Biology Topic 3: Organisms exchange substances with their environment

how is the size of an organism related to its SA:volume

• large organisms have a low sa:vol

• small organisms have a high sa:vol

how are large organisms adapted to facilitate gas exchange?

• specialised exchange surfaces (alveoli, gills)

• transport systems

• thin, folded membranes within exchange surfaces

how is metabolic rate related to the sa:vol

higher sa:vol > higher metabolic rate

• smaller organisms lose heat more rapidly, so heat is generated via metabolic processes

[and vice versa]

how are exchange surfaces adapted in unicellular organisms?

• exchange occurs through the cell membrane

• high sa:vol ratio to maximise rate of diffusion

• concentration gradients in/out of the cell to transport substances via diffusion or active transport

do insects have a transport system?

no - oxygen is transported directly into respiring muscle cells

how is O₂ exchanged in insects?

• O₂ enters the trachea via the spiracles

• O₂ dissolves into tracheal fluid, allowing O₂ to be transported faster

• when all O₂ in the fluid is used up, muscle cells go into anaerobic respiration. this produces lactic acid, reducing the water potential of muscle cells

• water from tracheal fluid enters the muscle cells via osmosis, so no more fluid is left in the trachea

• this allows O₂ to diffuse directly into the muscle cells without dissolving. this shortens the diffusion path, increasing diffusion rate

how is the tracheal system adapted for gas exchange?

• spiracles can open or close to avoid water loss/regulate air flow

• tracheal fluid allows gases to dissolve, facilitating diffusion

• abdominal pumping increases pressure within the tracheal system, forcing air into the trachea from the spiracles, providing a rapid supply of O₂ and fast removal of CO₂

how does ventillation occur in fish?

ventillation achieves a unidirectional flow of blood

• fish pushes its tongue down, opening the buccal cavity floor. this allows water to enter the fish

• fish closes the mouth, raising buccal cavity floor and increasing internal pressure causing the operculum to open

• pressure gradient between mouth and operculum cavities causes water to move over the gill filaments

• O₂ from the water is absorbed into the blood via lamellae in the gill filaments

what is the counter-current principle?

• the lamellae capillary system ensures deoxygenated blood flows in the opposite direction to the flow of water

• this maintains a steep O₂ concentration gradient across the entire length of the capillary, so maximal O₂ is diffusing into the blood

how are dicotyledonous leaves adapted for gas exchange?

• stomata open and close, letting gases in/out of the leaf. stomata are close to the cells, reducing diffusion path

• spongy mesophyll has air spaces and a large surface area. mesophyll cells absorb CO₂ for photosynthesis, and release O₂ as a product.

how are xerophytes adapted to minimise water loss?

• sunken stomata - minimise water loss

• curled leaves + stomata hairs - trap moist air around the plant, minimising water loss via transpiration or osmosis

• thick waxy cuticle - prevents water from evaporating out of the cell by increasing diffusion distance

• thin leaves/spindles - reduce surface area, preventing photosynthesis and thus transpiration

how are insects adapted to prevent water loss?

• waterproof exoskeleton

• spiracle hairs - trap moist air around the spiracles, preventing water loss via osmosis

which structures form the human gas exchange system?

• alveoli

• bronchioles

• bronchi

• trachea

• lungs