Biology - Topic 3 Mass transport

• What is surface area to volume ratio (SA:V)

  Relationship between the size and structure of an organism and its surface area to volume ratio
• As size increases, SA:V tends to decrease
• More thin / flat / folded / elongated structures increase SA:V
• How to calculate SA:V

  Divide surface area (size length x side width x number of sides) by volume (length x width x depth)
• SA:mass


  Easier / quicker to find / more accurate because irregular shapes
• Metabolic rate


  Amount of energy used up by an organism within a given period of time
• Metabolic rate is often measured by oxygen uptake as used in aerobic respiration to make ATP for energy release
• As SA:V increases (smaller organisms)

  Metabolic rate increases
• Adaptations that facilitate exchange as SA:V reduces in larger organisms


  • Changes to body shape (eg. long / thin)
  • Development of systems, such as a specialised surface / organ for gaseous exchange e.g. lungs
• Increases (internal) SA:V and overcomes (reduces) long diffusion distance / pathway
• Maintain a concentration gradient for diffusion eg. by ventilation / good blood supply
• As size increases, SA:V decreases because although both volume and surface area increase, volume increases faster than surface area
• Energy can't be produced, only released
• Gas exchange in single-celled organisms

  Thin, flat shape and large surface area to volume ratio, short diffusion distance to all parts of cell → rapid diffusion eg. of O2 / CO2
• Tracheal system of an insect


  1. Spiracles = pores on surface that can open / close to allow diffusion

  2. Tracheae = large tubes full of air that allow diffusion

  3. Tracheoles = smaller branches from tracheae, permeable to allow gas exchange with cells
• How an insect's tracheal system is adapted for gas exchange


  • Tracheoles have thin walls so short diffusion distance to cells
  • High numbers of highly branched tracheoles so short diffusion distance to cells and large surface area
  • Tracheae provide tubes full of air so fast diffusion
  • Contraction of abdominal muscles (abdominal pumping) changes pressure in body, causing air to move in / out so maintains concentration gradient for diffusion
  • Fluid in end of tracheoles drawn into tissues by osmosis during exercise (lactate produced in anaerobic respiration lowers ψ of cells) so diffusion is faster through air (rather than fluid) to gas exchange surface
• Structural and functional compromises in terrestrial insects that allow efficient gas exchange while limiting water loss

  • Thick waxy cuticle / exoskeleton → Increases diffusion distance so less water loss (evaporation)
  • Spiracles can open to allow gas exchange AND close to reduce water loss (evaporation)
  • Hairs around spiracles → trap moist air, reducing ψ gradient so less water loss (evaporation)
• How the gills of fish are adapted for gas exchange


  • Gills made of many filaments covered with many lamellae to increase surface area for diffusion
  • Thin lamellae wall / epithelium so short diffusion distance between water / blood
  • Lamellae have a large number of capillaries to remove O2 and bring CO2 quickly so maintains concentration gradient
• Counter current flow in fish gills


  1. Blood and water flow in opposite directions through/over lamellae

  2. So oxygen concentration always higher in water (than blood near)

  3. So maintains a concentration gradient of O2 between water and blood

  4. For diffusion along whole length of lamellae

  
• If parallel flow, equilibrium would be reached so oxygen wouldn't diffuse into blood along the whole gill plate
• How the leaves of dicotyledonous plants are adapted for gas exchange


  • Many stomata (high density) → large surface area for gas exchange (when opened by guard cells)
  • Spongy mesophyll contains air spaces → large surface area for gases to diffuse through
  • Thin → short diffusion distance

  
• Structural and functional compromises in xerophytic plants that allow efficient gas exchange while limiting water loss


  • Thicker waxy cuticle → Increases diffusion distance so less evaporation
  • Sunken stomata in pits / rolled leaves / hairs → 'Trap' water vapour / protect stomata from wind so reduced water potential gradient between leaf / air and less evaporation
  • Spines / needles → Reduces surface area to volume ratio
• Essential features of the alveolar epithelium that make it adapted as a surface for gas exchange

  • Flattened cells / 1 cell thick → short diffusion distance
  • Folded → large surface area
  • Permeable → allows diffusion of O2 / CO2
  • Moist → gases can dissolve for diffusion
  • Good blood supply from large network of capillaries → maintains concentration gradient
• Gas exchange in the lungs


  1. Oxygen diffuses from alveolar air space into blood down its concentration gradient

  2. Across alveolar epithelium then across capillary endothelium

  3. Carbon dioxide = opposite

  
• Ventilation


  Brings in air containing higher conc. of oxygen & removes air with lower conc. of oxygen, maintaining concentration gradients
• How humans breathe in and out (ventilation)


  Inspiration (breathing in): 1. Diaphragm muscles contract → flattens, 2. External intercostal muscles contract, internal intercostal muscles relax (antagonistic) → ribcage pulled up / out, 3. Increasing volume and decreasing pressure (below atmospheric) in thoracic cavity, 4. Air moves into lungs down pressure gradient

  Expiration (breathing out): 1. Diaphragm relaxes → moves upwards, 2. External intercostal muscles relax, internal intercostal muscles may contract → ribcage moves down / in, 3. Decreasing volume and increasing pressure (above atmospheric) in thoracic cavity, 4. Air moves out of lungs down pressure gradient

  
• Expiration is normally passive at rest because internal intercostal muscles do not normally need to contract and expiration is aided by elastic recoil in alveoli
• How different lung diseases reduce the rate of gas exchange


  • Thickened alveolar tissue (eg. fibrosis) → increases diffusion distance
  • Alveolar wall breakdown → reduces surface area
  • Reduce lung elasticity → lungs expand / recoil less → reduces concentration gradients of O2 / CO2

  
• How different lung diseases affect ventilation


  • Reduce lung elasticity (eg. fibrosis - build-up of scar tissue) → lungs expand / recoil less, reducing volume of air in each breath (tidal volume) and reducing maximum volume of air breathed out in one breath (forced vital capacity)
  • Narrow airways / reduce airflow in & out of lungs (eg. asthma - inflamed bronchi), reducing maximum volume of air breathed out in 1 second (forced expiratory volume)
  • Reduced rate of gas exchange → increased ventilation rate to compensate for reduced oxygen in blood

  
• How to analyse and interpret data to the effects of pollution, smoking and other risk factors on the incidence of lung disease


  • Describe overall trend → eg. positive / negative correlation between risk factor and incidence of disease
  • Manipulate data → eg. calculate percentage change
  • Interpret standard deviations → overlap suggests differences in means are likely to be due to chance
  • Use statistical tests → identify whether difference / correlation is significant or due to chance, eg. correlation coefficient, Student's t test, Chi-squared test
• How to evaluate the way in which experimental data led to statutory restrictions on the sources of risk factors


  • Analyse and interpret data as above and identify what does and doesn't support statement
  • Evaluate method of collecting data, eg. sample size, participant diversity, control groups, control variables, duration of study
  • Evaluate context → has a broad generalisation been made from a specific set of data?
  • Consider other risk factors that could have affected results
• Correlation


  Change in one variable reflected by a change in another - identified on a scatter diagram
• Causation


  Change in one variable causes a change in another variable
• Correlation does not mean causation → may be other factors involved
• It is the epithelial cell layer of the tracheoles that is thin, not the tracheoles themselves
• Most gas exchange surfaces have short diffusion pathways and large surface areas, but to get the mark you need to identify the feature that results in these
• Hairs trap water vapour which reduces the water potential gradient between the leaf and air, not just trap water
• A thick waxy cuticle increases diffusion distance which reduces water loss, not just reduces water loss
• In counter current flow, blood and water flow in opposite directions, not blood and oxygen
• Xerophytes have needles or spines to reduce their SA:V, not small leaves