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?

AP Biology

Generalized Negative Feedback Model

high

low

hormone 1

lowersbody condition

hormone 2

gland

specific body condition

raisesbody condition

gland

AP Biology

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

AP Biology

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

AP Biology

Generalized Positive Feedback Model

high

hormone 1

raisesbody condition

gland

specific body condition

Or…

AP Biology

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

AP Biology

Cell SignalingSignals can be

stimulatory…

or inhibitory.

AP Biology

Cell Signaling Cell signaling (sometimes just called

“signal transduction”) has three general stages: Reception Transduction Response

AP Biology

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!)

AP Biology

Step 1 - Reception

Receptor proteins may be either: embedded in the cell membrane

Examples: G protein receptors, ligand-gated ion channels

AP Biology

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

AP Biology

Step 1 - Reception The ligand binding to the receptor

changes the receptor’s conformation (shape), which initiates the next step, transduction

AP Biology

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…

AP Biology

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

AP Biology

(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!

AP Biology

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

AP Biology

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!

AP Biology

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

AP Biology

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

AP Biology

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?

AP Biology

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

AP Biology

Phosphorylation Cascade

AP Biology

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