3.3.2 Gas Exchange

• Insects cannot use their bodies as an exchange surface because they have a waterproof chitin exoskeleton and a small surface area to volume ratio in order to conserve water.
• The three main features of an insect’s gas transport system are the spiracles, trachea, the tracheoles, and the respiring muscle cells.
• Leaves are thin and flat to provide a short diffusion pathway and a large surface area to volume ratio, which allows for efficient gas exchange.
• Many minute pores in the underside of the leaf, known as stomata, allow gases to easily enter.
• Stomata are regulated by guard cells which allows them to open and close as needed, with most staying closed to prevent water loss while some open to let oxygen in.
• Air spaces in the mesophyll allow gases to move around the leaf, facilitating photosynthesis.
• Lamellae are at right angles to the gill filaments, giving an increased surface area.
• The countercurrent exchange system maximises oxygen absorbed by the fish by maintaining a steep concentration gradient, as water is always next to blood of a lower oxygen concentration.
• The water passes over the lamellae, and the oxygen diffuses into the bloodstream.
• Spiracles are holes on the body’s surface which may be opened or closed by a valve for gas or water exchange.
• Blood and water flow across the lamellae in opposite directions, creating a countercurrent exchange system.
• The rate of exchange is affected by…

  • Size of organism (therefore SA:V ratio)
  • Metabolic rate (how quickly chemical reactions take place in the body.) 
• An efficient gas exchange surface has…

  • a large surface area = increases SA:V ratio
  • thin (membrane) = decreases diffusion pathway
  • a steep concentration gradient = maintained by blood supply or ventilation.
• Tracheal system: gas exchange

  • Gases enter and leave the tracheae through the spiracles via diffusion
  • Spiracles are opened and closed by valves
  • When open water vapour can evaporate from the insect
• Respiratory gases can move in and out of the tracheae by 3 different methods:

  1. Along a diffusion gradient (insect at rest)
  2. By abdominal pumping (high activity)
  3. Ends of tracheoles are filled with water (high activity)
• How does an insect perform gas exchange when at rest?

  • Muscle cells use up O2
  • Muscle cells produce CO2

  this creates a concentration gradient.
• How does an insect perform gas exchange at periods of high activity?

  Via mass transport

  • Some species of insect can contract their abdominal muscles (abdominal pumping)
  • This forces large volumes of air out of the tracheae.
  • A new supply of air enters the tracheae on relaxation.
• How do insects perform gas exchange during prolonged periods of high activity?

  • Muscle cells at the end of tracheoles respire anaerobically during major activity
  • This produces lactic acid
  • Lactic acid is soluble and so dissolves in cells, lowering their water potential
  • Water moves into the cells by osmosis (from the tracheoles)
  • There is now a lower volume of water in the tracheoles, and so space for more air to be drawn further into the tracheal system
• Structure of fish gills in bony fish:

  • Series of gills on each side of the head
  • Each gill arch is attached to two stacks of filaments
  • On the surface of each filament, there are rows of lamellae 
  • The lamellae surface consists of a single layer of flattened cells that cover a vast network of capillaries
• The lamellae is the site of gas exchange in fish, they provide a large surface area for gas exchange and are thin to provide a short diffusion pathway.
• The main gas exchange surface in plants is the mesophyll cells in the leaf.
• Name and describe three adaptations of a leaf that allow efficient gas exchange.

  1. Thin and flat to provide short diffusion pathway and large surface area to volume ratio.
  2. Many minute pores in the underside of the leaf (stomata) allow gases to easily enter/leave
  3. Air spaces in the mesophyll allow gases to move around the leaf, facilitating photosynthesis.
• Guard cells control the rate of gaseous exchange by opening and closing the stomata. Plants may also close their stomata to limit the amount of water that evaporates from their leaves
• How do the stomata open?

  1. Water moves into guard cells via osmosis, down a water potential gradient 
  2. Causes guard cells to become turgid 
  3. The guard cells bend away and this opens the stomata.
• How do the stoma close?

  1. Water moves out of guard cells by osmosis
  2. This causes the guard cells to become flaccid 
  3. The guard cells no longer bend away from each other causing the stoma to close
• Xerophytes are plants adapted to live in warm, windy or dry environments (harsh conditions) where water loss is a problem.
• Xerophytic adaptations include:

  • Layer of hairs on their epidermis to trap water vapour around the stomata, reducing water potential gradient.
  • The stomata are sunken in pits capable of trapping water vapour. This also lowers the water potential between the air and the leaf, lessening the amount of water evaporating from leaf
  • Fewer stomata to reduce places where water can evaporate.
  • The leaves have a waxy, waterproof cuticle to reduce evaporation.
• Between each alveoli, collagen and elastic tissue is found, which is important for maintaining the structure and elasticity of the alveoli
• Adaptations for efficient gas exchange in the lungs:

  • Many alveoli and many capillaries provide a large surface area, increasing the SA:V ratio for fast diffusion
  • Alveolar and capillary walls are only one cell thick, creating a short diffusion gradient between alveoli and blood for fast diffusion
  • Alveolar walls made of squamous (flattened) epithelial cells
  • Ventilation and circulation maintain a diffusion gradient for fast diffusion
• Sequence of structures an oxygen molecule passes through from air to gas exchange surface: trachea, bronchi, bronchioles, alveoli
• Tissues allowing alveoli to stretch and recoil are collagen and elastic tissue
• Lungs have a large SA:V ratio due to the presence of many alveoli and capillaries, increasing the efficiency of gas exchange by providing a larger surface area for diffusion
• Gas exchange occurs as air enters via the trachea, bronchi, and bronchioles, where oxygen diffuses through the alveolar epithelium and capillary endothelium into the blood, down a diffusion gradient, while carbon dioxide diffuses through the capillary endothelium and alveolar epithelium into the alveoli, down a diffusion gradient
• Further adaptations for rapid diffusion of gases is that the alveolar surface is moist so gases dissolve and red blood cells are compressed against capillary wall so reduces diffusion pathway.
• Describe how oxygen in the air reaches capillaries surrounding alveoli in lungs (4)

  • Air enters the trachea, bronchi and bronchioles
  • oxygen moves down a pressure gradient
  • oxygen moves down a diffusion gradient
  • across alveolar epithelium
  • across capillary endothelium
• Explain why death of alveolar epithelium cells reduces gas exchange in human lungs (3)

  • reduced surface area
  • increased distance for diffusion
  • reduced rate of gas exchange
• Ventilation is controlled by the ribcage, intercostal muscles and the diaphragm.
• Maintenance of a large diffusion gradient across the alveolar epithelium is achieved by ventilation
• Breathing in is called inspiration, while breathing out is called expiration
• Two basic physical laws to understand before looking at ventilation:

  • The relationship between volume and pressure: as volume increases, pressure decreases
  • Gases move from a region of higher pressure to regions of lower pressure, down a pressure gradient