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 efficient exchange
• 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
• Alveolar epithelium


  • Very thin to minimize diffusion distance
  • Surrounded by capillary network to maintain concentration gradients
• Insect tracheal system


  • Spiracles as openings
  • Trachea as tubes with rings to prevent collapse
  • Tracheoles extending into tissues
• Insect gas exchange methods

  1. Diffusion down concentration gradients

  2. Ventilation by abdominal muscle contraction

  3. Tracheal volume changes during flight
• Insect adaptations to limit water loss

  • Small surface area for gas exchange
  • Spiracles can open and close like plant stomata
• 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
• Leaf gas exchange


  • Palisade and spongy mesophyll tissues
  • Stomata open during the day, close at night to reduce 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
• Structures in the leaf

  • Palisade mesophyll
  • Spongy mesophyll
  • Stomata
• Palisade mesophyll


  Where photosynthesis mainly happens
• Spongy mesophyll


  Lots of air spaces
• Stomata


  Where gases diffuse in and out
• Oxygen diffuses out of stomata


  If not being used in respiration
• Carbon dioxide diffuses in through stomata


  Because it's needed for photosynthesis
• Stomata close at night

  To reduce water loss by evaporation
• Stomata open in the daytime


  When it's bright
• This is linked to the light-dependent reaction of photosynthesis
• Adaptations of xerophytic plants to minimize water loss

  • Leaves roll up
  • Stomata are deep and sunken in
  • Tiny hairs sticking out
  • Thicker cuticle
  • Longer root network
• Digestion


  Large biological molecules are hydrolyzed into smaller soluble molecules which can be absorbed across the cell membranes
• Biological molecules digested


  • Carbohydrates
  • Lipids
  • Proteins
• Carbohydrate digestion


  1. Amylase in mouth

  2. Amylase in duodenum hydrolyzes polysaccharides to maltose

  3. Sucrase and lactase hydrolyze disaccharides to monosaccharides
• Protein digestion


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

  2. Exopeptidases hydrolyze peptide bonds at the ends of the chain

  3. Dipeptidase hydrolyzes dipeptides
• Lipid digestion


  1. Lipase hydrolyzes ester bonds in triglycerides

  2. Bile salts emulsify lipids to form micelles
• Micelle


  Vesicle formed of fatty acids, glycerol, monoglycerides and bile salts
• Lipid absorption


  1. Monoglycerides and fatty acids diffuse across epithelial cell membrane

  2. Reconstituted into triglycerides in ER and Golgi

  3. Packaged into vesicles and released into lacteals
• Ileum adaptations for absorption

  • Covered in villi and microvilli
  • Thin walls
  • Network of capillaries
• Co-transport


  Monosaccharides and amino acids absorbed by active transport due to higher concentration in epithelial cells
• Hemoglobin


  Quaternary structure protein involved in oxygen transport
• Myoglobin


  Oxygen-binding protein found in muscle tissue and fetal hemoglobin
• Oxyhemoglobin dissociation curve


  Shows how hemoglobin binds and releases oxygen at different partial pressures
• Oxygen loading


  Occurs in regions with high partial pressure of oxygen, e.g. alveoli