7 Transport in plants
• Flowering plants do not have compact bodies like those
of many animals.
• Leaves & extensive root systems spread out to obtain
the light energy, water, mineral ions & carbon dioxide
that plants gain from their environment to make organic
molecules, such as sugars & amino acids.
• Transport systems in plants move substances from
where they are absorbed or produced to where they are
stored or used.
• Plants do not have systems for transporting oxygen &
carbon dioxide; instead these gases diffuse through air
spaces within stems, roots & leaves.
Learning outcomes
Candidates should be able to:
7.1 Structure of transport
tissues
Plants have two transport
tissues: xylem and phloem.
a) draw and label from prepared slides plan diagrams of
transverse sections of stems, roots and leaves of herbaceous
dicotyledonous plants using an eyepiece graticule to show
tissues in correct proportions (see 1.1c)
b) draw and label from prepared slides the cells in the
different tissues in roots, stems and leaves of herbaceous
dicotyledonous plants using transverse and longitudinal
sections
c) draw and label from prepared slides the structure of
xylem vessel elements, phloem sieve tube elements and
companion cells and be able to recognise these using the light
microscope
d) relate the structure of xylem vessel elements, phloem
sieve tube elements and companion cells to their functions
TRANSPORT IN PLANTS
– move substances from absorption site to required site
– move substances from production site to metabolism site
– move substances to storage site
Xylem
– carries water & mineral salts
– transports from roots to rest of plant
Phloem
– carries products of photosynthesis
– transports from leaves to rest of plant
STRUCTURE OF
STEM, ROOTS,
LEAVES
– main organs involved in
transport in plants
– organelles > cells > tissue >
organs > organisms
monocotyledons & dicotyledons
– monocot: long, narrow leaves
– dicot: has leaves with blades &
stalks (petioles), broad leaves
name
structure
function
epidermis
continuous layer on the outside of the plant; one cell thick;
covered with waxy cuticle; in leaves, has pores called stomata;
in roots, may have extension called root hairs
provides protection; cuticle is waterproof and protects
organ from drying/infection; stomata allows entry of CO2;
root hairs increase surface area
parenchyma
made up of thin-walled cells used as packing tissue; has air
spaces between cells; contain chloroplasts in leaves (palisade/
spongy mesophyll)
very metabolically active; for storage of foods like starch;
help support plant; prevent wilting when turgid; air spaces
allow gas exchange; water/mineral salts transported
through walls/loving contents of cells; forms cortex (outer
region) in roots and stems, pith (central region) in stems;
chloroplast for photosynthesis
collenchyma
modified form of parenchyma with extra cellulose deposited at
corners of cell
extra cellulose provides extra strength; midrib of leaves
contain collenchyma
endodermis
one cell thick; surrounds vascular tissue in stems and roots
contains Casparian strip (band of suberin) which forms an
impenetrable barrier to water in walls of endodermis cells
Mesophyll
made up of specialized parenchyma cells found between lower
and upper epidermis of leaf; contain chloroplast;
palisade(column shaped)/spongy (round); spongy has large air
spaces between cells; palisade near upper surface of leaf,
contain more chloroplast than spongy
specialized for photosynthesis; palisade in upper surface to
get more sunlight
Pericycle
layer of cells, one/several layers thick; just inside
lignified cells for extra strength; new roots can grow
endodermis; next to vascular tissue; one-cell thick in roots; from the layer in roots
in stems, forms tissue called sclerenchyma, has dead
lignified cells
vascular tissue
made up of xylem and phloem; both contain more than one
type of cell; in stems, xylem and phloem found as vascular
bundles; the outside of bundles has caps made of
sclerenchyma fibers
sclerenchyma fibers provide extra support for stem
xylem: contains tubes called vessels made from dead cells
called xylem vessel elements; walls of cells are reinforced
with strong waterproof material called lignin; in roots, in
center and X-shaped
xylem: allows long distance transport of water/mineral
salts; provides mechanical support and strength
sclerenchyma fibers are long, dead, empty cells with
lignified walls (similar to xylem); only have mechanical
function and do not transport water (unlike xylem)
sclerenchyma only have mechanical function and do
not transport water (unlike xylem)
phloem: contains tubes called sieve tubes made from living
cells called sieve tube elements
phloem: allow long distance transport of organic
compounds like sucrose
Learning outcomes
Candidates should be able to:
7.2 Transport mechanisms
Movement of xylem sap and phloem
sap is by mass flow. Movement in
the xylem is passive as it is driven by
evaporation from the leaves; plants
use energy to move substances in
the phloem.
Xylem sap moves in one direction
from the roots to the rest of the
plant. The phloem sap in a phloem
sieve tube moves in one direction
from the location where it is made to
the location where it is used or
stored. At any one time phloem sap
can be moving in different directions
in different sieve tubes.
a) explain the movement of water between plant cells, and between them and their
environment, in terms of water potential (see 4.2. No calculations involving water
potential will be set)
b) explain how hydrogen bonding of water molecules is involved with movement in the
xylem by cohesion-tension in transpiration pull and adhesion to cell walls
c) describe the pathways and explain the mechanisms by which water and mineral ions
are transported from soil to xylem and from roots to leaves (include reference to the
symplastic pathway, apoplastic pathway and Casparian strip)
d) define the term transpiration and explain that it is an inevitable consequence of gas
exchange in plants
e) investigate experimentally and explain the factors that affect transpiration rate using
simple potometers, leaf impressions, epidermal peels and grids for determining surface
area
f) make annotated drawings, using prepared slides of cross-sections, to show how
leaves of xerophytic plants are adapted to reduce water loss by transpiration
g) state that assimilates, such as sucrose and amino acids, move between sources (e.g.
leaves and storage organs) and sinks (e.g. buds, flowers, fruits, roots and storage organs)
in phloem sieve tubes
h) explain how sucrose is loaded into phloem sieve tubes by companion cells using
proton pumping and the co-transporter mechanism in their cell surface membranes
i) explain mass flow in phloem sap down a hydrostatic pressure gradient from source to
sink
TRANSPORT OF WATER
1. water uptake near root tips
2. water enters xylem
3. water moves up xylem
4. water moves from xylem to leaf
cells
5. evaporation of water into leaf air
spaces
6. transpiration of water vapor
through open stomata into air
(mainly underside leaf)
TRANSPIRATION
the loss of water vapor from a plant to its environment by
diffusion down a water potential gradient; most transpiration
takes place through stomata in leaves
Water movement through leaf :
1. water vapor diffuses from air space through open stomata
2. water carried away from from the leaf surface by air
movements
3. this reduces water potential inside the leaf
4. water evaporates from mesophyll cell wall into the air space
5. water moves through the mesophyll cell wall or out of the
mesophyll cytoplasm into cell wall
6. water leaves xylem vessel through non-lignified area such as a
pit; may enter the cytoplasm or cell wall of a mesophyll cell
7. water moves up the xylem vessels to replace water lost from
the lead
Factors
affecting
transpiration
1. humidity
– if water potential gradient between in/out of leaf
becomes steeper, rate increase
– low humidity = increase rate
2. wind speed/temperature
– rate increase when wind speed/temperature increase
3. light intensity
– stomata opens during day, closes at night
– increase in light intensity = increase in rate
4. very dry conditions
– plant partially closes stomata
– this prevents turgidity
Xerophytes
• plants that live in places where water is in short
supply; keep water loss minimal
1. curled leaf
2. sunken stomata
3. cuticle contains cutin, a waterproof substance
4. hairs
5. leaves are needles (reduced surface area)
6. swollen succulent stems (to store water)
7. stomata only at underside of leaf
Symplastic pathway
• 1. water enters cytoplasm by
osmosis through partially
permeable membrane
• 2. water moves into sap in
vacuole, through the
tonoplast by osmosis
• 3. water may move from cell
to cell via plasmodesmata
• 4. water may move from cell
to cell via adjacent cell
surface membranes/cell
walls
Apoplastic pathway
1. water enters cell wall
2. water moves through cell
walls
3. water may move from cell
wall to cell wall via intracellular
spaces
4. water may move directly
from cell wall to cell wall
Xylem tissue
1. made from cells joined end to end to form
tubes
2. cells are dead
3. walls of cells are thickened with hard strong
material called lignin
4. functions: support & transport
5. in flowering plants, xylem tissue contains:
– vessel elements & tracheids: cells that
are involved with the transport of ware
– sclerenchyma fibers: elongated, dead,
empty cells with lignified walls to
support plant
– parenchyma cells
Xylem vessels & vessel
elements
– vessels made up of elongated cells (vessel elements)
– vessel element begun as normal cell, but wall lignin laid down
– lignin is hard, strong, impermeable to water; builds up around cell;
contents die to leave hollow lumen inside
– no lignin laid down in plasmodesmata; seen as ‘gaps’ in thick walls of
xylem vessel (pits)
– pits not open pores; crossed by permeable/unthickened cellulose
wall; in one cell link with those in neighboring cells, so water can pass
between
– end walls of neighbor vessel elements break down, forms continuous
tube (xylem vessel)
FROM ROOT TO STEM AND LEAF IN XYLEM
– hydrostatic pressure difference between top and bottom of xylem vessels
cause water to move up (great pressure below, low on top)
– lignified walls strong enough to prevent collapse from pressure
– all the water molecules move up xylem together as a body of liquid (mass
flow)
– cohesion vs. adhesion
1. cohesion: water molecules attached to each other by hydrogen bonding
2. adhesion: water molecules attracted to cellulose/lignin in walls of xylem vessels
– dead cells mean no protoplasm can get in way of transport
– narrow lumen helps prevent air bubbles; ‘air lock’, water cannot move with
bubble
Root pressure
– increase pressure difference by raising water pressure at the base of
vessels
– pressure raised by active secretion of solutes into the water in xylem
vessels in roots
– cells surrounding xylem vessels use energy to pump solutes across
membranes & into xylem by active transport
– presence of solutes lowers water potential of the solution in xylem,
draws water from surrounding root cells
– water transport is passive process, driven by transpiration
FROM ROOT HAIR TO XYLEM (in order)
1. water potential inside root lower than in soil, causes water to move down
gradient and into root through root hair
2. water either moves by apoplastic/symplastic pathway through cortex
3. normally, more water travels by symplast, but at high transpiration rates, more
water travels by apoplastic pathway
4. apoplastic pathway blocked once water reaches endodermis due to thick
waterproof waxy band of suberin in cell walls (Casparian strip; see table above)
5. older endodermal cells have greater suberin deposits, but water can pass freely
through passage cells instead
6. this lets plant control entrance of mineral ions into xylem; helps with generating
root pressure
7. once across endodermis, water moves through pericycle and into xylem through
pits
FROM SOIL INTO ROOT HAIR
• 1. epidermis drawn out into long, thin root hairs to increase surface
area, thus increasing rate of water absorption; reach into spaces
between soil particles and absorb water
• 2. water moves into root hairs by osmosis through membrane, into
cytoplasm and vacuole
• 3. have fungi to assist with water absorption, called mycorrhizas; act
like roots
TRANSPORT OF MINERAL IONS
– mineral ions in solution absorbed along with water by roots
– same as water; apoplastic/symplastic; mass flow
TRANSLOCATION
– can be applied to transport in both xylem and phloem
– moving from one place to another
– transport assimilates (sugars from photosynthesis) to sink for storage
through sieve elements and companion cells
Sieve tubes & sieve
elements (Phloem)
– phloem contains unique tube-like structures
(sieve tubes)
– made of living cells
– sieve tube made up of many sieve elements
joined vertically to form continuous tube
– sieve element has cellulose cell wall, surface
membrane, and cytoplasm with ER,
mitochondria; cytoplasm amount very small and
only forms thin layer inside cell wall; no nucleus
or ribosomes
– sieve plate is where sieve element walls meet;
perforated (visible through light mic.)
Companion cells
• – each sieve element has one companion cell beside it
• – have ‘normal’ plant cell structure
• – very closely associated with sieve element; single functional unit
Contents of phloem sieve tubes
• – liquid inside called phloem sap
• – sieve plates can block itself; prevent escape of sap when cut; then
sealed with carbohydrate called calls (clotting)
How translocation occurs
1. moves by mass flow; requires
ATP (active process)
2. pressure difference caused by
active loading of sucrose from
source to sieve element
3. sends sucrose to sink
4. entrance of sucrose in sieve
elements decreases water
potential, causing osmosis;
builds up more pressure
5. high pressure difference
between source and sink causes
mass flow towards low press.
6. flows up/down/side unlike
xylem (which only moves up)
Loading sucrose into phloem
1. photosynthesis sometimes produce triode sugars which are then
converted to sucrose
2. sucrose then moves from mesophyll cell, across leaf, to phloem
(symplastic/apoplastic)
3. sucrose loaded into companion cell or directly into sieve element by active
transport
4. H+ ions are pumped out of the companion cell into its cell wall, using ATP
5. H+ along with sucrose can then move back into cell through carrier protein
6. sucrose molecules then move from companion cell to sieve tube via
symplastic pathway
Unloading sucrose from phloem
1. unloading occurs into any tissue requiring sucrose
2. both symplastic/apoplastic
3. requires ATP; similar methods to loading
4. once in tissue, sucrose converted by enzyme to decrease
concentration and maintain concentration gradient (sucrose > glucose
+ fructose; by invertase)
DIFFERENCES BETWEEN SIEVE TUBES AND XYLEM
VESSELS