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Summary Topic 3- organisms exchange substances with their environments £4.76
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Summary Topic 3- organisms exchange substances with their environments
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The internal environment of a cell or organism is different from its external environment. The exchange of substances between the internal and external environments takes place at exchange surfaces. To truly enter or leave an organism, most substances must cross cell plasma membranes. In large ...

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  • January 27, 2025
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3.3 Organisms exchange substances with their
environment


3.3.1 Surface area to volume ratio



The relationship between the size of an organism or structure and its
surface area to volume ratio.

● need for specialised exchange surfaces arises as size of organism
○ SA: V ratio increases
● in single-celled organisms
○ substances can easily enter cell
■ distance that needs to be crossed over short
● in multicellular organisms
○ distance larger
■ high SA:V ratio
● multicellular organisms required specialised exchange surfaces for efficient gas
exchange
○ of CO2 and oxygen



Changes to body shape and the development of systems in larger
organisms as adaptations that facilitate exchange as this ratio reduces.

● feature of efficient exchange surface
○ large SA
■ root hair cells
■ folded membranes
○ thin
■ distance that needs to be crossed short
○ good blood supply/ventilation
■ maintain steep gradient
■ alveoli

,3.3.2 Gas exchange



Adaptations of gas exchange surfaces, shown by gas exchange:



• across the body surface of a single-celled organism

● distance that needs to be crossed over short



• in the tracheal system of an insect (tracheae, tracheoles and spiracles)

● insects do not possess transport system
○ oxygen needs to be transported directly to tissues undergoing respiration
● achieved with help of spiracles
○ small openings of tubes either
■ bigger trachea
■ smaller tracheoles
■ run into body of insect and supply it with required gases
● gases move in and out through
○ diffusion
○ mass transport
■ result of muscle contraction
■ result of volume changes in tracheoles



• across the gills of fish (gill lamellae and filaments including the
counter-current principle)

● Fish have
○ small SA: V ratio for gas exchange
○ impermeable membrane
■ gases cant diffuse through skin
● fish need specialised gas exchange surface
● Bony fish
○ four pairs of gills
■ each gill supported by an arch
○ along each arch multiple projections

, ■ Gill filaments with lamellae
■ participate in gas exchange
○ Blood and water flow across lamellae in counter-current direction
■ flow in opposite directions to one another
■ ensures steep diffusion gradient maintained
■ so max amount of oxygen diffusing into deoxygenated
blood from water
■ projections held apart by water flow
■ in absence of water, they stick together
■ fish cannot survive very long out of water



• by the leaves of dicotyledonous plants (mesophyll and stomata).

● stomata
○ allow gases to enter and exit leaves
○ large number
■ no cell far from stomata
■ diffu\sion distance reduces
○ air spaces
■ allow gases to move around leaf
■ easily come into contact with photosynthesising mesophyll cells



Structural and functional compromises between the opposing needs for
efficient gas exchange and the limitation of water loss shown by
terrestrial insects and xerophytic plants.

● Insects do not possess a transport system
○ therefore oxygen needs to be transported directly to tissues undergoing
respiration
■ achieved with help of spiracles
■ small openings of tubes either bigger trachea or smaller
tracheoles
■ run into body of an insect and supply it with required gases
○ gases move in and out through
■ diffusion
■ mass transport
■ as a result of muscle contraction and volume changes in the
tracheoles

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