Conditions that cells require to work properly: temperature pH humidity water concentration (of enzyme/substrate) cofactors and prosthetic groups inhibitors aqueous environment - keeps the substrate and products in solution freedom from toxins Stimulus: is any change in the environment that causes a responseE.g.: External - temperature, pH, salinity. length of day, sunlight Internal - temperature, toxins in blood, blood glucose levelsResponse: a change in behaviour or physiology as a result of a change in the environmentE.g. sweating, shivering Hibernating due to food shortage Horse loses its coat due to increase in temperatureRapid change: bright light to dim light - pupil sizeA good communication system is required to ensure that these different parts of the body work together effectively.A good communication system will: cover the whole organism enable cells to communicate with each other enable specific communication (target cells) enable sufficiently rapid communication enable both short term and long term responses Cell SignallingTwo major systems of communication that work by cell signalling: neuronal system - usually rapid hormonal system - enables long term responses Once cells have received an external signal (stimulus) there will be a sequence of changes inside the cell, involving enzymes, leading to a change (response).HomeostasisConditions that need to be kept constant: body temperature blood glucose concentration blood salt concentration water potential of blood blood pressure carbon dioxide concentratin Negative FeedbackStimulus -> receptor -> communication pathway -> effector -> responseReceptors - temperature, pH, pressureEffectors - muscle tissue or glandsCommunication pathway - nervous tissue, endocrine tissue (glands) and circulatory system (coordination centres in CNS)Coordination involves comparing the input from the receptor with a set value to determine whether or not a response is required.Positive FeedbackLess commonOptimum condition -> change away from optimum -> receptor detects change -> communication system informs effector -> effector reacts to increase change -> change away from optimum (repeats)E.g.As the cervix begins to stretch the change is signalled to the anterior pituitary gland, stimulating it to secrete the hormone oxytocin. Oxytocin increases the uterine contractions, which stretch the cervix more, which causes more secretion of oxytocin.