Homeostasis

• Why do multicellular organisms need communication systems?

  >for survival and activity

  >to respond to changes in external and internal environments

  >to coordinate the activities of different organs through cell signalling
• Survival and activity in organisms

  living things need to maintain certain conditions inside their cells such as:

  >suitable temperature and pH

  >aqueous environment which keeps substrates and products in solution

  >freedom from toxins and excess inhibitors
• An external environmental change is a stimulus. The way an organism changes its behaviour or physiology is a response.
• Internal environment in organisms

  The environment of cells is tissue fluid. Activities of cells can alter this. For example if excess co2 builds up in fluid outside cells, it will alter its pH and disrupt action of enzymes and other proteins.

  Accumulation of excess waste/toxins in tissue fluid acts as a stimuli, causing the removal of these through excretion so the cells can survive.

  This accumulation in blood is monitored preventing excretion of useful substances which need to be supplied to cells.
• Coordinating activities

  Multicellular organisms are more efficient as their cells are differentiated. A good communication system will:

  >cover the whole body

  >enable cells to communicate with each other

  >enable specific and rapid communication

  >enable both short-term and long-term responses
• Two types of communication through cell signalling

  Neuronal system: connected network of neurones signalling across synapse junctions. It can conduct signals quickly and enable rapid responses to stimuli.

  Hormonal system: uses blood to transport signals. Cells in an endocrine organ can release a hormone which is transported around body and recognised by specific target cells. It enables long-term responses to be coordinated.
• Homeostasis is used to maintain a constant internal environment despite changes in internal and external factors.
• Factors maintained by homeostasis in the body

  >body temperature

  >blood glucose concentration

  >blood salt concentration

  >water potential of the blood

  >blood pressure

  >co2 concentration
• Mechanism of homeostasis

  Stimulus->Receptor->Effector->Response

  Specialised structures are required for the pathway to work:

  >Sensory receptors: when these detect a change, it will be stimulated to send a message to an effector

  >Hormonal/Neuronal communication systems: these send signals between cells and transmit messages from receptor cell to effector cells via a coordination centre

  >Effector cells: liver and muscle cells which bring about a response
• Negative feedback

  Mechanism that reverses a change, bringing a system back to optimum.

  Receptor detects a change ->communication system informs Effector-> Effector reacts to reverse change -> system returns to optimum condition.
• Negative feedback occurs within most organisms. Some range is allowed around optimum conditions. Conditions must remain warm enough for efficient enzyme function and cool enough to avoid protein damage.
• Positive feedback

  Mechanism that increased a change, taking the system further away from optimum conditions. The activity of neurones rely on this feedback.
• How positive feedback can destabilise the system

  > As core body temperature enzymes become less active, the exergonic reactions are slower and release less heat.

  > This allows the body to cool further and slows enzyme-controlled reactions more.

  > Then the body temperature spirals downwards.
• How positive feedback benefit the system

  > In birth as the cervix begins to stretch, it causes the posterior pituitary gland to secrete Oxytocin.

  > This increases uterine contractions, stretching the cervix more.

  > Thus causing the gland to secrete more Oxytocin.

  > Once the cervix is fully dilated, the baby can be born
• Temperature control in ectotherms

  Ectotherm- organism that relies on external sources of heat to maintain its body temperature. Examples: snakes, locust, lizard

  Their temperature regulation relies on behavioural responses that alter amount of heat exchanged with the environment.

  > When they are cold they: move into a sunny area, lie on a warm surface, expose a larger SA to the sun.

  > When they are hot they: move out of the sun, move underground, reduce body surface exposed to the sun.
• Advantages of Ectothermy

  They don't use energy to keep warm so..

  >less food is used in respiration

  >more of the energy and nutrients gained from food is converted to growth

  >need to find less food

  >can survive for longer periods of time without food
• Disadvantages of ectothermy

  >They are less active in cooler temperatures so are at risk from predators when cold

  > They cant take advantage of food that is available when cold
• Temperature control in endotherms

  Endotherm- organism that uses heat from metabolic reactions to maintain its body temperature.
• Physiological mechanisms to maintain body temperature

  >Vasoconstrict/Vasodilate vessels to and from the skin altering amount of heat lost to the environment

  >Use glands to sweat

  >Hairs

  >Panting contracts in skeletal muscles

  >Respiration in muscle and liver cells
• Behavioural mechanisms to maintain body temperature

  >When hot: hide away from sun in shade, orientate body to reduce surface area of body exposed to sun, remain inactive and spead out limbs to enable greater heat loss, wet skin and use evaporation to help cool body.

  >When cold: lie in the sun, orientate body towards sun to increase surface area exposed, move about to generate heat in muscles, roll into ball reducing surface area and heat loss, remain dry.
• Advantages of endothermy

  >organisms can maintain a fairly constant body temperature whatever temperature externally.

  >they can remain active despite low external temperatures, so they can take advantage of prey that may be available and escape from potential predators.

  >inhabit colder parts of earth
• Disadvantages of endothermy

  >they have to use a significant part of their energy intake to maintain body temperature

  >need more food

  >lower proportion of the energy and nutrients gained from food is put towards growth

  >they may overheat in hot weather
• Control of temperature regulation in endotherms

  The hypothalamus coordinates homeostatic responses. Temperature receptors in the hypothalamus of the brain detects changes in core temperatures which sends out impulses causing different responses to reverse this change. For example if the body's core temperature is too low the hypothalamus will bring out vasoconstriction, increased metabolism for exergonic reactions and muscular contraction to release heat.

  The neuronal system transmits these impulses rapidly whereas the hormonal system transmits responses that need to be longer term.
• The thermoregulatory centre in the hypothalamus monitors blood temperature and detects changes in core body temperature. Early warnings that the body's condition could change allows the hypothalamus to respond quicker reducing variation.

  Peripheral temperature receptors in the skin monitor the temperature in extremities which is sent to the thermoregulatory centre in the hypothalamus.