10. Transport in Plants

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  • Created by: zoelaad
  • Created on: 30-12-17 02:22

The Structure and Function of Vascular Systems 1

Roots, Stem and Leaves:

Multicellular plants are large organisms so they need a transport system. Xylem tissue moves water and minerals from the roots to the leaves. Phloem tissue moves assimilate up and down the plant from the source to sinks.

Xylem Vessels:

Xylem vessels are adapted to enable the free flow of water along the vessels. Xylem vessels several adaptations:

  • end walls are removed to form long tubes
  • no cytoplasm or organelles present
  • cell walls are impregnated with lignin to make vessel wall waterproof and to strengthen the vessel
  • spiral, annular and recirculate thickening strengthens the wall to prevent collapse
  • bordered pits between the vessels allow the movement of water between vessels
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The Structure and Function of Vascular Systems 2

Phloem:

The phloem is adapted to transport assimilates actively by mass flow. There are two cell types involved:

  • Sieve Tube Elements- long sieve tubes that transport the assimilates
  • Companion Cells- support cells that provide all the metabolic functions for the sieve tube elements and are involved in actively loading the sieve tube

Adaptations of Phloem:

Sieve Tube Elements:                                                                Companion Cells:

- form long tubes                                                                       - closely associated with sieve tube elements
- end walls are retained                                                             - connected to sieve tube elements by plasmodesmata
- end walls contain many sieve pores (sieve plates)                 - dense cytoplasm with many mitochondria 
a thin layer of cytoplasm                                                        - large nucleus
- very few organelles and no nucleus

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Transpiration as a Consequence of Gaseous Exchange

Transpiration is the loss of water vapour from the upper part of the plant, mainly the leaves. Although some water evaporates and diffuses through the leaf surface, most water vapour is lost via the stomata. Transcription involves three stages:

1. Water moves by osmosis from the xylem to the mesophyll cells in the leaf

2. Water evaporates from the surfaces of the spongy mesophyll cells into the air spaces inside the leaves

3. Water vapour diffuses out of the leaf via the stomata

The stomata open during the day to allow gaseous exchange - carbon dioxide enters the leaf and oxygen is released. As the stomata are open, water vapour is lost. Transpiration is, therefore, a consequence of gaseous exchange. 

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Factors that Affect the Rate of Transcription

There must be a water potential gradient between the air spaces in the leaf and the surrounding air to make water vapour leave the leaf. The steeper the gradient, the more rapid the loss of water vapour (transcription). 

Factors that increase transcription rate include:

  • higher temperatures - this increases evaporation so there will be a higher water potential inside the leaf
  • more wind - this blows water vapour away from the leaf, reducing the water potential in the surrounding air
  • lower relative humidity - this increases the water potential gradient between the air inside the leaf and outside
  • higher light intensity - this causes the stomata to open wider
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Measuring the Rate of Transcription

Transcription can be estimated using a bubble potometer. A potometer actually measures water uptake by the stem, but you can assume that water uptake equals water loss from the leaves in most cases. 

Care must be taken when setting up the potometer to ensure that there are no leaks and no air in the system, expect the bubble used for measuring.

Once the shoot has been allowed to acclimatise, the movement of the bubble along the capillary can be measured under different conditions.

Transpiration rate is calculated by dividing the distance moved by a set time.

(http://www.passmyexams.co.uk/GCSE/biology/images/potometer.jpg)

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Transport of Water

  • The cell wall of a plant is permeable to water, the cell surface membrane is selectively permeable
  • As a result, plant cells can contain mineral ions in solution which reduces the water potential inside the cell
  • The more concentrated the mineral ions, the lower the water potential
  • Water moves by osmosis from a cell with a higher water potential to a cell with a lower potential because water molecules move down their water potential gradient
  • Water can now enter a cell from its environment if the water potential in the cell is lower than the water potential in the environment
  • This is how root hair cells absorb water from the soil
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Pathways in Water Transport

Once in the plant, water can move across a tissue such as the root cortex by different pathways:

1. The apoplastic pathway carriers water between the cells through the cell walls- the water does not enter the cytoplasm or pass through the cell surface membrane

2. The symplastic pathway takes water from cell to cell through the cytoplasm of each cell. Water often passes through plasmodesmata linking the cytoplasm of adjacent cells

3. The vacuolar pathway carries water through the cytoplasm and vacuole of each cell

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The Transpiration Stream

Water movement from the roots up to the leaves in the xylem is known as the transpiration stream. There are three mechanisms that move water up the stem: root pressureadhesion or capillary action and transpiration pull.

Root pressure and capillary action combined can only raise water by a few meters. Therefore, transpiration and the pull it creates are essential to move water all the way up a tall stem.

Root Pressure:

The pressure created by the action of the endodermis

Adhesion:

The attraction between water molecules and the walls of the xylem

Transpirational Pull:

Also known as cohesion-tension theory accounts for the movement of water up the xylem

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Adaptations of Plants to the Availability of Water

Xerophytes:

Are plants that are adapted to living in dry places. The following adaptations help them to reduce the loss of water vapour:

  • thick waxy cuticles on the leaves
  • smaller leaf area
  • stomata in pits
  • hairy leaves
  • rolled leaves

Hydrophytes:

Are plants that are adapted to living in water. The following adaptations help them to do this:

  • leaves and leaf stems have a large air space (to help them float)
  • the stomata may be on the upper surface of the leaf (to gain CO2 from the air)
  • the stem may be hollow (to allow gases to move the the roots easily) 
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The Mechanism of Translocation

Translocation is an energy-requiring process for transporting assimilates (mostly sucrose) around a plant. It occurs in the sieve tubes, but the companion cells are important as they actively load the assimilates into the sieve tubes.

Translocation is achieved by mass flow. It is caused by creating a high hydrostatic pressure at the source and a lower hydrostatic pressure at the sink. The fluid in the phloem sieve tube then moves from high to low pressure (down its pressure gradient)

Sources:

A tissue or organ that supplies assimilates to the phloem. This could be... a leaf that's made sucrose during photosynthesis, a root that has stored starch and converted it to sucrose or any other storage organ.

Sink:

A tissue or organ that removes assimilates from the phloem and uses them. This could be... the buds or stem tips that needs energy to grow, the leaves in spring as they grow and unfold, the roots in the summer and autumn when storing sugar as starch or any organ that may store starch. 

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Creating High Pressure in Sources

This process is called active loading. Sucrose is moved into the sieve tube by a complex process involving active transport:

1. Hydrogen ions are pumped actively out of the companion cells

2. The hydrogen ions diffuse back into the companion cells through special co-transport proteins carrying sucrose molecules into the companion cells

3. The sucrose builds up in the companion cells and diffuses into the sieve tubes through the many plasmodesmata

4. The water potential in the sieve tubes is reduced

5. Water flows into the sieve tube by osmosis, increasing the pressure

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Creating Lower Pressure in Sinks

1. As sucrose is used in respiration or converted to starch in the cells of the sink, the concentration decreases

2. This creates a concentration gradient between the sieve tubes and the cells in the sink

3. Sucrose diffuses out of the sieve tubes into the cells and the water potential in the sieve tube increases

4. Water then moves out of the sieve tube by osmosis

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