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Homeostasis
Homeostasis ​the maintenance of a constant internal environment
is
Changes in your external environment can affect your internal environment
Homeostasis involves control systems that keep your internal environment roughly constant
Keeping your internal environment constant is vital for cells to function normally and to stop
them being damaged
It’s particularly important to maintain the right core body temperature and blood pH
This is because temperature and pH affect enzyme activity
Enzymes control the rate of metabolic reactions
Temperature
If body temperature is too high, enzymes may become denatured
The enzyme’s molecules vibrate too much
This breaks the hydrogen bonds that hold them in their 3D shape
The shape of the enzyme’s active site is changed
It no longer works as a catalyst
This means metabolic reactions are less efficient
If body temperature is too low enzyme activity is reduced
This slows the rate of metabolic reactions
The highest rate of enzyme activity happens at their optimum temperature
In humans, this is 37O​
​
C
pH
If blood pH is too high or too low, enzymes become denatured
The hydrogen bonds that hold them in their 3D shape are affected
The shape of the enzyme’s active site is changed
It no longer works as a catalyst

This means metabolic reactions are less efficient
The highest rate of enzyme activity happens at their optimum pH
This is usually around pH 7
Some enzymes work better at other pHs
For example enzymes found in the stomach work better at a low pH
It is important to maintain the right concentration of glucose in the blood
Cells need glucose for energy
Blood glucose concentration also affects the water potential of blood
Concentration
If blood glucose concentration is too high the water potential of blood is reduced to a point
where water molecules diffuse out of cells into the blood by osmosis
This can cause the cells to shrivel up and die
If blood glucose concentration is too low, cells are unable to carry out normal activities
This is because there isn’t enough glucose for respiration to provide energy
Homeostatic systems detect a change
They respond by negative feedback
Homeostatic systems involve receptors, a communication system and effectors
Receptors detect when a level is too high or too low
The information’s communicated via the nervous system or the hormonal system to effectors
The effectors respond to counteract the change
This brings the level back to normal
The mechanism that restores the level to normal is called a negative feedback mechanism
Negative feedback keeps things around the normal level
Negative feedback only works within certain limits
If the change is too big the effectors may not be able to counteract it

Homeostasis involves multiple negative feedback mechanisms for each thing being controlled
This is because having more than one mechanism gives more control over changes in your
internal environment
Having multiple negative feedback mechanisms means you can actively increase or decrease a
level
This ensures it returns to normal
If you only had one negative feedback mechanism, all you could do would be turn it on or turn
it off
A level could only actively change in one direction
This ensures it returns to normal
Only one negative feedback mechanism means a slower response and less control
Some changes trigger a positive feedback mechanism
This amplifies the changes
The effectors respond to further increase the level away from the normal level
Positive feedback is useful to rapidly activate something
For example a blood clot after an injury
Platelets become activated and release a chemical
This triggers more platelets to become activated
Platelets very quickly form a blood clot at the injury site
The process ends with negative feedback, when the body detects the blood clot has been formed
Positive feedback can also happen when a homeostatic system breaks down
For example when you’re cold for too long
Hypothermia involves positive feedback
Hypothermia ​a low body temperature, below 35O​
is
​
C
It happens when heat is lost from the body quicker than it can be produced
As body temperature falls, the brain doesn’t work properly

Shivering stops
This makes body temperature fall even more
Positive feedback takes body temperature further away from the normal level
It continues to decrease unless action is taken
Positive feedback is not involved in homeostasis
It does not keep your internal environment constant

Control of Body Temperature
Temperature is controlled differently in ectotherms and endotherms
Animals are classed as either ectotherms and endotherms
This depends on how they control their body temperature
Ectotherms cannot control their body temperature internally
They control their temperature by changing their behaviour
For example, reptiles gain heat by basking in the sun
Their internal temperature depends on the external temperature
Their activity level depends on the external temperature
They’re more active at higher temperatures
They’re less active at lower temperatures
They have a variable metabolic rate
They generate very little heat themselves
Endotherms control their body temperature internally by homeostasis
They can also control their temperature by behaviour
For example, finding shade
Their internal temperature is less affected by the external temperature
Their activity level is largely independent of the external temperature
They can be active at any temperature

They have a constantly high metabolic rate
They generate a lot of heat from metabolic reactions
Mammals have many mechanisms to change body temperature
Heat loss
Sweating - more sweat is secreted from sweat glands when the body is too hot
The water in sweat evaporates from the surface of the skin
This takes heat from the body
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
When it’s hot, erector pili muscles relax
This allows the hair to lie flat
Less air is trapped
The skin is therefore less insulated
Heat can then be lost more easily
Vasodilation - when it’s hot, arterioles near the surface of the skin dilate
This is called vasodilation
More blood flows through the capillaries in the surface layers of the dermis
This means more heat is lost from the skin by radiation
The temperature is lowered
Heat production
Shivering - when it’s cold, muscles contract in spasms
This makes the body shiver
More heat is produced from increased respiration
Hormones - the body releases adrenaline and thyroxine

These increase metabolism
More heat is produced
Heat conservation
Much less sweat - less sweat is secreted from sweat glands when it’s cold
This reduces the amount of heat loss
Hairs stand up - erector pili muscles contract when it’s cold
This makes the hairs stand up
This traps more air
This prevents heat loss
Vasoconstriction - when it’s cold, arterioles near the surface of the skin constrict
This is called vasoconstriction
Less blood flows through the capillaries in the surface layers of the dermis
This reduces heat loss
Body temperature in mammals is maintained at a constant level by the hypothalamus
The hypothalamus receives information about both internal and external temperature from
thermoreceptors
Information about internal temperature comes from thermoreceptors in the hypothalamus
that detect blood temperature
Information about external temperature comes from thermoreceptors in the skin that detect
skin temperature
Thermoreceptors send impulses along sensory neurones to the hypothalamus
This sends impulses along motor neurones to effectors
The neurons are part of the autonomic nervous system
This is all done unconsciously
The effectors respond to restore the body temperature back to normal

Control of Blood Glucose Concentration
All cells need a constant energy supply to work
So blood glucose concentration must be carefully controlled
3​
The concentration of glucose in the blood is normally aroun 90mg per 100cm​ blood
of

It’s monitoeed by cells in the pancreas
Blood glucose concentration rises after eating food containing carbohydrates
Blood glucose concentration falls after exercise
More glucose is used in respiration to release energy
Insulin and glucagon control blood glucose concentration
The hormonal system controls blood glucose concentration using two hormones called insulin
and glucagon

They’re both secreted by clysters of cells in the pancreas called the islets of Langerhans:
Beta (ß) cells secrete insulin into the blood
Alpha (å) cells secrete glucagon into the blood
Insulin and glucagon act as effectors
They respond to restore the blood glucose concentration to the normal level
Insulin lowers blood glucose concentration when it's too high
Insulin binds to specific receptors on the cell membranes of liver cells and muscle cells
It increases the permeability of cell membranes to glucose
Cells can now take up more glucose
Insulin activates enzymes that convert glucose into glycogen
Cells are able to store glycogen in their cytoplasm as an energy source
The process of forming glycogen from glucose is called glycogen ess
Insulin also increases the rate of respiration of glucose, especially in muscle cells
Glucagon raises blood glucose concentration when it's too low
Glucagon binds to specific receptors on the cell membranes of liver cells
It activates enzymes that break down glycogen into glucose
The process of breaking down glucagon is called glycogenolysis
Glycogen also promotes the formation of glucose from fatty acids and amino acids
The process of forming glucose from non-carbohydrates is called gluconegogenesis
Glucagon decreases the rate of respiration of glucose in cells
DIAGRAM
Adrenaline is hormone that's secreted from adrenal glands
It's secreted when:
There's a low concentration of glucose in the blood
Somebody's stressed

During exercise
Adrenaline binds to receptors in the cell membrane of liver cells:
It activates glycogenolysis
It inhibits glycogenesis
It activates glucagon secretion and inhibits insulin secretion
This increases glucose concentration
Adreninaline gets the body ready for action by making more glucose available for muscles to
respire
Both adrenaline and glucagon can activate glycogenolysis inside a cell
This is done even though they bind to receptors on the outside of the cell
Adrenaline and glucagon bind to their specific receptors
They activates an enzyme called adenylate cyclise
Activated adenylate cyclise converts ATP into a chemical signal called a 'second messenger'
This second messenger is called cyclic AMP
cAMP activates a cascade that break down glycogen into glucose through glycogenolysis
Diabetes ​a condition where blood glucose concentration cannot be controlled properly
is
In type one diabetes, β cells in the islets of Langerhans do not produce any insulin
After eating, the blood glucose level rises and stays high
This is known as hyperglycaemia
It can result in death if left untreated
The kidneys cannot re absorb all this glucose
Some of it is excreted in the urine
Type two diabetes is usually acquired later in life
It is linked with obesity
It occurs when:

n't produce enough insulin
When the body's cells don't respond properly to insulin
Cells don't respond properly because the insulin receptors on their membranes don't work
properly
Cells therefore don't take up enough glucose
This means the blood glucose concentration is higher than normal
It can be treated by controlling simple carbohydrate intake and losing weight
Glucose-lowering tablets can be taken if diet and weight-loss can't control it

Control of the Menstrual Cycle
The human menstrual cycle last ps about 28 days
It involves:
A​
follicle ​an egg and it's surrounding cells developing the ovary
is
Ovulation - an egg being realeased
The uterus lining becoming thicker also that a fertilised egg can implant
A structure called a corpus luteum developing from the remains of the follicle
If there's no fertilisation, the uterus lining breaks down and leaves the body through the
vagina
This is known as menstruation
This marks the end of one cycle and the start of another
The menstrual cycle's controlled by the action of four hormones:
Follicle-stimulating hormone stimulates the follicle to develop
Luteinising hormone stimulates ovulation and stimulates the corpus luteum to develop
Oestrogen stimulates the uterus lining to thicken
Progesterone maintains the thick uterus lining, ready for the implantation of an embryo
FSH and LH are secreted by the anterior pituitary gland
Oestrogen and progesterone are secreted by the ovaries

Hormone concentrations change during different stages of the cycle
1
...
Rising concentrations of oestrogen
Oestrogen stimulates the uterus lining to thicken
Oestrogen inhibits FSH being released from the pituitary gland
3
...
LH surge
Ovulation is stimulated by LH
The follicle ruptures and the egg is released
LH stimulates the ruptures follicle to turn into a corpus luteum
The corpus luteum releases progesterone
5
...
Falling concentration of progesterone
FSH and LH concentrations increase as they're no longer inhibited by progesterone
The uterus lining isn't maintained
It breaks down
Menstruation happens
The cycle starts again

Negative and positive feedback mechanisms control the level of hormones
For example:
FSH stimulates the ovary to release oestrogen
Oestrogen inhibits further release of FSH
After FSH has stimulated follicle development, negative feedback keep the FSH concentration
low
This makes sure than no more follicles develop
DIAGRAM
LH stimulates the corpus luteum to develop
This produces progesterone
Progestoerone inhibits further release of LH
Negative feedback makes sure no more follicles develop when the corpus luteum is developing
It also makes sure the uterus lining isn't maintained if no embryo implants
DIAGRAM
Oestrogen stimulates the anterior pituitary to release LH
LH stimulates the ovary to release more oestrogen
Oestrogen further stimulates the anterior pituitary to release LH
High oestrogen concentration triggers positive feedback to make ovulation happen
DIAGRAM



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