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Week 13 Content

Overview of Week 13 Inquiry Question – How is an organism’s internal environment maintained in response to changing external environment?

Learning Objective #1 – Construct and interpret negative feedback loops to show homeostasis by using a range of sources including but not limited to:

  • Temperature
  • Blood glucose level

Learning Objective #2 – Investigate the various mechanisms used by organisms to maintain their internal environment within tolerance limits, including:

  • Trends and patterns in behavioural, structural and physiological adaptations in endotherms that assist in maintaining homeostasis
  • Internal coordination systems that allow homeostasis to be maintained, including hormones and neural pathways
  • Mechanisms in plants that allow water balance to be maintained 
NEW HSC Biology Syllabus Video – Homeostasis 

Week 13 Homework Questions 
 
Week 13 Curveball Questions
 
Week 13 Extension Questions
 
Solutions to Week 13 Questions

Overview of Week 13 Inquiry Question

Learning Objective #1 - Construct and interpret negative feedback loops to show homeostasis by using a range of sources including but not limited to:

- Temperature
- Blood Glucose Level

What is Homeostasis?

Homeostasis is the maintenance of a constant or almost constant internal environment, despite fluctuations in the external environmental.

The following diagram is a negative feedback loop for thermoregulation

That is, the regulation of temperature!

Homeostasis comprise of two stages:

1. Detecting the change (stimulus) from the stable state.

2. Counteracting the change (stimulus) to return to the stable state.

A stimulus is a change in the ambient or internal environment such as increase or decrease in temperature.

Thermoregulation is an example of homeostasis mechanism that you would expect to find in endotherms.

HSC-Biology-Notes-On- Thermoregulation-Homeostasis-And-Temperature

NOTE: The above diagram is labelled according to thermoreceptors on the skin detecting an INCREASE in ambient temperature as being step 1. 

Depending on the exam question, you may number or label in the opposite direction. 

  • That is, step 1 would involve thermoreceptors on skin detecting a DECREASE in ambient temperature. In that case, the numbering order would be reversed in direction, however, the flowchart will look the SAME.
The above mechanism of maintaining core body temperature within a narrow range is achieved using a negative feedback mechanism. 

The process of achieving homeostasis involves a negative feedback loop or mechanism which entails an effector being stimulated and performing an action directed by a control centre to reverse the original stimulus, effectively minimising the change (minimising deviation from set point e.g. core body temperature of 37 C). 

Moving on to human’s regulation of blood glucose level as another example of homeostasis occurring inside our body. 

Let’s examine the negative feedback loop! 

HSC-Biology-Blood-Glucose-Level-Homeostasis

Learning Objective #2 - Investigate the various mechanisms used by organisms to maintain their internal environment within tolerance levels, including:

- Trends and patterns in behavioural, structural and physiological adaptations in endotherms that assist in maintaining homeostasis

- Internal coordination systems that allow homeostasis to be maintained, including hormones and neural pathways

- Mechanisms in plants that allow water balance to be maintained

As explored in the Preliminary HSC Biology Course and Module 5, we examined how adaptations are inherited. However, in Module 6, we explored how favourable mutation can also give rise to adaptations that assist in the organism’s (or mutant) survival. If such mutation occurs in germ-line cells, it is possible for such mutation to be passed on to offspring in the next generation and be inheritable. 

Endotherms are organisms uses internal mechanisms to maintain their core body temperature within a narrow range, despite fluctuations in external environment. 

  • These animals have internal mechanisms allow the generation or production of internal heat energy to regulate their core body temperature. Hence, the term ‘warm-blooded’ animals.

Ectotherms are organisms that do not have internal mechanisms 

  • These animals DO NOT have internal mechanisms to generate heat and so they must relate on heat from the external environment to regulate their core body temperature. Hence, the term ‘cold-blooded’ animals.
  • Indeed, thermoregulation is NOT possible for ectotherms but performed by endotherms.

Now, we will move on to tackle the section of learning objective #2 in which we will examine some trends and patterns in behavioural, structural and physiological adaptations in endotherms that assist in maintaining homeostasis.  

Structural Adaptations

Mountain Pygmy Possum – Lives in cold, windy, mountainous regions of Australia. 

  • It has short legs, round body and small ears to minimise heat loss. This is because it will minimise the surface area in which the blood in blood vessel under the skin is exposed to the cold temperature of the environment which will result in heat being carried away convection (moving air carrying heat away).
Fairy Penguin – Lives in cold, southern seas of Australia
  • Again, it has short legs and round body similar to the Mountain Pygmy Possum. They also do not have external ear flaps which also minimises contact or exposure with cool surroundings to help maintain its core body temperature.
Red Kangaroo (Defying the above trend) – Lives in hot, arid areas of Australia such as deserts & grasslands.
  • They have a lot of blood vessels under their forearm and paws. This encourages heat loss via convection to keep their core body temperature within a narrow range, despite the hot ambient surroundings. 
 

Physiological Adaptations

Mountain Pygmy Possum

During long winters, the possum can enter a state of torpor whereby rate of metabolic activities and core body temperature are reduced to conserve energy. When it is in this state, it is able to tolerate surrounding temperatures of 2 degrees celsius. 

  • It can also curl into a ball to minimise the surface contact in which it is exposed to the its surroundings in cold conditions.

Fairy Penguin

Fairy Penguin are able to thermoregulate their core body temperature as their muscle glands are activated, resulting in involuntary shivering to produce heat energy.

Red Kangaroo

The muscle glands can also be activated via thermoregulation in kangaroos to generate heat energy in response to cold temperature stimulus detected by thermoreceptors on skin and in hypothalamus.

Panting is performed by Kangaroo whereby heat energy is loss through ventilation. The process of panting allows water on the tongue and mouth surface to be evaporated as the blood under these surfaces are able to be transferred to the cooler water via conduction. 

Behavioural Adaptations

Mountain Pygmy Possum

  • The possums are nocturnal. That is, they sleep during the day and are active during the night to escape the high temperatures of the body to avoid overheating. 
  • During the day, they can seek shade in holes within rocks or gaps under rocks. 

Fairy Penguin

  • To avoid overheating, Fairy Penguins are able to move into the cool or cold waters to lower their core body temperature or seek shade under rocks.
  • The penguins also hug each other to minimise each penguin’s surface area exposure to the cold environment.
Red Kangaroo
  • Similar to the mountain pygmy possum, the red kangaroos are also nocturnal. 
  • They also seek shade to avoid overheating during the day.

Internal coordination systems that allow homeostasis to be maintained, including hormones and neural pathways

It is critical for living organism to respond to both internal and external environmental changes. 

  • It is through responding to external environmental changes whereby we can keep our balance when walking on a footpath. If not, we will have great difficult walking in a straight line. 
  • We respond to internal changes to achieve thermoregulation. 

There are two main ways that we use respond to the environment. 

These two ways are through the nervous system and endocrine systems which are both internal coordination systems in our body to help achieve & maintain homeostasis. 

The nervous system is utilises electrochemical impulses to relay messages regarding to information such as the stimulus detected by receptors and appropriate reaction that is to be performed by effectors to counteract the stimulus and maintain homeostasis. 

Comparatively, the endocrine system employs hormones which are molecules that interact with specific receptors that are located on or within a specific target cell or tissue to initiate a response from the target cell/tissue. It is this response that allows homeostasis to be achieved and maintained. 

We will first explore the different parts of the nervous system before diving into the endocrine system.

The nervous system involves reception of stimulus, transmission of messages, interpretation and generation of response(s). Respectively, these functions can be performed using receptors, neurons, CNS and effectors. We will now explore each of these at a detail as required at HSC Biology level. 

A stimulus is a change in the internal or external environmental.

A receptor is one or a group of specialised cell that is responsible for detecting stimulus. They are also activated by the stimulus by converting physical feedback into electrochemical pulses or signals. 

In a negative feedback mechanism, an effector is an organ in the organism (such as a muscle or gland) that is activated by a neuron to carry out a response to counteract or oppose the stimulus. 

A neuron is a nerve call that consists of a cell body, dendrites as well as an axon that is enveloped by a myelin protein sheath. 

A neuronal fibre is comprised of many neurons that are connected together from one end to another. 

A nerve is a collection of neuronal fibres that is bundled up together. 

The CNS, also known as the central nervous system, is made up of the brain and spinal cord. 

The brain is involved regulating and coordination the internal environment of the organism. However, it is the hypothalamus that plays the biggest role in such regulation and coordination. 

  • The hypothalamus is involved in interpreting the stimulus detected by the receptors in the form of electrochemical signals and decide on an appropriate response to be carried out by relevant effectors to counteract the stimulus in a negative feedback mechanism. This interpretation of signals is performed using large numbers of interneurons in the CNS.
  • The hypothalamus is also involved in relaying the appropriate response to be carried out to effectors in the form of electrochemical signals. 

The spinal cord provides the neural pathway through which information detected by receptors pertaining to stimulus can be relayed to the brain in the form of electrochemical impulses via interneurons. It is also the pathway through which information regarding the appropriate response generated from the brain is relayed to effectors in the form of electrochemical impulses. 

The PNS, which is short for Peripheral Nervous System, are essentially a network of nerves that can be further divided into two categories which are sensory nerves and motor nerves. Recall that a nerve is essentially bundle of neuronal fibres whereby the fibres themselves are made up of many neurons or nerve cells joined together end-to-end. 

The role of sensory neurons (making up the sensory nerves) is to relay information detected by receptors about a stimulus, in the form of electrochemical impulses, to the CNS. Comparatively, the role of motor neurons is to transmit information regarding the appropriate response to be performed by effectors that is generated from the CNS (e.g. hypothalamus).

  • NOTE: Together, the CNS and PNS makes up the nervous system which provides neural pathways through which receptors can interact and coordinate with effectors to help achieve & maintain homeostasis.

The way the receptor relays information about a stimulus is that it conveys physical feedback into electrochemical signals which are carried by neurons in nerves (i.e. neural pathways) to the CNS. It is at the CNS where the stimulus is interpreted and an appropriate response message is produced and sent in the form of electrochemical message. 

This message is transmitted via effector neurons to effectors resulting in the effector to be activated upon receiving the nerve impulse (the electrochemical message). This activation or stimulation of the effector results it to carry out the appropriate response instruct by the CNS to counteract or oppose the stimulus in a negative feedback mechanism. 

The generation of electrochemical signals

So, how are message regarding the stimulus (sent from receptors to CNS) conveyed in the form of electrochemical signals? 

Similarly, how is the message regarding the appropriate response (sent from CNS to effectors) delivered in the form of electrochemical impulses?

Let’s see how this electrochemical impulse is produced, shall we? Let’s go!

So a nerve impulse is essentially a wave electrical charges (or depolarisation) that is propagated along a nerve cell’s axon as well as to another nerve cell. 

NOTE: Neurotransmitters are required for the wave of electrical charges to be passed from one neuron to another. When the electrochemical signal reaches the end of a neuron, it converted into a chemical known as a neutrotransmitter. This enables the signal to travel across synapses between neurons. 

These charges are generated as a result of sodium ions moved into the nerve cell. Hence, the we can refer to nerve impulses are essentially electrochemical impulses.

Now, let’s explore the mechanism behind the movement of sodium into neuron or nerve cell. 

When the neuron is not stimulated (i.e. not propagating a wave of electrical charges), it is at its rest (normal) state. 

At this state, there is an electrical difference (or potential) across the cell membrane of the neuron. When the neuron is at rest or unstimulated, we call this electrical difference as the ‘resting potential’ of the neuron’s membrane. Since there are more negatively charged organic ions inside the neuron and more sodium ions outside the neuron’s cell membrane, the rest potential of the neuron’s membrane is negative which ranges from -40mV to -90mV. 

When the neuron is stimulated by a stimulus (such as a change in temperature) that is of high enough intensity, it will result in sodium pumps in the cell membrane to open. As a result of this, sodium ions will diffuse across the cell membrane and into the neuron due to concentration difference. As a result, the membrane potential will be equalised to zero millivolts. 

If the membrane potential exceeds the -55mV threshold due to sodium ions flowing into the neuron, an action potential will be generated. 

  • NOTE: Sometimes, a membrane potential may also be produced if the change (decrease) in membrane potential is greater than 15mV.

Either way, if the membrane potential exceeds -55mV or have a change in membrane potential of greater than 15mV, a wave of electrical charge to be produced and propagated along the neuron.

The generation and propagation of the wave of electrical charge is an ‘all or nothing’ event. That is, if the -55mV threshold is not exceeded (or depolarisation is less than 15mV), there will be no nerve impulse generated. 

This effectively prevents insignificant events to continuously triggering nerve impulses and making the body overly sensitive.

A neuron cannot generate and propagate a new wave of electrical charge until the prior wave of electrical charge is complete. 

After the wave of electrical charge has been propagated across the axon of the nerve cell, the sodium pump will close and positively charged potassium ions will diffuse out of the cell due charge difference inside (positive) and outside the cell (negative). 

This diffusion process will eventually arrive to an equilibrium where the concentration of potassium ions inside and outside the cell are equal which will become the neuron’s resting potential, ranging from -40mV to -90mV. 

The neuron will ‘fire’ an impulse again when the axon of the neuron is stimulated by another stimulus that depolarises its cell membrane potential (or resting potential) by more than 15mV or increasing it above -55mV.

 

The workings of the Endocrine System

Hormones are secreted by endocrine glands (and some minor glands) into and transported by blood. 

These glands secrete hormones as a response to a specific stimulus. This stimulus can be divided into three categories being humoral, neural or hormonal. 

  1. Humoral: The changing concentration of specific chemicals in the chemical can result in the glands secreting hormones. 
  2. Neural: The stimulation of the glands via neurons can result in the secretion of hormones.
  3. Hormonal: Hormones can be secreted by the pituitary gland to control the amount of hormones secreted by other endocrine glands.Therefore, the pituitary gland is known as the ‘master gland’
We have already mentioned in the above section that hormones interact with specific receptors that are located on or within a specific target cell or tissue to initiate a response from the target cell/tissue. It is this response that allows homeostasis to be achieved and maintained. 
  • NOTE: Most of the cells in our body have surface receptor proteins that can interact with different hormones.

By interacting with the receptors of the target cell or tissue, the hormone is could:

  • Alter the cell membrane permeability to certain substances or
  • Modify the metabolic pathway of the cell (e.g. cause the cell to start or stop producing certain enzymes) or
  • Influence the rate of cell division.
 

The hypothalamus joins the nervous and endocrine systems together. 

This is because, in the nervous system, the hypothalamus interprets the stimulus detected by the receptor. In the endocrine system, the hypothalamus is responsible for controlling the endocrine glands as it connects to the pituitary gland. 

As mentioned above, we learnt how the pituitary gland (the ‘master gland’) is responsible for controlling the amount of hormones secreted by other endocrine glands.

Hormones' role in internal coordination system to achieve homeostasis!

Hormones are molecules that is produced and secreted to initiate cellular reactions in specific target cells, tissues or organs in the body of an organism. 

Osmoregulation is regulation of water and salt (or solute) concentration in the blood to achieve homeostasis. To achieve this, the Aldosterone and Anti-Diuretic Hormones are used. Let’s explore them. 

Hormone: Aldosterone 

1. Low water in Blood = Low blood pressure.

2. Release Aldosterone (by adrenal glands).

3. More sodium ions in urine flowing in kidney are re-absorbed back the blood in capillaries surrounding kidney.

4. Water in urine flowing in kidney is reabsorbed back into the blood via osmosis. 

Explanation of steps and how Aldosterone works:

Aldosterone is produced and secreted by the adrenal glands which is located above the kidneys. We have two kidneys and so we have two glands. There are stretch receptors that are located in the blood vessels. When these receptors detect a drop in blood pressure, it will cause cells (or granular cells) in the kidney to secrete an enzyme called renin. 

This enzyme is able to catalyse a reactions that cause the adrenal glands to secrete aldosterone. 

This hormone is able to act on the distal tubules, resulting in sodium ions to be actively reabsorbed back into the blood from the kidney (e.g. sodium ions in the urine flowing in kidney). As a result, the blood in the capillaries surrounding the kidney’s distal tubule will have a higher solute concentration due to the active re-absorption of sodium ions as a result of aldosterone. This will result in water travelling via osmosis from the urine in the kidney back into the blood, i.e. reabsorbed or conservation of water. This effectively increases the blood level and thus blood pressure of the organism, achieving homeostasis via osmoregulation. 

 

Hormone: Anti-Diuretic Hormone (ADH)

1. Low Water in Blood = High solute concentration.

2. Release Anti-Diuretic Hormone (by pituitary gland)

3. More water in urine flowing in the kidney is re-absorbed back into the blood in capillaries surrounding the kidney. 

Explanation of steps and how ADH works:

ADH is produced by the hypothalamus and stored in the pituitary glands. When osmoreceptors in the hypothalamus detect an increase solute concentration in the blood, it causes the pituitary gland to secrete ADH into the blood stream to reach the kidneys. 

It is at the kidneys whereby ADH acts on the distal tubules and collecting duct to increase their permeability to water. This increases the amount of water reabsorbed back into the blood from the kidney (water is present in the urine passing through kidney). As a result, the amount of water in the blood increases. 

The amount of ADH secreted will decrease when the solute concentration in the blood returns to normality. This allows homeostasis in between water and salt (or solute) concentration in the blood to be achieved, i.e. osmoregulation. 

NOTE: ADH does not directly influence or control the solute concentration in the blood. The hormone only indirectly affects the solute concentration in blood by controlling the amount of water being reabsorbed back into the blood from urine in the kidney.

NOTE: The secretion of ADH does not come to a stop, it only decreases or increases. The same is for Aldosterone, i.e. secretion of aldosterone does not come to a stop. 

Mechanisms in plants that allow water balance to be maintained

Alrightie, to wrap up this week’s notes, we will explore some mechanisms in plant that allows them to avoid dehydration ~

Crassulecean Acid Metabolism Pathway: The Stag Horn Fern is able to use the crassulecean acid metabolism pathway which allows it to close its stomata during the day and open it during night to acquire carbon dioxide for photosynthesis. This effectively minimises water loss via the stomata during the day where the sun is up, hitting the plant with its heat energy. This is possible because plants has large vacuoles in which it can store the carbon dioxide and used during the day when there is sunlight.

Leaf Fall: The eucalyptus tree are able to drop their leaves during summer where it is hot and dry. This effectively minimises the total surface area in which the tree is exposed to the ambient environment so that less heat is absorbed. By minimising the heat that is absorbed, the rate of transpiration is lowered and, thus, such leaf fall mechanism by the eucalyptus tree reduces its risk of dehydration.

Hanging leaf orientation: The eucalyptus tree has its leaves hanging in a vertical position so it reduces the surface area in which the leaves are exposed to the sun’s heat at its zenith. This effectively reduces the heat that is absorbed during the hottest time of the day and so it reduces the tree’s risk of dehydration. 

Controlled stomata opening and closing: The eucalyptus tree is able to control the time at which its stomata are opened and closed. During early morning and late afternoon, where the ambient temperature is cool and the sunlight is less intense, it is able to open its stomata to to obtain carbon dioxide necessary for photosynthesis to occur. This controlled manner of stomata opening and closing at the most suitable time allows the plant to avoid dehydration.

Curling of leaves: The Flax Lily plant is able to curl their leaves when temperature is high. Since its stomata are on the upper-side of its leaves, when the plant’s leaves curl up, the stomata will not longer be exposed to the external environment conditions. This effectively reduces the air flow across the stomata which would otherwise encourage transpiration.  Also, since the coiling of the leaf will trap a thin layer of humid air in the coil (i.e. between the leaf’s surface and the stomata) which help reduces transpiration. This because an increase in humidity reduces diffusion of water vapour out of the plant’s stomata. 

Thick, waxy cuticle: Eucalyptus and Banksia plants have thick cuticle that insulates water from excessive sunlight as it reduces the internal temperature. By doing so, the thick cuticle reduces the rate of evaporation.

Shiny Leaves: Eucalyptus plants have shiny leaves that help reflect sunlight and therefore help reduce the spike in internal temperature due to heat energy from sunlight. Again, this minimises the rate of transpiration and therefore serves to help minimise dehydration.

Hairs on leaf and stem: The paper daisy plant have hairs on its leaves and stem that help trap a thin layer of humid air. This increase in humidity reduces diffusion of water out of the plant’s cell which helps reduce transpiration. 

 

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