Mass Transport in Plants

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What is the definition of mass transport and some facts about it?
Movement of fluid over large distances in bulk, carrying dissolved substances, everything moves at the same rate in the same direction and it is caused by the pressure gradient (high -> low)
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What do xylem vessels do?
Transport water (and dissolved mineral ions) through the stem and leaves of the plant
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What process is the xylem heavily involved in?
Transpiration
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What is water needed for within the plant?
Fills vacuole to make cell more turgid and more supported, reactants in photosynthesis, hydrolysis reactions, transport medium e.g ions from roots to leaves
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What is the xylem made out of when fully formed?
Dead cells
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What happens to the horizontal end walls of the cells in the xylem?
They are broken down so the whole structure becomes a hollow continuous unbroken column
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What makes the walls of the xylem so strong?
Lignin
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What is another property of lignin which makes it suitable to be part of the xylem?
It is waterproof so keeps all of the water inside the xylem
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What process occurs when the stomata are open, which causes water to be 'pulled' out of the top of the xylem vessels?
Transpiration
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What is transpiration?
Water evaporates from leaf mesophyll cells + water vapour diffuses into air via stomata
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Where does the water come from which replaces the water lost in transpiration?
The xylem
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What is water molecules held together by that creates the unbroken column of water?
Hydrogen bonds
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What does transpiration do to the column of water?
'pull' stretches column of water and puts it under tension
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What would happen to the xylem if the lignin wasn't present during the 'pulling' times of the lignin?
The xylem would collapse
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What does the lignin cause to happen instead of the xylem collapsing?
The xylem lumen just gets narrower
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What between the walls of the xylem and the water molecule helps the column of water remain unbroken and rise upwards?
Adhesion
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Where does the water that replaces the water lost from the xylem come from?
The soil
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What are some external factors that make transpiration rate quicker, so the water rises up the xylem quicker?
Hotter temperature - water evaporates more as it has more K.E, Lower Humidity - steeper water potential gradient, Lots of wind - steeper water potential gradient as water carried away from leaves, Higher light intensity - stomata open more
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What does the phloem do?
Transport sucrose (+amino acids) from leaves or wherever photosynthesis occurs (source) to areas of growth/storage (sink)
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What process is the phloem heavily involved in?
Translocation
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Why do plants need sucrose?
Growing areas and once hydrolysed glucose can be used for respiration and cellulose
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Why does the plant transport it as sucrose and not glucose?
Sucrose is less reactive so less likely to be used up
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What is the structure of the phloem?
Sieve cells with sieve plates horizontally between them, with companion cells surrounding the phloem
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What is the structure of a sieve cell?
Cytoplasm stuck to the sides with a small amount of organelles in the cytoplasm, mainly empty.
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What is the function of the companion cells?
To produce enough ATP for the active transport processes in translocation as the sieve cell doesn't have enough room to make the ATP
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How is the sucrose made and how does it move into the phloem?
Made from condensation of glucose and fructose, and facilitated diffusion into companion cell, then active transport into the sieve tube cell
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How does sucrose effect the water potential?
Lowers it
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How does the lower water potential affect the hydrostatic pressure of the phloem?
Cause water from xylem to enter phloem via osmosis, so then the hydrostatic pressure of the phloem increases
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Why does mass flow occur to the sink?
It has a lower hydrostatic pressure so moves down pressure gradient
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How does the sucrose enter the sink once it gets there?
Facilitated diffusion into companion cell out of sieve tube cell, then Active transport into the sink
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What maintains the hydrostatic pressure gradient?
Once the sucrose has diffused out of the sieve tube cell at the sink, the water potential increase so the water leaves via osmosis back to the xylem where the water potential will be lower, meaning the HP remains the same
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What is the ringing experiment?
Cut the ring of bark and phloem from a tree and leave it a week, there should be a bulge of sucrose at the top of the ring, and a lower concentration of sucrose underneath the ring
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How can we use aphids to prove sucrose concentration?
Let aphid pierce phloem with mouth piece, remove aphid from mouthpiece, repeat further down tree, should see more fluid flowing from top mouthpiece which also has highest concentration of sucrose
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How can we use radioactively labelled substances, like CO2 with Carbon-14?
Place bottle with CO2 14 over leaf, track movement of the carbon-14 from the leaf to the sink
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What is some evidence that support mass flow?
Sucrose concentration rises in phloem shortly after rise in leaves, pressure in phloem (sap pours out and aphid mouthpiece not sucked in), Respiratory poisons stops translocation (no ATP for active transport)
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What is some evidence against mass flow?
Sieve plates create a barrier for mass flow, Dissolved ions/molecules don't move at same rate, sucrose reaches all areas of plant at same time (should reach areas with lower concentration quicker)
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Other cards in this set

Card 2

Front

What do xylem vessels do?

Back

Transport water (and dissolved mineral ions) through the stem and leaves of the plant

Card 3

Front

What process is the xylem heavily involved in?

Back

Preview of the front of card 3

Card 4

Front

What is water needed for within the plant?

Back

Preview of the front of card 4

Card 5

Front

What is the xylem made out of when fully formed?

Back

Preview of the front of card 5
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