7 Transport in plants

Question 1

Why do plants need a transport system (instead of using diffusion and osmosis)?

Answer 1

1.Transportation of assimilates from source to sink.
2.Transport of water and mineral ions across far distances e.g., the roots to leaves of a very tall tree.
3.Plant cannot be supported by only short-distance transport methods (osmosis and diffusion). Would take too long for water, mineral ions and assimilates to reach destinations. Therefore, not efficient enough.

Question 2

Assimilates

Answer 2

– organic products synthesised as a result of photosynthesis

Question 3

What are the features of plant transport systems?

Answer 3

1. Require vascular tissue made of xylem and phloem.
   * Xylem – transports water and mineral ions from roots to stems and leaves.
   * Phloem – transports assimilates from source to sink.
2. Long distance bulk transport systems. (Mass flow theory).
3. Transport tissue distributed between roots, stem and leaves so that all cells can receive their required mineral ions and assimilates.
4. A transport medium in which to transport dissolved substances. (Xylem or phloem sap).
5. Movement of transport medium using pressure differences.
6. Gaseous exchange does not occur in transport tissues (unlike mammals).
7. Transport system is unidirectional, as in, it is not a circular flow (like mammal circulatory system). However, the phloem can transport assimilates both up and down.

Question 4

Xylem

Answer 4

transports water and mineral ions from roots to stems and leaves

Question 5

Phloem

Answer 5

transports assimilates from source to sink.

Question 6

The location of vascular bundles

Answer 6

• xylem, phloem, sometimes associated tissues like sclerenchyma fibres) in the root vs stem vs leaves differs greatly

Question 7

Herbaceous plants

Answer 7

plants that have a soft and non-woody stem, ALSO, plants that have a life cycle of only one year

Question 8

Dicotyledonous plants

Answer 8

• flowering plants developed from seeds that have two embryonic leaves/cotyledons.
• Their leaves are in multiples of 4 or 5 and have a reticulated vein network.
• In stems, vascular tissues arranged in bundles in a ring pattern.

Question 9

Monocotyledonous plants

Answer 9

• herbaceous flowering plants developed from seed with only one embryonic leaf.
• Leaves are in multiples of 3 and have parallel veins.
• In stems, vascular tissues is arranges in bundles scattered around stem

Question 10

Why should plants be structured to maximise SA:V?

Answer 10

Plant leaves and root systems need to maximise their SA:V as much as possible in order to facilitate the uptake of water and mineral ions, as well as the absorption of sunlight and carbon dioxide.

Question 11

How do herbaceous vs woody plants facilitate gaseous exchange differently?

Answer 11

• In herbaceous plants, the stems (and leaves) contain stomata which facilitate gaseous exchange.
• In woody plants, there are lenticels to facilitate gas exchange.

Question 12

How are the stem and roots structured?

Answer 12

• Stem and roots are both organs containing different tissues.
• Above soil, plant is structured to withstand different mechanical stresses, such as wind.
• Plants are supported by sclerenchyma, parenchyma and collenchyma tissues.
• Vascular bundle arrangement differs to perform particular function.

Question 13

Epidermis

Answer 13

• outermost layer of stem and roots.
• Protective layer of cells that is covered by wax cuticle

Question 14

Parenchyma

Answer 14

• living undifferentiated cells in stem and roots functioning as packing tissue (fill empty spaces) AND a storage tissue.

Question 15

Features of parenchyma

Answer 15

*Spherical cells.
*Thin cellulose cell walls.
*Large vacuoles.
*Some have chloroplasts.
*Can be found in cortex of stem sand roots and pith of stems.
*Turgid parenchyma cells provide hydrostatic support.

Question 16

Collenchyma

Answer 16

living tissue specialised for support and found commonly as a layer directly beneath epidermis

Question 17

Features of collenchyma

Answer 17

*Cellulose cell walls with uneven thickening.
*Cell walls also contain pectin.
*Adapted for areas of growth as cell walls able to stretch

Question 18

Sclerenchyma

Answer 18

dead cells that compose tissue; specialised for support and strength in areas that are no longer elongating

Question 19

Features of sclerenchyma

Answer 19

*Two distinct groups: sclereids and fibres.
*Cellulose walls very thick, some walls contain lignin.
*Fibres are long.
*Sclereids are shorter and of varying shapes.

Question 20

How is vascular tissue distributed in a dicotyledonous stem?

Answer 20

*Vascular bundles close to outside of stem.
*Lateral forces acting on plant are best resisted by a cylindrical non-continuous circle arrangement of vascular bundle (especially for herbaceous stems).
*Because the stem is non-continuous the stem is still flexible.
*Xylem inside, phloem outside.
*Between xylem and phloem is cambium

Question 21

Cambium

Answer 21

meristematic tissue containing stem cells for division that can specialise into either xylem or phloem cells

Question 22

How is vascular tissue distributed in a dicotyledonous root?

Answer 22

*Situated centrally because roots are subject to pulling forces.
*Central column of supporting tissue.
*Xylem arranged in star (4 point) shaped block of tissue surrounded by separate phloem groups.
*Around both is the pericycle and endodermis.

Question 23

How is the structure of root hair cells adapted to their function?

Answer 23

*Epidermis in area with plentiful root hair cells known as piliferous layer.
*Root hair cell is a modified epidermal cell.
*Cell has a large vacuole.
*Long cytoplasmic extension projecting.
*Large surface area for water and mineral uptake.
*Root hair delicate - does not live for long.
*As root grows, new root hair cells nearer to tip replace old ones.
*No waxy cuticle.
*Thin cellulose cell wall.
*Many aquaporins (channel proteins for water uptake) in cell surface membrane.
*Many mitochondria to provide ATP for active uptake.

Question 24

Plan diagram

Answer 24

shows the distribution of tissues in a section

Question 25

What to include in plan diagram?

Answer 25

*Individual cells not drawn.
*Cells not drawn but tissues labelled.
*Shows proportions of different tissues.
*Annotations should be added if required, and include description of shape, size and colour. (E.g., differences in staining).
*Lines should be clear and continuous.
*Make proportions shown realistic.
*Use all available space.
*Do not shade or colour.
*Label lines should not have arrowheads or be crossed.

Question 26

How is vascular tissue distributed in a leaf?

Answer 26

*Vascular tissue is situated in reticulated vein network in lamina (blade of leaf) form side veins.
*Veins often surrounded by ring of parenchyma cells.
*Side veins merge into central main vein or midrib.
*Leaf stalk also known as petiole.
*Xylem on top, phloem on the underside.
*Point where leaf is attached to stem is known as node.

Question 27

How is the leaf structured (overview of components)?

Answer 27

*Epidermis
*Palisade mesophyll
*Spongy mesophyll
*Phloem
*Xylem
*Parenchyma, sclerenchyma

Question 28

Structure and function of epidermis?

Answer 28

*Colourless cells that allow light to enter the leaf.
*Protects cell.
*Specialised epidermal cells = guard cells.
*Guard cells enclose stomata.
*Gaseous exchange takes place through stomata.
*Water vapour evaporates in spongy mesophyll but is LOST through stomata via diffusion.
*Outer surface of epidermis covered by waxy waterproof cuticle to limit water loss.

Question 29

Which colours will lignin and cellulose stain in observation of plant specimens?

Answer 29

*Lignin – stains pink (e.g., xylem and sclerenchyma)
*Cellulose – stains blue or green.

Question 30

Structure and function of palisade mesophyll tissue?

Answer 30

*Adapted to carry out photosynthesis.
*Main tissue that carries out photosynthesis.
*Closely packed and thin-walled so absorbs maximum light.
*Arranged vertically so fewer cross walls to filter out light.
*Numerous chloroplasts that are arranged in best positions to absorb light.
*Large vacuole pushes cytoplasm and chloroplasts to edge of cells – absorb light and SHORT diffusion distance for CO2.
*Large surface area and moist, thin walls for rapid diffusion of gases.

Question 31

Structure and function of spongy mesophyll tissue?

Answer 31

*Loosely packed to create large intercellular air spaces.
*Air spaces facilitate evaporation, whereby water vapour then diffuses out from stomata.
*Spongy mesophyll can photosynthesise and store starch.

Question 32

Function of xylem?

Answer 32

*Main water conducting tissue in vascular plants.
*Transports xylem sap (watery solution of dissolved minerals, plant hormones and other nutrients).
*Provides hydrostatic and structural support for plants.

Question 33

Why are xylem elements termed ‘elements’ and not cells?

Answer 33

Cell is a living structure but mature xylem vessels are dead.

Question 34

How are xylem vessel elements structured?

Answer 34

*Vary in structure.
*All hollow and elongated.
*As they mature walls become impregnated with lignin.
*Lignin causes cell to die because it is unable to obtain water.
*Cell walls contain cellulose within lignin.
*End walls broken down – forms continuous tube known as xylem vessel.
*Non-lignified areas termed pits – not completely open, still retain cellulose cell wall.

Question 35

What do xylem pits allow for?

Answer 35

Lateral movement of water

Question 36

In what two ways does lignin thicken the xylem vessel? Which is better?

Answer 36

*Annular thickening (forms rings).
*Reticulate thickening (forms a spiral or network).
*Reticulate thickening better because allows for elongation of vessels as plant grows.

Question 37

Rules for drawing xylem vessel elements?

Answer 37

*If high powered drawing: do NOT include xylem parenchyma or sclerenchyma fibres.
*Make sure walls appear thick (use eyepiece graticule to give correct proportion of wall to lumen).
*Lumen has no contents and can be annotated as hollow.
*Pits too small to be seen.
*No cell surface membrane.

Question 38

What are xylem fibres?

Answer 38

Elongated sclerenchyma cells with walls that are thickened with lignin to provide support.

Question 39

What is lignin?

Answer 39

A biological polymer that provides mechanical strength

Question 40

How does structure of xylem vessel elements relate to function?

Answer 40

*Continuous elongated cells = unobstructed flow.
*Vessel is hollow and dead = no cytoplasmic contents to obstruct flow.
*Secondary thickening = can withstand tension; hydrophobic so waterproofs.
*Annular, spiral and reticulate lignin thickening = mechanical support; allow xylem vessel to elongate during growth.
*Pits = lateral flow of water. Can connect adjacent xylem vessels. Water can use pits to detour around air bubbles obstructing flow.
*Cellulose walls = adhesion. Helps water resist gravity and maintain upward flow.

Question 41

What is secondary thickening?

Answer 41

When cellulose cell wall is thickened with lignin.

Question 42

Describe xylem parenchyma?

Answer 42

*Unspecialised cells act as packing tissues.
*Roughly spherical in shape but when TURGID press upon and flatten each other.
*Therefore, provide hydrostatic support.

Question 43

Describe tracheids?

Answer 43

*Other cell type in xylem tissue.
*Basically the same as xylem vessels.
*Longer, thinner, tapering ends.
*Thickened with lignin.
*Pits to allow lateral movement of water.

Question 44

Phloem sap

Answer 44

mineral ions, sugars, amino acids, plant hormones dissolved in water

Question 45

What is the structure of the phloem (overview)?

Answer 45

*Sieve tube elements
*Companion cells

Question 46

Structure of sieve tube elements:

Answer 46

*Elongated cells form long tube.
*Peripheral cytoplasm found under cell surface membrane. Mitochondria and modified form of endoplasmic reticulum can be found. No nucleus, no ribosomes, no Golgi body. These are present in companion cells so fewer cellular structures obstruct flow of phloem sap.
*NB: sieve tube cells are living, unlike mature xylem vessels.
*End walls of sieve tubes perforated (pierced with holes) by large pores (2-6 um in diameter). These are termed sieve plates.
*Central space in sieve tube is termed the lumen.

Question 47

What is the relation between companion cells and phloem sieve tube elements?

Answer 47

*Comes from same cell division as corresponding phloem sieve tube element.
*Companion cells carry out life processes necessary to sustain phloem sieve tub elements and itself. (Somewhat of a surrogate cell).

Question 48

What is the structure of a companion cell?

Answer 48

*Many plasmodesmata for loading of assimilates.
*Specific membrane carrier proteins for areas lacking plasmodesmata.
*Many organelles, dense cytoplasm, thin cellulose cell wall.

Question 49

What are transfer cells?

Answer 49

*Specialised companion cells found in leaf tips.
*Very folded cell walls and cell surface membranes. (increases SA:V)
*Large SA increases rate of transfer of sucrose into sieve tube elements.

Question 50

How does the structure of phloem sieve tube elements relate to their function?

Answer 50

*Elongated and arranged to form continuous tube.
*Missing many organelles for unobstructed flow, open lumen.
*Sieve plates perforated, reducing resistance to flow.
*Sieve plates hold walls of sieve tube elements together – prevents lysis from pressure.
*Individual cell walls contain cellulose microfibrils, provides strength.

Question 51

How does the structure of companion cells relate to their function?

Answer 51

*Contain organelles to physiologically support sieve tube.
*Many mitochondria to release ATP needed for translocation.
*Plasmodesmata in regions of loading and unloading.

Question 52

How to identify phloem sieve tube elements in a prepared slide?

Answer 52

*Phloem sieve tube elements have thinner walls proportionate to their lumen than xylem vessel elements.
*Cell wall contains cellulose so should stain blue/green.
*Longitudinally appear as elongated individual cells joined by cross walls (sieve plates).

Question 53

What colour will xylem vessel wall stain in a prepared slide?

Answer 53

Red. (Lignin stains pink.)

Question 54

How to identify companion cells in a prepared slide?

Answer 54

*Longitudinally appear much narrower than phloem sieve tube element.
*Smaller diameter.

Question 55

How to draw phloem sieve tube elements and companion cells from a prepared slide?

Answer 55

Draw cells touching each other unless specified otherwise.

Question 56

Starch

Answer 56

insoluble, therefore does not affect water potential.

Question 57

What is the outlined pathway of water from root to leaf?

Answer 57

Root hair cells take up water -> water passes through root cortex into xylem -> xylem carries water up to aerial parts.

Question 58

How does water get taken up by root hair cells?

Answer 58

*Root hairs extend into spaces around soil particles.
*Surrounded by soil solution (mostly water and little bit of mineral ions).
*Soil solution has higher W.P than root hair cell so moves via osmosis into cell.
*From root hair cell water moves across cortex through either apoplast pathway or symplast pathway.

Question 59

How does the apoplast pathway work?

Answer 59

*Basically: water is transported through the cell walls of cortical cells.
*Water (starts off in root hair cell walls) -> root cortex -> endodermis (ring around xylem and phloem).
*As water is drawn into endodermal cells, PULLS more water along with it. Due to COHESION.
*This causes a tension that draws water along cell walls of cortical cells and along intercellular spaces between cortical cells.
*Moves from cell to cell in this manner.

Question 60

Why can apoplast pathway be described as non-living?

Answer 60

Water does not cross any cell surface membranes or enter cytoplasm of any cells.

Question 61

How does the symplast pathway work?

Answer 61

*Basically: water moves across cytoplasm of cortical cells as a result of W.P differences between cells.
*Water moves from cell to cell through PLASMODESMATA so does NOT cross through cell wall or membrane.
*Water enters root hair cell and makes W.P higher.
*That root hair cell then has higher W.P than adjacent cortical cell.
*Water moved from root hair cell into cortex via plasmodesmata (down W.P gradient).
*Water moves towards core of cortical from cell to cell down W.P gradient (like dominoes).
*AT SAME TIME, W.P in first cortical cell is lowered as water moves towards cortex so more water is drawn up to replaces it. Goes on and on and on: (water potential gradient is established).

Question 62

Why is the symplast pathway slower?

Answer 62

flow is obstructed by cytoplasm and organelles

Question 63

Protoplast

Answer 63

all contents of cell excluding cell wall

Question 64

How does water move from apoplast/symplast pathway into xylem?
Note: xylem situated in centre of root, surrounded by phloem.

Answer 64

*Water from apoplast pathway prevented from moving further along cell wall. Casparian strip in endodermal cells (made of waterproof suberin) stops it. *Remember endodermis is ring around xylem and phloem in root.
*Water from apoplast must join protoplast via osmosis and join up with symplast flow in cytoplasm.
*Water leaving endodermis passes through pericycle (layer of parenchyma cells).
*Once it enters the xylem it defaults back to apoplastic pathway. (so just takes a little detour with symplast pathway through endodermis).

Question 65

How do mineral ions move into the root?

Answer 65

*Move into root mostly via active transport because very low concentration in soil compared to root hair cell.
*Specific membrane proteins facilitate this.

Question 66

Transpiration

Answer 66

the loss of water vapour from the aerial parts of a plant

Question 67

What is the main force that actually pulls water up the stem?

Answer 67

the evaporation of water vapour from spongy mesophyll cells into intercellular air spaces, and then water diffuses out of open stomata into atmosphere.

Question 68

How does water move across the leaf?

Answer 68

*A water potential gradient is established that pulls water from xylem, across leaf mesophyll, then into the atmosphere.
*Humidity of atmosphere is usually less and has lower water potential than sub-stomatal air space so water vapour diffuses out of air spaces. Water vapour lost from air spaces replaced by water vapour evaporating from spongy mesophyll cells. Means that intercellular air spaces are saturated.
*Water evaporating from cell walls of spongy mesophyll replaced by water coming from xylem via apoplast or symplast pathway.

Question 69

How can plants control water loss?

Answer 69

Plant control water loss by closing stomatal pored with guard cells.

Question 70

How does water move up stem in xylem?

Answer 70

Moves up by mechanism of cohesion-tension theory.

Question 71

How does cohesion-tension theory work?

Answer 71

*Water evaporates from spongy mesophyll cells due to heat from the sun. Leads to transpiration.
*Water molecules stick together by cohesion due to hydrogen bonds.
*Water forms continuous, unbroken column across mesophyll cells and down xylem. Known as transpiration stream.
*As water evaporates from mesophyll cells in leaf into intercellular and then sub-stomatal air spaces, more molecules of water drawn up behind as a result of cohesion.
*Column of water pulled up from xylem as result of transpiration – known as transpiration pull.
*Transpiration pull puts xylem under tension.

Question 72

Tension

Answer 72

negative inwards pressure, positive pressure pushes outwards

Question 73

What is evidence for cohesion-tension theory?

Answer 73

1. Change in tree trunk diameter
   * During height of day when transpiration is highest, there is most tension in xylem. Pulling inwards – so diameter shrinks. Lignin prevents collapse inwards of xylem.
   * When transpiration is at lowest then not as much pressure – tree trunk relaxes.
2. Water does not leak out if xylem vessel is broken
   * Air is pulled in instead because pulling pressure not pushing pressure.

Question 74

Reasons for adhesion of water to xylem vessel?

Answer 74

*Cellulose lining of xylem cell wall is hydrophilic.
*Lignin (hydrophobic molecule) has hydrophilic groups.

Question 75

Xerophytes

Answer 75

plants structurally and physiologically adapted to living in areas where water losses due to transcription may be higher than their water uptake.

Question 76

What would happen if plants didn’t have xerophytic adaptations

Answer 76

• plants would become plasmolysed and desiccated.
• When a plant cell becomes plasmolysed the cytoplasm pulls away from the cell wall

Question 77

Cuticular evaporation

Answer 77

when water is lost from the cuticle of a plant.

Question 78

Xerophytic adaptations of leaves that reduce transpiration?

Answer 78

*Having a thick cuticle – prevents cuticular evaporation
*Curling up of leaves – traps a region of still air within the curled leaf, region becomes saturated with water vapour so there is only a very slight water potential gradient between sub-stomatal air space and atmosphere. Transpiration reduced considerably.
*Having stomata in pits or grooves – trap moist air next to lead, reduce water potential gradient.
*Having sunken stomata – stomata located just below surface of epidermis. Not directly exposed to air movements.
*Having needle-like leaves – surface area to volume ratio reduced. Reduction in surface area balanced against need for sufficient area for photosynthesis.
*Having trichomes (hairs) on leaves – thick layer of hair on lower epidermis traps moist air next to the leaf surface. Water potential gradient reduced.
*Closing stomata when transpiration rates are very high – e.g., C4 plants (split light dependent and light independent phase of photosynthesis, makes efficient use of CO2) or production of plant hormone abscisic acid in response to stress and dehydration. Causes stomata to close.
*Having a multi-layered epidermis – reduce cuticular transpiration to increase diffusion distance of water vapour from intercellular air spaces.
*Epidermal cells and hypodermal cells may be impregnated with cutin.

Question 79

What do all these different adaptations all aim collectively to do?

Answer 79

*Decrease transpiration rate and water loss by reducing the water potential gradient between the sub-stomatal air space and the external atmosphere.
*Increasing diffusion distance of water vapour from saturated intercellular air spaces.
*Maintaining volume of leaf reducing SA exposed for transpiration.

Question 80

What purposes does transpiration serve?

Answer 80

*Supplies leaf cells with water and mineral ions
*Supply water as a reactant
*Maintains turgor pressure in cells for structural support
*Supply water for phloem sap, thereby enabling translocation

Question 81

Evaporation of water requires …

Answer 81

heat energy and so has a cooling effect on the plant

Question 82

Hypodermis

Answer 82

one or more layers of cells between the epidermis and palisade mesophyll

Question 83

Cutin

Answer 83

waxy, water-repellent substance in the cuticle of plants

Question 84

How to make annotated drawings of transverse sections of leaves from xerophytic plants?

Answer 84

*Cuticle separate, continuous transparent (not individual cells).
*Trichomes shown in proportion to dimensions of epidermal cells, should not be a single line.

Question 85

If you are asked to annotate drawing with notes explain in terms of:

Answer 85

*Reducing water potential gradient between saturated sub-stomatal air spaces and the external atmosphere e.g., trichomes, sunken stomata, stomata in grooves or pits, curled or rolled leaves.
*Increasing distance for diffusion of water vapour from saturated intercellular air spaces e.g., thick walled cells at epidermis or below epidermis, cuticle, hypodermis.
*Maintaining volume of leaf but reducing SA exposed for water vapour, e.g., needle-shaped leaves.

Question 86

What are the different habitats of xerophytic plants?

Answer 86

*Desert plants
*Sand dunes (rainfall drains away through pores in sand)
*Dry, windy places
*Salt marshes
*Cold regions (water freezes) *Note: may have curled or rolled leaves that limits transpiration but at the same time minimises SA for photosynthesis. This is because in cold regions enzymes can’t work as optimally (due to temperature) so photosynthesis takes place at a slower rate and there is a reduced need for the SA.

Question 87

Halophytes

Answer 87

plants adapted to live in saline habitats like salt marshes

Question 88

How does plant use glucose synthesised from photosynthesis?

Answer 88

Synthesis of…
*starch
*cellulose
*amino acids
*plants hormones

Question 89

Sources

Answer 89

site of synthesis

Question 90

Sinks

Answer 90

sites in plant requiring assimilates

Question 91

Examples of sinks?

Answer 91

Highly metabolic areas such as:
*Buds
*Shoot tips
*Root tips
*Flowers

Question 92

Translocation

Answer 92

can occur in either direction of phloem and at any one time phloem sap can be moving in different directions in different sieve tubes.

Question 93

What is in phloem sap?

Answer 93

*Assimilates (sucrose, amino acids)
*Plant hormones
*Inorganic ions (potassium, chloride, phosphate)
*Water

Question 94

Mass flow theory

Answer 94

explains mechanism of how translocation of assimilates is achieved.

Consists of three phases, namely:
1.Transfer of sucrose into sieve tube elements from photosynthesising tissue
2.Movement of phloem sap in sieve tube elements from source to sink
3.Transfer of sucrose from sieve tube elements into storage or other sink cells

Question 95

Apoplastic loading

Answer 95

the way that plants load sucrose into phloem sieve tubes

Question 96

Process of apoplastic loading/active loading:

Answer 96

• Sucrose manufactured from products of photosynthesis in cells that have chloroplasts.
• Hydrogen ions/protons are actively pumped from companion cells into apoplast (cell walls and spaces between cells) using ATP.
• Hydrogen ion concentration builds up in apoplast, hydrogen ions flow back to companion cells down concentration gradient through carrier proteins (facilitated diffusion).
• Sucrose molecules transported with hydrogen molecules in process known as cotransport. Carrier proteins known as cotransporter proteins.
• In cotransport hydrogen ions are flowing back down their concentration gradient but sucrose ions are going AGAINST their concentration gradient. The slow of sucrose ions back into companion cells is powered by flow of protons.
• Sucrose molecules then move by simple diffusion through plasmodesmata from companion cell into phloem sieve tube element.

Question 97

First phase of mass flow theory

Answer 97

Transfer of sucrose into sieve tube elements from photosynthesising tissue

Question 98

Second phase of mass flow theory

Answer 98

movement of phloem sap in sieve tube element from source to sink

Question 99

Mass flow

Answer 99

bulk movement of substance through given channel or area in a specified amount of time

Question 100

How does mass flow occur?

Answer 100

*Apoplastic loading causes sieve tubes to have lower W.P. than xylem. Water therefore moves from xylem to sieve tube element by osmosis – this creates a high hydrostatic pressure within them.
*At the sink, sucrose moves out of phloem sieve tubes to be used where needed.