L2: transport in plants

what classifies a plant?

• chloroplast: plants seen as green

• roots present

• evolution: based on common ancestry/ phylogeny

ways of transporting water (4)

1. forces regulating water movement

2. structure and function: the path of water through a vascular plant

3. mechanisms of water transport

4. experimental evidence for the cohesion- tension theory

• vascular plant: plant that has specialized vascular tissues (xylem and phloem) to transport water/ nutrients/ food, structural support for them to grow complez & large

• cohesion- tension theory: explanation how water moves upward through xylem in plants

vascular plant

plant that has specialized vascular tissues (xylem and phloem) to transport water/ nutrients/ food, structural support for them to grow complex & large

plant vascular tissues (2 main)

plant vascular tissues: specialized tissues in vascular plants that transport water, nutrients, and organic cmps throughout the plant. They also provide structural support

1. xylem

2. phloem

(1) vascular tissue: xylem

• vascular tissue

• transports H2O

• roots —> shoots

stomata/ stomatal pore

• tiny pores

• regulate gas exchange at the leaves

• water vapour diffuses from leaves and exits —> atmosphere

(2) vascular tissue: phloem

• vascular tissue

• transports sugars (sucrose)

• bidirectional (depends on source/site)

(1) ways of transporting water: forces of regulating water movement

water potential energy resulting from

1. Differences in solute concentration

2. physical pressure

• can predict direction of water movement

water potential

• what is it? pure water vs solutions? equation? how to know directionality?

potential energy of water: Ψ (psi)

• in a particular env, compared to PE of pure water @ atmospheric pressure and room temp

• pure water Ψ= 0 MPa

• solutions Ψ<0 MPa (-ve values)

equation:

• Water potential (Ψ) = solute (osmotic) potential (Ψs) + pressure potential (Ψp )

know directionality

• look @/ calculate Ψ for two environments, H2O moves towards more -ve Ψ

• FACT CHECK 10:12

solute (osmotic) potential (Ψs)

• what is it?

• why is it always -ve and reduces Ψ?

• high/ low [solute] in cell?

solute (osmotic) potential (Ψs): the effect of dissolved solutes on water potential. tendency of water to move in response to diff in solute conc

• always -ve

  • addition of solutes reduces water free energy (movement)

• reduces Ψ

  • pure water has the highest possible water potential Ψ=0 MPa, so when the amount of solute increases in the water they take up space and disrupt free movement of water molecules. the solutes reduce the potential energy of water

in pure water there should not be any pressure, so solute P= waterP

• Water potential (Ψ) = solute (osmotic) potential (Ψs) + pressure potential (Ψp )

• Water potential (Ψ) = solute (osmotic) potential (Ψs)

high solute concentration in cell

• low [H2O] inside cell, high [H2O] in env, water flows in cell from env by osmosis

• Ψs is very -ve

• turgor pressure Ψp builds up to balance Ψs

• cell is hypertonic to the env

• env is hypotonic to cell

• low solute potential (more -ve value), high solute concentration. water will move to MPa that is lower, it has more solute and less H2O

low solute concentration in cell

• opposite

hypotonic/ hypertonic/ isotonic

describe how solute concentration of an external solution compares to the inside cell, it affects osmosis (all relative to something else)

hypotonic: low relative solute conc

• high H2O conc

• water moves out/// net movement H2O out

• Ψs

  • in a hypotonic solution, an animal cell will burst because it does NOT have a cell wall, no enhancement to keep its organelles and plasma membrane will easily burst

  • in a hypotonic solution, a plant cell will NOT burst because it does have a cell wall, it is firm enough to withstand the turgor pressure

Hypertonic: high relative solute concentration

• low H2O conc

• water moves in/// net movement H2O in

• Ψs increases (cytoplasm gets diluted)

• Ψp increases (turgor pressure builds up). turgid cell

Isotonic: same solute concentration

• 2 spaces have the same H2O/ solute concentration

• NO net movement of H2O

turgid cell/ turgor pressure

result of pressure potential

water intake of cell produces a large hydrostatic pressure against the cell wall

• interior of the cell is putting pressure on the cell wall

• cytoplasm full of H2O, more than usual

• pressure is put onto cell wall

• drought (water stress), loss of turgor pressure, leads to wilting leaves

pressure potential (Ψp )

pressure potential (Ψp ): physical pressure exerted by water. tendency of water to move in response to a physical pressure

• pressure potential in a plant cell is the turgor pressure. the turgor pressure is on the plasma membrane/ cell wall, and the cell wall pushes back with an equal and opposite force that cancels the pressure out.

  • the wall pressure prevents cell expansion when turgor pressure is achieved

  • the cell wall: is made up of cellulose mesh and is selectively permeable

• +ve OR -ve

• +ve: turgor pressure in cells (swollen)

  • pushing water

  • water intake produces large hydrostatic pressure

  • turgor pressure, root pressure

• -ve:: tension in xylem (transpiration pull)

  • pulling water

  • wilting plants

calculating water potential

Water potential (Ψ) = solute (osmotic) potential (Ψs) + pressure potential (Ψp )

• calculate outside env potential

• calculate cell potential

• compare

  • same value? no net movement

  • diff value? H2O spontaneous net movement towards lower value

(summary) (1) ways of transporting water: forces of regulating water movement

• what forces determine the movement of water in plants?

• terminologies used in the process of water movement

• water moves from high —> low water potential

• solute potential (osmotic potential) and pressure potential contribute to water potential in plant cells (the sum)

(2) ways of transporting water: structire and function— the path of water through a vascular plant

roots —> leaves

• driven by physical forces/ plant structures

root hairs,

roots in relation to water

water and nutrients enter the plant by the roots/ root hairs, in the section called the zone of maturation

• root hairs: protrusions from root epidermal cells. they elongate further into the soil, reaching more H2O and nutrients

• epidermal cells: the outermost layer of cells in the root, a barrier against pathogens and selective permeability

• zone of maturation: region above the zone of elongation, cells fully differentiate into specialized tissues (epidermal cells, cortex cells, endodermis, vascular tissue)

• zone of elongation: root tip that cells rapidly expand to drive root growth deeper into the soil

relevent tissue layers in roots

outer skin

• epidermis/ epidermal cells: produces root hairs

• cortex:

inner skin

• endodermis/ endodermal layer:

• vascular tissue: phloem & xylem (they are located in the central root tissue, in vascular bundles). to get here the H2O has to get past the other tissue layers

  • phloem: sugar transport

  • xylem: H2O transport, root—> shoot

relationship of central root tissue, vascular tissue, vascular bundles

central root tissue is everything inside the endodermis

vascular tissue is inside the central root tissue

vascular bundles are inside vascular tissue

structures/ functions/ characteristics of xylem cells that help transporting H2O (water- conducting cells)

• dead cells at maturity, does not have cytoplasm/ cytosol or organelles. essentially a hollow pipe for H2O transfer

• water moves by bulk flow (through soil, cell walls, through xylem)

  • bulk flow/ mass flow: large- scale movement of fluids driven by pressure gradients

• thick secondary walls (very thick to withstand the strong -ve pressure from pulling water through xylem) and has lignin.

  • lignin: water-repellent, does not let water in. this forces water to go through many membranes to filter solute ions out. strengthens cell walls (red)

2 water- conducting cells

1. tracheids:

   1. all vascular plants

   2. long and tapered cells with pits

2. vessel elements:

   1. shorter and wider with perforation plates and pits

• perforation plates and pits:

  • NOT holes

  • NO lignin

  • H2O can flow through

  • thinner than cell wall

  • entry/exit between adjacent cells, minerals can pass through

pits (relate to problem that can occur in xylem cells)

• water conducting cell

• in tracheids and vessel elements (xylem cells)

• enable xylem loading: water and minerals are loaded into the xylem through pits

problem that can occur in xylem cells: embolism

• embolism: air bubble forms in the xylem and blocks the flow of water. occurs during drought, freezing (low solubility), physical damage

• pits allow the maneuvering of H2O flow

structures/ functions/ characteristics of phloem cells that help transporting H2O

pathways for movement of water into the root (3)

1. apoplastic pathway

2. symplastic pathway

3. transmembrane pathway

• ALL move from (high H2O potential) in the soil, —> (low H2O potential) in the vascular tissue

(1) pathways for movement of water into the root: apoplastic pathway

• H2O is moving through only the porous cell walls until endodermal layer is reached

• cell wall is EXTERIOR to the cell

• never entering the cell

(2) pathways for movement of water into the root: symplastic pathway

• moves cell- to- cell by plasmodesmata

  • plasmodesmata: microscopic channels that connect between cells for direct communication and transport

• moves across plasma membranes

apoplastic vs symplastic transport

apoplastic: cnts cell walls allow movement of water outside of cells

symplastic: movement of water by cytoplasm

how water passes through cells in the endodermal layer

casparian strip: a continuous band of lignin in the cell walls of root endodermal cells. dark yellow

• water is prevented from moving along the apoplastic pathway (cell wall), it cannot diffuse through

• must enter endodermal cells by symplastic or transmembrane route

• forces H2O to cross 2+ membranes to filter ions/ metals/ solvents in H2O before going to xylem vessels

• lignin: strong, water repellent/ hydrophobic, found in secondary cell wall

EXPLAIN

passive exclusion and endodermis

passive exclusion: non- selective, physical barrier that blocks substances based on size, charge or solubility (no E require)

• large heavy metals/ toxins prevented

• essential ions are actively transported

EXPLAIN

(3) pathways for movement of water into the root: transmembrane pathway

• moves cell- to- cell by aquaporins

  • aquaporins: membrane proteins that facilitate rapid transport of water across biological membranes. found in all living organisms to maintain water balance

plasmodesmata vs aquaporins

plasmodesmata:

• microscopic cytoplasmic channels between plant cells

• in cell walls or plasma membrane of plant cells

aquaporins:

• membrane- embedded proteins that form water channels

• in cell membranes

how does stomata control gas exchange?

guard cells: surround stomatal pores and control if open/ closed

• open: CO2 diffuse in, water vapour and O2 diffuse out

• closed: minimizes water loss

stomata open in response to light

• open during the day

• exception: crassulacean acid metabolism (CAM) plants stomata open at night to take in CO2

  • minimizes water loss

  • these are plants living in hot environments

(summary) (2) ways of transporting water: structire and function— the path of water through a vascular plant

• what cellular structures of plants are designed and involved to regulate water transport?

• how are the structure/ features of xylem cells related to their function of water transport?

• water moves across the root tissue in 3 distinct pathways (apoplastic, symplastic, transmembrane)

• xylem consists of tracheids and vessel elements

  • tracheids are long and thin, contain pits

  • vessel elements are shorter and wider, contain pits and perfoations

• guard cells control stomatal opening and closing, diffusion of water vapour goes out of the leaf

(3) ways of transporting water: mechanisms of water transport

• water moves passivly, no E needed, from roots —> leaves, along a water potential gradient

  • uses the difference in H2O potential to defy gravity

• water at the air- water interface in leaves is under -ve pressure (tension) which pulls water up from the roots through the xylem

-ve vs +ve pressure

-ve

• pressure is lower than env

+ve

• pressure is higher than env

transpiration

transpiration: movement of water through the plant and subsequent evaporation (water loss) through aerial parts (leaves)

• process is needed for the plant to photosynthesize

• strong pull

• -ve pressure/ tension

mechanisms that drive water movement through xylem over long distances (3)

1. root pressure

2. capillary action

3. cohesion- tension

(1) mechanisms that drive water movement through xylem over long distances: root pressure

Root pressure/ pressure potential is built up when stomata close at night and transpiration stops. the transport of mineral nutrients continues at night and decreases xylem Ψ further and increasing root pressure. therefore water moves up the xylem

• push force water up xylem

• mineral nutrients are actively transported into xylem

  • bc plant is concentrating nutrients inside cells to use in plant. needs to move against gradient

• high solutes concentrations lead to decreased water potential Ψ

  • more solutes are being actively transported into the root, more solute= less water, water potential decreases

• low Ψ drives water intake from the soil

  • root has low Ψ, and water goes from highΨ —> lowΨ

• pressure potential is built up/ increases when stomata close at night

  • transpiration stops, diffusing of CO2 and H2O stop, water accumulates in the plant

  • transport of mineral nutrients still continue at night, this decreases xylem Ψ further and increases root pressure (+ve pressure). more nutrients = more water diffused into roots, plants accumulate more water bc it is not being released

• limitation: water moves up the xylem, but not enough for tall plants

(2) mechanisms that drive water movement through xylem over long distances: capillary action/ xylem as a capillary tube

capillary tube: thin, narrow tube

• xylem cells are dead at maturity, they are hollow without cytoplasm and organelles so water can move freely through them

capillary action: flow of a liquid through narrow spaces, can occur against gravity, NEEDS 3 FORCES

• pulling force water up xylem

1. cohesion

   1. interactions between H2O molecules (H-bonds bc of unequal distribution of e- of O and H)

   2. upward push

2. adhesion

   1. interaction with glassware, results in a curved meniscus

   2. upward pull

3. surface tension

   1. interaction of air- water, results in a curved meniscus

   2. upward pull, works better for narrower tubes. the forces are transmitted to molecules lower down in the water column by cohesion

(3) mechanisms that drive water movement through xylem over long distances: cohesion- tension

cohesion- tension theory: explaination of how water moves upward through plants, from roots —> leaves, defies gravity

• pulling force water up xylem

• water evaporates in air spaces inside the leaves, in air pockets in the leaves and outside of cells

• also forms a curved meniscus

• water vapour diffuses through stomata out into drier air by the water potential gradient

• more water evaporates from the water- air interfaces inside the leaf

• as water evaporates, surface tension increases at the menisci of the air-water interfaces inside the leaf

  • menisci: meniscus

cohesion- tension theory and tension

tension:

• driving force

• -ve pressure in the xylem created by water evaporation in the leaves

• develops at air- water interface outside of cell walls of leaf cells

• when there is more curvature, there is more tension (more -ve)

• The tension from the air- water interface is transmitted from water at the interface to water in leaf cells (including leaf xylem)

• Water is pulled from nearby leaf cells and xylem???

1. inside leaf cell walls water escapes into the air as vapour (tranpiration)

2. remaining water forms a curved meniscus, the curve increases surface tension and pulls on remaining H2O molecules

3. creates tension & -ve pressure in liquid water

4. tension spreads to adjacent water molecules

cohesion- tension theory and cohesion

cohesion: (between water molecules)

• transmits tension from water in xylem of leaves —> stems —> roots

  • cnts pulling because H2O molecules are always bonded

• water is pulled from the root xylem up the stem —> leaves

• At the same time, mineral nutrients are being actively transported into the root, and then to the root xylem

• Water from the root cortex moves into the root xylem

• Water from the soil moves into the root epidermis and cortex

recall embolisms (now in a vascular plant)

problem that can occur in xylem cells: embolism

• embolism: air bubble forms in the xylem and blocks the flow of water. occurs during drought, freezing (low solubility), physical damage

• pits allow the maneuvering of H2O flow

PROBLEM OCCURING IN A VASCULAR PLANT

• blocks xylem

• prevents cnts column of H2O, stops H2O cohesion & pulling

forces involved in water transport

1. root pressure: pushing water up xylem

2. capillary action: pulling water up xylem (cohesion, adhesion, surface tension)

3. cohesion- tension: pulling water up xylem, water evaporation —> surface tension transmitted through cohesion

water potential gradient (soil- plant- air)

less -ve —> more -ve

soil is close to 0

h2o moves passively

water potential and relative humidity

• both describe energy state of water and tendency to mvoe

drought stress

• low humidity, increase Ψair negativity, faster transpiration, wilting

(summary) (3) ways of transporting water: mechanisms of water transport

• what mechanisms make it possible to move water from roots to leaves?

• how does the cohesion- tension theory explain the movement of water in plants?

• Water moves passively from soil to roots to leaves to atmosphere,
  along a water potential gradient

• Transpiration generates a negative pressure (tension) at the air-water interfaces in leaves, which pulls up water from the roots through xylem, due to the cohesion between water molecules

• Root pressure and capillary action can result in short-distance
  transport of water, but do not fully explain long-distance transport

(4) ways of transporting water: experimental evidence for the cohesion- tension theory

diff experiments

experiment 1: cut petiole

• petiole: stalk connecting a leaf blade to the stem of a plant. (leaf stem)

Test: if cohesion- tension theory can explain movement of water from roots to shoots, you can cut the petiole attached to an actively transpiring leaf.

Let’s make a prediction about the direction of water movement in the part of the petiole still attached to the leaf

the water will continue to move from the petiole —> leaf because the leaf is still transpiring

experiment 2: tree trunk diameter

When under negative pressure (tension), xylem will tend to contract

• ...a bit like when you drink
  through a straw

test: if cohesion- tension theory can explain movement of water from roots to shoots, we can measure the diameter of a tree trunk (stem).

Let’s make a prediction about how the diameter will change with changing rates of transpiration.

• incr transpiration rate, induces transpiration pul/ tension on the water column

• support cohesion-tension theory of long- distance water transport

incr transpiration, incr H2O

trubk diameter shoule decr when trenspirationrate incr

experiment 3: light intensity and xylem pressure

• xylem pressure: hydrostatic pressure within the xylem vessels. roles in water transport, nutrient distribution, structural support

test: if cohesion-tension theory can explain movement of water from roots to shoots, Wei et al. (1999) manipulated light levels

They knew that transpiration rates increase with increasing light intensity.

• incr transpiration rate, induces transpiration pul/ tension on the water column

• support cohesion-tension theory of long- distance water transport

incr surface tension, incr tranpiration

incr surface tension = decr pressure

(summary) (4) ways of transporting water: experimental evidence for the cohesion- tension theory

• How the cohesion-tension theory of water movement in plants is
  supported by experimental evidence

• How to interpret experimental data pertaining to the cohesion tension theory

• How to make predictions about relative plant transpiration rates under different environmental conditions

• Experimental observations indicate that conditions favouring an
  increased transpiration rate induce a transpiration pull (tension) on the water column

• These experiments support the cohesion-tension theory of long-
  distance water transport

contributors for sugar transport (4)

1. Source and sink relationship
2. Structure and function of phloem
3. Phloem loading and unloading
4. Mechanism of sugar transport: pressure-flow model

(1) contributors for sugar transport: Source and sink relationship

source-sink relationship depends on developmental stages

(major) source: mature, photosynthesizing leaves

sinks: young growing leaves, flowers, fruits, seeds

• growing tissue that is NOT actively photosynthesizing

plant vascular tissues

recall// xylem (transports water) & phloem

phloem: transports sugars

• both directions roots ←→ shoots. this depends on where sucrose is available

transport is from source tissues to sink tissues

• source tissues: sucrose is available

• sink tissues: sucrose is needed

recall// transpiration, why is sugar synthesis important?

transpiration: movement of water through plant and subsequent evaporation through aerial tissues (leaves)

• the process supports photosynthesis

photosynthesis produces sugars in the leaves and are translocated by phloem to other parts of the plant

• for cellular function and growth

antiporters vs sumporters

• membrane transport proteins that move sucrose across plant cells

• differ in directionality and energy coupling

symporters:

• co- transporters

• move sucrose + other molecule in the same direction

• powered by a pre-established electrochemical gradient

• load sucrose into phloem from leaves, and uses proton gradient from ATPase pumps

antiporters:

• exchangers

• move sucrose in one direction, other molecule in the other direction

• indirectly ises gradients

• regulate sucrose storage/ release

summary: (1) contributors for sugar transport: Source and sink relationship

• How the direction of transport of sucrose is determined in plants

• The source-sink relationship in plants

• In vascular plants, sugars can move in both directions between roots and shoots

• Sugars are moved from source tissues to sink tissues (direction will depend)

(2) contributors for sugar transport: structure and function of phloem

recall// other vascular tissue xylem: DEAD @ maturity, just a cell wall

phloem consists of : ——————> ALIVE @ maturity

1. sieve- tube elements (SE):

   1. no organelles & nucleus, but alive

   2. sugar- conducting cell

   3. sieve plates: perfoorated connections between sieve tube elements

2. companion cells (CC):

   1. contain organelles (filled space) & nucleus

   2. responsible for metabolic needs of sieve- tube elements

   3. involved in loading and unloading of sucrose

phloem: how sieve- tube elements relate to companion cells

• Sieve-tube element (SE), with residual organelles

• Companion cell (CC) and SE are connected via plasmodesmata

• PP: phloem parenchyma cells (storage role)

xylem cells vs phloem cells

Feature	Xylem	Phloem
Function	Transports water + minerals (upward, roots → leaves)	Transports sugars + nutrients (bidirectional, leaves → roots/fruits)
Direction of Flow	Unidirectional (upward only)	Bidirectional (up and down)
Cells Involved	Dead at maturity (tracheids, vessel elements)	Alive at maturity (sieve tubes, companion cells)
Cell Walls	Thick, lignified (for strength)	Thin, non-lignified
Pressure Mechanism	Cohesion-tension (transpiration pull)	Pressure flow (osmotic pressure gradients)
Energy Requirement	Passive (no ATP needed)	Active (requires ATP for loading/unloading sugars)
Speed of Transport	Fast (up to 45 m/hour)	Slow (~1 m/hour)
Appearance	Hollow, rigid tubes (visible as "wood grain")	Soft, flexible tissue (e.g., inner bark)

summary: (2) contributors for sugar transport: structure and function of phloem

• What plant cells are involved in sugar transport?

• How is the structure of phloem cells related to its function of sugar transport?

• Sugars are translocated from source to sink via the phloem tissue

• Phloem consists primarily of sieve-tube elements, and companion cells

• Sieve-tube elements are long, thin cells with perforated ends (sieve plates); they lack most organelles, which facilitates sugar translocation

• Companion cells are connected to sieve-tube elements by plasmodesmata, and help maintain the metabolic activity of sieve tube elements

• Companion cells are important in sucrose loading and unloading

(3) contributors for sugar transport: phloem loading and unloading

loading: transfer of sugars from source tissues to the phloem, where they are concentrated

unloading: transfer of sugars from phloem to sink tissues

phloem loading: sugar transport photosynthetic cell —> phloem

• photosynthetic cell: plant cell that has chloroplasts to convert light E —> chemical E through photosynthesis

• how? steps? results?

(SECONDARY) Active transport moves sucrose from photosynthetic source cells into companion cells (steps)

• Once in companion cells, sucrose builds up in sieve-tube element cells via diffusion

• want to keep sucrose in phloem, so particles from source cells —> companion cells/ sieve- tube elements are moving against the conc gradient

• ATP is used to accumulate high [H+ ] outside the cell into source cell with a proton pump protein

• as H+ particles move back down its conc gradient into companion cell, it brings a sucrose molecule from the source cell

• This establishes an electrochemical gradient that allows the transport of sucrose into the phloem cell against a [sucrose] gradient
  

• secondary active bc ATP is used INDIRECTLY, 2 step process

• active bc it uses H+ and moves it across a membrane against its electrochemical gradient

(results)

• High [sucrose] in the sieve-tube element leads to low solute potential (Ψs )

• Water flows in from adjacent xylem (high —> low Ψ)

• This increases turgor pressure in the phloem near the source

phloem unloading: sugar transport from phloem to sink cell

• Near the sink tissue, sucrose can be exported from phloem to sink cells by facilitated diffusion or active transport

• This decreases [sucrose] in sieve tube elements and increases
  water potential

• Water moves out of the phloem and into the xylem tissue, decreasing turgor pressure

whole picture: phloem loading and unloading

• pressure- flow hypothesis

• leads to a pressure potential gradient in phloem

• high turgor pressure near source tissue, low turgor pressure near sink tissue

• water moves down pressure gradient, sugar is transported by bulk flow

  

metabolically active sink (facilitated diffusion)

• Phloem unloading into growing sink tissues (e.g., new leaves, elongating roots)

• In growing cells, sucrose is rapidly depleted by metabolic activity, maintaining low [sucrose]

• uses sucrose and keeps the conc gradient

storage sink (active transport)

• in vacuole, active transport is needed

• Sucrose accumulates in storage cells

• To maintain a favourable [sucrose] gradient for continued uptake from companion cells, sucrose must be actively transported to the vacuole

summary: (3) contributors for sugar transport: phloem loading and unloading

• How do plants regulate phloem loading and unloading?

• How do plants control the capacity of sink tissues to accept sugars?

• Sugars move from source to sink by water moving down a pressure gradient; sugars are transported by bulk flow

• Vascular tissues are organised into vascular bundles; the xylem can thus provide water needed for bulk flow in the phloem

• Sink capacity is controlled by:

  • Using sucrose in metabolically active tissues

  • Storing sucrose in vacuoles in storage tissues

(4) contributors for sugar transport: mechanism of sugar transport: pressure- flow model

concept: pressure- flow model

1. Source: high [sucrose] and water flow into the phloem generate high turgor pressure

2. Sink: low [sucrose] and flow of water out of the phloem generate low turgor pressure

3. Bulk flow: transport of sugars from source to sink is due to the difference in turgor pressure

evidence for pressure- flow model

this bug is essentially a tube for plant liquid

summary: (4) contributors for sugar transport: mechanism of sugar transport: pressure- flow model

• How the pressure-flow model explains sugar translocation in the
  phloem

• How the pressure-flow model is supported by evidence

• The pressure-flow model of sugar transport involves the generation of high turgor pressure in phloem due to high [sucrose] in the sieve-tube element near the source

• The bulk flow transport of sugars from source to sink is due to the
  difference in turgor pressure