Homeostasis

• changes in the external environment can affect you internal environment - the blood and tissue that surrounds your cells
• Homeostasis is the maintenance of a stable internal environment. It involves control systems that keep you internal conditions roughly constant (within certain limits) - means that your internal environment is kept in a state of dynamic equilibrium (i.e. fluctuating around a normal level)
• Keeping your internal environment stable is vital for cells to function normally and to stop them being damaged
• It is particularly important to maintain the right core body temp and blood pH - this is bc temp and pH affect enzyme activity and enzymes control the rate of metabolic reactions
• metabolic reactions are chemical reactions in living cells
• It is also important to maintain the right blood glucose concentration bc cells need glucose for energy and blood glucose concentration affects the water potential of blood
• Temp: the rate of metabolic reactions increases when the temperature is increased. More heat means more kinetic energy, so molecules move faster. This makes the substrate molecules more likely to collide with the enzymes' active sites. The energy of these collisions also increases, which means each collision is more likely to result in a reaction
• But if the temp gets too high (e.g. over 40 degrees), the reaction eventually stops. The rise in temperature makes the enzyme's molecules vibrate more. If the temp goes above a certain level, this vibration breaks some of the hydrogen bonds that hold the enzyme in its 3D shape. The active site changes shape and the enzyme and substrate no longer fit together - the enzyme is denatured - it no longer functions as a catalyst
• If body temperature is too low, enzyme activity is reduced, slowing the rate of metabolic reactions. The highest rate of enzyme activity happens at their optimum temp - about 37 degrees in humans
• If blood pH is too high or too low (highly alkaline or acidic) enzymes become denatured.

  • the ionic bonds and hydrogen bonds that hold them in their 3D shape are broken, so the shape of the enzyme's active site is changed and it no longer works as a catalyst
• The highest rate of enzyme activity happens at their optimum pH, so this is when metabolic reactions are fastest. Optimum pH is usually around pH 7 (neutral), but some enzymes work best at a low pH
• pH is calculated based on the concentration of hydrogen ions (H+) in the environment. the greater the concentration of H+, the lower the pH will be and the more acidic the environment
• pH = -log10 [H+]
• Log10 - the pH is expressed on a logarithmic scale
• • A logarithmic scale is a scale that uses the logarithm of a number instead of the number itself.
  • each value on a logarithmic scale using log10 is 10 times larger than the value before - so a solution of pH 3 contains 10x more H+ ions than a solution of pH 4
  • this is bc the concentration of H+ can vary enormously and so it is easier to compare values on a logarithmic scale
• [H+] is the concentration of hydrogen ions in a solution, measured in moldm-3 - so if you know the hydrogen ion concentration of a solution you can calculate its pH using the formula
• when an enzyme is denatured the reaction may still happen but it'll be too slow for the body's needs
• increasing the number by 1 on a log10 scale is the same as multiplying by 10 on a linear scale. So the numbers 1,2,3 and 4 on a long10 scale represent 10,100,1000 and 10,000 on a linear scale
• if blood glucose concentration is too high, the water potential of blood is reduced to a point where water molecules diffuse out of cells into the blood by osmosis (diffusion of water molecules from an area of higher water potential to an area of lower water potential, across a partially permeable membrane) - this can cause the cells to shrivel up and die
• if blood glucose is too low, cells are unable to carry out normal activities bc there isn't enough glucose for respiration to provide energy
• Negative feedback - homeostatic systems involve receptors, a communication system and effectors
• Receptors detect when a level is too high or too low and the information is communicated via the nervous system or the hormonal system to effectors
• the effectors respond to counteract the change - bringing the level back to normal - the mechanism that brings the level back to normal is called a negative feedback mechanism
• • normal level
  • level changes from normal
  • receptors detect change
  • communication via nervous system or hormonal system
  • effectors respond
  • level brought back to normal
• negative feedback only works within certain limits - if the change is too big then the effectors may not be able to counteract it e.g. a huge drop in body temp caused by prolonged exposure to cold weather may be too large to counteract
• homeostasis involves multiple negative feedback mechanisms for each thing being controlled. This is because having more than one mechanism gives more control over changes in your internal environment than just having one negative feedback mechanism
• having multiple negative feedback mechanisms means you can actively increase or decrease a level so it returns to normal e.g. there is a negative feedback mechanism to reduce body temp and also a mechanism to increase it
• If there was only one negative feedback system, all you could do would be turn it on or turn it off - only be able to actively change a level in one direction so it returns to normal - only one negative feedback mechanism means a slower response and less control
• Positive feedback - some changes trigger a positive feedback mechanism, which amplifies the change. The effectors respond to further increase the level away from the normal level. The mechanism that amplifies a change away from the normal level = positive feedback mechanism
• • normal level
  • normal level changes
  • receptors detect change
  • communication via nervous or hormonal system
  • effectors respond
  • change amplified
• positive feedback is not involved in homeostasis bc it does not keep your internal environment stable. Positive feedback is useful to rapidly activate processes in the body
• During the formation of a blood clot after an injury, platelets become activated and release a chemical - this triggers more platelets to be activated and so on - this means that platelets very quickly form a blood clot at the injury site (the process ends with negative feedback, when the body detects the blood clot has been formed)
• positive feedback can also happen when a homeostatic system breaks down
• hypothermia is low body temperature (below 35 degrees) - happens when heat is lost from the body quicker than it can be produced

  • as body temp falls the brain doesn't work properly and shivering stops - makes body temperature fall even more
  • positive feedback takes body temperature further away from the normal level and it continues to decrease unless action is taken
• water potential is the potential (likelihood) of water molecules diffuse out of or into a solution
• glucose is a solute. It lowers the water potential of the blood. If the blood glucose concentration is too high, water molecules move by osmosis from the cells (the area of higher water potential) to the blood (an area of lower water potential)
• Glucose is a respiratory substrate - a substance that can be broken down during respiration to release energy