Plant Transport

• Need for plant transport systems


  - Metabolic demands (Cells of the green parts of the plant make their own glucose + oxygen by photosynthesis, but this must be transported to the rest of the plant as well, as must nutrients absorbed by the roots.)

  - Size (While some plants are small, many become very large, such as trees like the Giant Redwood, and so these plants need a transport system to move substances up and down the plant

  - Surface Area : Volume (SA:V ratios vary throughout the plant, being larger in leaves and smaller in trunks, but have a relatively small SA:V on average, meaning they cannot rely on diffusion alone)
• Dicotyledonous plants (dicots)


  - Make seeds that contain two cotyledons, organs that act as food stores for the embryo plant and form the first leaves when the seed germinates

  - Herbaceous dicots have soft tissues and a short lifespan

  - Arborescent (woody) dicots have tough, lignified tissues and live far longer
• Vascular system

  - Collection of specialized tissues in dicotyledonous plants that transports substances throughout the plant.

  - In herbaceous dicots, this is made up mainly of phloems and xylems

  - These tissues are arranged together in vascular bundles
• Xylem

  - Largely non-living (made up of dead cells)

  - Two main functions: transport of water and mineral ions + support

  - One-way

  - Long, hollow structures made by several columns of cells fusing together end to end

  - Xylem fibres are long cells with lignified secondary walls that provide extra strength, but do not transport water

  (- Lignin can form spirals or relatively solid tubes with lots of unlignified bordered pits)
• Phloem


  - Living (made up of live cells)

  - Two-way

  - Transports sugars, water and amino acids

  - Contains sieve tube elements and companion cells
• Epidermis

  An outer layer of cells designed to provide protection
• Exodermis


  The outer layer, one or more cells in depth, of the cortex in some roots; characterized by Casparian strips within the radial and transverse cell walls, following development of Casparian strips, a suberin lamella is deposited on all walls of the exodermis.
• Endodermis


  The innermost layer of the cortex in plant roots; a cylinder one cell thick that forms the boundary between the cortex and the vascular cylinder.
• Root hair

  Small hairs on a root that produce a large surface area through which water and minerals can enter
• Parenchyma

  Packing and supporting tissue
• Palisade mesophyll


  Layer of tall, column-shaped mesophyll cells just under the upper epidermis of a leaf and the main photosynthetic tissue
• Midrib of the leaf


  Large, central vein in the leaf carrying the vascular bundle and providing support
• Xylem Parenchyma packs

  Around the xylem vessels, parenchyma packs store food and contain tannin
• Tannin


  A bitter chemical plants secrete to deter predators
• Sieve tube elements

  - Found in the phloem

  - Stacked end to end to form a long, hollow structure but are not lignified

  - In the areas between cells, the walls become perforated to form sieve plates which allow the phloem contents to flow through

  - As large pores appear in these cell walls, the tonoplast _ nucleus as well as some other organelles break down, resulting in the mature phloem cells further up having no nucleus
• Tonoplast


  membrane enclosing the central vacuole
• Companion cells

  - Joined to the sieve tube elements by many plasmodesmata adjacent to them

  - They maintain their nucleus

  - Very active and act as life support systems for the nucleus-less sieve tube cells
• Role of water transport in plants

  - Hydrostatic pressure (or turgor pressure) as a result of plant cell osmosis provides a hydrostatic skeleton to support the stems and leaves

  - Turgor also drives cell expansion and allows plants to force their way through tarmac and concrete

  - Loss of water by evaporation helps to keep plants cool

  - Mineral ions and photosynthesis products are transported in aqueous solutions

  - Water is used in photosynthesis
• Movement of water into the root

  - Soil water has a very low concentration of minerals/high water potential whereas the root hair cells have a high concentration of minerals/low water potential, creating a water potential gradient. As a result, water flows into the root hair cell
• Adaptations of root hair cells

  - They are microscopic, allowing them to penetrate between soil particles

  - They have a high SA:V with thousands growing on each root tip

  - Each hair has a thin surface layer and so a short diffusion/osmosis pathway

  - Concentration of solutes in root hair cytoplasms maintains a water potential gradient between soil water and cell
• Symplast Pathway

  - Pathway in the root

  - Water moves through the symplast (continuous cytoplasm connected by plasmodesmata)

  - The root hair cell has a higher water potential than the next cell due to water flowing in by osmosis, causing water to move to the next door cell by further osmosis, lowering the water potential in the root hair cell and allowing for more osmosis from the soil
• Apoplast pathway

  - Movement of water through cell walls and intercellular spaces.

  - The water fills spaces between the loose network of fibres and as water moves into the xylem, more water is pulled through the apoplast by cohesion, causing tension which produces a continuous flow of water through the resistance-less cellulose wall structure
• Movement of water into the xylem

  - Water reaches the endodermis and cannot pass through the Casparian strip, forcing it to join the apoplast pathway in the cytoplasm and pass through cell surface membranes, causing potentially toxic molecules in the water to be filtered out

  - Water enters the vascular bundle by the symplast pathway before being returned to the apoplast pathway once inside the Xylem

  - Root pressure pushes water up the xylem, but is not the major factor in the movement of water up from root to leaf.
• Casparian strip


  A water-impermeable ring of waxy Suberin in the endodermal cells of plants that blocks the passive flow of water and solutes into the vascular bundle by way of cell walls.
• Root transport

  The active transport of minerals into the xylem to produce osmosis
• Evidence for active transport in root pressure


  - Some poisons, e.g. cyanide, affect mitochondria and there is no production of ATP. If cyanide is applied to root cells, preventing energy supply, root pressure disappears

  - Root pressure increases and decreases with temperature, suggesting chemical reactions are involved

  - If oxygen/respiratory substrates fall, root pressure falls

  - Xylem sap may exude from the cut end of stems as certain times. In the natural world, xylem is forced out of special pores at the ends of leaves in some conditions - e.g. when transpiration is low overnight. This is called guttation
• Stomata


  Small openings on the underside of a leaf through which oxygen and carbon dioxide can move
• Guard cells


  cells that control the opening and closing of stomata
• Transpiration


  Loss of water vapour through gas exchange by the stomata
• Transpiration stream

  Soil -> root hair cells (-> apoplast pathway) -> symplast pathway -> apoplast pathway -> symplast pathway -> Xylem -> apoplast pathway -> cells of the leaf -> walls of mesophyll cells -> stomata along a diffusion gradient -> external air
• Adaptations of leaves

  - Large surface area for capturing light

  - Covered in a waxy cuticle to make them waterproof and prevent rapid water loss by evaporation
• Cohesion-tension theory

  - Water molecules evaporate from mesophyll cell surface into the air spaces in the leaf and diffuse out of the stomata down a concentration gradient

  - The loss of water by evaporation lowers the water potential of the mesophyll cell, causing water to move into the cell by osmosis from an adjacent cell by apoplast and symplast pathways

  - This is repeated across the leaf to the xylem, where water moves out of the xylem to the leaf

  - Water molecules form hydrogen bonds with the carbohydrates in the walls of the xylem vessels by adhesion while they also form hydrogen bonds with each other by cohesion. The combined effects of cohesion and adhesion result in water exhibiting capillary action. Water is drawn up the xylem in a continuous stream to replace water lost by evaporation in a process called the transpiration pull

  - The transpiration pull results in tension in the xylem, which helps move water across the roots from the soil
• Capillary action


  The process by which water rises up a narrow tube against gravity
• Evidence for cohesion-tension theory

  - Change in diameter of trees (When transpiration is high during the day, tension is also high and the tree shrinks in diameter. When transpiration is low at night, tension is also low and the tree grows in diameter)

  - Broken xylems suck in air (When you cut a flower stem, water is drawn into the xylem rather than water leaking out)

  - If air is pulled in, water can no longer be pulled up the stem (This is because the continuous stream of water molecules held together by cohesive forces has been broken by air)
• Measuring transpiration practical

  - It's difficult to make direct transpiration measurements due to difficulties collecting all of the evaporated water from the plant without gathering that from the soil

  - Instead, we measure the uptake of water by a plant as about 99% is lost by transpiration anyway

  - The rate of water uptake can be measured with a carefully set up potometer with waterproof jelly on the joints to prevent any water loss

  - Rate of water uptake = distance moved by air bubble/time taken (cm s-1)

  - The plant is placed in three different conditions: normal, bright light and Vaseline
• Controlling the rate of transpiration

  - Controlled by stomatal pores

  - Stomata are opened and closed through turgor with guard cells pumping in solutes by active transport to make the stomata swell to open and turgor loss when the stomata must close
• Guard cell structure

  - Diameter of 15-20 micrometres

  - Chloroplast, cytoplasm, vacuole, nucleus

  - An outer cell wall on the outside of the stomata and an inner cell wall around the stomatal aperture
• Factors affecting transpiration

  - Light intensity (High light intensity increases Rate of Transpiration)

  - Relative humidity (High relative humidity decreases Rate of Transpiration)

  - Temperature (High temperature increases Rate of Transpiration)

  - Air movement (High air movement increases Rate of Transpiration)

  - Soil-water availability (High soil-water availability increases Rate of Transpiration)
• Translocation


  Movement of sugars from sources to sinks
• Source


  Areas of high sugar concentration from which sugars are transported, create sugar