MRS COLVILLE MOD 3

• Plants require a transport system to ensure that all the cells of a plant receive a sufficient amount of nutrients. This is achieved through the combined action of xylem tissue which enables water as well as dissolved minerals to travel up the plant in the passive process of transpiration, and phloem tissue which enables sugars to reach all parts of the plant in the active process of translocation.
• The vascular bundle in the roots:

  • Xylem and phloem are components of the vascular bundle, which serves to enable transport of substances as well as for structural support.
  • The xylem vessels are arranged in an X shape in the centre of the vascular bundle.
  • This enables the plant to withstand various mechanical forces such as pulling.
  • The X shape arrangement of xylem vessels is surrounded by endodermis, which is an outer layer of cells which supply xylem vessels with water.
  • An inner layer of meristem cells known as the pericycle

  
• The vascular bundle in the stem:

  • Xylem is located on the inside in non-wooded plants to provide support and flexibility to the stem
  • Phloem is found on the outside of the vascular bundle
  • There is a layer of cambium in between xylem and phloem, that is meristem cells which are involved in production of new xylem and phloem tissue

  
• The vascular bundle in the leaf:

  • The vascular bundles form the midrib and veins of a leaf
  • Dicotyledonous leaves have a network of veins, starting at the midrib and spreading outwards which are involved in transport and support

  
• Xylem vessels have the following features:

  • They transport water and minerals, and also serve to provide structural support
  • They are long cylinders made of dead tissue with open ends, therefore they can form a continuous column.
  • Xylem vessels also contain pits which enable water to move sideways between the vessels.
  • They are thickened with a tough substance called lignin, which is deposited in spiral patterns to enable the plant to remain flexible
  • Water can only flow upwards
• Features of phloem:

  • tubes made of live cells
  • Translocation - movement of nutrients to storage organs and growing parts of the plant
  • Consist of sieve tube elements and companion cells
  • Sieve tube elements form a tube to transport sugars in the dissolved form of sap. This transport can be up or down
  • Companion cells are involved in ATP production for active processes. Eg: loading sucrose into sieve tubes
  • Cytoplasm of sieve tube elements and companion cells is linked through plasmodesmata - gaps between cell walls that allow communication and flow of substances like minerals between cells
• Transpiration is the process where plants absorb water through the roots, which then moves up through the plant and is released into the atmosphere as water vapour through pores in the leaves. Carbon dioxide enters, while water and oxygen exit through a leaf’s stomata.
• The transpiration stream, which is the movement of water up the stem, enables processes such as photosynthesis, growth and elongation as it supplies the plant with water which is necessary for all these processes. Apart from this, the transpiration stream supplies the plant with the required minerals, whilst enabling it to control its temperature via evaporation of water
• Transpiration involves osmosis, where water moves from the xylem to the mesophyll cells. Transpiration also involves evaporation from the surface of mesophyll cells into intercellular spaces and diffusion of water vapour down a water vapour potential gradient out of the stomata.
• Xerophytes are plants adapted to dry conditions. They can survive because of various adaptations which serve to minimise the water loss. The adaptations include smaller leaves which reduce the surface area for water loss. Both densely packed mesophyll and thick waxy cuticles prevent water loss via evaporation. Xerophytes respond to low water availability by closing the stomata to prevent water loss. They contain hairs and pits which trap moist air, reducing the water vapour potential. Xerophytes also roll the leaves to reduce the exposure of lower epidermis to the atmosphere, trapping air.
• Hydrophytes are plants that actually live in water such as water lillies. As such they need their own adaptations for this environment. They have a very thin or absent waxy cuticle as they don’t need to conserve water. Many constantly open stomata are found on the upper surfaces of leaves to maximise gas exchange. Wide, flat leaves give a large surface area for light absorbtion. Air sacs are found in some hydrophytes to enable leaves to stay afloat. Many large air spaces to make leaves and stems more buoyant.
• Movement of water in the root

  • Water enters through root hair cells and moves into the xylem tissue located in the centre of the root.
  • This movement occurs as a result of a water potential gradient, as the water potential is higher inside the soil than inside the root hair cells, due to the dissolved substances in the cell sap.
  • Therefore, the purpose of root hair cells it to provide a large surface area for the movement of water to occur.
  • Minerals are also absorbed through the root hair cells by active transport, as they need to be pumped against the concentration gradient.
• 2 ways water can move across the root to xylem:

  • symplast pathway, water enters cytoplasm through plasma membrane & passes from one cell to the next by plasmodesmata, channels that connect cytoplasm of cells
  • apoplast pathway, water moves through water filled spaces between cellulose molecules in cell walls. Water doesn’t pass through membrane so can carry dissolved mineral ions and salts. At the endodermis, it encounters a layer of suberin, Casparian strip, can't be penetrated by water
  • To cross the endodermis, water that has been moving through the cell walls must enter the symplast pathway
• Once it has moved across the endodermis, the water continues down the water potential gradient from cell to cell until it reaches a pit in the xylem vessel which is the entry point of water.
• Water moving in the xylem up the stem

  • The water is removed from the top of the xylem vessels into the mesophyll cells down the water potential gradient.
  • The push of water upwards is aided by the root pressure which is where the action of the endodermis moving minerals into the xylem by active transport, drives water into the xylem by osmosis, thus pushing it upwards
• Water moving in the xylem up the stem

  • The flow of water is also maintained with the help of surface tension of water and the attractive forces between water molecules known as cohesion.
  • The action of these two forces in combination is known as the tension-cohesion theory, which is further supported by capillary action where the forces involved in cohesion cause the water molecule to adhere to the walls of xylem, thus pulling water up.
• Light:

  • Required for photosynthesis 
  • In the light stomata open for gas exchange
  • In the dark, most stomata will close
  • Increase light intensity, increase open stomata, increase rate of water vapour diffusing out and increasing evaporation from the surfaces of leaves
  • Increasing light intensity increases the rate of transpiration
• Humidity

  • Measure of the amount of water vapour in the air compared to total concentration of water the air can hold
  • High relative humidity will lower the rate of transpiration because of the reduced water vapour potential gradient between inside and outside the leaf
  • Very dry air increases the rate of transpiration
• Temperature

  • Increase in temperature increases kinetic energy of the water molecules and increases rate of evaporation from the spongy mesophyll cells into the air spaces of the leaf 
  • Increase in temperature increases the concentration of water vapour that the external air can hold before it becomes saturated (so decreases its relative humidity and its water potential)
• Air movement

  • Each leaf has a layer of still air around it trapped by the shape of the leaf and features such as hairs on the surface of the leaf decreased air movement close to the leaf
  • Water vapour that diffuses out of the leaf accumulates here and so the water vapour potential around the stomata increases, reducing the diffusion gradient
  • Anything that increases the diffusion gradient will increase the rate of transpiration
  • So air movement or wind increases rate of transpiration and conversely a long period of still air will reduce transpiration
• Soil-water availability 

  • The amount of water available in the soil can affect transpiration rate 
  • If it is very dry the plant will be under water stress and the rate of transpiration will be reduced
• Stomata

  • Main way rate of transpiration controlled is by the opening & closing of stomatal pores
  • Turgor-driven process
  • turgor low - asymmetric configuration of guard cell walls close the pore
  • environmental conditions optimum - guard cells pump in solutes actively, increasing turgor
  • Cellulose hoops stop cells swelling in width, so they extend lengthways
  • Inner wall of guard cells are less flexible than outer walls, cells become bean-shaped and pore opens
  • When water becomes scarce, hormonal signals from roots can trigger turgor loss from guard cells, close the stomatal pore, conserve water
• potometer

  A) rubber

  B) vaseline

  C) capillary tube

  D) air bubble

  E) scale

  F) water

  G) water

  H) air bubble

  I) leafy shoot

  
• Potometer

  • the bubble potometer is an instrument used to indirectly determine the rate of water loss from plants (ie. transpiration)
  • this instrument actually measures the rate of water uptake. The water uptake only approximates the rate of transpiration as some of the absorbed water is retained by the plant or utilised in processes such as photosynthesis
• Potometer

  • A number of precautions are necessary when setting up the potometer for experimentation
  • cutting the leafy shoot underwater
  • minimise the risk of air entering the xylem
  • submerging the apparatus in water to fill it
  • applying vaseline to all joints to make it air-tight
• measuring transpiration Rates by change in mass

  • nalternative method for determining transpiration rates
  • directly recording the change of mass of a leafy shoot over a period of time
  • if there is a decrease in mass, the plant takes up water from the system and water evaporates out of stomata
• Translocation

  • the transport of organic compounds by plants from sources to sinks
• Source:

  • place where things are made
  • high sucrose concentration
  • leaves and stems
  • storage organs
  • seed stores when germinating
• Sink:

  • place where things are used / stored
  • low sucrose concentration
  • growing shoot and root tips (meristems)
  • any area involved in laying down food stores
• the products of photosynthesis are transported around plants in the phloem from sources (where they are produced) to sinks (where they are used or stored)
• the main substance transported in translocation is sucrose

  • glucose (produced in photosynthesis) to sucrose (transported around the plant in phloem)
• movement of sucrose via phloem can be up or down the plant stem
• mass flow theory - explains how movement of sucrose occurs in phloem
• Mass Flow in Phloem

  1. sucrose is actively loaded (active transport - ATP) into the sieve tube element of the phloem and reduces water potential
  2. water follows via osmosis and increases the hydrostatic pressure in the sieve tube element
  3. water moves down the sieve tube from high hydrostatic pressure at source to lower hydrostatic pressure at sink
  4. sucrose is removed from the sieve tube by the surrounding cells and increases the water potential in the sieve tube
  5. water moves out of the sieve tube and reduces the hydrostatic pressure
• Loading at the source

  • most sucrose is actively loaded into the phloem and some moves passively through the cytoplasm and plasmodesmata (symplast route)
• Sucrose into companion cells

  1. Hydrogen ions are actively pumped out of companion cells using ATP (active transport)
  2. Creates a concentration gradient (higher concentration of H+ outside the cell)
  3. Hydrogen ions move back in companion cells via a co-transporter protein located in companion cell membrane. H+ move back into the cell with sucrose molecules (facilitated diffusion)
  4. sucrose concentration in the companion cells increases, creating a concentration gradient
  5. sucrose molecules diffuse into sieve element of the phloem passively, down the concentration gradient (simple diffusion)
• unloading sucrose from the phloem at the sink is passive (relies on diffusion)
• Describe the structures and functions of sieve tubes and companion cells.

  Sieve tubes are elongated cells that line up to form tubes with thin cytoplasm and perforated cross walls / sieve plates. They form the phloem and allow the flow of sap. Companion cells are cytoplasmically

  linked to sieve tube cells and contain many mitochondria to produce ATP.
• Translocation is an energy requiring process which serves as a means of transporting assimilates such as sucrose in the phloem between sources which release sucrose such as leaves and sinks e.g. roots and meristem which remove sucrose from the phloem.
• Sucrose enters the phloem in active loading where companion cells use ATP to transport hydrogen ions into the surrounding tissue, creating a diffusion gradient, which causes the H+ ions to diffuse back into the companion cells. It's a form of facilitated diffusion involving cotransporter proteins, allows the returning H+ ions to bring sucrose molecules into companion cells, causing sucrose concentration in companion cells to increase. As a result, sucrose diffuses out of companion cells down the concentration gradient into the sieve tube elements through links known as plasmodesmata.