Plant and animal responses GapFill

Plants can respond to directional stimuli by regulating their growth, e.g. changing the distribution of  auxinsATPetheneabscisic acid in different areas to determine the relative elongation of the roots and shoots. This plant hormone is also important in controlling  chemotropismapical dominancecorneal reflexesthigmotropism, the inhibition of side shoot growth. Shoots display negative  nitrotropismexotropismgeotropismchemotropism and positive phototropism as they generally grow upwards towards the sun.

In animals, information received from chemoreceptors and pressure receptors can be used to modify the heart rate via the  independentvoluntaryautonomicmechanochemo nervous system. In contrast, the part of the brain responsible for emotions, conscious thought and memory is the  pituitary glandcerebellumponscerebrum.

A similar mechanism to that in synapses is used in  Pacinian corpusclesphospholipid bilayerssemilunar valvesneuromuscular junctions to initiate the contraction of voluntary  smoothskeletalcompressedfast-twitch muscles. They are made from bundles of  cone cellscellulosemyofibrilscardiac cells, which contain different types of filament. Thick  myoglobinmyosintroponinhaemoglobin filaments have globular heads bound to ADP. These can bind to  starchphosphocreatinetroponinactin filaments only when  NADPtropomyosinADPphosphocreatine moves out of the way of the binding sites in response to the action of  sodium ionsphospholipidsmRNAcalcium ions.

Hormones may also be involved in cell signalling. For example,  TRHadrenalinecortisolthyroxine, the fight-or-flight hormone, binds to liver cells to activate the enzyme  cyclokinaseadenylate hydrogenaseadenylyl cyclaseadenosine phosphatase, which converts ATP to cyclic AMP (cAMP). cAMP is referred to as a  second messengercascade messengeractivator substratechannel protein in this case as it binds to a protein kinase enzyme, changing its shape and activating it.