Communication

Organisms need to respond to external stimuli, e.g. temperature, oxygen concentration and levels of sunlight. These may be over time, e.g. winter fur to summer fur, or quickly, e.g. changing size of pupils.Internal environments change too - the build up of carbon dioxide as a result of respiration changes the pH of the tissue fluid, and therefor inhibits enzyme activity. Multicellular organisms need to coordinate different organs, so this requires a good communication system which will:

• Cover the whole body
• Enable cells to communicate with each other
• Enable specific communication
• Enable rapid communication
• Enable both short and long-term responses

Cells need to communicate with each other by a process called cell signalling.

Neuronal and hormonal systems are examples of cell signalling.

Negative feedback: A process in which any change in a parameter brings about the reversal of that change so that the parameter is kept fairly constant.

Positive feedback: A process in which any change in a parameter brings about an increase in that change.

Homeostasis: The maintenance of a constant internal environment despite changes in the environment.

Any change is detected by receptors, the communication system transmits a message from the receptor to the effector and, through negative feedback, the effectors reverse the change.

Ectotherms:

• Physiological: The horned lizard expands its ribcase and the frilled lizard uses its frill to expand its surface area to absorb more heat from the sun
• Behavioural: Snakes expose their body to the sun so more heat is absorbed, locusts orientate their body towards the sun to expose a larger surface area and so more heat is absorbed. By orientating their body away from the sun, more heat is lost. Lizards hide in burrows to prevent heat absorption by staying out of the sun

Endotherms:

• Physiological (temperature drop): Peripheral skin thermoreceptors are stimulated by a decrease in external temperature. Impulses are sent to the hypothalamus. Vasocontriction of arterioles to reduce heat loss by radiation, conduction, convection. Increased metabolic rate (respiration) to generate heat energy. Release of adrenaline. Shivering to generate heat energy. Erector pilli muscles raise hair to trap and insulating layer of air and therefore heat. Sweating or panting is reduced
• Behavioural: Hot - move into shade or hide in burrow, orientate body to decrease surface area exposed to sun, remain inactive. Cold - move into sunlight, move about, huddle

Endotherms monitor blood temperature in the thermoregulatory centre in the hypothalamus. If the core temperature drops or rises it sends signals to the effectors to reverse the changes. Peripheral temperature receptors monitor the extremities. The information is fed to the thermoregulatory centre. If it signals a temperature change to the brain, it can initate behavioural mechanisms for maintaining body temperature.