Biology 🧬 Unit 7 - The Xylem

What is the function of the xylem

Transport water and mineral ions up the plant (one directional transport)

Provide support to the plant

what are sclerenchyma fibres (xylem fibres)

elongated sclerenchyma cells with lignified walls to help support the plant. They strengthen the whole bundle

They are dead cells and have no living contents

as xylem vessels mature the deposition of lignin hardens the cellulose thickening on the inside of the xylem’s lateral walls making them ………… to water (hydrophobic)

The lignin makes cross links with the ……… that fill the gaps between cellulose fibres

impermeable, hemicelluloses

As lignin builds up the contents of the cell die meaning…

no cytoplasm or nucleus to hinder water flow leaving a completely empty space or ‘lumen’ inside

What does lignin do/prevent

Ensure xylem does not fall in on itself when water moves up through it (resists negative pressure - suction)

Lignin disposition prevents inward collapse due to transpiration pressure

The process of lignified action causes the end walls of neighbouring vessel elements to break down completely to form a ….

continuous tube

What makes the xylem permeable

Pits - permit lateral movements of water to the surrounding tissues

What are pits

Non-lignified areas can be seen as gaps in the thin i walls of the xylem vessel, these pits were where plasmodesmata were found in the original cell walls so no lignin is laid down

What helps increase the adhesion of water molecules

The cellulose in the walls of all xylem elements (pitted and non-pitted areas) this helps to resist the effects of gravity and keep the column of water moving upwards

What are the different types of thickening in xylem vessels

Annular, spiral, reticulate

What happens to mineral ions once absorbed by the root

Are transported across the cortex of the root

They converge on the central stele and are actively secreted into the water column flowing up the xylem

The continuing movement of ions across the cortex of the root is partly responsible for the gradient in water potential between soil solution (higher WP) and the cells of the stele (lower WP)

What is transpiration

The loss of water vapour from the aerial parts of the plant, mainly the mesophyll cells of the leaves

What 2 things causes water to move up the xylem

1) root pressure

2) the transpiration stream

What has a strong cooling effect for a plant

Evaporation of water from the cells of the leaf in the light

What is plasmolysis

The loss of water via osmosis and accompanying shrinkage of the cytoplasm away from the cell wall; cell is said to be plasmolysed

‘Can’ be reversed if placed in fresh water however there is a point where it becomes permanent/fatal

Explain how root pressure causes water to move up the plant

Transpiration reduces the water (hydrostatic) pressure at the top of a xylem vessel compared with the pressure at the base, so causes water to flow up the vessels

Plants may also increase the pressure difference between top and bottom by raising water pressure at base of vessels

Epidermal/endodermal cells actively transport mineral ions into the xylem vessels; reducing water potential inside the xylem vessels and causes water to move into the xylem vessels through the symplast down the water potential gradient via osmosis

The entry of water into the xylem vessels of the roots causes a ‘root pressure’ the continual movement of water into the xylem vessels serves to push the water molecules already in the xylem vessel further upwards

The pushing/upwards force of root pressure is sufficient to push water up the xylem vessel of the stem to a height of 2-3 metres

When is root pressure particularly important

In spring before the growing season when the leaves have not formed and are therefore not transpiring

how does hydrostatic pressure cause water to move up the xylem

Removal of water from xylem vessels in the leaf reduces the hydrostatic pressure in the xylem vessels

The hydrostatic pressure at the top of the xylem vessel becomes lower than the pressure at the bottom

This pressure difference causes water to move up the xylem vessels - like sucking on a straw causing a pressure difference

The lower the hydrostatic pressure the ……. The water potential

lower

What is the main mechanism by which water moves up the xylem

cohesion-tension hypothesis (the transportation stream)

What is cohesion

the force by which individual molecules stick together. Inside xylem vessels water molecules are bonded together by hydrogen bonds (force of cohesion) forming a continuous unbroken column of water

What does cohesion of water molecules ensure and what does it create

How does adhesion help with this

Ensures that water is lost at the leaf by evaporation, it pulls the column of water further up the stem to replace the water molecules being lost. This creates a tension inside the xylem vessels

This process is helped by adhesion:

Many short-lived hydrogen bonds from between the water molecules and the lignin and cellulose molecules of the xylem vessels

What is adhesion

The force by which individual molecules (in this case water) cling to surrounding material and surfaces

What do cohesion and adhesion of water help with

Help to keep the water in a xylem vessel moving as a continuous column of water

Water is drawn up the stem so water flow in the xylem is always upwards

Water mostly evaporates from the surface of …….. ……. Cells, a process known as ……… and water vapour leaves through the stomata

Spongy mesophyll, transpiration

As water leaves the xylem vessels in the leaf and then evaporates a …… is set up on the entire water column in the xylem tissue (the tension acts upwards as water is being removed from the top of the plant

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This loss of water …… the water potential of the spongy mesophyll cells, causing water to move out of the xylem vessel into the surrounding cells down a water potential gradient by ……..

tension

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Lowers, osmosis

During the transpiration pull there is a …… pressure within the xylem

The more tension, the ….. the diameter of the xylem becomes

Negative

Smaller

Is the apoplast pathway living or non living

Non living as water does not enter the cytoplasm

Describe the apoplast pathway

occurs between the mesh-like cellulose fibres in the plant cell walls (the gaps between each side of the cell wall) these gaps are water-filled so offer little resistance to the flow of water

Water travels from cell wall to cell wall without ever entering the cytoplasm of the cortical cells (cells of the cortex)

The apoplast also includes the water-filled spaces of dead cells of the xylem vessels

Most water passes through the …….. pathway most of the time, a case of transport by ‘…….. …….’

Apoplast, mass flow

Are cellulose and lignin hydrophobic or hydrophilic

both hydrophilic

The casparian strip is a waterproof strip found in the ……… it blocks the ……… pathway so that water and ions have to be transported by the …….. pathway. It’s responsible for selective mineral uptake and controls the water …….. - it lowers the water potential in the xylem so that water moves up the xylem

endodermis, apoplast, symplast, potential

The only way for water to cross the endodermis is through the ….. of the endodermal cells not through the cell walls

As endodermal cells get older the Suberin deposits become more extensive except in certain cells called …….. cells, through which water can continue to pass freely, this is the route water must take

This gives plants control over what mineral ions pass into the xylem as everything has to cross the cell surface membrane here (preventing harmful toxins/viruses entering xylem)

Cytoplasm (symplast pathway)

Passage

once through the selective membrane of passage cells water passes into the ……. Pathway via osmosis and into the cytoplasm

Symplast

What happens to water once through the casparian strip and the short distance of the symplastic pathway

Most water returns to the apoplast pathway and travels the remaining short distance to reach the xylem vessels

How does water move into the xylem vessels

through the pits in their walls

Where are the apoplastic and symplastic pathways found

They occur in the root, stem and leaf

Describe how water moves in the symplastic pathway

1. Water enters the cytoplasm by osmosis through the partially permeable membrane
2. Water moves into the sap in the vacuole through the too plastic by osmosis
3. Water may move from cell to cell through the plasmodesmata
4. Water may move from cell to cell through adjacent cell surface membranes and cell walls

Describe how water moves in the apoplastic pathway

1. Water enters the cell wall
2. Water moves through the cell wall
3. Water may move from cell wall to cell wall through the intercellular spaces
4. Water may move directly from cell wall to cell wall

What are plasmodesmata

Strands of cytoplasm that directly connect one cell to the adjacent cell so that water can take a cytoplasmic route across the cortex, towards the endodermis, essentially creates a continuous cytoplasm extending from the root hair to the xylem at the centre of the root

When transpiration rates are especially high, more water travels by the ……. Pathway

Apoplastic

How does water move from the xylem across the leaf

Once water is lost from the spongy mesophyll cells and into the air spaces around them, they have a relatively lower water potential as they have lost water from their cytoplasm.

So water enters from neighbouring cells

These adjacent cells have also lost water so they also need more water from neighbouring cells

In this way a water potential gradient is established that pulls water from the xylem across the leaf mesophyll and finally into the atmosphere