3.3- Transport in Plants (copy)

the need for plant transport systems

• large transport distances→ diffusion would not be fast enough to meet metabolic requirements of cells

• surface area: volume ratios→ SA:V ratio decreases so less surface area for absorption, greater volume= long diffusion distance

• increasing activity levels→ higher metabolic rate so more demand for oxygen and nutrients

adaptations of plants to increase SA:V ratio

• branching body shape

• flat and thin leaves

• root hairs on roots

mass transport systems in plants

• xylem→ transports water and mineral ions

• phloem→ transports sucrose and other nutrients

functions of the xylem

• carries dissolved minerals and water up the plant

• provides structural support

• food storage

function of the phloem

• transport organic compounds from source to sink

• transport can occur up or down plant

where are vascular bundles found in the plant

• Roots:

  • found in centre

  • xylem in centre core

  • phloem on edges of centre core

• Stems:

  • found around the outside

  • xylem found closest to centre of stem

  • phloem found closest to epidermis

• Leaves

  • vascular bundles form midrib and veins→ spread from centre of leaf

  • xylem found on upper side of epidermis

  • phloem found on lower side of epidermis

structure of the xylem (5)

• lignified cell walls→ adds strength to withstand hydrostatic pressure

• no end plates→ allows mass flow of water and dissolved solutes- cohesive and adhesive forces are not impeded

• no protoplasm (cells are dead when mature)→ doesn’t impede mass flow of water and dissolved solutes

• pits in walls→ lateral movement of water-. allows continual flow in case of air bubbles forming in the vessels

• small diameter of vessels→ helps prevent water column from breaking

structure of phloem sieve tubes

• sieve plates with sieve pores→ allows for continuous movement of organic compounds

• cellulose cell walls→ strengthens wall to withstand hydrostatic pressure

• no nucleus, vacuole, ribosomes→ maximises space for translocation of assimilates

• thin cytoplasm→ reduces friction to facilitate movement of assimilates

structure of companion cells (4)

• nucleus and other organelles present→ provides metabolic support to help with loading and unloading of assimilates

• transport proteins in plasma membrane→ moves assimilates in and out of sieve elements

• lots of mitochondria→ provides ATP for active transport of assimilates

• Plasmodesmata→ link to sieve tube which allows organic compounds to move from companion cells to sieve tubes

dicotyledonous plants

• seed networks contain two cotyledons (seed leaves)

• network of veins

• leaves that have broad blades and petioles (stalks)

• tap root with lateral branches

what is transpiration

• the loss of water vapour from a plant to its environment by evaporation and diffusion

advantages of transpiration

• cools plant

• transpiration stream is helpful in uptake of mineral ions

• turgor pressure of cells provides support to leaves and stems of non-woody plants

factors affecting rate of transpiration

• Movement of air:

  • air currents can sweep water molecules away from the leaf surface- maintains concentration gradient and increases rate of transpiration

• Temperature:

  • higher temp=higher E_k of molecules= water moves out of leaf at faster rate

  • if temp too high, stomata close to prevent excess water loss

• Light intensity:

  • stomata close in dark→ reduces rate of transpiration

• Humidity:

  • high humidity=more water in air= less of a concentration gradient= lower rate of transpiration

transpiration pathways

• apoplastic

• symplastic

apoplastic pathway

• most water travels through apoplastic pathway→ spaces running through cellulose cell walls, dead cells and xylem tubes

• water moves by diffusion either from cell wall to cell wall or through intercellular spaces

• casparian strip→ band of suberin that blocks apoplastic pathway when water reaches endodermis

  • when water reaches casparian strip it must take symplastic pathway

symplastic pathway

• smaller amount of water travels via symplastic pathway→ cytoplasm and plasmodesmata of cells

• water moves by osmosis into cell, through the vacuole and between cells through plasmodesmata

• symplastic movement slower than apoplastic movement

cohesion-tension theory

• water vapour from leaves causes water to move through xylem

• transpiration pull puts xylem under tension

• water has H-bonds with each other so will drag adjacent molecules with it

• continuous stream of water being pulled across mesophyll cells and up xylem

movement of water through leaves

• water vapour lost by transpiration lowers WP in air spaces surrounding mesophyll cells

• water in mesophyll cell walls evaporate into air spaces→ transpiration pull

• transpiration pull results in water moving through mesophyll cell wall or out of mesophyll cytoplasm into cell wall

• results in water leaving xylem vessels→ causes water to move up xylem vessels→ transpiration stream

role of stomata

• transpiration controlled by pairs of guard cells that surround stomata

• guard cells open stomata when turgid, close stomata when they lose water

• greater rate of transpiration when stomata open

• stomata generally open during day

translocation

• movement of assimilates through the plant through phloem

• movement form source to sink

• active process

sources of assimilates

• green leaves and green stem→ photosynthesis produces glucose which is transported as sucrose

• storage organs e.g. tubers and tap roots

• food stores in seeds

sinks of assimilates

• meristems that are dividing

• roots that are growing and/ or actively absorbing mineral ions

• any area where assimilates are being stored.

loading of assimilates

1. companion cells pump H+ ions out of cytoplasm and into cell walls- active process

2. high conc. of H+ ions in cell wall= move down conc. gradient back to cytoplasm of companion cell through cotransporter protein→ carries sucrose molecules with it.

3. sucrose molecules move into sieve tubes via plasmodesmata from comp. cells

unloading of assimilates

1. co-transport of sucrose and H+ ions into phloem lowers WP→ water moves in

2. high hydrostatic pressure at source= assimilates move down phloem

3. sucrose diffuses into sink cells

4. water potential in phloem is increased, so moves back to xylem

mass flow hypothesis/ pressure flow gradient in phloem

1. pressure difference created by loading sucrose→ lower WP in sap

2. water moves into sieve elements as it travels down WP gradient by osmosis

3. water in sieve elements= higher hydrostatic pressure

4. hydrostatic pressure gradient= mass flow of water from source to sink

xerophytes

plants adapted to dry and arid condtions

xeromorphic features

• succulent leaves→ stores water

• hinge cells shrink when flaccid→ leaves roll to expose waterproof cuticle- creates humid spaces in middle of rolled leaf

• reduced leaves i.e. spines, needles→ reduced SA so reduced transpiration

• stomata closed during day→ minimises daytime water loss

• sunken stomata→ water loss minimised by trapping moist air close to area of water loss, reducing diffusion gradient

• thick waxy cuticle→ water loss via cuticle is reduced

hydrophytic plants

plants adapted to live in freshwater

adaptations of hydrophytes

• cell walls prevent excess water uptake

• floating leaves→ more light for photosynthesis

• thin waterproof waxy cuticle→ little need to prevent water loss

• stomata on upper surface of leaves→ allows for gas exchange with air rather than water

• reduced root system→ only small roots required as nutrients can be extracted from surrounding water through their tissues

• reduced veins in leaves→ no need to transport water throughout the plant