module 3

• why do multicellular organisms need specialised exchange surfaces but single-celled organisms don't?

  single-celled organisms can exchange materials across their CSM to meet requirements

  • metabolic activity low → O2 needs low + CO2 produced is low
  • large SA:V ratio

  Multicellular organisms are the opposite

  • high metabolic rate (active + maintain temp)
  • small SA:V ratio
• 3 main features of an efficient exchange surface

  • increased surface area

  • thin layer

  • good blood supply/ventilation to maintain gradient
• how does increased SA aid diffusion?

  • larger SA:V ratio
  • bigger area for exchange
  • e.g root hair cells, villi
• how do thin layers aid diffusion?

  reduces diffusion distance, e.g. in alveoli
• how does a good blood supply aid diffusion?

  • Increases concentration gradient
  • substances constantly delivered and removed
  • e.g alveoli, gills, villi
• nasal cavity features

  • large SA with good blood supply
  • goblet cells secrete mucus to trap dust + bacteria
  • moist surface so gases dissolve helping them to pass across the gas exchange surface
• traches structure

  • supported by incomplete rings of strong flexible cartilage
  • lined with ciliated epithelium which uses synchronised movement to move bacteria out of lungs to throat
  • also lined with goblet cells which produce mucus to trap bacteria
  • smooth muscle and elastic fibres
• bronchiole structure

  • smooth muscle - contracts to constrict airways to control air flow
  • elastic fibres
  • ciliated epithelium
  • goblet cells
• alveoli structure

  • layer of thin flattened epithelial cells → short diffusion pathway
  • elastin + collagen → stretch and recoil
  • large surface area
  • surrounded by capillaries → good blood supply
  • covered in surfactant which speeds up transport of gases + reduces the surface tension of fluid in alveoli
  • good ventilation
• what happens in the alveoli?


  main gas exchange surfaces
• what is the purpose of cartilage in the trachea?

  prevents it collapsing on itself
• the route taken by air as it is inhaled

  • through the mouth
  • down trachea
  • into bronchi
  • into bronchioles
  • alveoli
• what is ventilation?


  movement of air into and out of the lungs
• what happens during inspiration?

  • diaphragm contracts, flattens + lowers
  • external intercostal muscles contract so rib cage moves upwards and outwards
  • volume of thorax increases so pressure in thorax decreased
  • pressure in thorax lower than than atmospheric pressure so air is drawn into lungs
• what happens during expiration?

  • diaphragm relaxes, moves up into dome shape
  • external intercostal muscles relax so rib cage moves downwards and inwards
  • volume of thorax decreases so pressure in thorax increased
  • pressure in thorax higher than than atmospheric pressure so air moves out of lungs
• is inspiration active or passive?

  active
• is expiration active or passive?

  passive
• what does a spirometer measure?


  record volumes of air inspired and expired over time, produces a spirograph
• tidal volume (TV)


  volume of air breathed in, in one breath at rest, around 500cm3
• expiratory reserve volume (ERV)

  volume of air that you can force out after a normal tidal expiration
• inspiratory reserve volume (IRV)


  volume of air that can be inspired over and above a tidal inspiration
• vital capacity (VC)


  greatest volume of air you can move into lungs in one breath, VC = IRV + ERV + TV
• what is vital capacity affected by?

  age, sex, exercise, posture
• residual volume (RV)


  volume of air left in lungs when you have exhaled as hard as possible, keeps alveoli partly inflated
• total lung capacity 


  vital capacity + residual volume
• what is the formula for ventilation rate?


  tidal volume x breathing rate, units = dm3min-1 (breathing rate = breaths per minute)
• how to calculate breathing rate?

  (no. of breaths x 60) ÷ no. of seconds
• what does air enter and leave an insect through?

  spiracles along insect abdomen
• what happens to air after it passes through spiracles?

  enters trachea and then tracheoles so O2 is directly delivered to tissues
• how is air drawn into the trachea?

  • insect pumps thorax and abdomen
  • these movements change volume of body and pressure in trachea
  • air drawn into the trachea or forced out as pressure changes
• what does the trachea in insects contain?

  chitin to strengthen it
• what is at the end of the tracheoles in insects?

  • tracheal fluid
  • oxygen diffuses faster in air than it does in water so tracheal fluid is a barrier to oxygen diffusion
  • when insect is active more SA of muscle is exposed so more O2 can diffuse
• structure of gills

  • gills composed of thousands of filaments
  • each filament is covered in lamallae
  • lamallae are thin so diffusion pathway of O2 from water into blood is short and they increase SA
  • large SA, good blood supply
  • operculum (bony flap) protects gills
• what do fish need to maintain for efficient gas exchange?

  continuous flow of water over the gills
• process of ventilation in fish

  water is constantly pushed over gill filaments for constant supply of O2:

  • fish mouth opens, buccal cavity lowered, increases volume of buccal cavity
  • pressure now lower in buccal cavity than outside pressure so water flows in
  • fish mouth closes, buccal cavity raised, increased pressure pushes water into gill cavity (has lower pressure)
  • pressure builds up in gill cavity which forces open the operculum and water is pushed out
  • when buccal cavity is lowered has effect of forcing operculum shut
• what is counter-current flow in fish?

  • water flowing over gills and blood in gill filaments flow in different directions
  • it ensures that all the way across the gill filament, blood constantly meets water with higher O2 conc than it has, maintains diffusion gradient
• how do fish slow the movement of water to allow more time for gas exchange?

  tips of adjacent gill filaments overlap
• how does a counter-current exchange system help fish with gas exchange?

  maintains steep concentration gradient
• purpose of transport systems

  • supply nutrients + oxygen
  • remove waste products
  • hormone circulation
  • temp maintenance
  • immune responses
• why do multi-cellular animals need transport systems?

  • metabolic demands high → greater demand for oxygen + waste removal
  • SA:V ratio smaller as animals get bigger so diffusion distances bigger
  • diffusion pathway increases as size of animal increases → need short diff. pathway to supply cells efficiently
  • if relied on diffusion would be too slow