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9.2 Transport in Angiospermophytes | BioNinja
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9.2 Transport in Angiospermophytes
9.2.1 Outline how the root system provides a large surface area for mineral ion and water uptake by means of
branching and root hairs
Plants take up water and essential minerals via their roots and thus need a maximal surface area in order
to optimise this uptake
The monocotyledon root has a fibrous, highly branching structure which increases surface area for maximal
absorption
The dicotyledon root has a main tap root which can penetrate deeply into the soil to access deeper
reservoirs of water and minerals, as well as lateral branches to maximise surface area
The root epidermis may have extensions called root hairs which further increase surface area for mineral
and water absorption
These root hairs have carrier proteins and ion pumps in their plasma membrance, and many mitochondria
within the cytoplasm, to aid active transport
Transport in the Root System
9.2.2 List the ways in which mineral ions in the soil move into the root
Minerals move into the root system via the following pathways:
Diffusion: Movement of minerals along a concentration gradient
Mass Flow: Uptake of mineral ions by means of a hydrostatic pressure gradient
Water being taken into roots via osmosis creates a negative hydrostatic pressure in the soil
Minerals form hydrogen bonds with water molecules and are dragged to the root, concentrating them for
absorption
Fungal Hyphae: Absorb minerals from the soil and exchange with sugars from the plant (mutualism)
9.2.3 Explain the process of mineral ion absorption from the soil into roots by active transport
Minerals that need to be taken up from the soil include K+, Na+, Ca2+, NH4+, PO43­ and NO3­
Fertile soil invariably contains negatively charged clay particles to which positively charged minerals may
attach
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Root cells contain proton pumps that actively pump H+ ions into the surrounding soil, which displaces the
positively charged minerals allowing for their absorption (the negatively charged minerals may bind to the
H+ ions and be reabsorbed with the proton)
Mineral Ion Absorption
This mode of absorption is called indirect active transport ­ it uses energy (and proton pumps) to establish
an electrochemical gradient by which mineral ions may be absorbed via diffusion
Alternatively, the root cells may absorb mineral ions via direct active transport ­ using protein pumps to
actively translocate ions against their concentration gradient
9.2.4 State that terrestrial plants support themselves by means of thickened cellulose, cell turgor and lignified
xylem
Three ways by which terrestrial plants may support themselves are:
Thickened cellulose: Thickening of the cell wall provides extra structural support
Cell turgor: Increased hydrostatic pressure within the cell exerts pressure on the cell wall, making cells
turgid
Lignified xylem: Xylem vessels run the length of the stem and branches, lignification of these vessels
provides extra support
9.2.5 Define transpiration
Transpiration is the loss of water vapour from the leaves and stems of plants
9.2.6 Explain how water is carried by the transpiration stream, including the structure of xylem vessels,
transpiration pull, cohesion, adhesion and evaporation
Some of the light energy absorbed by leaves changes into heat, converting water in the spongy mesophyll
into vapour
This vapour diffuses out of the stomata and is evaporated, creating a negative pressure gradient in the leaf New water is drawn from the xylem (mass flow), which is replaced by water from the roots (enters from soil
via osmosis)
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The flow of water through the
xylem from the roots to the leaf is
called the transpiration stream
Water rises through xylem vessels
because of two qualities:
Cohesion: Water molecules
are weakly attracted to each
other via hydrogen bonds
Adhesion: Water molecules
form hydrogen bonds with the
xylem cell wall
These properties create a suction
effect (or transpiration pull) in the
xylem
The xylem has a specialised
structure to facilitate transpiration:
The inner lining is composed of
dead cells that have fused to
create a continuous tube
These cells lack a cell
membrane, allowing water to
enter the xylem freely
The outer layer is perforated (contains pores), allowing water to move out of the xylem into the leaves
The outer cell wall contains annular lignin rings which strengthens the xylem against the tension created
by the transpiration stream
9.2.7 State that guard cells can regulate transpiration by opening and closing stomata
The transpiration pull is generated by the negative hydrostatic pressure created by the evaporation of water
vapor from the leaf
Guard cells line stomata and regulate transpiration by controlling how much water vapor can exit the leaf
When stomata are open the rate of transpiration will be higher than when they are closed
9.2.8 State that the plant hormone abscisic acid causes the closing of the stomata
When a plant begins to wilt from water stress, dehydrated mesophyll cells release the plant hormone
abscisic acid (ABA)
Abscisic acid triggers the efflux of potassium from guard cells, decreasing the water pressure within these
cells and making them flaccid
This causes the stomatal pore to close
Opening and Closing of Stomata by Abscisic Acid
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9.2.9 Explain how the abiotic factors light, temperature, wind and humidity, affect the rate of transpiration in a
typical terrestrial plant
Light Increasing the intensity of light increases the rate of transpiration
Light stimulates the opening of stomata (gas exchange required for photosynthesis to occur) Some of the light energy absorbed by leaves is converted into heat, which increases the rate of water
evaporation
Temperature Increasing the temperature increases the rate of transpiration
Higher temperatures cause an increase in water vaporisation in the spongy mesophyll and an increase in
evaporation from the surface of the leaf
This leads to an increase in the diffusion of water vapour out of the leaf (via the stomata) which increases
the rate of transpiration
Wind
Greater air flow around the surface of the leaf increases the rate of transpiration
Wind removes water vapour (lower concentration of vapour on leaf surface), increasing the rate of diffusion
from within the spongy mesophyll
Humidity
Increasing the humidity decreases the rate of transpiration
Humidity is water vapour in the air, thus a high humidity means there is a high concentration of water
vapour in the air
This reduces the rate of diffusion of water vapour from inside the leaf (concentration gradient is smaller
resulting in less net flow)
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9.2.10 Outline four adaptations of xerophytes that help to reduce transpiration
Xerophytes are plants that can tolerate dry conditions (such as deserts and high altitudes) due to a number of
specialised adaptations:
Reduced leaves: Reducing the surface area of the leaf will reduce the area for water loss and thus reduce
transpiration
Rolled leaves: Rolling up leaves (lower epidermis inside) reduces exposure of stomata to air and thus
reduces transpiration
Thick waxy cuticle: A thickened cuticle prevents water loss from the surface of the leaf and thus reduces
transpiration
Stomata in pits: Having stomata in pits, surrounded by hairs, concentrates water vapour near the
stomata, reducing the rate of transpiration
Low growth: Plants located near the ground are less exposed to wind and may be shaded, reducing the
rate of transpiration
C4 / CAM physiology: Plants with C4 or CAM physiology require less amounts of CO2, meaning stomata
can stay closed for longer 9.2.11 Outline the role of the phloem in active translocation of sugars (sucrose) and amino acids from source
(photosynthetic tissue and storage organs) to sink (fruits, seeds, roots)
Organic molecules (sugars, amino acids) move from their source (photosynthetic tissue or storage organs)
into a tube system called the phloem
Sugars are transported as sucrose (because it is soluble but metabolically inert) in the fluid of the phloem
(called the sap)
They are actively loaded into the phloem by companion cells, creating a high concentration which draws
water from the xylem via osmosis
The sap volume and pressure consequently increase to create mass flow which drives the sap along the
phloem
The organic molecules are actively unloaded by companion cells and stored in the sink (fruits, seeds, roots)
Sucrose is stored as starch (insoluble), while the water in the phloem is released (now that solute
concentration is low) and returned to the xylem
Active Translocation in the Phloem
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