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Plant Transport
Stem Support
1. Ground tissues: living cells that make up the bulk of stem / root of non-woody (herbaceous) plants
Parenchyma  support of herbaceous stem through turgidity
 sites of starch storage
Collenchyma additional layers of cellulose thickening laid down unevenly
[ Parenchyma : air spaces between living cells ; Collenchyma : thick cellulose wall at corners, thin cellulose
elsewhere ]
2. Xylem tissue: cellulose hardening with lignin ( transport of water from root to shoot only, transpiration
pull)
cambium
collen
phloem
epidermis
xylem
paren
Vascular bundles
pith
Transport of water
-
Begin : elongated cells with cellulose walls & living contents connected
End : dissolved away to become long, hollow tubes (mature)
Because deposit of cellulose thickening to
inside of lateral walls of vessel + hardening
with lignin
-
Tough tissues & strengthens itself internally  X collapse in itself
(spirally thickened)
-
Centrally placed vascular tissue is contained by endodermis, layer of cells unique to the root
At endodermis, waxy strip in radial walls blocks passage of water by apoplast route momentarily
(CASPARIAN STRIP) water passes endodermis through osmosis
At leaves, transpiration occurs ( evaporation of water vapour through stomata of green plant leaves &
stems)
-
Water uptake (roots)
- Transport : nutrients, oxygen, elaborated foods, plant growth substances
1. Mass flow (most significant) : through interconnecting free spaces between the cellulose fibres of plant
cell walls. The pathway APOPLAST entirely avoids living contents of cells, hence including dead cells (e.g.
xylem vessels)
2. Diffusion : through cytoplasm of cells and via the cytoplasmic connections between cells (plasmodesmata),
a.k.a. SYMPLAST. (less significant because plant cells have organelles  resistance)
3. Osmosis : from vacuole to vacuole of cells, driven by gradient in osmotic pressure ( recall definition for
osmosis). A means for individual cells to absorb water.
Roots absorb water from soil by osmosis whereas absorption of minerals is mainly
achieved by active transport. The absorbed water and minerals are transported
through xylem vessels.
Process of absorption of water
1. Concentration of solute in root
hair cells is higher than soil
water  water potential of root
hair cell < soil water
2. Water enter root hair cells by
osmosis
3. Water potential in root hair
cells > adjacent cells, water
movement through osmosis
and so on (due to gradient in of
water potential)
4. Water enters xylem vessels in
Distribution of tissues in different parts of
dicotyledonous plants
Leaf
Upper epidermis
xylem
Lower
epidermis
cambium
phloem
phloem
Ste
m
epidermis
pith
phloem
cortex
xylem
the middle of the root
cambium
epidermis
Root
phloem
xylem
Guard cells :
- Contain chloroplasts that carry out photosynthesis in the presence of light.
- Glucose is formed and thus lower the water potential of the cell
- Water enters the guard cells from adjacent cells by osmosis, guard cells  turgid
-
Inner wall is thicker than outer wall, cell stretches to outer side  stoma opens
At night, no photosynthesis, guard cells become flaccid  stoma closes
Xerophytes ( plants in dry envt. )  reduce water loss by evaporation
- Numerous epidermal hair  trap moistureRoot
- Sunken stomata  trap moisture
- Rolling leaves enclosing stomata  reduce contact between stomata and envt
- Spiny leaves  reduce surface area
- Fleshy stems / leaves  help store water
- Shed leaves during dry seasons  reduce rate of transpiration
Ion uptake (ions in soil solution are at low conc.)
Active transport: 1. Tranport of molecules against concentration gradient (from low to high)  hoarding of
valuable ions in cytoplasm
2. Selective process :
(reflects needs of plants)
3. Involves pumps : globular protein carriers that traverse the lipid bilayer. Requre energy
from reaction with ATP to change shape of such carriers ( pumping action)  specific &
“active”
* Supply for ATP energy maintained through aerobic cell respiration by roots
Adequate supply of ions
1. Mass flow of water: deliver fresh soil solution
2. Active uptake of ions through apoplast : maintains concentration gradient, ions diffuse outside apoplast
adjacent to protein pumps
3. Mutual relationship with soil-inhabiting fungi:
a) fungal hyphae:
receive supply of sugar from plant root  excessive sugar
b) plant roots:
root cell ions taken up when they were available in soil
Uptake of minerals
-
Mineral uptake is
achieved by active
transport which requires
energy  because
concentration of minerals
in the cytoplasm of root
hair cell is lower than that
in the soil water
(transport against
concentration gradient)
\ Transpiration pull / stream : ( losing water from leaf through stomata)
- An evaporation of water in the form of water vapour from plant body surface into the atmosphere
* Transpiration pull  process
;
transpiration stream  column of water
-
A thin film of water covers each mesophyll cell
Water evaporates from the film of water surrounding the mesophyll cell into the air spaces
Higher water vapour concentration inside the leaf than the atmosphere, water vapour diffuse out through
the stomata
Water potential within the cell will decrease  draw water from adjacent cells by osmosis
Water is drawn from xylem vessels due to the water potential difference
-
As water if drawn from xylem vessels, a suction pressure will develop and this pressure will pull water up
(transpiration stream)
* A water potential difference set up between mesophyll cells and the xylem vessels due to continuous
water loss resulted in a suction pressure. This helps to pull water up the xylem vessels from the root is
known as transpiration pull.
The process is responsible for transport of water and minerals.
* Hydrostatic pressure is pressure exerted by a fluid at equilibrium due to force of gravity.
Phloem:
-
-
Solutes flow through phloem from region of high hydrostatic pressure ( in & around photosynthesizing
cells & phloem sieve tubes nearby) to region of low hydrostatic pressure ( ground tissue cells of stems /
root / phloem sieve tubes nearby)
Source area: presence of sugar
- Sink area: starch storage cells
* presence of sugar raises osmotic pressure  water flows in (vice versa)
Xylem
Phloem
Contains dead cells arranged from end-to-end
Living cells arranged end-to-end
Hollow tubes (no cytoplasm)
End of each cells is formed by a cross-wall of
cellulose with holes, i.e. sieve plates
cell walls of cells derive most strength from lignin
Retain cytoplasm and extend through holes of sieve
plates to form sieve tubes (NO nucleus)
Provide support to plant
Companion cells help regulate metabolic activites of
sieve tube elements
Transport water and minerals (one direction
because of transpiration pull)
Transport of organic substances ( Translocation)
Factors affecting transpiration
1. Light intensity
 temperature
 diffusion & evaporation
 rate of transpiration
2. Temperature
 diffusion & evaporation
 rate of transpiration
3. Relative humidity
 diffusion rate
 rate of transpiration
4. Wind speed
 diffusion & evaporation
 rate of transpiration
5. Availability of water
 stomata closes, plants wilt  rate of transpiration