Plant Transport

Why are xylem and phloem needed for plant transport

- diffusion is too slow to distribute materials within the plant

Describe and explain the structure of the vascular bundle in roots

- xylem is central and star-shaped with phloem between groups of xylem cells

- This arrangement resists vertical stresses (pull) and anchors the plant in the soil

Define xylem

- tissue in the plant conducting water and dissolved minerals upwards

Define phloem

- plant tissue containing sieve tube elements and companion cells, translocating sucrose and amino acids from the leaves to the rest of the plant

Describe and explain structure of vascular bindle in stem

- vascular bundles are in a ring at the periphery, with xylem towards the centre and phloem towards the outside

- gives flexible support and resists bending

Describe and explain structure of vascular bundles in leaf

- vascular tissue is in the middle (midrib) and in a network of veins

- flexible strength and resistance to tearing

What are the main cell types in xylem

- vessels and tracheids

Define vessels in xylem

- water-conducting structures in only angiosperms comprising cells fused end to end making hollow tubes with thick lignified cells

Define tracheids

- splindle shaped, water conducting cells in the xylem of ferns, conifers and angiosperms

- not mosses - poorer at transporting water so can't grow as tall

What happens to vessels as they mature in xylem

- perforated end walls breakdown and so they become more efficient

- because they have a higher surface area for transport

- as lignin builds up in their cell walls, the contents die, leaving a large, hollow lumen. As tissue develops, the end walls of cells break down leaving a long,hollow tube. Water can move through this up the plant. Lignin is laid down in a characteristic spiral pattern.

Functions of xylem

- transport of water and dissolved mineral ions

- providing mechanical support

Describe tracheids and vessels

- occur in ferns, conifers and angiosperms comprising cells

- dead, hollow cells specialised to transport water and mineral ions

- tracheids: have tapered ends with pits

- Vessels: perforated ends which break down

- walls contain lignin for mechanical support, waterproofing and adhesion

What are phloem made up of

- sieve plates and companion cells, phloem fibres and parenchyma

What are sieve tube elements

- component of phloem, lacking a nucleus, but with cellulose cell walls, perforated with sieve plates

- products of photosynthesis can move up and down through these sieve plates

Why do sieve tube elements not have a nucleus

- to allow for more space to transport organic materials, sucrose and amino acids

Define translocation

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

Function of sieve plates

- cytoplasm filaments containing phloem proteins extend through the pores in the sieve plates to the next sieve tube element

Outline companion cell structure and function

- control metabolism of sieve tube elements and are connected via the plasmodesmata

- very biochemically active

- seen by their large nucleus, dense cytoplasm with lots of rough endoplasmic reticulum and mitochondria

Plasmodesmata function

- are small channels that directly connect the cytoplasm of neighbouring cells, establishing living bridges between cells

- allow certain molecules to pass directly from one cell to another

- are important in cellular communication

Why do terrestrial plants need to uptake water by roots

- risk dehydration and must conserve water

- water is taken up from the soil from the roots and transported to the leaves, where it maintains turgidity and is used in photosynthesis

- but much is lost through the stomata (transpiration)

What is the region fo greatest water uptake in a plant and why

- root hair zone

- surface area is high due to root hairs and water uptake is enhanced by thin cell walls

Where are root hair cells found

- lare numbers in a zone behind the root tip (meristem)

- as growth needed

Structure of a root hair

- individual epidermal cell with a thin cell wall

How does water enter a root

- soil water has a dilute solution of mineral ions and has a high water potential

- vacuole and cytoplasm of root hairs has lower, more negative, water potential and concentrated solution of mineral ions

- water passes down a water potential gradient from the soil into the root hairs by osmosis

The root hair grows between soil particles which are covered with a film of water and the soil has air spaces, what does this mean about the soil and for uptake of water

- water is polar so soil must be polar

- increases water potential of soil

- air spaces maintains a steep concentration gradient of oxygen for respiration which releases ATP which can be used for active transport - needed for uptake of mineral ions

State the ways water can move into the xylem and be transported around the plant

- apoplast pahtway

- symplast pathway

- vacoular pathway

What is the apoplast pathway?

- water and dissolved mineral ions move through the freely permeable cell walls from one cell wall to the next due to space between the cellulose fibres

Outline symplast pathway?

- water crosses the membrane and moves from the cytoplasm of one cell to the next via the plasmodesmata

What is the slowest pathway for water in a plant

- vacuolar pathway, but it is needed for turgor pressure

Outline the vacuolar pathway

- small% of water also enters the vacoule

Outline the role of the endodermis in transport

- vascular tissue in the root centre is surrounded by a region called the pericycle which is surrounded by a single layer of cells called the endodermis

- cell walls of endodermal cells are impregnated with a waxy material called Suberin forming a distinctive waterproof band called a Casparian strip

- Casparian strip prevents water movement by the apoplastic route driving it into the cytoplasm

Role of the Casparian strip

- water must enter the endodermal cells by osmosis and join the symplast route

- will increase water potential

- mineral ions are selectively absorbed by active transport

- water cannot enter the xylem via the apoplast pathway as the lignified xylem walls are waterproof, water can only enter through the symplast and vacoular pathways

How do mineral ions enter the xylem

- actively transported from the endodermis and pericycle into the xylem

- so Casparian strip has mitochondria

Define endodermis in transport of water and minerals

- a single layer of cells around the pericycle and the vascular tissue of the root and each cell has a waterproof barrier in it's cell walls

What is the casparian strip?

- impermeable, waterproof band of suberin in the cell walls of endodermal cells so that water leaves the apoplast pathway and enters the cytoplasm

Explain how water moves from the root endodermis to the xylem

- one explanation: increased hydrostatic pressure in root endodermal cells - pushes water into the xylem. The hydrostatic pressure is increased by active transport of ions (especially Na) into endodermal cells reducing water potential, drawing in more water by osmosis. Diversion of water into endodermal cells drom the apoplast pathway by the Casparian strip.

- decreased water potential in the xylem: below that of the endodermal cells, draws water in by osmosis across endodermal cell membranes - wp decreased by water being reasons above (water diverted due to Casparian strip and active transport of ions from endodermis and pericycle to xylem)

State 3 mechanisms for water movement from the soil through the plant into the air

- capillary, cohesion-tension and root pressure

Outline uptake of minerals

- active transport from soil to the cytoplasm

- or they move along the apoplast pathway, in solution. When they reach the endodermis, the Casparian strip prevents them from further movement in the cell walls. The mineral ions enter the cytoplasm by active transport and then diffuse down or are actively transported into xylem.

What is capillarity

- capillarity is the movement of water up narrow tubes by capillary action

- capillarity operates over short distance so may be important in mosses but contributes only a little to water movement in higher plants taller than a few cm in height

Outline role of root pressure in movement of water from roots to leaf

- caused by active transport of mineral ions into xylem and the water that follows by osmosis from endodermal cells

-operates over a short distance in living plants

Define cohesion

- attraction of water molecules for each other, seen as hydrogen bonds, resulting from the dipole structure of water molecule

Define adhesion

- attraction between water molecules and hydrophilic molecules in the cell walls of the xylem

Define cohesion-tension theory

- the theory of the mechanism by which water moves up the xylem, as a result of the cohesion and adhesion of water molecules and tension of water column, all resulting from water's dipole structure

What does cohesion-tension mechanism depend on

- cohesion: due to hydrogen bonds between adjacent water molecules

- adhesion: due to hydrogen bonds between water molecules and hydrophilic cell walls of xylem vessels

Tension: a column of water molecules has high tensile strength - it can be pulled without breaking

Define root pressure

- the upward force on water in roots, due to the osmotic movement of water into the root xylem

How to measure root pressure

- liquid will exude from the stem of a well-watered plant cut off just above ground level

- if stump is attached to manometer, the exuded liquid causes the mercury in manometer to rise, measuring root pressure

Explain the cohesion-tension theory

- the evaporation of water from leaf cells into air spaces and diffusion out through stomata into the air, draws water through leaf cells (by apoplast, symplast and vacuolar pathways) from the xylem which pulls up adhesive and cohesive water molecules in the xylem producing tension in the water column

- this movement of water up and out of the xylem of the leaf = cohesion-tension theory

Define transpiration stream

- continual flow of water in roots, up stem to leaves and out to the air

Define transpiration

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

Outline need for transpiration

- the continual evaporation of water taken in by the plants is needed to balance water uptake with loss

- causes continuous water uptake, water distribution;ion distribution, evaporative cooling

Effect of too much water loss

- leaves wilt, stomata close and plant cannot regain its turgor and dies

State factors that affect rate of transpiration

- genetic factors

- environmental factors:

- temperature, tight intensity, humidity, air movement

Effect of genetic factors in transpiration

- such as those controlling number, distribution and size of stomata

Effect of temperature on rate of transpiration

- as temp increases, KE increases, faster evaporation from walls of mesophyll cells and if stomata are open, speeds up their rate of diffusion out into the atmosphere, reducing the wp in atmosphere. The higher temperature causes water molecules to diffuse away from the leaf more quickly, reducing the water potential around the leaf.

Effect of humidity on the rate of transpiration

- the air inside a leaf is saturated with water vapour, it's relative humidity is 100%. The humidity of the atmosphere surrounding a leaf varies, but is never greater than 100%.

- transpiration in still air results in accumulation of a layer of saturated air at the surface of leaves. The water vapour diffuses away, leaving concentric rings of decreasing humidity the further away from the leaf you go = diffusion shells. The higher the humidity, the higher the water potential, water vapour diffuses down this gradient of relatvie humidity, which is also a gradient of water potential away from the leaf.

effect of air speed on transpiration

- movement of the surrounding air blows away the layer of humid air at the leaf surface

- wp gradient between leaf inside and outside increases water vapour diffuses out through the stomata more quickly

- the faster the air is moving, the faster the concentric shells of water vapour get blown away, the faster transpiration occurs.

Effect of light intensity on transpiration

- in most plants, stomata are open wider as the light intensity increases, increasing rate of transpiration. So stomata tend to open widest in the middle of the dat and less widely in morning and evening and closed at night

Outline how environmental factors affecting transpiration interact

- more water is lost on a dry, windy day than on a humid, still day. This is because the walls of the spongy mesophyll cells are saturated with water which evaporates and moves down a gradient of water potential from the leaf to the atmosphere, which has a low humidity, the wind having reduced the thickness of the layer of saturated air at the leaf surface

How to measure rate of transpiration

-potometer

-cut a shoot at a slant underwater to prevent air entering the xylem and increase surface area

-assemble potometer and insert shoot underwater so no air can enter

-remove apparatus from water but keep the end of capillary tube submerged in a beaker

-ensure the apparatus is watertight using vaseline

-dry the leaves and allow time for the shoot to acclimatise

-remove the tube from the beaker until one air bubble forms then put it back in

-record the starting position of the air bubble, start a stopwatch and record the distance moves per unit time eg per hour

-the rate of bubble movement is the transpiration rate

Function of potometer

- measures how much water is taken in by plant but as most is lost, so can also measure transpiration

Why do you cut leafy shoot under water when using a potometer

- to prevent air bubbles which would block xylem

Why do you seal joints of potometer with vaseline

- to allow transpiration and water loss is only from leaf and during the experiment

What are mesophytes

- plants that have evolved to live in conditions of adequate water supply

Xerophytes

Plants that have adapted to dry climates

Hydrophytes

plants that have special adaptation's to allow them to live in open water

Adaptations of mesophytes

- adequate water supply, adapted to well drained soils and moderately dry air

- lost water is readily replaced by uptake from soil

- so nospecial ways of conserving it

- if plant loses too much water, it wilts, leaves droop, stomata close, leaf surface area for absorbing light is reduced so photosynthesis is less efficient

- excessive water loss is prevented by stomata closing in the night

Adaptations of xerophytes

- adapted to dry areas, lots of water loss

- rolled leaves: reduce exposure to air, reduced transpiration

- sunken stomata: trap humid air, lower wp gradient, reduced diffusion, transpiration, evaporation

- hairs: trap water vapour, decrease wp gradient, reduced D,T,E

- thick cuticle: waterproof, reduced D,T,E

- sclerenchyma fibres: maintain shape of rolled leaf even when cell is flaccid

Adaptations of hydrophytes

- hydrophytes grow partially/wholly submerged in water

- little/no lignified tissue in leaves: supported by water

- poorly developed xylem: absorbs water from surroundings

- little/no cuticle: don't need to prevent water loss

- stomata on upper surface of floating leaves: gas exchange from air not water

- stems and leaves have large air spaces that are continuous down to the roots: provide buoyancy

Number of stomata on upper and lower surface of leaves for types of plants

- mesophytes: upper few, lower many

- xerophyte: upper many, lower few

- hydrophyte: upper many, lower none

Number of cuticles on upper and lower surface of leaves for different types of plants

- mesophytes: upper thick, lower thin

- xerophytes: upper thin on rolled leaves; thick on flat leaves, lower thick

- hydrophytes: upper none, lower none

Define translocation

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

- solutes may move up or down a plant

State experimental evidence for transport in the phloem

- ringing experiments

- radioactive tracers and autoradiography

- aphid experiments

- aphids and radioactive tracers

Outline ringing experiment evidence for transport in phloem

- early evidence was obtained from ringing experiments

- cylinders of outer bark tissue were removed from all the way around a woody stem, in a ring: this removed the phloem

- after leaving the plant some time, while it photosynthesised, the phloem contents above and below the ring were analysed

- above the ring, there was a lot of sucrose, suggesting that it had been translocated in by the phloem, also slight swelling

- below the ring, no sucrose, suggesting that it had been used by the plant tissues but not replaced, because the ring prevented it from being moved downwards

Outline use of radioactive tracers and autoradiography in studying translocation

- a plant photosynthesises in the presence of a radioactive isotope, such as C14 in CO2

- a stem section is placed on a photographic film, which is fogged if there is a radiation source, producing an autoradiograph.

- the position of fogging, and therefore the radioactivity, coincides with the position of the phloem, indicating that it is the phloem that translocates the sucrose made from 14CO2 in photosynthesis

Outline use of aphid experiments for evidence for translocation

- an aphid has a hollow, needle like mouthpart = stylet

- this inserts into sieve tube and the phloem contents, the sap, exude under pressure into aphid stylet

- in some experiments, the aphid was anaesthetised and removed

- It's exuded from the stylet and was collected and analysis showed the presence of sucrose

Outline use of aphids and radioactive tracers

- the aphid experiments were extended to plants which had been photosynthesised with 14CO2. This showed that the radioactivity and therefore the sucrose made in photosynthesis, moved at a speed of 0.5-1m h-1. This is much faster than the rate of diffusion alone so some additional mechanism had to be considered

Name of most common theory of translocation

- mass flow hypothesis

Define mass flow hypothesis

- passive mass flow of sugars from the phloem of the leaf with a high concentration (source) to other areas with lower concentration (sink) eg storage of growing tissues

Explain mass flow hypothesis

- leaf cells: source of sucrose made in photosynthesis. The sucrose makes the water potential very negative, so water moves in by osmosis. Water moves in more than into the sink.

- as water enters source, hydrostatic pressure increases and forces sucrose solution into phloem, movement = mass flow

- phloem connects source to sink

- sucrose is removed from sink by: respiration (cell division) , stored as starch, converted to cellulose and other cell wall polysaccharides, stored as nectaries

- increased pressure into phloem sink forces water out of sink into xylem (transpiration stream takes back to source)

Limitations of mass flow hypothesis

- doesn't explain everything and other mechanisms can be used

- phloem has high oxygen consumption and slows or stops at low temperatures and in presence of respiratory inhibitors (eg cyanide) but this could be linked to loading and unloading of sieve tubes or - active process

- presence of sieve tubes: translocation would expected to be faster without - potassium ions

- sucrose and amino acids move at different rates: could be due to differential loading rather than flow rate - protein filaments

- different substances may move in opposite directions within phloem although, it may happen simultaneously within one sieve tubes, has not been conclusively shown - cytoplasmic streaming

- phloem translocates solutes to the top of trees but the mechanism described does not allow enough pressure to be developed to transfer material that high

State and explain the different possible mechanisms for translocation

- active process: explains inhibition by low temps and cyanide, indicates energy generated by respiration used

- protein filaments: run through the pores of the sieve plates and may be involved in carrying different solutes along different routes through the same sieve tubes elements

- cytoplasmic streaming: organised flow of cytoplasm within a cell, this could carry solutes in different directions in individual sieve tube elements with a mechanism to transport the solutes through the sieve plate

- potassium ions: found in high concentrations particularly around the sieve plate have been found to establish a potential difference across the sieve plate which increases the rate of sucrose transport; although this may also be linked to loading and unloading at source and sink

how is tension generated

water evaporates/transpiration at top of water column, water molecules replaced from below and due to cohesion this creates an upward force of water throughout the whole column (tension)

how do minerals enter xylem from the soil

• can enter symplast or apoplast pathway (as ions in solution) by active transport

• apoplast: diffuses through cortex

• forced into symplast pathway as can’t enter casparian strip

• endodermis cytoplasm takes up ions by active transport

• enters xylem by diffusion or active transport (eg, nitrates through plasmodesmata)