mass transport

• Haemoglobin


  Globular, water soluble. Consists of four polypeptide chains, each carrying a haem group (quaternary structure)
• Role of haemoglobin


  Present in red blood cells. Oxygen molecules bind to the haem groups and are carried around the body to where they are needed in respiring tissues
• Factors affecting oxygen-haemoglobin binding


  • Partial pressure/concentration of oxygen
  • Partial pressure/concentration of carbon dioxide
  • Saturation of haemoglobin with oxygen
• Partial pressure of oxygen increases


  Affinity of haemoglobin for oxygen increases, so oxygen binds tightly to haemoglobin. When partial pressure is low, oxygen is released from haemoglobin
• Partial pressure of carbon dioxide increases


  Conditions become acidic causing haemoglobin to change shape. The affinity of haemoglobin for oxygen therefore decreases, so oxygen is released from haemoglobin (Bohr effect)
• Saturation of haemoglobin with oxygen


  It is hard for the first oxygen molecule to bind. Once it does, it changes the shape to make it easier for the second and third molecules to bind (positive cooperativity). It is then slightly harder for the fourth oxygen molecule to bind because there is a low chance of finding a binding site
• Oxygen binds to haemoglobin in the lungs


  Partial pressure of oxygen is high, low concentration of carbon dioxide, positive cooperativity
• Oxygen is released from haemoglobin in respiring tissues


  Partial pressure of oxygen is low, high concentration of carbon dioxide
• Oxyhaemoglobin dissociation curves


  Saturation of haemoglobin with oxygen (in %), plotted against partial pressure of oxygen (in kPa). Curves further to the left show the haemoglobin has a higher affinity for oxygen
• Carbon dioxide affects the position of an oxyhaemoglobin dissociation curve


  Curve shifts to the right because haemoglobin's affinity for oxygen has decreased
• Common features of a mammalian circulatory system

  • Suitable medium for transport, water-based to allow substances to dissolve
  • Means of moving the medium and maintaining pressure throughout the body, such as the heart
  • Means of controlling flow so it remains unidirectional, such as valves
• Structure of the heart chambers


  Atria: thin-walled and elastic, so they can stretch when filled with blood. Ventricles: thick muscular walls pump blood under high pressure. The left ventricle is thicker than the right because it has to pump blood all the way around the body
• Structure of the blood vessels

  Arteries have thick walls to handle high pressure without tearing, and are muscular and elastic to control blood flow. Veins have thin walls due to lower pressure, therefore requiring valves to ensure blood doesn't flow backwards. Have less muscular and elastic tissue as they don't have to control blood flow
• Why two pumps (left and right) are needed instead of one
• Cardiac diastole


  The heart is relaxed. Blood enters the atria, increasing the pressure and pushing open the atrioventricular valves. This allows blood to flow into the ventricles. Pressure in the heart is lower than in the arteries, so semilunar valves remain closed
• Atrial systole


  The atria contract, pushing any remaining blood into the ventricles
• Ventricular systole


  The ventricles contract. The pressure increases, closing the atrioventricular valves to prevent backflow, and opening the semilunar valves. Blood flows into the arteries
• Nodes involved in heart contraction


  • Sinoatrial node (SAN)= wall of right atrium
  • Atrioventricular node (AVN)= in between the two atria

  
• Myogenic


  The heart's contraction is initiated from within the muscle itself, rather than by nerve impulses
• Ventricular systole


  1. Ventricles contract

  2. Pressure increases

  3. Atrioventricular valves close

  4. Semilunar valves open

  5. Blood flows into arteries
• Nodes involved in heart contraction


  • Sinoatrial node (SAN) - wall of right atrium
  • Atrioventricular node (AVN) - in between the two atria

  
• How the heart contracts

  1. SAN initiates and spreads impulse across the atria, so they contract

  2. AVN receives, delays, and then conveys the impulse down the bundle of His

  3. Impulse travels into the Purkinje fibres which branch across the ventricles, so they contract from the bottom up
• Structure of capillaries


  • Walls are only one cell thick; short diffusion pathway
  • Very narrow, so can permeate tissues and red blood cells can lie flat against the wall, effectively delivering oxygen to tissues
  • Numerous and highly branched, providing a large surface area
• Tissue fluid


  A watery substance containing glucose, amino acids, oxygen, and other nutrients. It supplies these to the cells, while also removing any waste materials
• Formation of tissue fluid

  As blood is pumped through increasingly small vessels, this creates hydrostatic pressure which forces fluid out of the capillaries. It bathes the cells, and then returns to the capillaries when the hydrostatic pressure is low enough
• Water transport in plants


  Through xylem vessels; long, continuous columns that also provide structural support to the stem
• Components of phloem vessels


  • Sieve tube elements - form a tube to transport sucrose in the dissolved form of sap
  • Companion cells - involved in ATP production for active loading of sucrose into sieve tubes
  • Plasmodesmata - gaps between cell walls where the cytoplasm links, allowing substances to flow
• Translocation


  The process whereby organic materials are transported around the plant
• How sucrose moves into the phloem

  Sucrose enters companion cells of the phloem vessels by active loading, which uses ATP and a diffusion gradient of hydrogen ions. Sucrose then diffuses from companion cells into the sieve tube elements through the plasmodesmata
• How phloem vessels transport sucrose


  As sucrose moves into the tube elements, water potential inside the phloem is reduced. This causes water to enter via osmosis from the xylem and increases hydrostatic pressure. Water moves along the sieve tube towards areas of lower hydrostatic pressure. Sucrose diffuses into surrounding cells where it is needed