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Cell Membrane
Cell Membrane (cont’d)
• The conditions within a cell must remain
relatively constant at all times
• The process of maintaining this constant
internal environment is called homeostasis
• The role of the cell membrane is to control
the movement of materials in and out of the
cell
Cell Membrane (cont’d)
• The cell membrane controls the movement of
materials in and out through a number of different
processes
• The cell membrane is selectively permeable – that
is it allows certain items into the cell and certain
items out of the cell
• Water is the solvent on both sides of the
membrane that provides a medium for the
movement of particles across the cell membrane
Cell Membrane (cont’d)
• Many single celled organisms exist only in
water, while cell of multi-cellular organisms
are bathed in a thin layer of extra cellular
fluid – mainly water
Cell Membrane (cont’d)
•
Cell Membrane (cont’d)
Molecule Transport
• Two main categories of molecule transport
exist in cells,
• active transport and
• passive transport
Diffusion
• Is a method of passive transport where
small molecules pass through the cell
membrane.
• It can be defined as the movement
molecules from a region of high
concentration to a region of low
concentration
Diffusion (cont’d)
• Passive transport refers to the movement of
material across the cell membrane without the cell
expending (using) any of its energy
• Small molecules such as carbon dioxide and
oxygen pass easily in this manner
• This works because molecules are in constant
motion and this random motion causes molecules
to collide with one another
Diffusion (cont’d)
• Molecules of a substance will bounce around
when added to water, this is due to the water
molecules bouncing off the molecules
• This motion will move the molecules round the
water until the molecules of the substance are
evenly distributed
• The particles of the substance (molecules) are said
to follow a concentration gradient – that is they
move from a region of high concentration to a
region of low concentration
Diffusion (cont’d)
• Diffusion works well where the distances
are short and the molecules are small
• The cell is always using oxygen so oxygen
moves into the cell and the cell is always
producing carbon dioxide so it is always
moving out of the cell
• Both carbon dioxide and oxygen are
following the concentration gradient
Osmosis
• Osmosis is the diffusion of water across a cell
membrane
• It is important that the water concentration be kept
the same on both sides of the cell membrane
• If too much water enters or leaves the cell then
problems can arise
• Just like other substances, water moves with the
concentration gradient
Osmosis (cont’d)
• Is a form of passive transport because the cell does
not use any of its energy.
• The cell membrane can’t stop the process because
it is permeable to water.
• There are three conditions that can exist for
osmosis
–
–
–
–
Isotonic condition
Hypotonic condition
Hypertonic condition
Osmosis Video
Isotonic Condition
• This occurs when the concentration on
water inside the cell is equal to the
concentration of water outside the cell.
• There is an equal amount of water entering
and leaving the cell.
Hypotonic Condition
• When the concentration of water outside the
cell is greater than inside the cell.
• More water will move into the cell causing
it to swell.
• Animal cells may burst and plant cells will
become very rigid
Hypertonic Condition
• When the concentration of water is greater
inside the cell than outside.
• More water will migrate (move) out of the
cell than enter it.
• This will cause the cytoplasm of both plant
and animal cell to shrink.
Facilitated Diffusion
• Not all molecules that enter the cell are small
enough to pass easily through the cell membrane
• These molecules are facilitated or helped” across
the membrane
• Specialized and highly specific transport proteins
help these molecules enter the cell
• This process is still an example of passive transport
because it is the concentration gradient that causes
the molecules to move.
• This is how glucose enters the cell.
Active Transport
• In many instances the internal environment
of a cell must be vastly different from its
external environment. The cell must:
– Concentrate nutrients for maintenance and
growth
– Carry out any specialized functions that a cell
might have
– Many toxic waste products have to be
completely removed
Active Transport (cont’d)
• Passive transport will not be able to handle
all of these processes. In some instances the
cell must expend or use some of its own
energy to cause material to move in the
direction required.
• Active transport is required to move
materials AGAINST the concentration
gradient.
Active Transport (cont’d)
• Examples of specialized cells that must use
active transport are:
– Kidney cells pump glucose and amino acids out
of urine and back into the blood
– Intestinal cells pump in nutrients from the gut
– Root tip cells pump in nutrients from the soil
– Gill cells in fish pump out sodium
• Two basic types of active transport
– Exocytosis
– Endocytosis
• Exocytosis involves sending large
molecules (macromolecules) out of the cell
• Endocytosis involves bringing large
molecules (macromolecules) into the cell
Exocytosis
• Macromolecules don’t simple drift towards the
cell’s membrane and squeeze their way out
• They are packaged in a vesicle that separates them
from the rest of the cell
• The vesicle fuses with the membrane releasing the
cell contents to the outside
• The vesicle becomes incorporated into the
membrane
Endocytosis
• Two types
– Pinocytosis (cell drinking)
– Phagocytosis (cell eating)
• Pinocytosis – involves ingesting small
molecules or fluids surrounding the cell
• Phagocytosis – involves ingesting large
molecules (ie. Microroganisms or cell
debris)
Your Turn
• Pg. 61 # 1, 4 – 11, 13- 17
Surface area to volume
• in order to survive, cells must constantly interact
with their surrounding environment.
• Gases and food molecules dissolved in water must
be absorbed and waste products must be
eliminated.
• For most cells, this passage of all materials in and
out of the cell must occur through the plasma
membrane.
• Each internal region of the cell has to be served by
part of the cell surface.
• As a cell grows bigger, its internal volume enlarges and the
cell membrane expands.
• Unfortunately, the volume increases more rapidly than
does the surface area, and so the relative amount of surface
area available to pass materials to a unit volume of the cell
steadily shrinks.
• At some point, there is just enough surface available to
service all the interior; if it is to survive, the cell must stop
growing.
• If the cell grows beyond a certain limit, not enough
material will be able to cross the membrane fast enough to
accommodate the increased cellular volume.