CIE AS Level Biology: Transport in plants

What do dicotyledonous plants have

Seeds that contain two cotyledons (seed leaves)
Network of veins
Leaves that typically have broad blades (leaf surface) and petioles (stalks)
Tap root with lateral branches

Herbaceous dicots have a relatively short life cycle (one growing season) and non-woody tissue

Transport systems in plants

Plants need transport systems to meet their metabolic demands (glucose, hormones, mineral ions are required for various processes within plants), to efficiently move substances up and down and to compensate for their relatively small SA:V ratio (generally plants cannot rely on diffusion alone)
Plants have a vascular system which involves a network of vessels (vascular tissue) running through the leaves, stem and roots. These three parts are the main organs involved in transport. The vascular system is comprised of two distinct types:
Xylem (transports water and mineral ions from the roots to the rest of the plant)
Phloem (transports substances from the source (eg. leaf) to the sink (eg.root))

The xylem and phloem are arranged together in vascular bundles. The bundles are laid out differently in the leaves, stem and roots

Function of xylem tissue

The functions of xylem tissue in a plant are:
Vascular tissue that carries dissolved minerals and water up the plant
Structural support
Food storage

Location of vascular bundles in xylem

Xylem tissue is found, along with phloem tissue and other tissues, in vascular bundles. The location of the vascular bundles is dependent on which organ they are in as the different organs are under different stresses:
In the roots the vascular bundle is found in the centre and the centre core of this is xylem tissue. This helps the roots withstand the pulling strains they are subjected to as the plant transports water upwards and grows
In the stems the vascular bundles are located around the outside and the xylem tissue is found on the inside (closest to the centre of the stem) to help support the plant
In the leaves the vascular bundles form the midrib and veins and therefore spread from the centre of the leaf in a parallel line. The xylem tissue is found on the upper side of the bundles (closest to the upper epidermis)

Function of the phloem

The function of phloem tissue in a plant is to: Transport organic compounds, particularly sucrose, from the source (eg. leaf) to the sink (eg. roots). The transport of these compounds can occur up and down the plant

Phloem is a complex tissue also made up of various cell types; its bulk is made up of sieve tube elements which are the main conducting cells and the companion cells. Other cell types of phloem tissue also include parenchyma for storage and strengthening fibres

Location of vascular bundles in Phloem

The location of the vascular bundles is dependent on which organ they are in as the different organs are under different stresses:
In the roots the vascular bundle is found in the centre and on the edges of the centre core is the phloem tissue
In the stems, the vascular bundles are located around the outside and the phloem tissue is found on the outside (closest to the epidermis)
In the leaves, the vascular bundles form the midrib and veins and therefore spread from the centre of the leaf in a parallel line. The phloem tissue is found on the lower side of the bundles (closest to the lower epidermis)

Cell types that xylem tissue are made of

Tracheids (long, narrow tapered cells with pits)
Vessel elements (large with thickened cell walls and no end plates when mature)
Xylem parenchyma
Sclerenchyma cells (fibres and sclereids)
Most of the xylem tissue is made up of tracheids and vessel elements, which are both types of water-conducting cell

Xylem

Xylem Sap moves in one direction

Phloem

Phloem saps moves in many directions

Which part of the plant has a higher water potential

Root hairs

Movement of water in plants

Water in the soil > Root hair > Root cortex > xylem > upward movement through xylem > Leaves > Diffusion through stomata

Transpiration

heat from the Sun causes the water to evaporate from the surface of the leaves through the stomata

Transpiration pull

By the effect of the water leaving via transpiration throguh the leaves there's a pulling effect in the root hair that causes it to absorb more water

Factors affecting transpiration

Humidity: The Lower the humidity, the faster the Rate of transpiration
Wind Speed: The higher the wind speed, the higher the rate of transpiratiom
Very dry conditions: a pant will close its stomata, Lower transpiration rate
Temperature: higher temperature, higher transpiration rate
Light intensity: stomata open during the day, so transpiration rate is higher. They close at night, which causes transpiration rate to be lower.

What are xerophytes

Xerophytes are plants that have been adapted to survive in regions that have very little water. They have various adaptations: thick cuticles that prevent water loss, few stomata smaller leaves, waxy coating in stem to prevent water loss.

Xylem Vessels

made from cell joined from one end to the other end to form tubes, The cells are dead, the cell walls are thickens with a strong material calls lignin

In flowering plant, xylem vessels contain vessel elements and tracheids- cells involved in water transport
They contain sclerenchyma fibres, which are elongated cells with lignified cell walls that help support the plant and parenchymq cells which are used for storage of food and starch.

How does water travel from the soil into the root hair?

Root hairs reach into spaces between soil particles to absorb water. The large no. of root hairs provides a large surface area for faster absorption of water.
Theyre important for the absorption of mineral ions such as nitrate and magnesium

2 pathways through which water can move

Apoplastic pathway: water soaks through the cell wall and moves from cell wall to cell wall without entering the cytoplasm of cells

Symplastic pathway: water moves into the cytoplasm or vacuous by osmosis and then into other cells through the plasmodesmata

How does water travel from the root hair to the xylem

Water travels through the root hair til it reaches the endodermis
The cells in the endodermis have a thick waterproof band called Casparian strip
The Casparian strip forms an impenetrable barrier to waters forcing water water to travel through the cytoplasm of cells of the endodermis
Through this, plants control the mineral ions that pass into the xylem vessles. This can also help generate root pressure
Once across the endodermis, water moves through the pericycle into xylem vessels.

Transpiration establishes a water potential gradient, whereby water continuously moves in the same direction through the leaves.

How does water move through the xylem

Water move through the xylem through mass flow.
Mass flow: waste molecules and salutes absorbed from the soil as one body of liquid. What helps this happen? Adhesive and cohesive forces of water molecules

Adhesion:water molecules will bind to the walls of the xylem
Cohesion: water molecules are able to bind together.

What happens if there's an air bubble in the xylem

the column of water breaks 2nd the hydrostatic pressure difference between the top and bottom will not be transmitted. it's called an air lock. Water stops moving upwards

How does the xylem prevent an air lock? The small diameter of the xylem helps prevent breaks between the water column.
Pits in the vessel walls allow water to move out into neighbouring vessels.
Pits also allow water to move into the neighbouring living cells.

Mineral ions

Mineral ions are absorbed with water by the root hairs. The pathway through which mineral ions move is the Symplastic and apoplastic pathway
Mineral ions can move through mass transport, diffusion, facilitated diffusion and active transport.

Transport in Phloem

Translocation is the movement of assimilates from one part of the plant to another area. Assimilates (sucrose) are transported in sieve tube elements. Sieve elements are found in the phloem tissue along with companion cells, parenchyma and fibres
Companion cells are alive, metabolically active and contribute to the low it of sucrose within the cells.
Sieve tubes and sieve elements are living cells. Sieve tubes are made up of several Sieve elements joined from one end to the other
Sieve elements have cellulose cell walls, cell surface membranes and cytoplams containing endoplamsic reticulum and mitochondria
Each sieve element has a minimum of one companion cell next to it
Plasmodesmata help with exchange between the cytoplasm of the companion cell and the sieve elements.

How does translocation occur

Mass flow in the phloem involves active loading of sucrose into sieve elements from the point where sucrose is to be transported e.g source, to any part of the plant where sucrose is taken our of the phloem to be used e.g sink. This loading decreases water potential in the sieve elements, causing water to enter the sieve element through osmosis. This creates a pressure differences which causes mass flow of water? to move through the sieve tubes.

Phloem sap can flow upwards and downwards but only follows one direction in any particular sieve tube at any one time.

Sucrose loading

Mesophyll cells in leaves make sugars during photosynthesis. These sugars are converted to sucorse.
Sucrose moves from the mesophyll cell to the phloem tissue either through the symplastic or apoplastic pathway.
This sucorse is loaded into a companion cell or directly into a sieve element by active transport. Hydrogen ions are pumped out of the companion cell into its cell wall using ATP
In order to re-enter the cell, the hydrogen ions move through a carrier protein called a co-transporter molecule which carries both hydrogen ions and sucrose(sucorse is carried against the concentration gradient)
The sucrose molecules can then move from the companion cell into the sieve use.

Differences between sieve tubes (Phloem) and xylem vessels

Sieve Tubes (Phloem): Made up of living cells, No lignified cell walls, End walls form sieve plates, sieve plates allow phloem to seal itself with callous if damaged.

Xylem vessels: Made up of dead cells, have lignified cell walls, no end walls, no sieve plates.

What's the importance of the differences between sieve tubes and xylem vessles?

If phloem cells were dead cells, sucrose would leak out becuase they wouldn't have membranes. Dead xylem cells are also empty, allowing water to flow without obstruction
The lignified cell walls of xylem vessles help them support the plant
Sieve plates in phloem sieve tubes help to maintain the pressure in the tubes.
Sieve plates allow phloem cells to quickly clot sap to prevent loss of sucrose and vulnerability to diseases.