7.0 Transport in Plants Flashcards
Transverse section of stem (dicotyledonous plant)
Transverse section of root (dicotyledonous plant)
Transverse section of leaf (dicotyledonous plant)
Vascular Bundle
XYLEM:
- transports water and mineral ions
- from root to other parts of the plant
PHLOEM:
- transports organic substances from source to sink
Dicotyledonous plants
Dicotyledonous plants: typically have leaves with blades and stalks
Pericycle (ground tissue)
ground tissue includes all tissue except vascular bundles and epidermis
PERICYCLE:
- a layer of cells, one to several cells thick
- inside the endodermis and next to the vascular tissue
IN ROOTS:
- one cell thick
- new roots can grow from this layer
IN STEMS:
- formed from sclerechyma cells
- has dead lignified cells for extra strength
SCLERENCHYMA FIBRES:
- long, dead, empty cells with lignified walls only a mechanical function
- do not transport water
PARENCHYMA:
* made up of thin-walled cells
* metabolically active
* storage of foods like starch.
* support the plant: preventing wilting
* air spaces: allow gas exchange.
* water and mineral salts are transported
through
* Parenchyma forms the cortex in roots and
stems, and the pith in stems.
* The cortex is an outer region of cells.
* The pith is made up of similar cells but is
the name given to the central region of
stems.
* Parenchyma contains chloroplasts in leaves,
where it is modified to form the palisade and
spongy mesophyll.
Epidermis
EPIDERMIS:
* One cell thick
* In stems and leaves: waxy cuticle
* waterproof
* protection
* In leaves: stomata
* allow entry of carbon dioxide for
photosynthesis.
* In roots: root hairs
* increase the surface area for
absorption of water and mineral
salts.
Mesophyll
- made up of specialised parenchyma
cells - contains chloroplast
- specialised for photosynthesis
- two types
- palisade mesophyll
- spongy mesophyll
Endodermis
- one cell thick
- surrounds the vascular tissue
in stems and roots
Xylem: vessels/elements
XYLEM
STRUCTURE:
➤ Cell wall have lignin bands, in addition to cellulose
➤ lignin = strong, hard, waterproof substance
➤ Bands have different patterns (e.g. rings, spiral, reticulated)
➤ Thicker cell wall observed
FUNCTION:
➤ Structural support
➤ Transport of water form root to leaves to atmosphere
XYLEM ELEMENT/VESSEL
STRUCTURE RELATED TO FUNCTION:
- thick cell walls made of cellulose: structual support, allows adhesion of water
- cell wall contains lignin: prevents inward collapse as xylem vessels is under tension, waterproof to prevent water loss
- made of elongated cells joined end to end
- non-living
- no cytoplasm, no organelles, hollow lumen: more space for greater volume of water to flow, less resistance towards water flow
- no end-walls: less resistance to flow of water, forms a continuous tube joined end to end
- large lumen: large volume of water can be transported
- pits: famed form plasmodesmata, no lignin, allows lateral movement of water, to connect to all parts of plants, If there is an air bubble locking vessel, pits allow water to move out into another xylem vessel and bypass airlock
Phloem / phloem seive tube elements
PHLOEM:
FUNCTION:
➤ Transport of assimilates (e.g. sucrose, amino acids)
➤ From source = site of synthesis of photosynthetic products
➤ To sink = site where assimilates are stored /used
➤ Via translocation
PHLOEM SEIVE TUBE ELEMENTS:
- elongated seive elements joined end to end to form continuous column
- seive elements are living cells
STRUCTURE RELATED TO FUNCTION:
- have many plasmodesmata: allows loading of sucrose from companion cells, for rapid entry of water from source
- strong cellulose walls: prevents excessive cell bulging under water
- have few organelles: has cellulose cell wall, plasma membrane, mitochondria, few ER, no nucleus, ribosomes or vacoules
- has peripheral cytoplasm that lines cell wall: less resistant to flow, maximum volume of of phloem sap containing assimilates transported
- has seive plates = perforated cell wall, has many seive pores, cytoplasm of cells connected:
➤ Reduce barrier to flow
➤ Prevent cell bulging under pressure for phloem
➤ Sieve pores become plugged with callose to prevent loss of phloem sap after damage
Companion cells
STRUCTURE related to FUNCTION:
- next to and closely associated with seive tube elements
- many mitochondria: for ATP production via aerobic respiration, for active transport in translocation
- many ribosomes & rER : for polypeptide production
- numerous plasmodesmata across cell wall
Transport in Plants
Main substances transported:
- water
- gases (O2, CO2): using simple diffusion, leaves are thin and flat, have branching shape & network of air spaces, high surface area to volume ratio, effective for diffusion
- products of photosynthesis (sucrose)
- mineral ions
Why do plants have slower transport than animals:
- lower requirement for oxygen & glucose
- lower energy requirements
- lower rate of respiration
Transport of water and mineral ions
- similar pathway
- from roots upwards
PROCESS:
1. uptake of water at roots through osmosis into root hairs: Symplast (plasmodesmata) and Apoplast pathway (Casparian strip)
2. root cortex into xylem
3. movement of substances through xylem: transpiration pull, cohesion and adhesion of water, root pressure (hydrostatic pressure)
4. xylem into mesophyll cell surface (leaves): through Symplastic and Apoplastic pathway
5. water evaporates into gas and travels through mesophyll layer
6. diffuses through stomata
Symplast and Apoplast in Xylem across leaf
SYMPLASTIC:
- water enters through cell surface membrane by osmosis
- diffuses through cytoplasm of cells and plasmodesmata
APOPLASTIC:
- interconnected intracellular spaces that occur between cellulose fibres in the plant cell wall
- this route entirely avoids living contents of cells
- when Casparian strip is reached, water is forced to merge with the contents in symplast pathway, contains suberin which is insoluble
NOTE:
- during great mass of water transport, apoplastic pathway is used more often as cell wall to cell wall diffusion is more efficient, no involvment of plasma membrane
Mechanisms of water in xylem
TRANSPIRATION PULL:
- water vapor diffuses out via stomata
- water evaporates from mesophyll cell wall surface and lowers water potential at leaves
- roots have a higher water potential than leaves
- water moves down water potential gradient
- water moves up xylem from roots to leaves
- transpiration from leaves creates transpiration pull
COHESION & ADHESION:
- tension is present in xyelm
- H bonds between water molecules (cohesion)
- H bonds between water molecule and cell wall of xylem vessels (adhesion)
- both result in the ability of the fluid to move upwards against gravity in narrow spaces
RESULT of cohesion & adhesion:
- creates continuous column of water (extending from root hairs to stomata in leaves)
- Mass flow: the movement of water up through xylem vessels
ROOT PRESSURE:
- casparian strip at the endodermis blocks apoplast pathway: water ions must pass through endodermal cell, solutes are actively pumped across membranes into xylem vessels in root, active transport (ATP) required
- xylem vessel in root increases in solute conc.: lowers water potential, results in more water uptake from soil, increases hydrostatic pressure at roots
Transpiration
- loss of water vapor from leaves
- VIA STOMATA:
- diffusion of water vapor form airspace to atmosphere
- only occurs when stomata is open
- for gas exchange
- entry of CO2 for photosynthesis and exit of O2
2.VIA CUTICLE:
- loss of water vapor through cuticle on leaf surface
- very small amount of water loss
Rate of Transpiration
HUMIDITY:
- at low humidity: steeper water potential gradient
- higher rate of diffusion of water vapor out of stomata
WIND SPEED (air movement):
- in moving air, water vapor around leaf is more likely to be blown away
- steeper water potential gradient
- high rate of diffusion of water vapor
WATER AVAILABILITY:
- more water available, steeper water potential gradient
- reduced water availability causes stomata to close
TEMPERATURE:
- rise in temp, higher kinetic energy
- hgiher rate of evaporation from surface of spongy mesophyll cells
- but at very high temp, stomata closes to reduce water loss so transpiration rate decreases
LIGHT INTENSITY:
- at high light intensity, increased rate of transpiration as stomata opens wider due to increased photosynthesis
- at very high light intensity, stomata closes to prevent loss of water, transpiration rate decreases
STOMATAL APERTURE:
- increased width of stomatal aperture allows more water vapor to diffuse out of cell
Xerophytes
- plants that live in places where water is finite/short in supply
- therefore evolved special adaptations at their leaves to minimize water loss
ADAPTATIONS TO REDUCE WATER LOSS:
- Rolled leaf: increases humidity around stomata, reduces water potential gradient
- Thick waxy cuticle: increases instance for diffusion, acts as a barrier for transpiration, shiny surface reflects heat, lowers temperature
- Hairs/trichomes on surface: traps moisture to reduce water potential gradient
- Sunken stomata/stomata in pits: moist air trapped in pits reduces water loss
- No stomata on upper surface: not exposed to sunlight, reduces evaporation rate
- small leaves, reduced to spines: reduce surface area for transpiration
ADAPTATIONS TO GAIN MORE H2O:
- Swollen stems: stores water, succulent cardon stems (cacti)
- flattened, photosynthetic cactus stems
- deep and extensive roots
- long white hairlike bristles along the stem: helps reflect sunlight
Translocation
MESOPHYLL CELL (SOURCE):
- soluble, organic substances made by plant via photosynthesis
- glucose produced, converted to sucrose
- sucrose diffuses through mesophyll cells towards phloem via symplast and apoplast pathway
- passive process
COMPANION CELLS (LOADING OF SUCROSE):
- ATP required
- H+ ions in companion cells are pumped out via proton pump (membrane protein) into mesophyll cell wall/ intracellular space
- H+ ion gradient builds up
- the co-transporter goes through conformational change to transport H+ ions back into companion cell down the H+ conc. gradient
- while sucrose is transported together into companion cell against conc. gradient
- this is facillitated diffusion of H+ ions
- sucrose transported via secondary active transport
SEIVE TUBE (TRANSLOCATION):
- transport of assimilates within plants
- transported as phloem sap in phloem tissue
- sucrose diffuses from companion cell into seive tube
- down the conc. gradient
- via plasmodesmata
- passive movement
NEAR SOURCE:
- presence of sucrose lowers water potential in seive tube element
- water enters seive tubes from xylem via osmosis down water potential gradient
- increases the hydrostatic pressure in seive tube
NEAR SINK:
- hydrostatic pressure is lowering
- due to diffusion of sucrose
- phloem sap containing sucrose moves from region of high hydrostatic pressure to low hydrostatic pressure towards sink
- down hydrostatic gradient
- results in mass flow : movement of fluid down a hydrostatic pressure gradient
- water in sieve tubes near sink has higher hydrostatic pressure than xylem vessels
- water moves back to xylem vessel via osmosis
- down the hydrostatic pressure gradient
TARGET SINK CELLS (UNLOADING OF SUCROSE):
- sucrose converted into glucose, fructose, starch
- catalyzed by enzymes
- used for respiration, growth or storage
- decreases the conc. of sucrose
- maintains conc. gradient for stability
