2.D: Growth and dynamic
homeostasis are influenced by
changes in the environment
Organisms Respond to External Events
• I.e. taxis or kinesis
– Many animals change
speed based on light
(pillbugs run wild
when exposed to
light)
– Many animals change
direction in response
to stimuli (sharks
swim towards blood)
• Plants grow towards light
(phototropism)
Biotic vs. Abiotic Factors
Biotic = Living
Abiotic = Non-living
• Animals
• Plants
• Fungi
• Bacteria/
Archaeans
• Sunlight
• Water
• Vitamins/
Minerals/
Nutrients
• Temperature
Cells Are Affected by Both Biotic/Abiotic Factors
• i.e. bacterial cells
attach to each other
to form biofilms
• Cells shrink/grow
depending on water
availability
• Cells stop growing
after a certain density
(except in the case of
cancers)
Organisms Interact with Biotic/Abiotic Factors
• Organisms access
to nutrients is
affected by other
factors
• i.e. symbiosis,
predator-prey,
nutrient
availability etc
Symbiosis-Two species living together
• Mutualism-both
benefit
• Commensalism- one
benefits, other
unaffected
• Parasitism- one
benefits at other’s
expense
"live microbial feed
supplement which
beneficially affects
the host animal by
improving its
intestinal microbial
balance".
Abiotic Effects on Growth
• Better
nutrition Taller
individuals
• More sunlight 
More plant
growth
Evolution of Homeostatic Mechanisms
• Some mechanisms
are conservedshowing common
ancestry
• Others have
changed,
demonstrating
divergent
evolution
i.e. nitrogenous waste removal
• All animals produce
nitrogen byproducts
• Animals in different
environments have
evolved different
mechanisms for
excreting these
wastes
Ammonia
• Toxic byproduct of
protein
degradation
• In dilute
concentrations is
no big deal, so
aquatic organisms
can excrete it into
water
Urea
• Mammals convert
ammonia into urea
• Urea travels through
the blood to the
kidneys and is
excreted with water
and other
substances as urine
Solid Uric Acid
• In birds and reptiles
(and some desert
mammals) nitrogen
waste is excreted as
a solid
• Prevents water loss
and buildup of water
wastes in the shell of
an egg
Evolution of Circulatory System Shows
Common Ancestry and Divergence
Evolution of Circulatory System Shows
Common Ancestry and Divergence
Disruptions at the Molecular/Cellular
Level Affect Organisms
• I.e. viral
infection of a
cell
• Allergies
• Dehydration
Disruption of One Organisms Affects
Others
• I.e. disrupting the
food chain
• If you break one
link of the chain,
everyone is
affected
• Toxins in one
organism get
passed on
Biomagnification
• Toxins get
concentrated as they
move up the food
chain
• So small amounts of
mercury can become
extremely toxic to
polar bears (or
humans!)
Invasive Species
• Usually have no
natural predators
and grow
uncontrollably
• Disrupt the food
chain and
outcompete native
species