Organisms exchange substances with their environments

• 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


  - Mouth / nose -> trachea ->

  bronchi -> bronchioles -> alveoli

  - crosses alveolar epithelium into

  capillary endothelium

  
• Alveoli structure


  - Tiny air sacs

  - highly abundant in each lung - 300 million

  - surrounded by the capillary network

  - epithelium 1 cell thick
• Why large organisms need specialised exchange surface?

  - They have a small surface area to volume ratio

  - higher metabolic rate - demands efficient gas exchange

  - specialised organs e.g. lungs / gills designed for exchange
• Fish gill anatomy


  - Fish gills are stacks of gill filaments

  - each filament is covered with gill lamellae at right angles

  
• 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


  - Involves trachea, tracheoles, spiracles

  
• How tracheal system provides large surface area?


  - Highly branched tracheoles

  - large number of tracheoles

  - filled in ends of tracheoles moves into tissues during exercise

  - so larger surface area for gas exchange
• 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 tracheoles to cells by osmosis

  - gases diffuse faster in air
• How do insects limit water loss?


  - Small surface area to volume ratio

  - waterproof exoskeleton

  - spiracles can open and close to reduce water loss - thick waxy cuticle - increases diffusion distance so less evaporation
• Dicotyledonous plants leaf tissues


  - Key structures involved are mesophyll layers

  - (palisade and spongy mesophyll)

  - stomata created by guard cells
• 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


  - swell - open stomata

  - shrink - closed stomata

  - at night they shrink, reducing water loss by evaporation

  
• 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 to trap moisture to increase humidity -> lowers water potential inside plant so less water lost via osmosis

  - sunken stomata

  - curled leaves

  - hairs

  - thick cuticle reduces loss by evaporation

  - longer root network
• 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


  Mouth -> duodenum -> ileum
• Locations of protein digestion


  Stomach -> duodenum -> ileum
• Endopeptidases


  - Break peptide bonds between amino acids in the middle of the chain

  - creates more ends for exopeptidases for efficient hydrolysis

  
• Exopeptidases


  - Break peptide bonds between amino acids at the ends of polymer chain
• 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


  - Produced in pancreas

  - Breaks ester bonds in triglycerides to form :

  - monoglycerides

  - glycerol

  - fatty acids
• 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 triglycerides

  - Golgi apparatus combines triglycerides with proteins to form vesicles called chylomicron
• 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?

  Via co-transport in the ileum

  
• Haemoglobin (Hb)


  - Quaternary structure protein

  - 2 alpha chains

  - 2 beta chains

  - 4 associated haem groups in each chain containing Fe2+

  - transports oxygen
• 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


  - oxygen is loaded in regions with high partial pressures (alveoli)

  - unloaded in regions of low partial pressure (respiring tissue)

  
• 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