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

Therefore the cohesion- tension theory

Describes water movement up the xylem by this combination of adhesion of water moelcueld and tension in the water column resulting from their cohesion

Mechanism 2: Capillarity

Movement of water up narrow tubes, in this case the xylem, by capillary action

Role of capillarity

Only operated over short distances, up to a metre, may have a role in mosses but only makes small contribution to water movement in plants more than a few cm high

Mechanism 3: root pressure

Operates over short distances in living plants and is a consequence of osmotic movement of water into the xylem pushing water already there further up

What is root pressure caused by

The osmotic movement of water down the WPG across the root and into base of xylem

Transpiration stream

Continual flow of water in at the roots, up the stem to the leaves and out to the atmosphere

In a transpiration stream

Water is drawn upwards by

1) the cohesive forces between water molecules

2) adhesive forced between water molecules and hydrophilic lining of xylem vessels

Transpiration

Evaporation of water vapour from the leaves out through the stomata into the atmosphere

Problem for plants

They must balance uptake with water loss , if they lose more water than they absorb they wilt , if only a small volume of water is lost the plant recovers when water is available , if an excessive water is lost plant cannot regain its turgir after wiliting and it dies

Another dilemma

Stomata must be open during the day to allow gas exchange between leaf tissue and atmosphere but this means plant lose valuable water

Rate at which water is lost from the plant

Transpiration rate

Two main factors affecting transpiration rate

Genetic factors such as those controlling the number , distribution and size of stomata

Environmental factors that affect the WPG between the water vapour in the leaf and the atmosphere so they affect the rate of transpiration

Temperature

A temp increases lower the WP of the atmosphere , causes water moelcueld to diffuse away from leaf more quickly reducing WP around the leaf

Another effect of temp increase

Increases KE of water molecules accelerating their rate of evaporation from walls of mesophyll cells and if the stomata is open it speeds up their rate of diffusion out into the atmosphere

Explain humidity

Air inside the leaf is saturated with water vapour, relative humidity is 100%, humidity of atmosphere surrounding leaf varies but never greater than 100%

Therefore

There is a WP gradient between the leaf and the atmosphere , when the stomata are open water vapour diffuses out the leaf down a WPG

Transpiration in still air

Results in the accumulation of a layer of saturated air at the surfaces of leaves

The water vapour

gradually diffuses away leaving concentric rings of decreasing humidity the further away you go from thr leaf

Therefore

The higher the humidity, the higher the water potential , water vapour diffuses down this gradient of relative humidity which is also a gradient of water potential away from the leaf

Graph showing the effect of humidity on transpiration

Air movement

Movement of surrounding air blows away the layer of humid air at the leaf surface

Therefore

Water potential gradient between the inside and outside of the leaf increased , water vapour diffuses out through the stomata more quickly

So overall affect of air movement of transpiration rate

The faster the air is moving , the faster the concentric shells of water vapour get blow away the faster transpiration occurs

How does light intensity affect the rate of transpiration

By controlling the degree of stomatal opening, the stomata open wider as light intensity increases increasing the rate of transpiration. Stomata open widest in the middle of the day, less widely in evening and close at night

Potometer (sometimes called a transpirometer)

Doesn’t primarily measure the rate of transpiration, measures water uptake

Why does that work

most of the water taken up by the leafy shoot is lost through transpiration, rate of uptake is almost the same as the rate of transpiration

Use of potometer

Measures rate of uptake by the same shoot under different conditions, can be used to compare the uptake by leafy shoots of different species under the same conditions

First 4 steps of how to set up a potometer

1- cut a leafy shoot under water so no air enters the xylem

2-under water, fill the potometer with water ensuring there are no air bubbles

3-fit the leafy shoot to the potometer with rubber tubing under water to prevent air bubbles forming in the apparatus or the xylem

4- remove the potometer and shoot from the water seal joints with Vaseline and dry carefully

Next 4 steps in settling up a potometer

Introduce an air bubble or meniscus into the capillary tube

Measure the distance the air bubble or meniscus moves in a given time

Use the water reservoir to bring the air bubble or meniscus back to the starting point, repeat measurement and calculate mean distance

Experiment may be repeated to compare the rates of water uptake under different conditions e.g. altered light intensity or air movement

Translocation

Transport of soluble products of photosynthesis such as sucrose and amino acids through the phloem from sources to sinks

The source and sinks

Transported away from the site of synthesis in leaves (sources) to all other parts of the (the sinks) where they are used for growth or storage

Unlike the xylem

Xylem transports water and dissolved minerals upwards, phloem can translocate upwards, down, sideways to wherever the products of PT are needed

Phloem

Living tissue, consists of several types of cells including sieve tubes and companion cells

Sieve tubes

Adapted for the flow of material comprising end to end cells called sisvs tube elements

Comparison with the xylem vessels

Their end walls don’t break down , they are perforated in areas called sieve plates and so are parts of the side walls sometimes

Then what

Cytoplasmic filaments containing phloem protein extend from one sieve tube element to the next through the pores in the sieve plate

Another adaptation of sieve tube elements

They lose their nucleus and most of their organelles during development allowing space for tarsnoroting materials

Metabolism of sieve tube elements

Controlled by at least one neighbouring companion cell , biochemically very active as indicated by the large nucleus and dense cytoplasm containing much RER and many mitochondria

Companion cells and sieve tube elements

Companion cells are connected to sieve tube elements by plasmodesmata

Evidence

Experimental evidence shows that organic molecules are trans located in the phloem

Technique 1

Ringing experiments: cylinder of outer bark tissues were removed from all the way around a woody stem in a ring which removed the phloem

Step 2

After leaving the plant some time whilst it PTed, the phloem contents above and below the ring were analysed

Results

Above the ring there was a lot of sucrose suggesting it had been translocated in the phloem

Below the ring there was no sucrose suggesting it had been used by plant tissues but not replaced

Why was the sucrose not replaced

The ring prevented it from being moved downwards

What also showed this

The bark above the ring swelled slightly because solutes were accumulating as they couldn’t move down below the ring

Technique 2

Radioactive tracers and autoradiography: plant PT in the presence of a radioactive isotope such as carbon-14 in carbon dicoidd

Method

A stem section is placed on a photographic film which is exposed if there is a radiation source producing an autotadiograoh q

Results

The position of exposure and therefore the radioactivity coincides with the position of the phloem indicating it is the phloem that translocates the sucrose made from the RA carbon dioxide in PT

Technique 3

Aphid experiment: aphid has a hollow needle like mouthpart called the stylet inserted into a sieve tube and the phloem contents (sap) exude under pressure into the aphid’s stylet

Step 2

In some experiments the aphid was anaesthetised and removed , its stylet remained embedded in the phloem

Results

As the sap in the phloem is under pressure it exudes from the stylet and is collected and analysis showed the presence of sucrose

technique 4

Aphid and radioactive tracers: aphid experiments were extended to plants which had been photosynthesising with carbon dioxide -14

Results

Showed that the radioactivity and therefore the sucrose made in PT move at a speed of 0.5-1 metres per hour

Problem with these results

This is much faster than the rate diffusion alone so some additional mechanism had to be considered

Mass flow hypothesis

Hypothesis proposed to explain translocation suggests there is a passive mass flow of sugars from the phloem of the leaf where there is the highest concentration (the source) to other areas such as growing tissues where there is a lower concentration (the sink)

Step 1 of the mass flow hypothesis

Sucrose made in PT inside leaf cells( source) makes the WP in leaf cells very negative, water passes into cells by osmosis