chapter 6 - exchange

• what must exchange surfaces have to make transport across surfaces more efficient?

  similar adaptations
• the relationship between the size of an organism & its surface area : volume plays a significant role in its type of adaptations (SPEC 3.3.1)
• the smaller an organism, the larger its surface area : volume ratio and smaller distance from the outside of the organism
• the larger an organism, the smaller its surface area : volume ratio and the larger its distance from the middle
• what do large organisms have?

  higher metabolic rate, which demands efficient transport of waste
• what are the features of exchange surfaces?

  1. large surface area : volume ratio - increases rate of exchange
  2. very thin - short diffusion pathway and rapid exchange of materials
  3. selectively permeable - allows selected materials to cross
  4. movement of the environmental medium - e.g. air to maintain a diffusion gradient
  5. transport system - ensures movement of internal medium e.g. blood in order to maintain a diffusion gradient
• what type of skeleton do terrestial insects and what is its function?

  Exoskeleton - provides protection
• what type of layer do insects have?

  lipid layer to prevent water loss
• what adaptations do insects have to prevent water loss?

  1. small surface area : volume ratio
  2. waterproof coating on exoskeleton : prevents water loss by evaporation
  3. spiracles : can open & close to reduce water loss
• what structures involve gas exchange in insects?

  • spiracles -> small holes on the surface of the exoskeleton & oxygen and carbon dioxide can enter & leave. They are attached to :
  • trachea -> are a network of tubes. They branch into :
  • tracheoles -> smaller tubes which extend throughout into the muscle tissue to deliver oxygen to respiring cells
• how does atmospheric air diffuse into the trachea & tracheoles?

  1. cells respire, using up oxygen. This creates a concentration gradient as there's a low concentration of oxygen in the cells so it diffuses into the trachea & tracheoles
  2. muscles contract & this squeezes the trachea, causing mass transport of air
  3. ends of the tracheoles are filled with water so when there is a lot of muscle activity, anaerobic respiration of muscle cells produces lactate & this lowers water potential of cells so water moves from tracheoles into cells by osmosis. This lowers volume of tracheoles, more air drawn in
• what adaptations do insects have for efficient gas exchange?

  1. lots of tracheoles -> large surface area
  2. thin tracheoles -> short diffusion pathway
• what gas exchange surface do fish have?

  Gills
• why do fish need to have gills as a gas exchange surface?

  because they are waterproof & they are relatively large, so they have a small surface area : volume ratio
• what is the structure of a fish gill?

  1. gill arch - has stacks of gill filaments
  2. gill filaments - covered in gill lamallae
  3. gill lamallae
• what are the adaptations for efficient gas exchange in fish?

  1. lots of gill filaments & lamallae -> creates large surface area
  2. thin lamallae & many capillaries in lamallae -> short diffusion pathway
  3. countercurrent flow mechanism -> maintains concentration gradient
• what is the countercurrent flow mechanism in fish?

  when water & blood flows in opposite directions to ensure equilibrium is not reached and a concentration gradient is maintained down the whole length of the gill lamallae
• EXAM QUESTION - Explain how the countercurrent flow mechanism ensures efficient gas exchange in fish gills

  blood and water flows in opposite directions and this maintains a concentration gradient down the whole length of the lamallae
• what are the 3 key features of the leaf for gas exchange?

  1. stomata - small pores formed between 2 guard cells & is the site of gas exchange
  2. spongy mesophyll - where CO2 diffuses into. Lots of spaces which allows gases to diffuse into & helps to maintain a concentration gradient
  3. palisade mesophyll - where CO2 continues to diffuse from the spongy mesophyll. Where most of photosynthesis occurs as it is closer to the surface so more direct sunlight can be received
• what is an advantage of the palisade mesophyll being closer to the surface?

  it can receive more direct sunlight for photosynthesis
• what is the site of gas exchange in a leaf?

  Stomata
• explain what happens in the stomata for gas exchange

  oxygen diffuses out of the stomata because it is a product of photosynthesis so there is a high concentration of oxygen inside the spongy mesophyll so it diffuses out down a concentration gradient.

  carbon dioxide diffuses in through the stomata because it is required for photosynthesis so it is constantly being used by the palisade mesophyll, so it maintains a concentration gradient, a low concentration in the spongy mesophyll and a high concentration outside of the leaf
• what are adaptations of xerophytic plants to enable gas exchange & limi water loss?

  • curled leaves - trap moisture to increase humidity
  • hairy leaves - trap moisture to increase humidity
  • sunken stomata - trap moisture to increase humidity
  • thicker cuticle - reduce evaporation
• what is the structure of the lungs?

  pair of lobed structures made up of bronchioles, which end in tiny air sacs called alveoli
• structure of trachea:

  • flexible airway, supported by rings of cartilage
  • cartilage prevents trachea from collapsing as the air pressure inside falls when inhaling
  • tracheal walls are made of muscle, lined with ciliated epithelium & goblet cells
• structure of bronchi:

  • 2 divisions of the trachea, each one leading up to the lung
  • produce mucus to trap dirt particles & have cilia that move dirt-laden mucus towards the throat
  • larger bronchi supported by cartilage
• structure of bronchioles:

  walls made of muscle-lined with epithelial cells

  muscles allow them to constrict so they can control the flow of air in & out of the alveoli
• structure of alveoli:

  • collagen & elastic fibres between the alveoli
  • lined with epithelium cells
  • elastic fibres allow the alveoli to stretch as they fill with air when breathing in
• what is the gas exchange surface in lungs?

  alveolar membrane
• what is ventilation?

  process of air constantly moving in & out of the lungs
• what does the contraction of the external intercostal muscles lead to?

  inspiration
• what does the contraction of the internal intercostal muscles lead to?

  expiration
• inspiration (inhalation):

  • diaphragm contracts & flattens
  • ribcage moves up & out
  • external intercostal muscles contracts
  • internal intercostal muscles relaxes
  • increases volume in the thorax
  • decreases pressure in the thorax
  • atmospheric pressure is greater than pulmonary pressure so air is forced into the lungs down a pressure gradient to a lower pressure
• expiration (exhalation):

  • diaphragm relaxes & expands
  • ribcage moves in & down
  • external intercostal muscles relaxes
  • internal intercostal muscles contracts
  • decreases volume in the thorax
  • increases pressure in the thorax
  • pulmonary pressure is greater than atmospheric pressure, so pressure moves down a pressure gradient so air is forced out of the lungs
• how do you calculate pulmonary ventilation rate?

  pulmonary ventilation rate = tidal volume x breathing rate
• what is pulmonary ventilation?

  total volume of air moved in the lungs
• what is tidal volume?

  total volume of air taken in at each breath
• what is the breathing rate?

  the number of breaths taken per minute.
• what helps the rate of diffusion of these gases to stay as high as possible?

  ventilation
• why do mammals have a high oxygen demand?

  because they require an increased rate of aerobic respiration to maintain a constant body temperature