Endocytosis
– Phagocytosis
– Pinocytosis
Exocytosis
• Wastes and cell products are packaged by
the golgi apparatus in sacs called golgi
vesicles.
• The vesicles then fuse with the cell
membrane and the materials in the vesicles
is secreted out of the cell.
• Ex: When we cry, tear glands use
exocytosis to secrete a salty solution
containing proteins.
Wastes
Golgi vesicle
Endocytosis
• The process by which a portion of the cell
membrane surrounds a desirable
macromolecule that is outside of the cell.
• The cell then pinches off the saclike
portion of its outer membrane to form a
tiny new vesicle.
• Membranous vesicle moves into the cell
where it can fuse with other organelles or
release its contents into the cytoplasm.
Desired macromolecules
Endocytosis (2 types)
• Pinocytosis (cellular drinking)
• Phagocytosis (cellular eating)
Pinocytosis
• Cell membrane
encloses a
product of fluid
and its solutes
and brings the
droplet into the
cell.
Phagocytosis
• The cell engulfs the food particle or solid
substance rather than a liquid.
• Many unicellular organisms such as
amoebas obtain food by means of
phagocytosis.
• Human white blood cells are phagocytes
that engulf and destroy bacteria.
Three tumor cells being engulfed by a white
blood cell!
The End