Exchange and Transport

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Exchange and Transport

• 13.10 Limiting water loss in plants

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Learning outcomes

• Students should be able to understand the
  following
• How terrestrial plants are adapted to efficiently
  exchange gases and conserve water
• How Xerophytic plants are adapted to live where
  water loss may exceed their water uptake
• How Xerophytes are adapted to limit water loss
  through transpiration

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Learning outcomes

• Candidates should be able to
• Explain 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

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Balancing gas exchange with water loss Insects
and plants compared

• Both have waxy / waterproof coverings over their
  gas exchange surfaces to limit water loss by
  evaporation
• Both can close the openings to their gas
  exchange systems (spiracles / stomata) to
  regulate water loss
• In plants stomata are predominantly on the
  underside of leaves to limit water loss by
  evaporation
• Insects have evolved small SA Volume ratios to
  limit water loss from the body surface
• But the leaves of plants require a large surface
  area in order to absorb light and exchange gases
  for photosynthesis

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Adaptations of plants

• Plants in different habitats are adapted to cope
  with different problems of water availability
• Mesophytes - plants adapted to a habitat with
  adequate water
• Xerophytes - plants adapted to a dry habitat
• Halophytes - plants adapted to a salty habitat
• Hydrophytes - plants adapted to a freshwater
  habitat

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Some adaptations of xerophytes are
Adaptation How it works Example
small leaf surface area less area for evaporation conifer needles, cactus spines
low stomata density fewer gaps in leaves
stomata on lower surface of leaf only more humid air on lower surface, so less evaporation most dicots
sunken stomata maintains humid air around stomata marram grass, pine
stomatal hairs maintains humid air around stomata marram grass, couch grass, heather
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Some adaptations of xerophytes are
Adaptation How it works Example
thick cuticle stops uncontrolled evaporation through leaf cells Most dicots Evergreens e.g. Holly
shedding leaves in dry/cold season reduce water loss at certain times of year deciduous plants
folded leaves maintains humid air around stomata marram grass,
succulent leaves and stem stores water cacti
extensive roots maximise water uptake cacti
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All Cacti are xerophytes
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Left and right Epidermis of the cactus Rhipsalis
dissimilis. Left View of the epidermis surface.
The crater-shaped depressions with a guard cell
each at their base can be seen. Right X-section
through the epidermis underlying tissues. The
guard cells are countersunk, the cuticle is
thickened. These are classic xerophyte
adaptations.
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Transverse Section Through Leaf of Xerophytic
Plant
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Marram grass
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Marram grass
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Marram grass
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Written tasks

• AQA AS Biology textbook pg 201
• Summary questions 1-4
• 2. Read Not only desert plants have problems
  obtaining water on page 201 then answer
  application questions 1-4
• 3. Complete the exam style question about
  Xerophytic plants

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Mark scheme - exam style question
Answers Xerophytes question   1.(a) E.
superba Largest proportion of shallow roots 2
(b) Roots go to greatest depths Able to get
water when surface soil dried out or Greatest
root mass Able to store more water 2
(c) Curled leaves Thick cuticle Sunken
stomata Hairs max 2 6  
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Homework
Referring to the AQA AS Biology textbook and
other (internet) sources Use pictures and brief
explanations to show how a variety of Xerophytic
plants are adapted conserve water in dry habitats
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Learning outcomes

• Students should be able to understand the
  following
• How terrestrial plants are adapted to efficiently
  exchange gases and conserve water
• How Xerophytic plants are adapted to live where
  water loss may exceed their water uptake
• How Xerophytes are adapted to limit water loss
  through transpiration