Unit 9: Plant Biology
9.1 Transport in the xylem of plants
∑ Transpiration is the inevitable consequence of gas exchange in the leaf
The waxy cuticle has low permeability to air. Hence, stomata is needed for gas
exchange, though they also allow water vapour to escape (transpiration)
Water loss is minimised by control of guard cells that control stoma aperture
Transpiration is affected by:
o Humidity, wind/air movement, temperature, light (more stomata open)
*A group of plants called liverworts do not have stomata
ß Applications- Models of water transport in xylem using simple apparatus including
blotting or filter paper, porous pots and capillary tubing
As water is lost from the porous pot through the pores (modelling leaves
and stomata), water is drawn up by capillary action (caused by adhesive
and cohesive properties of water)
ß Skills- Measurement of transpiration rates using potometers
ß Skills- Design of an experiment to test hypotheses about the effect of temperature
or humidity on transpiration rates
A potometer measures transpiration by noting the
distance a bubble travels along a capillary tube
over time, as water is being taken up by a plant’s
roots.
∑ The cohesive property of water and the structure of the xylem vessels allow
transport under tension
∑ The adhesive property of water and evaporation generate tension forces in leaf cell
walls
Xylem vessels are long continuous tubes with thickened walls impregnated with
lignin, which strengthens them and prevents wall collapse at low pressures.
Mature xylem vessels are non-living, without plasma membrane or cytoplasm
Capillary Action
o Water’s polar property leads to cohesion between water molecules, and
adhesion to hydrophilic parts of xylem cell walls.
o This allows water to be pulled up in xylem in a continuous stream.
Transpiration-pull (passive process):
o As water evaporates from leaf, adhesion causes water to be drawn through
cell wall, from xylem vessels in the leaf
o This reduces pressure in xylem, generating a pulling force that is transmitted
through the xylem all the way to the roots.
o Cavitation where column of liquid breaks is uncommon due to water’s
cohesive properties.
ß NOS- Use models as representations of the real world: mechanisms involved in
water transport in the xylem can be investigated using apparatus and materials that
show similarities in structure to plant tissues
, Unit 9: Plant Biology
∑ Active uptake of mineral ions in the roots causes absorption of water by osmosis
Active transport is used to pump in mineral ions from soil into root cells, increasing
concentration gradient and difference in water potential to allow water absorption by
osmosis
Mutualistic relationship between fungi and plants
o Plants
Some ions move slowly through the soil very slowly because the ions
bind to soil particle surface, preventing efficient entry into plant.
Some plants have fungi growing at root surface and even into cells of
root. Thread-like hyphae of fungus grows into soil and absorbs mineral
ions from soil particle surface and supplies them to roots.
This is important for plant growth in mineral-deficient soils
o Fungi
Plants supply sugars and other nutrients to fungus
Root hair cell adaptations:
o Long narrow tubular extension
Increase SA:V ratio for absorption
o High sugar and mineral ion concentration in cell sap
High water potential gradient facilitates osmosis of water into cell
o Large vacuoles to take in water
o Lots of mitochondria
To produce ATP from aerobic respiration for use in active transport
o Actively take in mineral salts
Maintain water potential difference.
∑ Plants transport water from the roots to the leaves to replace losses from
transpiration
Plants transport water from the roots to the leaves to replace losses from
transpiration
o Transmembrane pathway: via water channels
o Apoplastic pathway: via porous cell wall
o Symplastic pathway: via plasmodesmata through cytoplasm
Casparian strip is a band of cell wall material at the endodermis, and is chemically
different from normal cell wall (Made of suberin rather than lignin).
The Casparian strip disrupts the apoplastic pathway, and water in that pathway
enters the cytoplasm and follows the symplastic pathway to make it through the strip
, Unit 9: Plant Biology
ß Applications- Adaptations of plants in deserts and saline soils for water
conservation
Xerophytes adapted for growth in dry habitats
o Ephemeral: short life cycles completed in brief periods when water is
available, while remaining dormant inside seeds until the next rain
o Perennial: rely on water storage in specialized leaves, stems or roots
Halophytes adapted to saline soils
Adaptations to dry/ saline environments:
o Leaves reduced to small scaly structures or spines
o Leaves shed when water is scarce and stem becomes green, functioning as
main photosynthetic organ
o Water storage structures
o Thick cuticle and multiple layered epidermis to reduce water loss
o Sunken stomata
o Long roots to increase water uptake
o Structures removing salt build-up
o Altered photosynthetic pathway
CAM plants: temporal separation of carbon dioxide absorption and
use. Stomata only open at night when it is cooler, hence less
transpiration.
C4 plants: spatial separation. Can store carbon dioxide as 4C
compound to reduce need for photorespiration when carbon dioxide
levels are low (eg. Maize and sugarcane)
Cactus, a xerophyte
o Reduced leaves present as spines reduce transpiration
o Stem contain water storage tissue
o Vertical stem don’t absorb light at mid-day when light is most intense
o Pleats on stem allow rapid volume expansion and contraction of stem
o Thick waxy cuticle on stem, with some stomata that open at night
o Wide root network
o CAM plant (Crassulacean acid metabolism), as carbon dioxide absorbed at
night is stored as 4C malic acid and converted back to carbon dioxide during
the day to allow for photosynthesis while stomata is closed.
Marram Grass (Ammophila arenaria), a xerophyte
o Rolled leaf creates a local humid environment with
water vapour trapped to reduce transpiration rate
by reducing evaporation rate of water from stomata
o Hairs on the inside of the folded leaves help to slow
or stop air movement around stomata and trap
moisture to reduce transpiration rate
o Hinge cells cause leaves to roll up.
*Hydrophyte: plant adapted to freshwater habitat
*Mesophyte: plant in habitat with adequate water
9.1 Transport in the xylem of plants
∑ Transpiration is the inevitable consequence of gas exchange in the leaf
The waxy cuticle has low permeability to air. Hence, stomata is needed for gas
exchange, though they also allow water vapour to escape (transpiration)
Water loss is minimised by control of guard cells that control stoma aperture
Transpiration is affected by:
o Humidity, wind/air movement, temperature, light (more stomata open)
*A group of plants called liverworts do not have stomata
ß Applications- Models of water transport in xylem using simple apparatus including
blotting or filter paper, porous pots and capillary tubing
As water is lost from the porous pot through the pores (modelling leaves
and stomata), water is drawn up by capillary action (caused by adhesive
and cohesive properties of water)
ß Skills- Measurement of transpiration rates using potometers
ß Skills- Design of an experiment to test hypotheses about the effect of temperature
or humidity on transpiration rates
A potometer measures transpiration by noting the
distance a bubble travels along a capillary tube
over time, as water is being taken up by a plant’s
roots.
∑ The cohesive property of water and the structure of the xylem vessels allow
transport under tension
∑ The adhesive property of water and evaporation generate tension forces in leaf cell
walls
Xylem vessels are long continuous tubes with thickened walls impregnated with
lignin, which strengthens them and prevents wall collapse at low pressures.
Mature xylem vessels are non-living, without plasma membrane or cytoplasm
Capillary Action
o Water’s polar property leads to cohesion between water molecules, and
adhesion to hydrophilic parts of xylem cell walls.
o This allows water to be pulled up in xylem in a continuous stream.
Transpiration-pull (passive process):
o As water evaporates from leaf, adhesion causes water to be drawn through
cell wall, from xylem vessels in the leaf
o This reduces pressure in xylem, generating a pulling force that is transmitted
through the xylem all the way to the roots.
o Cavitation where column of liquid breaks is uncommon due to water’s
cohesive properties.
ß NOS- Use models as representations of the real world: mechanisms involved in
water transport in the xylem can be investigated using apparatus and materials that
show similarities in structure to plant tissues
, Unit 9: Plant Biology
∑ Active uptake of mineral ions in the roots causes absorption of water by osmosis
Active transport is used to pump in mineral ions from soil into root cells, increasing
concentration gradient and difference in water potential to allow water absorption by
osmosis
Mutualistic relationship between fungi and plants
o Plants
Some ions move slowly through the soil very slowly because the ions
bind to soil particle surface, preventing efficient entry into plant.
Some plants have fungi growing at root surface and even into cells of
root. Thread-like hyphae of fungus grows into soil and absorbs mineral
ions from soil particle surface and supplies them to roots.
This is important for plant growth in mineral-deficient soils
o Fungi
Plants supply sugars and other nutrients to fungus
Root hair cell adaptations:
o Long narrow tubular extension
Increase SA:V ratio for absorption
o High sugar and mineral ion concentration in cell sap
High water potential gradient facilitates osmosis of water into cell
o Large vacuoles to take in water
o Lots of mitochondria
To produce ATP from aerobic respiration for use in active transport
o Actively take in mineral salts
Maintain water potential difference.
∑ Plants transport water from the roots to the leaves to replace losses from
transpiration
Plants transport water from the roots to the leaves to replace losses from
transpiration
o Transmembrane pathway: via water channels
o Apoplastic pathway: via porous cell wall
o Symplastic pathway: via plasmodesmata through cytoplasm
Casparian strip is a band of cell wall material at the endodermis, and is chemically
different from normal cell wall (Made of suberin rather than lignin).
The Casparian strip disrupts the apoplastic pathway, and water in that pathway
enters the cytoplasm and follows the symplastic pathway to make it through the strip
, Unit 9: Plant Biology
ß Applications- Adaptations of plants in deserts and saline soils for water
conservation
Xerophytes adapted for growth in dry habitats
o Ephemeral: short life cycles completed in brief periods when water is
available, while remaining dormant inside seeds until the next rain
o Perennial: rely on water storage in specialized leaves, stems or roots
Halophytes adapted to saline soils
Adaptations to dry/ saline environments:
o Leaves reduced to small scaly structures or spines
o Leaves shed when water is scarce and stem becomes green, functioning as
main photosynthetic organ
o Water storage structures
o Thick cuticle and multiple layered epidermis to reduce water loss
o Sunken stomata
o Long roots to increase water uptake
o Structures removing salt build-up
o Altered photosynthetic pathway
CAM plants: temporal separation of carbon dioxide absorption and
use. Stomata only open at night when it is cooler, hence less
transpiration.
C4 plants: spatial separation. Can store carbon dioxide as 4C
compound to reduce need for photorespiration when carbon dioxide
levels are low (eg. Maize and sugarcane)
Cactus, a xerophyte
o Reduced leaves present as spines reduce transpiration
o Stem contain water storage tissue
o Vertical stem don’t absorb light at mid-day when light is most intense
o Pleats on stem allow rapid volume expansion and contraction of stem
o Thick waxy cuticle on stem, with some stomata that open at night
o Wide root network
o CAM plant (Crassulacean acid metabolism), as carbon dioxide absorbed at
night is stored as 4C malic acid and converted back to carbon dioxide during
the day to allow for photosynthesis while stomata is closed.
Marram Grass (Ammophila arenaria), a xerophyte
o Rolled leaf creates a local humid environment with
water vapour trapped to reduce transpiration rate
by reducing evaporation rate of water from stomata
o Hairs on the inside of the folded leaves help to slow
or stop air movement around stomata and trap
moisture to reduce transpiration rate
o Hinge cells cause leaves to roll up.
*Hydrophyte: plant adapted to freshwater habitat
*Mesophyte: plant in habitat with adequate water
