Feedback Systems
Feedback Loop Video
Fig. 1-13
Negative
feedback 
A
Enzyme 1
Negative Feedback
• Most common form of
regulation
• Accumulation of end product
slows the process
B
D
Excess D
blocks a step
D
Enzyme 2
D
C
Enzyme 3
D
(a) Negative feedback
W
Enzyme 4
Positive Feedback
• End product SPEEDS UP
production of that product
• Blood clotting
• Childbirth
• The idea is for the process to
go to completion
Positive
feedback +
X
Enzyme 5
Excess Z
stimulates a
step
Z
Y
Z
Z
Enzyme 6
Z
(b) Positive feedback
Fig. 8-22
Initial substrate
(threonine)
Feedback Inhibition
• Metabolic pathway
• Switched off by inhibitory
binding of end product to an
enzyme earlier in the pathway
of production
Active site
available
Isoleucine
used up by
cell
Threonine
in active site
Enzyme 1
(threonine
deaminase)
Intermediate A
Feedback
inhibition
Isoleucine
binds to
allosteric
site
Enzyme 2
Active site of
enzyme 1 no
longer binds Intermediate B
threonine;
pathway is
Enzyme 3
switched off.
Intermediate C
Enzyme 4
Intermediate D
Enzyme 5
End product
(isoleucine)
Fig. 40-8
Response:
Heater
turned
off
Nonliving Example of What
type of Feedback to maintain
homeostasis
Room
temperature
decreases
Stimulus:
Control center
(thermostat)
reads too hot
Set
point:
20ºC
Stimulus:
Control center
(thermostat)
reads too cold
Room
temperature
increases
Response:
Heater
turned
on
Fig. 40-16
Sweat glands secrete sweat,
which evaporates, cooling the
body.
How is this an example of
feedback?
Is it positive or negative
feedbac
Thermostat in hypothalamus
activates cooling mechanisms.
Blood vessels in
skin dilate:
capillaries fill;
heat radiates from
skin.
Increased body
temperature
Body temperature
decreases;
thermostat
shuts off cooling
mechanisms.
Homeostasis: Internal
temperature of 36–
38°C
Body temperature
increases; thermostat
shuts off warming
mechanisms.
Decreased body
temperature
Blood vessels in skin
constrict, reducing heat
loss.
Skeletal muscles contract;
shivering generates heat.
Thermostat in
hypothalamus
activates warming
mechanisms.
Fig. 40-17
External
environment
Animal
body
Organic molecules
in food
Digestion and
absorption
Heat
Energy lost
in feces
Nutrient molecules
in body cells
Carbon
skeletons
Cellular
respiration
Energy lost in
nitrogenous
waste
Heat
ATP
Biosynthesis
Cellular
work
Heat
Heat
Blood Sugar Levels
Example of Hormonal Feedback System
Test Review Info
Every Pyruvate produces
• 3 NADH
• 1 ATP
• 1 FADH2
• HOW MANY PYRUVATES DO YOU GET FROM 1
SUGAR MOLECULE?
Fig. 10-9
RESULTS
Chlorophyll a
Chlorophyll b
Carotenoids
(a) Absorption spectra
400
500
600
700
Wavelength of light (nm)
(b) Action spectrum
Aerobic bacteria
Filament
of alga
(c) Engelmann’s
experiment
400
500
600
700
Can you fill this out?
Photosynthesis
Organelle for Process
Reactants (needed material)
Cycle of Chemical Reactions
Electron Transport Chain
Products (what is made)
What is the structure of a Mitochondria?
What is the structure of a Chloroplast?
Cellular Respiration
Comparison of Photosynthesis &
Cellular Respiration
Photosynthesis
Cellular Respiration
Organelle for
process
Chloroplast
Mitochondrion
Reactants
CO2 and H2O
Sugars (C6H12O6) and O2
Cycle of chemical
reactions
Calvin cycle in stroma Krebs cycle in matrix of
of chloroplasts builds mitochondria breaks
sugar molecules
down carbon-based
molecules
Electron Transport
Chain
Proteins within
thylakoid membrane
Proteins within inner
mitochondrial membrane
Products
Sugars (C6H12O6) and
O2
CO2 and H2O