plants

• Active loading

  1. H+ ions actively pumped out of companion cell using energy from ATP.
  2. High conc of H+ ions causes facilitated diffusion back into companion cell. Sucrose carried with H+ ions through cotransport proteins in plasma membrane
  3. increasing conc of sucrose in companion cell causes it to diffuse through plasmodesmata into sieve tube element
• phloem loading

  -different routes

  • symplast
  • apoplast
  • vacuolar
• • sucrose moved against conc. Gradient from cells where its made or being stored - why its an active process (need ATP)
  • ATP used to create conc. gradient for H+ ions pumped out of companion cells - increases conc outside cells
• translocation: transport of assimilates throughout a plant
• source: parts of a plant which load assimilates such as sugars, into phloem sieve tubes
• sinks: parts of the plant which remove assimilates from phloem sieve tubes
• pressure flow hypothesis: most widely accepted explanation of sap movement in plants
• hydrophytes: plant adapted to living in water or where its very wet.

  • keep leaves afloat so they're in the air and absorb light
  • large air spaces in leaves to keep them afloat
  • flexible stems and leaves to reduce damage by water current
• cacti (xerophyte)

  • store water in stem which is ribbed or fluted so it can expand
  • leaves reduced to spines to reduce surface area (limits water loss)
  • can trap moisture (reduces wp gradient)
  • stem is green can photosynthesise
  • roots are widespread or deep
  • close stomata at hottest time of day
• xerophytes: plants that are adapted to living in very dry conditions

  • can close stomata when water availibility is low
  • some plants maintain a high salt conc in leaves to reduce wp
  • have long roots that can reach water deep underground
• cohesion tension theory

  • water is a polar molecule - positive and negative charges not evenly distributed
  • in xylem, water molecules arrange so that positive and negative charged poles lie next to eachother - causes molecules to cohere
  • if some leave by transpiration others are pulled behind them
• • xylem tissue made of vessels to transport water and mineral ions and fibres (provide support for plant)
  • living cells called parenchyma cells - act as packing tissue to separate and support vessels
  • ligning forms patterns in cell walls: spiral, annular (weakest, most flexible), reticulate (strongest, least flexible)
• • xylem able to stretch as plant grows due to lignin being deposited in different patterns
  • places where lignification is not complete gaps called bordered pits form
• tracheids

  • transport water, mineral salt
  • not perforated at end
  • found in all vascular plants
  • ends overlap
  • help provide some support
  • xylem: big tubes surrounded by tracheids
• medulla


  makes up 'bulk' of root
• cortex


  found outside epidermis. provides strength and can also photosynthesise if chloroplasts are present
• pericycles


  just inside endodermis and contains meristem cells which retain the ability yo divide
• endodermis


  special sheath/ covering of cells that play a role in getting water to xylem vessels
• monocots

  • parallel lines in leaves
  • floral part in threes
  • one cotyledon
  • vascular bundles throughout
  • pollen grain has one pore
• dicots

  • vein like pattern in leaves
  • floral parts in fours or fives
  • two cotyledons
  • vascular bundles arranged in rings
  • pollen grains have 3 pores
• cotyledon


  and embyronic leaf in seed-bearing plants, one or more of which are the first leaves
• dicotyledonous


  collenchyma and sclerenchyma may also be found to produce strength and support - vascular bundles found in centre of roots
• companion cells

  • small cells found inbetween sieve tube elements
  • have a large nucleus and dense cytoplasm
  • produce ATP needed for active processes
  • responsible for carrying out metabolic processes needed to load sucrose into sieve tube elements
• parenchyma


  packing cells
• phloem


  transport assimilates and made of sieve tubes and companion cells
• sieve tube elements

  • elongated cells - line up to form sieve tubes
  • lack a nucleus and have little cytoplasm - depends on companion cells for providing proteins and ATP
  • sieve plates are perforated (have pores)
  • have thin cell walls - allows cell sap to move easily from one cell to the next
• transpiration: loss of water vapour from aerial parts of plant. - limited by waxy cuticle

  1. water enters leaf through xylem and moves into spongy mesophyll cells by osmosis and passes along cells by apoplast pathway
  2. water evaporates from cell walls of spongy mesophyll cells
  3. water is lost from leaf through stomata. Relies on there being a water vapour potential gradient
• schlerenchyma fibres

  • allow fluids/ nutrients to move through small lumen back and fourth
  • provide strength and support to plant
  • mature cells are dead
• parenchyma cells

  • living cells, intracellular shape
  • in roots, can store starch
  • in leaves can have chloroplasts called chlorenchyma
• chlorenchyma


  living cells, little/ no intracellular shape - provide support with some flexibility
• • water inside cell exerts a pressure on the cell wall - pressure potential - as pressure potential builds up, reduces the influx of water
  • cells placed in solutions with negative wp, water lost from inside cell
  • flaccid=plasmolysed
• reasons for transpiration

  • transports mineral ions up plant
  • maintains cell turgidity
  • photosynthesis
• water uptake - solutions moves to where its more negative
• vacuolar pathway


  enters the cell cytoplasm through plasma membrane ans is able to enter and pass through the vacuoles. water moves by osmosis
• apoplast pathway

  passes through spaces in cell walls and between cells. does not enter cells and therefore moves by mass flow (not osmosis)
• symplast pathway

  water enters cytoplasm through plasma membrane and moves through plasmodesmata, from one cell to next. Water moves by osmosis
• transpiration stream

  • movement of water from soils, through plant, to air surrounding leaves
  • main force driving this is water potential gradient
• root pressure


  minerals pumped into medulla and xylem which draws water into medulla by osmosis. pressure in root medulla builds up and pushes water up xylem.
• transpirational pull


  water molecules attracted to each other (cohesion). As water lost from leaves, whole column of water pulled upwards

  -pull from above creates tension in column of water. This is why xylem vessels are strengthened with lignin or they'd collapse- cohesion tension theory.
• casparian strip

  • water and minerals enter root hair cells by osmosis
  • water moves down wp gradient from root cortex to endodermis of vascular bundle
  • water moves via apoplast and symplast pathways
  • when water reaches endodermis, it is blocked by casparian strip which contains waxy material (suberin) through which water cannot pass