Water transport in plants

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The passage of water through a plant

Flowering plants have two distinct transport systems, both of which consist of tubes or vessels.

Phloem tissue is concered with the transport of sugars and other soluble products of photosynthesis. 

Xylem tissue is concerned with the transport of water and dissolved minerals, from the soil through the roots, the stem and to the leaves. 

Overview:

  • Water moves across the mesophyll cells of the leaf and is lost through the stomata.
  • Water moves through the xylem from the root, up the stem and into the leaf.
  • Water moves through the root cortex and into the xylem.
  • Water is taken in by the root hair cells in the epidermis of younger parts of roots.
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Uptake and movement of water across the root

Structure of a dicotyledenous root:

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Structures found in a root

Epidermis: Single outer layer of cells

Root hair cells: Epidermal cells with a large surface area for uptake

Cortex: Tissue making up the bulk of the root 

Endodermis: Single layer surrounding vascular bundle. Walls have a Casparian strip of a waxy and waterproof material called SUBERIN, which is impermeable to water. 

Vascular bundle: Contains xylem and phloem.

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Root hair cells

Root hair cells are part of the epidermis (outer single layer of cells). They are examples of specialised plant cells. They are adapted to their function in the following ways:

  • Hair like extension of cell: Large surface area for absorption
  • Thin cell wall: Shorter diffusion pahtway (e.g. for water)
  • Many mitochondria: ATP. Release lots of energy by respiration for active transport 
  • Specific carrier proteins embedded in the membrane: For active transport of ions (e.g. nitrates)
  • Relatively high conc. of ions/solutes in cytoplasm and vacuole: Low water potential inside cells to allow osmosis
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Transport of water from soil across the root

Mineral ions are actively transported from soil into the root hair cells. This reduces the water potential in cells which allows water to enter the cell by osmosis. 

To reach the endodermis water can move across the cortex by diffusion down a water potential gradient. There are two pathways the water can ake:

SYMPLASTIC pathway: 

  • Consists of LIVING parts of the cell (CYTOPLASMS).
  •  Water moves through the symplastic pathway by diffusing between the cytoplasm of neighbouring cells down a water potential gradient. 
  • The cytoplasms of neighbouring cells are often directly interconnected by plasmodesmata (microscopic channgels which traverse the cell walls and increase the rate of movement between cells) 

MOVEMENT OF WATER THROUGH THE CYTOPLASM BY OSMOSIS DOWN A WATER POTENTIAL GRADIENT

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Transport of water from soil across the root

APOPLASTIC pathway:

  • Consists of the CELL WALLS. 
  • Cell walls are porous, freely permeable and in direct contact with cell walls of neighbouring cells, so warer can move easily from cell to cell via this pathway.

MOVEMENT OF WATER ALONG CELL WALLS BY COHESION TENSION - NOT OSMOSIS 

  • When water reaches the endodermis the apoplastic pathway is blocked by the Casparian strip. This is a band of waterproof material called SUBERIN. All the water has to diffusde into the cytoplasm of the endodermal cells by osmosis, and continues towards the xylem in the symplastic pathway.
  • Cells of the endodermis ACTIVELY TRANSPORT ions into the xylem.
  • This LOWERS THE WATER POTENTIAL of the xylem so water enters from the cytoplasm of the endodermal cells by OSMOSIS. 
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Movement of water through the leaf

When stomata are open, water vapour diffuses from the air spaces in the lead, out through the stomata, down a diffusion gradient. This loss of water is called TRANSPIRATION. 

To replace this, water EVAPORATES from the walls of mesophyll cells into the air spaces forming water vapour, which builds up in the air spaces. 

The water potential in the mesophyll cells falls, and they replace their lost water from neighbouring cells by diffusion. 

As water vapour is lost by transpiration, a WATER POTENTIAL GRADIENT builds up across the leaf, allowing water to diffuse out of the xylem (higher water potential) and reach the mesophyll cells. 

As in the root water can move from cell to cell across the leaf via the symplastic or apoplastic pathways 

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Movement of water through the leaf

Capilarity - the movement of a liquid in tubes of small cross sectional area, like the space between microfibrials. 

2 routes which water can evaporate from the plant:

  • STOMATA - by EVAPORATION of water from the cell walls of the mesophyll, followed by diffusion of water vapour through open stomata in leaves and green stems. About 90% of water is lost this way. 
  • Cuticle - Waterproof waxy layer covers upper and lower epidermis of leaves. Some water evaportes.
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Movement of water from roots to leaves

Water is carried in xylem tissue. This is a series of CONTINUOUS TUBES called XYLEM VESSELS. These are made VESSEL ELEMENTS arranged end-to-end.

Features of xylem vessel:

  • Their cell walls are impregnated with LIGNIN: Lignin strengthens the xylem walls against the tension force within them, and makes them waterproof. 
  • The walls of xylem vessels contain tiny holes called PITS: If a vessel becomes blocked or damaged the water can be diverted laterally (sidewats) so the upwards movement of water can continute in a neighbouring vessel.
  • The lignified vessel walls cause the cell contents to DIE: This leaves a hollow lumen (no cytoplasm) that offers little resistance to the flow of water and minerals. 
  • The vessels also lose their end walls: They form a continuous column for water movement from root to leaves.
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The ascent of water in the xylem

Any theory of water movement up the xylem has to account for the movement oof water against gravity, in very narrow tubes (xylem vessels), at relatively high rates.

1) THE COHESION TENSION THEORY

  • EVAPORATION OF WATER FROM THE LEAVES PULLS UP MORE WATER FROM THE XYLEM IN THE STEM
  • Water molecules are polar and so have many weak hydrogen bonds form between them making them stick together. - COHESION.
  • Water molecules also form hydrogen bonds with the walls of the xylem. - ADHESION.
  • As TRANSPIRATION occurs through open stomata, water is lost from the mesophyll cells of the leaf and is replaced by water from the xylem. 
  • This creates low pressure at the top of the xylem in the stem. 
  • Water in the xylem is under TENSION and so is pulled up towards the leaves.
  • Continuous columbs of water are maintained due to COHESION between water molecules.
  • There is also adhesion pull on the vessel walls as the water is pulled up, causing xylem vessels to decrease in diameter. 
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Cohesion Tension Theory

This process is entirely PASSIVE as no ATP is required for this to occur. 

Evidence:

  • Tension has been measured in the xylem as plants transpire.
  • Lignified walls in xylem are strong enough to withstand this tension.
  • The diameter of trees decreases when they are transpiring, (tension pulls xylem walls in), and more so when temp and light intensities are higher. This can be measured using a dendrograph. 
  • Air bubbles in the xylem stop upward movement of water as they prevent cohesion.
  • Respiratory inhibitors, such as cyanide or a lack of oxygen, do not inhibit this process. 

Trunk diameter low at noon: stomata open, high transpiration rates, increased tension so xylem walls pulled in and xylem vessels decrease in diameter.

Trunk diameter highest at midnight: stomata closed, transpiration rates low, little tension so little inwards pull on walls of xylem vessels.

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2. Root pressure - active process

A freshly cut stem cut near to the ground continues to exude sap (water and minerals). The force generated to do this can be measured by attaching a manometer to the cut end. Respiratory inhibitors, such as cyanide or a lack of oxygen, inhibit this process. Cohesion tension theory cannot account for this as the stem has been cut there's no transpirational pull as no leaves from which water will evaporate. Cohesion tension theory requires no ATP, but the fact that cyanide or lack of oxygen inhibits the process suggests it's an active process. 

  • This force is known as ROOT PRESSURE. Root pressure is known to be due to the osmotic flow of water from the root endodermis into the xylem. 
  • The water potential gradient necessary for this is due to the active transport of mineral ions from the endodermis into the xylem. 
  • This reduces the water potential of the xylem contents and water follows by osmosis. 
  • As the volume of water increases inside the xylem, the pressure increases, proving a force that pushes the water up the stem. 

Respiratory inhibitors prevent root pressure as they inhibit active transport of ions and so no water potential gradient is set up between the endodermis and xylem and so no osmotic movement of water occurs into the xylem.

Root pressure provides an outward force/push on walls of xylem. Xylem vessels would become wider (or stay same as force is small)

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Factors affecting the rate of transpiration

Rate of transpiration can be measured by using a potometer. This piece of equipment measures the rate of uptake of water, the assumption being that this is the same as the rate at which water leaves the plant. This assumption might not be correct as not all water is lost in transpiration. Some water is lost in photosynthesis. Water is also produced in respiration, some water is needed to keep the plant turgid. 

  • Cut stem of a leafy shoot, under water preferably to prevent air bubbles entering xylem (use same cutting to control for SA of leaf.- diff leaves=diff SA=diff rate of transpiration)
  • Submerge the potometer and fill with water. This prevents air bubbles entering the xylem or potometer itself.
  • Attach the leafy shoot, using a piece of rubber turbing to the potometer. 
  • Seal the joints around the rubber tube with vaseline to maintain an airtight seal. 
  • Introduce an air bubble into capillary tube and time how long it takes for the bubble to travel a set distance along the tube. 
  • Repeat the procedure a number of times, in order for reliability and identify anomalies.
  • Calculate the rate of uptake, in mm3 per minute, using the means of the results obtained 
  • The experiment can be repeated changing environmental conditions, such as air,temp,wind speed etc.
  • Calculation: 
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Environmental(External)factors affecting Transp. r

  • Humidity: When the external air has a HIGH HUMIDITY there is MORE water vapour and so a HIGHER WATER POTENTIAL. Therefore the WATER POTENTIAL GRADIENT IS LOWERED and LESS WATER DIFFUSES OUT of the leaf through stomata. There's a LOWER rate of transpiration.
  • Temperature: If the temperature increases the water particles have more kinetic energy and therefore move around faster. As they move around faster, the RATE OF DIFFUSION WILL INCREASE thus INCREASING THE RATE OF TRANSPIRATION. Rates of EVAPORTION WILL ALSO INCREASE inside the leaf. (At higher temps, however, water loss is so great that the stomata close, so reducing rate of transpiration)
  • Light intensity: STOMATA OPEN IN THE LIGHT, to allow more gas exchange for photosynthesis. Thus is follows that for most plants, the GREATER THE LIGHT INTENSITY THE GREATER THE RATE OF TRANSPIRATION. 
  • Wind speed/air movement: AIR CURRENTS REMOVE WATER VAPOUR FROM THE AREA AROUND THE STOMATA. This reduces the water potential outside the leaf, INCREASING THE WATER POTENTIAL GRADIENT and INCREASING DIFFUSION. HIGHER wind speeds thus INCREASE THE RATE OF TRANSPIRATION.
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Xerophytic adaptations

A xerophyte is a plant that is adapted to live in dry conditions. Xerophytes are found in any region where water is scarce, e.g. deserts. Xerophyte examples = cactus, maram grass (on sand dunes)

These plants have STRUCTURAL adaptations, which allow them to survive in dry conditions either by reducing transpiration loss or by storing water. E.g.

  • Sunken stomata: Lower water potential gradient in leaf and higher water potential outside, less water is lost and also the stomata is not in direct contact with sunlight. 
  • Leaves reduced to spines, succulent stems: Succulent tissue can store water, the spines have a reduced SA and reduced stomata numbers and reduced SA of cuticle. 
  • Epidermal hairs: Trap water which reduces the water potential gradient so less water is lost by transpiration.
  • Rolled leaves: Water will stay inside the leaf. This reduces the water potential gradient. Increases humdity so higher water potential. The stomata is also trapped. 
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Xerophytic adaptations to reduce water loss

  • Thicker cuticle on leaves and stems: the cuticle is waxy so reduces water loss (from epidermis). greater thickness increases length of diffusion pathway for water to reach the atmosphere, and so decreases rate of diffusion of water through the cuticle. 
  • Reduction in the SA of leaves: reduces the number of stomata and reduces overall SA, give lower rates of diffusion by Fick's law. 
  • Curling or rolling of leaves: water collects just outside stomatal pore so reduces water potential gradient between air spaces inside leaf and the atmosphere. a lower rate of diffusion and less evaporation occurs.
  • Sunken stomata:water collects just outside stomatal pore so reduces water potential gradient between air spaces inside leaf and the atmosphere. a lower rate of diffusion and less evaporation occurs.
  • Reduced density/number of stomata: reduces the number of stomata and reduced overall SA gives lower rate of diffusion by Fick's law. 
  • Stomata confined to underside of leaf: cooler so less evaporation and less less (less diffusion of water therefore).
  • Daylight closure of stomata: reduces evaporation during the hottest part of the day.
  • Presence of epidermal hairs: the hairs trap a layer of still moist air so reducing water potential gradient between air spaces inside the leaf and the atmosphere. A lower rate of diffusion occurs.
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