6.5- LIMITING WATER LOSS Flashcards
In terrestrial organisms i.e. insects + plants where can problems arise from?
opposing needs of efficient gas-exchange system + requirement to conserve water
What are the features that make a good gas-exchange system also? (hint water loss)
also same features that increase water loss
What must terrestrial organisms be able to do?
limit their water loss without compromising efficiency of gas-exchange system
Where are the gas exchange systems of terrestrial organisms?
inside the body
What is the air at the exchange surface of terrestrial organisms like?
more or less 100% saturated with water vapour
As the air at the exchange surfaces of terrestrial organisms is more or less 100% saturated with water vapour, what does this mean?
there’s less evaporation of water from exchange surface
Where do most insects live and what is this called?
live on land-terrestrial
What is the problem for all terrestrial organisms?
water easily evaporates from surface of their bodies + they can become dehydrated
What does efficient gas exchange require?
thin, permeable surface with large area
What does a thin, permeable surface with a large area need for efficient gas exchange conflict with?
need to conserve water
What adaptations have insects evolved to reduce water loss? (3)
small SA:V
waterproof coverings over their body surface
spiracles
How does a small SA:V reduce water loss in insects?
minimise area over which water lost
What is the waterproof covering in insects?
rigid outer skeleton of chitin that’s covered with waterproof cuticle
What are spiracles?
opening of tracheae at body surface
Why can spiracles be closed?
reduce water loss
What does the closing of spiracles conflict with?
need for oxygen
As closing of spiracles conflicts with need for oxygen, when does closing of oxygen largely occur?
occurs largely when insect at rest
What do the features of insects (small SA:V, waterproof covering, spiracles) mean insects cannot do?
cannot use their body surface to diffuse respiratory gases in way single-celled organisms does
What do insects have as respiratory gases cannot diffuse through their body surface?
tracheae
What does the tracheae do?
carry air containing oxygen directly to tissues
Why can’t plants have a small SA:V?
they photosynthesis + photosynthesis requires large leaf SA
Why does photosynthesis require a large leaf SA?
for capture of light + for exchange of gases
How do terrestrial plants limit water loss? (2)
waterproof covering over parts of leaves + ability to close stomata when necessary
What have certain plants with restricted supply of water evolved?
range of other adaptations to limit water loss through transportation
What are plants that have restricted supply of water that have evolved adaptations to limit water loss through transpiration called?
xerophytes
What are xerophytes?
plants that adapted to living in areas where water is in short supply
What is the main way of plants to survive in habitats where there’s a high rate of water loss + limited water supply?
reduce rate at which water can be lost through evaporation
Where does the vast majority of water lost through in plants?
through leaves
What modifications do the leaves of xerophytes have? (5)
thick cuticle rolling up of leaves hairy leaves stomata in pits or grooves reduced SA:V of leaves
Why is a thick cuticle a modification of leaves of xerophytes?
less water can escape
Where do most leaves have their stomata largely, or entirely confined to?
lower epidermis
What does the rolling of leaves do for xerophytes?
protect lower epidermis from outside helps to trap region of still air within rolled leaf
What does the region of still air in the rolled leaves becoming saturated in?
saturated with water vapour + so has very high water potential
How does the rolled leaves prevent water loss in xerophytes?
no water potential gradient between inside + outside of lead and so no water loss
Example of leaf with thick cuticle?
holly
Example of leaf that rolls its leaves?
marram grass
What does a thick layer of hair on leaves, especially on the lower epidermis do?
trap still, moist air next to leaf surface
How do hairy leaves prevent water loss?
water potential gradient between inside + outside of leaves reduced + so less water lost by evaporation
Example of plant that has hairy leaves?
one type of heather plant
How do stomata in pits or grooves reduce water loss?
trap still, moist water next to leaf + reduce water potential gradient
Example of plant with stomata in pits or groves?
pine trees
What happens to rate of diffusion the smaller the SA:V?
slower the rate of diffusion
What shaped leaves would reduce water loss?
small + roughly circular in cross-section
What is the reduction in surface area of leaves balanced against?
balanced against need for sufficient area of photosynthesis to meet requirement of plant
