9,1 Transport in the Xylem of Plants Transpiration

● Transpiration: loss of water vapour from the leaves and stems (occurs as a consequence of gas exchange)

● Guard cells (located on either side of the stomata) minimise transpiration by control the opening of the stomata (pores)

Using a potometer

● Potometer: device used to measure water uptake in plants (measuring rate of transpiration)

Xylem structure helps withstand low pressure

● Xylem (vessel which transports water) walls are thick and contain lignin → withstand low pressure without collapsing ● Water is attracted to each other (cohesion) and is also attracted to hydrophilic parts of the cell wall (adhesion) →

supports water uptake Tension in leaf cell walls maintains the transpiration stream

● Water evaporates from leaves → adhesion and cohesion causes water uptake ● Low pressure generates pulling force upwards (transpiration-pull)

Active transport of minerals in roots

● Active transport of mineral ions into root hairs allow osmosis to occur (from soil to roots) ● Because some ions move through soil slowly, some fungi grow on plants roots to absorb these ions for them

Transport of water from roots to leaves

1. (Water evaporates from leaves) 2. Water moves from soil into roots through osmosis 3. Water gets into xylem through cell walls 4. Water climbs up xylem (adhesion and cohesion) 5. Water evaporates from leaves

Xerophytes and halophytes adaptations for water conservation

● Xerophytes: plants adapted to growing in dry habitats (e.g. cacti) ● Cacti adaptations (xerophyte):

○ Reduced leaf (spines) → prevent transpiration ○ Swollen stems → water storage ○ Pleats (folds) → allow for rapid contraction/expansion ○ Thick waxy cuticle → prevent evaporation of water ○ Stomata on stem which opens up during night instead of day →

slower transpiration (CO2 stored as malic acid at night and is used during day for photosynthesis)

● Marram grass (right) adaptations (xerophyte): ○ Rolled leaves → prevent water loss ○ Stomata in pits → prevent water loss ○ Hairs inside leaves → prevent air movement

● Halophytes: plants adapted to growing in saline soil ● Halophytes adaptations:

○ Reduced leaves ○ Leaves shed off in dry conditions, stems take over photosynthesis ○ Water storage structures ○ Thick cuticle and multiple epidermis ○ Sunken stomata ○ Long roots → seek for water ○ Structures for removing salt build-up

Drawing xylem vessels

● Larger phloem located outside with smaller xylem inside ● Xylem vessels: large lumen and thickened cell walls

9.2 Transport in the Phloem of Plants Translocation

● Translocation: transport of organic solutes (amino acids, carbohydrates) in a plant (from source [site of production/storage] to sink [site of usage]) by phloem

● Sinks and sources can interchange → phloems must be able to transport in either directions Phloem loading

● Phloem loading: actively transporting organic compounds (sucrose) into phloem

● Process (phloem loading [apoplast route]): 1. H+ ions are actively pumped (ATP) from companion

cell to source 2. (Because of high H+ concentration in source) H+ flows

out from source, simultaneously carrying sucrose into companion cell

3. Sucrose transported into phloem ● Process (translocation)

4. Sucrose enters sieve tubes from the source → high solute concentration

5. Osmosis occurs from corresponding xylem to sieve tubes 6. High water pressure near source causes movement of phloem

content from source to sink Sieve tube structure and function

● Sieve tube cells are living (unlike vascular elements of xylem) → membranes control concentration of organic molecules

● Sieve tube cells associated with companion cell → companion cells perform genetic and metabolic functions for sieve tube cell (which has smaller cytoplasm and no nucleus)

● Plasmodesmata (opening in cell wall) connecting sieve tube cell and companion cell → allow movement of carbohydrates and genetic materials between the 2 cells

● Infolding of plasma membrane (companion cell) → increases phloem loading ● Rigid cell wall → building up high pressure for internal flow ● Pores in sieve plates → prevent reducing the flow speed

Using aphid stylets to measure phloem transport rate

● Aphids use a stylet (part of mouth) to penetrate the stem and to feed on the phloem sap

● Scientists cut the stylet off while the aphid is feeding on the sap to measure the rate of flow and composition of the phloem sap

Identifying xylem and phloem in light micrographs

● Xylems are larger than phloem ● Phloems are located closer towards the outside and xylems closer to the inside ● Dicot if the vascular bundles are aligned in a ring shape, monocot if scattered

9.3 Growth in Plants

Meristems ● Meristems: region in plants undergoing active cell division by undifferentiated cells (region of growth) ● Apical meristems: growth occurring at the root or stem tip (shoot) ● Lateral meristems: growth occurring at the cambium (thickening)

Mitosis in plant growth

● Root apical meristem ○ Responsible for growth of root

● Shoot apical meristem ○ Produces both cells for stems and leaves/flowers ○ One daughter cell remains in the meristem whereas the other one increases in size and differentiates into a

specific cell

Hormones in shoot growth ● Auxin: hormone which can initiate root growth, allow development of

fruits/leaves, and also regulate leaf development (inhibits growth in roots, promotes growth in stems)

● → Auxin produced by the shoot apical meristem inhibits growth of axillary bud (shoots which form at plant nodes). At nodes further away from the meristem, growth of axillary buds are not inhibited

● Cytokinins: hormone which promote axillary bud growth ● → Levels of auxin and cytokinins determine axillary bud development

Plant tropisms

● Tropism: directional growth responses to directional external stimuli (e.g. gravity, light)

● Phototropism: growth towards light ● Gravitropism: growth in response to gravitational force

Auxin in gene expression

● Cells contain an auxin receptor ● Process:

1. Specific genes are transcribed when auxin binds to the receptors 2. Gene expression causes release of hydrogen ions 3. Hydrogen ions loosen the cellulose fibres → cell expansion

Intracellular pumps

● In phototropism: 1. Phototropins in tips of plants detects greater light intensity from a

particular direction 2. Auxin moves away from the light 3. Auxins cause the side with less light to grow (promote growth) 4. Stem curves towards the light source

● In gravitropism: 1. Root is placed on one side 2. Cell organelles (statoliths) accumulate at the bottom of the cell 3. PIN3 transport proteins move auxin to the lower side of the roots 4. Auxin causes inhibition of growth on lower side 5. Root curves downwards

Micropropagation of plants

● Micropropagation: process of making large numbers of identical plants from a stock plant

● Process: 1. Tissues (meristems) from stock plant are sterilised 2. Tissues are cut up into pieces (explant) 3. Explant is placed on sterilised growth medium (agar) with hormones (auxin and cytokinins)

a. 1 auxin : 1 cytokinins forms undifferentiated mass (callus) b. 1 auxin : 10 cytokinins allows roots to develop c. 10 auxin : 1 cytokinins allows shoots to develop

4. Roots and shoots develop → planted in soil ● Advantages:

○ Produces virus-free strain of plants ○ Faster and consumes less space compared to traditional methods ○ Produce plants with desirable characteristics (preservation of rare species) ○ Micropropagated plantlets can be stored in liquid nitrogen for cryopreservation

9.4 Reproduction in Plants

Flowering and gene expression ● Vegetative phase: seed germinates and the plant grows roots, stems, and leaves ● Reproductive phase: plant forms flowers and produces seeds ● Floral initiation is caused by changes in temperature and light (changes as length of days change)

Photoperiods and flowering

● Phytochrome are pigments in leaves which can switch between PR and PFR depending on the time of the day (used to measure night length)

● Conversion of forms ○ PR absorbs red light at 660 nm (more available) → becomes PFR (more produced) ○ PFR absorbs far-red light at 730 nm (less available) → becomes PR (less produced) ○ PFR converted back into PR in the dark (because no light)

● Effect of phytochrome on flowering (PFR promotes flowering in long-day plants, inhibits in short-day plants) ○ Long-day plants: PFR remains (after short night) → binds to receptor → flowers ○ Short-day plants: very little PFR remains (after long night) → not enough to inhibit → flowers

Inducing plants to flower out of season

● Growers manipulate the length of days and nights to force flowering ● E.g. Chrysanthemums

○ Chrysanthemums don’t flower during summer ○ Plant is covered in opaque black cloth for 12-15 hours → floral initiation

Draw an animal pollinated flower

● Petals: attract pollinators ● Sepal: cover and protect developing flower (before blooming) ● Stamen (male):

○ Anther: contains pollen ○ Filament: supports anther

● Pistil (female): ○ Stigma: Pollen landing site ○ Style: has tubes to transport pollen from stigma to

ovary ○ Ovary: contains ovules ○ Ovules: contains egg nuclei, develops into seeds

Mutualism between flowers and pollinators

● Sexual reproduction in plants require transfer of pollen from one flower to another flower’s stamen ● Mutualism: association between two organisms (benefit from each other)

Pollination, fertilisation and seed dispersal

● Process 1. Pollination: pollen lands on stigma 2. Pollen tube carries male gametes to the ovary (through style) 3. Fertilisation: Ovules fuse with pollen 4. Fertilised ovule develops into seed, ovary into fruit 5. Seed dispersal: seed travels long distances (prevent competition between offspring and parent)

Drawing structure of seeds

● Cotyledon: contains nutrients, serves as embryo leaves (dicotyledons have 2, monotorylegons 1)

● Testa: Seed coat ● Micropyle: hole through which pollen fertilises ovule (before

fertilisation) ● Radicle: embryo root ● Plumule: embryo shoot

Factors affecting germination

● Water ○ Many seeds are dehydrated (cells need water) ○ Some have hormones which inhibits germination (water washes off hormone) ○ Growth of embryo and shoot requires water

● Oxygen for aerobic respiration to produce ATP ● Warmth for metabolic enzymes ● Gibberellin (hormone):

○ Stimulates mitosis and cell division ○ Stimulates production of amylase (break down starch into maltose for further catabolism)