Summary:Water is transported through a plant in xylem vessels. This is a passive process, in which water moves down a water potential

gradient from soil to air. Water enters root hairs by osmosis, crosses the root either through the cytoplasm of the cells or via their cell

walls, and enters the dead, empty xylem vessels by mass flow, as a result of pressure differences caused by loss of water from leaves

by transpiration. Root pressure can also contribute to this pressure difference.

Transpiration is an inevitable consequence of gaseous exchange in plants. Plants have air spaces within the leaf linked to the

external atmosphere through stomata, so that carbon dioxide and oxygen can be exchanged with their environment. Water vapour,

formed as water evaporates from wet cell walls, also diffuses through these air spaces and out of the stomata.

The rate of transpiration is affected by several environmental factors, namely temperature, light intensity, wind speed and

humidity. It is difficult to measure the rate of transpiration directly, but water uptake can be measured using a potometer. Plants

that are adapted to live in places where the environmental conditions are likely to cause high rates of transpiration, and where soil

water is in short supply, are called xerophytes. They have often evolved adaptations that help to reduce the rate of loss of water vapour

from their leaves.

Translocation of organic solutes, such as sucrose, occurs through living phloem sieve tubes. The phloem sap moves by mass flow, as

a result of pressure differences produced by active loading of sucrose at sources such as photosynthesising leaves.

(1) Carbon dioxide (for photosynthetic plant cells)

(2) Oxygen (for respiration)

Organic nutrients(3)

(4) Inorganic ions and water (taken up from the soil, by roots)

Particular requirements of plant cells:

Plants have two transport systems: one for carrying mainly water and inorganic ions from roots to the parts above the ground, and

one for carrying substances made by photosynthesis from the leaves to other areas. Oxygen and carbon dioxide travel to and from

cells and their environment by diffusion alone.

The transport of water

From soil to root hairCells of the epidermis of a root are drawn out into long thin extensions called root hairs. Water diffuses down its water potential

gradient, through the partially permeable plasma membrane, into the cytoplasm and vacuole of the root hair cell.

Many plants, especially trees, have fungi located in or on their roots forming associations called mycorrhizas, which serve a similar

function to root hairs. Mutualism is the name given to a relationship between two different organisms in which both benefit.

From root hair to xylem

Water from the soil enters a plant through its root hairs and

then moves across the root into the xylem tissue in the centre.

Once inside the xylem vessels, the water moves upwards

through the root to the stem and from there into the leaves.

Chapter 10: Transport in multicellular plants

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Water taken up by root hairs crosses the cortex and enters the xylem in the centre of the root. It does this because the water

potential inside the xylem vessels is lower than the water potential in the root hairs (water moves down its water potential gradient).

Water can take two possible routes through the cortex. The cells of the cortex are surrounded by cellulose cell walls into which

water can soak and seep across the root from cell wall to cell wall (directly or through intercellular spaces) without ever entering the

cytoplasm of the cortical cells -> apoplast pathway. Another possibility is for the water to move into the cytoplasm or vacuole of a

cortical cell, and then into adjacent cells through the interconnecting plasmodesmata (or through adjacent plasma membrane and

cell walls) -> symplast pathway.

Once the water reaches the stele, the apoplast pathway is barred. The cells in the endodermis have a thick waterproof band of

suberin in their cell walls. This band, the Casparian strip, is an impenetrable barrier to water in the walls of the endodermis cells. The

only way for water to pass through is by symplast pathway. As endodermis cells get older, suberin deposits become more

extensive, except in passage cells, through which water can pass freely.

Water continues to move down the water potential gradient across the pericycle and towards the xylem vessels.

Xylem tissue

(1) Vessel elements and tracheids (cells involved with the transport of water)

(2) Fibres (elongated cells with lignified walls that help support the plant; they are dead cell with no living contents)

(3) Parenchyma cells (have unthickened cell walls and contain all the organelles plant cells contain but not chloroplasts as they

are not exposed to light)

Xylem tissue has the dual functions of support and transport. It contains several different types of cells:

Xylem vessels and tracheids

From leaf to atmosphere - transpiration

Vessels are made up of many elongated vessel elements arranged end to

end. Their walls are lined with lignin, a hard substance impermeable to

water. Types of lignified cell wall thickenings are: spiral, annular, reticulate

and pitted. Inside there is an empty space, or lumen. However, there are

non-lignified areas seen as "gaps" in the thick walls. These are called pits.

The end-walls of neighbouring vessel elements break down completely, to

form a continuous non-living tube -> xylem vessel.

Tracheids are dead cell with lignified cell walls, but they do not have open

ends so they don't form vessels. They are elongated cells with tapering

ends. They have pits in their walls, so water can pass from one tracheid to

another.

In the root, water which has crossed the cortex, endodermis and pericycle

moves into the xylem vessels through the pits in their walls. It then moves

up the vessels towards the leaves. Whereas the xylem vessels are in the

centre of the root, in the stem they are nearer to the outside.

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From leaf to atmosphere - transpiration

The cells in the mesophyll layers are not tightly packed, and have air spaces between them. The walls of the mesophyll cells are wet

and some of this water evaporates into the air spaces, so that the air inside the leaf is saturated with water vapour. This air in the

internal spaces of leaf has direct contact with the air outside the leaf, through small pores or stomata. If there is a water potential

gradient between the air inside the leaf and the air outside, then water vapour will diffuse out of the leaf down this gradient.

Transpiration is the process of loss of water vapour from the leaves of a plant (via the stomata).

(1) An increased water potential gradient between the air spaces in the leaf and the air outside

(2) Low humidity

Increased wind speed(3)

(4) Rise in temperature

The rate of transpiration is greater in the conditions of:

The stomata are the means of contact between photosynthesising mesophyll cells and the external air, and must be open to allow

carbon dioxide for photosynthesis to diffuse into the leaf. On a bright sunny day, stomata must be open. In especially dry

conditions, a plant compromises by closing its stomata to prevent its leaves from drying out. In hot conditions, transpiration plays

an important role in cooling the leaves. As water evaporates from the cell walls inside the leaf, it absorbs the heat energy from

these cells, thus reducing their temperature.

From xylem to leafAs water evaporates from the cell walls of mesophyll cells, more water is drawn into them to replace it. The source of this water is

the xylem vessels in the leaf. The removal of water from the top of xylem vessels reduces the hydrostatic pressure (pressure exerted

by a liquid). The hydrostatic pressure at the top of the vessel is lower than the pressure at the bottom. This pressure difference

causes the water to move up the xylem vessels.

The water in the xylem vessels is under tension. The movement of water up through xylem vessels is by mass flow. This means that

all water molecules move together, as a body of liquid. This is helped by the fact that water molecules are attracted to each other

by cohesion forces and to the lignin in the walls of xylem vessels by adhesion forces. Cohesion and adhesion help the water in the

xylem vessel moving as a continuous column.

If an air bubble is formed in the column then we say there is an air lock because the difference in pressure cannot be transmitted through

the vessel. The water stops moving upwards. Small diameter of vessels help to prevent this from occurring. Pits act as a bypass in such

situations (air cannot move through pits) and they are important in allowing water to move out of xylem vessels to surrounding living

cells.

Root pressure

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Root pressure

Plants may increase the pressure difference between top and bottom by raising the water pressure at the base of the vessel. The

pressure is raised by the active secretion of solutes into the water in the xylem vessels in the root. Cells surrounding the xylem

vessels use energy to pump solutes across their membranes and into the xylem by active transport. The presence of solutes

reduces the water potential of xylem solution, thus drawing in the water from surrounding root cells. This increases the water

pressure at the base of the xylem vessel.

Total lack of cell contents provides an uninterrupted pathway for the flow of water.

Lack of end walls in individual xylem elements provides an uninterrupted pathway for the flow of water.

A diameter between 0.01mm and 0.2mm is a compromise between two requirements. The wider the diameter, the more

water can be moved up thorugh a xylem vessel per unit time. However, if the vessels are too wide, there is an increased

tendency for the water column to break.

The lignified walls provide support, preventing the vessels from collapsing inwards (at higher pressures).

Pits in the walls of the vessels allow water to move into and out of them.

Water transport in plants is largely a passive process, fuelled by transpiration from the leaves. The water simply moves down a

continuous water potential gradient from the soil to the air.

Comparing rate of transpiration

It is relatively easy to measure the rate at which a plant stem takes up water. As a very high proportion of the water taken up by a

stem is lost in transpiration, and as the rate of transpiration directly affect the rate of water uptake, this measurement can give a

very good approximation of the rate of transpiration. The apparatus used for his is called a potometer

Xerophytes

(1) Rolled up leaves, exposing a waterproof cuticle to the air outside the leaf, while the stomata open into the enclosed, humid

space in the middle of a roll.

Leaves reduced to spines, reducing the surface area from which transpiration can take place.(2)

(3) Leaves in the form of needles, greatly reducing the surface area available for water loss; covered in a layer of waterproof w ax

and have sunken stomata.

(4) Leaves in a form of tiny hair-like structures that act as a physical barrier to the loss of water.

(5) Swollen, succulent stems coated in wax (to cut down water loss) that store water and photosynthesise.

Xerophytes are plants that live in spaces where water is in short supply. They often have adaptations to reduce the rate of

transpiration:

Translocation

Translocation is the term used to describe the transport of soluble organic substances within a plant. Assimilates are substances

which the plant itself has made, for example sugars which are made by photosynthesis in the leaves. Assimilates are transported in

sieve elements. Sieve elements are found in phloem tissue, along with other types of cells: companion cells, parenchyma and fibres.

Sieve elements

Companion cellsEach sieve element has at least one companion cell lying close beside it. Companion cells have the structure of a "normal" plant cell

with cellulose cell wall, a plasma membrane, cytoplasm, a small vacuole and a nucleus. The number of mitochondria and

ribosomes is larger than normal, and cells are metabolically very active. Numerous plasmodesmata pass through cell walls of a

companion cell and a neighbouring sieve element, making direct contact between the cytoplasms of the companion cell and sieve

element.

The contents of phloem sieve tubes

The liquid inside phloem sieve tubes is called phloem sap or just sap. It is not easy to collect enough phloem sap to analyse its

contents. Aphids may be used to sample sap. Aphids feed using tubular mouthparts called stylets which are inserted through the

surface of the plant's stem or leaves into the phloem. If the stylet is cut near the aphid's head, the sap continues to flow.

A sieve tube is made up of many elongated sieve elements,

joined end to end vertically to form a continuous column.

Each sieve element is a living cell and it has a cellulose cell

wall, a plasma membrane and cytoplasm containing

endoplasmic reticulum and mitochondria. Cytoplasm only

forms a thin layer lining the inside of the wall of the cell.

Sieve elements have no nucleus, tonoplast or ribosomes.

Where the end walls of two sieve elements meet, a sieve

plate is formed. This is made up of the walls of both

elements, perforated by large pores. In living phloem, the

pores are open, presenting little barrier to the free flow of

liquids through them.

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surface of the plant's stem or leaves into the phloem. If the stylet is cut near the aphid's head, the sap continues to flow.

How translocation occursPhloem sap moves by mass flow. To create the pressure differences needed for the mass flow in phloem, the plant has to use

energy. Phloem transport is therefore an active process.

The pressure difference is produced by active loading of sucrose into the sieve elements at the place from which sucrose is to be

transported. This is usually in a photosynthesising leaf. As sucrose is loaded into the sieve element, this decreases the water

potential in the sap inside it. Therefore, water follows the sucrose into the sieve elements, moving down a water potential gradient

by osmosis.

At another point along the sieve tube, sucrose may be removed by other cells, for example in the root. As sucrose is removed,

water again follows by osmosis. Thus, in the lief, water moves into the sieve tube; in the root, water moves out of it. This creates a

pressure difference: hydrostatic pressure is high in the lief, and lower in the sive tube in the root. This pressure difference causes

water to flow from the high pressure area to the low pressure area, taking with it any solutes.

Any area of a plant in which sucrose is loaded into the phloem is called a source (usually a photosynthesising leaf). Any area where

sucrose is taken out of the phloem is called a sink (growing point or storage point e.g. fruit, root)

Sap flows both upwards and downwards in phloem (flow in xylem is only upwards). Within a vascular bundle, sap can flow in any

direction, but it can flow one way in any particular sieve tube at any one time.

Loading of sucrose into phloem

In leaf mesophyll cells, photosynthesis in chloroplasts produces trioses which are converted into sucrose. The sucrose, in solution,

moves from the mesophyll cell, across the leaf to the phloem tissue by symplast or apoplast pathway. Sucrose is loaded into a

companion cell by active transport.

(1) In both xylem and phloem case, liquid moves by mass flow along a pressure gradient, through tubes formed by cells stacked

end to end.

Unlike water transport through xylem, which occurs through dead xylem vessels, translocation through phloem sieve tubes

involves active loading of sucrose at sources, thus requiring living cells.

(2)

Xylem vessels have lignified cell walls, whereas phloem tubes do not. The presence of lignin in a cell wall prevents the

movement of water and solutes across it, so kills the cell . This does not matter in xylem vessels as they do not need to be

alive; indeed it is a positive advantage to have a empty tube for unimpeded flow of water & the dead xylem vessels with

(3)

Differences between sieve elements and xylem vessels

One possible way is carrying sucrose molecules through a co-transporter

molecule into the companion cell, against their concentration gradient.

Hydrogen ions are moved out of the cells using ATP which creates a large

excess of H+ ions outside the companion cells. They can move back into the

cell down their concentration gradient, through a carrier protein for both H+

ions and sucrose at the same time. The sucrose molecules can then move from

the companion cell into the sieve tube thorugh the plasmodesmata connecting

them.

ATP is present in phloem sieve elements in quite large amounts as it is required for

the active transport of H+ ions. Phloem sap always has a relatively high pH (8)

which would be expected if H+ ions were actively being transported out of the cell.

Unloading of sucrose from phloem

Unloading occurs into any tissue which requires sucrose. It is probable that

sucrose moves out of the phloem into these tissues by diffusion. Once in the

tissue, sucrose is converted into glucose and fructose by enzyme invertase, so

decreasing its concentration and maintaining a concentration gradient.

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alive; indeed it is a positive advantage to have a empty tube for unimpeded flow of water & the dead xylem vessels with

strong walls also support the plant.

(4) End walls of xylem elements disappear completely, whereas those of phloem sieve elements form sieve plates. These sieve

plates prevent the phloem tube from collapsing; xylem already has sufficient support from its lignified walls. The sieve plat es

also allow the phloem to rapidly seal itself up if damaged.

Phloem sap has high turgor pressure and would leak out rapidly if the holes in the sieve plate were not quickly sealed. Phloem sap has

valuable substances which the plant cannot afford to lose in large quantities. The "clotting" of the phloem sap also prevents the entry of

microorganisms which could feed on nutritious sap or cause disease.

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