Communication

Why do cells in multicellular organisms need to communicate?

• To organise activities within the body
• To respond to external stimuli
• The communication between cells is called cell signalling.

How can communication take place?

• The nervous system - This involves transport through neurones and across synapses. The action potentials can travel very quickly through the nervous system, enabling a rapid response to external stimuli.
• The endocrine system - This involves the release of hormones from specific organs, which travel through the blood plasma to be recognised by specific target cells, the endocrine system produces a slower, but longer term response.

What is homeostasis?

• Homeostasis is the ability to maintain a stable internal environment, regardless of the external environment.

Which conditions are controlled by homeostasis?

• Temperature - this is called thermoregulation (in endotherms, heat can be generated from within the body, whereas in ectotherms, external heat sources are relied upon)
• Water volume - this is called osmoregulation 
• Glucose concentration 

How are these conditions controlled?

• Negative feedback helps to achieve control over the internal conditions of the body, changes within the body are detected, and a response is produced that counteracts the change that has occured. 

The body has a normal core temperature of 37 degrees, if this temperature rises, a number of things will occur to bring it back down again, this is negative feedback-

1. The detector (in this case the hypothalamus) detects the increase in body temperature.

2. The coordinator (in this case also the hypothalamus) responds, and sends a message to the      receptor.

3. The receptor (in this case the sweat glands and arterioles) puts a response in place, for example the sweat glands will begin to produce more sweat, and the arterioles will vasodilate.

4. The body may now become too cool, and the process will begin again. 

There are very few examples of positive feedback, as the process involves taking a parameter further away from the norm, for example:

• Labour- oxytocin (a hormone) is released, causing uterine contractions. As the labour progresses, more oxytocin is released, increasing the rate and intensity of contractions.
• Carbon dioxide- If higher than normal levels of carbon dioxide are breathed in, the heart beats faster to try and remove it as quickly as possible from the body, this increases the breathing rate, and thus the body takes in more carbon dioxide, taking the levels further from the norm.