AQA A_level Biology Exchange A* Notes

Surface Area
To Volume
Ratio

, Exchge Of Substces
Subances

RESPIRATORY GASES NUTRIENTS EXCRETORY PRODUCTS HEAT
E.G: E.G: E.G: • lose if have too
• oxygen • Glucose • Urea much
• Carbon Di • AAs • Carbon Di • Gain if have too less


2 Ways of Exchange

PASSIVELY ACTIVELY:
E.G: E.G:
• Diffusion • Active Transport
• Facilitated Diffusion • Co Transport


Why is SA:V ratio Impoant?
• Organisms requirements ( resources from environment) depend on its metabolic rate and
its total volume
• Amount of heat energy released (waste energy from resp + other met activity) depends on
metabolic rate of volume

• An organisms ability to obtain resources and get rid of waste products and heat
depends on its surface area in contact with the surroundings (+ diff dist )


* SA:V ratio depends on… SIZE and SHAPE

Calculations
SA (mm2) = length x width Volume (mm3) = length. X width x depth


• As the surface area and volume of an organism
increase (and therefore the overall 'size' of the
organism increases), the surface area : volume
ratio decreases
• This is because volume increases much more
rapidly than surface area as size increases

, Gas Exchge
Fick’s Law


Rate of diffusion & Surface area x Concentration gradient
Thickness of exchange surface

• All gas exchanges diffusion so is governed by Ficks law of diffusion

• Gas exchange system is try to…
• increase the surface area and concentration gradient
• decrease the diffusion pathway

Propeies of Exchange Surfaces

1. Large surface area E
2. Thin Short diffusion pathway &
3. Selectively permeable (Small uncharged mols simply diff through in between phospholipid mols)
4. Moist (Exch surf consists of living cells with permeable cell mems water will diff out )
Gases reaching surf dissolve in moisture




Problems with exchange surfaces

• due to large SA, thinness , permeability required and OTHER subs and energy easily cross

• Water often lost by terrestrial organisms sometimes resulting in damage

• heat energy easily lost by org with constant temp

• thin - easily suffer physical damage —> Exchange surf internal ,protected by body

E.G: MAMMALIAN GAS EX SURF = ALVEOLI
• enclosed space in body —> reducing loss of water and heat
—>reducing risk of physical damage

, Gas Exchange: Unicellular organisms

Very high SA:V ratio —> Very small
Sho Diffusion Pathway — > Thin surface mebrane
Diffusion —> small mols , non-polar , conc grad . Maintained thru respiration.
(Oxygen + carbon dioxide)
Only survive in water —> would lose too much water without - due to high SA:V ratio
(Except if dormant state with a special coating)

Easily able to cross cell wall:
• cellulose freely permeable to most subs
• Large gaps in cell wall

CELLULOSE IS LIKE A WOOLY JUMPER
• keeps you warm but not dry
• Allows water in


Insects
Position

• exoskeleton provides:
1. Support P
2. Protection from water loss
3. Prevents entry of harmful things e.g microbes

• exoskeleton covered by waterproof and gas proof cuticle made of chitin

• gas exchange surface is internal
• Reduces water loss

Syem of tubes
• many highly branched tracheae divide into LARGE SA 9
many smaller branching tracheoles
• Opening to tubes = spiracles
• Spiracles closed often - reduce water loss
• Opened + closed by valves

* SPIRACLES LEAD INTO TRACHEAE WHICH BRANCH
INTO NARROWER TRACHEOLES, WHICH EXTENT
INTO ALL TISSUES