BY2 - Adaptations for transport in plants

It transports materials around the body

In animals it is blood

In plants it is xylem and phloem

The xylem is central and star-shaped, phloem is between groups of xylem

Arrangement resists vertical stresses, anchors plant into soil

They are in a ring at the periphery

Xylem towards centre, phloem towards outside

Gives flexible support, resists bending

It is in the midrib and in a network of veins

Gives flexible strength, and resistance to tearing

Vessels and tracheids

Vessels occur only in angiosperms. Lignin builds up in cell walls, content dies, leaves empty space, lumen. Tissue develops and end walls of cell break down to leave a long hollow tube which water moves up through. Lignin is laid in a spiral pattern, stains red

Tracheids occur in ferns, confier and angiosperms, not mosses

Water is taken up from soil, through roots and to leaves

Much is lost through the stomata, during transpiration

Constant replacement must occur through the soil

Region of greatest uptake is root hair as surface area is very large and they have thin cell walls

Soil water has a higher water potential than the root hair, so water moves into root hair by osmosis, down water potential gradient 

Three different routes:

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

Symplast pathway - moves through cytoplasm and plasmodesmata. Plasmodesmata are strands of cytoplasm through pits in cell wall

Vacuolar patheway - water moves from vacuole to vacuole

Two main pathways are symplast and apoplast, apoplast is faster 

Water can only pass into xylem from symplast or vacuolar pathways, leaves the apoplast pathway

Centre of the root is a region called pericycle, surrounded by a single layer of cells, the endodermis. They are impregnated with a waxy material, suberin, forms a distince band called Casparian *****

Suberin is waterproof, Casparian ***** prevents warer moving further in apoplast and drives it into cytoplasm

Water moves from root endodermis, into xylem by osmosis across endodermal cell membranes

Water potential of endodermis cells is raised by water being driven in by Casparian *****

Water potential of xylem is decreases by active transport of things from endodermis and pericycle into xylem

Minerals are absorbed into cytoplasm by active transport, against a concentration gradient

Mineral ions can also move along apoplast pathway in solution and when they reach endodermis, the Casparian ***** prevents further movement in cell walls, and they enter cytoplasm by active transport then diffuse/active transport into xylem

Water always moves down a water potential gradient, air has low water potential, soil high water potential, water moves from soil through plant to air

Cohesion-tension: water vapour evaporates from leaf cells into air spaces, diffuses out through stomata into atmosphere. As water molecules leave xylem cells, pull up others due to cohesion, provides a continuous pull, producing tension in water column. Charges on water molecules attract hydrophilic lining of vessels, adhesion, contributes to water movement

Capilarity: movement of water up narrow tubes, by capillary action. Only over short distance, up to 1m

Root pressure: operates over short distances in living plants, consequence of osmotic movement of water into xylem, pushing water already there further up

The evaporation of water vapour frm the leaves or other above-ground parts of the plant, out through stomata into the atmosphere

Genetic facotrs such as distribution and size of stomata

Environmental factors such as temperature, humidity, air movement and light intensity

Increases the kinetic energy of water molecules, and lowers water potential of atmosphere

Water molecules evaporate faster, higher temperature causes water molecules to diffuse away faster

The higher the humidity, the higher the water potential and water potential diffuses down the gradient of relative humidity, also gradient of water potential, away from the leaf

It blows away the layer of humid air on the leaf surface

Water potential gradient between inside and outside of leaf increases, water vapour diffuses out through stomata quicker 

Faster air is moving, faster vapour blown away, faster transpiration

It controls the degree of stomatal opening

Stomata tend to be open widest in the middle of day, less widely in morning and evening and closed at night

Wider stomata open, faster transpiration

1) Cut shoot under water, so no air enters xylem

2) Fill potometer with water, under water, so no air bubbles

3) Fit leafy shoot to potometer to prevent air bubbles forming in apparatus/xylem

4) Remove potometer and shoot from water, seal joints with Vaseline

5) Introduce an airbubble into capillary tube

6) Measure the distance air bubble moves in a given time

7) Use water reservoir to bring air bubble back to start point and repeat to calculate a mean

The are land plants, have an adequate water supply and it is readily replaced 

They are adapted to grow in well-drained soils and moderately dry air

They survive unfavourable times of the year because:

Many shed their leaves so they don't lost water as it will be scarce

Aerial parts of non-woody plants die off so they aren't exposed to frost/cold winds, underground organs survive

They are dormant seeds over winter, with low metabolic rate so no water is required

Xerophyes are plants with xeromorphic characteristics

Adapted to living with low water availability, modified structures to prevent water loss

Live in hot, dry desret regions, cold regions and windy locations

May have the following modifications:

Rolled leaves to reduce the leaf area exposed to air, and reduce transpiration

Sunken stomata to reduce water potential gradient between inside and outside of leaf, reducing rate of diffusion of water out through stomata

Hairs to trap water vapour, reduce water potential gradient

Thick cuticle which is waxy so is waterproof and reduces water loss, thicker cuticle, lower rate of transpiration

Fibres of sclerenchyma are still so leaf shape is maintained

They grow partially or wholly submerged in water

They have the following adaptations:

Little or no lignified support tissues as water is a supportive medium

Surrounded by water, little need for transport tissue, poorly developed xylem

Leaves have little/no cuticle as no need to prevent water loss

Stomata on upper surface as lower surface is in the water

Stems and leaves have large air spaces, forming a reservoir of oxygen and carbon dioxide to provide buyoancy

Translocation is the active movement of soluble products of photosynthesis, such as sucrose and amino acids, through phloem, from sources to sinks

It is a living tissue consists of sieve tubes and companion cells

Sieve tubes comprise end-to-end cells called sieve tube elements, sometimes parts of the side walls are perforated, called sieve plates

Cytoplasmic filaments containg phloem extend from one sieve tube to another and there elements lose their nucleus and organelles during development to allow space for transporting materials

Metabolism is controlled by neighbouring companion cells, which are very active due to large nucleus, dense cytoplasm with much RER and many mitochondria, connected to sieve tube via plasmodesmata

Cylinders of outer bark were removed, removing phloem

Left plant for some time to photosynthesis, phloem contents above/below ring were analysed

Above ring, lots os sucrose, it had been translated in phloem

Below ring, no sucrose, it had been used by plant tissues but not replaced

A plant photosynthesises in the presence of a radioactive isotope, such as 14C in carbon dioxide, 14CO2

Stem section is placed on photographic film, exposed if there's a radiation source, producing an autoradiograph

Position of exposure, and radioactivity, coincides with position of phloem, indicates phloem translocates the sucrose made from 14CO2 in photosynthesis

Aphid has a hollow, nededle-like mouth called a stylet

It's inserted into a sieve tube, phloem contents, sap, exude under pressure into aphid's stylet

Some experiments, aphid was anaesthetised and removed and stylet remained embedded in phloem, as sap in phloem is under pressure it exuded from stylet and was collected and analysised showing presence of sucrose

Aphid experiments were extended to plants which had been photosynthesising with 14CO2

Showed radioactivity and sucrose made in photosynthesis moved at a speed of 0.5-1mh-1

Much faster than rate of diffusion alone, additional mechanism had to bec onsidered

Suggests there is a passive mass flow of sugars from phloem of leaf where there is highest concentration (source), to other areas, such as growing tissues, where there is a lower concentration (sink)

Rate of phloem transport is about 10000 times faster than if substances were moving by diffusion

Doesn't take into account sieve plates

Sucrose and amino acids move at different rates and in different concentrations in same tissue

Phloem has a relatively high oxygen consumption, translocation is slowed/stopped at low temperature

Companion cells are biochemically very active

An active process may be involved

Protein filamenets pass through sieve pores

Cytoplasmic streaming could be responsible for movement