Gas exchange

• The larger the organism

  The lower the surface area to volume ratio
• Surface area to volume (SA/V) ratio


  Affects transport of molecules
• The lower the SA/V ratio


  The further the distance molecules must travel to reach all parts of the organism. Diffusion alone is not sufficient in organisms with small SA/V ratios
• Larger organisms

  • Require mass transport and specialised gas exchange surfaces
• Features of an efficient gas exchange surface

  • Large surface area
  • Short diffusion distance
  • Steep diffusion gradient
  • Ventilation mechanism
• Gas exchange mechanism in the Amoeba

  Unicellular organism with a large SA/V ratio. Thin cell membrane provides short diffusion distance. Simple diffusion across the cell surface membrane is sufficient to meet the demands of respiratory processes
• Gas exchange mechanism in flatworms

  Multicellular organisms with a relatively small SA/V ratio (in comparison to the Amoeba). However, flat structure provides a large surface area and reduces the diffusion distance. Simple diffusion is sufficient to meet the demands of respiratory processes
• Gas exchange mechanism in earthworms

  Cylindrical, multicellular organisms with a relatively small SA/V ratio (in comparison to the flatworm). Slow moving and low metabolic rate ∴ require little oxygen. Rely on external surface for gas exchange. Circulatory system transports oxygen to the tissues and removes carbon dioxide, maintaining a steep diffusion gradient
• Ventilation


  The movement of fresh air into a space and stale air out of a space to maintain a steep concentration gradient of oxygen and carbon dioxide
• Gill filaments


  Main site of gaseous exchange in fish, over which water flows. They overlap to increase resistance to flowing water - slowing it down and maximising gaseous exchange. Found in large stacks, known as gill plates, and have gill lamellae which provide a large surface area and good blood supply for exchange
• Counter current flow


  Blood and water flow in opposite directions across the gill plate
• Counter current flow


  Maintains a steep diffusion gradient. Keeps rate of diffusion constant and enables 80% of available oxygen to be absorbed
• Parallel flow


  Water and blood flow in the same direction across the gill plate
• Insect gas transport system

  • Spiracles - small, external openings along the thorax and abdomen through which air enters, and air and water leave the gas exchange system
  • Tracheae - large tubes extending through all body tissues, supported by rings of chitin to prevent collapse
  • Tracheoles - smaller branches dividing off the tracheae
• Main site of gas exchange in insects

  Tracheoles
• Adaptations of the insect tracheal system to a terrestrial environment

  • Spiracles can be opened or closed to regulate diffusion
  • Bodily contractions speed up the movement of air through the spiracles
  • Highly branched tracheoles provide a large surface area
  • Impermeable cuticle reduces water loss by evaporation
• Ventilation of the tracheal system in insects

  1. Expansion of the abdomen opens the thorax spiracles (through which air enters) and closes the abdominal spiracles

  2. Compression of the abdomen closes the thorax spiracles and opens the abdominal spiracles (through which air is expelled)
• Mammalian adaptations for gas exchange

  • Alveoli provide a large surface area and thin diffusion pathway, maximising the volume of oxygen absorbed from one breath. They also have a plentiful supply of deoxygenated blood, maintaining a steep concentration gradient
• Larynx

  A hollow, tubular structure located at the top of the trachea involved in breathing and phonation
• Trachea

  Primary airway, carries air from the nasal cavity down into the lungs
• Maintaining a steep concentration gradient.
• Larynx


  A hollow, tubular structure located at the top of the trachea involved in breathing and phonation.
• Trachea

  • Primary airway, carries air from the nasal cavity down into the chest
  • Wide tube supported by C-shaped cartilage to keep the air passage open during pressure changes
  • Lined by ciliated epithelial cells which move mucus, produced by goblet cells, towards the back of the throat to be swallowed. This prevents lung infections
• Bronchi


  • Divisions of the trachea that lead into the lungs
  • Narrower than the trachea
  • Supported by rings of cartilage and lined by ciliated epithelial cells and goblet cells
• Bronchioles

  • Many small divisions of the bronchi that allow the passage of air into the alveoli
  • Contain smooth muscle to restrict airflow to the lungs but do not have cartilage
  • Lined with a thin layer of ciliated epithelial cells
• Alveoli


  • Mini air sacs, lined with epithelial cells
  • a layer of endothelium
  • Good blood supply to maintain a steep diffusion gradient
  • 300 million in each lung
• Pleural membranes


  Thin, moist layers of tissue surrounding the pleural cavity that reduce friction between the lungs and the inner chest wall
• Pleural cavity

  The space between the pleural membranes of the lungs and the inner chest wall
• Ventilation


  • The movement of fresh air into the lungs and stale air out of the lungs via inspiration and expiration
  • Via negative pressure breathing
• Internal intercostal muscles

  A set of muscles found between the ribs on the inside that are involved in forced exhalation
• External intercostal muscles


  A set of muscles found between the ribs on the outside that are involved in forced and quiet inhalation
• Inspiration


  1. External intercostal muscles contract (while internal relax), raising the ribcage

  2. Diaphragm contracts and flattens

  3. Outer pleural membrane moves out, reducing pleural cavity pressure and pulling the inner membrane out

  4. The alveoli expand. Alveolar pressure falls below air pressure so air moves into the trachea
• Surfactant


  A fluid lining the surface of the alveoli that reduces surface tension and prevents collapse of the alveoli during exhalation
• Identify the structures of the dicotyledonous leaf labelled in the diagram below.
• Waxy cuticle


  Reduces water loss from the leaf surface
• Upper epidermis


  • Layer of transparent cells allow light to strike the mesophyll tissue
  • Epidermal cells also synthesise the waxy cuticle, reducing water loss
• Palisade mesophyll layer

  • Directly below the upper epidermis
  • Receives the most light so contains the greatest concentration of chloroplasts
• Spongy mesophyll layer

  • Contains air spaces that reduce the diffusion distance for carbon dioxide to reach the chloroplasts in the palisade layer
  • Contains some chloroplasts
• Vascular bundle


  The vascular system in dicotyledonous plants. It consists of two transport vessels, the xylem and the phloem
• Vascular bundles


  They form a large network to deliver water and nutrients to photosynthetic tissues and remove glucose