Ch 9

• • Transpiration is the inevitable consequence of gas exchange in the leaf
• • Plants transport water from the roots to the leaves to replace losses from transpiration.
• • The cohesive property of water and the structure of the xylem vessels allow transport under tension.
• • The adhesive property of water and evaporation generate tension forces in leaf cell walls.
• • Active uptake of mineral ions in the roots causes absorption of water by osmosis
• • Adaptations of plants in deserts and in saline soils for water conservation

  desert (xerophyte) :leaves reduced to spines, thick waxy cuticle, CAM physiology, stomata in pits

  
  

  saline (halophyte): long roots that search for water, leaves are shed when water is scarce stem becomes green and takes over photosynthesis, thick cuticle, water storage structures in leaves
• • Models of water transport in xylem using simple apparatusincluding blotting or filter paper, porous pots and capillary tubing
• capillary action - Water has the capacity to flow along narrow spaces in opposition to external forces like gravity
• water is transported across the paper towel via capillary action

  
• transverse section of stem

  A) phloem

  B) cambium

  C) xylem

  D) vascular bundle

  E) epidermis

  F) cortex

  G) pith

  
• • Measurement of transpiration rates using potometers (air bubble moves along as water absorbed by shoot)
• • Design of an experiment to test hypotheses about the effect oftemperature or humidity on transpiration rates.
• • Plants transport organic compounds from sources to sinks
• • Incompressibility of water allows transport along hydrostatic pressure gradients
• • Active transport is used to load organic compounds into phloem sieve tubes at the source
• • High concentrations of solutes in the phloem at the source lead to water uptake by osmosis
• • Raised hydrostatic pressure causes the contents of the phloem to flow towards sinks.
• companion cells load stuff from source into the phloem sieve tubes using active transport (so they have a lot of mitochondria)
• • Sieve elements are connected by sieve plates at their transverse ends, which are porous to better enable flow between cells
  • Sieve elements have no nuclei and reduced numbers of organelles to maximise space for the translocation of materials
  • The sieve elements also have thick and rigid cell walls to withstand the hydrostatic pressures which facilitate flow
• companion cells: infolding plasma membrane -> higher SA:V

  lots of mitochondria for active transport

  contain the appropriate proteins for transport
• • Identification of xylem and phloem in microscope images of stem and root. (remember xylem walls have lignin rings these are NOT sieve plates)
• • Analysis of data from experiments measuring phloem transport rates using aphid stylets and radioactively-labelled carbon dioxide. (closer the severed stylet is to the sink the slower the rate at which phloem sap will come out)
• • Undifferentiated cells in the meristems of plants allow indeterminate growth
• • Mitosis and cell division in the shoot apex provide cells needed for extension of the stem and development of leaves.
• • Plant hormones control growth in the shoot apex
• • Plant shoots respond to the environment by tropisms
• • Auxin efflux pumps can set up concentration gradients of auxin in plant tissue
• • Auxin influences cell growth rates by changing the pattern of gene expression.
• • Micropropagation of plants using tissue from the shoot apex, nutrient agar gels and growth hormones.

  explants (small cuts of the meristem, most undifferentiated tissue works best) are added to sterilised growth medium greater than 10:1 ratio of auxin to cytokinin ROOTS develop (more auxin = roots) if ratio of "a" to "c" is less than 10:1 this is shoot media and shoots develop -> once root+shoots developed cloned plant is transferred to soil
• • Use of micropropagation for rapid bulking up of new varieties, production of virus-free strains of existing varieties and propagation of orchids and other rare species. (meristem has no vascular tissue so is free of viruses )
• micro propagated plantlets can be preserved in liquid nitrogen
• phototropins -> absorb light of appropriate wavelength -> conformation changes -> bind to receptors in the cell which control transcription of specific genes -> likely the genes that code for PIN3 proteins that transport auxin
• gravotropism: statoliths are heavy organelles that fall -> leads to distribution of PIN3 proteins at bottom -> auxin is transported to the bottom of the root where it inhibits growth -> root grows downwards
• • Flowering involves a change in gene expression in the shoot apex
• • The switch to flowering is a response to the length of light and dark periods in many plants.
• Pr -> Pfr
• in short day plants the night is long enough for enough Pfr to convert back to stable Pr and plant can flower as Pfr inhibits flowering in SDP
• in long day plants the night is too short for enough Pfr to convert back to Pr and the plant flowers since Pfr promotes the transcription of the genes for flowering in LONG day plants
• • Success in plant reproduction depends on pollination, fertilization and seed dispersal. (feathery to catch wind, fleshy and attractive for animals, hooks to hook onto animal coats)
• fertilization: pollen grain on stigma grows down the style to ovary, this pollen tube carries male gametes to fertilize the ovary