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UNIT THREE
BIOLOGY
AREA OF STUDY #2: DETECTING &
RESPONDING
EXAM REVISION LECTURE
CHP 5: HOMEOSTASIS - INTRODUCTION
CHP 5TOPIC 1: HOMEOSTASIS - INTRODUCTION
In order for cells to function properly they require an environment in which their
requirements are met and conditions remain stable. This is true for all life forms!
Homeostasis is: the maintenance of the internal environment in a relatively stable
state despite changes in either the external or internal environment.
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is the condition of a relatively stable internal, maintained within narrow limits
when changes occur in the internal environment, homeostatic mechanisms act to restore it to the
‘normal’ state
if the body deviates too far from the normal steady state of a variable, death can occur.
Factors Controlled Include:
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core body temperature
blood glucose concentration
water levels in body tissues
ph (hydrogen ion concentration)
ions, such as sodium, calcium and chloride ions
blood oxygen concentration
carbon dioxide concentration
blood volume
blood pressure
In this Petri dish, tiny green Volvox colonies
have moved towards the light source, allowing
increased levels of photosynthesis.
CHP 5TOPIC 1: HOMEOSTASIS - INTRODUCTION
Key Body Systems Contributing To Homeostasis
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nervous system
endocrine system
respiratory system
circulatory system
digestive system
excretory system
integumentary (skin)system
CHP 5TOPIC 1: HOMEOSTASIS - INTRODUCTION
Defining The Internal Environment
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the body’s internal environment consists of the tissue fluid (surrounding cells) and blood plasma
plasma is the liquid part of the blood.
both tissue fluid and plasma are located outside of cells. Together they are called extracellular fluid
the composition of the extracellular fluid is regulated so that body cells can operate at their optimum
fluids located outside cells are extracellular.
fluids located inside the cells are intracellular.
CHP 5TOPIC 2: STIMULUS – RESPONSE MODEL
In order to be able to maintain stable conditions, organisms need to be able to
detect changes in the internal or external environmenst, transmit information
about those changes to a control centre , coordinate a plan to deal with the
challenge and carry out an appropriate response. A simple model can dmonstrate
this principle:
Stimulus-Response Model
• a change in the internal or external environment acts as a stimulus that is detected by
receptors
• if the intensity of the stimulus is sufficient (threshold), messages are transferred to a
control centre
• messages are then passed to effectors which produce a response
CHP 5TOPIC 2: STIMULUS – RESPONSE MODEL
Receptors are able to detect changes in the internal and external environments. Most act as
misalignment or disturbance receptors, detecting a change from normal conditions. Some
detect changes in the internal environment, and others, changes in the external
environment which have the potential to alter the internal environment.
Type Of Receptor
Receptor
Location
Detects
Chemoreceptors
(detect chemicals)
• Olfactory lining
• Taste buds
• Oxygen concentration
• osmoreceptors
• glucose level receptors
•pH/ CO2 receptors
• nose
• tongue, epiglotis and pharynx
• aorta
• hypothalamus
• pancreas
• medulla, aorta and carotid
artery
• Certain gases and vapours
• certain chemicals
•Low oxygen conc. in blood
• fall in water conc. in blood
•Rise or fall in blood glucose
• pH and CO2 levels in blood
Mechanoreceptors
(detect pressure and
movement)
• ear
•Touch & pressure
receptors
•Muscle length receptors
• Arterial pressure rec.
• ear drum and inner ear fluid
• skin, muscles, connective
tissue
• skeletal muscles
• aorta
•Vibrations and balance
•Compression
Photoreceptors
(detect light)
• eye
• head
• visible light
Thermoreceptors
(detect
temperature)
• heat receptors
•Cold receptors
• body temp receptors
• skin
• skin
• hypothalamus
• heat input
• heat loss
• core body temp
• muscle stretching
• blood pressure
CHP 5TOPIC 2: STIMULUS – RESPONSE MODEL
Think about the example of your body’s response to raised CO2 levels!
Receptors: brain (medulla), aorta and carotid artery
Control Centre: the brain (hypolthalamus)
Effector: Respiratory muscles in lungs
Consider -High CO2 levels: your response – faster and deeper breathing, but it
doesn’t go on for ever. Eventually your breathing rate returns to normal. This is
because increased breathing rate tends to lower your CO2 levels; that is, it
produced an effect opposite to the original stimulus. This constitutes negative
feedback. Your body detects that the original stimulus has been reversed and
switches off the response.
Topic CHP 5.1 Questions:
1. Homeostasis is: the____________ of the ______________environment is a relatively
____________ state despite _______________ in either the external or internal
environment.
2. Five factors maintained by homeostatic mechanisms include: ____________, ___________
______________, ____________________ & __________________
3. Another name for the integumentary system is the ______________
4. Fluid outside the cell is known as:________________ and includes: ___________ &
________________. This is also known as the internal environment.
Topic CHP 5.2 Questions:
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Fluxuations within tolerance levels is known as maintaining the _________________ state
As stimulus is: ________________________________________
True / False: A stimulus, no matter how minor will trigger a response from the control centre
A receptor is: _________________________________________
Receptors that detect light are known as: ___________________receptors
Receptors that detect pressure and movement are known as: ________________receptors
CHP 5TOPIC 3: FEEDBACK
Negative Feedback:
The maintenance of a stable internal environment (homeostasis) relies on negative feedback
systems. In the stimulus-response model, negative feedback occurs when the effector
brings about a response that counteracts the original stimulus, so that the variable within
the internal environment is returned to its optimal level.
Transmission to nerves
Stimulus
Increase in blood carbon dioxide
Receptor
In arteries and brain
Feedback
Control Centre
Response
Respiratory centre in brain
Decreased carbon dioxide
In blood
Effector
Respiratory muscles in
lungs (increased
ventilations)
(Negative feedback is normal, good!)
Transmission to nerves or
hormones
CHP 5TOPIC 3: FEEDBACK
Negative Feedback
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Despite the fact that it is called negative feedback its effects are positive
Without negative feedback homeostatic mechanisms would not work
The word negative comes from the fact that the receptors are receiving signals opposite to
those caused by the original stimulus
Positive Feedback
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Positive Feedback systems do exist in nature, but they are not mechanisms which maintain
homeostasis
The feedback is positive as the response causes an increase in the intensity of the original
stimulus, not a reversal of it, as in negative feedback
Example: production of oxytocin
• produced by the pituitary gland during childbirth
• once birthing process commences, oxytocin stimulates uterine contractions that help push
baby out of the uterus
• also acts on the pituitary to produce more oxytocin
• continues until baby is born
CHP 5TOPIC 4: SIGNALLING MOLECULES
SIGNALLING MOLECULES
• Body cells communicate with each other via chemical signals
• are chemical signals, produced in cells, which affect the function of other body cells
• they include:
– Hormones
– Neurotransmitters
– Pheromones
– Plant hormones
CHP 5TOPIC 4: SIGNALLING MOLECULES
Signalling Molecule
Hormones
• some are fatty acids/ steroids
others are peptides/ simple
amino acids some are water
soluble/ some not!
• are produced in endocrine glands
• travel in the general circulation
or extracellular fluid
• exert an effect on specific body
tissues/ cell called target cells
Neurotransmitters
• most are peptides or modified
amino acids
• examples: serotonin,
Acetylcholine, dopamine
Pheromones
• may be hydrocarbons or more
complex molecules
Plant Hormones
• vary from simple molecules like
ethylene (C2H4), complex organic
molecules
Production
Are usually produced
in endocrine glands.
Some neurones also
produce hormones
e.g those of the
hypothalamus
transport
Target Cells
General circulation – (blood)
or extracellular fluid
Specific cells in the body
respond to each hormone.
Target cells have a specific
receptor for each hormone
they respond to
Produced in
neurones, stored in
synaptic vesicles
Travels across synaptic gap
The dendrites of another
neuron- to continue an
impulse’ and cells simulated
by neurons- muscles and
glands
Produced in exocrine
glands
Secreted into the external
environment
Other members of the same
species.
Produced in
specilalised cells
Most are carried in plant’s
vascular tissue. Ethylene
diffuses through tissues
Cells with appropriate
receptors. Some hormones
affect a variety of plant
tissue
CHP 5TOPIC 4: SIGNALLING MOLECULES
Hormone Facts continued …
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Some hormones cause an increase in a cellular function
Others may cause a decrease or turn the function off
Hormones are usually produced in very small quantities, yet exert large effects on target cells –
this is due to signal amplification - the ability of one molecule of hormone to cause the
production of many molecules inside the cell
Responses to signals include: activation of DNA to produce proteins
Once hormones have delivered their signal to target cells and the desired effect has occurred,
they are degraded by cell enzymes and excreted via the kidneys or faeces
Gland
Hormone
Action
Hypothalamus
Many
Many body activities
Pituitary
Growth Hormone
The master gland
Thyroid
Thyroxine
Metabolism Growth
Adrenals
Cortisol, Adrenaline
Metabolism, Responds to stress
Pancreas
Insulin, Glucagon
Blood glucose concentration
Gonads
Testosterone, Oestrogen
Fertility and sex characteristics
CHP 5TOPIC 4: SIGNALLING MOLECULES
SIGNAL TRANSDUCTION:
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Cascade of events linking an external signal (such as a hormone) to a particular cellular
response, examples include:
• Signal Transduction: Lipid Soluble Hormones (Hydrophobic)
- example: sex hormones,
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- steroid hormones enter cell through phospholipid bilayer (not possible for other hormones)
- in target cells, there are specific receptors forming a hormone-receptor complex – in cytoplasm
- hormone-receptor complex enters nucleus – it attaches to a specific region on a chromosome
- hormone-receptor complex activates a particular gene, causing production of mRNA
- mRNA travels to ribosomes – assembles a new protein (many copies)
- protein produced (enzyme, growth factor or structural protein) affects cell function
- typically, steroid hormones regulate long-term development
CHP 5TOPIC 4: SIGNALLING MOLECULES
Signal Transduction: Peptide Hormones (Hydrophillic/ Lipophobic)
Remember: The change from an extracellular (outside) signal to an intracellular (inside) signal happens
by a process known as signal transduction
Unlike lipids, peptide hormones cannot pass through the phospholipid bilayer of cell membranes.
1. The hormone (first messenger) binds with a specific receptor molecule on the cell surface (specific!)
2. Some have binding site on outside of cell and an enzyme site on the inside
3. The binding of receptor, hormone (first messenger) stimulates the enzymatic end of the protein
molecule
4. The enzyme activates the formation of many molecules which act as second messengers. The signal is
amplified
5. Second messengers cause a change in cell functioning- a response
In another type of Signal Transduction, an enzyme doesn’t exist beneath the membrane protein but a
nearby protein molecule known as a G protein is activated which in turn activates a third protein, an
enzyme. This enzyme (as in 4 above) activates the formation of many molecules which act as second
messengers. The signal is amplified . As in 5, second messengers cause a change in cell functioning- a
response
Once a hormone response is initiated, the hormone is degraded
CHP 5TOPIC 4: SIGNALLING MOLECULES
Pheremones
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signalling molecules that are released by animals into their environment enabling them to communicate
with members of their own species
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pheromones can be used for:
• attracting mates
• inducing mating activity
• marking territories
• signallng alarm
• marking food trails
Plant Hormones
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The growth of plants is regulated by environmental factors and plant hormones
They are similar to animal hormones in that :
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They are produced in response to a stimulus
They are produced in particular parts of the plant and travel to cells, which will respond, in a different part of the
plant
They bind with specific receptors which cause a response within cells
They also differ from animal hormones in an number of ways:
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The same hormone can cause many different hormones (animal hormones are very specific)
A plant hormone may cause one response at low concentrations and the opposite at high concentrations
The actions of plant hormones overlap a lot, with many hormones contributing to one observed response
Topic CHP 5.3 & 5.4 Questions:
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True/ False: All homeostatic mechanisms maintain homeostasis with ‘negative feedback’
Positive feed back mechanisms cause: an increase/ a decrease in the initial stimulus.
List four signalling molecules: ________________, ________________,
_____________________, _______________
4. Where are neurotransmitters produced? ______________
5. How are hormones transported around the body: _______________
6. What cells to signalling molecules react with? _______________
7. What is signal amplification? _____________________________________
________________________________
8. True/ False: hormones can direct the activation of DNA to produce proteins
9. What happens to hormones once they have delivered their signal to target cells?
______________________
10. What is signal transduction: ____________________________
11. How does signal transduction differ between hydrophillic and hydrophobic
messengers?___________________________________________________________
12. Name two functions of pheremones: ________________________
CHP 5TOPIC 5: PLANT HORMONES
Plant Hormones
Classifications of plant hormones:
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Auxins- regulate growth – elongates cells in stems, roots
Cytokinins – growth promoting – promote cell reproduction - shoots
Gibberellins- speed growth, germination
abscic acid- dormanct, close stomata, falling leaves and falling fruit
Ethylene (gas)- ripens fruit
Florigen- control of flowering?
Jasmonates
Brassinosteroids
Plant Hormones & Growth Response
Tropisms
– is a growth response towards (positive) or away from (negative) a given stimulus
Examples:
- Positive Phototropism (Light): is growth towards a stimulus (light)
- Thigmotropism (Touch): change in the direction of growth because of contact another object
- Geotropism (Gravity): is a growth response due to gravity (positive (grows down!- with)
- Hydrotropism (Water): growth response due to stimulus of water
CHP 5TOPIC 5: PLANT HORMONES
Hormone
Auxins
Refers to many
hormones,
including IAA
• Water soluble
Site of
Production
Produced in shoot
apical tips, leaves,
& seeds
Action
Transport
-is to control enlargement
and elongation of cells
- phototropism
- growth of flowers/ fruit
- cell differentiation
-at high concentrations
inhibits root growth
- apical dominance
Moves primarily
through
parenchyma cells
surrounding
vascular tissue
Target Cell
•Shoot, roots
Gravity sensing mechanism
Auxin in higher amounts on
“lower” side of organ
Root more sensitive to auxin inhibits elongation
Statoliths at bottom
of cells of pea root
CHP 5TOPIC 5: PLANT HORMONES
Hormone
Site of
Production
Action
Gibberellins
Formed in young
leaves, apical
tips, embryo
. Stimulates flowering
in some plants
• affects fruit develop.
• stimulates seed
germination
• stimulates cell
division
Cytokinins
Formed in roots
auxin/cytokinin ratio
is important
• cell
reproduction
Abscisic Acid
•Inhibits growth
Levels increase in
response to cold, drought,
and high salt levels
•Influences stomatal
closure
Transport
Target Cell
Translocated in
xylem & pholem
Tissues of stem and
leaves
Translocated
upward in xylem
Shoots, roots,
growing fruits
Transported from Stomata,
leaves in phloem Deciduous leaves
•Lead abscission
Ethylene
gas
Florigen
Fruits
• Ripening of fruit
• Primarily synthesized
in response to stress
Diffuses through
tissues
fruits
. Control of flowering
- Regulates the initiation of flowering and so is involved in
photoperiodism.
Topic CHP5.5 Questions:
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True/ False: Auxins regulate growth through cell elongation
True/ False: Auxins always promote cell growth
True/ False: Auxins are one plant hormone that is responsible for plant tropism
What is a tropic response: ________________________________________________
Define positive phototropism______________________________________________
The effect of cutting a growing coleoptile tip from a growing shoot would be?
______________________________________________________
7. What is apical dominance? __________________________________
8. Gibberelins are responsible for: _________________________________________
9. Gibberelins are transported through the plant within:________________________
10. What is the role of the hormone ethylene? ___________________________________
11. How is ethylene transported through a plant?_______________________________12. High levels of Abscisic Acid in a plant could indicate:
_________________________________________________________________