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Flashcards in Plant Transport Deck (74):

What is the function of sieve tube cells?

To transport organic solutes e.g sucrose and amino acids


Sieve tube cell structure:-

Formed from cells called sieve elements placed end to end. End wall perforated by pores (area called sieve plates). Cytoplasm forms thin layer around periphery + cytoplasmic filaments extend between each sieve cell through the pores.
Where sieve elememts meet = sieve plates


Companion cell function:-

Provide the energy needed to move sucrose into the sieve tube cells.


Companion cell:-

Closely associated with each sieve tube, has dense cytoplasm w/ a large central nuclei, many mitochondria (to produce ATP for active transport of sucrose into phloem) connected to sieve tube by plasmodesmata.


What is translocation?

The transport of soluble organic molecules in plants. Can take place to wherever photosynthesis products are needed.


Phloem structure:-

Living tissue, consists of several cell types. 2 main are sieve tube cells and companion cells. Structural support provided by fibres and parenchyma cells.


Why have plants developed a mass transport system made from columns of plant cells?

They are long and multicellular so diffusion would be too slow to distribute materials.


What is transpiration?

Loss of water from leaves


What are the 2 distinct types of vascular tissue in plants' mass flow system?

Xylem and Phloem


What do Xylem transport and where?

Water + mineral ions from roots to leaves


What do phloem transport and where?

Sugars and amino acids (made by photosynthesis) from the leaves to the rest of the plant.


How is vascular tissue distributed in the root and why?

Located within central stele. Resists vertical stress (pull) and anchors the plant in the soil.


How is vascular tissue distributed in the stem and why?

Located within separate bundles towards the periphery. Arrangement gives flexible support and resists bending.


T.S section of a stem structure, outside to in:

Outer ring= epidermis.
Next ring (thick)=cortex.
Blobs in this =phloem.
Small ring = cambium.
Next blobs = xylem.
Centre area = pith.


T.S of root outside to in:-

Root hair(coming off).
Epidermis (thin outer line).
Exodermis (slightly further in thin line).
Cortex (large area).
Endocyte (next ring).
Pericycle (next ring).
Phloem (small blobs).
Cambium and xylem (star shape).


What are the 2 types of conducting cell in xylem?

Vessels and tracheid.


What two components also found in xylem provide mechanical support?

Fibres and parenchyma cells


How do xylem vessels maintain structure?

Walls of ringed lignin.


Pericycle function:-

Contains vascular tissue (xylem and phloem)


Epidermis function:-

Outer layer of cells, some of which are specialised into root hair cells. Provide increased SA for ion and water uptake.


Root cortex function:-

Made of paremchyma cells, which provide mechanical support to the root.



Layer of cells that surround pericycle. Have a Casparian strip around them, made of suberin (waxy substance) that waterproofs the cell walls.


Where do cessels occur?

Angiosperms (flowering plants).


Vessel structure:-

Made of long columns strengthened by lignin.



Prevents tubes collapsing inwards during transpiration due to negative pressure in the vessels.


What gives the characteristic appearance of vessels under microscopes?

The ringed structure of the ligning.


Vessels impermeable:-

Water and solute can't pass into xylem cells so protoplasm dies, leaving a hollow tube (lumem) through which water can climb up the plant.


Tracheid structure:-

Elongated cells w/ tapering ends. Strengthened by waterproof lignin.


Where do tracheids occur?

Ferns, conifers + angiosperms but not in mosses.


How is water passed cell to cell through tracheids?

via gaps (pits) as don't have open ends.


Why can't mosses grow as tall as other plants?

No water conducting tissue and are therefore poorer at transporting water.


Why have angiosperms become the dominant plant type on Earth?

Water moving straight up the plant in vessels is much more efficient than the twisting path through tracheids.


2 functions of xylem:-

Transport of water and dissolved mineral ions and providing mechanical strength and support to the plant.


Overview of root-xylem transport:- (3)

•Water taken up from soil through roots and transported through xylem to the leaves where it is used in photosynth.
•howev, much water is lost through stomata by transpiration.
•area of greatest uptake = root hair zone.


Adaptations for root water uptake:- (4)

•Large SA.
•Thin cell walls (short diff pathway.
•Many mito (produced ATP 4 active transport e.g dissolved mineral ions.
•lower water pot than soil (generates gradient for osmosis).


How does water enter the root? (3)

Water in soil contains only a weak solution of mineral ions (so high water pot). Vacuole and cytoplasm of root hair cell has a strong solution of mineral ions, solutes and glucose (low water pot). Water passes into root hair cell by osmosis, down water pot grad.


Water movement across root:-

Water pot grad across root cortex (highest in root hair, lowest in xylem) so water moves down grad across the root. Can do this via 3 different routes (other card).


3 routes of water movement:-

Symplast pathway:- through the cytoplasm and plasmodesmata.
Apoplast:- in the cell walls (fastest).
Vacuolar (small amount):- vacuole to vacuole in adj cells. Used less often due to resistance as it has to pass through tonoplast and plama membrane.



Water can't enter xylem from apoplast PW because lignin makes xylem walls waterproof. Water must pass into xylem via symp/vacuolar.
Vascular tissue in centre of root is surrounded by endodermis.


Endodermal cell suberin:-(4)

Waxy. Contained in cell walls. Forms bands around the cells, known as casparian strips. Waterproof and so prevents water moving further along the apoplast and causes it to enter the cytoplasm and continue along the symplast pathway.


Mineral uptake:-

Active as surrounding soil water has a weak concentration of mineral mineral ions compared to the inside of the root hair cell.


3 possible theories for water movement up the xylem:-

•cohesion-tension theory.
•root pressure theory.
Capillarity-adhesion theory.


Cohesion tension theory:-

Water evaporates from the surface of cells of the spongy mesophyll into air spaces and then diffuses out of stomata down water potential gradient-leaving higher water pot in xylem, lower in air spaces. Tension is created on column of water molecules as they are drawn out of xylem because cohesion. So, transpiration from leaves pulls water up the stem in continuous column (transpiration stream).


Root pressure theory:-

Water moving into the xylem pushes water that is already there further up. Operates over short distances in living plants



The tendency for water to rise in narrow tubes.


Capillarity/adhesion theory:-

Xylem vessels =narrow w/ hydrophilic lining. Water molecules are attracted and adhere to the hydrophilic walls, causing the water to move up the vessels. Only operates over short distances up to 1m.


Transpiration stream:-

The continual flow of water in at the roots, up the stem and out to the atmosphere.


Factors affecting transpiration rate:-(4)

Light intensity.
Wind speed.


Light intensity on transpiration:1

Controls the degree of stomatal opening for photosynthesis gas exchange.


Temp on transpiration rate:-

Rise = additional kinetic energy for water molecules movement. Accelerates evaporation rate from mesophyll cell walls and speeds up diffusion rate if stomata are open.


Humidity on transpiration rate:-

Humidity = % saturation of water molecules in the air. Leads to variation in water pot grad.


Wind speed on transpiration rate:-

Can blow away layer of humid air at leaf surface, therefore increasing water pot grad.


Rate of water uptake (mm^3/s) with potometer:-

Speed of air bubble movement (mm/s) x cross sectional area of capillary tube (mm^2)


Potometer use:-(7)

•Cut leafy shoot underwater to prevent air entering xylem.
•under water, fill potometer w/ water. Ensure no air bubbles.
• fit under water w/rubber tubing to prevent air bubbles.
•remove from water, seal joints w/ vaseline and dry carefully.
•inteoduce single air bubble.
•measure distance moved in x time.
•use water resorvoir to bring bubble back to start point. Repeat as necessary.


Phloem movement theory:-

Mass flow theory


Mass flow theory part 1

Photosynthesising cells in the leaves ( source cells) produce glucose which is converted into sucrose before being actively transported into the phloem using ATP from the companion cells


Mass flow theory part 2:-

By increasing the level of solutes (sucrose) in the phloem, the water potential is lowered causing water to moving from the adjacent xylem by osmosis down a water potential gradient


Mass flow theory part 3:-

The volume of water in the phloem has now been increased which in turn raises the hydrostatic pressure. Sucrose and dissolved solutes now move by mass flow from a high to low hydrostatic pressure down a pressure gradient


Mass flow theory part 4

At the roots/growing points (sink cells) the sucrose is removed from the phloem sieve tubes by active transport. Here it is converted to starch for storage or converted to glucose for respiration. The loss of sucrose from phloem raises the water potential higher than the xylem.


Mass flow theory part 5:-

This causes water to enter the xylem by osmosis and as the volume of water decreases so does the hydrostatic pressure. At the same time, ions are being pumped into the xylem from the soil by active transport, also reducing the water potential in the xylem


Mass flow theory part 6

Water moves from the phloem to xylem, down water potential gradient, causing a reduction in hydrostatic pressure in the phloem


Mass flow theory part 7

Water moves up the xylem by transpiration.


Problems with mass flow theory (5):-

•phloem transport rate much faster than if substances moving by diffusion.
•doesn't account for sieve plates.
•sucrose + AA move at diff rates and directions in the same tissue.
•phloem consumes O2 + translocation is slowed/stopped at low temps/in presence of resp inhibitors.
•companion cells role aren't considered even though they are biochemically very active.


Other phloem/translocation theories:- (4)

•Active process may be involved (affected by resp inhibitora and low temp).
•protein filaments pass through sieve pores so perhaps diff solutes are carried along diff routes in same sieve tube cell.
•cytoplasmic streaming could be responsible for movement in diff directions in individual sieve tube elements.
•at present, no clear understanding of translocation process.



Grow submerged or partially submerged in water.
E.g water lily which is rooted to mud at bottom of pond and had floating leaves on surface.


Hydrophyte adaptations:-(5)

•stomata on upper surface of leaves.
•leaves have little cuticle.
•large air spaces in stem and leaf tissue provide buoyancy.
•water supports the plant so little lignified plant tissue is needed.
•poorly developed xylem-surrounded by water so don't need transport tissues.


Mesophyte (normal flower) adaptations:- (3)

In prolonged dry periods, they survive by:-
•shedding their leaves to reduce transpiration.
•producing dormant seeds.
•ariel parts die off in winter but underground organs e.g bulbs survive.



Lives where water is in short supply e.g deserts, cold regions where water is frozen, exposed windy conditions.
Structurally adapted to reduce water loss. E.g marram grass in sand dunes.


Marram grass adaptations:- (4)

•thick cuticle on leavess-reduces water loss.
•stomata in pits, water vapour not moved so humid air maintained around stomata, reducing water pot grad and therefore diff rate.
•hairs surrounding stomata-traps water molecules, maintaining humid air around stomata.
•hinge cells-cause leaves to roll up, reducing SA and maintaining humidity around.


Translocation ringing experiment:-

•cylinders of bark removed from stem, removing phloem but leaving xylem.
•after photosynthesis, phloem analysis showed a lot of sucrose above the ring but none below, suggesting translocation in phloem.
•bark above swelled due to solute accumulation.


Translocation radioactive tracers + autoradiography experiment:-

•plant photosynthesises in presence of radioactive CO2 (^14CO2).
•section of stem placed on photographic film + exposed to radiation source, producing an autoradiograph.
•position of exposure + therefore radioactivity coincides with the phloem position, indicating that the phloem translocates the sucrose made from ^14CO2 in photosynthesis.


Aphid translocation experiments:-

•aphids, such as greenfly, have hollow, needle-like mouthparts called stylets. This is inserted into a sieve tube and the phloem contents (sap) exude under pressure into aphid's mouth.
•in some experiments, the aphid was anaesthetised + removed, leaving stylet embedded in phloem. As sap is under pressure, it exuded from stylet. Collection and analysis showed sucrose presence.


Translocation aphid + radioactive tracer experiment:-

•aphid experiments extended to plants which had been photosynthesising with ^14CO2.
•these showed that the radioactivity + therefore the sucrose made in photosynthesis, moved at a speed too fast to be explained by diffusion alone so some additional mechanism had to be considered.


Aphids + radioactive tracers showed:- (3)

•sucrose is transported in phloem.
•sucrose is transported bi-directionally to sinks.
•translocation is a rapid process (too rapid to occur by diffusion alone).