Exchange of Substances

• what are features of a good gas exchange surface?

  -large surface area

  -thin for short diffusion distance

  -selectively permeable
• how does mass transport of oxygen occur in insects?

  contraction of muscles through abdominal pumping enabling mass movements if air in and out
• how does oxygen reach the working muscles in an insect?

  through the tracheoles
• what is the advantage of having water at the end of the tracheoles ?


  -during high activity anaerobic respiration creates lactate which reduces the water potential of respiring cells

  -causes them to take up water during osmosis

  -the loss of water from the tracheoles will pull gas into them
• what are spiracles?

  tiny pores in the insects exoskeleton that allow gasses to enter and leave
• how do insects reduce water loss ?


  - small surface area to volume ratiowaterproof coverings

  -spiracles that can be opened and closed
• how does the tracheal system limits the size of insects?

  -insects rely on diffusion to bring oxygen to the tissues

  -if an insect was too large it would take too long
• what are gills made up of ?

  gill filaments
• what increases the surface area of gills ?

  they have many gill filaments with many lamellae on them
• how does counter current flow allow for effective gas exchange?

  -opposite flow of water and blood so there is a concentration gradient

  -equilibrium is not reached so diffusion is constantly take place

  -along the whole length of the gill
• describe two features of a leaf that increases gas exchange?


  -spongy mesophyll contains air spaces so short diffusion pathway

  -many stomata to allow diffusion throughout the leave so shorter diffusion pathway
• where are stomata found?

  underside of the leaf to limit evaporation and transpiration
• what cells open and close the stomata?


  guard cells which close when water run out
• what plants are well adapted to dry environments?

  xerophytes
• what adaptions do xerophytes have to reduce water loss?


  -a thick waxy cuticle

  -rolling of leaves

  -stomata sunken in pits or grooves

  -a reduced surface area to volume ratio
• how do the adaptations of xerophytes reduce water loss?


  -reduce water potential gradient

  -therefore slower diffusion of water vapour from air spaces

  -so reduced evaporation of water
• how does a thick cuticle reduce transpiration?

  increases diffusion distance
• name the structures involved in human gas exchange?


  trachea, lungs, bronchi, bronchioles and alveoli
• what is the role of goblet cells in the trachea?

  produce mucus to trap microorganisms
• why can the alveoli strech and recoil?

  because they have elastic fibres due to the protein elastin
• what do bile salts do?

  emulsify lipids into micelles increasing their surface area
• how are the alveoli adapted for gas exchange?


  -squamous epithelial cells that are 1 cell thick so short diffusion distance

  -capillaries run close to alveoli to maintain concentration gradient

  -contain elastic fibres so can stretch to prevent bursting and recoil which helps to repel stale air

  -many alveoli to give a large surface area
• describe the mechanism for inhaling


  -diaphragm contracts to become flatter and moves down

  -external intercostal muscles contract and ribcage moves up and out

  -volume of thoracic cavity increases

  -pressure in thoracic cavity drops below atmospheric pressure

  -air moves into the lungs
• describe the mechanism for expiration


  -diaphragm relaxes and is pushed up by displaced organs underneath

  -internal intercoastal muscles contract and ribcage moves down and in

  -volume of thoracic cavity decreases

  -pressure in thoracic cavity increases and rises above atmospheric pressure

  -air moves out of lungs
• how do you find pulmonary ventilation rate?


  ventilation rate x tidal volume
• what is the primary structure of haemoglobin?

  the order of amino acids in the polypeptide chains
• what is the secondary structure of haemoglobin?

  the coiling of the polypeptide chains in to an alpha helix or a beta pleated sheet via hydrogen bonds
• what is the tertiary structure of haemoglobin?


  the folding of the polypeptide chains into a specific shape with hydrogen bonding, ionic bonding and disulphide bridges
• what is the quaternary structure of haemoglobin?


  the linking of four polypeptide chains into a spherical molecule, contains a haem group
• what is loading?


  when haemoglobin binds to oxygen in the lungs
• what is unloading?


  when haemoglobin releases its oxygen in the respiring tissues
• what is a high affinity?


  -when haemoglobin takes up oxygen more easily but releases it less easily
• what is a low affinity?


  when haemoglobin takes oxygen less easily but releases it more readily
• what is partial pressure?

  the pressure exerted by a gas if it was in a container on its own
• how does haemoglobin change its affinity for oxygen?

  it will change shape in the presence of certain molecules
• what shape does the oxygen dissociation curve take?


  sigmoidal curve- s
• why does the oxygen dissociation curve increase slowly to begin with?


  the shape of the haemoglobin molecule makes binding the first oxygen molecule difficult
• why does the oxygen dissociation curve increases sharply after the beginning?


  -the first oxygen molecule changes shape of the quaternary structure

  -this makes binding of the other oxygens easier
• why does the oxygen dissociation curve increases slowly at the end?


  less likely for the fourth oxygen to find a binding site
• what is positive cooperativity?


  the increase in ease of binding the oxygen molecules