Mass transport

• Red blood cells

  Contain lots of haemoglobin, no nucleus, biconcave, high SA:V, short diffusion path
• Oxygen transport

  1. Haemoglobin associates with/binds/loads O2 at gas exchange surfaces where pO2 is high

  2. Forms oxyhaemoglobin which transports O2

  3. Haemoglobin dissociates from/unloads O2 near cells/tissues where pO2 is low
• Haemoglobin

  • Protein with a quaternary structure
  • Made of 4 polypeptide chains
  • Each chain contains a Haem group containing an iron ion (Fe 2+)
• Haemoglobins are a group of chemically similar molecules found in many different organisms
• Oxygen loading, transport and unloading

  1. In areas with low pO2 (respiring tissues), Hb has a low affinity for O2 so O2 readily unloads/dissociates

  2. In areas with high pO2 (gas exchange surfaces), Hb has a high affinity for O2 so O2 readily loads/associates
• Oxyhaemoglobin dissociation curve

  • S-shaped (sigmoid) due to cooperative nature of oxygen binding
• Cooperative nature of oxygen binding

  Binding of first oxygen changes tertiary/quaternary structure of haemoglobin, uncovering Haem group binding sites and making further binding of oxygens easier
• Evidence for cooperative nature of oxygen binding

  • At low pO2, little/slow increase in % saturation of Hb with oxygen
  • At higher pO2, big/rapid increase in % saturation of Hb with oxygen
• Bohr effect

  Effect of CO2 concentration on dissociation of oxyhaemoglobin - curve shifts to right
• Effect of CO2 concentration on dissociation of oxyhaemoglobin

  Increasing blood CO2 lowers blood pH, reducing Hb's affinity for oxygen as shape/structure changes slightly, so more/faster unloading of oxygen to respiring cells at a given pO2
• Bohr effect

  More dissociation of oxygen → faster aerobic respiration / less anaerobic respiration → more ATP produced
• Different types of haemoglobin

  Made of polypeptide chains with slightly different amino acid sequences, resulting in different tertiary/quaternary structures/shapes and different affinities for oxygen
• Haemoglobin type

  • Curve shift left → Hb has higher affinity for O2, more O2 associates readily at gas exchange surfaces
  • Curve shift right → Hb has lower affinity for O2, more O2 dissociates readily at respiring tissues
• Mammalian blood circulation

  Closed double circulatory system - blood passes through heart twice for every circuit around body
• Importance of double circulatory system
• Blood vessels entering and leaving the heart and lungs

  • Vena cava - transports deoxygenated blood from body to heart
  • Pulmonary artery - transports deoxygenated blood from heart to lungs
  • Pulmonary vein - transports oxygenated blood from lungs to heart
  • Aorta - transports oxygenated blood from heart to body
• Blood vessels entering and leaving the kidneys

  • Renal arteries - oxygenated blood to kidneys
  • Renal veins - deoxygenated blood from kidneys to vena cava
• Coronary arteries

  Carry oxygenated blood to the heart muscle, located on surface of heart, branching from aorta
• Left ventricle wall

  Thicker than right ventricle to generate higher pressure to pump blood around entire body
• Cardiac cycle

  1. Atrial systole - atria contract, blood pushed into ventricles

  2. Ventricular systole - ventricles contract, blood pushed out of heart

  3. Diastole - atria and ventricles relax, blood fills atria and flows passively to ventricles
• Cardiac cycle graphs

  • Pressure in artery higher than ventricle → semilunar valves closed
  • Pressure in ventricle higher than artery → semilunar valves open
  • Pressure in ventricle higher than atrium → atrioventricular valves closed
  • Pressure in atrium higher than ventricle → atrioventricular valves open
• Cardiac output

  Volume of blood pumped out of heart per minute = stroke volume x heart rate
• Heart rate

  Calculated as 60 seconds / length of one cardiac cycle
• Arteries

  • Thick smooth muscle and elastic tissue to withstand and control high pressure
  • Narrow lumen to maintain high pressure
  • Smooth/folded endothelium to reduce friction
• Arterioles

  • Thicker smooth muscle layer than arteries to control blood flow to capillaries
• Veins

  • Wider lumen, little elastic and muscle tissue, valves to prevent backflow - carry blood at lower pressure
• Capillaries

  • Thin endothelial cell wall, large network, narrow lumen - allow efficient exchange of substances
• Tissue fluid formation

  Higher blood pressure in capillaries forces water and dissolved substances out, but large plasma proteins remain in capillary
• Tissue fluid return

  At venule end, decreasing capillary pressure and increasing plasma protein concentration draws water back into capillary from tissue fluid
• Exchange surface between blood and tissue fluid

  • Wall is a thin (one cell) layer of endothelial cells → reduces diffusion distance
  • Capillary bed is a large network of branched capillaries → increases surface area for diffusion
  • Small diameter / narrow lumen → reduces blood flow rate so more time for diffusion
  • Pores in walls between cells → allow larger substances through
• Formation of tissue fluid

  1. Higher blood / hydrostatic pressure inside capillaries (due to contraction of ventricles) than tissue fluid (so net outward force)

  2. Forcing water (and dissolved substances) out of capillaries

  3. Large plasma proteins remain in capillary
• Return of tissue fluid to the circulatory system

  1. Hydrostatic pressure reduces as fluid leaves capillary (also due to friction)

  2. (Due to water loss) an increasing concentration of plasma proteins lowers water potential in capillary below that of tissue fluid

  3. Water enters capillaries from tissue fluid by osmosis down a water potential gradient

  4. Excess water taken up by lymph capillaries and returned to circulatory system through veins
• Low concentration of protein in blood plasma

  • Water potential in capillary not as low → water potential gradient is reduced
  • So more tissue fluid formed at arteriole end / less water absorbed at venule end by osmosis
• High blood pressure (eg. caused by high salt concentration)

  • Increases outward pressure from arterial end AND reduces inward pressure at venule end
  • So more tissue fluid formed at arteriole end / less water absorbed at venule end by osmosis
  • Lymph system may not be able to drain excess fast enough
• Risk factor

  • An aspect of a person's lifestyle or substances in a person's body / environment
  • That have been shown to be linked to an increased rate of disease
• Risk factors for cardiovascular disease

  • age
  • diet high in salt or saturated fat
  • smoking
  • lack of exercise
  • genes
• Xylem tissue

  Transports water (and mineral ions) through the stem, up the plant to leaves of plants
• Xylem tissue

  • Cells joined with no end walls forming a long continuous tube → water flows as a continuous column
  • Cells contain no cytoplasm / nucleus → easier water flow / no obstructions
  • Thick cell walls with lignin → provides support / withstand tension / prevents water loss
  • Pits in side walls → allow lateral water movements
• Cohesion-tension theory of water transport in the xylem

  1. Water lost from leaf by transpiration - water evaporates from mesophyll cells into air spaces and water vapour diffuses through (open) stomata

  2. Reducing water potential of mesophyll cells

  3. So water drawn out of xylem down a water potential gradient

  4. Creating tension ('negative pressure' or 'pull') in xylem

  5. Hydrogen bonds result in cohesion between water molecules (stick together) so water is pulled up as a continuous column

  6. Water also adheres (sticks to) to walls of xylem

  7. Water enters roots via osmosis
• Setting up a potometer

  1. Cut a shoot underwater at a slant → prevent air entering xylem

  2. Assemble potometer with capillary tube end submerged in a beaker of water

  3. Insert shoot underwater

  4. Ensure apparatus is watertight / airtight

  5. Dry leaves and allow time for shoot to acclimatise

  6. Shut tap to reservoir

  7. Form an air bubble - quickly remove end of capillary tube from water