2.1 c - Transport and exchange in plants

What transport tissue does root/stem contain?

vascular tissue

Two main types of transport tissue in plants

xylem and phloem

structure of roots

vascular bundle specialised for water, ion and organic molecules transport, cortex cells have air spaces and starch grains, steel mainly xylem tissue with some phloem

Structure of stems

vascular bundle around outside for support, protoxylem central and metaxylem to edge, vascular tissue extends into leaf as midrib, branching into veins

Where are leaf veins typically found?

spongy mesophyll

Respiratory tree

trachea, bonchi, bronchioles, alveolar duct, alveoli

What does xylem tissue move and where?

water/dissolved minerals up roots

Is mature xylem dead or alive?

dead

What does phloem tissue move and where?

food from photosynthetic tissue to roots for respiration or growing tips

Is mature phloem dead or alive?

alive

Is phloem simple or complex tissue and why?

complex as has supporting fibres, sieve tubes and companion cells

Structure of xylem vessels

vessel elements stacked into long hollow pipes, with lignin rings/spirals as secondary wall inside primary cellulose wall

What are tracheids?

cells with end wall and no open end so rely on pits for water movement

Adaptations of xylem

no cytoplasm so faster flow, no end walls for easy water movement, lignin impregnated walls are strong/flexible, pits allow water to leak

Sieve tube elements structure

lie end to end in continuous stack, end walls perforated into sieve plates for easy movement between elements

What happens when sieve tube elements mature?

living, no nuclei/reduced cytoplasm and few organelles. Microtubules extend throughout and aid sugar translocation

Structure and function of companion cell

dense cytoplasm with full organelles, linked to sieve tube elements by plasmodesmata, carry out metabolic activities to support sieve tube elements

Function of waxy cuticle

warerproofs leaf reducing evaporation from epidermal cells

How does transpiration happen?

water evaporates from mesophyll cell walls, vapour builds up in air spaces, stomata open for CO2 uptake but water vapour diffuses out too

Factors affecting transpiration

humidity, temperature, wind speed, light and water availability

Two pathways of water/mineral transport

apoplast and symplast

Fastest pathway for water/mineral transport

apoplast

Journey through apoplast pathway

water moves along cell walls and air spaces by capillary action. Blocked at stele as suberin forcing use of symplast pathway

Symplast pathway

movement through cytoplasm and kplasmodesmata. As water enters xylem draws water from neighbouring cells

Transport across roots

water move across root cortex down water potential gradient, root hairs give large SA. Ions moved into xylem lowers water potential, drawing water in by osmosis creating root pressure

What does the endodermis do in water movement?

forces water through symplast controlling ion entry to vascular bundle

How do ions enter root hair cell?

facilitated diffusion or active transport depending on conc grad

Cohesion-tension theory

evaporated water replaced by water pulled up xylem under tension. Aided by cohesion and adhesion

Define cohesion

attractive forces between water molecules

Define adhesion

water molecules sticking to cell wall

Describe translocation of organic materials in phloem?

sugars in leaves lower water potential, drawing water in raising pressure. At roots, sucrose converted to starch, raising water potential. Pressure gradient causes mass flow through sieve tube elements

Two features of translocation

energy requiring, occur in two directions

Evidence for energy expenditure

companion cells have high metabolic activity and actively load sucrose into sieve tube elements. Metabolic inhibitors stop respiration disrupting translocation

Evidence of mass flow in phloem hypothesis

cutting phloem releases soln under pressure, sucrose conc gradient, movement in living sieve tube elements. Viruses only translocated to roots when leaf photosynthesising

Adaptations of zerophytes

folded leaves enclose stomata, reduced SA and thick cuticle reducing water loss

Adaptations of hydrophytes

stomata only on upper surface, large air spaces enable floating

What are hydrophytes?

plants that grow in water