gas exchange

• What is gas exchange?

  The process by which gases such as oxygen and carbon dioxide move passively by diffusion across a surface. Down a concentration gradient.
• What establishes the concentration gradients needed for gas exchange?

  Aerobic respiration - since oxygen is used and carbon dioxide is produced
• How do organisms like bacteria carry out gas exchange?

  - high surface area to volume ratios

  - short diffusion pathways within them

  - they can rely on gas exchange with their external environment via simple diffusion across the cell surface membrane
• What gas exchange system do insects often use?

  Tracheal system
• How does the tracheal system work?

  - oxygen diffuses into the insect through pores called spiracles

  - the gas then enter wide tubes called tracheae

  - the tracheae then branch into narrower tubes called tracheoles

  - tracheoles allow oxygen to diffuse directly into the cells of the insect

  - the end of a tracheoles is fluid-filled

  - carbon dioxide diffuses out of the insect in the opposite direction
• What are the adaptations of the tracheal system for gas exchange?

  - thin walls so short diffusion distance to cells

  - highly branched tracheoles so short diffusion distance to cells and large surface area for gas exchange

  - tracheae provide tubes full of air so fast diffusion through insect tissues

  - water from fluid in the end of the tracheoles moves into tissues during exercise so faster diffusion through air (than through liquid) to the exchange surface

  - body can be moved by muscles to pump air so maintains concentration gradient for oxygen/carbon dioxide
• The high surface area of the tracheal system means that it has the potential to allow rapid water loss from the insect - how does it minimise this water loss?

  - spiracles have valves that can periodically close to reduce water loss

  - hairs around spiracles further reduce loss of water vapour
• Where does gas exchange occur in a fish?

  Across the gills
• What does the gill consist of?

  Gill filaments attached to a single gill arch
• Describe countercurrent flow and why it is good?

  - Water and blood flows in opposite directions

  - Blood is always passing water with a higher oxygen concentration

  - this ensures that the concentration gradient of oxygen is maintained across the entire length of each lamella therefore across the entire length of the gill

  - oxygen is diffusing into the blood across the whole length of the gill

  - more oxygen is released into the blood and more aerobic respiration to release energy for swimming
• Describe concurrent flow and why it is less effective than countercurrent

  - blood and water go in the same direction

  - the oxygen gradient is steep at first but it soon disappears as the oxygen concentration reaches equilibrium

  - at this point there is no further net uptake of oxygen into the blood by diffusion

  - for this reason fish gills have evolved countercurrent flow instead of
• What are the adaptations of fish gills for efficient gas exchange

  - each gill has many gill filaments each with many lamellae. This creates a large surface area for gas exchange

  - each lamella is lined by a thin epithelium. This ensures a short diffusion pathway to the blood
• What is the importance of gases for plants?

  - oxygen is needed for aerobic respiration

  - carbon dioxide is needed for photosynthesis

  
  
• What organelle do plants use to photosynthesise?

  Chloroplasts
• What’s the upper part of the mesophyll called?

  Palisade mesophyll
• Why does most of the photosynthesis take place in the palisade mesophyll?

  It is where the sunlight arrives and therefore has lots of chloroplasts
• What are the adaptations of the upper epidermis?

  - Although they don’t photosynthesise they are transparent to let light pass through

  - they secrete a waxy cuticle that reduces water loss from the leaf

  - the cuticle is also transparent

  
  
• What are the pores called that let gas pass through by diffusion into the leaf?

  Stomata
• Where are the stomata usually on the leaf?

  On the underside
• What are the stomata surrounded by that control their opening and closing?

  Guard cells