9. Plant biology

9. Plant biology

9.1 Transport in the xylem of plants

Nature of science:

• Use models as representations of the real world - mechanisms involved in water transport in the xylem can be investigated using apparatus and materials that show similarities in structure to plant tissues.

Understandings:

• 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.

Applications and skills:

• Application: Adaptations of plants in deserts and in saline soils for water conservation.

• Application: Models of water transport in xylem using simple apparatus including blotting or filter paper, porous pots and capillary tubing.

• Skill: Drawing the structure of primary xylem vessels in sections of stems based on microscope images.

• Skill: Measurement of transpiration rates using potometers. (Practical 7)

• Skill: Design of an experiment to test hypotheses about the effect of temperature or humidity on transpiration rates.

9.2 Transport in the phloem of plants

Nature of science:

• Developments in scientific research follow improvements in apparatus - experimental methods for measuring phloem transport rates using aphid stylets and radioactively-labelled carbon dioxide were only possible when radioisotopes became available.

Understandings:

• 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.

Applications and skills:

• Application: Structure–function relationships of phloem sieve tubes.

• Skill: Identification of xylem and phloem in microscope images of stem and root.

• Skill: Analysis of data from experiments measuring phloem transport rates using aphid stylets and radioactively-labelled carbon dioxide.

9.3 Growth in plants

Nature of science:

• Developments in scientific research follow improvements in analysis and deduction - improvements in analytical techniques allowing the detection of trace amounts of substances has led to advances in the understanding of plant hormones and their effect on gene expression.

Understandings:

• 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.

Applications and skills:

• Application: Micropropagation of plants using tissue from the shoot apex, nutrient agar gels and growth hormones.

• Application: 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.

9.4 Reproduction in plants

Nature of science:

• Paradigm shift - more than 85% of the world’s 250,000 species of flowering plant depend on pollinators for reproduction. This knowledge has led to protecting entire ecosystems rather than individual species.

Understandings:

• 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.

• Success in plant reproduction depends on pollination, fertilization and seed dispersal.

• Most flowering plants use mutualistic relationships with pollinators in sexual reproduction.

Applications and skills:

• Application: Methods used to induce short-day plants to flower out of season.

• Skill: Drawing internal structure of seeds.

• Skill: Drawing of half-views of animal-pollinated flowers.

• Skill: Design of experiments to test hypotheses about factors affecting germination.