5.1.1 - homeostasis

• multicellular organisms need communication systems to respond when their internal and external environment changes and to coordinate organ function.
• Cell signalling is communication between cells.

  Electrical signals carried By neurones or chemical signals as hormones.
• cell signalling exists as:

  long distance endocrine signalling

  paracrine signalling between adjacent cells occurs directly or aided by extracellular fluid

  in autocrine signalling where the cell releases signals to stimulate its own receptors and triggers a response within itself.
• Homeostasis is maintaining a constant internal environment despite changes in internal and external factorS.
• Negative feedback is when self regulatory mechanisms return internal environment to optimum when there is a fluctuation.
• positive feedback is when a fluctuation triggers changes that results in an even greater deviation from the normal level.
• receptors are specialised cells located in sense organs that detect a specific stimulus.
• effectors are usually muscles or glands which enable a physical response to a stimulus.
• An ectotherm is an organism that cannot increase its respiration rate to increase the internal production of heat.
• ectotherms rely on external sources to regulate body temperature. respond to temperature changes behaviourally

  eg, may orient itself to minimise/maximise sun exposure
• Endotherms are organisms that can regulate their body temperature independently of external sources.

  Thermoreceptors send signals to the hypothalamus which trigger a behavioural or physiological response.
• Behavioural methods used by Endotherms:

  • Basking in the sun
  • pressing against warm surfaces
  • digging burrows
  • hibernation / aestivation
  • panting
• Autonomic nervous system enables Endotherms to thermoregulation by negative feedback.

  Peripheral thermoreceptors detect changes in skin temperature. Receptors in hypothalamus detect changes in blood temperature.

  Hypothalamus sends impulses to effectors in skin and muscles.
• phyisological responses to temperature control in Endotherms:

  vasodilation

  vasoconstrictin

  sweating

  shivering
• in thermoregulation,

  vasodilation / constriction of arterioles supplying skin capillaries controls heat loss to skin surface

  hair erector muscles contract and follicles protrude to trap air for insulation

  evaporation of sweat cools skin surface
• receptor: detect change in the body
• effector: brings about change in the body (muscle or gland)
• Plants need to respond to external environment:

  grow towards sunlight or water

  avoid herbivory
• animals need to respond to external environment:

  move to eat / catch prey

  move to avoid predation.
• organisms need to respond to external environment due to enzyme activity and enzyme controlled reactions involved in metabolic reactions.
• Internal environmental changes organisms respond to:

  blood glucose concentration

  internal temperature

  water potential

  cell ph
• external environmental changes organisms respond to:

  humidity

  external temperature

  light intensity

  new or sudden sound
• The main way we detect something going on in our body is through the neuronal system
• receptors are specific - they only detect one particular stimulus
• Receptors are transducers

  they convert one form of energy into another.
• Neuronal reflex are:

  stimuli - receptor - sensory neurone - cns - relay neurone - motor neurone - effector - response
• Hormones can be one of two types:

  lipid based

  protein based
• lipid hormones can diffuse straight through the phospholipid bilayer because it is lipid soluble

  the hormone binds with receptor in cytoplasm forming receptor hormone complex

  receptor hormone complex enters nucleus and triggers gene transcription

  transcribed mRNA is translated to proteins that alter cell activity
• Protein based hormones cannot diffuse through the phospholipid bilayer as it is too large
• protein hormones use first messenger and second messenger response
• Cell signalling is communication between cells. where cells work together to trigger a response.
• What is homeostasis?

  maintaining a stable internal environment even though the environment is changing
• Factors need to be kept constant in the body:

  core body temperature = 37 degrees

  blood ph = 7.3 - 7.5

  blood glucose concentration = 4 to 7 mmol

  water potential of blood
• Enzymes and other proteins are sensitive to changes in temperature. They function best at their optimum temperature. At low temperatures they may not work efficiently.

  If enzyme activity is not at optimum, metabolic reactions will not be as efficient.
• Blood ph is monitored because:

  enzymes are sensitive to ph and work best at their optimum ph.

  A shift too far in ph can lead to denaturing of enzymes so metabolic reactions such as respiration will be slow or will not occur (death)
• Blood glucose concentration is monitored:

  we need a decent level of glucose for aerobic respiration to generate ATP used for muscle contraction and active transport.

  keeping blood glucose level constant also ensures a constant Water potential.
• Water potential of blood is monitored:

  changes in water potential may cause cells to shrink or expand as water leaves or enters by osmosis. The water potential must be kept constant otherwise cells cannot function properly.
• Negative feedback is when an effector causes a change that brings the environment back to its optimum conditions.
• Positive feedback is when the situation is amplified until there is a change brought about. Eg, pregnancy.
• Endotherms are warm blooded animals