Homeostasis and thermostability:

The need for communication:

Outline the need for communication systems within multicellular organisms, with reference to the need to respond to changes in the external and internal environment and to coordinate the activities of different organs

State that cells need to communicate with each other by a process called cell signalling

State that neuronal and hormonal systems are examples of cell signalling

Keeping Cells Active

All living cells need to maintain a certain limited set of conditions inside their cells. This is because the cellular activities rely on the action of enzymes.

Enzymes need a specific set of conditions in which to work efficiently. These include:

A suitable temperature A suitable pH An aqueous environment that keeps the substrates and products in solution Freedom from toxins and excess inhibitors

Stimulus and response: External environments

STIMULUS: any change in the environment that causes a response

RESPONSE: a change in behaviour or physiology as a result of a change in the environment

All living organisms have an external environment that consists of the air, water or soil around them. This external environment will change, as it changes it may place stress on the living organism. - For instance, a cooler environment will cause greater heat loss. If the organism is to remain active and survive, the changes in the environment must be monitored and the

organism must change its behaviour or physiology to reduce the stress. The environment change is a stimulus and the way in which the organism changes its behaviour or

physiology is it response The environment may change slowly- As the seasons change- Global warming These changes will elicit a gradual response However the environment may change more quickly. The changes from day to night or walking from bright light into an unlit room are rapid changes. The change (stimulus) must be monitored and the organism must respond to the change.

Internal environments:

Most multicellular organisms have a range of tissues and organs. Many of the cells and tissues are not exposed to the external environment, they are protected by epithelial

tissues and organs such as skin and bark. In animals the internal cells and tissues are bathed in tissue fluid. This is the environment of the cells. As cells undergo their various metabolic activities they use up substrates and produce products Some products may be unwanted or toxic These substances diffuse out of the cells into the tissue fluid.

Therefore, the activities of the cells alter their own environment. One waste product is Carbon Dioxide- If this is allowed to build up in the tissue fluid outside the cells it could disrupt the action of enzymes by

changing the pH of the environment around the cell. Accumulation of waste or toxins in this internal environment must act as a stimulus to cause removal of

these wastes This stimulus may act directly on the cells which respond by reducing their activities so that less waste is

produced However, this response may not be good for the whole organism.

External environments:

Include environments that are subject to change such as :- Air- Water- Soil Changes in the environment must be monitored

Maintaining the internal environment of the cells

EXCRETION: the removal of metabolic waste (waste from the reactions inside cells) from the body

The composition of the tissue fluid is maintained by the blood. Blood flows throughout the body and transports substances to and from the cells. Any wastes or toxins accumulating in the tissue fluid are likely to enter the blood and be carried away. In order to prevent their accumulation in the blood they must be removed from the body (excretion) It is important that the concentrations of wastes and all substances in the blood are monitored. This ensures that the body does not excrete too must of any useful substance but removed enough of the

wastes to maintain good health It also ensures that all the cells of the body are supplied with the substrates they need.

Coordination:

A multicellular organism is more efficient that a single celled organism, as its cells can be differentiated. Its cells are specialised to perform particular functions. Groups of cells specialised in this way form tissues and organs. The cells that monitor the blood may be in a different part of the body and well away from the cells that

release a substance into the blood or well away from the organs that removes the substance from the blood. Therefore a good communication system is required to ensure that these different parts of the body work

together effectively.

A Good communication system will:

Cover the whole body Enable cells to communicate with each other Enable specific communication Enable rapid communication Enable both short-term and long- term responses. Coordinate activities of different organs Respond to changes in the internal environment

Cell signalling:

Cells communicate with each other by the process of cell signalling. This is a process in which one cell will release a chemical that is detected by another cell. The second cell will respond to the signal released by the first cell.

There are two major systems of communication that work by cell signalling:

1. The neural system - An interconnected network of neurones that signal to each other across synapse junctions- The neurones can conduct a signal very quickly and enable rapid responses to stimuli that may be changing

quickly.2. The hormonal system- Uses the blood to transport its signals- Cells in an endocrine organ release the signal (a hormone) directly into the blood- It is carried all over the body but is only recognised by specific target cells. - The hormonal system enables longer term responses to be coordinated.

Homeostasis and Negative Feedback:

Define the terms negative feedback, positive feedback and homeostasis

Explain the principles of homeostasis in terms of receptors, effectors and negative feedback

Homeostasis:

HOMEOSTASIS: the maintenance of the internal environment in a constant state despite external changed

Homeostasis is keeping the internal environment constant despite external changes Many living organisms have to keep a great number of conditions constant inside the body. These may

include:- Body temperature- Blood glucose concentration- Blood salt concentration- Water potential of the blood- Blood pressure- Carbon Dioxide Concentration

Negative Feedback:

NEGATIVE FEEDBACK: a process that brings about a reversal of any change in conditions. It ensures that an optimum steady state can be maintained, as the internal environment is returned to its original set of conditions after any change. It is essential for homeostasis.

In order to maintain a constant internal environment a number of processes must occur.

Any change to the internal environment must be detected. Any change must be signalled to other cells There must be a response that reverses the change.

The reversal of a change in the internal environment to return to a steady state or optimum position is known as negative feedback.

For negative feedback to work effectively there must be a complex arrangement of structures that are all coordinated through cell signalling

These structures are part of the standard pathway that is used to produce a suitable response to a stimulus.

Stimulus Receptor Communication Pathway (cell signalling nerves/hormones) Effectors Response

There are a number of structures required for this pathway to work:

Sensory receptors: - Such as temperature receptors or glucose concentration receptors- These are internal and monitor conditions inside the body- Detect changes and will be stimulated to send a message. A communication system: - Such as the nervous system or the hormonal system- This acts by signalling between cells- It is used to transmit a message from the receptor cells to the effector cells- The message may or may not pass through a coordination centre such as the brain. Effector cells: - Such as liver cells or muscle cells- These bring about a response that reverses the change detected by the receptor cells.

This type of system in which the level of a factor triggers a corrective mechanism is called a self-adjusting system

Control mechanisms:

Homeostatic control mechanisms use negative feedback

All have output that is controlled & a set point Set point is usually determined genetically Detectors monitor output (receptors) Comparator compares actual output with set point- Produces error signals and conveys difference between actual and optimum output- Usually nerve impulses or hormones Effectors restores output to set point Thermoregulation is separate but coordinated mechanisms control deviations in different directions from

set point for greater control

Homeostasis is a dynamic process- it works by continual adjustments to compensate for fluctuations of output

Positive Feedback:

POSITIVE FEEDBACK: a process that increases any change detected by the receptors. It tends to be harmful and does not lead to homeostasis.

Positive feedback is less common than negative feedback When positive feedback occurs the response is to increase the original change This destabilises the system and is usually harmful E.g. when the body get too cold- Below a certain core body temperature the enzymes become less active- If they are less active the exergonic reactions that release heat are slower and release less heat- This allows the body to cool further and slows down the enzyme controlled reaction even more, so that the

body temperature spirals downwards. There are, however, some occasions when positive feedback can be beneficial. Positive feedback is used to stimulate an increase in a change E.g. at the end of pregnancy to bring about the dilation of the cervix- As the cervix begins to stretch the change is signalled to the anterior pituitary gland, stimulating it to

secrete the hormone oxytocin- Oxytocin increases the uterine contractions, which stretch the cervix more, which causes the secretion of

more oxytocin- Once the cervix is fully dilated, the baby can be born.

The meaning of constant:

A negative feedback system can maintain a reasonably constant set of conditions However, the conditions will never remain perfectly constant There will be some variation about the mean or optimum condition As long as this variation is not too great then the conditions will remain acceptable Conditions inside a living organisms will remain within a relatively narrow range

Maintaining body temperature – Ectotherms:

Describe the physiological and behavioural responses that maintain a constant core body temperature in ectotherms

The need to maintain body temperature:

Changes in body temperature can have a dramatic effect upon the structure of proteins, including enzymes Enzymes are globular proteins, and their structure is very specific to their function The activity of enzymes is affected if they are not kept at, or close to, their optimum temperature, so

temperature affects their ability to function inside cell If enzymes do not function properly, the level of activity that can be achieved by the organism will be

dramatically affected. The core temperature is the important factor as all the vital organs are found within the main parts of the

body Peripheral parts of the body may be allowed to increase or decrease in temperature without affecting the

survival of the individual.

Ectotherms or Endotherms:

Endotherms:

Endotherms can maintain the temperature of their bodies within fairly strict limits They are largely independent of the external temperature.

Ectotherms:

ECTOTHERM: an organism that relies on external sources of heart to regulate its body temperature.

The body temperature of an ectotherm tends to fluctuate with the external temperature Ectotherms are unable to increase respiration rates to generate heat internally They rely on external sources of heat to keep warm Despite this, many ectotherms can successfully regulate their body temperatures under all but the most

extreme conditions. Metabolic rate varies

Advantages:

Ectotherms use less of their food in respiration They need to find less food and may be able to survive for long periods without eating For example, a snake can last weeks between meals. A greater proportion of the energy obtained from food can be used for growth.

Disadvantages:

They are less active in cooler conditions, and may need to warm up in the morning before they can be active This puts them at greater risk of predation. Lizards can often be seen basking in the sun during the early morning. They may be incapable of activity during the winter as they never warm up sufficiently This means that they must have sufficient stores of energy to survive over the winter without eating.

Temperature regulation in ectotherms:

Ectotherms do not use internal energy sources to maintain their body temperature when cold However, once they are active their muscle contractions will generate some heat from increased

respiration.

Temperature regulation relies upon increasing the heat exchange with their environment:

When an ectotherm is cold it will change its behaviour or physiology to increase absorption of heat from its environment

When an ectotherm is hot it will change its behaviour or physiology to decrease absorption of heat and increase heat loss to its environment.

In order to warm up they will bask in the sun or lie on a warm surface If they are too hot they will stay underground or in the shade

Some Ectotherms possess physiological or anatomical adaptations to help exchange heat with their environment.

Adaptation How it helps Regulate Temperature

Example

Expose Body To Sun Enables more heat to be absorbed SnakesOrientate Body To Sun Exposes larger surface area for more

heat absorptionLocusts

Orientate Body Away From Sun Exposes lower surface area so that Locusts

less heat is absorbedHide In Burrow Reduces heat absorption by keeping

out of the sunLizards

Alter Body Shape Exposes more or less surface area to the sun

Horned Lizards

Increase Breathing Movements Evaporates more water Locusts

Maintaining Body Temperature – Endotherms:

Describe the physiological and behavioural responses that maintain a constant core body temperature in endotherms, with reference to peripheral temperature receptors, the hypothalamus and effectors in skin and muscles.

ENDOTHERMS: an organism that can use internal sources of heat, such as heat generated from metabolism in the liver, to maintain its body temperature.

Use internal sources of heat to maintain their body temperature. Many chemical reactions in the body are exergonic – they release energy in the form of heat. Endotherms can increase the rate of respiration in the liver (an exergonic reaction), simply to release heat;

they are using some of their energy intake to stay warm. Endotherms, like ectotherms, can use behavioural mechanisms as well to help maintain body temperature

(basking in the sun, for example). They also have useful physiological mechanisms, such as redirecting blood to or away from the skin.

There are many advantages of endothermy:

A fairly constant body temperature whatever the temperature is externally Activity possible when external temperatures are cool – such as at night, early in the morning or during the

winter. Ability to inhabit colder parts of the planet.

Disadvantages:

A significant part of the energy intake is used to maintain body temperature in the cold More food required Less of the energy from food is used for growth, or more food is needed in order to grow.

Temperature regulation in endotherms:

Several behavioural and physiological responses are used by endothermic organisms to maintain their constant body temperatures.

Physiological mechanisms to reduce body temperature:

Sweating:

More sweat is secreted from the sweat glands when the body is too hot The water in sweat evaporates from the surface of the skin and removes heat from the body (high latent

heat of evaporation) The skin is cooled

Hairs lie flat:

Mammals have a layer of hair that provides insulation by trapping air (air is a poor conductor of heat)

Erector pili muscles relax so the hairs life flat when the body is hot Less air is trapped, so the skin is less insulated and heat can be lost more easily

Vasodilation:

When it’s hot, arterioles near the surface of the skin dilate More blood flows through the capillaries in the surface layers of the dermis More heat is lost from the skin by radiation and the temperature is lowered

Mechanisms to increase body temperature:

Shivering: (thermogenesis)

When it’s cold, muscles contract in spasms This makes the body shiver and more heat is produced from increased respiration

Hormones: (thermogenesis)

The body releases adrenaline and thyroxine These increase metabolism so more heat is produced

Less sweat:

Less sweat is secreted from the sweat glands Reduces the amount of heat loss

Piloerection:

Erector pili muscles contract when it’s cold which makes the hairs stand up This traps more air and so prevents heat loss

Vasoconstriction:

When it’s cold, arterioles near the surface of the skin constrict Less blood flows through the capillaries in the surface layers of the dermis This reduces heat loss

Physiological Mechanisms To Maintain Body TemperatureComponent Of Body Involved Response If Core Body Temperature

Is Too HighResponse If Core Body Temperature Is Too Low

Sweat Glands In Skin Secrete more sweat onto skin: water in sweat evaporates, using heat from blood to supply latent heat of evaporation

Less sweat is secreted; less evaporation of water, so less loss of latent heat.

Lungs, Mouth and Nose Panting increases evaporation of The animal does not pant, so less

water from the lungs, tongue and other moist surfaces, using latent heat as above.

water evaporates

Hairs On Skin Hairs lie flat, providing little insulation and thus more heat can be lost by convection and radiation

Hairs are raised to trap a layer of insulating air, reducing the loss of heat from the skin

Arterioles Leading To Capillaries In Skin

Vasodilation allows more blood into the capillaries near the skin surface; more heat can be radiated from the skin, which, in pale skinned people, may look red.

Vasoconstriction reduces the flow of blood through capillaries near the surface of the skin; less heat is radiated.

Liver Cells Rate of metabolism is reduced; less heat is generated from exergonic reaction such as respiration

Rate of metabolism is increased, therefore respiration generates more heat, which is transferred to blood

Skeletal Muscles No Spontaneous Contractions Spontaneous contractions generate heat as muscle cells respire more.

Behavioural Mechanisms To Maintain Body TemperatureBehaviour If Too Hot Behaviour If Too ColdMove into shade or hide in burrows Move into the sunlightOrientate body to decrease surface area exposed to the sun.

Orientate body to increase surface area exposed

Remain inactive and spread out the limbs to increase the surface area.

Move about to generate heat in mulches (except in the cold when it is better to keep still and roll into a ball to reduce the surface area)

Control of temperature:

The maintenance of core body temperature is important.

Endotherms monitor the temperature of their blood in the hypothalamus of the brain The hypothalamus receives information about both internal and external temperature from

thermoreceptors- Thermoreceptors in the hypothalamus detect internal temperature (temperature of the blood)- Thermoreceptors in the skin (peripheral temperature receptors) detect external temperature (temperature

of the skin) Thermoreceptors send impulses along sensory neurones to the hypothalamus which sends impulses along

motor neurones to effectors If the core temperature drops below an optimum the hypothalamus sends signals to reverse the change.

This will involve several changes:

Increased rate of metabolism in order to release more heat from exergonic reactions. Release of heat through extra muscular contraction. Decreased heat loss to the environment.

If the core temperature rises above the optimum the hypothalamus sends signals that bring about the opposite changes. This is an example of negative feedback.

The hypothalamus:

Contains 2 thermoreceptor centres:- A heat loss centre in the anterior hypothalamus which is activated by increases in blood temperature. It

uses nerve impulses and hormones as the error signals to activate responses that can increase heat loss- A heat gain centre in the posterior hypothalamus is activated by decreases in blood temperature. This uses

error signals to initiate a variety of corrective mechanisms which conserve body heat and raise the blood temperature

Rise in core temperature

Thermoregulatory centre in the

hypothalamus detects change

Nervous system and hormonal system carry

signals to skin liver and muscles

Less heat generates and more heat lost Temperatre Falls

Fall In Core Temperature

Thermoregulatory centre in the

hypothalamus detects change

Nervous system and hormonal

system carry signals to skin liver and

muscles

Moreheat generates and more heat lost

Temperatre Rises

The role of peripheral temperature receptors:

The thermoregulatory centre in the hypothalamus monitors the blood temperature and detects any change in the core body temperature

An early warning that the body temperature may change helps the hypothalamus to respond more quickly and avoid too much variation in core body temperature.

If the extremities start to cool down or warm up this may eventually affect the core body temperature The peripheral temperature receptors in the skin monitor the temperature in the extremities This information is fed to the thermoregulatory centre in the hypothalamus This is part of the autonomic nervous system The brain can initiate behavioural mechanisms for maintaining body temperature, such as moving into the

shade.

STIMULUS: any change in the environment that causes a response

RESPONSE: a change in behaviour or physiology as a result of a change in the environment

EXCRETION: the removal of metabolic waste (waste from the reactions inside cells) from the body

HOMEOSTASIS: the maintenance of the internal environment in a constant state despite external changed

NEGATIVE FEEDBACK: a process that brings about a reversal of any change in conditions. It ensures that an optimum steady state can be maintained, as the internal environment is returned to its original set of conditions after any change. It is essential for homeostasis.

POSITIVE FEEDBACK: a process that increases any change detected by the receptors. It tends to be harmful and does not lead to homeostasis.

ECTOTHERM: an organism that relies on external sources of heart to regulate its body temperature.

ENDOTHERMS: an organism that can use internal sources of heat, such as heat generated from metabolism in the liver, to maintain its body temperature.