9.1 - Xylem
Plant diagram
● Cambium, xylem, phloem, cell wall
Transpiration
● The loss of water from leaves and stems of plants
● Affected by wind, temperature, humidity
○ Greatest rates when dry and hot with wind
● Process/cohesion tension theory:
● Water is heated in the mesophyll by sunlight and becomes vapour
○ This vapour transpires out the stomata
● Loss of water generates negative pressure and a transpiration pull on water
molecules in the xylem. More water is drawn into the leaf.
● Cohesion means that transpiration keeps happening throughout the plant (knock-on
effect)
Stomata closed
Stomata open
CO2 uptake low, water loss low
CO2 uptake high, water loss high
Low pressure in cytoplasm
High pressure in cytoplasm
Guard cells flaccid
Guard cells turgid
Caused by water shortage & darkness
Caused by sunlight/high photosynthesis &
reduced CO2 concentration
Xylem
● Made of dead cells to form a hollow tube
● Provides support to plant w lignin walls
● Transports water from roots to leaves
○ Osmosis occurs due to roots having higher solute concentration than the soil
Xerophytes
● Plants that can survive in extreme conditions by reducing transpiration
Life cycle adaptations:
● Perennial plants bloom in wet seasons
● Dormant seeds can survive for many years until ideal growth conditions
Metabolic adaptations:
● Crassulacean acid metabolism (CAM) plants: CO2 is absorbed at night and stored as
a C4 compound. During the day, photosynthesis can occur with the stomata closed
by using these carbon stores.
Physical adaptations
● Fewer leaves or stomata
● Rolled leaves or spines
● Stomata in pits with hairs
● Deeper roots to reach water
● Waxy cuticle reduces transpiration
Roots
● Water enters the root hair cells by osmosis
● Water moves down the concentration gradient
● Mineral ions enter through active transport
○ Active uptake of mineral ions results in a higher concentration of minerals in
plants than in the surrounding soil
● Ca2+ and K+ enter by ion exchange using a proton pump
● NO3- enters by symport with H+ ions
Measuring transpiration
● Water transport in xylems can be modelled in various ways:
○ Capillary tubing (water moves along tubing via surface tension)
○ Filter paper (absorbs water due to adhesive properties)
○ Porous pots (semi-permeable containers can model osmosis)
● Transpiration rates in plants are measured with potometers
○ Potometers measure movement of air bubble /meniscus
○ More movement represents increased transpiration rate
9.2 - Phloem transport
Active translocation
● From source (leaf) to sink (fruits, roots, seeds)
Phloem
● Transports sugars and organic molecules
○ Loaded into phloem by companion cells at the source and sink
○ Sugar is actively transported into phloem
● Bidirectional
● Sieve transport cells
○ Transports water and solute
○ Have no nucleus, ribosomes, or vacuoles
○ Rigid cell walls allow for the building of high pressures needed to generate
hydrostatic pressure
● Companion cells
○ Help perform certain metabolic functions for sieve elements
○ Plasmodesmata (microscopic pores in the cell walls) between companion and
sieve element cells are larger than in most plant cells to allow for exchange
like ATP
● Parenchyma
○ Between other cell types and helps transfer materials to the sieve elements
and companion cells
● Sclerenchyma cells
○ Provides structural support for the plant
Sugars
Amino Acids
Source: Green stems and leaves; storage
tissue in seeds
Source: roots or tubers; storage in
germinating seeds
Sink: growing roots and stems; roots
absorbing minerals
Fruit production or other energy storage;
flowering and reproduction
Sink: growing roots and stems
Developing leaves, fruits; flowering and
reproduction
Active transport
● H+ ions are actively transported out of the phloem cell
● A higher concentration gradient is established outside the cell
● H+ ions flow back into the cell
● The energy released is used to cotransport sucrose into the phloem cell
Osmosis
● Concentration of sucrose in phloem is relatively high, water conc. Low
● Water moves down the concentration gradient from the xylem, through the
membrane, into phloem cells
Hydrostatic pressure gradients
● Relatively high conc. Of sucrose and water in phloem sieve cells
● Water is incompressible, and the sieve walls are rigid
○ Causes a build up of hydrostatic pressure at the source
● Water and the solutes (sucrose & amino acids) flow down the concentration gradient
to the sink where the pressure is relatively low (root)
○ Due to the active unloading of sucrose and hence loss of water by osmosis at
the sink
Aphids
● Feed primarily on plant sap
● Used to measure rates of phloem transport
1. A plant is grown in the lab and one leaf is exposed for a short time to CO2 containing
the radioactive isotope 14C.
2. The 14CO2 will be taken and incorporated into glucose by the process of
photosynthesis. Glucose is converted into sucrose for translocation via the phloem.
3. Aphids are encouraged to feed on the phloem in different locations of the stem at
different times.
4. The phloem is then analysed for 14C content and the results can be used to
calculate the rate at which substances move through the phloem
9.3 - Growth in plants x
Meristems
● Undifferentiated cells that have specific regions of growth and development
● Allow for regrowth and vegetative propagation
● Apical meristem
○ Occurs in shoots and roots
○ Responsible for primary growth (lengthening) and leaf development
● Lateral meristems
○ Occurs at the cambium
○
Responsible for secondary growth (widening) and production of bark
Auxins - hormones
● Controls growth in the shoot apex by stimulating or inhibiting cell division
● Released by the shoot apical meristem
● Coordinates both apical and directional growth (tropism)
○ Influences cell growth rates by changing gene expression in plant tissue
● Promotes cell elongation away from light, hencing allowing the plant to grow towards
the light
○ Inhibits cell elongation in the roots
Apical growth
● Allows for extension of the stem and leaf development
● Growth occurs in sections of the stem (nodes), with the remaining meristem tissue
forming inactive axillary buds
○ Axillary buds have the potential to form new shoots
● Auxins promote growth in the apex, but inhibit growth in buds
○ This is called apical dominance
Micropropagation
● Used to produce large numbers of identical plants (clones) from a stock plant
● Produces virus-free stains of existing varieties and helps to propagate rare plant
species (orchids)
● Tissue sample is grown in agar and treated with growth hormones
● Growing shoots are divided and transferred to soil to form new plants
9.4 - Reproduction in plants
Pollination
● Pollen carried from anther of a flower to stigma of another flower
○ Can rely on wind or water for pollination, primarily animals
○ cross-pollination is preferred bc variation
Fertilisation
● Pollen tube grows down the stigma to the ovary through the style
○ Style is not hollow
● Pollen is delivered to the ovum and fertilisation occurs
Seed dispersal
● Once the seed has developed in the ovule, it is ready for dispersal
○ Wind, animal, water
Long day and short day flowers
● Flower depends on hours of darkness
● Can manipulate flowering by exposing plants to light/darkness (photoperiodism)
Long-day
● Irises
● Need high levels of Pfr to bloom
Short-day
● Chrysanthemums
● Need low levels of Pfr to bloom
Phytochromes
● Leaf pigments that can be used to 'measure' the length of night
● Pr (red) produced slowly in the dark
● Pfr (far-red) produced quickly in the sunlight