Transport in plants

  • Created by: Abi9ai1
  • Created on: 05-09-19 12:14

Why do plants need a transport system?

  • All living things need to be able to take substances from, and return wastes to their environment. Large plants have a small SA:vol so they need a specialised exchange surface and a transport system.
  • Plants demands for oxygen is low, as the plant isnt very active, so it doesn't do lots of respiration. Therefore demands can be met by diffusion alone. 
  • However, demands for water and sugars are still high. 
  • Plants can absorb water and minerals at the roots, but they cannot absorb sugar from the soil.
  • The leaves can perform gaseous exchange and make sugars from photosynthesis, but they cannot absorb water from the air.
  • Therefore, a plant needs a transport system to move water and minerals from the roots to the leaves; and to move sugars from the leaves to the rest of the plant.
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The vascular tissues

The transport system in plants consists of specialised vascular tissue:

  • Water soluble mineral ions travel upward in the xylem tissue.
  • Assimilates, such as sugars, travel up or down in phloem tissue.

Distribution of vascular tissue

  • The vascular tissue is distributed throughout the plant in a very distinctive way in dicotyledonous plants.
  • The xylem and phloem are found together in vascular bundles. 
  • These bundles may also contain other types of tissue ( e.g. collenchyma and sclerenchyma) that give the bundle strength and help support the plant.
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Xylem and phloem in the young root

  • The vascular bundle is in the centre of the young root. There is a central core of xylem, often in an "X" shape. The phloem is found in the arms of the "X" of xylem tissue. 
  • This arrangement provides strength to withstand the pulling forces to which roots are exposed.
  • Around the vascular bundle is the endodermis, which has a key role in getting water into the xylem vessels.
  • Just inside the endodermis is a layer of meristem cells, called the pericyle.

Image result for root cross section      Image result for xylem in root

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Xylem and phloem in the stem

  • The vascular bundles are found near the edge of the stem. 
  • In young woody plants and non woody plants the vascular bundles are seperate.
  • However, in older woody plants the bundles are one continuous ring. This means there is a complete ring of vascular tissue under the bark of a tree, providing strenghth and felixibility to the branches.
  • The xylem is found towards the inside of the vascular bundle, and the phloem on the outside.
  • In between the phloem and xylem there is a layer of cambium, which is made of meristem (plant stem cells).

Image result for stem cross section

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Xylem and phloem in the leaf

  • The vascular bundles form the midrib and veins of a leaf.
  • A dicotyledonous leaf has a branching network of veins that get smaller as they spread away from the mid rib.
  • Within each vein, the xylem is located on top of the phloem.
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Transport in plants definitions

Dicotyledonous plants- Plants with 2 seed leaves and a branching pattern of veins in the leaf.

Meristem- A layer of dividing cells, here it is called the pericycle.

Phloem- Transports dissolves assimilates.

Vascular tissue- Consists of cells specialised for transporting fluids by mass flow.

Xylem- Transports water and minerals.

Plasmodesmata- Gaps in the wall containing cytoplasm that connects 2 cells.

Potometer- a device that can measure the rate of water uptake as a leafy stem transpires.

Transpiration- The loss of water vapour from the arial parts of the plant, mostly from the stomata in the leaves.

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The structure and function of xylem

Xylem is a tissue used to transport water and mineral ions from the roots up to the leaves and other parts of the plant. Xylem tissue consists of:

  • Vessels to carry water and dissolved mineral ions
  • Fibres to help support the plant
  • Living parenchyma cells which act as packing tissue to separate and support the vessels.
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Xylem vessels

  • As xylem vessels develop, lignin impregnates the walls of the cells, making the walls waterproof.
  • This kills the cells. The end walls and contents of the cells decay, leaving a long column of dead cells with no contents ( xylem vessel).
  • The lignin strengthens the vessel walls and prevents the vessel from collapsing.
  • This keeps the vessels open even when water is in short supply.
  • The lignin thickening forms patterns in the cell wall. These may be spiral, annular (rings) or reticulate (broken rings). 
  • This prevents the vessel being too rigid and allows some flexibility.
  • In some places lignification is not complete, leaving gaps in the cell wall. These form bordered pits
  • The bordered pits in 2 adjacent vessels are aligned to allow water to leave the xylem and pass to the living parts of the plant.
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Adaptations of xylem to its function

Xylem vessels can carry water and mineral ions from the roots to the very top of the plant because:

  • They are made from dead cells alligned end to end to form a continuous column.
  • The tubes are narrow, so that the water column doesn't break easily and capillary action can be effective.
  • Bordered pits in the lignified walls allow water to move sideways from one vessel to another.
  • Lignin deposited in the walls in spiral, annular or reticulate patterns allows xylem to stretch as the plant grows, and enables the stem or branch to bend.

The flow of water is not impeeded, because:

  • There are no cross-walls.
  • There are no cell contents, nucleus or cytoplasm.
  • Lignin thickening prevents the walls from collapsing.
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Phloem and companion cells

  • Phloem is a tissue used to transport assimilates (mainly sucrose and amino acids) around the plant. The sucrose is dissolved in water to form sap.
  • Phloem consists of sieve tubes- made up of sieve tube elements- and companion cells.

Companion cells

  • In between the sieve tubes are small cells, each with a large nucleus and dense cytoplasm. These are the companion cells.
  • They have lots of mitochondria to produce the ATP needed for active processes.
  • The companion cells carry out the metabolic processes needed to load assimilates into the sieve tubes.
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Sieve tube elements

  • Elongated sieve tube elements are lined up end to end to form sieve tubes.
  • They contain no nucleus and very little cytoplasm, leaving space for mass flow of sap to occur.
  • At the end of the sieve tube elements are perforated cross-walls called sieve plates.
  • The perforations in the sieve plate allow movement of sap from one element to the next. 
  • The sieve tubes have very thin walls and when seen in transverse sections they are usually 5 or 6 sided.
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Pathways taken by water

  • The cellulose cell walls of plant cells are fully permeable to water.
  • Water molecules can move freely between the cellulose molecules or even in gaps between the cell.
  • Water can also pass across the cell wall and through the partially permeable plasma membrane into the cytoplasm or into the vacuole.
  • Many plant cells are joined by special cytoplasmic bridges.
  • These are cell junctions at which the cytoplasm of one cell is connected to that of another through a gap in their cell walls. These junctons are called plasmodesmata.
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3 pathways water takes

The apoplast pathway

  • Water passes through the spaces in the cell walls and between cells. 
  • It doesn't pass through any plasma membranes into the cell.
  • This means water is moved by mass flow, not osmosis.
  • Dissolved mineral ions can also be carried with the water.

The symplast pathway

  • Water enters the cell cytoplasm through the plasma membrane.
  • It can then pass through the plasmodesmata from one cell to the next.

The vacuolar pathway

  • This is similar to the symplast pathway but water isnt confined to the cytoplasm of the cells.
  • It is able to enter vacuoles as well.
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Water potential

  • Water potential is a measure of the tendency of water molecules to move from one place to another.
  • Water always moves from a region of higher water potential to a region of lower water potential.
  • The water potential of pure water is 0.
  • In a plant cell, the cytoplasm contains mineral ions and sugars (solutes) that will reduce the water potential.
  • This is because there are fewer "free" water molecules available than in pure water.
  • Therefore, the water potential in plant cells is always negative.
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Water uptake

  • If you place a plant cell in pure water, it will take up water molecules by osmosis.
  • This is because the water potential in the cell is more negative than the water potential of water.
  • Water molecules will move down the water potential gradient into the cell.
  • The cell will not continue to absorb water until it bursts, due to there being a strong cellulose cell wall.
  • Once the cell is full of water it is described as being turgid.
  • The water inside the cell starts to exert pressure on the cell wall, called the pressure potential.
  • As the pressure builds up it reduces the influx of water.
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Water loss

  • If a plant cell is placed in salt solution, with a very negative water potential, the plant will lose water by osmosis.
  • This is because the water potential of the cell is less negative than the water potential of the solution, so water moves down the water potential gradient and out of the cell.
  • As water loss continues, the cytoplasm and vacuole shrink.
  • Eventually the cytoplasm no longer pushes against the cell wall, and the cell is no longer turgid.
  • If water continues to leave the cell, then the plasma membrane will lose contact with the wall    - a condition known as plasmolysis.
  • The tissue is now flaccid.
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Transpiration

  • Transpiration is the loss of water vapour from the above ground parts of the plants ( mainly leaves)
  • Some water may evapourate through the upper leaf surface, but this loss is limited by the waxy cuticle.
  • Most water vapour leaves through the stomata, which open to allow gaseous exchange for photosynthesis.
  • Since photosynthesis occurs only when there is sufficient light, most water vapour is lost during the day.
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Typical pathway taken by water when leaving the le

  • Water enters the leaf through the xylem, and moves by osmosis into the cells of the spongy mesophyll. It may also pass along the cell walls via the apoplast pathway.
  • Water evapourates from the cell walls of the spongy mesophyll.
  • Water vapour moves by diffusion out of the leaf through open stomata.
  • This relies on a difference in the concentration of water vapour molecules in the leaf compared with the outside of the leaf.
  • This is known as the water vapour potential gradient.
  • There must be a less negative (higher) water vapour potential inside the leaf compared to the outside of the leaf.
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The importance of transpiration

Transpiration is essential for the plants survival. As water vapour is lost from the leaf it must be replaced from below. This draws water up the stem as a transpiration stream.

This movement:

  • Transports useful mineral ions up the plant
  • Maintains cell turgidity
  • Supplies water for growth, cell elongation and photosynthesis
  • Supplies water that, as it evapourates, cools the plant on a hot day.
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Environmental factors that affect the transpiratio

Light intensity 

  • In light the stomata open to allow gaseous exchange for photosynthesis.
  • Higher light intensity increases the transpiration rate.

Tempurature

  •   Increase the rate of evapouration from the cell surfaces so that the water vapour potential in the leaf rises.
  • Increase the rate of diffusion through the stomata because the water molecules have more kinetic energy.
  •  Decrease the relative water vapour potential in the air, allowing more rapid diffusion of molecules out of the leaf.
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Environmental factors that affect the transpiratio

Realtive humidity

  • High relative humidity in the air will decrease the rate of water loss.
  • This is because there will be a smaller water vapour potential gradient between the air spaces in the leaf and the air outside.

Air movement 

  •  Air moving outside the leaf will carry away water vapour that has just been diffused out of the leaf, maintaining the high water potential gradient.

Water availability

  • If there is little water in the soil, then the plant cannot replace the water that is lost.
  • If there is insufficient water then the stomata close and the leaves wilt.
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Water uptake and movement across the root

  • The outermost layer of cells (epidermis) of a root contains root hair cells.
  • These are cells with a long extention (root hair) that increases the surface area of a root.
  • These cells absorb mineral ions and water from the soil.
  • The water then moves across the root cortex down a water- potential gradient to the endodermis of the vascular bundle.
  • Water may also travel through the apoplast pathway as far as the endodermis, but then enter the sympast pathway, as the apoplast pathway is blocked by the Casparian *****.
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The role of the endodermis

  • The movement of water across the root is driven by an active process that occurs in the endodermis. 
  • The endodermis is a layer of cells surrounding the medulla and xylem.
  • This layer is also known as the starch sheath, as it contains granules of starch.
  • The Casparian ***** blocks the apoplast pathway between the cortex and the medulla.
  • This ensures that dissolved mineral ions (especially nitrates) have to pass into the cell cytoplasm through the plasma membranes.
  • The plasma membranes contain transporter proteins, which actively pump mineral ions from the cytplasm of the cortex cells into the medulla and xylem. 
  • This makes the water potentail of the medulla and xylem more negative, so that water moves from the cortex cells into the medulla and xylem by osmosis.
  • Once the water has entered the medulla, it cannot pass back into the cortex, as the apoplast pathway of the endodermal cells is blocked by the Casparian *****.
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Movement of water up the stem

Movement of water through the xylem is by mass flow. This is a flow of water and mineral ions in the same direction. 

There are 3 processes that help to move water up the stem:

  • Root pressure
  • Transpiration pull
  • Capillary action
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Root pressure

  • The action of the endodermis moving minerals into the medulla and xylem by active transport, draws water into the medulla by osmosis.
  • Pressure in the root medulla builds up and forces water into the xylem, pushing water up the xylem.
  • Root pressure can push water a few metres up a stem, but cannot account for water getting to the top of tall trees.
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Transpiration pull

  • The loss of water by evapouration from the leaves must be replaced by water coming up from the xylem.
  • Water molecules are attracted to eachother by forces of cohesion.
  • These cohesion forces are strong enough to hold the molecules together in a long chain or column.
  • As molecules are lost at the top of the column, the whole column is pulled up as one chain.
  • The pull from above creates tension in the column of water.
  • This is why the xylem vessels must be straightened by lignin.
  • The lignin prevents the vessel collapsing under tension.
  • This is cohesion tension theory.
  • It relies on the plant maintaining an unbroken column of water all the way up the xylem.
  • If the water column is broken in a xylem vessel, then the water column can still be maintained through another vessel via the bordered pits.
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Capillary action

  • The same forces that hold water molecules together also attract the water molecules to the side of the xylem vessel.
  • This is called adhesion.
  • Because the xylem vessels are very narrow, these forces of attraction can pull the water up the sides of the vessels.
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How water leaves the leaf

  • Most of water that leaves the leaf is water vapour exiting through the stomata.
  • Only a tiny amount of water leaves through the waxy cuticle.
  • Water evapourates from the cells lining the cavity immediatly above the guard cells ( the sub-stomatal air space).
  • This lowers the water potential in these cells, causing water to enter them by osmosis through neighbouring cells.
  • In turn, water is drawn from the xylem in the leaf by osmosis.
  • Water may also reach these cells by the apoplast pathway from the xylem.
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Terrestrial plants

  • Getting access to water can be an issue for land plants
  • Water is lost by transpiration, because plants exchange gases with the atmosphere via their stomata.
  • During the day plants take up lots of carbon dioxide and remove oxygen due to photosynthesis, so the stomata must be open
  • However this means it is easy for water to be lost. 
  • This water must be replaced.

Plants living on land must be adapted to:

  • Reduce loss of water
  • Replace water that is lost.
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Terrestrial plants adaptations

  • A waxy cuticle on the edge of the leaf will reduce water loss due to evapouration through the endodermis.
  • The stomata are often found on the under surface of leaves, this reduces evapouration due to direct heating of the sun.
  • Most stomata are closed at night, when there is no light for photosynthesis.
  • Deciduous plants lose their leaves in winter, when the ground may be frozen ( making water less available) and when temperatures may be too low for photosynthesis.
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Marram grass

  • Specialised living in sand dunes where the conditions are paticularly harsh because any water in the sand often drains away quickly. 
  • The sand may be salty and the leaves are often exposed to very windy conditions.
  • It is a xerophyte - a plant adapted to living in arid conditions.

The adaptions of marram grass include:

  • The leaf is rolled longitudinally so that air is trapped inside - this air becomes humid, which reduces water loss from the leaf. The leaf can roll more tightly in very dry conditions.
  • There is a thick waxy cuticle on the outer side of the rolled leaf (upper epidermis) to reduce evapouration.
  • The stomata are on the inner side of the rolled leaf (lower epidermis), so they are protected by the enclosed air space.
  • The stomata are in pits in the lower epidermis, which is also folded and covered by hairs. These adaptation help to reduce air movement and therefore loss of water vapour.
  • The spongy mesophyll is very dense, with very few air spaces, so there is less surface area for evapouration of water.
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Cacti

  • Cacti are succulents - they store water in their stems which become fleshy and swollen. The stem is often ribbed or fluted so that it can expand when water is available.
  • The leaves are reduced to spines. This reduces the surface area of the leaves. When the total leaf surface area is reduced, less water is lost by transpiration.
  • The stem is green for photosynthesis.
  • The roots are very wide spread, in order to take advantage of any rain that falls.
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Xerophytic features

  • Closing the stomata when water availabilty is low will reduce water loss and so reduce the need to take up water.
  • Some plants have a low water potential inside their leaf cells. This is acheived by maintaining a high salt concentration in the cells. The low water potential reduces the evapouration of water from the cell surfaces as the water potential gradient between the cells and the leaf air spaces is reduced.
  • A very long tap root that can reach water deep underground.
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Hydrophytes

  • Plants that live in water, e.g water lilies.
  • These plants have easy access to water, but are faced with other issues such as getting oxygen to their submerged tissues and keeping afloat - they need to keep their leaves in the sunlight for photosynthesis.

The adaptations of a water lily include:

  • Many large air spaces in the leaf. This keeps the leaf afloat so that they are in the air and can absorb sunlight.
  • The stomata are on the upper epidermis, so that they are exposed to the air and allow gaseous exchange.
  • The leaf stem has many large air spaces. This helps with buoyancy, but also allows oxygen to diffuse quickly to the roots for aerobic respiration.
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How do hydrophytes transpire?

  • Transpiration is the loss of water vapour from the surfaces of the leaves - but water will not evapourate into water or into air that has a high humidity.
  • If water cannot leave the plant, then the transpiration stream stops and the plant cannot transport mineral ions up to the leaves.
  • Many plants contain specialised structures at the tips or margins of their leaves called hydathodes.
  • These structures can release water droplets which may then evapourate from the leaf surface.
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Translocation

  • Translocation occurs in the phloem, and is the movement of assimilates throughout the plant.
  • Assimilates are substances made by the plant, using substances absorbed from the environment.
  • These include sugars (mainly transported in sucrose) and amino acids.
  • A part of the plant that loads assimilates into the phloem sieve tubes is called a source.
  • A part of the plant that removes assimilates from the phloem sieve tubes is called a sink.
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Active loading

  • Sucrose is loaded into the sieve tubes by an active process. This involves the use of energy from ATP in companion cells.
  • The energy is used to actively transport H+ out of the companion cells, increasing the conc of outside the cell and decrease their conc on the outside, causing a concentration gradient.
  • The H+ diffuse back into the companion cells through special cotransporter proteins.These proteins only allow movement of the H+ into the cell if they are accompanied by sucrose molecules. This is known as cotransport.
  • It is also known as secondary active transport , as it results from active transport of the H+ out of the cell and moves sucrose against its conc gradient.
  • As the conc of sucrose in the companion cell increases, it can diffuse through the plasmodesmata into the sieve tube.
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Movement of sucrose

  • Movement of sucrose along the phloem is by mass flow.
  • A solution of sucrose, amino acids and other assimilates flows along the tube.
  • The solution is called sap, and it can be made to flow either up or down the plant as required.
  • The flow is caused by a difference in hydrostatic pressure between the two ends of the tube, which produce a pressure gradient.
  • Water enters the tube at the source, increasing the pressure, and it leaves the tube at the sink, reducing the pressure.
  • Therefore the sap flows from the source to the sink.
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The source

  • Sucrose entering the sieve tube element makes the water potential inside the sieve tube more negative.
  • As a result, water molecules move into the sieve-tube element by osmosis from the surrounding tissues.
  • This increases the hydrostatic pressure in the sieve tube at the source.
  • A source is any part of the plant that loads sucrose into the sieve tube, e.g. the leaf.
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The sink

  • A sink is anywhere that removes sucrose from the phloem sieve tubes.
  • The sucrose could be used for respiration and growth in a meristem, or it could be converted into starch for storage in a root.
  • Where sucrose is being used in cells, it can diffuse out of the sieve tube via the plasmodesmata.
  • It may also be removed by active transport.
  • The removal of sucrose from the sap makes the water potential less negative, so that water moves out of the sieve tube into the surrounding cells.
  • This reduces the hydrostatic pressure in the phloem at the sink.
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Along the phloem

  • Water entering the sieve tube at the source increases the hydrostatic pressure.
  • Water leaving the sieve tube at the sink reduces the hydrostatic pressure.
  • Therefore a pressure gradient is set up along the sieve tube, and the sap flows from higher pressure to lower pressure.
  • This could be in either direction, depending upon where sucrose is being produced and where it is needed.
  • It is even possible that sap could be flowing in opposite directions in different sieve tubes at the same time.
  • Since the sap in one tube is all moving in the same direction, this is mass flow.
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