1.2.3 transport in plants

Unit F211: Cells, Exchange and Transport 1.2.3 Transport in Plants

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After studying this section, Student should be able to:a) explain the need for transport systems in multicellular plants in terms of

size and surface area : volume ratio; b) describe, with the aid of diagrams and photographs, the distribution of

xylem and phloem tissue in roots, stems and leaves of dicotyledonous plants;

c) describe, with the aid of diagrams and photographs, the structure and function of xylem vessels, sieve tube elements and companion cells;

d) define the term transpiration; e) explain why transpiration is a consequence of gaseous exchange; f) describe the factors that affect transpiration rate; g) describe, with the aid of diagrams, how a potometer is used to estimate

transpiration rates (HSW3); h) explain, in terms of water potential, the movement of water between

plant cells, and between plant cells and their environment. (No calculations involving water potential will be set);

i) describe, with the aid of diagrams, the pathway by which water is transported from the root cortex to the air surrounding the leaves, with reference to the Casparian strip, apoplast pathway, symplast pathway, xylem and the stomata;

j) explain the mechanism by which water is transported from the root cortex to the air surrounding the leaves, with reference to adhesion, cohesion and the transpiration stream;

k) describe, with the aid of diagrams and photographs, how the leaves of some xerophytes are adapted to reduce water loss by transpiration;

l) explain translocation as an energy-requiring process transporting assimilates, especially sucrose, between sources (eg leaves) and sinks (eg roots, meristem);

m) describe, with the aid of diagrams, the mechanism of transport in phloem involving active loading at the source and removal at the sink, and the evidence for and against this mechanism (HSW1, 7a).

Multicellular plants, just like multicellular animals are large, complex organisms. Their cells need a constant supply of nutrients to survive. They also need to get rid of metabolic wastes which may become toxic if they build up.

Because of their size, multicellular plants have a small surface are compared to the volume. They cannot therefore meet all their nutrient supply and waste disposal needs by diffusion alone and they need a transport system.

OCR AS Biology Revision Notes by M Idriss Page 31


Distribution and function of Transport Tissues in Plants

Plants have fewer types of tissues than animals. The tissues of a plant are organized into three tissue systems: the dermal tissue system, the ground tissue system, and the vascular tissue system.

1. Dermal tissue system - protects the soft tissues of plants and controls interactions with the plants' surroundings. It is called Epidermis and may be covered by a cuticle.

2. Ground tissue system – made of parenchyma, collenchyma and sclerenchyma cells. Its function for photosynthesis, storage, regeneration, support, and protection.

3. Vascular tissue – made of xylem and phloem, which function to transport water and dissolved substances. Vascular tissue may also contain meristematic tissue called cambium which divides to make secondary xylem and phloem during growth or repair.

Tissue System (+ components)

Tissue Functions Location of Tissue Systems

Dermal Tissue SystemEpidermisPeriderm (older stems + roots)

• protection• prevents water loss

Ground Tissue SystemParenchyma tissueCollenchyma tissueSclerenchyma tissue photosynthesis

• food storage• regeneration • support• protection

Vascular Tissue SystemXylem tissuePhloem tissue

• transports water & minerals• transports food

For AS Biology we are only going to look at the tissues involved in transport

The Vascular tissue is contained in bundles made of xylem, phloem and cambium tissues. Their function is support and transport of water, mineral ions and manufactured carbohydrates. In roots they are in the centre to provide support as the root burrows into the soil. In stems they are found near the outside to withstand bending forces.



Distribution of Xylem and Phloem tissues in stem, root and Leaf.

Longitudinal section of stem showing structure of cell types in vascular bundles

Individual cell types of the xylem (left) and phloem (right) as seen from the outside

Xylem Tissue consists of Xylem Vessels, tracheids, fibres and Parenchyma Cells. Xylem Vessels conduct water and mineral ions from the roots upward through the stem to the leaves and are adapted for their function:

They are made of elongated vessel members, have open end walls so that successive cells form continuous tubes. The cells are dead, with no cytoplasm so the vessels have a wide Lumen.

They are reinforced by having deposits of Lignin in their walls thereby preventing collapse. The lignin may form rings or spirals

The lignin also makes the walls waterproof so water is not lost

OCR AS Biology Revision Notes by M Idriss 3


They have pits – areas in their walls with no thickening, this allows water to move laterally between vessel members should one of the vessel members become blocked

Phloem Tissue transports sap (sugars manufactured by photosynthesis and dissolved in water) from the leaf to other parts of the plant. It is made up of Sieve Tubes, Companion Cells (parenchyma) and fibres. The phloem is also well adapted for this function.

Individual sieve tube cells (sieve tube elements) are elongated and have perforations (sieve pores) in their end plates (Sieve Plates) through which substances can move.

Because sieve tube elements have very little cytoplasm and no nuclei, they have wide lumens. Companion Cells with lots of cytoplasm and mitochondria, lie next to sieve tube cells and allow them to stay alive.

The wall between companion cells and sieve tube elements has many plasmodesmata. Plasmodesmata are channels in cell walls that link adjacent cells together – the cytoplasm of one cell is continuous with another through the plasmodesmata and this allows transport and communication between cells.

Both xylem and phloem have some cells that have no lumen or have a blocked lumen and which are not involved in transport. The structures labelled ‘fibres’ in the diagram above refer to sclerenchyma fibres - various kinds of hard, woody cells that serve the function of support in plants. Mature sclerenchyma cells are dead cells that have heavily thickened walls containing lignin.

What is Transpiration and why is it important?

Transpiration is the loss of water vapour from the aerial parts of the leaf, mainly the leaves. Most transpiration occurs through openings on the leaves called stomata.

During the day (in presence of light) leaves open their stomata to absorb CO2 to make glucose by photosynthesis (gas exchange). Whilst the stomata are open, water vapour escapes – transpiration is therefore a side effect of gas exchange!

Later, we will see that transpiration has another importance. It plays a role in helping the plant to pull water up the stem

Factors that affect transpiration rate

The rate of transpiration can be affected by several factors

Light . Light makes the stomata to open allowing gas exchange for photosynthesis, and so the brighter it is the faster the transpiration rate. This is a problem for some plants as they may lose too much water during the day and wilt.

Temperature . The higher the temperature, the faster the transpiration rate High temperature means higher kinetic energy of water molecules so evaporation of water


http://www.britannica.com/EBchecked/topic/529124/sclerenchyma-cell


from the spongy cells increases and this increases the water potential gradient between the inside and the outside of the leaf.

Humidity . High humidity means a higher water potential in the air, so a lower water potential gradient between the leaf and the air, so less evaporation.

Wind speed . Wind blows away saturated air from around stomata, replacing it with drier air, so increasing the water potential gradient and increasing transpiration.

Measuring (Estimating) Rate of Transpiration

A potometer (drinking meter) is an instrument used to measure the rate of water uptake by the stem.

We can use the potometer to estimate the rate of transpiration. We assume that all of the water taken up by the plant is also lost as vapour by transpiration. This is not strictly correct because

a) During the day, some of the water taken up by the plant is used in photosynthesis. So transpiration will be slightly lower than uptake

b) during the night plants may take up more water than they transpire (i.e. they store water and become turgid

Transpiration rate measured with a potometer is therefore only an estimate but it is a good estimate because the difference is too small for small plants which are used in experiments.

The potometer can be used to investigate the various environmental factors that affect the rate of transpiration (light, temperature, wind speed and relative humidity of the air).

Diagram of a potometer. Taken from

http://revisionworld.co.uk/a2-level-level-revision/biology/transport/transport-substances-plant


http://revisionworld.co.uk/a2-level-level-revision/biology/transport/transport-substances-plant


How do you set the potometer up?

How do you measure the rate of transpiration?

What Precautions must be taken when using the potometer?

Avoid having air bubbles or leaks in the system/tubes Safety – when cutting stem or inserting into (sharp) glass tubing

Movement of Water through the Plant

Water enters plants from the soil through root hairs. From root hair cell, it travels across the root cortex and the endodermis into xylem tissues in the vascular bundles. The water then moves up the plant within the xylem.

The movement of water through a plant can be split into three parts: 1. through the roots; 2.

through the stem and 3. through the leaves:

1. Movement through the Roots

The movement of water from root hair cells into the xylem can occur by two different pathways: the apoplast and symplast pathways.

The Apoplast pathway – water moves from cell to cell through the cell walls. Water does not enter the cytoplasm or pass through the cell surface membrane or any other membrane. This pathway offers the least resistance to water flow and accounts for 90% of water movement across the root cortex. The apoplast pathway cannot however occur across the endodermis- a layer inside the root cortex with its waterproof casparian strip walls. At this point water has to cross the cell membrane by osmosis and enter the symplast. This allows the plant to have some control over the uptake of water into the xylem.

The Symplast pathway – water moves from cell to cell through the cytoplasm of each cell. Water often passes through plasmodesmatat that link the cytoplasm of adjacent cells. About 10% of transport across the root cortex occurs this way. The cytoplasms of all cells in the root are connected by plasmodesmata there are no further membranes to cross until the water reaches the xylem, and so no further osmosis.



The uptake of water by osmosis actually produces a force that pushes water up the xylem. This force is

called root pressure, which can be measured by placing a manometer over a cut stem, and is of the order

of 100 kPa (about 1 atmosphere). This helps to push the water a few centimetres up short and young

stems, but is not enough pressure to force water up a long stem or a tree. Root pressure is the cause

of guttation, sometimes seen on wet mornings, when drops of water are forced out of the ends of leaves.

2. Movement through the Stem

The xylem vessels form continuous pipes from the roots to the leaves. Water can move up through

these pipes at a rate of 8m h-1, and can reach a height of over 100m. Since the xylem vessels are

dead, open tubes, no osmosis can occur within them. The driving force for the movement is

transpiration in the leaves. This causes low pressure in the leaves, so water is sucked up the stem to

replace the lost water. The column of water in the xylem vessels is therefore under tension (a

stretching force). Fortunately water has a high tensile strength due to the tendency of water

molecules to stick together by hydrogen bonding (cohesion), so the water column does not break

under the tension force. This mechanism of pulling water up a stem is sometimes called

the cohesion-tension mechanism.

The very strong lignin walls of the xylem vessels stops them collapsing under the suction pressure, but in

fact the xylem vessels (and even whole stems and trunks) do shrink slightly during the day when

transpiration is maximum.



3. Movement through the Leaves

The xylem vessels ramify in the leaves to form a branching system of fine vessels called leaf veins.

Water diffuses from the xylem vessels in the veins through the adjacent cells down its water potential

gradient. As in the roots, it uses the symplast pathway through the living cytoplasm and the apoplast

pathway through the non-living cell walls. Water evaporates from the spongy cells into the sub-

stomatal air space, and diffuses out through the stomata.

Movement of sugars - Translocation

Translocation refers to the movement of the end products of photosynthesis from areas where they are made (source) to areas where they are made use (sink). The main product of photosynthesis is glucose. This converted into the disaccharide, sucrose and dissolved in water for transport as plant sap. Once the sucrose arrives at the sink it can be used for respiration or converted into other molecules like cellulose, lignin etc.


http://3.bp.blogspot.com/-nyZdXyvfZsk/UYntgm3ClzI/AAAAAAAACEM/0_EZPBhXDSg/s1600/translocation.png


Transpiration from leaves of plants helps to pull water up the stem. Most of the water absorbed by roots is transpired (99%). Only a small proportion (1%) is used for photosynthesis and to make plant cells turgid.

Adaptations to dry habitats

Plants in different habitats are adapted to cope with different problems of water availability.

Mesophytes= plants adapted to a habitat with adequate water; Xerophytes = plants adapted to a dry habitat; Halophytes = plants adapted to a salty habitat; Hydrophytes = plants adapted to a freshwater habitat

Some adaptations of xerophytes are:

Adaptation How it works Example

thick cuticle stops uncontrolled evaporation through leaf cells

most dicots

small leaf surface area less area for evaporation conifer needles, cactus spines

low stomata density fewer gaps in leaves

stomata on lower surface more humid air on lower surface, so most dicots



of leaf only less evaporation

shedding leaves in dry/cold season

reduce water loss at certain times of year

deciduous plants

sunken stomata maintains humid air around stomata marram grass, pine

stomatal hairs maintains humid air around stomata marram grass, couch grass

folded leaves maintains humid air around stomata marram grass,

succulent leaves and stemstores water Cacti

extensive roots maximise water uptake Cacti


1.2.3 transport in plants

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