HOMEOSTASIS
In Biology, the term homeostasis refers
to the ability of a cell or organism to
maintain internal stability. Homeostasis is a
state of balance or steady state between a
living thing and its environment. Failure to
respond effectively to a failure of
homeostasis can result in disease or death.
In an individual eukaryotic cell, there are
a number of organelles that contribute to
the cell’s ability to maintain homeostasis.
Besides packaging and exporting
substances from the cell, the Golgi
apparatus creates lysosomes. Lysosomes
contain enzymes that break down wastes
and cellular debris. The endoplasmic
reticulum acts as a “highway” for moving
particles around a cell, but it also regulates
calcium concentration and helps with drug
detoxification. The nucleus is probably the
most important organelle for maintaining
homeostasis, since it contains the DNA
which directs the all the functions of a cell.
Multicellular organisms must maintain
homeostasis not only as individual cells, but
as an entire organism. In mammals, there
are mechanisms in place to keep variables
such as body temperature, blood glucose
levels, and blood pH within tolerable limits.
For humans, our blood pH has a normal
range of 7.35-7.45. That’s it - a range of 0.1
is all we can tolerate! If the pH falls below
6.8 or goes above 7.8, death is very likely.
Our body has a number of buffers
(chemicals) that can adjust the
pH of the blood as needed. The kidneys
also remove excess chemicals from the
blood. Excess CO2 in the blood causes
acidity, and the lungs remove CO2 from the
blood. As you can see, many systems work
together to maintain homeostasis! There
are many other variables our body must
regulate… how many can you think of?
In order to maintain these variables
within acceptable ranges, your body has
feedback mechanisms. A feedback
mechanism occurs when the level of one
substance influences the level of another
substance or the activity of another organ.
A simple example of a feedback mechanism
is when you exercise. Your cells need more
oxygen when you
exercise. When the
oxygen levels in
your blood drop,
your heart will
pump faster and
you will also
breathe faster
and/or deeper in
order to supply the
oxygen needed.
Another
example of homeostasis is
thermoregulation - our ability to maintain a
steady body temperature. Do you know
what feedback mechanisms help regulate
our body temperature?
Most feedback mechanisms are negative
feedback mechanisms. This means that
when our body temperature goes up, our
body tries to bring it back down (the
opposite direction). Since the body brings
the temperature in the opposite direction,
it’s called a negative feedback mechanism.
Even plants maintain homeostasis. A
response mechanism in plants is the ability
to open tiny holes in their leaves called
stomata. Opening the stomata allows gases
to be exchanged, but also allows water to
evaporate. When water is limited, the plant
can close the stomata and maintain water
homeostasis.
Name:_____________________________Period:_________
Biology I
Homeostasis Questions
1. What is homeostasis? ________________________________________________
_________________________________________________________________
_________________________________________________________________
2. Name an organelle that helps the cell maintain homeostasis:______________________
How does it help maintain homeostasis? ___________________________________
_________________________________________________________________
_________________________________________________________________
3. Human red blood cells eject their nucleus before they begin circulating in your bloodstream
and also lack organelles. They are also among your body’s shortest lived cells, living only 100120 days (the average is 7-10 years for most of your cells). Explain why red blood cells are
short-lived. _________________________________________________________
_________________________________________________________________
_________________________________________________________________
_________________________________________________________________
4. The article mentions variables such as body temperature, blood glucose levels, and blood
pH. These all need to be maintained at constant levels. List at least three other variables your
body regulates:
_________________________________________________________________
_________________________________________________________________
_________________________________________________________________
5. What is a feedback mechanism?_________________________________________
_________________________________________________________________
_________________________________________________________________
6. What feedback mechanisms help you regulate your body temperature?_______________
_________________________________________________________________
_________________________________________________________________
_________________________________________________________________
7. Why can’t plants just keep their stomata closed? Then they wouldn’t have to worry about
losing too much water. Explain why they have to be open: _________________________
_________________________________________________________________
_________________________________________________________________
8. Write a conclusion paragraph for this article. It seems to be missing.
_________________________________________________________________
_________________________________________________________________
_________________________________________________________________
_________________________________________________________________
_________________________________________________________________
Name:_____________________________Period:______
Biology I Honors
Homeostasis Questions
1. What is homeostasis? ________________________________________________
_________________________________________________________________
_________________________________________________________________
2. Name an organelle not mentioned in the article that helps the cell maintain
homeostasis:______________________How does it help maintain homeostasis?______
_________________________________________________________________
_________________________________________________________________
_________________________________________________________________
3. Human red blood cells eject their nucleus before they begin circulating in your bloodstream
and also lack organelles. They are also among your body’s shortest lived cells, living only 100120 days (the average is 7-10 years for most of your cells). Explain why red blood cells are
short-lived. _________________________________________________________
_________________________________________________________________
_________________________________________________________________
_________________________________________________________________
4. The article mentions variables such as body temperature, blood glucose levels, and blood
pH. These all need to be maintained at constant levels. List at least three other variables your
body regulates:
_________________________________________________________________
_________________________________________________________________
_________________________________________________________________
5. What is a feedback mechanism?_________________________________________
_________________________________________________________________
_________________________________________________________________
6. What feedback mechanisms help you regulate your body temperature?_______________
_________________________________________________________________
_________________________________________________________________
_________________________________________________________________
7. Differentiate between a negative feedback mechanism and a positive feedback mechanism:
_________________________________________________________________
_________________________________________________________________
_________________________________________________________________
_________________________________________________________________
_________________________________________________________________
8. When your blood glucose gets too high, your pancreas releases insulin to help bring the
levels down. Is this an example of a negative feedback mechanism? ________ Explain:____
_________________________________________________________________
_________________________________________________________________
9. Blood clotting is initiated when injured tissue releases signal chemicals that activate platelets
in the blood. An activated platelet releases chemicals to activate more platelets, causing a rapid
cascade and the formation of a blood clot. Is this an example of a negative feedback
mechanism?____________ Explain:______________________________________
_________________________________________________________________
10. Is positive feedback a mechanism for maintaining homeostasis? ________ Explain your
answer:___________________________________________________________
_________________________________________________________________
_________________________________________________________________
_________________________________________________________________
_________________________________________________________________
_________________________________________________________________
11. Why can’t plants just keep their stomata closed? Then they wouldn’t have to worry about
losing too much water. Explain why they have to be open: _________________________
_________________________________________________________________
_________________________________________________________________
12. Write a conclusion paragraph for this article. It seems to be missing.
_________________________________________________________________
_________________________________________________________________
_________________________________________________________________
_________________________________________________________________
_________________________________________________________________
_________________________________________________________________
_________________________________________________________________
_________________________________________________________________
Questions