Component 3: Plant Transport (new) Flashcards
Draw the structure of a dicotyledon root and show positions of vascular tissue
Outer layer = epidermis Cortex Inner circle = endodermis X shape = xylem small circles = phloem
Steele/vascular bundle = endodermis, phloem and xylem
Where does most water absorption take place?
Through the root hair cells
What are the characteristics of root hair cells for water absorption?
- large surface area for water to enter by osmosis
- permeable, cellulose cell wall is freely permeable to water
What are the characteristics of root hair cells for mineral ion uptake?
- lots of mitochondria to provide ATP for active transport
- lots of protein carriers in the membrane for active transport
What are the two things plants have to do to survive?
- transport water from the soil via roots to photosynthesising leaves
- transport products of photosynthesis to all respiring cells
How does water enter the root hair cell from the soil?
- soil water has a very dilute solution of mineral ions, creating a high water potential
- root hair cells have a low water potential as vacuoles have concentrated solution of sugars/salts
- water passes into the root hair cell by osmosis down the water potential gradient into the walls of epidermal cells
What are the 3 ways water can travel through the cells of the cortex?
- apoplast
- symplast
- vacuolar
What is the apoplast pathway?
Water travels across the cortex through cells walls or through spaces between cells
What is the symplast pathway?
Water moves through the cytoplasm and plasmodesmata
What is the vacuolar pathway?
water moves from vacuole to vacuole
Is there a water potential gradient in the cortex?
- there is
- highest in the root hair cells
- lowest in the xylem
- so water moves down the water potential gradient across the root
What is the endodermis?
A single layer of cells around the pericycle and vascular tissue of the root.
Each cell has an impermeable waterproof barrier in it cells wall
What is the Casparian strip?
The impermeable band of suberin in the cell walls of the endodermal cells, blocking the movement in the apoplast, driving it into the cytoplasm
What is the endodermis apoplast blocked by?
Casparian band in the cell wall
Why can’t the water enter xylem from the apoplast pathway?
The cell walls of xylem are made out of lignin which makes the walls waterproof
- water can only enter by symplast or vacuolar pathways
What does water do a the casparian band?
Water passes across the plasma membrane and continues along the symplast pathway
How does water move from the xylem from the roots?
By osmosis, water potential of xylem must be more negative than the water potential of the endodermal cells
How is the water potential gradient established between the endodermal cells and the xylem?
- water being driven into the cytoplasm by the casparian strip, increases water potential of the cells
- active transport of mineral salts into the xylem decreases water potential of the xylem
How are mineral ions absorbed into the plant?
active transport, against the concentration gradient
Describe the uptake of nitrogen in a plant
- enters the plant as nitrate or ammonium ions
- diffuse down a concentration gradient in the apoplast pathway
- enters symplast pathway by active transport against the concentration gradient
- then they flow via plasmodesmata in the cytoplasmic stream
What happens to ions at the endodermis?
Ions must be actively taken up to by-pass the casparian band which allows the plant to selectively take up ions at this point (lowers water potential of the xylem)
What type of movement is the movement of water up through xylem?
Passive process
What are the 3 main mechanisms driving the movement in the xylem?
- Cohesion-Tension (pull of transpiration)
- Capillary action
- Root pressure (push of root pressure)
What is the biggest driver in water in the xylem?
Cohesion-tension (pull of transpiration)
What is the effect of transpiration in the xylem?
The continues removal of water molecules from the top of the xylem vessels results in a tension causing a pull on the xylem column
What in the cohesion-tension mechanism in the xylem?
- water molecules are pulled out in the transpiration stream due to cohesion between polar H2O molecules forming hydrogen bonds
- this results in unbroken columns being continuously drawn up by transpiration stream
- charges on water molecules cause attraction between water and xylem cell wall in adhesion
What is the capillary action mechanism in the xylem?
- movement of water up narrow tubes
- xylem cells are narrow and have small spaces between cellulose molecules
How large is the capillary action mechanism?
- small contribution as it only operates over short distances
- really important in small plants
What is the root pressure mechanism in the xylem?
- caused by the osmotic movement of water into the xylem down a water potential gradient due to active transport of mineral ions across endodermis
- this produces positive hydrostatic pressure inside the xylem, forcing water upwards
How large is the root pressure mechanism?
only operates over short distances, not large
How does water pass through the plant?
Water passes through the root to the xylem, up through the stem to the leaves where most evaporates
Draw the structure of a dicotyledon stem and show positions of vascular tissue
Epidermis
Collenchyma
Cortex
Medulla
Vascular bundle in cortex: sclerenchyma phloem cambium xylem
What is transpiration?
- Transpiration is the loss of the water vapour from leaves giving rise to the transpiration stream
- the continued removal of water molecules from the top of the xylem vessels results in a tension causing a pull on the xylem column (drawn up by cohesive forces between H2O molecules and adhesive forces if molecules and hydrophilic lining)
What are the 4 external factors that affect transpiration?
- Light intensity
- Temperature
- Humidity
- Air movement
How does light intensity affect transpiration rate?
- light controls the degree of opening and closing of the stomata
- increasing light intensity, stomata open wider, increasing rate of transpiration (i.e. highest in the middle of the day)
How does temperature affect transpiration rate?
- an increase in temp results in a lower water potential of the atmosphere (warm air has more kinetic energy so can hold more water)
- this also increased KE of water molecules, causing H2O to diffuse away from the leaf quicker (walls of mesophyll cells)
How does humidity affect transpiration rate?
- low humidity outside the leaf creates a steeper diffusion gradient between internal air spaces of the leaf and the atmosphere outside, increases transpiration
- high humidity reduces the rate of transpiration
How does air movement affect transpiration rate?
- maintains a diffusion gradient by blowing humid air away from leaf surface (around the stomata), increasing rate of transpiration as replacement air is less saturated with water
(water potential gradient between inside and outside leaf increases)
How does water move out of the leaf?
Water always moves down a water potential gradient, it does so by evaporation when it transpires out of the leaf
How do you measure the rate of water uptake?
Using a potometer
How can you measure rate of transpiration?
A potometer measures the rate of water uptake but if cells are fully turgid then the rate of water uptake = rate of transpiration
- as most of the water taken up the leafy shoot is lost by transpiration
How do you set up a potometer?
- Cut leafy shoot at an angle underwater
- Fill potometer will water underwater
- Fit shoot into rubbing tubing of potometer, underwater
- Remove apparatus from water, seal joints with vaseline and dry carefully
- Introduce air bubble in capillary tube
- Measure time taken of air bubble across the scale
- Open reservoir tap to return the bubble to the starting point
- Repeat measurements and calculate mean for reliability
- Repeat experiment under different conditions
What are hydrophytes?
Partially or completely submerged in water (water plants)
What are mesophytes?
Plants living in conditions of adequate water supplies
What are xerophytes?
Plants that live in conditions where water is scare
How are hydrophytes adapted for their environment?
- Little support (lignified) tissue as it is supported by the water
- Little transport tissue, surrounded by water so xylem is poorly developed
- Little or no cuticle as water loss doesn’t need to be prevented
- Stomata on the upper surface of the leaves
- Large air spaces in both stem and leaf tissue to form an oxygen reservoir and provide buoyancy
What is an example of a hydrophyte?
Water Lily
How are hydrophytes situated in the water?
Live with their roots submerged at the bottom of the pond with leaves floating on the surface
What is an example of xerophytes?
Marram Grass
What is the overall function of xerophyte adaptations?
To prevent excessive water loss
Adaptations of marram grass: Leaf Shape
Rolled Leaves
- reduces leaf air exposed to air
- reducing transpiration
- roll inwards when there is excessive transpiration
Adaptations of marram grass: Stomata
Sunken Stomata
- depressions of the adaxial surface
- humid air is trapped outside the stomata
- reducing the water potential gradient and reduces transpiration
Adaptations of marram grass: cuticle
Thick Cuticle
- covers abaxial leaf surface
- wax is waterproof
- reduces rate of transpiration (water loss)
Adaptations of marram grass: hairs
Stiff, interlocked hairs trap water vapour reducing rate of transpiration
Adaptations of marram grass: fibres
Fibres of sclerenchyma
- stiff to maintain the leaf shape if cells become flaccid
What are some general features of mesophytes?
- excessive water loss is prevented by stomatal closure at night
- grow best in well-drained soils and dry air
- the water taken up at night compensated for water loss during the day
How do mesophytes survive in unfavourable times of the year?
- shedding their leaves, don’t lose water by transpiration as water in ground may be frozen
- aerial parts die off so it’s not exposed and survive underground as bulbs
- annual mesophytes survive as dormant seeds (low metabolic rate, less water required)
What are the 2 main types of conducting cell in the xylem?
Tracheids and vessels
- these form a continuous system of channels for water transport
- both contain no living material, no cytoplasm
What type of transport system is in the xylem?
One-way transport system
What is the structure of tracheids?
- more narrow than vessels
- made of cells that have perforated end walls
- contains lignin, hard, strong and waterproof
- gaps (pits) through which water travels
- spindle-shaped (twisting path)
(- provides lots of support)
What does the xylem consist of?
Tracheids, vessel, supporting fibres and living parenchyma
What is the structure of vessels?
- main conducting tube
- made up of wide cells with no end walls (continuous tube)
- lignified
What is the function of fibres in the xylem?
Support role
What is the function of parenchyma?
Living packing tissue (keeps all xylem elements in place)
How is lignin deposited?
Deposited as rings/spirals
What is the role of lignin?
Mechanical strength (supports plant) and allows adhesion of water molecules
What happens if there is ever any breaks or bubbles in the xylem?
pits between adjacent vessels allow lateral movement of water allowing the column to bypass bubbles
What would breaks or bubbles do to the plant?
Breaks cohesion and prevent upwards movement and these occur frequently
What is the role of the arrangement in the root?
Central arrangement anchors the plant
What is the role of the arrangement in the stem?
Peripheral arrangement to resist bending
What are the 2 main cell types in the phloem?
sieve tubes and companion cells
What are sieve tubes adapted for?
For the flow of material
What is the structure of the sieve tubes?
- form end-to-end cells called sieve tube elements and end walls don’t break down
- end walls form sieve plates where strands of cytoplasm can pass through the pores
- sieve tube elements are alive but have no nucleus and few organelles
What is function of the companion cells?
- they control the metabolism (ie respiration, excretion…) of sieve elements as they have greatly reduced contents
What is the structure of the companion cells?
- cytoplasm of the companion cells and their sieve tube element are joined through plasmodesmata
- have a very dense cytoplasm , large nucleus, many mitochondria, RER and ribosomes
What is the function of the phloem fibres?
support role only
What is the function of the parenchyma?
packing tissue (keeps phloem elements in place)
What is translocation?
The movement of the soluble products of photosynthesis, such as sucrose and amino acids, through the phloem from source to sink
What happens in the ringing experiment?
- outer ring of bark is scraped away which removes the phloem
- after leaving the plant for a while, allowing for it to photosynthesise, a bulge of sugar forms above the ring
- below the ring there was no sucrose
What do the results of the ringing experiment tell us?
- the bulge formed suggests that sugar moves down the stem in the phloem
- the lack of sucrose below the ring suggests that sucrose was being used by plant tissues and not replaced
What is the aphid experiment?
- aphids have specialised mouthparts called stylets that can penetrate phloem tubes
- aphids are then anaesthetised with CO2
- they’re removed from the stylet so it remains embedded in the phloem
- pure phloem sap can be collected through the stylet for analysis, shows presence of sucrose
What is the radioactive tracer and autoradiography experiment?
- radioactive labelled CO2 is placed in a bag surrounding a leaf, allowing the plant to photosynthesise with it
- a stem section is placed on photographic film
What do the results of the radioactive tracer and autoradiography experiment tell us?
- when the film develops the presence of radioactivity can be seen where the phloem was
- indicating that its the phloem that translocates the sucrose made from carbon 14 in photosynthesis
What happens in the aphids and radioactive tracers experiment?
- radioactive labelled CO2 is placed in a bag surrounding a leaf, allowing the plant to photosynthesise with it
- CO2 is incorporated into sugars and transported into the phloem
- aphids feeding on the sugar in the phloem can be used to trace the movement of the sugar in the plant from source to sink
What is mass flow theory?
- main theory put forward to explain the transport of organic solutes
- suggests that there is a passive mass flow of sugars from the leaves (source) to other growing tissues (sink), high to low conc of sugars
- products of pss are transported in soluble form to all parts of the plant
What are the problems with mass flow theory?
- doesn’t take into account sieve plates these would impede flow
- rate of transport in phloem is faster than substances moving by diffusion
- sucrose and a.a. are moving at different rates and directions in the same tissue
- phloem has a high rate of oxygen consumption & translocation is slowed by respiratory poisons
- companion cells contain many mitochondria, no suggested role for these cells
Other theories of mass flow: active
- Active process may be involved
- sucrose is loaded into phloem
- using energy generated by respiration
Other theories of mass flow: cytoplasmic streaming
cytoplasmic streaming could be responsible for bi-directional movement in individual sieve tubes
- this is providing that there is a mechanism to transport solutes through sieve plates
Other theories of mass flow: protein filaments
protein filaments pass through sieve pores
- suggests that different solutes are carried along different routes through the same sieve tube elements
What is amended mass flow?
- H+ pumped out of companion cells
- H+ return to cells with sucrose down diffusion gradient , occurs through co-transporter proteins
- sucrose diffuses into s.t.e through plasmodesmata
- ψ in s.t decreases, water moves in by osmosis
- hydrostatic pressure in s.t at source increases, sugary fluid moves down tube from high to lower h.p (source to sink)
- sucrose moves from s.t into surrounding cells by facilitated diffusion/a.t. Sucrose enters root cell (sink) to be used in resp or to be converted into starch for storage
- water moves out of the sieve tube by osmosis, hydrostatic pressure at the sink
