Section 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 surfaces
• 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 to maintain concentration gradients
• Leaf gas exchange


  • Palisade and spongy mesophyll tissues
  • Stomata open/close to regulate gas exchange and 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
• Leaf structures


  • Palisade mesophyll (where photosynthesis mainly happens)
  • Spongy mesophyll (lots of air spaces)
  • Stomata (where gases diffuse in and out)
• Stomata open in daytime


  When it's bright
• Stomata close at night


  When it's dark
• Stomata opening and closing is linked to the light-dependent reactions of photosynthesis
• Adaptations of xerophytic plants

  • Leaves roll up
  • Stomata are deep and sunken in
  • Tiny hairs sticking out
• Adaptations of xerophytic plants are to reduce water loss
• Other adaptations of xerophytic plants

  • Thicker cuticle
  • Longer root network
• Digestion of carbohydrates


  1. Amylase in mouth and pancreas hydrolyses polysaccharides into disaccharides

  2. Disaccharidases hydrolyse disaccharides into monosaccharides
• Digestion of proteins


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

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

  3. Dipeptidase hydrolyses dipeptides
• Digestion of lipids


  1. Lipase hydrolyses ester bonds in triglycerides

  2. Bile salts emulsify lipids into micelles
• Micelle


  Vesicle formed of fatty acids, glycerol, monoglycerides and bile salts
• Lipids are absorbed as monoglycerides and fatty acids which diffuse into epithelial cells</b>
• Adaptations of ileum for absorption

  • Villi and microvilli increase surface area
  • Thin walls for short diffusion distance
  • Network of capillaries maintains concentration gradients
• Monosaccharides and amino acids are absorbed by co-transport
• Hemoglobin


  Quaternary structure protein involved in oxygen transport
• Oxyhemoglobin dissociation curve


  • Shows how hemoglobin binds and releases oxygen at different partial pressures
  • Demonstrates cooperative binding and Bohr effect
• Fetal hemoglobin has higher affinity for oxygen than adult hemoglobin
• Hemoglobin adaptations in different animals
• Closed and double circulatory system

  Blood remains in blood vessels and passes through heart twice in each circuit
• Key blood vessels


  • Coronary arteries
  • Vena cava
  • Aorta
  • Pulmonary artery
  • Pulmonary vein
  • Renal artery
  • Renal vein
• Cardiac muscle


  • Myogenic (can contract without nervous/hormonal stimulation)
  • Never fatigues
• Heart structures


  • 4 chambers (2 atria, 2 ventricles)
  • Atria have thin walls, ventricles have thick walls
  • Right ventricle pumps to lungs at low pressure, left ventricle pumps to body at high pressure
  • Semilunar valves
  • Atrioventricular valves
• Blood vessel types


  • Arteries
  • Arterioles
  • Capillaries
  • Veins