UNIT THREE

BIOLOGYAREA 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.

• 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.

Key Body Systems Contributing To Homeostasis

• nervous system• endocrine system• respiratory system• circulatory system• digestive system• excretory system• integumentary (skin)system

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.

CHP 5TOPIC 1: HOMEOSTASIS - INTRODUCTION

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 MODELReceptors 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

• muscle stretching• blood pressure

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

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 arteryControl 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:5. Fluxuations within tolerance levels is known as maintaining the _________________ state6. As stimulus is: ________________________________________7. True / False: A stimulus, no matter how minor will trigger a response from the control centre8. A receptor is: _________________________________________9. Receptors that detect light are known as: ___________________receptors10. Receptors that detect pressure and movement are known as: ________________receptors

Transmission to nerves

StimulusIncrease in blood carbon dioxide

Feedback

ResponseDecreased carbon dioxideIn blood

Transmission to nerves or hormones

(Negative feedback is normal, good!)

ReceptorIn arteries and brain

Control CentreRespiratory centre in brain

EffectorRespiratory muscles in

lungs (increased ventilations)

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.

CHP 5TOPIC 3: FEEDBACK

Negative 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

CHP 5TOPIC 3: FEEDBACK

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 MOLECULESSignalling Molecule 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

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

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

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 ActionHypothalamus 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:• 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,

1. - steroid hormones enter cell through phospholipid bilayer (not possible for other hormones)2. - in target cells, there are specific receptors forming a hormone-receptor complex – in cytoplasm3. - hormone-receptor complex enters nucleus – it attaches to a specific region on a chromosome4. - hormone-receptor complex activates a particular gene, causing production of mRNA5. - mRNA travels to ribosomes – assembles a new protein (many copies)6. - protein produced (enzyme, growth factor or structural protein) affects cell function7. - 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 inside3. The binding of receptor, hormone (first messenger) stimulates the enzymatic end of the protein

molecule4. The enzyme activates the formation of many molecules which act as second messengers. The signal is

amplified5. 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• 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

Topic CHP 5.3 & 5.4 Questions:1. True/ False: All homeostatic mechanisms maintain homeostasis with ‘negative feedback’2. Positive feed back mechanisms cause: an increase/ a decrease in the initial stimulus.3. 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 proteins9. 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 HormonesClassifications 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 ResponseTropisms– 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 HORMONESHormone Site of

ProductionAction Transport Target Cell

AuxinsRefers 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

Auxin in higher amounts on “lower” side of organ

Root more sensitive to auxin - inhibits elongation

Statoliths at bottom of cells of pea root

Gravity sensing mechanism

CHP 5TOPIC 5: PLANT HORMONESHormone Site of

ProductionAction Transport Target Cell

Cytokininsauxin/cytokinin ratio is important

Formed in roots • cell reproduction

Translocated upward in xylem

Shoots, roots, growing fruits

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

Abscisic AcidLevels 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

Ethylenegas

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.

Topic CHP5.5 Questions:1. True/ False: Auxins regulate growth through cell elongation2. True/ False: Auxins always promote cell growth3. True/ False: Auxins are one plant hormone that is responsible for plant tropism4. What is a tropic response: ________________________________________________5. Define positive phototropism______________________________________________6. 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:

_________________________________________________________________