1. Dynamic Equilibrium Constant small changes to help maintain homeostasis.

2. What does it all mean?  Let’s deconstruct the term “dynamic equilibrium”.  Dynamic means that it is always in motion. The opposite to dynamic is “static”.  Equilibrium means that it exists within a narrow range.  In other words, homeostasis and dynamic equilibrium are synomyms.

3. Graphing dynamic equilibrium  A common example of dynamic equilibrium in the human body is the maintaing of blood sugar levels.  After eating a meal, blood sugar levels increase.  In response to this, the body secretes insulin to remove sugar.  If the blood sugar level goes to low, glucagon is secreted which converts glycogen back into sugar.

4. Guard Cells in plants  Plants also need to maintain homeostasis.  The the bottom side of a plants leaf are small openings known as stomates.  The stomate is a pore through which excess water vapor is released by the plant.

5. Guard cells cont.  On either side of the stomate are two specialized cells called guard cells.  During dry times, the guard cells change shape and close over the stomate.  With the stomate closed, the plant retains more water.  If there is enough water present, the guard cells revert to their original shape and the stomate remains open.

6. Feedback mechanisms  There are two types of feedback mechanism, negative and positive.  Negative feedback mechanism: a mechanism of the endocrine system that chemically corrects an imbalance; called “negative feedback” because when the level of the imbalance decreases, the feedback stops the correcting effects.  The pancreas regulating blood sugar level is an example of a negative feedback mechanism.  Negative feedback mechanisms promote stability.

7. Feedback mechanism cont.  Positive feedback mechanism: A mechanism that enhances the process that causes the mechanism in the first place  These do not promote stability.  Much rarer in living organisms.  Childbirth is an example of a positive feedback mechanism. The childs head pressing on the uterus causes a contraction, which increases pressure which leads

8. Regents questions on dynamic equilibrium  What usually results when an organism fails to maintain homeostasis?  (1) Growth rates within organs become equal. (2) The organism becomes ill or may die. (3) A constant sugar supply for the cells is produced. (4) The water balance in the tissues of the organism stabilizes.

9. Question 2  If a human system fails to function properly, what is the most likely result?  (1) a stable rate of metabolism (2) a disturbance in homeostasis (3) a change in the method of cellular respiration (4) a change in the function of DNA

10. Question 3  An increase in the level of insulin in the blood would most directly result in  (1) a decrease in the amount of glucose in the blood (2) a decrease in the amount of protein in the blood (3) an increase in the amount of fat in cells (4) an increase in the amount of carbon dioxide in cells

11. Question 4  Which statement describes a feedback mechanism involving the human pancreas?  1 The production of estrogen stimulates the formation of gametes for sexual reproduction. 2 The level of oxygen in the blood is related to heart rate. 3 The level of sugar in the blood is affected by the amount of insulin in the blood. 4 The production of urine allows for excretion of cell waste

12. Question 5  Negative feedback mechanisms are best described as processes that help  (1) reduce hormone levels to below normal in the blood (2) destroy hormones in the blood (3) directly control muscle contraction in the leg (4) keep body conditions near a normal, steady state