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Endocrine & Cell Communication Part IV: Maintaining Balance (Homeostasis) TEACHER NOTES needs coding
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Endocrine & Cell Communication Part IV:
Maintaining Balance (Homeostasis)
AP Biology Curriculum Framework
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EK 3.D.2 Cells communicate with each other through
direct contact with other cells or from a distance via
chemical signaling.
c. Signals released by one cell type can travel long
distances to target cells of another cell type.
1. Endocrine signals are produced by endocrine
cells that release signaling molecules, which are
specific and can travel long distances through the
blood to reach all parts of the body.
illustrative example-insulin
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When you ingest carbohydrates your blood
glucose level rises, which stimulates your
pancreas to secrete insulin which in turn
promotes cellular uptake of glucose into the liver
and muscle cells where it is stored as glycogen.
When your blood sugar level decreases between
meals, the pancreas secretes glucagon which
promotes the hydrolysis of glycogen to release
glucose and fatty acids to raise your blood sugar
levels.
Simple Hormone Pathways
• Hormones are released from an endocrine cell, trave
through the bloodstream, and interact with specific
receptors within a target cell to cause a physiological
response
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Simple Hormone Pathways
• For example, the release of acidic contents of the
stomach into the duodenum stimulates endocrine
cells there to secrete secretin.
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• This causes target cells in the pancreas, a gland
behind the stomach, to raise the pH in the duodenum
• The increased pH results in a decrease of secretin
secretion.
Simple Hormone Pathways
Example
Stimulus
Low pH in
duodenum
Endocrine
cell
Negative feedback
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Pathway
S cells of duodenum
secrete the hormone
secretin ( ).
Hormone
Target
cells
Response
Blood
vessel
Pancreas
Bicarbonate release
The pancreas releases sodium bicarbonate to
raise the pH which neutralizes acid chyme from
the stomach thereby raising the pH (making the
environment more alkaline).
In this simple endocrine pathway a low
duodenum pH stimulates endocrine cells in the
small intestine (S cells) to secrete the hormone
secretin. Secretin travels through the blood
stream to its target cells (pancreatic cells)
causing them to release bicarbonate solution
resulting in an increase in the pH. The increase
serves as a negative feedback mechanism
resulting in lower levels of secretin released.
Endocrine & Cell Communication: Maintaining Balance (Homeostasis) TEACHER NOTES needs coding
Negative Feedback
• Secretin secretion regulation is an example of
negative feedback in action.
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Feedback Regulation
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• A negative feedback loop inhibits a response by
reducing the initial stimulus, thus preventing excessiv
pathway activity.
• Positive feedback reinforces a stimulus to produce an
even greater response.
• For example, in mammals oxytocin causes the release
of milk, causing greater suckling by offspring, which
stimulates the release of more oxytocin.
An example of positive feedback
Oxytocin stimulates the uterus to contract. This
causes the placenta to make more prostaglandins
which signal more vigorous uterine contractions
which cause more oxytocin to be produced
thereby amplifying the contraction process.
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Insulin and Glucagon: Control of Blood Glucose
• Hormones work in pairs to maintain homeostasis.
• Insulin (decreases blood glucose) and glucagon
(increases blood glucose) are antagonistic hormones
that help maintain glucose homeostasis.
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• The pancreas has clusters of endocrine cells called
pancreatic islets with alpha cells that produce
glucagon and beta cells that produce insulin.
Endocrine & Cell Communication: Maintaining Balance (Homeostasis) TEACHER NOTES needs coding
Figure 45.13
Insulin
Body cells
take up more
glucose.
Beta cells of
pancreas
release insulin
into the blood.
Blood glucose
level declines.
Liver takes
up glucose
and stores it
as glycogen.
STIMULUS:
Blood glucose level rises
(for instance, after eating a
carbohydrate-rich meal).
Homeostasis:
Blood glucose level
(70–110 mg/100mL)
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STIMULUS:
Blood glucose level
falls (for instance, after
skipping a meal).
Blood glucose
level rises.
Liver breaks
down glycogen
and releases
glucose into
the blood.
Alpha cells of pancrea
release glucagon into
the blood.
Describe the actions that occur when blood
glucose levels decline and when they rise.
Glucagon and insulin are paired hormones that
work together to maintain blood glucose levels
between 70 and 110 mg/100mL
Glucagon
AP Curriculum Framework
• EK 3.D.4 Changes in signal transduction
pathways can alter cellular response.
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– A. Conditions where signal transduction is blocked
or defective can be deleterious, preventative or
prophylactic.
• Illustrative example - diabetes
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Out of Balance: Diabetes Mellitus
• Diabetes mellitus is perhaps the best-known
endocrine disorder.
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Ask students to explain how a lack of insulin
leads to elevated levels of glucose in the blood.
Then ask them to suggest reasons this increased
level of glucose is harmful to the person with
diabetes.
• It is caused by a deficiency of insulin or a decreased
response to insulin in target tissues.
• It is marked by elevated blood glucose levels.
Out of Balance: Diabetes Mellitus
• Type 1 diabetes mellitus (insulin-dependent) is an
autoimmune disorder in which the immune system
destroys pancreatic beta cells.
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Type 1 has in the past been referred to as
Juvenile Diabetes. Just as a point of interest, the
incidence varies from 8 to 17 per 100,000 in
Northern Europe and the U.S. with a high of
about 35 per 100,000 in Scandinavia to a low of
1 per 100,000 in Japan and China.
• Type 2 diabetes mellitus (non-insulin-dependent)
involves insulin deficiency or reduced response of
target cells due to change in insulin receptors.
Action of Insulin
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When insulin receptors respond properly to the
presence of insulin, the result is the transport of
glucose from outside the cell to inside the cell
via transport protein. People with Type I diabetes
do not produce sufficient insulin to maintain a
proper level of glucose transport. The disorder is
typically treated by providing the patient with
insulin.
Endocrine & Cell Communication: Maintaining Balance (Homeostasis) TEACHER NOTES needs coding
Insulin & Glucose Regulation
Scroll across the bottom of the to activate the
animation controls and press PLAY
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Increases Ca2+
uptake in
intestines
Active
vitamin D
Stimulates Ca2+
uptake in kidneys
PTH
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Stimulates
Ca2+ release
from bones
Parathyroid
gland (behind
thyroid)
STIMULUS
Falling blo
Ca2+ leve
Blood Ca2+
level rises.
Blood calcium levels need to be approximately
10 mg/100 mL. Two hormones, PTH and
calcitonin work in tandem to regulate the blood
glucose in mammals.
Homeostasis:
Blood Ca2+ level
(about 10 mg/100 mL)
Homeostasis in blood calcium levels
• PTH increases the level of blood Ca2+
Describe how calcitonin and PTH work together
to maintain blood calcium levels. High calcium
– It also has an indirect effect, stimulating the kidne
to activate vitamin D, which promotes intestinal levels can cause mental confusion, nausea,
uptake of Ca from food.
fatigue. Low blood calcium causes muscle
Calcitonin decreases the level of blood Ca
– It stimulates Ca deposition in bones and secretio cramps, spasms, twitching and tingling in the
by kidneys.
fingers and around the mouth.
– It releases Ca2+ from bone and stimulates
reabsorption of Ca2+ in the kidneys.
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2+
•
2+
2+
Homeostasis In Blood Calcium Levels
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This animation has more detail than we actually
need. The next is illustrative of the amount of
detail students need to know. Emphasize it is the
“homeostasis” aspect of this process established
by cell to cell communication that is important.
Scroll across the bottom of the to activate the
animation controls and press PLAY
Endocrine & Cell Communication: Maintaining Balance (Homeostasis) TEACHER NOTES needs coding
Practice
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•
Blood calcium levels
•
Blood calcium level
•
Parathyroid release
•
Thyroid releases
calcitonin
•
If calcium rises abov
point
•
If calcium falls below
point
Ask students to match the events on the right
with the numbers in the picture. The next shows
the answers.
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Solution
1. Blood calcium levels r
3. Blood calcium level fa
5. Parathyroid releases P
2. Thyroid releases calci
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6. If calcium rises above
point
4. If calcium falls below
point
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Created by:
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Debra Richards
Coordinator of Secondary Science Programs
Bryan ISD
Bryan, TX
The two hormones, calcitonin and parathyroid
hormone work together to keep blood calcium
levels within a homeostatic range.(10 mg/100
mL)