1. BIOLOGY UNIT THREE AREA OF STUDY #2: DETECTING & RESPONDING
EXAM REVISION LECTURE
CHP 5: HOMEOSTASIS - INTRODUCTION

2. 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.
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:
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.

3. CHP 5TOPIC 1: HOMEOSTASIS - INTRODUCTION
Key Body Systems Contributing To Homeostasis
nervous system
endocrine system
respiratory system
circulatory system
digestive system
excretory system
integumentary (skin)system

4. CHP 5TOPIC 1: HOMEOSTASIS - INTRODUCTION
Defining The Internal Environment
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.

5. 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

6. 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
Vibrations and balance
Compression
muscle stretching
blood pressure
Photoreceptors
(detect light)
eye
head
visible light
Thermoreceptors
(detect temperature)
heat receptors
Cold receptors
body temp receptors
skin
heat input
heat loss
core body temp

7. 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.

8. Topic CHP 5.1 Questions: Topic CHP 5.2 Questions:
Homeostasis is: the____________ of the ______________environment is a relatively ____________ state despite _______________ in either the external or internal environment.
Five factors maintained by homeostatic mechanisms include: ____________, ___________
______________, ____________________ & __________________
Another name for the integumentary system is the ______________
Fluid outside the cell is known as:________________ and includes: ___________ & ________________. This is also known as the internal environment.
Topic CHP 5.2 Questions:
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

9. Feedback CHP 5TOPIC 3: FEEDBACK Stimulus Response
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
Feedback
Response
Decreased carbon dioxide
In blood
Transmission to nerves or hormones
(Negative feedback is normal, good!)
Receptor
In arteries and brain
Control Centre
Respiratory centre in brain
Effector
Respiratory muscles in lungs (increased ventilations)

10. CHP 5TOPIC 3: FEEDBACK Negative Feedback Positive Feedback
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
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

11. CHP 5TOPIC 4: 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

12. CHP 5TOPIC 4: SIGNALLING MOLECULES
Production
transport
Target Cells
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
Are usually produced in endocrine glands. Some neurones also produce hormones e.g those of the hypothalamus
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
Neurotransmitters
most are peptides or modified
amino acids
examples: serotonin, Acetylcholine, dopamine
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
Pheromones
may be hydrocarbons or more
complex molecules
Produced in exocrine glands
Secreted into the external environment
Other members of the same species.
Plant Hormones
vary from simple molecules like
ethylene (C2H4), complex organic
molecules
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

13. CHP 5TOPIC 4: SIGNALLING MOLECULES
Hormone Facts continued …
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

14. CHP 5TOPIC 4: SIGNALLING MOLECULES
SIGNAL TRANSDUCTION:
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,
- 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

15. 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.
The hormone (first messenger) binds with a specific receptor molecule on the cell surface (specific!)
Some have binding site on outside of cell and an enzyme site on the inside
The binding of receptor, hormone (first messenger) stimulates the enzymatic end of the protein molecule
The enzyme activates the formation of many molecules which act as second messengers. The signal is amplified
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

16. CHP 5TOPIC 4: SIGNALLING MOLECULES
Pheremones
signalling molecules that are released by animals into their environment enabling them to communicate with members of their own species
pheromones can be used for:
attracting mates
inducing mating activity
marking territories
signallng alarm
marking food trails
Plant Hormones
The growth of plants is regulated by environmental factors and plant hormones
They are similar to animal hormones in that :
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:
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

17. Topic CHP 5.3 & 5.4 Questions:
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: ________________, ________________, _____________________, _______________
Where are neurotransmitters produced? ______________
How are hormones transported around the body: _______________
What cells to signalling molecules react with? _______________
What is signal amplification? _____________________________________
________________________________
True/ False: hormones can direct the activation of DNA to produce proteins
What happens to hormones once they have delivered their signal to target cells? ______________________
What is signal transduction: ____________________________
How does signal transduction differ between hydrophillic and hydrophobic messengers?___________________________________________________________
Name two functions of pheremones: ________________________

18. CHP 5TOPIC 5: PLANT HORMONES
Classifications of plant hormones:
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

19. CHP 5TOPIC 5: PLANT HORMONES
Site of Production
Action
Transport
Target Cell
Auxins
Refers to many hormones, including IAA
Water soluble
Produced in shoot apical tips, leaves, & seeds
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
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

20. CHP 5TOPIC 5: PLANT HORMONES
Site of Production
Action
Transport
Target Cell
Gibberellins
Formed in young leaves, apical tips, embryo
. Stimulates flowering in some plants
affects fruit develop.
stimulates seed germination
stimulates cell division
Translocated in xylem & pholem
Tissues of stem and leaves
Cytokinins
auxin/cytokinin ratio is important
Formed in roots
cell reproduction
Translocated upward in xylem
Shoots, roots, growing fruits
Abscisic Acid
Levels increase in response to cold, drought, and high salt levels
Inhibits growth
Influences stomatal closure
Lead abscission
Transported from leaves in phloem
Stomata,
Deciduous leaves
Ethylene
gas
Fruits
Ripening of fruit
Primarily synthesized in response to stress
Diffuses through tissues
fruits
Florigen
. Control of flowering
- Regulates the initiation of flowering and so is involved in photoperiodism.

21. Topic CHP5.5 Questions:
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? ______________________________________________________
What is apical dominance? __________________________________
Gibberelins are responsible for: _________________________________________
Gibberelins are transported through the plant within:________________________
What is the role of the hormone ethylene? ___________________________________
How is ethylene transported through a plant?_______________________________-
High levels of Abscisic Acid in a plant could indicate: _________________________________________________________________