MASS TRANSPORT IN ANIMALS

• EXCHANGE IN MULTICELLULAR ORGANISMS

  In multicellular organisms, diffusion across the outer membrane is too slow, for two reason:

  1. Some cells are deep within the body so there’s a big distance between them and the outside environment
  2. larger animals have a slow surface area to volume ratio so it’s difficult to exchange enough substances to supply a large volume of animal through a relatively small outer surface

  So rather than using straightforward diffusion to absorb and excrete substances, multicellular organisms need specialised exchange organs
• EXCHANGE IN MULTICELLULAR ORGANISMS

  A transport system is required to take materials between cells and exchange surfaces. Materials have to be transported between exchange surfaces and the environment.

  • mass transport maintains final diffusion gradients bringing substances to and from cells
  • mass transport helps maintain relatively stable immediate environment of cells that’s tissue fluid
• MASS TRANSPORT: an efficient system to carry substance to and from their individual cells

  • In mammals, ‘mass transport’ normally refers to the circulatory system, which uses blood to carry glucose and oxygen around the body.
  • It also carries hormones, antibodies and waste like CO2.
  • Mass transport in plants involves the transport of water and solutes in the xylem and phloem.
• OXYGEN TRANSPORT IN THE BODY

  DISSOLVED OXYGEN: oxygen that’s dissolved in the blood plasma
• OXYGEN TRANSPORT IN THE BODY

  HAEMOGLOBIN: a protein in red blood cells that binds to oxygen and carries it to the cells
• OXYGEN TRANSPORT IN THE BODY

  MYOGLOBIN: a protein in muscle cells that stores oxygen for cellular respiration, one polypeptide chain which holds oxygen at low partial pressures
• ROLE OF HAEMOGLOBIN MOLECULES

  • HAEMOGLOBIN MOLECULES: transport oxygen in red blood cells from the lungs to the rest of the body
  • they’re proteins that have evolved to make it efficient at loading oxygen under one set of conditions but unloading it under a different set of conditions
  • the haemoglobins are a group of chemically similar molecules found in many different organisms
• ROLE OF HAEMOGLOBIN MOLECULES

  To be efficient at transporting oxygen, haemoglobin must:

  • readily associate with oxygen at the surface where gas exchange takes place
  • readily dissociate from oxygen at those tissues requiring it

  This is because haemoglobin can also bind to carbon dioxide and other waste products, allowing them to be carried away from the tissues.
• STRUCTURE OF HAEMOGLOBIN

  QUATERNARY STRUCTURE PROTEIN

  • made of 4 polypeptide chains (4 globular subunits) associated with each other
  • the four globular subunits are held together by disulphide bonds and arranged so that their hydrophobic R groups are facing inwards (helping preserve the 3D spherical shape) and the hydrophilic R groups are facing outwards (helping maintain its solubility)
  • each polypeptide chain contains a haem group containing iron ion (Fe^2+) which combines with oxygen
• STRUCTURE OF HAEMOGLOBIN

  • found in red blood cells
  • no nucleus - meaning it can contain more haemoglobin
  • biconcave shape - to increase surface area for rapid diffusion / absorption of oxygen
• HOW THE STRUCTURE OF HAEMOGLOBIN OCCURS: 1

  • PRIMARY STRUCTURE: sequence of amino acids in 4 polypeptide chains. Determined by the genes
  • SECONDARY STRUCTURE: each polypeptide chain coiled into a helix, initial folding
  • TERTIARY STRUCTURE: each polypeptide chain folded into precise shape - important factor in its ability to carry oxygen. Bonds form between R groups: Hydrogen, ionic bonds and weak van der waals forces
• HOW THE STRUCTURE OF HAEMOGLOBIN OCCURS: 2

  • QUATERNARY STRUCTURE: all 4 polypeptide chains linked to together to form spherical molecule. Each polypeptide associated with a haem group - which contains Fe^2+ molecule. Each Fe^2+ ion can combine with a single oxygen molecules (O2), making a total of 4 O2 molecules (8 atoms) that can be carried by a single haemoglobin molecule in humans
• AFFINITY FOR OXYGEN AND PO2 (PARTIAL PRESSURE OF OXYGEN)

  AFFINITY: tendency a molecule has to bind with oxygen

  • haemoglobin affinity for oxygen varies in the conditions it’s in

  PO2: partial pressure (concentration) of oxygen

  • The greater the concentration of dissolved oxygen in cells, the higher the partial pressure
• AFFINITY FOR OXYGEN AND PO2 (PARTIAL PRESSURE OF OXYGEN)

  As PO2 increases, haemoglobins affinity for oxygen also increases:

  • oxygen loads onto haemoglobin to form Oxyhaemoglobin where there’s a high PO2
  • oxyhaemoglobin unloads its oxygen where there’s a lower PO2
• LOADING AND UNLOADING OF OXYGEN

  ASSOCIATION / LOADING IN THE LUNGS

  • high oxygen concentration so high PO2 as oxygen enters blood capillaries at the alveoli in the lungs
  • high affinity for oxygen to bind
  • oxygen molecule binds / loads to the iron in the haem group forming Oxyhaemoglobin
  • this slightly changes the shape of the haemoglobin (undergoes conformational change) and makes binding sites more available so more oxygen binds easier
  • it can carry 4 oxygen molecules (8 oxygen atoms)
• LOADING AND UNLOADING OF OXYGEN

  DISSOCIATION / UNLOADING IN THE TISSUES

  • low oxygen concentration so low PO2 as when cells respire, they use up oxygen which lowers the PO2
  • low affinity for oxygen so it unloads
  • also, concentration of carbon dioxide is high, increasing the rate of unloading (Bohr effect)
  • oxygen is released from the haemoglobin molecule into respiring tissues
  • the haemoglobin then returns to the lungs to pick up more oxygen
• EQUATIONS FOR ASSOCIATION / DISSOCIATION OF OXYGEN

  OVERALL: Haemoglobin + oxygen $⇌$oxyhaemoglobin

  Hb + 4O2 ⇌ HbO8

  1. Hb + O2 ⇌ HbO2
  2. HbO2 + O2 ⇌ HbO4
  3. HbO4 + O2 ⇌ HbO6
  4. HbO6 + O2 ⇌ HbO8
• AFFINITY

  • the first oxygen molecule is the hardest to pick up and least likely to unload (high affinity)
  • As oxygen binds and changes the conformation, more binding sites are available, so easier to pick up
  • when it’s saturated at HbO8 it will easily give up the oxygens to respiring tissues (low affinity)
• SUMMARY OF AFFINITY OF HAEMOGLOBIN FOR O2

  • REGION IN BODY: gas exchange surface (lungs)
  • O2 CONCENTRATION (PO2): high
  • CO2 CONCENTRATION (PCO2): low
  • AFFINITY FOR O2: high
  • RESULT: oxygen associates
• SUMMARY OF AFFINITY OF HAEMOGLOBIN FOR O2

  • REGION IN BODY: respiring tissues
  • O2 CONCENTRATION (PO2): low
  • CO2 CONCENTRATION (PCO2): high
  • AFFINITY FOR O2: low
  • RESULT: oxygen dissociates
• EXPLAINING THE CURVE : 1

  1. Shape of the haemoglobin molecules makes it difficult for the first oxygen molecule to bind with one of the sites on its 4 polypeptide subunits because they’re closely united. Therefore at low PO2 (respiring tissues), low affinity, so little oxygen binds to haemoglobin. The gradient of the curve is shallow initially.
  2. the binding of this first oxygen molecule changes the conformation / quaternary structure of the haemoglobin molecule, causing it to change shape

  
• EXPLAINING THE CURVE: 2

  3. This change makes it easier for the other subunits to bind to an oxygen molecule, it induces the binding of subsequent oxygen molecules as more binding sites are available.

  4. It therefore takes a smaller increase in the PO2 to bind the second oxygen molecule than it did to bind the first one so the gradient of the curve steepens. This is positive cooperativity because binding of the first molecule makes binding of the second easier.

  
• EXPLAINING THE CURVE: 3

  5. After the binding of the third molecule this changes. With the majority of the binding sites occupied, it’s less likely that a single oxygen molecule will find an empty site to bind to. The gradient of the curve reduces and the graph flattens off.

  6. At high PO2 (alveoli), high affinity, oxygen loads, it’s completely saturated so the gradient is flat. Useful to load lots of oxygen to be transported to areas that require it

  
• THE BOHR EFFECT - EFFECT OF CARBON DIOXIDE: 1

  1. haemoglobin has a reduced affinity for o2 in the presence of co2. The greater the concentration of co2, the more readily the haemoglobin release / dissociates with its o2
  2. high co2 concentration causes the Oxyhaemoglobin curve to shift right. For the same PO2, the saturation of o2 is much lower as the affinity decreases so more o2 unloads in higher pco2
  3. this is bc water (in blood) co2 forms carbonic acid and this acidic co2 slightly changes the tertiary structure of the haemoglobin. This happens at respiring tissues as respiration produces co2

  
• THE BOHR EFFECT - EFFECT OF CARBON DIOXIDE: 2

  3. (Carbon acid). In alveoli there’s a lower partial pressure of carbon dioxide as it diffuses out of the blood so there’s a higher affinity for oxygen.

  
• EFFECT OF CARBON DIOXIDE ON HAEMOGLOBIN: 1

  1. In tissues, carbon dioxide is produced by respiring cells
  2. carbon dioxide forms carbonic acid in solution, so pH of the blood within tissues is lowered
  3. the lower pH changes the conformation / shape / tertiary structure of the haemoglobin into one with a lower affinity for oxygen
  4. haemoglobin releases its oxygen to the respiring tissues
  5. at the gas exchange surface (alveoli) carbon dioxide is constantly being removed
  6. the pH is raised due to the low level of carbon dioxide at the exchange surface
• EFFECT OF CARBON DIOXIDE ON HAEMOGLOBIN: 2

  7. this higher pH changes shape of haemoglobin into one that enables it to load oxygen readily to transport it to the respiring tissues

  8. The haemoglobin has a high affinity for oxygen when its transporting it to the tissues so it’s not released before the tissues
• ACTIVITY OF TISSUES

  The more active a tissue, the more oxygen is unloaded

  1. The higher the rate of respiration
  2. the more carbon dioxide the tissues produce
  3. the lower the pH
  4. the greater the haemoglobin shape change
  5. the more readily oxygen is unloaded
  6. the more oxygen is available for respiration
• DIFFERENT HAEMOGLOBINS

  • many animals are adapted to their environment by possessing different types of haemoglobin with different oxygen transport properties
  • Having a particular type of haemoglobin is an adaptation that helps the organism to survive in a particular environment
• DIFFERENT HAEMOGLOBINS

  • structure of haemoglobin is coded for by DNA
  • if there’s a mutation that changes the shape of haemoglobin which is beneficial for the animal, by natural selection this allele will increase in the population
  • there are also mutations which can cause harm ie. sickle cell anaemia
• SICKLE CELL ANAEMIA: a genetic disorder that causes abnormal haemoglobin, resulting in some red blood cells assuming an abnormal sickle shape and can’t carry as much oxygen
• DISSOCIATION CURVES

  • the curve is sigmoidal, meaning that the rate of oxygen binding to haemoglobin starts off slow as the shape restricts binding increases as the PO2 increases until a certain point, after which it levels off.
  • this point is known as the plateau region or the saturation point, where almost all of the haemoglobin is fully saturated with oxygen
• DISSOCIATION CURVES

  • CONCENTRATION OF OXYGEN: higher the concentration of oxygen, the more oxygen that can be transported by haemoglobin
  • ALTITUDE: at higher altitudes, the air pressure and oxygen concentration decreases, making it harder for haemoglobin to transport oxygen
  • TEMPERATURE: higher temperatures increase the solubility of oxygen in the blood, making it easier for haemoglobin to transport oxygen
  • ACIDITY: changes in the acidity of the blood can affect the ability of haemoglobin to bind to oxygen
• DISSOCIATION CURVES

  • the further to the left the curve, the greater the affinity of haemoglobin for oxygen (so it readily loads oxygen but unloads it less easily)
  • the further to the right the curve, the lower the affinity of haemoglobin for oxygen (so it loads oxygen less readily but unloads easily)
• FETAL HAEMOGLOBI: 1

  • the haemoglobin of a developing fetus has a higher affinity for oxygen than adult haemoglobin
  • this is vital to allow a fetus to obtain / attract oxygen from its mothers blood at the placenta. Fetus can’t inhale / exhale, mothers haemoglobin is the only o2 source.

  -fetal haemoglobin can bind to o2 at low PO2

  -at this low PO2 the mothers haemoglobin is dissociating with o2

  • on a dissociation curve, the curve for fetal haemoglobin shifts left of adult haemoglobin

  -this means at any PO2, fetal haemoglobin has a higher percentage saturation than adult haemoglobin

  
• FETAL HAEMOGLOBIN: 2

  • after birth, a baby begins to produce adult haemoglobin which gradually replaces fetal haemoglobin.
  • This is important for the easy release of oxygen in the respiring tissues of a more metabolically active individual

  
• LOWER PO2 (HIGH ALTITUDE/UNDERGROUND)

  • the PO2 is lower at higher altitudes and underground
  • organisms that live in environments with a low concentration of oxygen have haemoglobin with a higher affinity for oxygen than human haemoglobin
  • this is beneficial as it allows them to obtain / attract a sufficient level of oxygen saturation in their blood when the PO2 in the air is low
  • the dissociation curve of their haemoglobin is to the left of ours, higher affinity, higher saturation at any PO2

  
• SIZE, SA:V AND METABOLISM

  • small smalls have a higher SA:V ratio than larger animals
  • this means they lose heat quickly, so they have a high metabolic rate to help keep them warm
  • This means they have a high oxygen demand
  • mammals that are smaller than humans have haemoglobin with a lower affinity for oxygen than human haemoglobin
  • they need their haemoglobin to easily unload oxygen to meet their high oxygen demand
  • the dissociation curve of their haemoglobin is to the right of the human one

  
• HIGH ACTIVITY LEVELS

  • organisms that are very active and have a high oxygen demand have haemoglobin with a lower affinity for oxygen than human haemoglobin
  • this is because they need their haemoglobin to easily unload / dissociate oxygen, so that its available for them to use
  • the dissociation curve of their haemoglobin is to the right of the human one

  
• MYOGLOBIN

  • myoglobin is a single polypeptide chain in a tertiary structure, it has only one oxygen binding site
  • that means it’s fully saturated after one oxygen molecule has bound to it
  • it also doesn’t do cooperative bonding
  • myoglobins affinity for oxygen is higher than the haemoglobin, specifically at lower levels
  • this is due to the fact that myoglobin has a simpler job than haemoglobin which is to store and release oxygen to the molecules whereas haemoglobin is also responsible for carrying and releasing the oxygen at the right places