9.1 transport in dicotyledonous plants Flashcards
the need for transport systems in multicellular plants x3
- metabolic demand
- size
- surface area and volume
plants need transport systems due to metabolic demands
Photosynthesis 🡪 glucose and oxygen
Glucose is then respired to produce ATP.
ATP needed by all cells for active transport, chemical reactions, cell division.
but
Internal and underground parts of plants don’t photosynthesise (no light)
Oxygen and glucose, hormones, proteins and ions transported around the plant to where they are needed.
plants need transport systems due to their size beacuse
Several layers of cells.
Only outermost cells get to use substances diffusing in.
Transport systems move substances to internal cells.
plants need a transport system because of their SA:V ratio
Leaves are adapted to have high SA:V
but
When considering stems, trunks and roots, they have small SA:V and cannot rely on diffusion alone
Vascular tissue/bundle
Made up of xylem and phloem tissue.
Each makes up a different transport system.
Distributed differently in different areas of the plant.
Transpiration System
Function?
medium?
passiv/active?
- The movement of water molecules and dissolved minerals ions
- Xylem vessels
- Passive - does not req metabolic energy
Translocation System
role?
transport medium?
active/passive?
The movement of sugars (Sucrose) & amino acids
Phloem vessel – sieve & companion cells
Active - reqs metabolic energy
Draw and compare cross section of vascular bundle in root,stem,leaf
root - +shape of xylem and triangular phloem and root hairs
stem- ploem on outside and xylem on inner seperated by the cambium
leaf - ploem below xylem on top
The phLOem sits beLOw the xylem tissue in the Leaf.
Xylem structure and function
Mostly non-living tissue
Functions
- transport water and mineral ions
- support
Several different cell types
**Vessel elements **
Joined end to end in several columns
No end walls, no cytoplasm
Cell walls thickened by woody substance called lignin (organic polymer)
Amount of lignin increases as plant get older
Water and mineral ions move in/out through non-lignified pits
Xylem parenchyma
Thick walled
Packed around vessel elements
Store of food
Stores tannin – protection against attack from herbivores
Distinct patterns of lignin spirals around the lumen
– mechanical strength
- allows flexibility
- prevents breaking and collapsing
phloem structure and function
Living tissue
Functions
* transport organic solutes (assimilates) around plant from the leaves where they are a product of photosynthesis
* supplies non-photosynthetic cells with sugars (for respiration) and amino acids (for synthesis of nitrogen containing compounds)
* Formed from cells arranged in tubes
* Not used for support
- Sieve tube elements
-Companion cells
Sieve tube elements
Living cells
Joined end to end
Sieve parts are the end walls with hole to allow solutes to pass through
No nucleus
Very thin layer of cytoplasm
Few organelles
The cytoplasm of adjacent cells is connected through holes in the sieve plates
Companion cells
The lack of nucleus in the sieve tube elements mean they cannot survive on their own.
Companion cells carry out ‘living functions’ for themselves and their sieve cells.
E.g. provide energy for active transport
plants vascular bundles we learn about are called
herbaceous dicotyledonous plants.
flowering and seeding plants witout bark
The cambium layer contains
meristem cells.
explanation to roots structure
As plants grow, their roots ‘push’ through soil.
Xylem tissues is the strongest so is in the centre – X structure
Phloem in four separate sections.
Stain
Toluidine blue O (TBO)
Lignin
blue/green
Phloem and rest of tissue varying shades of pink
which stain
Lignin
blue/green
Phloem and rest of tissue varying shades of pink
Toluidine blue O (TBO)
Transpiration
The term given to the movement through and loss of water from plants.
transpiration mechanism
- Water evaporates to become water vapour.
- Water vapour diffuses out the stomata.
- Water potential of the air space decreases.
- Water osmotes from the adjacent cells into the air spaces.
- Water osmotes out of the xylem into the cells of the leaf.
- Water molecules ‘hydrogen bonds’ to itself (cohesion) resulting in tension.
- Water molecules bond to the walls of the xylem vessel (adhesion) resulting in capillary action.
- Water loss from the leaves causes transpiration.
- Thus, the rate of water loss determines the rate of transpiration.
- Most water is lost from the underside of a leaf, where there are stomata.
- Most plants control their water intake by opening and closing their stomata.
- Water levels change in the guard cells around each stoma.
- Water levels change either passively by osmosis, or by active transport of solutes.
- Transpiration rates also vary naturally in response to environmental factors such as temperature and humidity.
how do plants Reduce water loss
Wilting
* Loss of turgor pressure causes cells to be flaccid. The plant will wilt.
* The leaves collapse and hang down. This reduces the surface area for water to be lost from.
* Wilting gives protection against further water loss.
Stomata close
* This prevents water loss by diffusion but stops photosynthesis.
Why does water move? x3
Osmosis: water moves from areas of high water concentration to areas of low water concentration. KE of molecules.
Mass flow: water also moves from areas of high hydrostatic pressure to areas of low hydrostatic pressure.
It is also affected by gravity and electrostatic forces, such as those that cause surface tension.
Evidence to support Cohesion-tension Theory
Changes in tree diameter – at high transpiration rates (e.g. warmer temps) tree diameter decreases. Higher rates of evaporation from the leaf cause extra tension. At night, during low transpiration rate, diameter increases.
**
Cut flowers** – often they draw air in rather than leaking water out, as water continues to move up the cut stem.
Broken xylems – broken or cut xylems stops drawing up water as the air drawn in breaks the transpiration stream – cohesion between water molecules.
time of day rate of water flow and tree trunk diameter graph analysis
D1: decrease in diameter
E1:
increase in rate of diffusion from leaf
causes more tension in the water column.
water moves up the xylem faster
creating lower pressure.
This draws the sides of the xylem inwards,
reducing diameter(and overall diameter of the trunk).
D2: increase in diameter
E2: decrease in rate of diffusion from leaf
causes less tension in the water column.
Water moves up the xylem slower.
The sides of the xylem are not drawn inwards, increasing diameter (and overall diameter of the trunk).
Capillary action
Capillary action occurs when the water molecule adhere to the walls of the xylem vessel.
Conclusion: The narrower the tube, the higher a liquid can settle.
This is a plausible explanation for plants having many narrow xylem vessels, rather than fewer wide xylem vessels.
Narrower xylem vessels offer a survival advantage. They are selected for by Natural Selection.
Root pressure
Root hair cells actively pump ions from the soil into the cells.
This creates a water potential gradient between soil and cytoplasm.
Water then osmotes from the soil to the cell.
This increases the hydrostatic pressure from below.
Water is ‘pushed’ up the xylem.
Evidence for Active Transport in Root Pressure x4
The effect of cyanide – Cyanide stops mitochondria from working.
The effect of changing temperature – respiration is an enzyme controlled chemical reaction.
The effect of changing reactant availability – oxygen is respiratory substrate.
Guttation – sap and water will move out of cut stems, suggesting they are actively pumped out not drawn up by transpiration.
Transpiration mechanisms
Three processes caused by changes in concentration and attractions pull water up the xylem
Root Pressure & osmosis [Active Process]
Transpiration Pull (Cohesion – tension theory)
Capillary Action (Adhesion)
three routes of water from root hair to xzylem
Symplast
* this is the movement of water through the living spaces of the cell – cytoplasm
* Changing cells through the plasmodesmata.
* Each cell further away from the roots has a lower water potential so water is drawn up the plant
Vacuolar pathway
* is the same as the symplast pathway when the water moves through the cells vacuoles in addition to the cytoplasm
This is the slowest route
Apoplast
* This is the movement of water through the cell wall and intracellular spaces
* Cohesive and tension forces acting on the cell walls pulls the water up the plant
* This is the fastest movement of water
Estimating the rate of transpiration
A potometer actually measures the rate of water uptake by the plant.
If cells are turgid, > 95% of water taken up is lost by transpiration, so this gives a reasonable estimate of transpiration rate.
rate of transpiration formula
rate = change/time taken
Water vapour lost by the leaves is replaced from the water in the capillary tube.
The movement of the meniscus at the end of the water column can be measured.
what do we use to measure transpiration
potometer
how to increase validity of transpiration rate pag
It is important to take certain precautions to ensure the results are valid:
Set it up under water to make sure there are no air bubbles inside the apparatus.
Cut the stem under water to prevent air entering the xylem.
Cut the stem at an angle to provide a large surface area in contact with the water.
Dry the leaves.
Species of xerophyte and description of adaptation
Marram grass - curved leaf
Marram grass - sunken stomata
Marram grass - hairs around the stomata
Cactus - spines instead of leaves
Cactus - succulent stems and ‘leaves’
Cactus - many shallow roots
Marram grass - curved leaf decreases the rate of diffusion becuase….
The curved leaf means the stomata are not exposed to moving air.
The curved leaf traps water diffusing out of the leaf. Decreases gradient between inside and outside leaf.
Reduces rate of diffusion of water out of the leaf and so water uptake.
Marram grass - sunken stomata decreases the rate of diffusion how?
The ‘dip’ means the stomata are not exposed to moving air.
The ‘dip’ traps water diffusing out of the leaf.
Reduces rate of diffusion of water out of the leaf and so water uptake.
Marram grass - hairs around the stomata decreases the rate of diffusion how?
he hairs trap water diffusing out of the leaf.
Reduces rate of diffusion of water out of the leaf and so water uptake.
Cactus - spines instead of leaves
decreases rate of diffusion how
By not having leaves, cacti reduce their surface area to volume ratio.
Reduces rate of diffusion of water out of the leaf and so water uptake.
Cactus - succulent stems and ‘leaves’
decreases rate of diffusion how?
Having succulent stems and leaves increases the pathway for diffusion of water from the plant to the environment.
Reduces rate of diffusion of water out of the leaf and so water uptake.
Cactus - many shallow roots
Can absorb a higher volume of surface water by osmosis.
Hydrophytes
Hydrophytes are plants that live in water.
Sinks
Parts of the plant which require sucrose.
Sucrose in low concentration.
Food storage organs (roots/tubers).
Meristems.
Sources
Parts of the plant where sucrose is made or stored.
Sucrose in high concentration.
Photosynthesising leaves.
Sometimes food storage organs.
relation between sinks and sources
During different growing seasons, sources and sinks can alternate.
The sink will always have the lower concentration of sucrose and the source will always have the higher concentration.
LOADING
H+ ions actively pumped out of the companion cell.
H+ ions diffuse back into the companion cells.
down their concentration gradient, through a co-transporter protein, with sucrose.
Sucrose molecules diffuse into the phloem.
Concentration of sucrose increases in the phloem, decreasing the water potential.
Water osmotes into the phloem from the xylem.
Water and sucrose diffuse into the sieve tube through plasmodesmata (passive).
UNLOADING
Cells of the sink convert sucrose to glucose to respire or convert sucrose to starch to store.
This maintains a concentration gradient for sucrose.
Sucrose diffuses out of the phloem into the companion cell, then to the sink cells.
This increases the water potential in the phloem.
Water osmotes back the the xylem.
This reduces the hydrostatic pressure.
MASS FLOW
Water osmotes from the xylem into the sieve tubes. This increases (hydrostatic) pressure near the source.
At the sink, sucrose diffuses out of the phloem. This creates a lower hydrostatic pressure at the sink.
Sucrose solution flows from high hydrostatic pressure to low hydrostatic pressure.
evidence for translocation
Ringing a tree causes sugars to collect above the ring
Aphids plug their piercing mouthparts (stylets) into sieve cells, and the pressure in the phloem pushes its contents into the insect’s gut – sometimes so quickly that it exudes from the aphid’s anus.
The contents of sieve cells are under positive pressure, as is shown by the feeding of aphids.
plants adapted to reduce water loss
Xerophytes
plants adapted to survive in water
Hydrophytes
Hydrophytes adaptations to….
help cope with low O2 levels
- air spaces help them float and store of O2
- Stomata only present on top
- flexible leaves and stem so supported by water and prevents damage from currents
Factors that affect transpiration rate
light
temp
humidity
wind
affect of light on transpiration
light stimulates the opening of the stomata (mechanism). Light also speeds up transpiration by warming the leaf. Plants transpire more rapidly at higher temperatures because water evaporates more rapidly as the temperature rises.
affect of temp on transpiration
higher temp the faster the transpiration rate, warmer water molecules have more energy to evaporate this increases the water potential gradient between inside and outside of leaf making the water diffuse out the leaf faster
