Homeostasis Lesson:
The machinery of life requires suitable biochemical conditions
• Living things are biochemical machines that need resources to run: life is a coordinated
symphony of chemical reactions
• The biochemistry of life is mediated by enzymes: Proteins that speed the rate of the
chemical reactions of life
o Enzymes speed the rate of reactions by providing an assist to the
chemicals
iomolecules involved
▪ Enzymes bind reactants and
ing them into the optimal orientation to
eact,
eak apart, whatever
o Without enzymes, the biochemical reactions would still occur but far too slowly
to drive life as it exists today
• Enzymes require particular physicochemical conditions to maintain their 3D structure
properly
• Living things need a fairly constant internal environment for their biochemistry to
function properly
The internal conditions of life
• What kinds of conditions must be maintained?
o Anything that impacts biochemistry/enzyme function must be maintained within
an appropriate range
▪ Examples:
• Molecule concentration/water balance (hydration)
• Kinetic energy of molecules (level of motion = temperature)
• Acid-base balance (pH)
• How constant does the internal environment need to be?
o It depends: organisms can often tolerate greater ranges for some variables, but
only na
ow ranges for others, and different organisms are adapted to different
conditions and ranges.
o The key point is that organisms can only tolerate a certain range for any given
variable: stress or even death will occur outside of this range
• Living things must maintain homeostasis (“same conditions”): a constant suitable
internal environment for their biochemistry.
Maintaining the internal conditions of life
• Homeostasis (from Greek, “standing the same”) refers to the maintenance of the
internal environment for proper biochemical and physiological function, at both the
cellular and organismal levels.
o Proper control of internal goods, water levels, temperature, pH...
o Proper control of internal biochemical conditions
• Homeostatic control relies on negative feedback loops: processes that are inhibited by
their own outcomes, resulting in self-regulating systems
o A change in the system elicits a response that opposes or resists that the same
change, thus keeping the output near a set point
o Examples:
▪ Body temperature goes up --> increased temperature elicits sweating -->
ody temperature goes down
▪ Body temperature goes up --> decreased temperature elicits shivering -->
ody temperature goes up
▪ Hydration level increases --> thirst is suppressed while the kidneys dump
more water --> hydration level decreases
▪ Hydration level decreases --> thirst is provoked while the kidneys retain
more water --> hydration level increases
• Negative feedback regulation in response to increased body temperature in humans:
o Increased body temperature causes the hypothalamus to trigger increases
sweating and blood vessel dilation, thus increasing the rate of heat dissipation so
ody temperature will fall.
Signaling feedbacks
• Some feedbacks are simple and highly specific to one process
o Example: the product of a metabolic pathway directly inhibits the enzymes or
genes involved in the pathway, so when the product level rises the pathway is
inhibited and when the product level falls the pathway speeds up
• Some feedbacks are complex and coordinate multiple processes
o Example: maintaining body temperature in humans involves the coordinated
esponses of sweat glands, muscles, blood vessels, heart rate,
eathing rate,
cellular respiration, metabolic responses in
own fat...
o Complex, multi-system responses usually involve communication between
processes, coordinated by some sort of a signal
• Signal control example: constraining blood sugar levels in humans
• The pancreas produces both the hormones insulin and glucagon
o Insulin: signals the liver and body cells to take. up sugar from the blood stream
o Glucagon: signals the liver to release more sugar into the blood stream
Hormonal signaling
• Hormones are long-distance signaling molecules in multicellular organisms that are
important in driving and coordinating cell activities and responses to the environment
(including homeostasis)
o Hormones represent a fairly long-term communication system: once produced
hormones tend to persist a while
o Hormones are typically ca
ied rapidly throughout the organism via its bulk flow
transport system (I.e., in the fluids)
o In the case of animals, hormones are produced by the endocrine system
▪ Glands and tissues secrete hormones that are ca
ied by the circulatory
system to signal cells throughout the body
▪ Endocrine = “secreted within”
• Hormones evoke targeted responses in key cells
o May act at the surface by binding a cell-surface receptor
o Hydrophobic (nonpolar) hormones may pass through the cell mem
ane into the
cell, where they can initiate responses such as turning on a transcription factor
(something that binds DNA to turn a gene on or off)
Kinds of hormones
• There are three main kinds of hormones: amines, peptides, and steroids
o Amines: derived from amino acids
▪ Must be built specially as a part of a dedicated metabolic pathway
▪ Hydrophilic: soluble in water, but not so much lipids
▪ Usually have names ending in –ine
o Peptides: short amino acid chains; essentially very short simple proteins
▪ Can be built simply by stringing together amino acids in different
combinations, as cells already do to build proteins
▪ Hydrophilic: soluble in water, but not really in lipids
▪ Frequently have names ending in –in
o Steroids: ring-structure molecules
▪ Must be built specifically as a part of a dedicated metabolic pathway
▪ Hydrophobic: soluble in lipids, but not so much in water
▪ Usually have names ending in –ol or –one
• What kind of hormone can evolve most rapidly?
o A peptide hormone.
o Because a peptide is a polymer that consists of a sequence of amino acid
uilding blocks, simply swapping out a single amino acid is enough to alter the
overall shape and structure of the hormone.
• Which hormone is likely to have the longest impact in terms of a response?
o One that acts inside the cell as a transcription factor, to turn certain genes on or
off
• Which kind of hormone is most likely to pass into the cell and act intracellularly, as
opposed to binding to a cell surface receptor?
o Steroid hormone
o Steroid hormones are hydrophobic biomolecules, which allows them to pass
through the hydrophobic interior of the cell’s lipid mem
ane with no problem—
no need for a channel opening, even.
• Amines and peptides bind their target cells at the cell surface, while steroids pass
through the cell mem
ane to bind receptors inside the cell
Hormonal responses vary between species
• Hormones are very ancient, with the basic hormone structures being found across all
life
• Because hormone structures are ancient, the same or highly similar hormones are often
found across different
anches of life... but they may use the same hormones in very
different ways
o Different species can respond wildly differently to the same hormones
o Example: thyroid-stimulating hormone (TSH)
▪ Mammals: TSH regulates metabolism via the thyroid
▪ Amphibians: TSH triggers metamorphosis
▪ Birds: TSH triggers feather molt
▪ Snails: TSH influences the number of eggs and sperm produced
Hormones often signal many responses
• Cells “hear” chemical messages via chemoreceptors: binding triggers the response
pathway
o No active chemoreceptor = no response
o Different chemoreceptors usually trigger different response pathways
o Different chemoreceptors may have different binding affinities = different
sensitivities to the same signal
• A single cell may have multiple active vs. Inactive chemoreceptors of different types and
varying sensitivities
• Different cells may respond to a signal differently, and even a single cell may exhibit
different responses at different times
o Depends on which chemoreceptors are active, and how sensitive they are
▪ Other hormones and their response pathways may influence which are
active
▪ Low hormone levels only trigger the most sensitive chemoreceptors;
when hormone levels are higher less sensitive receptors and their
esponses kick in too
Hormonal responses vary between cells
• Hormones elicit specific responses in target cells
o The nature of the response depends on which hormone receptors and
associated response (signal transduction) pathways they express
• A single hormone can evoke a wide variety of responses in different cells, such that
different tissues of the body often respond differently to the same hormone
o Example: the plant hormone auxin causes shoot cells to elongate but inhibits
oot cells from elongating, amid other myriad functions
o Example: antidiuretic hormone (ADH) is best known for regulating urination in
mammals, but an ADH receptor in the rodent
ain seems to play a role in
mating behavior
o Example: estrogen, testosterone, and other hormones associated with
eproduction have different effects in different tissues of men vs. women
• Mice: promiscuous—do not form lasting pairings
• Prairie voles: highly monogamous—pair bond for life
• Insertion of a
ain receptor gene for ADH (along with its regulatory sequences) from
the prairie vole into mice results in less promiscuous mice!
Animal homeostasis and communication systems
• Most animals have both endocrine and nervous communication systems, and both of
these are key in homeostatic monitoring and response
o Both are systems of long-distance communication in animals
▪ Endocrine: generally slow, long-lasting chemical signaling achieved via
hormones distributed by the circulatory system
▪ Nervous: rapid, short-lived electrical signaling achieved by a network of
neurons
o The two systems work together: hormones signal the
ain to send certain
coordinated nervous signals, and the
ain sends nervous signals that trigger the
elease of appropriate hormones
▪ Example: you encounter a lion, and a flood of hormones are released as
part of the “fight or flight” response which further amp you up
Positive vs. Negative feedbacks
• We've noted the importance of negative feedback loops in homeostasis, which respond
to a stimulus in a way that reverses trends
o These are very important in homeostasis, because they help keep conditions
within a certain range by driving a system back to the set point
o As soon as the conditions wander outside of the range, the negative feedback
loop reins them back in
• Positive feedbacks also occur in biological systems, but they are effectively anti-
homeostatic: they drive the system further and further away from a set point
o Positive feedback loop: the outcome of a process amplifies rather than
diminishes the process, thus creating a “runaway train” type of scenario that
drives a system away from the set point
▪ Not involved in homeostasis, since they drive the system out of balance...
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