Structure and Transport in Plants

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Plant Cell Wall

  • The plant cell wall is rigid, containing cellulose microfibrils running through a matrix of hemicelluloses and pectic substances.
  • Cellulose is made of beta glucose (one inverted after the other), joined by 1,4 glycosidic bonds.
  • Hydrogen bonding between cellulose molecules (-OH and O) creates cross-linking between cellulose chains, forming cellulose microfibrils.
  • The hemicelluloses and pectic substances act as a glue, holding cellulose microfibrils in place. 
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Plant Cell Wall (cont)

The plant cell wall:

  • Provides mechanical support (when turgid) and protection.
  • Prevents osmotic bursting of the cell.
  • Provides a pathway for movement of water and mineral salts.
  • Can become lignified.
  • Matrix of hemicelluloses and cellulose microfibrils is fexible, so the plant can bend with the wind.  
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Middle Lamela

  • The middle lamela is a thin layer of pectic substances.
  • It is located between the cell walls of 2 neighbouring plant cells.
  • Middle lamela glues neighbouring cells together
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  • Plasmodesmata are fine threads linking the cytoplasms of two neighbouring cells together.
  • The plasmodesmata run from pores in a cell well, through the middle lamela, and through the neighbouring cell wall, linking 2 cytoplasms.
  • They enable a continuous system of cytoplasm, the symplast, to be formed between neighbouring cells for transport of substances
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  • The tonoplast is the large central vacuole in a plant cell.
  • It is a single-membrane bound sac containing cell sap.
  • Cell sap contains mineral salts, sugars and enzymes.
  • Tonoplast also stores waste products, and can sometiimes function as a lysosome
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Secondary Cell Wall

  • When a plant cell is finished growing, it can deposit excess material and form a secondary cell wall.
  • The excess material may be lignin, which makes wood.
  • It may also be suberin, which makes cork.
  • The secondary cell wall serves supporting or waterproofing roles. 
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  • Amyloplasts are specialised plant organelles.
  • They store amylopectin and are used to provide energy when the cell needs it.
  • They are formed from leucoplasts, which are unspecialised plant cells.
  • Amyloplasts are an example of plastids (storage organelles). 
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Primary Plant Body

  • The body of a vascular plant is divided into 3 principle organs: the leaves, the stem and the roots.
  • The shoot system of a plant consists of all the stems and leaves.
  • The points where leaves attach to the stem are nodes.
  • Regions between leaves (nodes) are called internodes
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The 2 functions of the stem are support and transport:

  • Support: to hold leaves in the best position for obtaining sunlight for photosynthesis.
  • Support: to support flowers in a way that maximises the chance of pollination.
  • Transport: transport of products from photosynthesis to other parts of the plant.
  • Transport: transport water and other reactants of photosynthesis to photosynthesising leaves.
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Growth of Primary Plant Body

  • Growth in plants is confined to certain regions called meristems.
  • A meristem is a group of cells that can divide by mitosis.
  • They produce daughter cells which grow and form the rest of the plant body. 
  • The daughter cells are unspecialed and totipotent. 
  • The three types of meristematic tissue are protoderm, procambium and ground meristem, which give rise to the epidermis, vascular tissues and ground tissues (parenchyma) respecitively. 
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  • The epidermis protects cells beneath it.
  • Epidermal cells secrete cutin, a waxy substance that forms the waxy cuticle.
  • The waxy cuticle helps prevent water loss from stem surface. It also prevents pathogens from entering.
  • Epidermal cells may form hairs, which act as an insulating layer, be hooked to help climbing plants, or be irritant for protection against preditors. 
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Ground Tissue

  • Ground tissue is found benieth the dermal tissue in the cortex.
  • The ground tissue system carries out photosynthesis, stores photosynthetic products, and helps support the plant
  • There are 3 types of ground tissue: parenchyma, collenchyma and slerenchyma.
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Ground Tissue- Parenchyma

  • Parenchyma are unspecialised living cells, with a primary cell wall, shared middle lamela and large central vacuole.
  • It serves as packing material and supports the stem.
  • They are modified to store various substances such as starch, or so that they can carry out photosynthesis.
  • Some parenchyma are modified to form collenchyma or sclerenchyma
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Ground Tissue- Collenchyma

  • Collenchyma are living cells that support the stem.
  • They have thick primary cell walls and no secondary cell wall.
  • They provide support for perioles and growing organs.
  • They are quite flexible, allowing a stem to sway in the wind without snapping
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Ground Tissue- Sclerenchyma

  • Sclerenchyma are dead cells.
  • They have a primary cell wall, and a thickened secondary cell wall.
  • Their main function is also support.
  • There are 2 forms of schlerenchyma: elongated fibres and sclerids
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Ground Tissue- Sclerenchyma: Elongated Fibres

  • Elongated fibres (made of cells) are found in bundles.
  • Lignin is deposited to form lingified secondary cell wall. This makes the fibres strong yet flexible.
  • Once the fibre is lignified the cell contents die to form a hollow tube (lumen) in middle of fibre.
  • Lignin is permeable to water, so water enters fibres and flows through hollow lumen. 
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Ground Tissue- Sclerenchyma: Sclerids

  • Sclereids are impregnated with lignin.
  • They are packed together very tightly.
  • They form very rigid support for plant stem. 
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Cross-Section of Stem


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Vascular Bundles

  • Vascular bundles contain the transport tissues xylem and phloem.


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  • The xylem is a water transport system.
  • Protoxylem is living tissue.
  • Protoxylem becomes lignified and impermeable to water, the contents of the cell die and hollow tubes are formed inside.
  • This lignified tissue is called metaxylem.
  • Water moves out of xylem into surrounding cells through non-lignified parts of the wall, or through pits in the wall. 
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  • The phloem is a long line of living cells that meet end to end, forming seive tubes,
  • The phloem transport food from their sources to tissues that either consume the food or store it.
  • The sieve tube elements (hollow end to end cells) have adjacent companion cells that maintain all the phloem's organalles and may regulate the preformance of the sieve tube elements. 
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