Download - Biology unit 5 (BIOL5) Homeostasis
Unit 5.4
Homeostasis: maintenance of a constant internal environment
Know homeostasis is the control of the internal environment within restricted limits and to appreciate its dynamic nature
Know core temperatures, blood pH will have an effect upon optimal rate of enzyme action
Understand the importance of maintaining a constant blood glucose concentration in terms of energy transfer and water potential of blood.
Understand homeostasis is achieved by negative feedback, i.e. when an increase from a set point is detected by a receptor and via a corrective mechanism leads to a decrease and vice versa
Know how positive feedback, where increase leads to further increase and decrease to further decrease is associated with breakdown of control systems e.g. temperature control
Interpret diagrammatic representations of negative and positive control systems
Know the role of the hypothalamus (as a detector), autonomic nervous system, methods of heat production, conservation and loss (radiation convection and conduction) through which negative feedback maintains constant body temperature
Contrast the mechanisms of temperature control in an ectothermic reptile and an endothermic mammal
Appreciate factors which affect blood glucose levels
Know the role of the pancreas, alpha and beta cells as detectors of blood glucose and producers of glucagon and insulin
Know the role of insulin and glucagon in controlling the uptake of glucose by cells and their activation of enzymes
Know the role of the liver in glycogenesis and gluconeogenesis involving the interconversion of glucose and glycogen
Understand the effect of adrenaline on glycogen breakdown and synthesis
Explain the second messenger model of adrenaline and glucagon in action
Appreciate how control of type 1 and 2 diabetes can be achieved by manipulating diet and use of insulin.
Key words
Homeostasis, negative feedback, positive feedback, endotherm, ectotherm, islets of Langerhans,
glucagon, insulin, glycogenesis, glycogenolysis, gluconeogenesis, hydrolysis, condensation.
Ways of heat gain/loss
Heat gain
o Respiration
o Conduction
o Radiation
o Convection
Heat loss
o Evaporation of water in sweat
o Conduction
o Radiation
o Convection
Ectotherm
o They control their body
temperature by adapting
their behaviour to changes in
the environment
o Gain heat from the
environment
Exposing themselves
to the sun
Taking shelter
Gaining warmth from
the ground
Generating metabolic
heat
Colour variations
Endotherm
o Have both physiological and behavioural responses to maintain body temperature
o Humans
Skin receptors (hot and
cold) send impulses
along sensory neurone
to hypothalamus
Thermoreceptors in the
hypothalamus which
monitors blood
temperature
Hypothalamus is
connected to our heat
gain centre
o If blood is too hot, the heat loss
centre is switched on
Increased sweating therefore more water is evaporated in sweat therefore
more body heat is used for evaporation therefore body temperature
decreases
o Arterioles in the skin vasodilate therefore more blood flows through capillaries in
skin therefore more heat energy is lost via radiation
o Hairs lie flat therefore less air is trapped therefore less insulation layer therefore
more heat energy lost via radiation, convection and conduction
o Also behavioural – clothing off
o Sprawl – less surface area to volume ration
o Long term metabolic rate decreases i.e. less thyroxin
If endotherms get too cold
o Hairs go up (erector muscles contrast) therefore insulation layer and therefore less
heat energy loss via C.C.R.
o Shiver – muscle contraction therefore increase respiration rate therefore generation
of heat
o Arterioles in the skin vasoconstrict therefore less blood flows through capillaries in
skin therefore less heat energy lost by radiation
o Decreased sweating
o Increased metabolic rate
Short increase of adrenaline
Long increase of thyroxin
o Behavioural
Warm environment
o Vasodilation
o Increased sweating
o Lowering of body hair
o Behavioural mechanisms
Hormones – chemicals made by the endocrine gland so secreted directly into the bloodstream.
Travel in the blood. Only affect target cells because these have receptor sites complementary in the
shape to the hormone; effective in small quantities, long lasting and have a widespread effect.
Produced by glands
Carried in blood plasma to target cells
Effective in small quantities
Widespread and long lasting
Hormones function in two main ways. One mechanism is known as the second messenger
model. This mechanism is used by two hormones involved in the regulation of blood
glucose, namely adrenaline and glucagon,
The second messenger model of hormone action works as follows;
o The hormone is the first messenger
o It binds to specific receptors on the cell-surface membrane of target cells to form a
hormone-receptor complex.
o The hormone-receptor complex thus produced activates an enzyme inside of the cell
that results in the production of a chemical that acts as a second messenger.
o The second messenger causes a series of chemical changes that produce the
required response.
o In this case of adrenaline, this response is the conversion of glycogen to glucose.
1. The hormone adrenaline approaches the receptor site
2. Adrenaline fuses to receptor site and in doing so activated an enzyme inside the membrane
3. Many ATP molecules make many cyclic AMP molecules. Each cyclic AMP molecule activates
many enzymes. Many enzymes activate glycogen glucose gland. Called the cascade effect.
Pancreas
Islets of Langerhans contain
alpha cells that make and secrete
glucagon and beta cells that
make and secrete insulin.
Some cells make and secrete
pancreatic juice.
Produces enzymes – protease,
amylase and lipase for digestion
Insulin and glucagon hormones
produced for regulating blood
glucose levels.
BRAIN/NERVOUR SYSTEM DOES NOT CONTROL BLOOD GLUCOSE LEVELS
Homeostasis of blood glucose
Normal – but if level increases of
glucose the beta cells of islet of
Langerhans receptors detect high
glucose level and secrete insulin
(effector)
Insulin reduces blood glucose levels
by
o Causing the carrier proteins
in the cell surface membrane
of liver/muscle to take up
more glucose
o Increase the number of
glucose carriers in the
membranes
o Stimulates enzyme to
convert glucose to glycogen
therefore glucose levels are
reduced
o A change in tertiary structure of the glucose transport protein channels, causing
them to change shape and open allowing more glucose into cells.
By increasing rate of absorption of glucose into cells, especially in muscle cells
Increasing respiratory rate of the cells, which therefore use up more glucose, thus increasing
their uptake of glucose from the blood.
By increasing the rate of conversion of glucose into glycogen (glycogenesis) in the cells of the
liver and muscles
By increasing the rate of conversion of glucose to fat.
Blood glucose pool
Removing glucose Adding glucose
Respiration Food and drink
Glucose glycogen GLYCOGENESIS
Fat + Protein GLUCONEOGENESIS
Fat/protein Glycogen glucose GLYCOGENOLYSIS
Normal blood glucose drops – alpha cells in the islets of Langerhans secrete glucagon
o Glucagon – activates enzymes to convert glycogen into glucose (glycogenolysis)
o Activates enzymes to convert fat/protein glucose (gluconeogenesis)
o Therefore increase in blood glucose therefore negative feedback has occurred
o Increases the conversion rate of amino acids and glycerol (gluconeogenesis)
Role of adrenaline in regulating the blood glucose level
Adrenaline – produced in adrenal glands;
Adrenaline raises the blood glucose levels by
o Activating an enzyme that causes the breakdown of glycogen into glucose in the
liver
o Inactivating an enzyme that synthesises glycogen from glucose.
Adrenaline and the second messenger model:
Second messenger model is one of the two models of hormone action. This model involves
peptide hormones that aren't soluble in the lipid bilayer but are soluble in water. What
happens is:
1) The hormone binds to a receptor protein on the cell surface membrane, forming a
hormone - receptor complex.
2) This activates an enzyme - adenyl cyclase - which converts ATP into cyclic AMP, or cAMP
for short. The cAMP is a second messenger with the hormone being the first.
3) The cAMP can go on to cause a cascade of chemical reactions. In the case of glucagon, it
can go on to cause glycogenolysis and gluconeogenesis. In the case of adrenaline it activates
enzymes involved in glycogenolysis and deactivates enzymes involved in glycogenesis.
Blood glucose comes from
o Diet
Resulting from the breakdown of other carbohydrates such as starch,
maltose, lactose and sucrose.
o From the breakdown of glycogen (glycogenolysis)
Stored in liver or muscle cells
o From gluconeogenesis
Production of NEW (neo=new) glucose that is glucose from sources other
than carbohydrates
The liver for example can make glucose from glycerol and amino acids.
Diabetes
The inability to regulate the blood glucose levels
o Either can’t make insulin
o Or insulin that is produced has a lack of response to the hormone
Type 1 (insulin dependent)
o Body unable to produce insulin
o May be a result of autoimmune response
o Develops quickly
Control
o Injection – protein therefore cannot be digested
Type 2 (insulin independent)
o Glycoprotein receptors losing their responsiveness
o Obesity and old age
Control
o Regulating carb intake
o Matching with exercise
o Drugs that slow glucose release in gut
o Injection
Homeostasis
Re-read tissue fluid notes (AS module)
Importance
o Enzymes that control biochemical reactions within cells, and other proteins, such as
channel proteins, are sensitive to changes in pH and temperature. Any change to
these factors reduces the efficiency of enzymes or may even prevent them working
altogether – denaturing them.
o Even small fluctuations in temperature or pH can impair the ability of enzymes to
carry out their roles.
o Maintaining a constant internal environment means that reactions take place at a
constant and predictable rate.
o Changes to water potential of blood and tissue fluid may cause cells to
shrink/expand as a result of osmosis.
Control mechanisms
o The set point which is the desired level or norm, at which the system operates. This
is monitored by…
o Receptor which detects any deviation from the set point and informs the…
o Controller, which coordinates information from various receptors and sends
instructions to an appropriate…
o Effector, which brings about the changes needed to return the system to the set
point. The return to normality creates a…
o Feedback loop, which informs the receptor of the changes to the system brought
about by effector.