Unit 6 - Organisms respond to changes in their internal and external environments
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Homeostasis
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Unit 6 - Organisms respond to changes in their internal and external environments
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AQA
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AQA Biology A Level Year 2 Student Book
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Unit 6 - Organisms respond to changes in their internal and external environments
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Section 6 ( Organisms Response to Changes within Environments ), Topics 16
Homeostasis is the maintenance of a stable internal environment. As organisms exchange
with their environment, changes in the external environment can affect the internal.
Homeostasis involves in control systems and mechanisms that regulate the internal
environment in an attempt to keep it constant, and in favour of the organism.
This is significant as it is crucial to keep cells functioning normally and prevent irreversible
damage being sustained. Factors such as fluctuating temperature and blood pH can affect
proteins structure, making them denature and useless. Blood glucose can affect respiration
and energy levels, as well as blood water levels affecting cells' water potentials.
Homeostasis works through negative feedback, which is brought about by receptors,
communication systems and effectors. Homeostasis regulates many internal factors, all
regulated by specific receptors:
Temperature
Blood pH
Blood Glucose
Blood Water
Carbon Dioxide and Oxygen
These all may be coordinated by the Nervous System or the Endocrine System. Both are
coordination systems aimed to allow effective communication between control centres,
receptors and effectors. Exocrine system is also used but to less of an extent.
Exocrine Glands
Exocrine glands are glands that contain ducts that carry substances straight to target cells.
These are external body surfaces or epithelial layers ( intestines, lungs ).
Exocrine glands do not produce hormones, but secrete other substances onto surfaces:
Mammary Glands
o Secrete milk onto breast surface.
Salivary Glands
o Secrete saliva into the mouth.
Sweat Glands
o Secrete sweat onto body surfaces.
Thermoregulation
Animals such as mammals derive most of their heat from metabolic activities that occur in
the internal environment. These animals are known as Endotherms. Animals that obtain
their heat from their surroundings are Ectotherms.
, Endotherm Thermoregulation
Endotherms gain their heat from internal metabolic activities. Body temperature remains
constant despite the external temperature fluctuating. In order to maintain this
temperature difference, physiological and behavioural processes are used.
In endotherms with cold environments:
Vasoconstriction
o Diameter of arterioles decrease, decreasing volume of blood near skin
surface capillaries.
Involuntary Muscle Contractions
o Rapid impulses sent to muscles, causing shivering of muscles increases
respiration rate, increasing heat produced.
Elongation of Hair Follicles
o Hairs gather a layer of insulating air around the surface, decreasing
temperature gradient.
Increased Metabolic Rates
o Pancreas and Liver release more glucose into blood, allowing higher
respiration rates in ATP demanding cells. Thyroid glands produce Thyroxine,
increasing metabolism.
Decrease in Sweat Gland Activity
o Exocrine system decreases sweat gland activity, so no evaporation of sweat.
Behavioural
o e.g. Standing in sunlight.
In endotherms in warm environments:
Vasodilation
o Diameter of arterioles increase, increasing volume of blood near skin surface
capillaries.
Increase in Sweat Gland Activity
o Exocrine system increases sweat gland activity, causing evaporation of sweat
on surface.
Lowering of Hair Follicles
o Layer of insulating layer of air removed, allowing wind to remove warm
surface temperatures.
Behavioural
o e.g. Standing in the shade.
Ectotherm Thermoregulation
Ectotherms gain their heat from their external environments, and have little internal control
mechanisms. Without behavioural processes, their internal temperature would fluctuate.
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