Plant exchange and transport

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1.2.3 Transport in plants
As plants become larger and more complex, transport systems become essential to supply nutrients to
and remove waste from individual cells
The supply of nutrients from the soil relies upon the flow of water through vascular system as does the
movement of the products of photosynthesis
Why do plants need a transport system?
Cells inside a multicellular plants would not receive enough water or nutrients to stay alive
Only the surface layer of epithelial cells would receive substances through diffusion
The main problem in plants is that
the roots can only take up water easily but not sugars
the leaves make sugars but cannot obtain water
Transport system in plants
Plants have special vascular tissues that move water
the xylem transports water and soluble material upwards
the phloem transports sugars up or down
both are highly specialised to carry out their function
Vascular tissues
Found throughout the plant
Xylem and phloem found in vascular bundles
Bundles often contain other types of tissue that give it strength and
support the plant
Distribution of Xylem and phloem
In young roots
Vascular bundles found in Centre of young root
Large central core ­ X- xylem
Phloem ­ between the arms of the X shaped xylem
Endodermis surround the vascular bundle and has a key role in
uptake of water
Meristems just inside the endodermis called pericycle
In stem
Vascular bundles found near outer edge
Non woody plants
Separate and discrete
Separate in young stems
Become continuous in older stems
Means there is a complete ring of vascular tissue just under the bark of the
Arrangement provides strength and flexibility
Xylem is found towards the inside of each vascular bundle
Phloem is found towards the outside
In between the xylem and phloem is a layer of cambium
Layer of meristem cells that divide to produce new xylem or phloem
Transport in plants

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In leaf
Vascular bundles form the midrib and veins
Has a branching network of veins that get smaller as they spread away from the
Within each vein the xylem can be seen on top of the phloem
A plant transport tissue that carries water from the roots to the rest of the plant. It consists of
hollow columns of dead cells lined end to end and reinforced with lignin. It provides important
support for the plant.…read more

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Companion cells
small cells with a large nucleus and dense cytoplasm in between the sieve tubes
have many mitochondria to produce the ATP needed for active processes
carry out the metabolic processes need by the sieve tube elements
includes using ATP as source of energy to load sucrose into the sieve tube elements
cytoplasm of companion cells and sieve tube element s are linked through many
gaps in the cell walls allowing communication and flow of minerals between cells
Plant cells and water
Water…read more

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Then passes through the plasodesmata from one cell to the next
Once inside the cytoplasm water can move through continuous cytoplasm from cell to
Vacuolar pathway (C)
Similar to the symplast pathway but water is not confined to the cytoplasm
It moves through the vacuoles as well
Water uptake from soil
Outermost layer of cells on the root are covered with root hair cells that Increase
surface area
These cells actively take up minerals from the soil using active transport
The minerals reduce the…read more

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Water movement up stem
3 processes that help water move up stem
Root pressure
Water is driven into the xylem by osmosis by the action of the endodermis moving
minerals into the xylem by active transport
This forces the water into and up the xylem
Root pressure can only account for water moving a few metres up the stem not to the
top of a tall tree
Transpirational pull
Water lost through the leaves must be replaced by the water travelling up the xylem
Water…read more

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Water keeps cells turgid
The flow of water can carry useful minerals
Evaporation of water can keep the plant cool
Measuring rate of transpiration
Important that there are no air bubbles
Water uptake is about 99% of water loss through transpiration
Water loss in the leave is replaced by the water in capillary tube
The movement of the meniscus at the end of the water column can be measured
Factors that affect rate of transpiration
Number of leaves
More leaves means bigger surface area…read more

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Most plants can reduces water loss by adaptations of structure and behaviour
A waxy cuticle on the leaf will reduce water loss due to evaporation through the
Stomata are often in the under suface of the leaf
Reduces evaporation due to direct heating from the sun
Stomata close at night
Plants loses leafs during winter when water is less available and temperatures are too
low for photosynthesis
Living in arid conditions
Xerophytes are plants that are adapted to reduce water loss so that it…read more

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Sugars are made during photosynthesis and converted to sucrose and loaded into
the phloem
This occurs during late spring summer and early autumn
Stores in other parts of the plants are sources during early spring when leaves are
Here the leaves act as sinks
Other sources include roots where stored of carbohydrates are released into the
Roots can act as sources at some points of the year and sinks during others
A sink removes sucrose from the phloem
How does sucrose enter the…read more

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If plant is supplied with radioactively labelled carbon dioxide it soon appears in the
Removing a ring of bark which contains the phloem results in sugars collecting above
An aphid feeding on a plant can be used to show that the mouth parts are taking food
from the phloem
How we know it needs ATP
Companion cells have many mitochondria
Translocation can be stopped by using an metabolic poison that inhibits the production
of ATP
Energy must be need as the rate of…read more


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