3.3 - Transport in plants Flashcards
Structure and function of xylem vessels
Continuous, hollow tubes with no end walls or contents = Less resistance to flow of water and more space due to lack of contents
Walls impregnated (reinforced) with lignin = strengthens wall - prevents the collapse of xylem vessel under tension waterproofs wall - reduces lateral flow of water improves the adhesion of water molecules to the wall - increases capillarity
Lignification in spiral pattern = pattern of lignin allows flexibility and stretching of stem
Bordered pits (pores) in walls of vessels = Allow lateral movement of water between vessels to get around blockage e.g. air bubbles (which prevent continuous columns of water)
Narrow lumen = More capillary rise (more effective capillary action)
Structure and function of phloem tissue - Structure of sieve tube elements
Little cytoplasm, most organelles absent inc. nucleus = Less resistance for transport and more space for transport
Sieve plates = Connects sieve tube elements to allow sucrose (as sap) through
Joined end to end to form tube = Allows continuous transport
Bi-directional flow = Allows sucrose to go go both up and down the plant
Living = Allows active processes
Structure and function of phloem tissue - companion cells
Many mitochondria = A lot of respiration is needed to provide large amounts of ATP for active processes e.g. actively loading sucrose into sieve tubes
Nucleus = Controls the functions of both the companion cell and sieve tube element
Plasmodesmata = Allows continuation of cytoplasm between companion cell and sieve tube element
Summarise how water moves from the soil to the xylem
- Minerals actively transported into root hair cell (through carrier proteins)
- Water moves via osmosis from soil into root hair cells across cell surface membrane (through aquaporins) down the water potential gradient
- Water can move via cell walls in the apoplast pathway
- Water can move via the cytoplasm in the symplast pathway, through plasmodesmata, linking
the cytoplasm in neighbouring cells - At the endodermis, the Casparian strip (made of suberin) blocks the apoplast pathway
- This makes the water enter the symplast pathway
- Water potential is most negative in the xylem due to the active transport of minerals into it
- This causes water to move into the xylem from the cells of the endodermis and cortex
How does transpiration result in the movement of water up a stem?
- Water evaporates from the surface of the mesophyll cells in the leaf and forms water
vapour - Water vapour diffuses from a high water potential to a lower water potential out of the leaf, through the stomata
- More water is drawn from the mesophyll cells via the symplast/apoplast pathways in the
leaf replacing the water that has evaporated - This occurs via osmosis down the water potential gradient
- This water is replaced by water from the xylem vessels (moving out via osmosis)
- The loss of water from the xylem causes a low hydrostatic pressure at the top of the
xylem - Water moves from a higher pressure (roots) to a lower pressure (down the pressure
gradient) under tension - Water is therefore pulled up the xylem by mass flow
- The cohesion of water molecules due to the hydrogen bonds between them causes them to stay as a long unbroken column of water during this process - the transpiration stream
Setting up a potometer
- cut a healthy shoot under water (to stop air entering xylem vessels)
- cut shoot at a slant (to increase surface area)
- check potometer is air bubble free
- insert shoot into potometer under water
- remove potometer from water and ensure, airtight joints around shoot
- dry leaves
- keep, conditions constant
- allow time for shoot to acclimatise
- shut screw clip
- keep ruler fixed and record position of air bubble on scale
- start timing and measure distance moved per minute
Why is using a photometer not an exact measure of the rate of transpiration
- transpiration is the loss of water by evaporation from leaves
- a potometer measures water uptake to replace loss
- some water may be used e.g. in photosynthesis
- rather than all evaporating from the leaves
- also uptake by detached shoot may not be same as that of the whole plant
Factors affecting the rate of transpiration
Number of leaves = more leaves = more water loss = larger SA over which water can evaporate out of plant (often more stomata)
Number and size of stomata = more/bigger stomata = more water loss = larger SA over which water can evaporate out of plant via stomata
Waxy cuticle present = Waxy cuticle present = less water loss
reduces water evaporating from leave surface as it is hydrophobic
Light = Lighter conditions = more water loss = stomata open wider in light (to allow gas exchange for photosynthesis), if they are open = larger SA for water to evaporate out
Temperature = Higher temperature = more water loss = more kinetic energy, water evaporates faster and water vapour diffuses out of leaf faster
Humidity = Higher humidity = less water loss air is more saturated (with water). Whilst saturation in air spaces in leaf is still higher, there is a shallower water potential gradient
Wind = More windy = more water loss = carries water vapour that has just diffused from leaf away, making the air immediately surrounding the leaf less saturated and maintaining a steeper water potential gradient
Water availability = More water in soil = more water loss cannot replace the water that is lost
Adaptation of xerophytes
epidermis covered in hairs = hairs trap water vapour which stops the wind removing wind vapour more humid air around leaf reduces water potential gradient so less evaporation and so less transpiration
Thicker waxy cuticle = hydrophobic prevents water passing through epidermis of plant so less evaporation and so less transpiration
Small leaves/needles = small surface area fewer stomata so less evaporation and so less transpiration
Sunken stomata (in pits) = hairs trap water vapour which stops the wind removing wind vapour more humid air around leaf reduces water potential gradient so less evaporation and so less transpiration
curled leaves = lower epidermis is not exposed to atmosphere traps water vapour which stops the wind removing wind vapour more humid air around leaf reduces water potential gradient so less evaporation & transpiration
Small air spaces in mesophyll = less water can evaporate into the air spaces quickly become full reduced area for loss of water
stomata shut in day, open in night = transpiration occurs via stomata warmer in day than night more evaporation and so more transpiration will occur in the day shutting stomata in day reduces this loss
Translocation - active loading…
- H+ ions are actively transported (requires ATP) out of the companion cells
- This produces a diffusion gradient for the H+ ions
- They move back into the companion cell via facilitated diffusion through co -transporter carrier proteins along with sucrose
- Sucrose has been actively loaded into the companion cell
- There is a high concentration of sucrose in the companion cell compared to the sieve tube element, so it diffuses into it down the concentration gradient through the plasmodesmata
How sucrose moves along the phloem
At the source
- Sucrose is actively loaded into the sieve tube elements at the source
- This reduces the water potential in the sieve tube element
- Water enters the sieve tube elements by osmosis
- This increases the hydrostatic pressure in the sieve tube element near the source
At the sink
- Sucrose is unloaded at the sink by diffusion (or active transport) and used in respiration/stored
- This increases the water potential in the sieve tube element
- Water moves into the sink via osmosis down the water potential gradient
- This reduces the hydrostatic pressure in the sieve tube element near the sink
- Water in the sieve tube element at the source moves down the hydrostatic gradient from source to sink
- This creates a flow which carries the sucrose and other assimilates along the phloem via massflow either up or down the plant
How the sieve tube elements adapted to allow mass flow to occur
- elongated elements, joined end to end to form a column
- sieve plates with pores in end walls allow sucrose through
- little cytoplasm and no nucleus - less resistance to transport
Why, if a ring is cut around the bark of a tree, a swelling can occur above the ring
- phloem is in the bark and so sucrose cannot pass the cut
- area above cut acts as a sink so water moves into cells
- damage triggers increased cell division
- to produce cells to store sugars
- cut causes infection
Evidence for this mechanism of translocation
How we know phloem used:
- Radioactively labelled CO2 supplied for photosynthesis appears in phloem
- Aphids feeding on plant stems insert mouthparts into phloem
- Sugars collect above ring when tree is ringed to remove phloem
How we know it needs ATP:
- Companion cells have many mitochondria
- Translocation is stopped if a poison which stops ATP production is given
- Flow of sugars is very high that ATP must be used - much faster than would be possible with diffusion
How we know this mechanism is used:
- pH of companion cells is higher than surrounding cells (H+ ions reduce pH)
- Concentration of sucrose is higher in source than sink
Evidence against this mechanism:
- note all solutes in phloem move at same rate
- Sucrose moved to all parts of plant at same rate and doesn’t go to places with lowest
concentration faster
