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i Understand how surface area to volume ratio affects transport of molecules in living
organisms.
Surface area and volume are both very important factors in the exchange of materials in
organisms. The volume is important to study as it tells you how much of a substance needs
exchanging. On the other hand, the surface area tells you how much of a substance can be
exchanged.
The lower the surface area to volume ratio the further distance molecules needs to travel to
reach all parts of the organisms
ii Understand why organisms need a mass transport system and specialised gas exchange
surfaces as they increase in size.
Small organisms (unicellular) have a large surface area to volume ratio so diffusion distance
is short and metabolic demand is lower.Large organisms have a low surface area to volume
ratio therefore large organisms have evolved a specialised gas exchange system such as
trachea,gills and lungs etc.
Topic 4.3- Gas exchange
i Understand how insects, fish and mammals are adapted for gas exchange.
Insects
● Insects have a tracheal system which allows for efficient gas exchange
● The trachea contains spiracles (which can open and close to prevent water loss) in
which air moves in
● Oxygen moves into the cell, down a concentration gradient (concentration of oxygen
outside the cell is higher than inside the cell).
● Gas exchange does not occur at the trachea as it is lined with rings of chitin making it
impermeable
● So oxygen diffuses in the tracheoles which are permeable Therefore gas exchange
takes place there
● Tracheoles have thin walls which means short pathway they are also highly branched
which increases surface area area
● Tracheoles contain fluids which allow oxygen to dissolve
,Fish
● Fish contain gills which allow gas exchange to occur
● The gills contain gill filaments and lamellae which increases surface area
● Fish also contain a countercurrent exchange system which means water and blood
flow in opposite directions. This means water will be next to blood with lower oxygen
and there maintains a steep gradient
Mammals
● Gas exchange takes place in the lungs which contain millions alveoli
● The alveoli are very efficient for gas exchange as they provide a large surface area
● They are made from short flattened epithelial cells which provide a short pathway
● Finally the alveoli are surrounded by capillaries containing deoxygenated blood so
there is a steep concentration
ii Understand gas exchange in flowering plants, including the role of stomata, gas exchange
surfaces in the leaf and lenticels.
Plants
● Leaves have a large surface area to volume ratio as they are broad and flat
● Inside the leaf they are spongy mesophylls which also increase the surface
area (they have thin walls so gas exchange can occur quickly
● Gas exchange occurs through diffusion in the stomata which opens and
closes.
● Guard cells open and close to allow the evaporation of water
● Waxy cuticles is impermeable to gas which prevents excessive water loss
Lenticels are spongy areas with loosely packed cells that are sites of gas exchange in woody
stems and roots e.g trees and barks .
, Topic 4.2 -Cell transport mechanisms
i Know the structure of the cell surface membrane with reference to the fluid mosaic model.
The way substances move in and out of the cell depends on the structure of the cell
membrane.
All cells are surrounded by a partially permeable membrane composed of phospholipids
with protein molecules in between them. The membrane proteins can act as transport,
receptor, structural or recognition protein.
Phospholipids have a hydrophilic head and a hydrophobic tail
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