Communication

...leads to communication and coordination between cells...

• The signalling cell releases molecuels by exocytosis which travel to target cells
• Target cells will have receptor molecules (Glycoproteins) on their surface which have a complementary shape to that of the signalling molecule

Types:

• Local Chemical mediator
• Neurotransmitter
• Physical Contact
• Hormones - use cell signallying to communicate with distant target cells

Cells in glands secrete a chemical messenger (hormone) and rely on the blood system to transporrt the hormone round the body. Target cells have a receptor which is complementary to the hormone so it will trigger the desired internal response in the target cell - other cells are not afected

...maintains a constant internal environment. it keeps the body in an equilibrium and allows a degreee of independence from the environment

it is important for the body to have a stable environment for cells to functions correcty:

• Temperature - too high and protein structures change, too low and enzymes don't operate properly
• Water - levels of water can affect the metaolism and osmosis
• Concentration of Glucose - affects osmosis and rate of respiration
• Concentration of nitrogenous waste - can become toxic in the body if levels are too high

Negative Feedback - where deviation away from the norm produces a response which returns the system back to the norm

Positive Feedback - where deviation away from the norm produces a response which further increases the level away from the norm

Temperature:

Norm -> Too hot -> detected by receptors in the brain and skin (Hypothalamus/thermoregulatory system) -> Corrective mechanism (increased sweating brings about a return to the norm) -> Norm

Blood Sugar Regulation

Norm ->too much -> Detected by receptors in the pancreas (Islets of Langaharn) ->Corrective mechanism (shivering) brings a return to the norm -> Norm

Dynamic equilibrium is maintained by negative feedback

Control mechanisms must be coordinated -> largely achieved by the brain. In the case of glucose, it is the pancreas that co ordinates the response

Ectothermic Organisms (reptiles, insects and invertebrates) - control their temperature mainly through behavioural means

• Body temperature fluctuates with the environment - though not generally to the same degree
• Behavioural and structural mechanisms are important in limiting variation
• They are found in a limited range of environments
• They need less food than endotherms to supply their metabolic needs

Endothermic Organisms (birds and mammals) - control their temperature througgh physiological and behavioural means

• Body temperature is more of less constant -largely independent of the environment
• Physiological mechanisms importnant in maintaining a constant temperature
• Found in an extremely wide range of environments
• Need more food than ectotherms to supply their metabolic needs

They rely on behaviours to seek out an area at their optimum temperature -> in early morning & late evening they are active as it is not too hot or cold. To warm up, they bask in the sun and at midday will lie in the shade. At night they lie in crevices/burrows so their immediate atmosphre is warm

Reorientation - varys the surface exposed to solar radiation

Thermal Gaping - an open mouth allows water to evaporate, reducing temperature

Body Raising  - reduces the ara in contact with hot surfaces

Colour changing - alters the ability to absorb heat

Burrowing - avoids temperature fluctuation of the environment

The thermoregulatory centrre in the hypothalamus recieves input from two sets of receptors:

• receptors in the hypothalamus monitor the temperature of the blood as it passes through the brain
• receptors in the skin monitor the external temperature

Smooth Muscle:

Low temp - muscles contract causing vasoconstriction - less heat is carried fromt he croe to the surface of the body, maintaining core temperatures. Extremities can turn blue and feel cold - can cause damage ie frostbite

High temp - muscles relax causing vasodilation - more heat is carried from the core to the surface, where it is lost by convection/radiation. Skin may turn red

Sweat Glands

• Low Temp - No sweat so heat is conserved
• High Temperatures - glands secrete sweat onto skin surface so it evaporates - water has a high latent heat of evaporation which takes heat from the body

Erector Pili

• Low Temp - Muscles contract, raising skin hairs, trapping an insulating layer of still, warm air next to the skin - not effective in humans as it just causes goosebumps
• High Temp - Muscles relax, lowering the skin hairs, allowing air to circulate over the skin, encouragin convection and evaporation

Behavioural

• Low Temp - put clothes on, stay indoors
• High temp - remove clothing, find shade

Adrenal and Thyroid Glands

• Low Temp - Glands secrete adrenaline and thyroxine, which increase the metabolic rate in different tissues, especially the liver, therefore generating heat (in brown adipose tissue)
• High Temp - Glands stop releasing adrenaline and thyroxine

Skeletal Muscles

• Low Temps - muscle contract and relax repeadtedly, generating heat by friction and the metabolic reactions
• High Temps - No shivering