Topic 9

• Reasons why plants need transport system

  Size, metabolic rate, surface area to volume ratio
• Why size affects a plant's need for a transport system

  Large plants need to move substances up and down the entire length
• Why metabolic demand affects a plant's need for a transport system

  Internal and underground plant parts need oxygen and glucose to get to them, hormones need transporting to where they have an effect, mineral ions need transporting to make proteins for enzymes
• Why surface area to volume ratio affects a plant's need for a transport system

  Plants have a low surface area to volume ratio so diffusion is not enough to supply the plant cells with everything they need
• Arrangement of vascular bundles in the stem

  Around the edge for strength and support
• Arrangement of vascular bundles in the root

  In the middle to help withstand the tugging strains from the wind
• Arrangement of vascular bundles in the leaf

  Midrib of a leaf supports the structure of the leaf
• Dicotyledonous plants


  Plants that make seeds that contain two cotyledons
• Cotyledons


  Organs that act as food stores for developing embryos
• Types of dicots


  Herbaceous, woody
• Structure of the xylem

  Long hollow structures made of columns of cells fused together end to end, thick-walled parenchyma around the xylem vessels, lignified secondary walls, bordered pits
• Role of thick walled parenchyma

  To store food, to store tannins
• Role of lignin in xylem

  To provide mechanical strength
• Arrangement of lignin in the xylem

  Rings, spirals, solid tubes
• Role of pits in the xylem

  To be where the water leaves the xylem for other cells in the plant
• Function of the xylem

  To transport water and mineral ions, to support the plant
• Structure of sieve tube elements

  Many cells joined end to end to form a hollow structure, not lignified, sieve plates
• Function of sieve tube elements


  Main transporting vessels of organic solutes
• Function of sieve plates

  To let phloem contents flow through
• Why mature phloem cells have no nucleus

  Large pores appear in the cell walls, tonoplast and nucleus and other organelles break down, phloem fills with phloem sap
• Structure of companion cells

  Linked to sieve tube elements by plasmodesmata, nucleus and organelles present
• Function of companion cells

  To act as the life support system for the sieve tube cells
• Structure of the phloem

  Sieve tube elements, companion cells, fibres, sclereids
• Sclereids


  Cells with very thick cell walls
• How to dissect stems to observe xylem

  Put material in water containing a strongly coloured dye for 24 hours, rinse it, make clean transverse cut with a sharp blade on a white tile, xylem show up as spots, make a clean longitudinal cut, xylem show up as coloured lines
• Limitations of dissection of stem to observe vascular bundles

  Can't be adjusted to see phloem, dependent on sharp blade and steady hand, if they aren't cut in the right place you won't see any xylem
• Process of transpiration


  Water evaporates from the surface of mesophyll cells into air spaces in the leaf and moves out of the stomata by diffusion, evaporation lowers water potential of the cell, water moves into cell by osmosis through apoplast and symplast pathways, repeated across the leaf to the xylem, water moves out of xylem by osmosis, water molecules form hydrogen bonds with each other resulting in cohesive forces causing capillary action, water drawn up the xylem to replace water lost by evaporation by the transpiration pull, transpiration pull causes tension in xylem
• Theory related to transpiration

  Cohesion-tension theory
• Capillary action


  Water moving up a narrow tube against the force of gravity
• What is transpiration an inevitable consequence of?

  Gaseous exchange for photosynthesis
• Evidence for the cohesion-tension theory

  Trees shrink in diameter when transpiration is at its highest because of the higher tension in the xylem, broken xylem vessels take up air rather than letting water out, broken xylem vessels can't move water because the continuous stream has broken
• How to measure transpiration rate

  Potometer
• Why is it difficult to measure transpiration directly?

  Hard to condense and collect all water that evaporates from leaves without collecting water from the soil, hard to separate water from transpiration and water vapour from respiration
• Precautions when setting up a potometer

  All joints sealed with waterproof jelly, airtight, calibrated, cut the shoot at a slant, set up underwater
• Which wall of the guard cell is more flexible?

  Outer layer
• Factors which will affect the rate of transpiration

  Light intensity, relative humidity, temperature, air movement, soil-water availability
• How does light intensity affect the rate of transpiration?

  Increased light intensity opens more stomata, increases evaporation from the surfaces of the leaf
• How does relative humidity affect the rate of transpiration?

  High relative humidity will lower the rate of transpiration, reduced water potential gradient
• How does temperature affect the rate of transpiration?

  Increased temperature increases the kinetic energy of water molecules and increases rate of evaporation, increased temperature increases concentration of water vapour that the external air can hold
• How does air movement affect the rate of transpiration?

  Air movement blows away diffusion shells, increases water potential gradient