Post on 06-Jan-2016
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AP Biology
Cell Communication and
Homeostasis
AP Biology
Dynamic Homeostasis
What is (dynamic) homeostasis?
Homeostasis = The property of a system that regulates its internal environment to maintain stable, (relatively) constant conditions In living things, often termed “dynamic
homeostasis” - what do you figure this indicates?
Feedback Control Homeostasis is often
maintained through the use of feedback systems (or loops).
A feedback system uses the consequences of the process (too much or too little produced) to regulate the rate at which the process occurs Consists of a sensor, a
control center, and an effector pathway
Positive vs Negative
Feedback loops may be positive or negative Negative feedback mechanism:
Maintains homeostasis by returning a changing condition back to its stable target point Discussion: although there are negative
and positive operons, both types are a negative feedback mechanism - why?
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Generalized Negative Feedback Model
high
low
hormone 1
lowersbody condition
hormone 2
gland
specific body condition
raisesbody condition
gland
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Controlling Body Temperature
high
low
nerve signals
sweat
nerve signals
body temperature(37°C)
shiver
dilates surfaceblood vessels
constricts surfaceblood vessels
Nervous System Control Feedback
hypothalamus
hypothalamus
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liver
pancreas
liver
Regulation of Blood Sugar
blood sugar level(90mg/100ml)
insulin
body cells takeup sugar
from blood
liver storesglycogen
reducesappetite
glucagon
pancreas
liver releasesglucose
triggershunger
high
low
FeedbackEndocrine System Control
islets of Langerhans beta islet cells
islets of Langerhansalpha islet cells
Positive vs Negative
Alterations in negative feedback mechanisms -> deleterious consequences
Discussion: People who are diabetic produce minimal insulin. What effect does this have on the blood sugar control feedback loop?
Positive vs Negative Positive feedback mechanism: Does not maintain homeostasis; instead, amplifies responses and processes, moving the system further and further away from starting conditions. Example: labor in childbirth
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Generalized Positive Feedback Model
high
hormone 1
raisesbody condition
gland
specific body condition
Or…
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Generalized Positive Feedback Model
low
hormone 1
lowersbody condition
gland
specific body condition
Discussion Describe a positive feedback loop in the case of
asthma, taking into account variables such as: airway swelling/narrowing aiway irritation blood oxygen levels cortisol increasing heart & breathing rates lung oxygen content nervous system recognition of blood oxygen levels oxygen available to brain panic release of stress hormones such as cortisol
AP Biology
Cell Signaling
AP Biology
Cell Signaling Every feedback loop in an organism that we
discussed, positive or negative, has one thing in common: cell signaling.
In a multicellular (and even unicellular!) organism, recognizing and responding to changes, internal or external, requires cell-to-cell communication
Cells do this by generating, transmitting, and receiving chemical signals
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Cell SignalingSignals can be
stimulatory…
or inhibitory.
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Cell Signaling Cell signaling (sometimes just called
“signal transduction”) has three general stages: Reception Transduction Response
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Step 1 - Reception Reception
Signaling begins with the recognition of a chemical messenger by a receptor protein Chemical messenger = a ligand
Different receptors “recognize” different ligands due to fit, in a one-to-one relationship (think enzymes!)
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Step 1 - Reception
Receptor proteins may be either: embedded in the cell membrane
Examples: G protein receptors, ligand-gated ion channels
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Step 1 - Reception or:
in the cytoplasm or even nucleus In these cases, a
hydrophobic ligand diffuses into the cell
Examples: steroid hormones, nitric oxide
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Step 1 - Reception The ligand binding to the receptor
changes the receptor’s conformation (shape), which initiates the next step, transduction
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Step 2 - Transduction Signal transduction is the process
by which a signal is converted to a cellular response. The activated receptor affects
another molecule, which affects another, which affects another…
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Step 2 - Transduction When the receptor protein changes
conformation, it may… Serve as an enzyme Open up a channel between cell interior and exterior
(like ion channels in neurons!) Release a polypeptide from itself into the cytoplasm
…which is the first in what will be a series of chemical reactions, largely involving proteins…
…but using at least one small, non-protein second messenger. Common second messengers: ions (Ca2+), cAMP
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(Remember cAMP?) Fun fact!
Glucose high = cAMP low ring a bell? That’s actually because of one such
multistep process! When glucose passes into the cell, one
step in the process involves inhibiting adenylate cyclase, an enzyme which otherwise is busy producing the second messenger cAMP!
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Step 2 - Transduction Signal transduction = efficiency! Due to signal amplification: some
steps in transduction activate multiples of the next step
So, a single ligand can trigger a large response
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Benefits of a 2° messenger system
Amplification!
signal
receptor proteinActivated adenylyl cyclase
amplification
amplification
amplification
amplification
GTP G protein
product
enzyme
protein kinase
cAMP
Not yetactivated
1
2
4
35
6
7
FAST response!
amplification
Cascade multiplier!
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Step 3 - Response End results could be
producing or destroying transcription factors (turns genes on/off)
activating enzymes cytoskeleton rearrangement many more!
http://bcs.whfreeman.com/thelifewire/content/chp15/15020.html
AP Biology
Signal Transduction Signal transduction diagrams can
follow some slightly different conventions, but common ones are: A stimulates B A inhibits B Translocation/Relocation
B to C is a larger (amplified) response than A to B
A
A
A
B
B
B C
A
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Signal Transduction A and B subunits join to make C
A separates into subunits B and C
Multistep pathway from A to B with some steps not shown
B
B
B
A
A
A
C
C
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Discussion Consider this very simple diagram of a
signal cascade (bigger image on next slide), and answer: What’s happening? What is the ligand?
What is the second messenger (hint: not necessarily named)? Does EGF trigger or inhibit gene regulation?
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AP Biology
Signal Transduction That example displayed a common signal
transduction method: a phosphorylation cascade A series of protein kinases adding a phosphate
group to the next protein in the sequence (remember kinase = “activator”)
Reception
Transduction
Response
mRNANUCLEUS
Gene
P
Activetranscriptionfactor
InactivetranscriptionfactorDNA
Phosphorylationcascade
CYTOPLASM
ReceptorGrowth factor
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Phosphorylation Cascade
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Cell Signaling Specificity
Which receptors and secondary messengers a cell possesses determines which signals it will respond to, and how This is why a liver and a
heart cell will do two different things when activated by the same hormone, like epinephrin