Plants

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  • Created by: Roisind65
  • Created on: 13-02-15 09:16
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  • Plants
    • Gas exchange in the leaf of a plant
      • When photosynthesis is taking place, although some carbon dioxide comes from respiration of cells, most of it has to be obtained from the external air. In the same way, some oxygen from photosynthesis  is used in respiration but most of it diffuses out of the plant.
      • When photosynthesis is not occurring oxygen diffuses into the leaf because it is constantly being used in cells during respiration. In the same way, carbon dioxide produced during respiration diffuses out.
      • Structure of a plant and gas exchange.
        • No living cell is far from the external air, and therefore a source of oxygen and carbon dioxide
        • diffusion takes place in the gas phase, which makes it more rapid than if it were in water.
        • Thin, flat shape that provides a large surface area.
        • Many small pores, Stomata, mostly in the lower epidermis
        • numerous interconnecting air-spaces that occur throughout the mesophyll.
      • Stomata
        • Minute pores which occur mainly on the leaves , especially the underside.
        • Each stomata is surrounded by a pair of guard cells.
          • These cells can open and close the stomatal pore.
            • In this way can control the rate of gaseous exchange.
              • This is important terrestrial organisms lose water by evaporation
      • Plants have to balance the conflicting needs of gas exchange and control of water loss. They do this completely or partly closing stomata at times when water loss would be excessive.
    • Movement of water through roots.
      • Uptake by root hairs
        • Roots hair are the exchange surfaces in plants that are responsible for the absorption of water and mineral ions.
        • Plants constantly lose water by the process of transpiration
        • They provide a large surface area as they are very long extensions and occur in thousands on each of the branches of a root
        • They have thin surface layer across which materials can move easily through
        • In damp conditions they are surrounded by soil solution which contains small quantities of mineral ions, however is contains mostly water and therefore has a very high water potential.
          • In contrast the root hairs have sugars, amino acids and mineral ions dissolved inside them. These cells therefore have a much lower water potential. As a result water moves by osmosis from the soil solution into the root-hair cells down this water potential gradient.
        • Apoplastic pathway: watre is drown into the endodermal cell; it pulls more water along behind it, due to the cohesive properies of the water molecules. This creates a tension that draws water along the cell walls of the cells of the root cortex.
          • The  mesh-like  structure of the cell walls of these cells has many water-filled spaces and so there is little or no resistance to this pull of water along the cell walls.
        • Symplastic Plathway
          • water entering by osmosis increases the water potential of the root-hair cell
            • the root-hair cell now has a high water water potential than the first cell
              • water therefore moves from the root-hair cell to the first cell in the cortex by osmosis, down the water potential gradient
                • The first cell now has a high water potential than it's neighbour to the inside of the stem.
                  • water therefore moves into this neighbouring cell by osmosis along the water potiential gradient
                    • This second cell now has a higher water potential than it's neighbour to the inside, and so water moves from the second cell to the third cell by osmosis a long the water potential gradient.
                      • at the same time, this loss of  water from the first cortical cell lowers ot's water potiental, causing more to enter it by osmosis from the root-hair cell
                        • In this way, a water potential gradient is set up across all the cortex, which carries water along the cytoplasm from the root-hair cell to the endodermis.
        • Caspasian Strip
          • a distinctive band of suberin around the endodermal cells of a plant root that prevents water passing through into the xylem via the cell walls. The water is forced through the living part of the endodermal cells
        • Movement of water into the xylem
          • the plant has control over the ions that enter its xylem vessels, since water must cross a plasma membrane to get there.The vacoular pathway moves molecules through the vacuoles only of the plant.
          • Xylem
            • The xylem is constructed of three main elements;Vessel elements, including tracheids - cells involved in water transportFibres - elongated cells with lignified walls that support the plantParenchyma cells - normal plant cells, except no chloroplasts.
    • Movement of water up stems
      • Movement of water across cells of a leaf
        • Water diffuses from the xylem vessels in the veins through the adjacent cells down its water potential gradient. As in the roots, it uses the symplast pathway through the living cytoplasm and the apoplast pathway through the non-living cell walls. Water evaporates from the spongy cells into the sub-stomatal air space, and diffuses out through the stomata.
      • Cohesion theory
        • This is the theory that water molecules are attracted to each other, making it easy for the water to move together, this also explains why water molecules are able to move up the xylem.
      • Root Pressure
        • The xylem vessels form continuous pipes from the roots to the leaves. Since the xylem vessels are dead, open tubes, no osmosis can occur within them. The driving force for the movement is transpiration in the leaves. This causes low pressure in the leaves, so water is sucked up the stem to replace the lost water. The column of water in the xylem vessels is therefore under tension (a stretching force). Fortunately water has a tendency of water molecules to stick together by hydrogen bonding (cohesion).
    • Transpiration and factors affecting it.
      • Light
        • Light stimulates the stomata to open allowing gas exchange for photosynthesis, and as a side effect this also increases transpiration. This is a problem for some plants as they may lose water during the day and wilt.
      • Temperature
        • High temperature increases the rate of evaporation of water from the spongy cells, and reduces air humidity, so transpiration increases.
      • Humidity
        • High humidity means a higher water potential in the air, so a lower water potential gradient between the leaf and the air, so less evaporation.
      • Air movement
        • Wind blows away saturated air from around stomata, replacing it with drier air, so increasing the water potential gradient and increasing transpiration
    • Xerophytic Plants
      • a thick cuticle
        • stops uncontrolled evaporation through leaf cells
      • rolling up the leaves
      • hairy leaves
        • maintains humid air around stomata
      • stomata in pits and grooves
        • maintains humid air around stomata
      • a reduced surface area to volume ratio of the leaves
        • less area for evaporation

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