TOPIC 3 - EXCHANGE OF SUBSTANCES

• Surface area to volume ratio

  • The surface area of an organism divided by its volume
  • the larger the organism, the smaller the ratio
• Factors affecting gas exchange

  • diffusion distance
  • surface area
  • concentration gradient
  • temperature
• Ventilation

  • Inhaling and exhaling in humans
  • controlled by diaphragm and antagonistic interaction of internal and external intercostal muscles
• Inspiration

  • External intercostal muscles contract and internal relax
  • pushing ribs up and out
  • diaphragm contracts and flattens
  • air pressure in lungs drops below atmospheric pressure as lung volume increases
  • air moves in down pressure gradient
• Expiration

  • External intercostal muscles relax and internal contract
  • pulling ribs down and in
  • diaphragm relaxes and domes
  • air pressure in lungs increases above atmospheric pressure as lung volume decreases
  • air forced out down pressure gradient
• Passage of gas exchange


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• Alveoli structure


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• Why large organisms need specialised exchange surface


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• Fish gill anatomy


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• How fish gas exchange surface provides large surface area

  • Many gill filaments covered in many gill lamellae are positioned at right angles
  • creates a large surface area for efficient diffusion
• Countercurrent flow

  • When water flows over gills in opposite direction to flow of blood in capillaries
  • equilibrium not reached 
  • diffusion gradient maintained across entire length of gill lamellae
• Name three structures in tracheal system


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• How tracheal system provides large surface area


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• Fluid-filled tracheole ends

  • Adaptation to increase movement of gases
  • when insect flies and muscles respire anaerobically - lactate produced
  • water potential of cells lowered, so water moves from tracholes to cells by osmosis
  • gases diffuse faster in air
• How do insects limit water loss


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• Dicotyledonous plants leaf tissues


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• Gas exchange in plants

  • Palisade mesophyll is site of photosynthesis
  • oxygen produced and carbon dioxide used creates a concentration gradient
  • oxygen diffuses through air space in spongy mesophyll and diffuse out stomata
• Role of guard cells


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• Xerophytic plants

  • Plants adapted to survive in dry environments with limited water (e.g. marram grass/cacti)
  • structural features for efficient gas exchange but limiting water loss
• Adaptations of xerophyte


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• Digestion

  • Process where large insoluble biological molecules are hydrolysed into smaller soluble molecules
  • so they can be absorbed across cell membranes
• Locations of carbohydrate digestion


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• Locations of protein digestion


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• Endopeptidases


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• Exopeptidases


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• Membrane- bound dipeptidases

  • Break peptide bond between two amino acids
• Digestion of lipids

  • Digestion by lipase (chemical)
  • emulsified by bile salts (physical)
  • lipase produced in pancreas 
  • bile salts produced in liver and stored in gall bladder
• Lipase


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• Role of bile salts

  • Emulsify lipids to form tiny droplets and micelles
  • increases surface area for lipase action - faster hydrolysis
• Micelles

  • Water soluble vesicles formed from fatty acids, glycerol, monoglycerides and bile salts
• Lipid absorption

  • Micelles deliver fatty acids, glycerol and monoglycerides to epithelial cells of ileum for absorption
  • cross via simple diffusion as lipid-soluble and non-polar
• Lipid modification

  • Smooth ER reforms monoglycerides / fatty acids into tryglycerides
  • golgi apparatus combines tryglycerides with proteins to form vesicles called chylomicrons
• How lipids enter blood after modification

  • Chylomicrons move out of cell via exocytosis and enter lacteal
  • lymphatic vessels carry chylomicrons and deposit them in bloodstream
• How are glucose and amino acids absorbed


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• Haemoglobin (Hb)


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• Affinity of haemoglobin

  • The ability of haemoglobin to attract / bind to oxygen
• Saturation of haemoglobin

  • When haemoglobin is holding the maximum amount of oxygen it can hold
• Loading / unloading of haemoglobin

  • Binding/detachment of oxygen to haemoglobin
  • also known as association and disassociation
• Oxyhaemoglobin dissociation curve


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• Oxyhaemoglobin dissociation curve shifting left

  • Hb would have a higher affinity for oxygen
  • load more at the same partial pressure
  • becomes more saturated 
  • adaptation in low-oxygen environments
  • e.g. llamas/ in foetuses