L1 - transport systems in multicellular plants

• Water moves through the plant from roots to leaves in the xylem and is lost to the air as water vapour
• The products of photosynthesis (sugars and assimilates) are transported through the phloem
• Large multicellular organisms cannot rely on diffusion alone to supply cells with substances (such as glucose and oxygen) and remove waste products. They require specialised transport systems
• Why do multicellular organisms require specialised transport systems:

  -plants require transport systems to move substances around their structures to stay alive

  -diffusion would not provide enough of these essential molecules fast enough
• Why do plants require water:

  -for photosynthesis (happens in mesophyll cells usually)

  -to transport materials (eg. minerals)
• Why do plants require glucose:

  -cellular respiration (sugar produced by photosynthesis in the leaves is transported up and down to other tissues via the phloem)
• Seeds come in two forms:

  -dicot

  -monocot
• Cotyledons store food. This food is needed by the developing embryo for respiration before the plant can photosynthesise
• Dicotyledonous plants are plants that have seeds made of 2 cotyledons
• A dicot example is a bean seed
• An example of a monocot plant is a corn seed
• Plant root structure:

  
• Xylem transports water
• Phloem transports assimilates
• Root structure:

  -endodermis surrounds vascular tissue, important in taking water to the xylem

  -pericycle is meristem cells that are unspecialised and keep dividing
• The cambium is made up of unspecialised meristem cells
• Stems are made up of:

  -epidermis

  -phloem

  -cortex

  -xylem

  -cambium

  -parenchyma
• The xylem, phloem, and cambium make up the vascular bundle
• Stem structure:

  
• Leaf structure:

  -the vascular bundles form the midrib and veins of a leaf

  
• Xylem is made of:

  -continuous, hollow tubes called vessels

  -fibres and living parenchyma cells
• Xylem have spirals of lignin running around the lumen to help reinforce the vessels so they don’t collapse

  
• Xylem structure:

  -cells are dead but their walls have been impregnated with lignin (lignified walls)

  -lignin strengthens the vessel walls

  -lignin can be laid down in spirals, annular rings, or reticulate patterns

  -areas of non lignificad wall are called pits
• Phloem structure:

  -made of sieve tube elements and companion cells

  -sieve elements have very little cytoplasm

  -no nucleus

  -forms a long tube

  -have sieve plates between cells
• Companion cell structure:

  -companion cells are next to the sieve tubes

  -contain lots of mitochondria to produce ATP, and a nucleus

  -linked to the sieve tubes by plasmodesmata, these link the cytoplasm of the two cells