unit 3

• transpiration definition 


  the loss of water from the stomata by evaporation
• factors affecting rate of transpiration

  light intensity, temperature, wind and humidity
• which has a negative effect on transpiration 


  humidity
• cohesion tension theory 


  cohesion - hydrogen bonds form between water molecules allowing them to stick together- allows water to travel up the xylem as a continous column. adhesion - water aheres to the xylem walls. root pressure - water moves into roots by osmosis so more water is pulled upwards
• transpiration


  1.water vapour evaporates out of the stomata of leaves. this loss of water creates a lower pressure. 2. when water is lost by transpiration, more water is pulled up the xylem to replace it. 3. due to hydrogen bonds between water molecules, they are cohesive creating a column of water within the xylem. 4. water molecules also adhere to the walls of the xylem. 5. as continuous column of water is pulled up, creates tension pulling xylem in to become narrower
• translocation definition 


  movement of solutes to where they are needed
• mass flow hypothesis 


  1.active transport of solutes from companion cells into sieve tubes of phloem at the source. 2. this lowers the water potential inside sieve tube so water enters by osmosis from the xylem. 3. this creates a high hydrostatic pressure inside sieve tubes at source. 4. at sink, solutes are removed from phloem to be used. 5. this increases water potential, so water leaves by osmosis. and lowers pressure.
• movement of soluble organic substances is due to the differences in hydrostatic pressure between the source and sink end of the sieve tube elements
• gas exchange in insects

  1.tracheal system. 2. spiracles for uptake of oxygen and release of carbon dioxide. 3. trachea. 4. tracheoles - increase surface area. 5. exoskeleton to prevent water loss
• adaptions of xerophytic plants 


  curled leaves, hairs, sunken stomata, thick cuticle, longer root network
• bile salts

  emulsify lipids to form tiny droplets, micelles. increases surface area for lipase to hydrolyse. micelles are vesicles containing fatty acids, glycerol. monoglycerides and bile salts
• adaptions of ileum

  1.covered in microvilli which increases surface area. 2. thin walls so short diffusion pathway. 3. network of capillaries so maintains a concentration gradient.
• oxyhaemoglobin dissociation curve

  oxygen is loaded in regions of high partial pressure of oxygen e.g. alveoli. and dissociates at low partial pressures e.g. respiring tissues
• bohr effect

  at high pH - there is low partial pressure of carbon dioxide, curve shifts to the left increasing affinity for oxygen so uploads more oxygen. at low pH - there is a high partial pressure of carbon dioxide, curve shifts to the right decreasing affinity for oxygen so unloads more oxygen
• Explain the advantage of the curve being to the left in an organism in a low oxygen environment:

  High percentage saturation of haemoglobin with oxygen at low partial pressure
• Explain the advantage of the curve being to the right in an organism with a high level of activity /high metabolic rate / high exercise

  Haemoglobin has a lower affinity for oxygen• so releases more oxygen more readily to respiring tissues• so more respiration can take place
• Why a person experiencing high blood pressure may get swelling:

  Hydrostatic pressure of blood is high so more water is forced out of arteriole end of capillary• some tissue fluid accumulates as not all of it can be reabsorbed
• Describe how tissue fluid is formed and how it is returned to the circulatory system.(6 marks)

  High hydrostatic pressure forced fluid out. Large proteins remain in capillary. Low water potential in capillary due to plasma proteins. Water re-enters capillary by osmosis. Lymphatic system return excess fluid to circulatory system.
• Events of the cardiac cycle:

  1.Atria contract 2. Increased pressure in atria 3. AV valves open (when pressure in atria is higher than ventricles) 4. Blood flows into ventricles 5. Ventricles contract 6. Increased pressure in ventricles 7. AV valves shut (when pressure in ventricles is higher than pressure in atria)8. Semi-lunar valves open (when pressure in ventricles is higher than pressure in arteries) 9. Blood moves into arteries fast at first and then more slowly 10.Semi-lunar valves close (when pressure in ventricles low and pressure in arteries is high) 11.Diastole