3 - ICH - Transport in plants Flashcards
Why do plants need transport systems?
As they become larger, SA : Vol decreases, diffusion distance increases and have a relatively high metabolic rate ∴ needs transport systems to:
- Supply nutrients
- Water, minerals and sugars
- Remove waste molecules from individual cells
What transport systems do plants have?
What does each transport?
Plants have 2 distinctive transport systems, both consist of tubes and together they make up the plants vascular tissue
XYLEM
- Carries water and mineral ions from roots through the stem to the leaves of the plant (UP)
PHLOEM
- Carries sugars and products from photosynthesis produced by the leaves to other parts of the plant (DOWN)
What is the plant’s vascular tissue made of?
Xylem and phloem
Where are xylem and phloem found?
Found together throughout the plant, sometimes associated with tissues e.g. sclerenchyma to form distinct structures called vascular bundles
What is the name given to flowering / seed bearing plants?
Dicotyledonous plants
What is a herbaceous dicotyledonous plant?
A non-woody flowering / seed bearing plant which contains 2 food reserves in the seed
Structure and distribution of vasuclar tissue in the leaf of herbaceous dicotyledonous plants
LEAF:
- Vascular tissues for a network of tiny vascular bundles throughout the blade or lamina of the leaf
- these merge to form side veins which in turn merge with the central main vein / midrib
Structure and distribution of vasuclar tissue in the stem of herbaceous dicotyledonous plants
STEM:
- Xylem and phloem form a series of vascular bundles arranged around the outside of the stem
- Between the xylem and phloem in each vascular bundle is a cambium layer - a thin layer of dividing cells which give rise to a new xylem and phloem vells
Structure distribution of vasuclar tissue in the root of herbaceous dicotyledonous plants
- What is the function of each part of the root?
FROM FURTHEST OUT TO IN:
Root hairs:
- Entensions os some epodermal cells
- Incease SA for water uptake
Epidermis:
- Single layer of cells around the root
- Physical barrier to protect moist inner tissues of stem from desiccation and pathogens
- Helps support by holding in the turgid parenchyma cells of the cortex
Cortex:
- Tissue that makes up the bulk of the root
Endodermis:
- Single layer of cells surrounding central vascular bundle
- Walls contain a band of suberin (casparian strip) - waxy substance that is impermeable to water
- Casparian strip forces water to travel down symplast pathway
Pericycle:
Central vascular bundle = Phloem and xylem:
- Phloem - Carries sugars and products from photosynthesis produced by the leaves to other parts of the plant (DOWN)
- Xylem - Carries water and mineral ions from roots through the stem to the leaves of the plant (UP)
What is this a diagram of?
Lable it
Leaf of a herbaceous dicotyledonous plant
What is this a diagram of?
Lable it
Stem of a herbaceous dicotyledonous plant
What is this a diagram of?
Lable it
Root of a herbaceous dicotyledonous plant
Structure (4) + function of xylem (2)
Function:
- Transporting water and mineral ions from roots through the stem to the leaves of the plant (UP the plant)
- Provide support to plant
Structure - Types of cells involved:
-
Xylem parenchyma
- Unspecialised cells wihch act as packing tissue around other components
- Provides support
-
Xylem fibres
- Elongated cells with walls that are thickened by lignin
- Provides support / waterproof walls
- Lignin forms patterns: rings and spirals which allow for flexibility
-
Xylem vessels
- Vary in structure but are hollow and elongated
- Greater diameter of any xylem cell
- Functional cylem vessels are DEAD - contain no cytoplasm
- Pits in lignified walls allow the lateral movement of water
- Water forms a continuous column from roots → leaves in xylem vessels - this is where the bulk of water is transported in flowering plants
-
Trachaeids
- Similar structure to vessels but are longer and thinner
- Has pitted tapering ends
- Found in all plants, they’re the main conducting tissue in ferns and conifers
EXAM HINTS: (4)
Key points for when a question asks you to relate the structure of xylem vessels to their function as the main water conducting tissue in plants
How is lignin layed out in xylem vessels?
Early thickining:
- Rings
- Spiral
Older thickening:
- Scalariform (ladder like)
- Reticulate (net like)
- Pitted
Define osmosis
Osmosis = The net movement of water molecules from an area of less negative ψ to an area of more negative ψ across a partially permeable membrane
What are the 3 routes that water can take across the cortex of the root?
Symplast pathway:
- Through living part of cell
Apoplast pathway:
- Through dead part of cell
- Through cell wall or between cells
Vacuolar pathway:
- Through the vacuole of cell - a special type of symplast pathway
How is water taken up across the root?
ROOT HAIRS:
- Root hairs grow into the spaces between soil particles which are filled by the soil solution of mostly water and a small quantity of mineral ions
- ∴ ψ is relatively high / less negative
- Soil around the root has a more negative ψ than inside the root hairs as water is constantly evaporating from it
- Water diffuses by osmosis down a ψ gradient into root cells
Explain the importance of the casparian strip in the transport of water around the plant
A band in the wall of the endodermis:
Casparian strip is made of a band of suberin - Impermeable to water ∴ blocks off apoplast pathway and forces water moelcules to take a symplast route through cells
Movement of water through a plant:
Describe the apoplast route
DEAD ROUTE!
- Cellulose cell walls are permeable to water
- Water moves through adjacent cell walls in the spaces between the fibres of cellulose
- Because there’s strong cohesive forces between water molecules when water molecules enter xylem, the tension set up drawm more water molecules through the cellulose cell walls
Movement of water through a plant:
Describe the symplast route (7) - what is it’s special case? (1)
LIVING ROUTE!
- Takes place through cytoplasm of the cortical cells as a result of osmosis
- Water passes through cell waslls through plasmodesmata (tiny pores)
Water moves along this pathway by:
- Water enters a root hair cell by osmosis
- Makes it’s ψ less negative
- Root hair cell’s ψ is less negative to it’s adjacent cortical cell’s ψ
- ∴ Water moves into this neighbouring cell by osmosis down a ψ grad
- Loss of water from first cortical cell raises it’s ψ causing more water to enter from root hair cell by osmosis
In this way a ψ grad is set up across all the cells of the cortex, as along as water is removes from the innermost cortical cells the ψ grad will be maintained and the movement of water will continue
Special case of symplast pathway = vacuolar pathway which is just a symplast route that goes through the vacuole
Movement of water through a plant:
How does water pass from the cortex into the xylem? (5)
- Water reaching the endodermis by the apoplast pathway finds its further progress prevented by the waterproof casparian strip
- At this point all the water is forced to move into the living cytoplasm of ther endodermal cels regardless of what pathway is followed beforehand
- Endodermal cells actively transport ions into the pericycle and then they diffuse into the xylem
- Makes ψ of xylem more negative than pericycle ∴ creates a osmotic grad, helps water movement into xylem through pits in xylem vessel walls by osmosis
- Root ahir cells actively take up mineral ions from the soil to help maintain as osmotic grad and diffusion grad for ions to diffuse across cortex into the xylem
State the 3 ways in which water moves from the roots → shoots
Cohesion-tension
Capillarity
Root pressure
Movement of water up the stem: (4)
Cohesion-tension
Cohesion-tension theory is the main way water moves up the xylem in the stem
Basically… loss of water from the leaves pulls more water up the xylem in the stem
- Water evaporates through the stomata in the leaves during transpiration
- Creates osmotic grad created causing water to move across the cells in the leaf → stomata
- Causes water to leave xylem vessels
- Reduces pressure - creating a tension in the xylem
- Column of water is drawn up the stem by adhesion of water molecules to the walls of the xylem - transpiration stream
- Column remains intact due to H bonds between water molecules - cohesion
Movement of water up the stem:
Root pressure
Root pressure is caused by the action of endodemal cells actively pumping ions into the xylem vessels in the root nad the influx of water that follow
- If a stem is cut close to where it emerges from the soil, liquid oozes from the cut surface - caused by root pressure
- Generates limited force and only very contributions to the ascent of water up the stem, but may play a role in small herbaceous plants
Movement of water up the stem:
Evidence for the action of root pressure (2)
- Sap exudes from xylem of a stem that is cut close to the ground
- Metabolic inhibitors, low temps and lack of O2 in the root all reduce root pressure
Movement of water up the stem:
Evidence to explain the action of cohesion-tension (4)
- Tension has been measured in the xylem as plants transpire
- Lignified walls of narrow xylem vessels are strong enough to withstand the tension measured
- Diameter of trees reduces when they’re transpiring (tension pulls xylem vessels inwards) and increases when they’re not transpiring
- Air bubbles in water column within xylem interfere with cohesion between water molecules and stop upward movement of water
Explain the movement of water through the leaf and how it loses / replaces it (6)
The main force that pulls water up the stem of a plant is the evaporation of water from the leaves - process called transpiration
- Humidity of atmosphere < hymidity of sub-stomatal air space
- i.e. ψ of atmosphere is more negative than ψ of sub-stomatal air space
- If stomata are open, water will diffuse out of sub-stomatal air chambers into atmosphere by osmosis down a ψ grad
Because water is lost by diffusion through the open stomata, there is a ψ grad through the whole leaf
- Water lost from the air spaces in the leaf is replaced by evaporation of water from the cell walls of the mesophyll cells surrounding the air spaces
- As their ψ becomes more negative, these mesophyll cells gain water by osmosis from adjacent cells and ultimately from the xylem vessles of the leaf
What apparatus is used to measure the rate of transpiration?
Potometer
What does a potometer measure?
It’s used the measure the rate of transpiration but to be mroe precise, it measures the rate of water uptake by a leafy shoot
Describe an experiment to measure and calcularte the rate of transpiration of a leaft shoot (7)
- A leafy shoot is cut under water to prevent air entering the xylem
- Care must be taken to keep leaves dry
- The potometer is completely filled with water
- Make sure there’s NO air bibbles
- Use a rubber tube to attach the leafy shoot to the potometer under water
- The potometer is removed from under water all and the joints are sealed with petroleum jelly
- Distance moved by the air bubble in a given time is measured
- Measurement is repeated until at ≥3 consistent readings are obtained
- Average volume of water lost can be calculated = πr2l
- Once the air bubbles near the end of the capillary tube, the tap on the water resevoir can be opened until the bubble is pushes back to the end of the capillary tube - can now repeat experiment
This experiment can be repeated on the same leafy shoot changing various conditions e.g. temps, light intensities, air movements, humidity…
Label the potometer and what happens at each part
How do these affect transpiration?
- Increase in temperature
- Increase in air movement e.g. wind
- Decrease in humidity
- Increase in light intensity
Structural + physiological modifications of xerophytic plants to help them adapt (6)
How does each modification help the plants to adapt?
Thick cuticle:
- Regular waxy cuticles still allow 10% of water to be lost during transpiration
- Many evergreen plants e.g. Holly have thicker cuticles to reduce water loss
- Can be a problem during winter when the ground may be frozen
Rolling up of leaves:
- Most plants have their stomata largely or entirely confined to the lower epidermis. Rolling of leaves hides lower epidermis and traps a region of still air within the rolled leaf
- This region becomes saturated with water vapour ∴ no ψ grad between sub-stomatal air space and the outside of the leaf ∴ reduced rate of transpiration
- E.g. Marram grass roll their leaves in hot windy conditions
Hairy leaves:
- Thich layer of hair on leaves, particuarly on lower epidermis traps air against leaf surface ∴ reduces the ψ grad between outside and inside the leaf ψ water loss is reduced considerably
Having stomata in pits or grooves:
- These trap moist air next to the leaf and reduce the ψ grad
- E.g. holly and pine
Reducing SA : Vol ratio of leaves:
- Reduces leaves → needles e.g. cacti
- Reduces SA of which water can be lost
Closing stomata when transpiration rate are very high:
- E.g. Cacti can close their stomata during the hottest part of the day ∴ reducing water loss
Summarise the adaptations of xerophytic plants:
What is the mechanism and give examples of each
- Deep root system
- Shallow root system
- Reduction in transpiration rate
- Storage of water
- Resistance to wilting
State the 2 different types of cells in the phloem
Sieve tubes
Companion cells
Phloem:
Describe the structure + function of sieve tube elements (6)
- Elongated cells joined end to end to form long tubes
- The cells are LIVING and retain a thin layer of cytoplasm, which lies next to the cellulose cell walls.
- Cytoplasm contains mitochondria but no nucleus, golgi apparatus or ribosomes
- The leaves the bulk of the cells relatively hollow ∴ reduces resistance to the flow of liquids in them
- End walls are perforated by large pores
- Perforated end walls = sieve plates
- Pores are lined with a carbohydrate (callose)
- If the sieve plate is damaged, callose seals the pores preventing the loss of valuable organic material
- Central space within the sieve tube = lumen
Phloem:
Describe the structure + function of companion cells (4)
- Closely associated wuth sieve tube elements
- Sieve tube elements lack organelles (nucleus, golgi apparatus and ribosomes) they’re unable to carry out many vital metabollic processes essential for their survival
- Companion cells carry out these activities for both themselves and the sieve tube elements
- Many plasmodesmata occur in the cell walls seperating the companion cells and sieve tube elements
- Very folded cell walls and CSM at the tips of leaves = Transfer cells
- Thought to actively transport sucrose into sieve tube elements
Translocation - Pressure flow hypothesis:
Describe the sequence of events that take place during phase 1 -Transfer of sucrose into sieve elements from photosynthesising tissue (7)
- Photosynthesis in the chloroplpasts of mesophyll cells produce triose sugars, some are converted into sucrose
- Sucrose in solution then moves from mesophyll cells to the phloem (apoplast or symplast route)
-
Sucrose is loaded into companion cell by active transport
- H+ actively transported out of companion cell into surrounding tissue
- Creates high conc of H+ outside companion cell
- H+ diffuse back into companion cell down a diffusion grad, passes through a co-transporter protein against conc grad - must bring back a sucrose as well
- Sucrose molecules can then move from companion cell → sieve element via the many plasmodesmata connecting them
Translocation - Pressure flow hypothesis:
Describe the sequence of events that take place during phase 2 - Mass flow of sucross through the sieve tube elements
- Sucrose that’s entered sieve tube elements cause them to have a more negative ψ
- Water moves into them by osmosis from xylem as xylem’s ψ is now less negative than ψ in sieve tube elements
-
AT SINK
- Sucrose is removed from sieve tube elements and used in repiration, stored as starch or converted into other organic compounds
- Sieve tube elements will have low sucrose content ∴ a less negative ψ than surrounding cells ∴ water leaves by osmosis
- As a result of water entering the sieve trube elements at the source and leaving them at the sink, a hydrostatic pressure gradient is created leading to the mass flow of the sucrose solution along phloem
Evidence that translocation of organic molecules occurs in phloem (5)
-
Radioactive CO2 is incororated into sugars during photosynthesis
- Radioactivity appears in the phloem
- When phloem is cut, a solution of organic molecules exude
-
Removal of a ring of a tree to a certain depth leads to removal of phloem but not xylem
- If done during the summer, sugars collect above the ring
- If done in the spring sugar collects below the ring
-
Aphids feed on plant sugars
- The sharp piercing mouthparts of the aphid are able to penetrate the phloem
- Poisoning mitochondria stops translocation
- Movement in the phloem is too fast to be explained by diffusion
