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Transport in plants
Explain the need for transport systems in multicellular plants in terms of size and
surface area: volume ratio
Describe, with the aid of diagrams and photographs, the distribution of xylem and
phloem tissue in roots, stems and leaves of dicotyledonous plants
Describe, with the aid of diagrams and photographs, the structure and function of
xylem vessels, sieve tube elements and companion cells
Define the term transpiration
Explain why transpiration is a consequence of gaseous exchange
Describe the factors that affect transpiration rate
Describe, with the aid of diagrams, how a potometer is used to estimate
transpiration rates
Explain, in terms of water potential, the movement of water between plant cells, and
between plant cells and their environment. (No calculations involving water potential will
be set)
Describe, with the aid of diagrams, the pathway by which water is transported from
the root cortex to the air surrounding the leaves, with reference to the Casparian strip,
apoplast pathway, symplast pathway, xylem and the stomata
Explain the mechanism by which water is transported from the root cortex to the air
surrounding the leaves, with reference to adhesion, cohesion and the transpiration stream
Describe, with the aid of diagrams and photographs, how the leaves of some
xerophytes are adapted to reduce water loss by transpiration
Explain translocation as an energy-requiring process transporting assimilates,
especially sucrose, between sources (e.g. leaves) and sinks (e.g. roots, meristem)
Describe, with the aid of diagrams, the mechanism of transport in phloem involving
active loading at the source and removal at the sink, and the evidence for and against
this mechanism…read more

Slide 2

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Requirements of Plants
Substance Purpose How obtained/removed How transported
CO2 Photosynthesis. Diffusion through open Diffusion; diffusion distances
stomata. are short because CO2 is
already where it is needed
at the leaf.
O2 For respiration in cells not Diffusion through open Diffusion. This is sufficient
actively photosynthesising. stomata or lenticels in because plants respire at a
Removed as a waste product stem/bark. lower rate relative to
of photosynthesis. mammals.
H2O Photosynthesis, turgidity, Uptake through roots via Mass flow through xylem
solvent for metabolic rxns, osmosis. Water vapour down hydrostatic pressure
cooling. removed through open gradient
stomata (transpiration).
Inorganic ions Metabolic rxns. Active transport through " "
roots hairs.
Organic nutrients e.g. Energy to carry out life Plants are autotrophic "self Via mass flow (pressure flow
glucose processes. feeding". Organic nutrients hypothesis) through phloem
needs to be transported tissue and active loading at
from photosynthetic cells sources and unloading at
(where synthesised) to sinks.
respiring cells.
· Large multicellular plants have a low SA:Vol.
· The cells are distant from the extracellular environment
· There is a greater demand for materials and the need to remove a greater amount of waste produced due
to the large number of metabolically active cells
Transport of certain substances via diffusion is inadequate to meet demands.
Therefore multicellular plants need specialised exchange surfaces and mass flow transport systems to obtain
essential materials from the environment, and transport these to the cells where they are needed. Waste also
needs to be transported from the cells for excretion into the external environment…read more

Slide 3

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Plant vascular tissue
Xylem: Transports water and dissolved minerals upwards from roots to aerial parts of the plant by transpiration
driven by solar energy. Also provides structural support.
Phloem: Transports soluble organic substances (e.g. sucrose and other assimilates) throughout the plant by
translocation from sources (where produced) to sinks (where used/stored) through a mechanism of pressure flow.
Dicot Root
· Xylem: located centrally in an `X' shape in CS. This provides
support for the plant and reduces frictional damage as the
root penetrates the soil.
· Phloem: Located between the arms of the X shaped xylem.
· Cortex: Made up of parenchyma cells: stores food. Thick ­
prevents crushing as root pushes through soil.
· Pericyle: Layer of meristem cells lining the inner surface of
the endodermis which differentiate into new lateral roots.
· Endodermis: Layer of parenchyma surrounding vascular
bundle some walls impregnated with suberin.
Dicot Stem bundle
Vascular bundle: arranged in ring around periphery to provide
support and flexibility.
Sclerenchyma: vascular bundle caps
Collenchyma: found beneath epidermis, flexible support
Chlorenchyma: may be found below epidermis allowing
Central pith: made up of parenchyma and air spaces Epiderm
C m
ma…read more

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Dicot leaf: organ of
Upper epidermis: Secrete cuticle to prevent water loss. Transparent to allow light
to reach mesophyll.
Vascular bundles: Form midrib and veins of leaf which support the thin leaves.
Xylem is on top of the phloem.
Palisade mesophyll (chlorenchyma): elongated parenchyma cells with many
chloroplasts for photosynthesis.
Spongy mesophyll: associated with air spaces to allow rapid diffusion of gases.
Lower epidermis: Contains guard cells and stomata which open to allow gas
Root Stem Leaf
Central vascular bundle with xylem Vascular bundles arranged around Vascular bundles form midrib and
in `x' and phloem between periphery veins of leaf.
Has endodermis and pericyle Xylem below phloem Xylem above phloem
May have root hairs Has collenchyma beneath epidermis Has chlorenchyma tissue (palisade
Has central pith mesophyll)
May have lenticels Upper epidermis
Lower epidermis with guard cells
and stomata.…read more

Slide 5

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Structure of Xylem
Consists of xylem vessel elements (tubes) for upward transport of water and dissolved
minerals, fibres for support, and parenchyma cells filling gaps between vessels.
· Vessel elements are made of dead cells joined end to end with no cross
walls to form empty continuous tubes
· Lignified cellulose cell walls.
· Provides flexibility during growth and allows bending of stem
· Prevents walls collapsing inwards due to tension
· Waterproofs vessels and improves adhesion of H2O
· Pits (unliginified regions) allow lateral movement of H2O and allows it to
bypass blockages.
· Narrow lumen: increases adhesion of H2O…read more

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Consists of many elongated sieve tube
Structure of phloem elements with associated companion cells.
Sieve tube elements Companion cells
· Joined end to end with perforated end Each sieve tube element connected to
walls called sieve plates through which own living companion cell via
phloem sap can pass. plasmodesmata (cytoplasmic strands that
· Are thin walled have no nucleus, pass through cell walls) which allow
tonoplast, vacuoles or ribosomes and exchange of materials.
lack other organelles. · Contain a nucleus, dense endoplasmic
· Cytoplasm is thin and peripheral so as reticulum, ribosomes and numerous
to provide resistance to flow of phloem mitochondria ­ mitochondria provide
sap. energy (ATP) for metabolic needs and
active transport.
· They use ATP and co-transporter
proteins to actively load sucrose into
the phloem.
N.B: If a sieve tube is damaged, the pores in
the sieve plate become plugged by callose ­
a polymer of glucose ­ prevents loss of
phloem sap (and assimilates) and prevents
Phloem sap is cytoplasmic stands
containing dissolved sucrose and other
assimilates. Due to callose the sap
must be collected using an aphid.…read more

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