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Transport in Plants (Translocation (Movement of Sucrose (Sucrose moves by…
Transport in Plants
Translocation
Active Loading
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The H+ ions are co-transported back into the companion cells with sucrose molecules (secondary active transport)
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As sucrose concentrations increase, they then diffuse into the sieve tube through the plasmodesmata
The movement of assimilates (substances which have become part of the plant, mainly sugars) throughout the plant
Movement of Sucrose
Sucrose moves by mass flow, it is moved within sap which contains other assimilates such as amino acids
The flow of sap is determined by the difference in hydrostatic pressure between the two ends of the tube
At the source, sucrose enters the sieve tube and lowers the water potential, causing water to enter via osmosis, increasing hydrostatic pressure
At the sink, the sucrose is removed from the sieve tubes, raising the water potential so water moves out, lowering the hydrostatic pressure at the sink
Sap will move down the pressure gradient created by the differences in hydrostatic pressure. This is from source to sink
The Source
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In early spring, this could be the roots where energy stored as starch is covered into sucrose
The leaf is the most obvious source, converting sugars made by photosynthesis into sucrose
The Sink
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Sucrose is used for respiration and growth in meristems, or can be converted into starch for storage
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Transpirtation
Pathway
Water enters xylem, moves by osmosis into cells of spongy mesophyll or through the apoplast pathway
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Water vapour diffuses out the stomata, relying on a water vapour potential gradient
The loss of water vapour from the aerial parts of a pant, mostly through the stomata in the leaves
Importance
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Supply water for growth, photosynthesis and cell elongation
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Potometer
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4. Cut stem at an angle, providing a large surface area
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Transpiration Stream
The movement of water from the soil, through the plant, to the air surrounding the leaves
Water Uptake in the Root
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Water travels through the cells by all 3 pathways, however, the endodermis is covered by a Casparian strip, blocking the apoplast pathway into the xylem
Transporter proteins actively transport mineral ions from the cytoplasm of the endodermis into the xylem (and medulla), causing a decrease in water potential
Water can then be transported by osmosis into the xylem and cannot pass back due to the apoplast pathway being blocked by the Casparian stirp
Movement up the Stem
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Root Pressure
Pressure in the medulla builds up from the increase in water and mineral ions, forcing the water into and up the xylem for a few metres
Transpiration Pull
Cohesion of water molecules (due to hydrogen bonding) cause the water to be pulled up the xylem as a column of water
The pull from above (due to transpiration) causes tension in the column of water, which is why lignin is important as to prevent the collapse of the xylem under pressure
This is the cohesion-tension theory, allowing an unbroken column of water in the xylem. If broken, the column can still be maintained through another vessel via bordered pits
Capillary Action
Adhesion is the attraction of the water molecules to the sides of the xylem. Due to the xylem being narrow, the forces of attraction pull the water up the sides of the vessel
Leaving the Leaf
Water evaporates above the guard cells (in the sub-stomatal air space), lowering water potential
This causes water to enter the cavity by osmosis and in turn, draws water into the leaf by osmosis (or the apoplast pathway) from the xylem
Transport Tissues
Xylem Structure
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Living cells act as packing cells, separating and supporting vessels
Lignin impregnates the walls, killing the cells, making the vessels waterproof and strengthens them
Lignin thickening forms patters in cell wall, allowing some flexibility
In some places liginification is not complete, forming gaps/ bordered pits, allowing water to leave the vessel into the next or into living cells
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Phloem
Consists of sieve tube elements, packing cells, and companion cells, which transport mainly sucrose and amino acids
Sieve Tube Elements
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They have no nucleus, and very little cytoplasm
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Callose blocks sieve tube elements quickly, inhibiting spread of pathogens and loss of sap
Companion Cells
In between sieve tubes, has large nucleus, dense cytoplasm and many mitochondria
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Transport in Plants
Why Transport is Needed
As with animals, larger plants have smaller surface area to volume ratios therefore need specialised transport system and exchange surfaces
There is low metabolic rate, therefor oxygen demand is low and can be met by diffusion
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Vascular Tissue
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Assimilates, such as sugars, travel up or down the phloem tissue
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In the Young Root
In the centre is the vascular bundle the xylem forming an x-shape at the core, with phloem found in between the arms of the xylem. This arrangement provides strength to withstand pulling forces which the roots are exposed to
The vascular bundle is surrounded by endodermis cells of which inside it are meristem cells (a layer of diving cells) called the pericycle
In the Stem
Vascular bundles are found near the outer edges of the stem, providing strength and flexibility to withstand bending forces to the stem.
The xylem is towards the inside of each vascular bundle and the phloem to the outside. In between the xylem and phloem is a layer of cambium containing meristems
In the Leaf
Vascular bundles are branching networks of veins that get smaller as they spread away from the midrib. In each vein, the xylem is located on top of the phloem
Dissection
Allow leafy stem to take up water/stain by transpiration then cut transversely or longitudinally, examine with lens or microscope
Cut thin section, place on slide, add stain, place cover slip and observe with light microscope
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Movement of Water
Pathways Taken
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Plasmodesmata are gaps in the cell walls, joining two cells together
Apoplast Pathway
Mass flow of water through the spaces in between cells. Dissolve alts and mineral ions can be carried with the water
Symplast Pathway
Water moves by osmosis from cell to cell in the cytoplasm, through the plasmodesmata
Vacuolar Pathway
Similar to the symplast pathway, however, water also move in and out of the vacuoles
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