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;

Transport in Plants

• Plants need a transport system so that cells deep within the plants tissues can receive the nutrients they need for cell processes

• The problem in plants is that roots can obtain water, but not sugar, and leaves can produce sugar, but can’t get water from the air

What substances need to be moved?

• The transport system in plants is called vascular tissue

• Xylem tissue transports water and soluble minerals

• Phloem tissue transports sugars

The Vascular Tissues

• Xylem and phloem are found together in vascular bundles, that sometimes contain other tissues that support and strengthen them

Root vs. stem vs. leafThe vascular bundle

differs depending on if it is a root or stem

Root• The vascular bundle is found

in the centre• There is a large central core of

xylem- often in an x-shape• This arrangement provides

strength to withstand the pulling forces to which roots are exposed

• Around the vascular bundle are cells called the endodermis which help to get water into the xylem vessels

• Just inside the endodermis is the periycle which contains meristem cells that can divide (for growth)

Stem• The vascular bundles are found near the outer edge of the stem• The xylem is found towards the inside of each vascular bundle, the

phloem is found towards the outside• In between the xylem and phloem is a layer of cambium• Cambium is a layer of meristem cells that divide to make new xylem

and phloem

Leaf

• The vascular bundles (xylem and phloem) form the midrib and veins of the leaf

• A dicotyledon leaf has a branching network of veins that get smaller as they branch away from the midrib

• Within each vein, the xylem can be seen on top of the phloem

Phloem

Xylem

Stem

A = Xylem

B = Phloem

C/D = Upper/Lower epidermis

Leaf

Xylem vessel wall

Phloem

Root

Endodermis

Xylem vessel lumen

Starch grains

Structure of Xylem

• Used to transport water and minerals from roots to leaves

• Consists of tubes for water, fibres for support and living parenchyma cells

Xylem vessels

• Obvious in dicotyledonous plants• Long cells with thick walls containing lignin• Lignin waterproofs walls of cells and strengthens

them• Cells die and ends decay forming a long tube• Lignin forms spiral, annular rings or broken rings

(reticulate)• Some lignification is not complete and pores are

left called pits or bordered pits, allowing water to move between vessels or into living parts

Adaptations of Xylem to Function

• Xylem can carry water and minerals from roots to shoot tips because:

• Made of dead cells forming continuous column• Tubes are narrow so capillary action is effective• Pits allow water to move sideways• Lignin is strong and allows for stretching• Flow of water is not impeded as: there are no

end walls, no cell contents, no nucleus, lignin prevents tubes collapsing

Structure of Phloem

• Function to transport sugars from one part to another

• Made of sieve tube elements and companion cells

Sieve Tubes

• Sieve tube elements not true cells as they have little cytoplasm

• Lined up end to end to form a tube

• Sucrose is dissolved in water to form a sap

• Tubes (known as sieve tubes) have a few walls across the lumen of the tube with pores (sieve plates)

Companion cells

• In between sieve tubes• Large nucleus, dense

cytoplasm• Many mitochondria to

load sucrose into sieve tubes

• Many plasmodesmata (gaps in cell walls between companion cells and sieve tubes) for flow of minerals

Water route between cells

• Apoplast: between cell walls of neighbouring cells

• Symplast: through plasma membrane and plasmodesmata to cytoplasms from cell to cell

• Vacuolar: same as symplast, but also through vacuoles

Water uptake from the soil

• Epidermis of roots contain root hair cells

• Minerals absorbed by active transport using ATP

• Minerals reduce the water potential in the cell cytoplasm (more negative) so water is taken up by osmosis

Movement across the root• Active process occurring at the endodermis (layer of cells surrounding the

xylem, some containing waterproof strip called casparian strip)• Casparian strip blocks the apoplast pathway (between cells) forcing water

into the symplast pathway (through the cytoplasm)• The endodermis cells move minerals by active transport from the cortex into

the xylem, decreasing the water potential (more negative), thus water moves from the cortex through the endodermal cells to the xylem by osmosis

• A water potential gradient exists across the whole cortex, so water is moved along the symplast pathway (through cytoplasm) from the root hair cells across the cortex and into the xylem

Casparian Strip

• Blocks the apoplast pathway (cell walls)• Water and dissolved nitrate ions have to pass

into the cell cytoplasm through cell membranes• There are transporter proteins in the cell

membranes that actively transport nitrate ions into the xylem lowering the water potential (more negative)

• Water enters xylem down concentration gradient and cannot pass back

Water movement up stem

• Root pressure: minerals move into xylem by active transport, forcing water into xylem and pushes it up the stem

• Transpiration Pull: loss of water at leaves replaced by water moving up xylem. Cohesion-tension theory- cohesion between water molecules and tension in the column of water (which is why xylem is strengthened with lignin) means the whole column of water is pulled up in one chain

• Capillary action: adhesion of water to xylem vessels as they are narrow

How water leaves the leaf

• Through stomata• Tiny amount through the waxy

cuticle• Water evaporates from the

cells lining the cavity between the guard cells, lowering water potential and meaning that water enters them by osmosis

from neighbouring cells, which is replaced by further neighbouring cells and so on

Transpiration

• Loss of water vapour from upper parts of the plant

• Water enters leaf from xylem and passes to mesophyll cells by osmosis

• Water evaporates from surface of mesophyll cells to form water vapour (air spaces allow water vapour to diffuse through leaf tissue)

• Water vapour potential rises in air spaces, so water molecules diffuse out of the leaf through open stomata

Transpiration: three processes

• Osmosis from xylem to mesophyll cells

• Evaporation from surface of mesophyll cells into intercellular spaces

• Diffusion of water vapour from intercellular spaces out through stomata

Water use in plant

• Photosynthesis

• Cell growth and elongation

• Turgidity

• Carriage of minerals

• Cools the plant

Measuring transpiration

• Potometer is used to estimate water loss

Factors affecting transpiration

• Leaf number: more leaves, more transpiration• Number, size, position of stomata: more and large, more

transpiration, under leaf, less transpiration• Cuticle: waxy cuticle, less evaporation from leaf surface• Light: more gas exchange as stomata are open• Temperature: high temperature, more evaporation, more

diffusion as more kinetic energy, decrease humidity so more diffusion out of leaf

• Humidity: high humidity, less transpiration• Wind: more wind, more transpiration• Water availability: less water in soil, less transpiration

(e.g. in winter, plants lose leaves)

Too much water loss

• Less turgidity

• Non-woody plants wilt and die

• Leaves of woody plants die first then it will die if water loss continues

Xerophytes• Smaller leaves reducing surface area e.g. pine tree• Densely packed spongy mesophyll to reduce surface area, so less

water evaporating into air spaces• Thick waxy cuticle e.g. holly leaves to reduce evaporation• Closing stomata when water availability is low• Hairs on surface of leaf to trap layer of air close to surface which

can become saturated with water, reducing diffusion• Pits containing stomata become saturated with water vapour

reducing diffusion• Rolling the leaves so lower epidermis not exposed to atmosphere

also traps air which becomes saturated• Maintain high salt concentration to keep water potential low and

prevent water leaving

Marram GrassLeaf rolled up to trap air inside

Thick waxy cuticle to reduce water evaporation from the surface

Trapped air in the centre with a high water potential (less negative)

Hairs on lower surface reduce movement of air

Stomata in pits to trap air with moisture close to the stomata

Movement of Sugars

• Translocation: movement of assimilates (sugars and other chemicals) through the plant

• Source: a part of the plant that releases sucrose to the phloem e.g. leaf

• Sink: a part of the plant

that removes sucrose from

the phloem e.g. root

Sucrose Entering the Phloem

• Active process (requires energy)• Companion cells use ATP to transport hydrogen

ions out of their cytoplasm• As hydrogen ions are now at a high

concentration outside the companion cells, they are brought back in by diffusion through special co-transporter proteins, which also bring the sucrose in at the same time

• As the concentration of sucrose builds up inside the companion cells, they diffuse into the sieve tubes through the plasmodesmata (gaps between sieve tubes and companion cell walls)

Sucrose movement through phloem

• Sucrose entering sieve tube lowers the water potential (more negative) so water moves in by osmosis, increasing the hydrostatic pressure (fluid pushing against the walls) at the source

• Sucrose used by cells surrounding phloem and are moved by active transport or diffusion from the sieve tube to the cells. This increases water potential in the sieve tube (makes it less negative) so water moves out by osmosis which lowers the hydrostatic pressure at the sink

Movement along the phloem

• Water entering the phloem at the source, moving down the hydrostatic pressure gradient and leaving at the sink produces a flow of water along the phloem that carries sucrose and other assimilates. This is called mass flow. It can occur either up or down the plant at the same time in different phloem tubes

Evidence for translocation• Radioactively labelled carbon from carbon dioxide can appear in the

phloem• Ringing a tree (removing a ring of bark) results in sugars collecting

above the ring• An aphid feeding on the plant stem contains many sugars when

dissected• Companion cells have many mitochondria• Translocation is stopped when a metabolic poison is added that

inhibits ATP• pH of companion cells is higher than that of surrounding cells• Concentration of sucrose is higher at

the source than the sink

Evidence against translocation

• Not all solutes move at the same rate

• Sucrose is moved to parts of the plant at the same rate, rather than going more quickly to places with low concentrations

• The role of sieve plates is unclear

Useful Revision Sites

• http://scienceaid.co.uk/

• http://www.s-cool.co.uk/

• http://www.sparknotes.com/biology/