Module 3.3 - Transport in Plants Flashcards
1
Q
Why do plants need a transport system?
A
- Move water + minerals up from roots up to the leaves
- Move sugars from leaves to the rest of the plant
2
Q
(MA) Describe the structure of the xylem vessels and each structure’s function.
A
- Continuous, hollow tubes w no end walls/contents: less resistance to water flow + more space as less contents
- Walls impregnated w lignin: strengthens wall to prevent collapse under tension. Waterproofs wall to reduce lateral flow of water. Improves adhesion of water molecules to wall to increase capillarity
- Lignification in spiral pattern: pattern of lignin allows flexibility + stretching of stem
- Bordered pits in walls of vessels: allow lateral movement of water between vessels to get around blockages e.g. air bubbles
- Narrow lumen: more capillary rise (more effective capillary action)
3
Q
What two components make up phloem tissue?
A
- Sieve tube elements
- Companion cells
4
Q
(MA) Describe the structure of the sieve tube elements and the function of each structure.
A
- Little cytoplasm, most organelles absent incl. nucleus: less resistance for transport + more space for transport
- Sieve plates: connect sieve tube elements to allow sucrose (as sap) through
- Joined end to end to form tube: allow continuous transport
- Bidirectional flow: allow sucrose to go up + down the plant
- Living: allow active processes
5
Q
(MA) Describe the structure of companion cells and the function of each structure.
A
- Many mitochondria: lots of respiration to provided lots of ATP for active processes e.g. active loading sucrose into sieve tubes
- Nucleus: controls functions of companion cells + sieve tube elements
- Plasmodesmata: allows continuation of cytoplasm between companion cells + sieve tube elements
6
Q
(MA) How does water move from the soil to the xylem?
A
- Minerals a ticket transported into RHC (via carrier proteins)
- Water moves via osmosis from soil into RHCs across cell surface membrane (through aquaporins) down a water potential gradient
- Water can move via cell walls in apoplast pathway
- Water can move via cytoplasm in symplast pathway, through plasmodesmata, linking cytoplasm in neighbouring cells
- Endodermis: Casparian strip (made of suberin) blocks apoplast pathway
- Makes water enter symplast pathway
- Water potential most negative in xylem she to active transport of minerals into it
- Causes water to move into xylem from cells of endodermis + cortex
7
Q
What makes the water potential in the xylem lower than that of the surrounding cells near the roots?
A
Active transport of minerals into xylem so higher solute concentration therefore lower water potential
8
Q
(MA) How does transpiration result in the movement of water up a stem?
A
- Water evaporates from surface of mesophyll cells in leaf + forms water vapour
- Water vapour diffuses from high water potential to a lower water potential out of leaf through stomata
- More water drawn from mesophyll cells via symplast/apoplast pathways in leaf to replace water lost through evaporation
- Occurs via osmosis down a water potential gradient
- Water replaced by water from xylem vessels (moving out via osmosis)
- Loss of water from xylem causes low HS pressure at top of xylem
- Water moves from a higher pressure (roots) to lower pressure (down pressure gradient) under tension
- Water is therefore pulled up the xylem by mass flow
- Cohesion of water molecules due to H bonds between them causes them to stay as long unbroken column of water during this process: transpiration stream
9
Q
(MA) How do you set up a potometer?
A
- Cut a healthy shoot under water (stop air entering xylem vessels). Cut at slant (increase SA)
- Check potometer is air bubble free
- Insert shoot into potometer underwater
- Remove potometer from after + ensure airtight joints around shoot
- Dry/blot leaves
- Keep conditions constant
- Allow time for shoot to acclimatise
- Shut screw clip
- Keep ruler fixed + record position of air bubble on scale
- Start timing + measure distance moved per minute
10
Q
(MA) Why is using a potometer not an exact measure of the rate of transpiration?
A
- Transpiration is the loss of water by evaporation from leaves
- Potometer measures water uptake to replace loss
- Some water may be used e.g. in photosynthesis/keeping cell turgid
- Rather than all evaporating from leaves
- Uptake of detached root may not be the same as that of the whole plant
11
Q
(MA) What are the factors that affect the rate of transpiration?
A
- No. of leaves: more leaves = larger SA over which water can evaporate from plant (often more stomata) so more water loss
- No. + size of stomata: more/bigger stomata = larger SA over which water can evaporate out of plant via stomata so more water loss
- Waxy cuticle present: reduces water loss as surface is hydrophobic
- Light: lighter conditions = stomata open wider (to allow gas exchange for photosynthesis) so larger SA for water to evaporate so more water loss
- Temp: higher = more KE so water evaporates faster + water vapour diffuses out of leaf faster
- Humidity: higher = air more saturated w water so shallower water potential gradient (but still higher in leaf) so less water loss
- Wind: more = water vapour carried faster just diffused from leaf away so air immediately around lead less saturated + maintaining steeper water potential gradient so more water loss
- Water availability: less in soil = can’t replace water lost so less water lost
12
Q
(MA) What are the adaptations of xerophytes and how do these reduce transpiration to help them survive?
A
- Epidermis covered in hairs: traps H2O (g) which stops wind removing it so more humid air around lead reduces water potential gradient so less evap so less transpiration
- Thicker waxy cuticle: hydrophobic prevents H2O passing through plant’s epidermis so less evap so less transpiration
- Small leaves/needles: small SA = fewer stomata so less evap so less transpiration
- Sunken stomata in pits: hair traps H2O (g) stopping wind removing H2O(g) so more humid air around leaf reduces water potential gradient so less evap so less trans
- Curled leaves: lower epidermis not exposed to atmosphere traps H2O(g) which stops wind removing it so more humid air around leaf rescues gradient so less evap so less trans
- Small air spaces in mesophyll: less water can evaporate into air spaces quickly become fully reduced area for loss of water
- Stomata shut in day open at night: trans occurs via stomata, warmer in day than night so more evap + therefore trans will occur, in day shutting stomata reduces this loss
13
Q
(MA) Describe the process of active loading as part of translocation.
A
- H+ ions actively transported (using ATP) out of companion cells
- Produces a diffusion gradient for H+ ions
- Move back into companion cell via facilitated diffusion through co-transporter carrier proteins along w sucrose (only works if both bound to co-transporter protein, secondary active transport as resulted from active transport of H+ ions out of cell)
- Sucrose has been actively loaded into companion cell
- Higher sucrose conc in companion cell compared to sieve tube element so diffuses down conc gradient through plasmodesmata
14
Q
(MA) How does sucrose move along the phloem at the source?
A
- Sucrose actively loaded into sieve tube elements at source
- Reduces water potential in sieve tube element
- Water enters sieve tube element via osmosis
- Increases HS pressure in sieve tube element near source
15
Q
(MA) How does sucrose move along the phloem at the sink?
A
- Sucrose unloaded at sink by diffusion (or active transport) + used in respiration or stored
- Increases water potential in sieve tube element
- Water moves into sink via osmosis down water potential gradient
- Reduces HS pressure in sieve tube element near sink
- Water in sieve tube element at source moves down HS gradient from source to sink
- Creates a flow which carries sucrose + other assimilates along phloem via mass flow up or down the plant
16
Q
(MA) How are the sieve tube elements adapted to allow mass flow to occur?
A
- Elongated elements joined end to end forming a column
- Sieve plates w pores in end walls allow sucrose through
- Little cytoplasm + no nucleus so less resistance to transport
17
Q
(MA) Why, if a ring is cut around the bark of a tree, can swelling occur above the ring?
A
- Phloem in bark + so sucrose can’t pass cut
- Area above cut acts as sink so water moves into cells
- Damage triggers increased cell division
- Produces cells to store sugars
- Cut causes infection
18
Q
(MA) How do we know the phloem is used in translocation?
A
- Radioactively labelled CO2 supplied for photosynthesis appears in phloem
- Aphids feeding on plant stems insert mouthpiece into phloem
- Sugars collect above ring when tree is ringed to remove phloem
19
Q
(MA) How do we know ATP is used in translocation?
A
- Companion cells have mitochondria
- Translocation stopped if poison which stops ATP production is given
- Flow of sugars is v high that ATP must be used, must faster than would be possible w diffusion
20
Q
(MA) How do we know the mechanism of translocation is used?
A
- pH of companion cells higher than surrounding cells (as H+ ions reduce pH)
- Concentration of sucrose is higher in source than sink
21
Q
(MA) What evidence is there against the mechanism of translocation?
A
- Not all solutes in phloem move at same rate
- Sucrose moved to all parts of plant at same rate, doesn’t go to places w lowest conc faster
22
Q
What is transported in the xylem tissue?
A
Water + soluble minerals
23
Q
In which direction do the water and soluble minerals travel in the xylem vessel?
A
Upwards from the roots to the rest of the plant
