HL Bio - Unit #5.4: Transport in plants

Transpiration

Transportation of water throughout the plant via the xylem 

• When light energy is absorbed by the leaves it is converted into heat which will evaporate the water → the vapor then leaves the plant forming an negative pressure gradient → Negative pressure gradient causes the transpiration pull where water is drawn up from the xylem 

Forces that pull the water molecules up

passive processes

1. Cohesion

2. Adhesion

Cohesion

Water being a polar molecule with a partially negative and positive end will attract to other water molecules which will continuously pull them up

Adhesion

The xylems walls are hydrophilic and attract the water molecules to adhere to it which will also bring the water up

Cavitation

Process when liquid is unable to resist the low pressure of the xylem and break

Xylem Structure

• A long continuous hollow tube made up of dead cells with 

• It allows for the free movement of water but only in one direction (up)

• Cell Wall Pits between each cell (these gaps) making up the tube allows for the transfer of water without being blocked 

• The walls of the xylem has thickened cellulose and is reinforced by lignin which makes up the “wood” of the plant

  • Lignified walls are impermeable but there are gaps through which water can pass

• The diameter of the xylem is normally larger than that of the phloem

Root Pressure within the Xylem:

• Plants will take up both water and minerals from the soil through the roots

• The higher level of water within the root will increase the the root pressure pushing the water up the xylem (called positive pressure)

This is done when the transpiration rate of the plant is insufficient and more transpiration is needed - Reasons for transpiration not occurring:

1. High atmospheric humidity

2. During the night when the stomata are closed

3. During the winter for deciduous trees that have lost their leaves and xylems need to refill with sap before the new leaves grow

Translocation

The process of organic molecules being transported around the plant through the Phloem Sieve tube in any direction from a source to a sink

Source

Any exporting region that produces sugars

• Storage organs that are unloading their sugar stores

  Eg, Photosynthetic tissues like mature green leaves 

Sink

Importing region that doesn’t produce sugar but still ended sugar

• Parts of the plant that are able to store sugars later use

• Eg, Developing fruits or seeds and growing leaves

Carbohydrates in sources and sinks

• Source: produces in the leaves/stem

• SInk: Then transported to growing roots, stems, fruits for energy storage

Amino Acids as sources and sinks

• Source: Produced in the roots and germinating seeds 

• Sink: Then are transported to growing roots, stems and fruits

Phloem Sieve Tube Cells

• Tissues that transport carbon compounds

  • The phloem tube is made up of living cells

  • The sieve tubes are made up of cellulose

    • Sieve plates are found between cells that have tiny pores within them to allow for the transport of the sap (remnants of the cell walls) → easier to have the pores patched up with protein to prevent the sap from being lost

  • Reduced cytoplasm and no nucleus → needs the help of parenchyma cells

Phloem Sieve tube transportation method is…

Bidirectional: Can move sap both up or down (not both at the same time thought)

Companion/ Parenchyma Cells

• Used with the sieve tube cells to help with active transport since the tube lacks the correct metabolic organelles to carry out some processes

  • Contains a large amount of mitochondria to produce energy for the sieve tube as well

  • Infolding within the plasma membrane increases the the loading capacity of the phloem using the apoplastic route

Hydrostatic Pressure Gradients

The force within the cell that pushes the plasma membrane against the cell wall

Hydrostatic Pressure Gradients at sources

High solute concentrations develop in the sieve tube at sources and draws water in by osmosis increasing the hydrostatic pressure

Hydrostatic Pressure Gradients at sinks

Root sinks compounds required by the tissues are unloaded by active transport lowering the solute concentration and dropping the hydrostatic pressure

Plasmodesmata

• Allows for the transport of sucrose from the companion cell to the sieve tube cell using the symplast pathway

  • Cytoplasmic connections that allows the transportation of ATP energy

Symplastic Route

ALlows for water to pass from cytoplasm from cytoplasm through the plasmodesmata

Apoplastic Route

Provides a route for the water to move through the spaces between the cells and the cell walls

Monocotyledon

1.  Stems have it’s vascular bundles (groups of phloem and xylem tubes) scatters throughout the stem

2. When growing its cotyledon (the embryonic leaf) grows as a singular straight leaf

3. The leaves a monocot normally has smooth edges

4. Flowers of monocots normally come in 3s or multiples of 3s

5. Has multiple veins of roots that extend throughout the ground

   • The xylem tubes in the roots is organized into rings while the phloem is scattered throughout 

Dicotyledon

1. Stems have its vascular bundles organized neatly into rings (most trees)

2. When growing its cotyledon grows with 2 leaves and rounded leaves

3. The leaves of a dicot normally has rough rigid leaves with visible veins

4. Flowers of dicots normally come in 4s or 5s

5. Has a clear major root that contains an x-shaped made of the xylem tubes

   • Phloem is scattered throughout

Epidermis of the root

Absorbs water and mineral ions from the soil often using long narrow outgrowths (root hairs)

Endodermis of the root

An inner skin of cells that water must pass through to reach the xylem

Cortex in the root

Unspecialized cells that bulk out the root to strengthen it and increase its surface area

Cambium in the stem

Produces more xylems and phloems

Pith in the stem

Cells that bulk out the stem to strengthen it

Cortex in the stem

Support for the stem and for photosynthesis

Epidermis

Waterproofing and protection