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.