Unit 4 - Key Questions

• Things that need to be exchanged between organisms and their environment

  • Respiratory gases
  • Nutrients
  • Excretory products
  • Heat
• Surface area to volume ratio

  The ratio of the surface area to the volume of an object
• Factors that affect the rate of diffusion of substances into cells

  • Surface area
  • Thickness of exchange surface
  • Permeability of the cell-surface membrane to the substance
  • Concentration gradient of the substance between the inside and outside of the cell
• Erythrocyte

  A red blood cell
• RBC's (red blood cells)

  • Biconcaved shape for greater surface area for oxygen diffusion
  • No organelles so maximum volume to carry haemoglobin
• Specialised exchange surfaces

  • Large surface area to volume ratio
  • Very thin so shorter diffusion distance
  • Movement of environmental medium to maintain concentration gradient
  • Effective internal transport system to ensure diffusion gradient is maintained
• Diffusion equation

  (surface area x difference in concentration) / length of diffusion path
• Mass transport of oxygen in insects

  Contraction of muscles through abdominal pumping enabling mass movements of air in and out
• Oxygen reaching working muscles in insects

  Through the tracheoles / tracheal tubes
• During periods of high activity in insects

  Anaerobic respiration will create lactate which will reduce the water potential of respiring cells causing them to take up water by osmosis. This loss of water from the tracheoles will pull gas into them
• Spiracles

  Tiny pores in an insect exoskeleton that allow gases to enter and leave
• Insect spiracles

  • They open and close to balance the need for gaseous exchange with the need to conserve water
• Terrestrial insects

  • Small surface area to volume ratio, waterproof coverings, spiracles that can be opened and closed
• The tracheal system limits the size of insects because it relies on diffusion to bring oxygen to the tissues. If an insect was too large it would take too long for oxygen to reach the respiring tissue rapidly enough to supply their needs
• Gills

  Made up of gill filaments
• Gills

  • They have many gill filaments with many gill lamellae on them
• Countercurrent flow in gills

  Ensures there is always a diffusion gradient between water and the full length of the capillaries in the gills allowing maximum uptake of oxygen
• Leaf

  • Spongy mesophyll contains air spaces so short diffusion distance from atmosphere to palisade mesophyll
  • Many stomata to allow diffusion throughout the leaf reducing diffusion distance
• Stomata

  Tiny pores mainly found on the underside of the leaf to limit evaporation and transpiration
• Guard cells

  The cells that control the opening and closing of stomata
• Having open stomata is a disadvantage as it can lead to excessive water loss
• Xerophytes

  Plants that are well adapted to dry environments
• Adaptations of xerophytes to limit water loss

  • Thick waxy cuticle
  • Rolling of leaves
  • Hairy leaves
  • Stomata sunken in pits or grooves
  • Reduced surface area to volume ratio of the leaves
• How adaptations of xerophytes reduce water loss

  Reduce water potential gradient and therefore slower diffusion of water vapour from air spaces and hence reduced evaporation of water
• Transpiration

  The process when a plant loses water
• Thick cuticle

  Increases the diffusion distance for water vapour, reducing transpiration
• Humans must have a high rate of gaseous exchange because they have a large volume of cells and have to maintain a high body temperature
• Structures of the human gas exchange system

  • Trachea
  • Lungs
  • Bronchi
  • Bronchioles
  • Alveoli
• Trachea

  Supported by a ring of cartilage to prevent it collapsing when air pressure inside is low
• Goblet cells in the trachea

  Produce mucus
• Alveoli

  • They can stretch and recoil due to elastic fibres made of the protein elastin
• Internal intercostal muscles

  Relax during inspiration
• Tidal volume

  The volume of air that enters and leaves the lungs during one normal breath
• External intercostal muscles and diaphragm

  Relax during expiration
• Calculating pulmonary ventilation

  Pulmonary ventilation = tidal volume x breathing rate
• During expiration

  Thoracic volume decreases
• During inspiration

  Thoracic volume increases
• During inspiration

  Pressure in the thoracic cavity drops below atmospheric pressure so air moves in down a pressure gradient
• In the capillaries during gas exchange

  Red blood cells flatten themselves against the side of the capillary, resulting in a shorter diffusion distance
• Digestion

  The breakdown of large, insoluble molecules into smaller, soluble molecules