Download Background information

Background information
Conditions within the body must be maintained at a constant level in order to achieve optimal metabolic
efficiency; enzymes, responsible for controlling all metabolic activities within the cell, will only operate
within a limited range of temperature and acidity. Solute concentration must in turn be kept constant in
order for these reactions to occur properly. Homeostasis is the process by which organisms maintain a
relatively stable environment.
Homeostasis consists of two stages:
detecting changes from the stable state - this is made possible by the presence of receptors in living
organisms. Receptors in animals consist of nerve cells that detect stimuli (a stimulus is any
information that causes a response). Receptors in plants usually include the shoot and root tips
working together with plant hormones.
counteracting changes from the stable state - this is brought about in living organisms by effectors. In
mammals, effectors are usually muscles or glands and in plants hormones such as auxins and
cytokinins act as effectors.
Changes in an organism's surroundings are detected by receptors, processed by a control centre
(usually the brain), and counteracted by effectors.
1)
Match the stimulus in column A with the receptors that detect it in column B below.
Stimulus
Changes in the concentration of blood
chemicals such as glucose, amino acids,
CO, oxygen and dissolved ions
Temperature changes
Light
_ Pressure, gravity, sound
2)
Receptor
Thermoreceptors
Mechanoreceptors
Chemorece tors
Photoreceptors
Match the effector in column A with the response it produces in column B below.
Effector
Response
Muscle
Gland
Plant hormones
Secretion of hormones or other chemicals;
examples of their action include control of
solute levels in the blood, diverting blood to
the muscles, secondary sexual characteristics.
Responses include control of flowering and
fruiting, growth of buds, stem elongation.
Examples include contraction to produce
movement, blood vessel constriction and
shivering.
3)
In a 'feedback loop' the receptors detect the response and send messages back to the control
centre to stop further adjustment. This is often referred to as 'negative feedback' because it results
in a negative response by the effector; for example the accumulation of a hormone in the blood
automatically cuts down its production.
Insert the words listed into the feedback loop shown below.
effector , stimulus , conductor (usually nerve fibres), receptor, response, control centre
A Feedback Loop
4)
Give a specific example of a negative feedback system which operates in mammals. Use a diagram
in your answer.
5)
List three things that must be maintained at a constant level in order to maintain a stable internal
environment in living organisms.
6)
Describe what a receptor is.
7)
Define the term 'stimulus'.
8)
State the two main stages in homeostasis.
THE ROLE OF THE NERVOUS SYSTEM 1N
DETECTING AND RESPONDING TO
ENVIRONMENTAL CHANGE
Read the following passage and answer the questions.
In endotherms a constant internal temperature is controlled by an inbuilt 'thermostat'; the hypothalamus,
which is located in the brain. The hypothalamus is a region of neurones and secretory cells and it
performs a variety of roles involving nervous and hormonal messages. In its role as a temperature
regulator, it acts by first detecting and then responding to changes in the surrounding temperature.
Receptor cells within the hypothalamus itself detect temperature changes from either nervous impulses
sent from 'thermoreceptor' cells in the skin, or from changes in the internal blood temperature of the
organism.
The hypothalamus responds to higher temperature signals by sending nervous messages to the
peripheral nervous system, which in turn stimulate cooling processes such as 'vasodilation' (blood vessels
dilate and move closer to the skin's surface, allowing heat to escape), sweating and panting. The resulting
effect is a lowering of the overall blood temperature.
When the hypothalamus detects lower temperatures it directs the peripheral nervous system to stimulate
heating mechanisms such as increased muscle activity (shivering), 'vasoconstriction' (blood vessels
constrict, so reducing heat loss to the surroundings) and the contraction of hair erector muscles to allow
hair, fur or feathers to stand on end. The result of these processes is a warmer blood temperature. The
involvement of the hypothalamus and peripheral nervous system in dealing with environmental change is
an example of a 'feedback loop', in which the response alters the stimulus, as shown in the diagram
below.
The Role of the Nervous System in Responding to Environmental Change
1
1)
Copy and complete the following passage, using the words listed below.
peripheral, receptors, blood, shivering, vasoconstriction, hypothalamus,
thermoreceptors, vasodilation, sweating, panting
The …………………… is an area of the brain involved in both nervous and hormonal control. It
contains sensory ……………….. that receive stimuli from …………….. in the skin and from
temperature changes in the ………………. . The hypothalamus then sends information via the
……………….. nervous system to produce heating responses such as ……………..
and
…………………, and cooling responses such as …………………. , ……………… and
……………….. .
3)
Draw and label a diagram to describe how both endotherms and ectotherms would use a simple
reflex action pathway to respond to extreme environmental stimuli such as heat or acidity. Include
the following terms in your labelling:
effector, receptor, motor neurone, sensory neurone, connecting neurone.
4)
Match column A with column B.
A
Hypothalamus
Thermoreceptors
Endotherm
Vasoconstriction
Peripheral Nervous System
Ectotherm
B
Stimulates sensory receptors in hypothalamus
to respond to heat and cold.
An organism that cannot regulate its body
temperature internally.
A mechanism in which blood vessels dilate to
allow heat loss to occur.
Mammal that utilises vasoconstriction to
reduce heat loss.
Area of brain that regulates internal
temperatures.
Mammal that utilises vasodilation to allow
heat loss in its desert environment.
Mitchell's Hopping Mouse
An organism capable of regulating its internal
environment.
Arctic Fox
A mechanism in which blood vessels constrict
to prevent heat loss.
Blood Temperature
Vasodilation
The neurones outside the central nervous
system.
Specialised heat sensitive neurones located in
the skin.
LIFE'S NARROW TEMPERATURE LIMITS
Read the passage below and answer the questions.
The temperature ranges in which life on earth can occur are dictated by the heat tolerance of enzymes and
other proteins within cells and by the need for a liquid medium in which molecules can react together.
At temperatures of only a few degrees below 0°C the cytoplasmic fluid of cells freezes. As a result,
molecules are either left at a concentration that is too high for satisfactory metabolic activity or immobilised
completely. Most proteins begin to denature at temperatures around 50°C - their three-dimensional
structure is altered and enzymes can no longer function.
The occurrence of life is limited to external environments within the narrow range of just below freezing to
45-50°C. Exceptions include some forms of cyanobacteria found in hot springs that have adapted so that
metabolism can occur at temperatures of about 100°C. Certain species of lichen and moss have been
known to withstand temperatures below -200°C and the seeds of some plants are capable of surviving
temperatures of over 100°C during bushfires.
Organisms living in large bodies of water such as oceans experience very little temperature variation
(usually no more than 10°C annually) because of the large volume to surface area ratio of their
surroundings. As a result, marine organisms do not need to posses as many adaptations for coping with
temperature changes. Instead, their body temperatures usually remain the same as the ambient water
temperature. Nevertheless, oceans in different areas of the world range from 0°C to around 40°C and
aquatic life forms can be found at all these temperatures.
Terrestrial organisms, on the other hand, must be able to adapt to wide temperature fluctuations, sometimes
within a few hours. In some places annual terrestrial temperatures may vary by as much as 70°C. Globally,
terrestrial temperatures range from approximately -60°C to 60°C. Temperatures in areas near the tropics
are usually warm and relatively constant whereas regions closer to the poles experience more seasonal
variation and larger temperature extremes.
1)
Explain why most forms of life cannot exist at temperatures a few degrees below 0°C.
2)
Outline the reasons why most forms of life cannot exist at temperatures above 50°C.
3)
Name some organisms which are exceptions in that they can survive outside the range of 0°C -50oC.
4)
Why don't aquatic organisms need to possess as many adaptations to temperature variations as
terrestrial organisms?
5)
Which areas of the earth experience greater seasonal variation and temperature extremes?
PLANT RESPONSES TO TEMPERATURE CHANGE
1)
Match Column A with Column B.
A
Waxy, shiny, hairy or light coloured leaf
surfaces
Vernalisation
B
Transpiration
Surface area
One of the major ways temperature
affects plants
Water loss and subsequent evaporation
from plants which helps
to cool the plant
Dormancy
Rhizomes, tubers and bulbs
This is reduced in the leaves of some
desert plants to reduce heat absorption
Help to reflect light and reduce the temperature
of the plant in hot conditions
Process in which some plants lose leaves
to avoid damage in cold conditions
Process in which plants must be subjected to cold
temperatures before the will flower
Underground reproductive structures
which survive when the plant above the
round dies
Eucalypt leaves hang this way to reduce
the surface area exposed to the sun
Some Australian plants need this for
their seed pods to open or for their
seeds to germinate
2)
Growth rate
The heat of a bushfire
Vertically
The table below shows the effect of temperature changes on growth in barley plants.
Temperature °C
20
22
24
26
28
30
Growth in mm/hr
4
6
6.8
7.2
8
9.1
Convert these figures into a line graph. Comment on the effect temperature has on the growth rate
of barley.
3)
Explain why heat loss through the evaporative cooling of transpiration poses a problem for
desert plants.