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, cells in cortex have small air spaces between them and cells rich in starch grains, steel mainly xylem tissue with less phloem tissue

Structure of stems

vascular bundle around outside of stem to provide greater support to branches/leaves, in vascular bundles protoxylem xylem section closer to center of stem and metaxylem closer to edge, leaves branch off stems and vascular tissue continues from stem into leaf as midrib which branches to form veins

Where are leaf veins typically found?

spongy mesophyll just below palisade mesophyll layer

What does xylem tissue move and where?

water/dissolved minerals up roots

What is structure of xylem tissue?

large vessels with thick/strong walls and hard fibre cells to add support

Is mature xylem dead or alive?

dead

What does phloem tissue move and where?

moved food from photosynthetic tissue/food store to root system as raw material for cellular respiration or up to feed growing tips

Is mature phloem dead or alive?

alive

Is phloem simple or complex tissue and why?

complex as it has supporting fibres and two special cell types : sieve tubes and companion cells

What do xylem vessels consist of?

vessel elements stacked to form long, hollow pipes thickened by rings/spirals of lignin located as secondary wall inside primary cellulose cell wall

What are associated with xylem vessels?

tracheids

What are tracheids?

cells with end wall and no open end so rely on pits to allow water to leak through

Adaptations of xylem

no cytoplasm so faster flow, no end walls so water flows easily between cells, secondary cell wall impregnated with lignin so strong but flexible and microscopic pits between cells allow water to leak through

What occurs in xylem vessels to resist vessels collapsing?

thickening occurs in inner surface of vessel walls

Two thickening patterns of protoxylem vessels

spiral vessels and annular vessels

How are thickening patterns in protoxylem vessels formed?

first formed xylem vessels found in short lived structures, lignin laid down doesn’t restrict elongation of xylem vessels as growth occurs

Spiral vessels

spirally arranged thickened bands of cellulose and lignin

Annular vessels

rings of thickened cellulose and lignin

What happens to protoxylem vessels patterns?

both patterns allow for expansion so protoxylem differentiates and matures and subject to stress and stretched so many destroyed

Two thickening patterns of metaxylem vessels

pitted vessel and reticulated vessels

Pitted vessels

uniformly lignified except at pores allowing movement of water laterally into adjacent cells

Reticulated vessels

interconnected bars of lignin give irregular areas of unthickened walls

What happens to metaxylem vessels patterns?

in more permanent structures protoxylem collapses and replaced by metaxylem vessels, metaxylem matures after stem elongation complete

What are sieve tube elements?

part of phloem that transports sugars

What are associated with sieve tube elements?

companion cells

Sieve tube elements structure

lie end to end to form continuous stack and cellulose cell walls at ends of cells perforated to form sieve plates so movement between sieve tube elements easier

What happens when sieve tube elements mature?

when mature cells living with no nuclei and reduced cytoplasm volume and few organelles. Microtubules extend throughout sieve tube elements and involved in translocation of sugars

Structure and function of companion cell

dense cytoplasm and normal cell organelles, linked to sieve tube elements by plasmodesmata strands, act as supporting cells carrying out metabolic activities

Function of waxy cuticle

warerproofs leaf surface reducing evaporation of water from epidermis cells

How does transpiration happen?

water evaporates from cell walls of mesophyll cells so vapour builds up in air spaces, during day stomata open to allow diffusion in of CO2 but water vapour diffuses out too

What is stomatal transpiration?

evaporation from mesophyll surface and diffusion of water vapour out open stomata

Factors affecting transpiration

humidity, temperature, wind speed, light and water availability

How does humidity affect transpiration?

higher humidity externally reduces water potential gradient so less water transpired

How is high humidity mimicked?

plastic bag over plant

How does temperature affect transpiration?

increased temp = increased KE of water molecules and evaporation rate