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UNIT 2 BIOLOGY. Organisms and Their Environment Area of Study 1: Adaptations of Organisms. WEEK 2: Learning Outcomes. By the end of the week, you should be able to: Describe the structure of the endocrine system. Explain hormonal control in complex multicellular organisms.

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unit 2 biology


Organisms and Their Environment

Area of Study 1: Adaptations of Organisms

week 2 learning outcomes
WEEK 2: Learning Outcomes

By the end of the week, you should be able to:

  • Describe the structure of the endocrine system.
  • Explain hormonal control in complex multicellular organisms.
  • Explain how organisms regulate water balance (osmoregulation).
feedback systems hormonal control
Feedback Systems: Hormonal Control
  • Feedback systems are the general mechanism of nervous or hormonal regulation in animals.
  • Feedback occurs when the response to a stimulus has an effect of some kind on the original stimulus. The nature of the response determines how the feedback is 'labelled'.
  • Negative feedback: is when the response reduces the effect of the original stimulus. It works in the opposite direction.
  • Positive feedback: is when the response enhances the original stimulus. It works in the same direction.
negative feedback

Draw a flowchart to illustrate this

Negative Feedback
  • Negative feedback is most common in biological systems. Examples of this are:
    • Blood glucose concentrations rise after a sugary meal (the stimulus), the hormone insulin is released and it speeds up the transport of glucose out of the blood and into selected tissues (the response), so blood glucose concentrations decrease (thus decreasing the original stimulus).
another example of negative feedback
Another example of negative feedback

Draw a flowchart to illustrate this

  • Exercise creates metabolic heat which raises the body temperature (the stimulus), cooling mechanisms such as vasodilation (flushed skin) and sweating begin (the response), body temperature falls (thus decreasing the original stimulus).
positive feedback
Positive Feedback
  • Positive feedback is less common, which is understandable, as most changes to steady state pose a threat, and to enhance them would be most unhelpful. However, there are a few examples:
    • A baby begins to suckle her mother's nipple and a few drops of milk are released (the stimulus). This encourages the baby and releases a hormone in the mother which further stimulates the release of milk (the response). The hungry baby continues to suckle, stimulating more milk release until she stops. (Positive feedback, it would not have helped the baby if suckling decreased milk flow, as in negative feedback!)
another example
Another example...
  • A ripening apple releases the volatile plant hormone ethylene (the stimulus). Ethylene accelerates the ripening of unripe fruit in its vicinity so nearby fruit also ripens, releasing more ethylene (the response). All the fruit quickly becomes ripe together. ("One 'bad' apple has ruined the whole lot." The biological explanation - positive feedback - for an old saying!)

Draw a flowchart!

yet another
Yet another...
  • Fever is an example of a positive feedback mechanism.

Explain to the person next to you how you think it works...

the endocrine hormonal system
The Endocrine (Hormonal) System
  • Hormones act by altering biochemical reactions in target cells.
  • Exocrine glands secrete their products straight onto the target tissue via a duct.
  • Endocrine glands secrete their products into the circulatory system via the capillary network.
    • These hormones are carried by the bloodstream, to target tissues elsewhere in the bodies, which then interpret the messages and act on them.
the endocrine system
The Endocrine System
  • There are many different glands in the body that release hormones.
    • Hypothalamus – midbrain.
    • Pituitary gland – base of the brain; beneath the hypothalamus.
    • Thyroid gland – in the throat.
    • Parathyroid gland – rear surface of the thyroid gland.
    • Thymus – just behind the sternum, over the heart.
    • Adrenal glands – above the kidneys.
    • Pancreas – organ attached to the small intestine.
    • Ovaries – in females.
    • Testes – in males.

Label me!


  • There are 2 types of hormones which differ slightly in their mode of action.
  • Amino acid hormones - these are proteins, which act by binding directly to receptors on the cell membrane. E.g. insulin and glucagon.
  • Steroid hormones - these are lipid based, they act by crossing the plasma membrane. E.g. testosterone and oestrogen.


  • The stimuli will only affect a particular group of hormone secreting cells.
  • The hormone released will then only affect those cells with the appropriate receptor.
  • E.g. growth stimulating hormone produced in the pituitary gland only affects bone and muscle and promotes protein synthesis.


  • The hormonal system is slower than the nervous system because the molecules have to be passed through blood or tissue to reach the target cells.


  • Once hormones have entered the cell they can bind to internal receptors.
  • They then cause the release of second messengers or they can enter the nucleus and regulate the production of other proteins by switching genes on or off.


  • Pairs of hormones with opposite effects.


  • E.g. insulin converts glucose to glycogen and glucagon converts glycogen back into glucose when needed.
interaction of glands hypothalamus
Interaction of Glands - Hypothalamus

The hypothalamus is:

  • located in the brain and controls the release of hormones from the pituitary gland.
  • an important link between the endocrine and nervous systems.
pituitary gland
Pituitary Gland


  • It secretes nine hormones that directly regulate many body functions and controls functions of other glands.
  • Two distinct portions:
    • Anterior (front)
    • Posterior (back)


  • Hypothalamic releasing hormones stimulate cells of anterior pituitary to release hormones.
  • Nerve impulses from hypothalamus stimulate nerve endings in the posterior pituitary gland to release hormones.
pituitary gland1
Pituitary Gland


  • Too much growth hormone (GH) in early childhood can result in a condition called gigantism from a hyperactive anterior pituitary gland.
  • Too little GH can result in Pituitary Dwarfism.

Robert Wadlow

1918 - 1940

2.7m tall

Thyroid Gland

Function: plays a major role in regulation the body’s metabolism.

thyroid gland
Thyroid Gland


  • If the thyroid gland produces too much thyroxin, it can cause a condition known as Hyperthyroidism- fast heart beat resulting in palpitations, a fast nervous system withtremor and anxiety symptoms, a fast digestive system resulting inweight loss and diarrhoea. – Graves’ disease
  • If too little thyroxinis produced it is called hypothyroidism– weight gain, fatigue and constipation – Hashimoto’s Thyroiditis
  • Function:Glycogen in the Pancreas help to keep the level of glucose in the blood stable.
  • Disorders:When the Pancreas fails to produce or properly use Insulin, it can cause a condition known as Diabetes Mellitus.

Adrenal Gland


  • The adrenal glands release Adrenaline in the body that helps prepare for and deal with stress.
  • Also regulates kidney function.


  • Pair of reproductive organs found in women that produce eggs.
  • Also secrete oestrogenand progesterone, which control ovulation and menstruation.


  • Pair ofreproductive glands that produces sperm.
  • Also secrete testosteroneto give the body its masculine characteristics.
pineal gland
Pineal Gland


  • Secretes Melatonin
    • Regulates circadian rhythms (helps with sleeping)
    • Promotes sexual development
    • Influences skin pigmentation
  • Produces the hormone thymosin, which promotes the development of T lymphocytes(white blood cells) that are involved in immunity.
  • The thymus helps establish the immune system in the first few years of life, but stops working after puberty. It will then gradually reduce in size as we get older.
  • What is an endocrine gland?
  • What is a hormone? How does it bring about a response?
words to know
Words To Know

Endocrine System


Endocrine gland

comparison of nervous system and endocrine system
Comparison of Nervous System and Endocrine System
  • Neurons release neurotransmitters into a synapse, affecting postsynaptic cells.
  • Glands release hormones into the bloodstream.
  • Only target cells of hormone respond.
complete this table
Complete this Table

Involves electrical transmission

Involves chemical transmission


Hormones in circulatory system



Specific location – muscles and glands

Tissues and glands



water balance
Water Balance
  • Water is the fluid medium in which everything happens at the cellular level in organisms.
  • There must be a balance between loss and gain.

How can water be lost?

How can water be gained?

  • The regulation of water is described as osmoregulation, a feedback mechanism that is under the control of hormones.
  • Effectively controlling the amount of water available for the cells to absorb.
osmoregulation in mammals
Osmoregulation in Mammals
  • Water balance in mammals is directly link to blood pressure
  • It is also link to maintaining the salt levels in the body
  • Involves the kidney
  • Increased water raises the blood pressure and decreased water lowers the blood pressure.
  • Two chemical compounds play a part:
    • The hormone vasopressin is an ADH (Anti-Diuretic Hormone) that aids reabsorption of water.
    • Renin is an enzyme that helps regulate sodium levels, and thus water levels in the blood.
  • McGraw-Hill animation
  • Mr Anderson explains
  • Detailed animation
how does vasopressin adh work when we are dehydrated
How does vasopressin (ADH) work when we are dehydrated?
  • Osmoreceptors in the hypothalamus detect high concentrations of solutes in the blood (low water).
  • A thirst sensation is generated by the osmoreceptors.
  • ADH is released from the hypothalamus.
  • ADH travels to the posterior pituitary gland where it is released into the blood.
  • ADH travels to the kidneys where it increases the permeability of the tubules to water.
  • The kidneys reabsorb more water; the solute concentration decreases.
  • Water concentration in the blood increases.
  • Negative feedback leads to a decreased secretion of ADH from the hypothalamus.
how does renin work when we are dehydrated
How does renin work when we are dehydrated?
  • Blood volume decreases and blood pressure falls.
  • This reduces filtration taking place in the glomerulus in the kidneys.
  • Pressure-sensitive receptors in the kidneys detect this.
  • Renin is released into the blood by the kidneys.
  • Renin initiates chemical reactions in the adrenal glands which releases the hormone aldosterone.
  • Aldosterone increases sodium ion reabsorption by the kidneys back into the blood.
  • Due to osmosis, water travels in the same direction as the sodium ions.
  • The blood pressure rises.
to sum it up
To sum it up
  • Draw a negative feedback diagram showing the stimulus, receptor, control centre, transmission of message, effectors and response.
osmoregulation in water

Do you remember this term?

Osmoregulation in Water
  • Some marine organisms body fluids are isotonic to there external environment
  • As long as they remain in their external environment their internal environmental will stay stable
  • These organisms are called Osmoconformers
  • What happens then when the external environments are hypotonic or hypertonic to the internal environment of an organism?

Lets have a look

Can you remember these terms?

osmoregulation in fish
Osmoregulation in Fish


Salt Water Fish

Murray Cod

Fresh Water Fish

With your knowledge of osmosis describe what would happen to each of these animals in terms of their salt and water levels if they did not have ways to control it?

osmoregulation in fish1
Osmoregulation in Fish


Murray Cod

Fresh Water Fish

  • Would take in too much water.
  • Would lose too much salt.


  • How does it maintain its salt water balance?
  • Gills that are highly permeable to water and salt.
  • Scales that are impermeable to water and salt.
  • Rarely drinking water.
  • Excreting large amounts of dilute urine.
  • Actively absorbing salts by specialised cells in the gills.

What types of adaptations are these?

osmoregulation in fish2
Osmoregulation in Fish



Salt Water Fish

  • Would lose too much water
  • Would take in to much salt


  • How does it maintain its salt water balance?
  • Scales that are impermeable to water and salt.
  • Gills that are highly permeable to water and salt.
  • Drinking almost continuously.
  • Producing small amounts of urine.
  • Actively excreting salts from specialise cells in the gills.

What types of adaptations are these?

adaptations to regulate water balance
Adaptations to Regulate Water Balance


Come up with a list of Structural, Physiological and Behavioural Adaptations that could help an organism regulate water balance.

adaptations to regulate water balance1

Waterproof layer

Hairs or valves guarding openings


Store it


Reabsorb water from faeces

Produce highly concentrated urine

Rely on metabolic water




Spend most time in burrows

Drinking water from environment

Adaptations to Regulate Water Balance
words to know1
Words To Know
  • Osmoregulation
  • Osmoregulator
  • Osmoconformer