3.3 Transport in plants

Transport in plants

  • Plants need transport system to move water and minerals from roots up to the leaves and sugars from the leaves to the rest of the plant.
  • Water and soluble mineral ions travel upwards in the xylem tissue.
  • Assimilates such as sugar travel up or down the phloem tube.
  • In dicotyledonous plants (two seed leaves), the xylem and phloem are found in vascular bundle along with tissues such as collenchyma and sclerenchyma.
  • In the young root, the vascular bundle is found in the centre with a central core of xylem in an X shape.The phloem is found inbetween the arms of X shape to provide strength. It is surrounded by the endodermis and a layer of meristem called the pericycle.
  • In the stem, the vascular bundles are found near the outer edge. In non woody plants the bundles are seperate but in woody plants as the stem ages it becomes continous providing strength and flexibility. Xylem is found towards the inside and phloem towards outside with inbetween a layer of cambium.
  • In the leaf, the vascular bundles form the midrib and veins of the leaf. A dicotyledonous has a branching network of veins that get smaller as they spread from the midrib. These veins have a xylem located on top of the phloem.
  • The dissection of plant material to examine vascular tissue requires staining of tissue.
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Transport tissues

  • Xylem tissue consists of vessels to carry the water and dissolved mineral ions, fibers to support plant and living parenchyma cells to seperate and support vessels.
  • As the xylem vessels develop they are impregnated by ligin which makes it waterproof and kills the cells. Lignin thickening forms patterns in the cell wall which can be spiral, annular (rings) or reticulate (broken rings) preventing the vessel from being too rigid and allows flexibility.
  • In some places lignification is not complete leaving bordered pits for water to pass through vessels. These tubes are narrow so capillary action can be effective.
  • Phloem tissue is used to transport assimilates (mainly sucrose and amino acids) around the plant. The sucrose is dissolved in water to form sap.
  • The tissue is thin and made up of sieve tube and companion cells. Elongated sieve tubes elements are lined up end to end to form sieve tubes. At the ends there are perforated sieve lates to allow movement of sap.
  • Inbetween sieve tubes, there are companion cells which have a large nucleus and dense cytoplasm. They have numerous mitochondria to produce to produce ATP for active processes such as the metabolic process needed to load assimilates into the sieve tubes.
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Movement of water through plants

  • Cellulose cell walls of a plant cell are fully permable to water meaning that it can move freely into cell cytoplasm or vacuole.
  • Many plant cells are joined by special cytoplasmic bridges called plasmodesmata meaning there are 3 pathways taken by water through a plant.
  • The apoplast pathway is where water passes through the spaces in the cell walls not any plasma membrane meaning water moves by mass flow rather than osmosis. Dissolved mineral ions and salts can be carried with the water.
  • The symplast pathway is where water enters the cytoplasm through the plasma membrane. It can then pass through the plasmodesmata from one cell to the next.
  • The vacuolar pathway is similar to the symplast pathway but the water is not confined to the cytoplasm of the cell, it is able to pass through the vacuole.
  • Water potential  (ψ) is the measure of  the tendency of water molecules to move from one place to another. Water always move from a region of high water potential to a region of lower water potential. The water potential of pure water is 0 kPa (high) and plants cells are always negative (low).
  • If you place a plant cell in pure water, it will take up water by osmosis down water potential gradient making the cell turgid.
  • If you place plant cell in salt solution, it will lose water by osmosis down water potential gradient making the cell go through plasmolysis.
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Transpiration

  • Transpiration is the loss of water vapour from the upper parts of the plant, usually the leaves.
  • Most water vapour leaves through the stomata which is open to allow gaseous exchange for photosynthesis.
  • Water enters the leaf through xylem and moves via osmosis to spongy mesophyll by the apoplast pathway. The water evaporates  from cell walls of the spongy mesophyll. Water vapour moves out by diffusion out of leaf via the stomata due to the water vapour potential gradient.
  • Transpiration transports useful mineral ions, maintains cell turgidity, supplies water for growth, cell elongation and photosynthesis and keeps plant cool on hot days.
  • The rate of transpiration can be affected by the environmental factors: light intensity, temperature, relative humidity, air movement and water availibility.
  • In light, the stomata opens to allow gaseous exchange for photosynthesis so higher light intensity increases transpiration rate.
  • A high temperature will increase the rate of evaporation, increase rate of diffusion and decrease the relative water potential.
  • Higher relative humidity will decrease the rate of water loss meaning there will be a smaller water vapour potential gradient.
  • Air moving outside leaf will carry water vapour meaning a high water vapour potential gradient is maintained.
  • If there is little water in soil, the stomata close and the leaves wilt.  
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The transpiration stream

  • The transpiration stream is the movement of water from the soil, through the plant, to the air surrounding the leaves. The main driving force is the water potential gradient.
  • The epidermis of a root contains root hair cells that increase surface area of the root. They absorb mineral ions and water from the soil. The water then moves across root cortex down water potential gradient to the endodermis of the vascular bundle. Water may also travel by apoplast pathway to endodermis but must then enter symplast pathway as apoplast is blocked by casparian strips.
  •  The endodermis is a layer of cells surrounding the medulla and xylem known as the starch sheath. The casparian strips blocks the apoplast pathway between cortex and medulla to ensure water and ions have to pass into cytoplasm through plasma membrane.
  • The plasma membrane contains transporter proteins to actively pump ions from cytoplasm of the cortex cells into the medulla and xylem. This makes the water potential of the medulla and xylem more negative so water moves from cortex cells by osmosis. 
  • Movement of water up through the xylem is by mass flow.
  • Root pressure in medulla builds up and forces water into the xylem, pushing it up.
  • Water molecules are attracted by forces of cohesion which hold molecules in long chain with the pull from above creating tension in column of water.
  • Adhesion is where water molecules are attracted to sides of xylem to pull water up vessel.
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The adaptations of plants to availability of water

  • Most terrestrial plants reduce water loss by having a waxy cuticle on the leaf to reduce evaporation through the epidermis, stomata are found on under-surface of leaves to stop evaporation from direct sunlight, most stomata are closed at night and decicuous plants lose their leaves in winter to retain water in cold temperatures, have a low water potential inside leaves and very long tap root.
  • Marram grass is a xerophyte and so is adapted to living in arid conditions. The adapatations include: the leaf is rolled longitudinally so air is trapped which becomes humid to reduce water loss, thick wacy cuticle on outside of rolled leaf to reduce evaporation, stomata are on the inner side of rolled leaf so are protected by enclosed air space and  inside pits in lower epidermis which is folded and covered in hairs and the spongy mesophyll is very dense with few air spaces.
  • Cactis are also xerophytes, they are succulents so they store water in their stems and can expand when water is available, the leaves are reduced to spines to reduce surface area of leaves, the stem is green for photosynthesis and the roots are very widespread.
  • Hydrophytes are plants that live in water, an example is the water lily who is adapted by having many large air spaces in leaf to keep leaves afloat to absorb sunlight, the stomata are on the upper epidermis so they are exposed to the air to allow gaseous exchange and the leaf stem also has many large air spaces to diffuse quickly to roots for aerobic respiration.
  • Many hydrophytes contain hydathodes to release water droplets which evaporate from the leaf surface.
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Translocation

  • Transloacation occurs in the phloem and is the movement of assimilates throughout the plant.
  • Assimilates are substances made by the plant such as sugars and amino acids.
  • The source loads assimilates into the phloem sieve tubes and the sink removes assimilates from the phloem sieve tubes.
  • Active loading involves use of ATP in the companion cells, the energy is used to actively transport hydrogen ions (H+)  out of the companion cells. This creates a concentration gradient.
  • Cotransport is where these hydrogen ions diffuse back into the companion cells through cotransporter proteins. It is a secondary active transport as it actively transports H+ ions and moves sucrose against the concentration gradient. As the concentration of sucrose in the companion cells increases, it can diffuse through the plasmodesmata into the sieve tubes.
  • The movement of sucrose along the phloem is by mass flow. A solution of sucrose, amino acids and other assimilates flow together and this is called sap.
  • The flow is caused by a difference in hydrostatic pressure between the two ends of the tube which produces a pressure gradient. Water enters the tube at the source by osmosis, increasing the pressure, and leaves the tube at the sink, reducing the pressure.
  • Sap flows from higher pressure to lower pressure.
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Key words & definitions

  • Dicotyledonous plants: plants with two seed leaves&a branching pattern of veins in leaf
  • Meristem: a layer of dividing cells, can be called pericycle here
  • Phloem: transports dissolved assimilates
  • Vascular tissue: consists of cells specialised for transporting fluids by mass flow
  • Xylem: transports water and minerals
  • Companion cells: the cells that help to load sucrose into sieve tubes
  • Sieve tube elements: make the tubes in phloem tissue that carry sap up&down the plant. Seperated by sieve plates
  • Plasmodesmata: gaps in the cell wall containing cytoplasm that connects two cells
  • Potometer: a device that can measure the rate of water upatke as leafy stem transpires
  • Transpiration: the loss of water vapour from the aerial parts of a plant, mostly through stomata
  • Adhesion: the attraction between water molecules and walls of xylem vessel
  • Cohesion: the attraction between water molecules caused by hydrogen bonds
  • Hydrophyte: a plant adapted to living in water or where the ground is very wet
  • Xerophyte: a plant adapted to living in dry conditions
  • Assimilates: substances that have become a part of plant
  • Sink: a part of plant where those materials are removed from transport system
  • Source: a part of plant that loads materials into transport system
  • Translocation: the transport of assimilates throughout a plant
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