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Summary Unit 3 - Organisms exchange substances with their environment
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3 - Organisms exchange substancesw/ their env.
Surface area to vol. ratio:
- Smaller organisms = higher SA:V
- Larger organisms need a specialised surface / organ for gaseous exchange e.g. lungs
o Because they have a smaller SA:V and a long diffusion pathway (and skin is waterproof / gas tight)
o As well as having a high demand for oxygen and to remove carbon dioxide
Gas exchange:
In insects:
- Tracheal system:
o Air to spiracles -> through tracheae -> at tracheoles to/from cells -> O2 diffuses down conc. gradient
to respiring cell + CO2 diffuses down conc. gradient from respiring cell
- Adaptations:
o Thin and flat body -> large SA:V and short diffusion pathway -> for rapid diffusion
o Lots of thin, branched tracheoles -> short diffusion pathway and SA:V -> rapid diffusion
In fish:
- Counter-current flow:
o Blood flows though lamellae and water flows over lamellae in opp. directions
o Always a higher conc. of O2 in water than blood
o Hence, a conc. gradient of O2 between water and blood is maintained along whole length of
lamellae -> equil. not met -> maximising diffusion of O2
- Adaptations:
o Gill is made of lots of filaments which are covered in many lamellae -> gill filaments provide large SA,
and lamellae incr. it even more
o Vast network of capillaries on lamellae -> remove O2 to maintain conc. gradient
o Thin epithelium -> shorter diffusion pathway betw. water and blood
In dicotyledonous plants:
- Process:
o CO2/O2 diffuse through stomata
o Stomata open by guard cells
o CO2/O2 diffuse into mesophyll layer into air spaces
o CO2/O2 diffuse down conc. gradient
- Adaptations:
o Lots of stomata that are close together -> large SA for gas exchange
o Mesophyll cells has large SA -> rapid diffusion of gases
o Thin -> short diffusion pathways
Adaptations of xerophytic plants:
- Thick waxy cuticle -> incr. diffusion distance -> less evaporation
- Pitted/grooved stomata + curled leaves -> WP gradient decr -> less evaporation
- Spindles -> reduces SA:V
- Hairs -> ‘trap’ water vapour -> WP gradient decr -> less evaporation
, Human gas exchange system:
- Trachea -> bronchi -> bronchioles-> alveoli
- Gas exchange in alveoli:
o O2 diffused from alveoli, down conc. gradient -> Across alveolar epithelium, -> then across capillary
endothelium, into the blood
o CO2 diffuses from capillary, down conc. gradient -> across capillary endothelium -> across alveolar
epithelium -> into alveoli
- Adaptations of alveoli:
o One cell thick -> short diffusion pathway -> fast diffusion
o Large SA:V -> fast diffusion
o Good blood supply from network of capillaries -> maintains conc. gradient
o Elastic tissue -> allows it to recoil after expansion
- Breathing in:
o External intercostal muscles contract, internal intercostal relax
o Ribcage moves up and out
o Diaphragm contracts -> flattens
o Vol. in thoracic cavity incr.
o Pressure in thoracic cavity decr.
o Atm. pressure higher than lung pressure
o Air moves down press. Gradient into lungs
- Breathing out:
o Internal intercostal muscles contract, external intercostal muscles relax
o Ribcage moves down and in
o Vol. of thoracic cavity decr.
o Pressure in thoracic cavity incr.
o Atm. Pressure lower than lung pressure
o Air moves down press, gradient out of lungs
- Fibrosis –
o Scar tissue in lungs → scar tissue is thicker and less elastic than normal
o Diffusion distance increased → rate of diffusion decreased
o Faster ventilation rate to get enough oxygen into lungs/blood
o Lungs can expand and recoil less → can’t hold as much air
Reduced tidal volume
Reduced forced vital capacity
- Asthma –
o Asthma = inflamed bronchi
o Asthma attack: smooth muscle lining bronchioles contracts
o Constriction of airways – narrow diameter → airflow in/out of lungs reduced
FEV reduced
o Less oxygen enters alveoli / blood
- Reduce rate of gas exchange in alveoli → less oxygen diffuse into blood → cells receive less oxygen → rate of
aerobic respiration reduced → less energy released → fatigue, weakness etc.
Digestion and absorption:
- Digestion of starch
o Amylase hydrolyses starch -> maltose
Amylase produced by salivary glands, released into mouth, and by pancreas, released into
small intestine
o Membrane-bound maltase hydrolyses maltose -> glucose = glycosidic bond hydrolysed
- Digestion of lipids:
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