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Transport in Plants

Transport in Plants

 

  • Need for a transport system

    • Same as in animals to an extent, as the size of the plant increases athe plant has a smaller surface area to volume ratio and as a result a transport system is needed as diffusion alone becomes inadequate to gain all the required nutrients required 

    • Furthermore as plants are not very active they do not require as much oxygen but require mineral ions, nutrients and water in every cell to assure that they can synthesise sugars and as these are largely collected from soil a method to get them to the leaves is needed 

  • Structures

    • Root:

      • Vascular bundle at centre in the shape of a X, the actual X is the xylem (THINK: Xit through the root Xylem) [obviously not correct but can aid remembering]

      • The phloem is the region found around this and 

      • THINK: Xylem LARGE, Phloem small 

      • The area surrounding the X is the medulla 

      • The overall circumference is the endodermis with a layer of meristematic tissue called the pericycle just within it 

    • Stem: 

      • Structure provides strength and support to bending or wind which they experience 

      • Ring structure in appearance (This is the structure with numerous ‘doritos’ around the outside doritos refer to the individual vascular bundles)

      • Central region around which the vascular bundles form is the medulla 

      • Vascular Bundle can be separated into 4 parts 

      • Remember the XYLEM IS FOUND NEAR THE INSIDE OF THE VASCULAR BUNDLE (they are the large circles) 

      • Remember the Phloem is the more middle region with small circles

      • Phloem and Xylem separated by the cambium 

      • The outermost region of the vascular bundle is the sclerenchyma which has the major role of providing support 

    • Leaf: 

      • There is a central midrib which is formed of the vascular bundles and some veins are also formed of the vascular bundles

      • The xylem is again the inner region the phleo the outer 

  • Can be shown using celery and cutting a thin section placing the section in a slide adding the stain of phloroglucinol and viewing under a microscope. The stain can also be placed in a beaker of water with the celery within it and as the transpiration stream occurs the xylem will be stained with a given colour 

  • Xylem: 

    • Carries water up a plant in a unidirectional transpiration stream has lignified cell walls and large lumens relative to the phloem but  arrow to prevent water breaking apart. The walls will have bordered pits which enable the lateral movement of water. There is no contents to the cell and the passage is continuous 

    • Associated parenchyma are often near the xylem which can store water and assimilates 

    • Also has fibres to support the plant 

  • Phloem: 

    • Role is to transport assimilates in a bidirectional transport method

    • Composed of individuals cells called sieve tube elements: these contain no nucleus and very little cytoplasm, there are often perforated cross walls at the end of each cell which is a sieve plate and allow the movement of sap from one cell to another  

    • Each associated with a companion cell: These have large nuclei and dense cytoplasm, they also have many mitochondria to produce the required ATP, they essentially carry out the metabolic processes required to load assimilated into the phloem 

    • There are parenchyma here as well 

  • Pathways of Water:

    • 3 core pathways 

      • Apoplast- Passes through the cell walls and between cells

      • Symplast - Water passes purely through the cytoplasm of the cells from one cell to the next

      • Vacuolar - Water passes through the vacuoles such that it goes periodically between the cytoplasm of cells and then through the vacuole etc.

    • Water will move from cell to cell via osmosis from high to low water potential and will be taken up through the soil in this fashion and also move out of cells though the same mechanism (ref previous notes on cell membranes) 

    • This process occurs when plants are in contact with each other 

  • Transpiration

    • It is the loss of water vapour from the leaves and aerial parts of the plant often via the stomata 

    •  Important to enable the transport of mineral ions ip the plant and maintain the turgidity as well as supply water for growth and photosynthesis and to keep the plant cool

    • Transpiration Stream (name of the mechanism) 

      • Outermost layer of cell - endodermis - in the root has root hair cells with a large surface area. Water and mineral ions diffuse via osmosis into the root hair cells. The movement of water into the root is an active process after the initial part to assure maximal water uptake. 

      • Water then moved down the root cortex to the endodermis down a water potential gradient by one of the pathways of water mentioned above 

      • If the apoplast pathway is taken the water will meet the casparian strip (impermeable region) in the endodermis which forces water to diffuse into the cell which in doing so filters the water and and collates it

      • Plasma membrane contains transporter proteins which pump mineral ions from the cytoplasm of the cortex into the medulla and xylem 

      • This makes the water potential of the medulla and xylem more negative so water moves from the cortex cell into the medulla and xylem by osmosis 

      • Once the water enters th medulal the Casparian strip prevents it from returning

      • As the endodermis moves mineral ions into the medulla and xylem via active transport more water is brought into the region which marks the pressure in the root medulla build up which forces water into the xylem 

      • Movement of water up the xylem occurs via mass flow  

      • There is also capillary action which occurs due to the adhesive nature of water and the fact that the xylem is narrow which enables the water to climb up the xylem to a degree and hld in place (based around hydrogen bonding and polarity of water etc, ref bio mols) 

      • There is a force of transpiration pull, as water molecules are attracted to each other through cohesive forces the molecules are held in a long chain so when water molecules diffuse out of the leaves the whole column of water is pulled and this creates a tension which is why the lignin is needed in the xylem 

        • This is called cohesion-tension theory

      • These 3 methods of movement contribute to mass flow which occurs as a result of the hydrostatic pressure gradient caused when water leaves the leaves at the top and cohesion tension theory meaning water moves from high hydrostatic pressure in the roots to low hydrostatic pressure in the leaves via mass flow down their pressure gradient 

      • Water evaporates from the surface of cells of the spongy mesophyll into air spaces and accumulates, as this occurs the water potential of the spongy mesophyll cells decreases drawing further water into the cell via osmosis causing the aforementioned forces ot act and water to move up the xylem which can then be used in cells for metabolic processes. 

      • As water accumulates in the airspaces a concentration gradient is formed and eventually the water vapour will diffuse out via the stoma

    • Numerous factors affect transpiration: 

      • Light intensity: With an increase the stoma open more enabling more gaseous exchange and more water vapour to diffuse out so transpiration increases

      • Temperature: As expected an increase in temp means more evaporation of water on the surface of the cells and an increase in diffusion as the conc grad is greater and the cells have more KE 

      • Humidity: As humidity increases the rate of water loss will decrease as the concentration gradient decreases which reduces the rate of transpiration 

      • Wind: An increase in the amount/intensity of the wind will cause water vapour that has diffused out of the stoma to be immediately moved away preventing the conc grad from reducing thus maintaining a high rate of water vapour diffusion and increasing the rate of transpiration 

      • Water availability: With low water levels in the soil the water canno t be replaces causing the leaves to wilt and the leaves to die which obviously decreases transpiration 

    • Experiment: Key points

      • Potometer 

      • Make sure to control all the other factors other than what is being tested 

      • Pi r ^2 times the length of the capillary tube for the volume of water 

      • Cut the shoot at an angle under water to prevent air bubbles 

      • Dry the leaves 

      • Make sure the shoot is healthy 

      • Fixed ruler 

      • Repeats 

  • Translocation: 

    • Occurs in the phloem and is the name given to the active loading of assimilates, which are substances made by the plant and those absorbed from the environment such as sugars and amino acids

    • The region where they are loaded in the Source and the region of the plant where they are unloaded is the sink 

    • Mechanism of Loading:

      • ATP from the companion cells is used to actively transport hydrogen ions out of the companion cell which leads to an H+ ion concentration gradient from outside the companion cell to inside (lower conc inside) 

      • The hydrogen ions thus diffuse back into the cell via cotransporter proteins which only activate to move the hydrogen ion across the cell if it is also accompanied by a sucrose molecules. Thus sucrose molecules have to be in the protein for H+ ions to diffuse 

      • This is called secondary active transport as the initial process is active in pumping H+ ions out of the cell but the latter part is passive as it occurs via a form of facilitated diffusion requiring the two molecules to be present 

      • This process increase concentration of sucrose in the companion cells, which then enables the diffusion of sucrose via the plasmodesmata between the companion cells and the sieve tube elements

    • Translocation: 

      • The movement of sucrose along the phloem occurs via mass flow whereby a solution of sucrose, amino acids and other assimilates forming a mixture called sap move via mass flow up or down a plant

      • It occurs due to a hydrostatic pressure gradient between the two ends of the phloem causing an overall pressure gradient. The sap then flows from high to low pressure up or down the phloem 

      • As the associates such as sucrose are loaded the water potential at the source is made more negative. As a result water then diffuses into the phloem from low to high water potential by osmosis which is what causes the hydrostatic pressure to build and the gradient to form 

      • A source is any point of loading of sucrose into the sieve tube elements this may be the leaves where glucose and sugars are made and converted to sucrose or it may be the roots where there are sucrose stores etc. 

      • A sink can be any point that removes sucrose from the phloem, this could be for growth in meristem or shoot. The movement of water out at the sink lowers hydrostatic pressure which maintains the gradient 

      • ESSENTIALLY: water entering at the source increases the hydrostatic pressure and water leaving at the sink reduced the pressure which causes a hydrostatic pressure gradient enabling sap to move from high to low pressure down the pressure gradient via mass flow  

 

Adaptation of Plants:

  • 2 types hydrophytes and xerophytes: hydrophytes are adapted to living in water while xerophytes are adapted to living in dry conditions (Think Hydro - Hydration - Water conditions) 

  • Xerophytes:

    • Must be adapted to reduce the water loss and replace nay water that is lost 

    • Examples are marram grass (M) and cacti (C)

    • Leaves may be rolled so air is trapped inside which increases humidity and decreases the diffusion distance preventing transpiration at a very high rate and thus reducing water loss (M)

    • There is a thick waxy cuticle on the outer side of the leaf to reduce evaporation (M)

    • Stoma on the inner side of the rolled leav sunken within pits in the lower epidermis and covered by hairs which act to increase the humidity and prevent water vapour which has diffused out leaving (M)

    • Fewer air spaces within the leaf and fewer stomata to reduce overall loss of water (M)

    • Can store water in stems so water is available if needed (C) 

    • Leaves can be very thin and even reduced to spines (C) 

    • Stem may be green and able to photosynthesise itself (C) 

    • Widespread roots to assure any rainfall is captured (C) 

    • Stoma may be closed when water availability is low to reduce water loss 

    • Plants may have a low water potential in the leaf cells having a high salt concentration in the cells which reduces evaporation of water from the cell surfaces

    • Long tap root to reach water underground 

  • Hydrophytes: 

    • Live in water so have to be able to get oxygen and remove water 

    • Example is a water lily which can:

      • Numerous large air spaces in the leaf to keep the leaves afloat so they are in the air and can absorb sunlight 

      • Stoma on the upper epidermis so exposed to the air to allow gaseous exchange

      • Many large airspaces in the leaf to aid buoyancy 

      • Many hydrophytes contain specialised hydathodes which can release water droplets which can evaporate from the leaf surface 

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