Transport in plants Flashcards
Transport systems in plants
-water and mineral ions: xylem- up only
-assimilates e.g. sucrose- phloem- up and down
Transport systems needed to:
-Meet metabolic demands:
Only areas with photosynthetic pigments can produce glucose
All cells need supply of O2 and glucose
All cells need to remove waste products of metabolism
Hormones need to be transported to site of use
Mineral ions need to be transported to cells for protein and nucleic acid production
-Cope with SA:V
Whole plants have a small SA:V ratio & too small to meet demand by simple diffusion (move substances up at quick rate)
Roles of water within a plant
-Photosynthesis
-Tugor pressure: provide hydrostatic skeleton to support stems & leaves, causes cell expansion- force enables roots to force way through the soil
-Main component of cytosol
-Main component of cell sap
-Enables mineral ions and assimilates to be transported to where they are needed
Xylem
-Made of hollow dead xylem vessels supported by living parenchyma cells
- Dissolved mineral ions and water up only
-No end walls, uninterrupted tube
-Walls thickened with lignin- more lignified as they age
-Cells linked by pits- cellulose but no lignin- allows lateral movements of water and mineral ions
Phloem
Consist of sieve tubes and companion cells
Phloem sieve tube element (PSTE)
-Living, narrow, connected end to end to form tubes
-End walls known as sieve plates- perforated to form sieve pores- allow solutes to mass through unimpeded
-Lack nucleus, RER, central vacuole, Golgi- reduces resistance to flow of phloem sap
-Connected to companion cells by plasmodesmata between sieve plates
Companion cell
-Living, carry out all living functions required by CC and PSTE e.g. ATP for active transport, so many mitochondria
-1 CC per PSTE
-Very metabolically active
Monocotyledons (eg. cereals and grasses)
-Stem:
Vascular bundles distributed evenly across all stem. Phloem outside, cambium middle, xylem inside
-Roots:
vascular tissue is at the centre arranged as a circle of xylem and phloem. pericycle around it. Endodermis around circle
-Leaves:
Veins run parallel along length of leaves. Xylem small circle, phloem around
1 type of mesophyll.
Vascular bundles
Where the xylem and phloem are. On the outside of them is the cortex, and the pith on the inside
Dicotyledons (broad-leaved crops e.g. carrots, potatoes)
-Stem:
Vascular bundles are in a ring just of circles below epidermis. Phloem on the outside, xylem on the inside
-Roots:
Vascular bundles occur in central Steele. X is xylem. Between arms is phloem. Pericycle a single layer of cells below endodermis.
-Leaves. Xylem upper, phloem lower. 2 types of mesophyll
waxy cuticle function
reduces water vapour loss from the leaf surface
Transpiration definition
-the loss of water vapour from the underside of the leaf via the open stomata
(water enters cells through osmosis in the root hair cells)
Transpiration stream
-the movement of water from roots to leaves via mass flow and transpiration. Also known as cohesion-tension theory
Mechanisms involved in the transpiration stream
-Cohesion: water molecules are attracted to each other via hydrogen bonds
-Adhesion: water molecules are attracted to walls of xylem which aids mass flow of water up xylem
-Tension: the evaporation of h2o from PMC to mesophyll air spaces generates a suction that draws more h2o into leaf
- Transpiration at the leaves
-water evaporates from cellulose cell walls of MC to the mesophyll air spaces
-Water vapour then diffuses from mesophyll air spaces to the sub-stomatal air spaces
-Water vapour then diffuses down the water potential gradient through the open stomata to surrounding external air
-This loss of water vapour is called transpiration
- Movement of water up the xylem
-xylem vessels transport water up the plant stem from roots to leaves in a continuous column.
-water is drawn up the xylem by capillary action (cohesion forces) due to hydrogen bonds between adjacent water molecules (polar+dipoles)
-at leaves, water leaves xylem by the apoplast pathway
-Adhesion of h2o molecules to xylem walls also aids in resisting gravity
- Water moves from root hairs to xylem tissue
-Different pathways exist for the water to move from RHC to xylem tissues
transmembrane route, apoplast route, symplastic route
Transmembrane route
via water channels
Apoplast route
Via cellulose cell walls
Walls v absorbent so water diffuses along cellulose fibres & osmosis through spaces between fibres
Most water travels this way- little resistance
NB: when water reaches the endodermis, pathway is blocked by Casparian strip (band of cells that have waxy Suberin layer in cell walls) So water has to cross via symplast pathway
Symplastic route
Water moves through protoplasm via plasmodesmata
Water uptake at RHC
-Water taken up from soil by RHC via osmosis
Adapted for role:
-Extended cytoplasm and cell wall to form protrusions (inc SA)
-Thin cell wall (water by osmosis, mineral ions by AT)
-High mitochondria (for AT)
-Absence of chloroplasts
Temperature on transpiration
Higher temp, h2o molecules have more kinetic enegy so greater evaporation from MC to air spaces. Higher diffusion gradient between inside of leaf to external air, so more diffusion from sub-stomatal air to outside
Also, higher temp= higher conc of water vapour can be held in the air, so lower humidity and water potential, which maintains diffusion gradient
light intensity on transpiration
higher light intensity, greater number of stomata open to obtain co2 for photosynthesis, so faster evaporation of water vapour through stomata
humidity on transpiration
lower humidity, external air is drier, greater conc gradient, increases rate of transpiration
wind speed on transpiration
wind moves water molecules away from stomata opening, greater conc gradient, faster rate of evaporation
number of leaves on transpiration
larger SA, more stomata, greater transpiration rate
cuticle on transpiration
thicker- less transpiration
stomata on transpiration
greater density or wider diameter of stomatal pore, more SA, so higher rate
Guard cells
stomata open= turgid
stomata closed= flaccid
How do stomata cells open and close
Guard cells (GC) control + Plant growth regulator (Absisic acid)
Turgid= stoma open
Flaccid= stoma closed
- Aba > specific Ca+ ions open
- Ca+ flood into guard cell
- ^Ca+ conc in cytosol= activates proton pumps.
- Influx of protons= PMF
- PMF opens K+ channels in membrane > they leave via FD.
- WP inc in cytosol
- H2O leaves GC via O, down WP grad.
- GC flaccid = stoma closes
Lenticels in bark of trunks and stems
-Pores that provide direct pathway for gas exchange between interior and external atmosphere
-Necessary as stems become woody and develop bark which is impermeable to gases
Rate of transpiration practical:
Procedure
Bubble moving
-Cut shoot underwater- to prevent air lock from forming on xylem
-Cut at 45 angle- increase SA for water uptake
-Connect potometer to shoot underwater
-Ensure apparatus is water tight e.g. seal with vaseline
-Dry leaves
-Record starting position of the bubble and record distance
Rate of transpiration practical:
Limitations
Assume all water movement is due to loss of evaporation of water vapour from stomata
however:
-water used in photosynthesis
-water produced in respiration
-water used in condensation reactions
etc
Rate of transpiration practical:
Fair test
Ensure all other variables are controlled: humidity, light intensity, temp, wind which are not being investigated
Rate of transpiration practical:
Errors
-Error when reading meniscus
-Leaves not being fully dried properly
-Not resetting air bubble to correct position
Translocation definition
The movement of dissolved organic solutes through the phloem around the plant from sources to sinks. This is an active process
Molecules that are transported by translocation
Sucrose, amino acids
Sources
-Site of photosynthesis where sucrose is made (glucose +fructose)
-green leaves, storage organs
Sinks
-site of assimilate storage
-Main sinks:
roots that are growing, meristems that are actively dividing, developing seeds or fruits etc
Phloem loading
a) Symplast route
-Sucrose moves from source through cytoplasm via plasmodesmata into sieve tubes
-This route is passive
-Lowers WP inside, so water moves in by osmosis
-Generates hydrostatic pressure that moves sucrose through phloem
-mass flow hypothesis
Phloem loading
b) uses both passive and active mechanisms
-Protons actively pumped out of companion cells into cell wall
-Using ATP
-Proton gradient builds up
-Protons diffuse back into companion cells via co-protein which also carries sucrose through facilitated diffusion
-Sucorse moved into cc agains its conc gradiet
-Sucrose enter STE by diffusion via plasmodesmata.
Phloem unloading
- Sucrose diffuses from phloem into surrounding tissues (sucrose also converted into glucose and starch)
-Loss of solute raises WP in STE
-So water exits by osmosis and some is drawn into xylem and enters transpiration stream
-so WPG between STE and xylem vessels recirculates solution
Source loading increases hydrostatic pressure, sink unloading reduces hydrostatic pressure
photosynthesis equation
water + co2-> glucose and O2
respiration equation
glucose + oxygen -> carbon dioxide + water + ATP
