exchange

• 2 ways exchange occurs in organisms

  passively - no metabolic energy required - diffusion and active transport

  actively - metabolic energy required - active transport
• surface area to volume ratio

  decreases with increasing organism size as volume increases more rapidly than surface area

  large surface area : volume ratio preferable
• adaptions of organisms in relation to surface area : volume

  flattened shape so no cell is far from surface

  specialised exchange surfaces with large areas to increase surface area to volume ratio
• characteristics of specialised exchange surfaces

  large surface area relative to volume of organism which increases exchange rate

  very thin so short diffusion distance

  selectively permeable

  movement of environmental medium to maintain concentration gradient e.g., air

  transport system to ensure movement of internal medium e.g., blood to maintain diffusion gradient
• why are specialised exchange surfaces located in the body

  thin and so are easily dehydrated and damaged
• gas exchange in single celled organisms

  small = large surface area to volume ratio

  oxygen absorbed by diffusion across body surface, which is only covered by a cell surface membrane

  carbon dioxide diffuses out across body surface
• gas exchange pathway in insects - anatomy

  spiracle

  trachea - internal network of tubes supported by strengthened rings

  tracheoles - dead end tube divisions of trachea, extend through all body tissues

  muscle cell
• 3 ways respiratory gases move in and out of insect tracheal system

  along diffusion gradient

  mass transport

  ends of tracheoles filled with water
• how respiratory gases move in and out of insect tracheal system - along diffusion gradient

  oxygen used up by respiring cells and so concentration towards end of tracheoles falls

  creates diffusion gradient causing oxygen to diffuse from atmosphere along trachea and tracheoles into cells

  carbon dioxide produced by cells creating concentration gradient in opposite direction

  causes carbon dioxide to diffuse along tracheoles and tracheo to atmosphere
• how respiratory gases move in and out of insect tracheal system - mass transport

  contraction of muscles in insects can squeeze trachea enabling mass movements of air in and out

  speeds up exchange
• how respiratory gases move in and out of insect tracheal system - ends of tracheoles filled with water

  periods of major activity = muscle cells around tracheoles respire via anaerobic respiration = lactate which is soluble and lowers water potential of muscle cells

  water therefore moves into cells from tracheoles by osmosis

  water in ends of tracheoles decreases in volume and in doing so draws air further into them

  increases rate of air movement into tracheoles but leads to greater water evaporation
• spiracles and water loss


  pores on insects body surface

  may be opened and closed by valve

  when open, water vapour can evaporate from insect and so insects tend to keep them shut
• limitations of tracheal system of gas exchange in insects

  relies mostly on diffusion to exchange gases between environment and cells

  for effective diffusion, diffusion pathway needs to be short = insects are small illustrating how length of diffusion limits size insects can obtain
• structure of gills

  located behind fish head

  made up of gill filaments stacked in piles

  at right angles to filaments are gill lamellae, which increase surface area of gills
• countercurrent principle in fish

  ensures maximum possible gas exchange achieved

  • blood and water flowing over lamellae in opposite directions
  • blood already well loaded by oxygen meets water, which has maximum concentration of oxygen, diffusion from water to blood
  • blood with little oxygen meets with water that has had most but not all of oxygen removed

  diffusion gradient for oxygen uptake maintained across entire gill lamellae width
• how do volumes and types of gases exchanged with external air differ due to balance of photosynthesis and respiration rates

  photosynthesis taking place, majority of carbon dioxide obtained from external air, majority of oxygen diffuses into air from plant

  no photosynthesis occurring, oxygen diffuses into leaf due to constant respiration and carbon dioxide diffuses out
• gas exchange in plants as similar to insects

  no living cell far from external air ( source of carbon and oxygen )

  diffusion takes place in gas phase making it more rapid than if it was in water
• adaptions of leaves for gaseous exchange

  many small pores - stomata - and so no cell is far from stoma = short diffusion pathway

  numerous interconnecting air spaces occurring throughout mesophyll so gases can readily come in contact with mesophyll cells

  large surface area of mesophyll cells for rapid diffusion
• stomata and gas exchange in insects

  minute pores occurring mainly on underside of leaf

  each stoma surrounded by guard cells that open / close stomatal pore

  control rate of gaseous exchange and water loss
• limiting water loss in insects

  water easily evaporated from body surfaces

  efficient gas exchange requires thin, permeable surface with a large area - conflicts with need to preserve water
• insects adaptations to reduce water loss


  small surface area to volume ratio - minimises surface water can be lost over

  waterproof coverings over body surface

  spiracles on openings of tracheae can be closed to reduce water loss but conflicts with need for oxygen
• limiting water loss in plants

  waterproof coverings over parts of leaves

  ability to closed stomata when necessary

  cannot have small surface area to volume ratio as photosynthesis requires large large leaf surface area
• 5 adaptations of plants in areas of high water loss / limited water supply

  thick cuticle

  rolling up of leaves

  hairy leaves

  stomata in pits or grooves

  reduced surface area to volume ratio of leaves
• adaptations of plants in areas of high water loss / limited water supply - thick cuticle


  forms waterproof barrier

  thicker cuticle = less water loss
• adaptations of plants in areas of high water loss / limited water supply -reduced surface area to volume ratio of leaves


  leaves that are small and roughly circular in cross section reduced date of water loss
• adaptations of plants in areas of high water loss / limited water supply - stomata in pits or grooves


  trap still, moist air next to leaf and reduces water potential gradient
• adaptations of plants in areas of high water loss / limited water supply - hairy leaves


  thick layer of hairs on leaves, especially lower epidermis, trap still moist air next to leaf surface

  water potential gradient between inside and outside of leaves reduced and therefore less water is lost by evaporation
• adaptations of plants in areas of high water loss / limited water supply - rolling up of leaves

  most leaves have stomata largely or entirely on lower epidermis

  rolling of leaves in a way that protects lower epidermis from outside helps trap region of still air within rolled leaf

  region becomes saturated with water vapour and so has high water potential = no water potential gradient between inside / outside of leaf and so no water loss
• why is the volume of oxygen that has to be absorbed and volume of carbon dioxide that must be removed large in mammals

  relatively large organisms with large volume of living cells

  maintain high body temperature which is related to them having high metabolic and respiratory rates
• why are mammalian lungs located in the body

  air not dense enough to support and protect these delicate structures

  body as a whole would otherwise lose great deal of water and dry out
• key anatomy of human gas exchange system

  lungs

  trachea

  bronchi

  bronchioles

  alveoli
• lungs structure and function

  pair of lobed structures

  made up of series of highly branched tubules = bronchioles which end in tiny air sacs called alveoli
• trachea structure and function

  flexible airway supported by rings of cartilage preventing collapse as air pressure inside falls when breathing in

  tracheal walls made up of muscle lined with ciliated epithelium and goblet cells
• bronchi structure and function

  two divisions of trachea each leading to one lung

  similar in structure to trachea and also produce mucus to trap dirt particles and have cilia moving dirt laden mucus to throat`

  larger bronchi supported by cartilage
• bronchioles structure and function

  series of branching subdivisions of bronchi

  walls made of muscle lined with epithelial cells

  muscle allows constriction to control air flow in and out of alveoli
• alveoli structure and function

  minute air sacs with diameter of 100 - 300 micrometers

  end of bronchioles

  between alveoli = collagen and elastic fibres

  lined with epithelium

  elastic fibres allow alveoli to stretch as they fill with air when breathing in and then spring back during breathing out to expel the carbon rich air

  alveolar membrane is gas exchange surface
• ventilation 


  process by which the diffusion of gases across alveolar epithelium is maintained by constantly moving air in and out of lungs
• inspiration 


  when air pressure of atmosphere is greater than inside lungs, air forced into lungs
• expiration 


  when air pressure in lungs is greater than that of atmosphere, air is forced out of lungs
• 3 sets of muscles influencing pressure changes within lungs

  diaphragm - sheet of muscle separating thorax and abdomen

  intercostal muscles - lie between ribs

  • INTERNAL INTERCOSTAL = contraction leads to expiration
  • EXTERNAL INTERCOSTAL = contraction leads to inspiration