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TRANSPORT IN XYLEM, STEPS, ---> - Coggle Diagram
TRANSPORT IN XYLEM
GAS EXCHANGE
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This is also mentioned in topic 6.4 as the exchange of carbon dioxide and oxygen gases in the alveoli
TRANSPIRATION STREAM
Definition transpiration stream = The flow of water through the xylem from the roots to the leaf, against gravity
- Some of the light energy absorbed by leaves (during photosynthesis) is converted into heat, which evaporates water within the spongy mesophyll
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- This vapour diffuses out of the leaf via the stomata, creating a negative pressure gradient within the leaf → leaves = low pressure // roots = high pressure
- This negative pressure creates a tension force in leaf cell walls which draws water from the xylem (transpiration pull)
- The water is pulled from the xylem under tension due to the adhesive attraction between water and the leaf cell walls
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COHESION
Definition = Force of attraction between two particles of the same substance (e.g. between two water molecules)
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Structure of xylem
- Hollow tube allows free movement of water and ions
- Xylem does NOT have cytoplasm → Provides a larger lumen which makes water transport more efficient
- Cells are non living, so transport must be passive
- Cell walls are impregnated with lignin → Provides strength as water is transported under tension
- Walls are thickened with cellulose & lignin to provide further strength
- Cell wall has pores / pits which enables water to be transported between cells
ADHESION
Definition = Force of attraction between two particles of different substances (e.g. water molecule and xylem wall)
- Water molecules tend to stick to other molecules that are charged / polar
- Xylem wall is polar & can form intermolecular associations with water molecules
- Individual hydrogen bonds = weak BUT large number of bonds gives adhesive forces great strength
- Capillary action = Caused by the combination of adhesive forces causing water to bond to a surface (sides of xylem vessel) & the cohesive forces bonding water molecules together
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ROOTS
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- Roots are HYPERTONIC to soil & Soil is HYPOTONIC to roots
ADAPTATIONS
Xerophytes
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b. Metabolic adaptations
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Plants can open their stomata at night, reducing water vapour evaporation
c. Physical adaptations
Rolled leaves, small leaves, low growth, thick waxy cuticle, stomata sunken in pits, hairs on leaves, extensive root system
Halophytes
Definition = Plants that survive in salty conditions (such as marshlands) due to the presence of numerous adaptations
d. Cellular sequestration – Halophytes can sequester toxic ions and salts within the cell wall or vacuoles
a. Tissue partitioning – Plants may concentrate salts in particular leaves, which then drop off (abscission)
e. Root level exclusion – plant roots may be structured to exclude ~95% of the salt in soil solutions
b. Salt excretion – Certain parts of the plant, like the stem, may contain salt glands which actively eliminate salt
c. Altered flowering schedule – Halophytes may flower at specific times, like during rainy seasons, in order to minimise salt exposure
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