adpatations for trnaport in plants

What do vascular tissues do

Transports materials around the body

In animals vs in plants

Animals- vascular tissues is blood

Plants- vascular tissues is xylem and phloem found adjacent to each other in vascular bundles

Distributions

They have different distributions in different parts of the plant

In roots

xylem is central, star shaped with phloem between group of xylem cells

This arrangement

Resists vertical stresses (pull) and anchors the plant in the soil

In stems

Vascular bundles are in a ring at periphery with xylem towards the centre and the phloem towards the outside giving flexible support and resists bending

In leaves

Vascular tissues is in the midrib and in a network of veins giving flexible strength and resistance to tearing

Main cell types in xylem

vessels and tracheids

Tracheids

Occur in ferns, conifers, angiosperms (flowering plants) not in mosses

Moses

Have no water conducting tissues and are poorer at transporting water and cannot grow as tall as these other plants

Vessels

Occur in angiosperms only

In vessel cells

As lignin builds up in their cell walls the contents die leaving an empty space which is the lumen

As the tissue develops

The end walls of the cells break down leaving a long hollow tube like a drainpipe through which water climbs straight up the plant

The lignin

is laid down in a characteristic spiral pattern and unlike cellulose of phloem cell walls , stains red so xylem is easy to identify in microscope sections

Two functions of the xylem

Transport of water and dissolved minerals

Providing mechanical strength and suppprt

terrestrial plants and water

e.g. animals risk dehydration and must conserve water

1) Water uptake by the roots

Water is taken up from the soil through the roots and transported to the leaves where it maintains turgidity and is a reactant in PT

However

Much water is lost through the stomata in process of transpiration

This loss

Must be offset by constant replacement from the soil

The region of the greatest uptake

Root hair zone where the SA of the root is increased by the presence of the root hairs and uptake is enhanced by their thin cell walls

Soil water

Has a very dilute solution of mineral salts and has a high WP

Whereas

The vacuole and the cytoplasm of the root hair cell contain concentr solution of solutes and have a lower more negative WP so water passes into the root hair cells by osmosis down a WP gradient

2) movement of water through root

Water must move into the xylem to be distributed around the plant. It can travel to the xylem across cells of the root cortex by 3 different routes

Route 1

Apoplast pathway - water moves in the cells walls, cellulose fibres in the cell wall are separated by spaces through which the water moves

Route 2

Symplast pathway- water moves through cytoplasm and plasmodesmata, so the symplast is a continual pathway across root cortex

Plasmodesmata

strands of cytoplasm through pits in the cell wall joining adjacent cells

Route 3

Vacuolar pathway - water moves form vacuole to vacuole

The difference between the pathways

Two main ones are the symplast and the apoplast pathways, apoplast is faster so is probs the most significant

however

Water cannot enter the xylem from the apoplexy because lignin makes xylem walls waterproof

Therefore water can only

Pass into the xylem from the symplast or vacuolar pathways so it must leave the apoplast pathways

How does this happen

the vascular tissues in the centre of the root is surrounded by a region called the pericycle which is surrounded by a single layer of cells- endodermis

Endodermis

Single layer of cells around the pericycle and vascular tissue of the root, each cell has an impermeable waterproof barrier in its cell walls

The cell walls of endodermis cells

Impregnated with a waxy material- suberin forming a distinctive band on the radial and tangential walls called the casparian strip

Casparian strip

Impermeable band of Suberin in cell walls of endodermal cells blocking the movement of water in the spillway driving it into the cytoplasm

Since the Suberin is waterproof

The casparian strip prevents water moving further in the apoplast and drives it into the cytoplasm

How does water then move from the root endodermis into the xylem

By osmosis across the endodermal cell membranes into xylem by down WP gradient for this to be efficient the WP of the xylem must be much more negative than WP of the endodermal cells

First way through which this is achieved

WP of the endodermis cells is raised by water being driven in the casparian strip

Second way through which this is achieved

The WP of the xylem is decreased by active transport of mineral salts mainly Na+ ions from the endodermis and the pericycle into the xylem

Water moving into the xylem

Generates an upwards push - root pressure on water already in the xylem

3) movement of water from roots to leaves

Water moves down a WPG , air has a very low WP and soil water (very dilute solution) has a very high WP so water moves from the soil through plant into the air

Mechanism 1: cohesion-tension

Water vapour evaporates from leaf cells into air spaces and diffuses out through stomata into the atmosphere drawing water across the cells of the leaf in the apoplast, symplast, vacuolar pathways from the xylem

As water moelcules leave xylem cells in the leaf

They pull up other water moelcules behind them in the xylem, moelcules move beacsue they show cohesion, this continuous pull produces tension in the water column

Cohesion

Attraction of water moelcules for each other seen as hydrogen bonds resulting from the dipole structure of water molecule

Contributing to water movement up the xylem

Charges on water molecules causing attraction to the hydrophilic lining of the vessels- this is adhesion