homeostasis

• why do multicellular organisms require communication systems?

  • to respond to stimuli in their internal and external environment
  • to coordinate organ function
• what is cell signalling

  • communication between cells: electrical signals by neurons or chemical signals as hormones
• cell signalling

  • endocrine - long distance signalling
  • paracrine - signalling between adjacent cells directly or by extracellular fluid
  • autocrine - cell releases signals to stimulate its own receptors and trigger a response within itself
• what is a nerve?

  a bundle of neurones
• factors that need to be controlled:

  (humans)

  • temperature
  • pH
  • blood glucose conc.
• pH

  enzymes are denatured beyond optimum pH

  • hydrogen + ionic bonds hold the tertiary structure of the enzyme
  • solutions with excess H+/OH- can cause bonds to break, altering the active site
  • ESC cannot form easily
  • complete denaturation occurs after a while
• blood glucose conc.

  glucose affects water potential of blood + availability of respiratory substrate in cells

  • essential for cellular respiration
• response of stomata

  • inc light intensity + low $CO_2$ conc. within leaf causes stomata to open
  • darkness, high $CO_2$ conc., low humidity, high temp = water stress causes stomata to close
• homeostasis

  the regulation of internal conditions of a cell or organism to maintain optimum conditions for function, in respinse to internal + external changes
• stages of cell signalling

  1. stimulus received by receptor cell
  2. stimulus converted to signal to be passed on (transduction)
  3. signal transmitted to target cell (effector)
  4. appropriate response made
• endotherms

  possess physiological mechanisms to maintain internal body temperature
• ectotherms

  rely on behavioural mechanisms e.g huddling together or moving into water
• thermoregulation in endotherms

  external temp detected by thermoreceptors in skin + mucous membranes

  • impulse from receptors sent to hypothalamus
  • info processed by hypothalamus, then homeostatic responses initiated
• vasodilation

  • muscles in the walls of arterioles relax - dilation - more blood flows to skin capillaries
  • heat lost to environment by radiation
• sweating

  secreted by glands in the skin

  • cools skin by evaporation
  • heat energy from body converts water to water vapour
  • less effective in humid areas due to reduced water vapour gradient
• flattening of hairs

  • hair erector muscles in skin relax, hairs lie flat
  • stops formation of insulting layer of air - air can circulate over skin removing heat lost by radiation
• vasoconstriction

  • muscle in arteriole walls contract, arterioles near skin constrict - less blood flows through capillaries
  • blood diverted to shunt vessels deeper in the skin so no heat lost to environment
  • heat lost by radiation at surface reduced
• increased metabolic rate

  most metabolic processes are exothermic - providing warmth

  • thyroxine can increase basal metabolic rate increasing heat production
• shivering

  quick contraction and relaxation of muscles

  • metabolic processes required for this release heat energy
• erection of hairs

  hair erector muscles in skin contract, so hairs stand on end

  • insulating layer of air trapped, reducing heat loss by radiation