12 Biology

Chapter 5&6- Homeostasis & regulatory mechanisms

Homeostasis is the maintenance of the internal environment in a relatively stable state in the face of changes in either the external or internal environment.

Organisms only survive, grow and reproduce when their external environment provides adequate levels of nutrients, water, oxygen, carbon dioxide and suitable physical conditions such as light and temperature.

These requirements usually must stay within narrow tolerance limits for an organism to function efficiently.

Can you think of an example of an organism exceeding a tolerance limit in some way?

Organisms must also regulate their internal environment in the face of internal & external factors which may occur according to their activities.

Can you think of an example of an organism doing this?.

• Homeostasis occurs in ALL living organisms, Uni-cellular & Multi-cellular.

Uni-cellular organism don’t have an internal environment. They are cells directly placed in fluids of their external environment, their cell membrane regulates their cytoplasm.

An advantage for multi-cellular organisms is that their cells are protected from the organisms external environment by the extracellular fluid.

This internal environment allows conditions inside the organism to be maintained for efficient cell functioning.

What is the environment external to a cell?

How are cells protected from this surrounding fluid?

External environment Internal environmentThe medium surrounding an organism

The extracellular fluid: is the fluid that surrounds cells in multi-cellular organisms.

For optimal functioning, cells regulate:Concentration of particular saltsTemperatureNutrient levelsWaste levelsPH

• Tight regulation of extracellular fluid & a stable internal environment is vital for optimal cellular function in multi-cellular organisms.

Examples of processes used to stabilise the internal environment include:

Lungs & exchange of carbon dioxide & oxygen

Animal circulatory systemsRemoval of wastes Root absorption of water & minerals

• Vertebrates have many complex homeostatic mechanisms which are ultimately controlled by the hormonal & nervous systems.

• Can you think of another system which is vital for delivering the messages of the nervous and hormonal system?

The stimulus response model

• LINK- http://www.phys.unsw.edu.au/biosnippets/

• Changes or stimuli are detected by receptors. The stimulus must reach a threshold of intensity of the specific receptor, before receptors can stimulate effectors to produce a response.

• These act to restore the variable to it’s original state.

• The response produced reduced the effect of the original stimulus, therefore provides negative feedback to that stimulus.

• Eg. A rise in body temp=physiological changes and behavioral responses to restore temp to original level (shivering & putting on clothing)

Misalignment detectors:These detectors detect when a particular variable is “out of line” or out of its optimal range. Eg. Oxygen content in blood

Disturbance detectors:These detectors warn of problems before they arise. They detect changes that are likely to to cause change in a variable.

Homeostasis is achieved by three important mechanisms:

STRUCTURAL – the organism has particular physical features to maintain homeostasis.

FUNCTIONAL – the metabolism of the organism is able to adjust to changes.

BEHAVIOURAL – the actions of the organism individually or with others help the organism to maintain homeostasis.

Stimuli to response• Stimuli are

environmental factors that organisms can detect and to which they can respond.

• Stimuli are detected by means of specialised effectors to produce a response.

receptorstimulus

Control centre

Effector

response

nerves

nervesHormones

Types of signals

Physical Stimuli

•Light•Heat•Touch/mechanical

Chemical Stimuli

•Nutrient molecules-glucose•Hormones•Neurotransmitters•Pheremones

Homeostatic Mechanisms

The most complex organisms to regulate their internal environment are the mammals and birds.

The mechanism used by these organisms is called ‘The Stimulus-Response mechanism’

The 3 main types are:1. Simple Stimulus-response2. Negative Feedback systems3. Positive Feedback systems

Stimulus-Response

• The general pattern of a stimulus–response mechanism is the withdrawal reflex.

Negative Feedback System

• Negative feedback systems are stimulus–response mechanisms that act to restore the original state. The response produced reduces the effect of the original stimulus; that is, the response provides feedback that has a negative effect on the stimulus.

Feedback System

Some systems controlled by homeostasis

Control of Requires regulation of

nutrient levels

(e.g. glucose)

•nutrient intake

•digestive and circulatory system functions

•storage and mobilisation of nutrients

•behaviour

body temperature

•general metabolism

•blood flow to tissues

•muscle activity and sweating

•behaviour

water and salt balance

•excretion of water and salts to maintain correct osmotic concentration of internal body fluids

•behaviour

metabolic rate

•lung ventilation and circulation to deliver adequate oxygen to tissues

•nutrient intake and storage

•behaviour

Regulating responses to stimuli To coordinate all the

different activities a multicellular organism will integrate and coordinate the activities of their cells.

There are two major systems for this:

1. ENDOCRINE SYSTEM (hormones) and

2. NERVOUS SYSTEM (nerves).

FEEDBACK LOOPS(*write onto a diagram)

This involves a six step process:

Stimulus : a change from ideal conditions

Receptor : the cells or tissue that detects the change

Transmission : method by which the message is carried

Effector : a gland or muscle which causes the response to happen

Response : an action that occurs due to the effect of the response

Feedback : the consequence of the response on the stimulus

FEEDBACK LOOP: BLOOD PRESSURE

STIMULUSBlood pressure

falls

TRANSMISSIONmessagesent tobrain

RECEPTORSin

the muscles of blood vessels note the

change

EFFECTORBrain sends message

to heart & bloodvessels

RESPONSEHeart rate increases

FEEDBACKBlood vessels

Constrict and blood pressure increases

Controlling Blood Pressure

MAINTENANCE OF BODY TEMPERATURE

The ability to control body temperature is extremely important if animals are to survive.

*(Recall from Area of Study 1 that enzymes have optimal temperatures for their activity.)

If the enzyme, or the cell it is in, is too cold, collisions between enzyme and substrate occur infrequently, and metabolic processes slow to rates which may no longer support life.

FEEDBACK LOOP: KEEPING WARM

STIMULUSBody temp

falls

TRANSMISSIONSkin muscles erect hairs

RECEPTORSin

the hypothalamus detectchange

EFFECTORBlood vessels in skin

constrictRESPONSE

Skeletal musclesshiver

FEEDBACKPituitary gland secretes

thyroxine

FEEDBACK LOOP: COOLING DOWN

STIMULUSBody temp

RISES

TRANSMISSIONBlood vessels

in skindilate

RECEPTORSin

the hypothalamus detectchange

EFFECTORSweat glands release

sweatRESPONSE

Pituitary gland secretesLess thyroxine

FEEDBACKMuscle contractions

are reduced

Regulating blood glucose Negative feedback by

hormones The control of blood

glucose levels involves two hormones:

1.insulin, which controls the upper limit and

2.glucagon which controls the lower limit.

The normal range is 3.6 to 6.8 mmol/L

Insulin and Glucagon

• The hormone Insulin controls the uptake by cells of glucose from the blood. Beta cells in the pancreas control insulin levels.

• The hormone Glucagon acts on the liver to release more glucose into the blood.

FEEDBACK LOOP: Fall of blood glucose

• Blood sugar falls• Receptors in the pancreas detect change• Alpha cells release glucagon• Beta cells suppress insulin production• Liver converts glycogen to glucose• Muscle glycogen converted to glucose• Fat tissue broken down for energy• Blood sugar level rises

*do on board

FEEDBACK LOOP: Blood sugar level rises

• Blood sugar rises• Receptors in the pancreas detect change• Beta cells increase insulin production • Alpha cells decrease glucagon production• Fat tissue increases conversion of glucose to

fat• Skeletal muscle increases uptake of glucose• Liver synthesises glycogen • Blood sugar level falls

*do on board

Diabetes – high blood glucose

High blood glucose

Pancreas (beta cells detect then secretes more insulin)

Liver increases glycogen synthesisRemoving glucose from blood

Lower blood glucose

Caused by eating

Diabetes – low blood glucose

Blood glucose drops (due to exercise, starving)

Beta cells detect glucose level Alpha cells in pancreas produce glucagon

Liver breaks down stored glycogen and glucose released into bloodstream

Blood glucose level rises.

Other examples of Regulation

Blood CO2 levels•Respiration is controlled involuntarily & is under the control of the respiration centre in the medulla. •The level of carbon dioxide (CO2) in the blood is one of the main stimuli that can alter the rate of respiration. •CO2 also binds with hydrogen, making blood more acidic (lowered pH). •Exercise is an example of when blood CO2 would increase. When exercising, your muscles require more energy, meaning your tissues consume more oxygen than when at rest. •More oxygen consumption=more carbon dioxide production. •If blood CO2 increases, chemoreceptors become stimulated which sends signals to the breathing centre, which sends nerve impulses back to the muscles of breathing (diaphragm & intercostals), causing them to relax quicker increasing breathing rate. •This aims to increase oxygen in the blood and reduce CO2= homeostasis.

Summary• Negative feedback systems produce stability.

They are stimulus– response mechanisms in which the response produced reduces the effect of the original stimulus.

• Disturbance detectors respond to changes in the environment that are likely to cause a change in the precise factor of the internal environment that is being controlled.

• Misalignment detectors monitor the precise factor of the internal environment that is being controlled.

(read pages 113-119 for other systems)

Summary• Homeostasis is the maintenance of a

constant internal state despite changes in the external environment.

• There are two major systems involved in homeostasis: nerves and hormones

Regulatory pathwaysIn both hormonal and nervous

systems signals are passed from one cell to the next by chemical communication.

Hormones and Neurotransmitters

SIGNAL TRANSDUCTION• Signal transduction refers to any process by

which a cell converts one kind of signal or stimulus into another.

• This often involves an ordered sequence of biochemical reactions inside the cell, that are carried out by enzymes and linked through second messengers resulting in what is thought of as a "second messenger pathway".

• Such processes are usually rapid.

Signal Transduction

• Signal transduction refers to the ways that receptors convert incoming signals into information that leads to an appropriately coordinated response.

• Receptors- convert incoming signals into messages usually carried by nerves or hormones.

• In many signal transduction processes, the number of proteins and other molecules participating in these events increases as the process eminates from the initial stimulus, resulting in a "signal cascade" and often results in a relatively small stimulus eliciting a large response.

Cell signallingThe signalling and response process is called the signal

transduction pathway and often involves many enzymes and molecules in a signal cascade which causes a response in the target cell.

These pathways are categorised on the distance the signal travels to reach its target cell.

1. Endocrine signalling – carried long distances by circulatory system

2. Paracrine signalling – released in immediate vicinity of target cells.

3. Autocrine signalling – cells produce and react to their own signals.

4. Pheromone signalling- chemical signals between members of the same species that travel through the external environment.

Signal Transduction pathwayApplying the stimulus response model to the

cell.1.The signal binds to receptor molecule.2.Receptor molecule changes shape or

confirmation. 3.Initiates a molecular cascade of secondary

messenger molecules to finally an effector molecule.

4.Effector molecule initiates the cellular response.

How do we detect changes in the environment?

Homeostasis and regulation are carried out by hormonal and nervous system. In animals responses are based on sensory information received from all parts of the body. Individual effector organs involve muscle and glandular tissue.

Our bodies have a number of different receptors that detect change:

1. Chemoreceptors – CO2 levels and hence pH

2. Mechanoreceptors – pressure/touch

3. Photoreceptors - light

4. Thermoreceptors – temperature

5. Baroreceptors – blood pressure

6. Proprioreceptors – stretch and tension

7. Olfactory receptors – detect airborne and dissolved chemicals

Types of Receptors• Chemoreceptors - detect chemicals

– Olfactory lining in nose; taste buds; oxygen concentration receptor in aorta; osmoreceptors in hypothalamus; glucose level receptors in pancreas; pH/CO2 receptors in medulla, aorta and carotid arteries

• Mechanoreceptors – detect pressure and movement– Ear; touch & pressure receptors in skin muscles, joints and

connective tissue; muscle length receptors in skeletal muscle; muscle tension receptors in tendons; joint receptors; venous pressure receptors; arterial pressure receptors; lung inflation receptors; lung deflation receptors; lung irritant receptors.

• Photoreceptors – detect light– Eye.

• Thermoreceptors - detect temperature– Heat receptors and cold receptors in skin; body temperature

receptor in hypothalamus.

Feedback systems

• Homeostasis or regulation therefore involves fluctuations around a set-point.

• The size of the fluctuations depends on the sensitivity and location of the sensory receptors, the tolerance of the control centre to variation from the set-point, and efficiency of the response mechanism.

• Most biological feedback systems are negative feedback systems which operate as proportional control systems – the size of the response is proportional to the size of the stimulus.

QuizQuiz

• Define Homeostasis• What are the two major systems

involved in homeostasis?• What are the characteristics of

hormones?• What is the difference between

Protein and Steroid hormones?

AnswersAnswers

• Homeostasis is the maintenance of a constant internal state despite changes in the external environment.

• There are two major systems involved in homeostasis nerves and hormones

• Hormones are proteins that are released in glands, they are specific, slow working substances.

• Protein hormones are made of amino acid (polypeptides) and Steroids are fatty acid based (cholesterol)

Nervous and Endocrine Systems

• The nervous and endocrine systems work together to coordinate the actions of all other systems of the body to produce behavior and maintain homeostasis.

• The endocrine system produces chemical messengers that are transported through the circulatory system. It requires seconds, minutes or hours.

• The nervous system is more rapid, requiring only thousandths of a second.

• In general, the endocrine system is in charge of body processes that happen slowly, such as cell growth.

• Faster processes like breathing and body movement are monitored by the nervous system.

Endocrine System(hormonal)

Hormones: are chemical messengers released in response to a stimulus

detected by a receptor. are substances produced by one cell that have an effect on

another. Only effect target cells with specific receptors for the hormone.

can regulate the growth or activity of specific cells. can transmit their signal by altering specific biochemical reactions

in cells. are released from glands directly into the blood stream. (no ducts) are slow working, BUT long lasting. take time to reach the target area, but once there the response

can be fast or slow.

Two Main Types of Hormones PROTEIN hormones – made of chains of amino acids

(polypeptide chains) which are too big to pass through the cell membrane. They must bind to the membrane using specific receptor sites triggering metabolic response.

STEROID (fatty acid based) hormones are small and lipid soluble, they can pass through the cell membrane and enter the cytoplasm. The hormone binds with receptor molecules in the cytoplasm which then diffuse into the nucleus. Once in the nucleus they then affect gene expression on the chromosomes (DNA). This initiates enzyme synthesis and a biochemical pathway.

Protein Hormones

Adrenaline, ADH, thyroxine and growth hormones.

Steroid hormones Testosterone,

oestrogen, progesterone and corticosteroids

Endocrine/hormonal Organs

Endocrine System• The foundations of the endocrine system are the hormones

and glands. • As the body's chemical messengers, hormones transfer

information and instructions from one set of cells to another. • Although many different hormones circulate throughout the

bloodstream, each one affects only the cells that are genetically programmed to receive and respond to its message.

• A gland is a group of cells that produces and secretes, or gives off, chemicals. It selects and removes materials from the blood, processes them, and secretes the finished chemical product for use somewhere in the body.

• Some types of glands release their secretions in specific areas. For instance, exocrine glands, such as the sweat and salivary glands, release secretions in the skin or inside of the mouth.

• Endocrine glands, on the other hand, release more than 20 major hormones directly into the bloodstream where they can be transported to cells in other parts of the body.

Endocrine System and Negative Feedback

• When hormone levels reach a certain normal or necessary amount, further secretion is controlled by important body mechanisms to maintain that level of hormone in the blood.

• Regulation of hormone secretion may involve the hormone itself or another substance in the blood related to the hormone.

• For example, if the thyroid gland has secreted adequate amounts of thyroid hormones into the blood, the pituitary gland senses the normal levels of thyroid hormone in the bloodstream and adjusts its release of thyrotropin, the pituitary hormone that stimulates the thyroid gland to produce thyroid hormones.

• Another example is parathyroid hormone, which increases the level of calcium in the blood. When the blood calcium level rises, the parathyroid glands sense the change and decrease their secretion of parathyroid hormone.

This turnoff process is called a negative feedback system.

Glands, Hormones & ActionGland Hormone ActionThyroid Thyroxine Stimulates metabolic

process

Ovaries Oestrogen Maintenance of female sex characteristics

Testes Androgens Maintenance of male sex characteristics

Pituitary Anti diuretic hormone (ADH) Promotes water retention by kidneys

Pituitary Follicle stimulating Hormone (FSH)

Stimulates production of egg & sperm

Pancreas Insulin Lowers blood glucose levels

Pancreas Glucagon Raises blood glucose levels

Hypothalamus Releasing hormones (RH) Hormones that stimulate the pituitary

Nervous Nervous SystemSystem

Peripheral Central

Somatic

Spinal cord

Autonomic

Brain

Motor Sympathetic

Parasympathetic

Sensory

Generally increases energy use and prepares the body for action by increasing heart and metabolic rate.Neurotransmitter is usually adrenaline.

Enhances activities that conserve energy, such as digestion and slowing heart rate. Restores resting state. Neurotransmitter is acetylcholine.

Nervous SystemSensory Input • Receptors are parts of the nervous system that sense

changes in the internal or external environments. • Sensory input can be in many forms, including

pressure, taste, sound, light, blood pH, or hormone levels, that are converted to a signal and sent to the brain or spinal cord.

Integration and Output • In the sensory centers of the brain or in the spinal

cord, the barrage of input is integrated and a response is generated.

• The response, a motor output, is a signal transmitted to organs than can convert the signal into some form of action, such as movement, changes in heart rate, release of hormones, etc.

Sensory neuronsSensory neuronsCategory of sensory receptor

What they detect

Chemoreceptor O2 , CO2, pH, ions, complex organic molecules (eg: hormones, neurotransmitters)

Mechanoreceptors

Sound, touch, pressure, gravity

Photoreceptors Light, infrared radiation

Thermoreceptors Heat, cold

Other Electric fields, magnetic fields

Sensory receptorsSensory receptors

Signal transmissionSignal transmissionThere are three basic steps involved in

the way signals are sent in the nervous system :

generation of an impulse (action potential),

conduction of the action potential and transmission across a synapse.

Neurons• The neuron is the functional unit of

the nervous system.

• Humans have about 100 billion neurons in their brain alone!

• Although they vary in size and shape, all neurons have three parts:

– Dendrites receive information from another cell and transmit the message to the cell body.

– Cell body contains the nucleus, mitochondria and other organelles typical of eukaryotic cells.

– Axon conducts messages away from the cell body. Axon terminals form synapses with other neurons or with a muscle or a gland.

Relationship between different types of neurons

The Nerve Message - Electrical

• In a resting nerve (one that is not responding to stimulus), a small difference exists between the electrical charge on the inside and outside its cell membrane. The outside of the cell membrane of the axon is positive compared with the inside.

• Stimuli of various kinds can activate neurons so that they transmit nerve impulses along their axons. Such a nerve cell is said to be ‘excited’.

• Nerve impulses involve changes in the charge across the axon membranes.

• As the impulse moves along the axon, a change occurs in the permeability of the membrane so that positive ions move into the cell.

• This results in the outside of the membrane becoming negative compared with the inside. The change in permeability travels along the neuron.

• After a nerve impulse has been transmitted by a neuron, the original distribution of ions across the cell membrane is restored.

Myelin sheath• Acts as insulator to minimize metabolic

expense while maintaining rapid conduction of electrical impulse through neurons.

• Sheaths are formed by glial cells:

– oligodendrocytes in the central nervous system

– Schwann cells in the peripheral nervous system.

• The myelin sheath in peripheral nerves normally runs along the axon in sections about 1 mm long, punctuated by unsheathed nodes of Ranvier which contain a high density of voltage-gated ion channels.

• An action potential is conducted more rapidly along a myelinated axon because it ‘jumps’ from one node to the next.

The Nerve Message - Chemical

• Neurons communicate with one another via synapses, where the axon terminal of one cell impinges upon a dendrite or soma of another (or less commonly to an axon).

• The human brain has a huge number of synapses. Estimates vary for an adult, ranging from 100 to 500 trillion synapses.

• The space between two cells is known as the synaptic cleft. For signals to cross the synaptic cleft neurotransmitters are required.

• The time for neurotransmitter action is between 0.5 and 1 millisecond.

• Acetylcholine is an example of a neurotransmitter, as is norepinephrine, although each acts in different responses.

The Synapse

How do neurotransmitters work?

• Neurotransmitters are stored in small synaptic vesicles clustered at the tip of the axon.

• Arrival of the action potential causes some of the vesicles to move to the end of the axon and discharge their contents into the synaptic cleft.

• Released neurotransmitters diffuse across the cleft, and bind to receptors on the other cell's membrane, causing ion channels on that cell to open and prompting transmission of the message along that cell’s membrane.

• Some neurotransmitters cause an action potential, others are inhibitory.

• Once in the cleft, neurotransmitters are active for only a short time.

• Enzymes in the cleft inactivate the neurotransmitters. Inactivated neurotransmitters are taken back into the axon and recycled.

What is a reflex arc?• The brain generally coordinates responses

to information from receptors.

• Sometimes however the body cannot wait for the transmission of information from the spinal cord to the brain and back from the brain to an effector. This is where the reflex arc comes in….

• The reflex arc is an automatic, involuntary reaction to a stimulus.

• Examples of reflex arcs include balance, the blinking reflex, and the stretch reflex.

Example of a reflex arc

Touching a sharp object

• Receptor detects pain

• Sensory neurons carry signal to spinal cord

• Interneurons in the spinal cord directly stimulate effectors (e.g. skeletal muscle) using motor neurons

• Spinal interneurons also transmit pain signal to the brain This is why you drop the object and then a moment later feel the pain

• Brain then coordinates the rest of your voluntary and involuntary responses to pain.

Reflex arc

Reflex arc

Reflex arcReflex arc

Grey matter

Sensory Sensory neuroneneurone

Relay neurone

Motor neurone

Effector

(muscle)

Receptors in skin cells

reflex arc bbc

PheromonesPheromonesPheromones are:released outside the body to stimulate other

organisms. chemicals released by an animal that acts as a

signal to other animals of the same species; usually sexual attractants or alarm signals.

used by different animals for various purposes. Examples include:

-social organisation and control (e.g., bee colonies);

-scent marking and territoriality (e.g., ring-tail possums and other mammals);

-alarm signals (e.g., some insects); -mating signals and inducing mating activity

(mammals).