Topic 3

• Surface area to volume ratio

  Formula: Surface area / Volume. Relationship between organism size/shape and this ratio
• Small organisms


  • Have very large surface area compared to volume, so can meet needs through simple diffusion
• Larger organisms


  • Have smaller surface area compared to volume, so need adaptations for mass transport and exchange across cells
• Key adaptations for exchange surfaces

  • Villi and microvilli in small intestine
  • Alveoli and bronchioles for gas exchange
  • Spiracles and tracheae for insect gas exchange
  • Gill filaments and lamellae for fish gas exchange
  • Stomata on plant leaves
• Breathing


  Movement of air in and out of the lungs
• Ventilation


  The scientific term for breathing
• Gas exchange


  Diffusion of oxygen and carbon dioxide in and out of cells
• Key structures of the human gas exchange system

  • Alveoli
  • Bronchioles
  • Bronchi
  • Trachea
  • Lungs
• Human ventilation


  1. Diaphragm muscle contracts

  2. External intercostal muscles contract, rib cage moves out, air flows in

  3. Internal intercostal muscles contract, rib cage moves in, air flows out
• Pulmonary ventilation


  Total volume of air moved into the lungs per minute
• Alveoli are tiny air sacs, with a thin epithelium to minimize diffusion distance, and surrounded by a capillary network
• Terrestrial insects


  • Have a large surface area to volume ratio, but a waterproof exoskeleton that prevents gas exchange
  • Use a tracheal system with spiracles, tracheae and tracheoles to deliver oxygen to cells
• Insect gas exchange methods

  1. Diffusion down concentration gradients

  2. Ventilation by abdominal muscle contraction

  3. Drawing in air during flight due to water potential changes
• Adaptations of insect gas exchange

  • Small surface area for exchange
  • Thin tracheal walls for short diffusion distance
  • Steep concentration gradients maintained
• Fish gills


  • Large surface area from many gill filaments and lamellae
  • Short diffusion distance due to capillary network
  • Counter-current flow mechanism maintains concentration gradient
• Fick's law of diffusion


  Diffusion rate is proportional to surface area, concentration gradient, and inversely proportional to diffusion distance
• Leaf gas exchange structures

  • Palisade mesophyll
  • Spongy mesophyll
  • Stomata
• Stomata open during the day for photosynthesis and close at night to prevent water loss
• Wing in opposite directions which means you should never actually have equilibrium and there will always be a higher concentration of oxygen in the water compared to the blood and that is why we maintain the concentration or the diffusion gradient across the entire gill lamellae
• Looking at gas exchange in leaves, the tissue layer palisade mesophyll is where photosynthesis mainly happens and the spongy mesophyll has lots of air spaces, and the stomata is where the gases actually diffuse in and out
• Oxygen will actually diffuse out of the stomata if it's not being used in respiration and carbon dioxide diffuses in because it's needed for photosynthesis
• Stomata close at night when it's dark and they'll open in the daytime when it's bright, and this is linked to the ideophase synthesis because it's a light dependent reaction
• Xerophytic plants (adapted to survive in environments with very limited water)


  • Leaves aren't flat, they roll up
  • Stomata are deep and sunken in
  • Lots of tiny hairs sticking out
  • Thicker cuticle
  • Longer root network
• During digestion, large biological molecules are hydrolyzed into smaller soluble molecules which can be absorbed across the cell membranes
• Biological molecules that need to be digested

  • Carbohydrates
  • Lipids
  • Proteins
• Carbohydrate digestion


  1. Amylase in mouth and pancreas hydrolyzes polysaccharides into disaccharides

  2. Sucrase and lactase hydrolyze disaccharides into monosaccharides
• Protein digestion


  1. Endopeptidases hydrolyze peptide bonds in the middle of the polymer chain

  2. Exopeptidases hydrolyze peptide bonds at the end of the chains

  3. Dipeptidase hydrolyzes peptide bonds between two amino acids
• Lipid digestion


  1. Lipase hydrolyzes ester bonds in triglycerides to form monoglycerides and fatty acids

  2. Bile salts emulsify lipids to form micelles, increasing surface area for lipase
• Micelle


  Vesicle formed of fatty acids, glycerol, monoglycerides, and bile salts
• Lipids are digested into monoglycerides and fatty acids, which can diffuse across the cell surface membrane to enter the epithelial cells
• In the epithelial cells, the monoglycerides and fatty acids are modified back into triglycerides and released as vesicles into the lacteal for transport around the body
• Ileum wall


  • Covered in villi with thin walls and a network of capillaries
  • Epithelial cells have microvilli, creating a large surface area
  • Short diffusion distance
  • Maintains concentration gradients
• Monosaccharides and amino acids are absorbed by active transport in the form of co-transport, due to the higher concentration already in the epithelial cells
• Hemoglobin


  Quaternary structure protein involved in the mass transport of oxygen around the body
• Myoglobin


  Hemoglobin found in muscle tissue and fetuses
• Oxyhemoglobin dissociation curve


  Shows how hemoglobin behaves in different oxygen partial pressures
• Hemoglobin loads oxygen in regions with high partial pressure of oxygen (e.g. alveoli) and unloads oxygen in regions with low partial pressure (e.g. respiring tissues)
• Cooperative binding - the first oxygen binding to hemoglobin makes it easier for subsequent oxygens to bind
• Bohr effect


  High CO2 concentration causes the oxyhemoglobin curve to shift to the right, decreasing hemoglobin's affinity for oxygen
• Different animals have hemoglobin adapted to their particular needs and environments