Plant transport

General Structure of Plants

 The structural organisation of a plant

is related to these core functions:

 Leaves contain chloroplasts

(photosynthesis) and stomatal pores (gas

exchange)

 Roots are branched to optimise the

uptake of water and minerals from soil

 Stems transfer materials between leaf

and root

 Bundles of vascular tissue are found

in all three sections

Vascular Tissue

 Transport involves two types of vessels –

xylem and phloem (arranged in bundles)

 Xylem vessels transport water from the

roots to the leaves

 This type of transport is called transpiration

 Phloem vessels transport nutrients from a

source to sink

 This transport is called translocation

 The distribution of the vessels differs

between the sections of the plant

Leaf Tissue

 Leaf tissue features:

 Epidermis – Outer layer of cells with a

waxy layer called a cuticle that limits

water loss/absorption

 Palisade mesophyll – Elongated cells with a

high concentration of chloroplasts for

photosynthesis

 Spongy mesophyll – Loosely arranged cells

with more air spaces to allow for gas

exchange

 Vascular bundles – Xylem (transport water)

and phloem (transport sugars)

 Stomata – Openings that allow for carbon

dioxide uptake and water loss

Stem Tissue

 Stem tissue features:

 Epidermis – Outer protective layer of cells

(with cuticle) that limits water loss/

absorption

 Ground tissue – The cortex (outer) and pith

(inner) assist in transport and storage of

materials

 Vascular bundles – Arranged in a ring near

the outer edge of the stem to resist

compression and bending

 Xylem – located to the interior side

 Phloem – located to the exterior side

Root Tissue

 Root tissue features:

 Epidermis – Outer protective layer of cells

(with cuticle) that has root hairs to increase

surface area

 Cortex – loosely packed cells that allow

water movement and storage of food

reserves

 Endodermis – includes the Casparian strip

and is impermeable to the passive flow of

water and ions, which allows the rate of

uptake to be controlled

 Stele – central region containing the vascular

bundles and pericycle/cambium

 Vascular bundles – Xylem and phloem

 Pericycle/cambium – provides strength

and allows for the development of lateral

roots

Transpiration

Xylem Structure

 Vessel elements

 End walls have become fused to form a continuous

tube, resulting in a faster rate of water transfer

 Tracheids

 Tapered cells that exchange water solely via pits,

leading to a slower rate

 When mature, xylem tissue is dead, so water

transport is a passive process

 The cell walls of the dead tissue remains and

are reinforced with a substance called lignin

Mass Flow

 The flow of water through the xylem (from roots

to leaves) is called the transpiration stream and

involves mass flow

 Mass flow is the movement of fluid down a

pressure gradient

 Leaves have lower pressure due to evaporation

 Roots have higher pressure due to osmotic uptake

 Hence, water will flow from the roots to the leaves

Transpiration

 Approximately 99% of the water a

plant absorbs from the soil and

transports through the xylem is lost by

evaporation (mostly through stomata)

due to the process of transpiration

 There are three main ways in which

water moves through the xylem:

 Evaporation and transpiration pull

 Capillary action (cohesion-tension)

 Root Pressure

Evaporation and Transpiration Pull

 Some of the light absorbed by leaves becomes heat, which can convert

water into vapour

 The vapour diffuses out of stomata and is evaporated, creating tension forces and

negative hydrostatic pressure in the leaf which draws new water out of the xylem

(transpiration pull)

 Water is pulled through the plant along a gradient of increasing solute

concentration due to sugar production

Capillary Action

 Capillary action is the ascension of

water through a tube against gravity

 Capillary action occurs in xylem

vessels due to the cohesive and

adhesive properties of water which

creates one unbroken column of

water through the plant

Root Pressure

 Water entering the stele from the soil

creates a root pressure

 This pressure provides a weak 'push'

effect for the water's upward

movement through the plant

 The root hairs increase surface area

for absorption

 Mineral uptake from the soil assists

osmosis

Mineral Uptake

 Minerals that need to be taken up from the soil include Mg2+ (for

chlorophyll) and nitrates (for amino acids), as well as Na+, K+ and PO4

3–

 Some are absorbed by diffusion along their concentration gradient

 Root cells also contain proton pumps that actively pump H+ ions into the

surrounding soil, which displaces the positively charged minerals from

negatively charged clay particles, allowing for their absorption

Water Uptake

 Water will follow the mineral ions

into the root via osmosis through

two different pathways:

 Symplast pathway – the water moves

continuously through the cytoplasm of

cells (connected via plasmodesmata)

 Apoplast pathway – the water moves

through the non-living spaces of the

plant such as the cellulose cell walls

 Water cannot cross the Casparian strip

and so must be transferred to the

cytoplasm of the endodermis

Water Uptake

Translocation

Translocation

 Plants transport sugars and other organic molecules

from source to sink

 Sources: Photosynthetic tissues (e.g. leaves)

 Sinks: Storage organs (e.g. fruits, seeds, roots)

 The organic molecules are transported via the

phloem, in a process referred to as active

translocation

 Apart from water, phloem sap comprises mainly

sucrose (up to 30%)

 It may also contain minerals, hormones, and amino acids,

in transit around the plant.

Dollar Photo Club

Maple syrup is made from

collected maple sap

Phloem Structure

 Phloem comprise of sieve elements and

companion cells

 Sieve elements connect to form a tube with porous

plates at their transverse ends (allows material flow),

have no nuclei (to maximise space), and have thick

and rigid cell walls (to withstand pressure)

 Companion cells possess a highly folded membrane

so as to maximise SA:Vol ratio (more material

exchange)

 Plasmodesmata connect the two cells (symplastic

flow)

Companion Cells

 Companion cells support phloem transport

by:

 Providing metabolic support for sieve elements

 Facilitate loading and unloading at source and

sink

 Companion cells move materials in two ways:

 Via interconnecting plasmodesmata (symplastic)

 By actively pumping materials from within the cell

wall space of the companion cells (apoplastic)

Phloem Loading

 Phloem loading is an active process that

occurs against a concentration gradient (and

needs ATP)

 Protons are pumped out of phloem cells

 They passively return via a co-transport protein

which facilitates the joint movement of solutes such

as glucose or sucrose

 The build up of solutes in the phloem creates

a hypertonic solution that draws water via

osmosis

Mass Flow

 High concentrations of solute in the phloem draws water from the xylem

due to osmosis

 The water uptake creates an increase hydrostatic pressure that forces the sap to

move along the phloem cells towards areas of lower pressure (sinks) due to the

gradient via mass flow

Phloem Unloading

 Solutes are unloaded by companion cells

and transported into sinks (roots, fruits,

seeds, etc.)

 This causes the sap at the sink to become

more hypotonic (lower solute concentration)

 Consequently, water is drawn out of the

phloem and back into the xylem by osmosis

 This ensures that the hydrostatic pressure at

the sink is always lower than at the source

 Hence, phloem sap will always move from

the source towards the sink