Download Tenlie Mourning November 20,2010 Investigation 5 Homeostasis is

Tenlie Mourning
November 20,2010
Investigation 5
Homeostasis is a collection of processes that maintain the constant stability of a cell. It is
important that these processes are carried out because a cell needs a constant state of stability to
most effectively carry out its other responsibilities in a living organism.
PASSIVE TRANSPORT:
As I said before, molecules need a way to get through the lipid bilayer. The easiest way
for a molecule to do this is a type of passive transport (movement of molecular substances across
the cell membrane) through diffusion (movement of molecules from an area of higher
concentration to an area of lower). Molecules are always moving around to try to reach a state of
equilibrium, which is when the concentration of molecules is equal on all sides of a cell. The
difference of conecentraion of molecules within a cell is called the concentration gradient. When
molecules diffuse they are moving to the top by bouncing off eachother and moving in random
straight lines. This movement is driven by kinetic energy generated through the constant state of
motion cells occupy.
Still, some cell membranes wont allow certain molecules to pass through it through
diffusion. This is dependent on the size and shape of the molecule. Molecules that are small but
cant diffuse across the membrane can go through the pores of the lipids.
OSMOSIS:
Just like anything else, cells need the hydration of water. Solutes are organic (sometimes
inorganic) compounds and solvent is water. The process of water molecules diffusing across a
cell membrane from an area of lower concentration to an area of higher concentration is called
osmosis (type of passive transport).
When the concentration of solute molecules outside of the cell is larger than the
concentration inside the cell, the cell is in a hypotonic environment. At this point, water will
diffuse inside the cell until equilibrium is reached. When the concentration of solute molecules is
higher inside the cell, the cell is in a hypertonic environment. In this case, water will diffuse out
until equilibrium is reached.
CELLS IN RELATION TO OSMOSIS:
The most common environment for a cell is hypotonic. Unicellular organisms function
this way all of the time, water is always diffusing into these organisms. Unicellular organisms
deal with excess water through osmosis. There are several ways for a cell to use osmosis.
Some use contractile vacuoles (organelles that remove excess water). These contractile
vacuoles expand and contract-removing the water. This process is not limited to unicellular
organisms as many multi-cellular organisms use this same operation. It is necessary for cells to
remove this water because this lowers the solute concentration in the cystol.
Plant cells that live in a hypotonic environment. Plant cells also use osmosis through
swelling until the cell membrane is pressed against the cell wall. The pressure that this process
creates is called turgor pressure. Cells shrink away from the cell wall and the turgor pressure is
lost.
Some cells aren’t equipped with the tools used to deal with hypotonic environments. For
example, human blood cells. When cells like this are put into hypotonic environments, they
become distorted. Water diffuses into the cell, which eventually causes it to burst. When cells
like this are put into hypertonic environments, too much water leaves the cell, causing it to shrink
and shrivel.
FACILITATED DIFFUSION:
Facilitated diffusion is a type of active transport (the counter to passive transport, its
when cells need assistance to move across a membrane) is for cells that can diffuse rapidly
through cell membranes. This could be because they are not soluble enough to pass through
lipids but too big to go through pores. In facilitated diffusion, molecules get through membranes
through carrier proteins. These carrier proteins take molecules through the concentration
gradient. The cell doesn’t need extra energy for this.
In this process the carrier proteins pick up molecules, change shape to shield them and
then carry them to the other side of the cell.
DIFFUSION THROUGH ION CHANNELS:
Another type of passive transport is ion channels. Ions such as sodium, potassium,
calcium, and chloride are important for a variety of cell functions. Ions can’t diffuse across
membranes without help. Some of these channels are always open, while some regulate which
ions pass through-through responding to three types of stimuli. Stimuli control the ability of ions
to cross through membranes. These stimuli are:
1. stretching the cell membrane
2. electric signals
3. chemicals in the cystol or external environment.
SODIUM POTASSIUM PUMPS:
So far I have only really talked about the way transport works in terms of plant and
unicellular organisms. However in animal cells, active transport is almost always used. An
example of active transport in animal cells is Sodium potassium pumps. To function, animals
must have higher concentrations of sodium and potassium. Sodium-potassium pumps keep these
levels high. This process works like this:
1. 3 sodium ions bond to the carrier protein
2. carrier protein slips molecule phosphate group and ATP.
a. splitting ATP supplies protein energy to change shape to protect molecule
3. protein carries sodium
4. protein picks up and carries potassium
5. protein changes shape again, releasing potassium into cell.
This process is important because it creates the electrical gradient. This is when the outside
of the membrane is positively charged and the inside negatively charged. This is important to
the electrical conduction across nerves.
ENDOCYTOSIS:
This is another function of cells in homeostasis. Endocytosis is the process of cells
ingesting external materials. These can include, other large cells, macromolecules and nutrients.
These external materials are enclosed in pouches. The pouches will attach to the membrane of
other cells. Once the pouch is attached it becomes a vesicle. These vesicles are ingested by
lysosomes, while others fuse with other organelles.
There are two types of endocytosis:
1. Pinocytosis: the ingestion of liquid or fluid.
2. Phagocytosis: movement of other large cells.
EXCYTOSIS
Excocytosis is the opposite process of endocytosis. In this process, extra, unneeded, cells
fuse with the inner part of the membrane and release the extra proteins into the cells external
environment.
Homeostasis is a crucial part of a cells survival. It is the process of the movement of the
things most vital to the life of a cell, thus the life of an organism.