3.2 gas exchange

• How do single celled organisms exchange gases?

  Directly across their body/external surface via diffusion.
• What are the adaptations of the gas exchange surface in the tracheal system of an insect?

  • tracheole is thin/one cell thick so short diffusion pathway and rapid diffusion
  • tracheoles enter and supply cells and tissues so diffusion directly into cells and short diffusion pathway
  • tracheoles are highly branched so there’s a large surface area for diffusion
• What features do exchange surfaces with an increased rate of reaction have?

  • large surface area:volume ratio
  • thin (short diffusion distance)
  • maintained large concentration gradient
• Why are the ends of tracheoles filled with water?

  • muscle cells may carry out anaerobic respiration, producing lactic acid (soluble lactate)
  • so the water potential of muscle cells is lowered
  • therefore water moves into the cells from tracheoles by osmosis
  • this decreases the pressure in the tracheoles, drawing more air in
• Describe the gas exchange system of insects.

  • gas (CO2 and O2) enters and leaves through tiny pores: spiracles which are found on the exoskeleton of insects. the spiracles can open and close with a valve
  • this leads to a trachea, supported and strengthened by rings to prevent collapse
  • these divide into tracheoles which extend through body tissue and lead to muscle. tracheoles have water filled ends
• How and why does oxygen enter an insect’s body?

  • in muscles, oxygen is used in aerobic respiration
  • so an oxygen concentration gradient is established
  • oxygen from the environment enters the insect via the spiracles and diffuses in
• How is abdominal pumping used to mass transport CO2 out of an insect?

  • CO2 is produced during aerobic respiration
  • abdominal pumping increases pressure in the body
  • muscle contraction squeezes trachea enabling mass movement of air in and out
  • this pushes out CO2 along a pressure gradient
• How are gills adapted to have a large surface area to volume ratio?

  Many gill filaments covered in many gill lamellae increases surface area to volume ratio.
• How are gills adapted to have a short diffusion distance?

  • capillary network in every lamellae so capillary network is close to external environment where oxygen is diffusing in from
  • gill lamellae is very thin
• How are gills adapted to maintain the oxygen concentration gradient?

  Countercurrent flow mechanism
• What is the countercurrent flow mechanism?

  • water flows over the gills in the opposite direction to the flow of blood in the capillaries
  • this ensures equilibrium is never reached and the concentration of oxygen is always higher in the water than in the blood
  • this ensures that a concentration gradient of oxygen is maintained across the entire length of the gill lamella
• How are dicotyledonous plants adapted for gas exchange?

  • air spaces in spongy mesophyll layer increases the surface area which increases the rate of diffusion of O2 and CO2
  • thin tissues within the leaf decrease the diffusion distance increasing the rate of gas exchange
  • concentration gradient maintained as the CO2 that enters through the stomata is immediately used by photosynthetic cells
  • stomata helps create a short diffusion pathway increasing rate of gas exchange as no active ventilation is required
• How are xerophytic plants adapted to survive in very dry conditions where water is limited?

  • curled leaves so smaller surface area and less evaporation
  • hairs on epidermis to trap water vapour around the stomata so the water potential gradient is decreased
  • fewer stomata so less evaporation of water
  • thicker waxy cuticle so greater diffusion pathway and so the rate of osmosis is decreased
  • stomata sunk in pits to trap water vapour which reduces the diffusion gradient of water between the leaf and air
• Describe inhalation (inspiration) in humans?

  • diaphragm contracts and flattens
  • external intercostal muscles contract causing the rib cage to move upwards and outwards
  • this increases the chest cavity (thorax) volume
  • air pressure in the lungs decreases
  • atmospheric air pressure is greater so air is drawn in from the atmosphere to the lungs along the pressure gradient
• Describe exhalation (expiration) in humans?

  • diaphragm relaxes and appears “C shaped”
  • external intercostal muscles relax causing the rib cage to move downwards and inwards
  • this decreases the chest cavity (thorax) volume
  • air pressure in the lungs increases
  • atmospheric air pressure is lower so air is forced out of the lungs along the pressure gradient
• What is the name of the cell surrounding capillaries?

  Endothelial cells
• What is the name of the cell surrounding alveoli?

  Epithelial cells
• What is the gross structure of the human gas exchange system?

  Trachea, bronchi, bronchioles, lungs, alveoli
• What is the pathway taken by oxygen as it enters the human gas exchange system?

  Mouth/nasal cavity —> trachea —> bronchi —> bronchioles —> alveoli —> red blood cells in blood
• What is the function of goblet cells?

  Secrete mucus to trap pathogens and microorganisms.
• What is the function of ciliated cells?

  Sweep away pathogens and microorganisms trapped in mucus.
• Describe the relationship between internal and external intercostal muscles?

  Antagonistic
• Describe the trachea and its function in the human gas exchange system.

  • the airway that connects the mouth to the lungs (bronchi)
  • ”C” shaped cartilage rings provide support and prevent the airways from collapsing during pressure changes
  • lined by ciliated cells and goblet cells
• Describe the bronchi and its function in the human gas exchange system.

  • “C” shaped cartilage rings provide support and prevent the airways from collapsing during pressure changes
  • lined by ciliated cells and goblet cells
  • narrower than trachea
  • allow passage of air into the bronchioles
• Describe the bronchioles and its function in the human gas exchange system.

  • narrower than the bronchi
  • have no supporting cartilage so mostly only have muscle and elastic fibres so they can easily contract and relax during ventilation
  • allow passage of air to the alveoli
• What is the surface where human gas exchange occurs?

  Alveolar epithelium
• What are the features of the alveolar epithelium that allow for efficient gas exchange?

  • alveoli walls are folded and a large number of alveoli - increases the surface area available for O2 and CO2 to diffuse across so faster rate of diffusion
  • alveoli and capillary walls are one cell thick - short diffusion pathway, a faster rate of diffusion
  • extensive capillary network/circulation of blood - constant flow of blood through the capillaries so oxygenated blood is brought away from the alveoli and deoxygenated blood is brought to them, this maintains a concentration gradient for diffusion
• How are the gases exchanged between the alveoli and the capillaries in the lungs?

  • simple diffusion
  • air in the alveoli contains a high concentration of oxygen, oxygen diffuse from the alveoli into the blood capillaries, before being carried away to the rest of the body for aerobic respiration
  • blood in the capillaries has a relatively low concentration of oxygen and a high concentration of carbon dioxide, the carbon dioxide diffuses from the blood and into the alveoli and is then exhaled
• How do terrestrial insects limit water loss while still maintaining an efficient gas exchange system?

  • insects possess a waterproof exoskeleton that prevents water loss however this waterproof, waxy exoskeleton makes gas exchange by diffusion very difficult
  • insects have evolved a breathing system (the tracheal system) which consists of many tubes that carry oxygen directly to all tissues and cells of the body
  • spiracles are openings in the exoskeleton of insects that are connected to the tracheal system
• Why is water loss an inherent problem with gas exchange?

  • cells need to be exposed to air in order for the oxygen to diffuse into the organism
  • many organisms bodies are made of a high percentage of water
  • when living cells are exposed to the air, water vapour and vaporises and cells dehydrate and shrivel leading to organism death
• Why is oxygen, carbon dioxide and water required in some organisms?

  • oxygen is required for respiration
  • carbon dioxide is required for photosynthesis
  • water is a solvent that facilitates the transport of essential nutrients, most cells are made of water and water is required as a metabolite in some reactions e.g. hydrolysis