Plant Structure, Growth, and Reproduction: Topic 9.3 and 9.4 Flashcards
Explain apical growth in plant shoots, including the role of auxin.
From the Notes:
- Occur at apex/tips of stems and roots
- Add vertical growth to roots and stems (increased light/ CO2 and nutrient absorption)
- Produce new leaves and flowers/ fruits
- Develops into primary xylem and phloem
- Apical meristems (at the tips of the shoot and roots) allow the stem and roots to lengthen
- Growth at these regions is due to cell elongation AND mitosis
- The shoot of the plant is the stem together with the leaves.
- At the very top of the shoot (the “shoot apex”) there is a meristem called the shoot apical meristem
- Cells in the shoot apical meristem actively carry out mitosis & cell division repeatedly to generate the cells needed for extension of the stem and development of the leaves (and flowers).
Understand that plant hormones control growth in the shoot apex
- Plant hormones are called phytohormones
- Plant hormones released from the shoot apex control stem growth and the formation of new nodes
- One of the main groups of these hormones, AUXINS, are produced by cells in the shoot apex (in the apical meristem)
- Auxin (IAA - Indole-3-acetic acid) promotes growth in the shoot apex by promoting cell division AND cell elongation by changing patterns of gene expression!
- Auxin also promotes growth in the shoot apex by inhibiting growth in the lateral/ axillary buds (a condition called apical dominance)
From a Markscheme:
a. growth in shoots is indeterminate/unlimited ✔
b. produces stem and leaves ✔ For mpb, both stem and leaves are needed and buds or branches should not be accepted as alternatives.
c. growth/cell growth/cell elongation controlled/affected by hormones/auxin/IAA ✔
d. new/extra cells produced by mitosis/cell division / apex is a meristem ✔
e. tropism/phototropism / grows towards the sun/light ✔
f. auxin moved away from sunny side/to shady side of shoot «apex»
OR
auxin efflux pumps set up concentration gradients ✔
Explain the process of phototropism in plants, including the role of auxin.
From the Notes:
- Phototropism is a positive tropism in plant stems (they grow toward the light) and is a negative tropism in plant roots (they grow away from the light - into the ground)
- Phototropism in plants is controlled by auxin
Understand that auxin efflux pumps can set up concentration gradients of auxin in plant tissue
- Auxin promotes growth by lengthening plant cells and altering gene expression (produced by the shoot tip in coleoptiles = the protective sheaths around apical meristems)
- Normally, auxin is distributed evenly throughout plant cells in stems so plant stems grow evenly
- Auxin efflux pumps (membrane proteins) can actively transport (using ATP) auxin out of cells to redistribute it within plant tissues, creating auxin concentration gradients and causing certain plant tissues to contain higher concentrations of auxin than others.
- In stems/ shoots, high concentrations of auxin stimulate cell elongation/ promote growth (cells become larger/ longer and stems grow) by changing patterns of gene expression!
- Note that auxin has the opposite effect in roots (causing a negative phototropism so that roots grow down into the soil and AWAY from light)
Understand that auxin influences cell growth rates by changing the pattern of gene expression: Auxin and Phototropism
- IN STEMS: If one side of a plant receives more light (detected by shoot tip), auxin efflux pumps redistribute auxin so that the SHADED side of the plant has a HIGHER concentration of it.
- More auxin = more growth/ cell elongation, so stem bends on shaded side (cells elongate) and plant stem grows toward light. HOW?
- Auxin activates proton pumps in cells; Protons (H+ ions) are pumped from cytoplasm to cell walls which decreases the pH, loosening cellulose fibers by breaking the bonds between them (so they become more flexible/ elastic)
- Auxin also upregulates the expression of expansin genes, further increasing elasticity of cell walls, so cells on SHADED side of stem elongate (turgor pressure), and plant stem bends TOWARD the light (positive phototropism)
From a Markscheme: (Explain the role of auxin in phototropism)
a. auxin/ IAA is a plant hormone;
b. produced by the tip of the stem/shoot tip;
c. auxin efflux pumps set up concentration gradients;
d. causes transport of hydrogen ions from cytoplasm to cell wall;
e. decrease in pH / H+ pumping breaks bonds between cell wall fibres;
f. makes cell walls flexible/extensible/plastic/softens cell walls;
g. auxin makes cells enlarge/grow;
h. gene expression also altered by auxin to promote cell growth;
i. (positive) phototropism is growth towards light;
j. shoot tip senses direction of (brightest) light;
k. auxin moved to side of stem with least light/darker side causes cells on dark side to elongate/cells on dark side grow faster;
l. Accept clearly annotated diagrams for phototropism marking points.
Discuss (pros and cons) the use of micropropagation to reproduce plants.
Pros:
- Rapid Bulking:
- Stock plants with desirable characteristics are cloned, which maintains ALL of the desirable characteristics (because new plants are genetically identical to original plants) - more reliable than selective breeding
- Used to quickly create large numbers of genetically modified plants too
- Production of Virus-Free Strains
- Plant viruses can wipe out crops, cause famine, and harm economies
- Plant viruses spread via vascular tissue, which meristems do not contain, so using micropropagation can quickly produce large numbers of virus-free plants from the original non-infected meristems
- Propagation of Rare/ Endangered Species
- Used to increase numbers of rare or endangered species or species that are difficult to breed (such as orchids, which have very small seeds), or species that are in “high commercial demand”
Cons:
- Expensive
- High-tech
Draw and Label a half-view of an animal pollinated flower (and know the functions of each part too).
Look at 9.4 Plant Reproduction notes! (Slide 3) - https://docs.google.com/presentation/d/1VN1bW5SDPC75xP8XaUbDB3TeXkYURbWZP7cQydoaf7w/edit#slide=id.p3
Stigma: Pollen “landing pad” (sticky)
Style: Supports the stigma
Ovary: Produces female sex cells (ovules) by meiosis
Anther: Produces male sex cells (pollen) by meiosis
Filament: Holds up/ supports the anther
Petals: Attract pollinators
Sepals: Protect developing flower (while in the bud)
Outline the relationship between flowers and pollinators (mutualistic).
Understand that most flowering plants (~80%) use mutualistic relationships with pollinators in sexual reproduction
- Animal-pollinated plants have large, brightly colored (“showy”), scented flowers (to attract birds, bats, bees/ other insects etc.), and “sticky” pollen grains (to adhere to pollinator bodies)
- Pollinators have a mutualistic relationship with flowering plants (both benefit - animal gets nectar/ pollen and flower is pollinated/ fertilized) and most flowering plants have coevolved with pollinator species
Distinguish between pollination and fertilization in flowering plants.
Pollination: When pollen (from the anther) is transferred to/ placed on the stigma of a flower (by means/ vectors of animals, wind, or water)
Fertilization: The fusion of haploid nuclei (the male pollen grain fuses with the female ovule to produce a diploid zygote)
Describe the conditions needed for seed germination.
From the Notes:
Necessary favorable conditions:
- Water (to rehydrate the seed, triggers gibberellin production, and triggers further metabolic reactions)
- Oxygen (for aerobic respiration – ATP!)
- pH (optimum in soil/ surrounding environment for enzyme function)
- Ideal temperature (for optimal enzyme activity)
- Typically warmer temps indicate optimal germination (spring)
Additional favorable conditions required by some seeds:
- Fire (sprouting occurs after intense heat exposure - fires clear areas for new seeds to germinate with less competition)
- Freezing (germination occurs in spring, only after exposure to winter/ extreme cold)
- Digestion (enzymes in animal digestive tracts help remove seed coat)
- Washing (inhibitors on some seeds must be washed away before germination can occur)
- Scarification (seed sprouts after seed coat is weakened by physical damage)
From a Markscheme:
a. water needed to rehydrate the seed;
b. gibberellin released / active after water absorbed;
c. gibberellin needed to produce amylase;
d. water needed to allow substances inside the seedling to be transported;
e. oxygen needed for (aerobic) cell respiration;
f. warmth needed to speed up metabolism/enzyme activity;
g. warmth indicates that it is a favourable season for germination/spring;
h. some seeds need a cold period to stimulate germination;
i. some seeds need fire to stimulate germination;
j. some seeds need to pass through an animal (gut) to stimulate germination;
Explain the process of photoperiodism in long-day and short-day flowering plants (including the role of phytochrome).
From the Notes:
- Flowering in (many, but not all) plants is controlled by light (photoperiodism – a plant’s response to the lengths of the night)
- Different plants flower during different seasons (they need to flower when their pollinators and resources are most active/ abundant), so flowering occurs due to different lengths of night (critical variable is length of darkness):
A. Long-day plants (short night plants): flower when days are longer and nights are shorter (midsummer)
B. Short-day plants (long night plants): flower when days are shorter and nights are longer (spring/ autumn) - Flowering in these different types of plants is controlled by a blue-green pigment (absorbs wavelengths of light) called phytochrome.
- Phytochrome exists in two different forms:
1. Pr (biologically inactive form): in the day, absorbs red light (660nm), which rapidly converts it into Pfr
2. Pfr(biologically active form): in the day, absorbs FAR-red light (730nm), which rapidly converts it back into Pr - Note: Pfr reverts back to Pr form in the dark (less energy to maintain), but very slowly
- Sunlight contains more red light (than far-red light), so * Pfr is more predominant during the day and Pr form is more predominant at night
- At night, Pfr produced during the day is SLOWLY converted back to Pr.
- The LENGTH of the NIGHT determines how much Pfr will remain in the plant from the day.
- The amount of Pfr remaining in the plant at the end of the night allows the plant to “time” how long the night was.
- The amount of Pfr in a plant at the end of the night (based on the LENGTH OF THE NIGHT) promotes flowering by activating specific genes in shoot apex cells. * These changes in gene expression (DNA transcription) allow flowers to be produced.
- In long-day (short night) plants: more Pfr promotes flowering – when night is LESS than a certain critical length, Pfr levels are higher due to more sunlight exposure and shorter nights; high levels of Pfr activate genes to promote flowering
- In short-day (long night) plants: more Pfr inhibits flowering (by inhibiting gene expression) – when night is GREATER than a certain critical length, Pfr levels are lower due to less sunlight exposure and longer nights; genes no longer inhibited = promotes flowering
From the Worksheet:
a. flowering affected by light;
b. phytochrome;
c. exists in two (interconvertible) forms/Pfr and Pr;
d. Pr (red absorbing/660 nm) converted to Pfr (far-red/730 mm absorbing) in red or day light;
e. sunlight contains more red than far red-light so Pfr predominates during the day;
f. gradual reversion of Pfr to Pr occurs in darkness;
g. Pfr is active form / Pr is inactive form;
h. in long-day plants, flowering induced by dark periods shorter than a critical length / occurs when day is longer than a critical length;
i. enough Pfr remains in long-day plants at end of short nights to stimulate flowering;
j. Pfr acts as promoter of flowering in long-day plants;
k. short-day plants induced to flower by dark periods longer than a critical length/days shorter than a critical value;
l. at end of long nights enough Pfr has been converted to Pr to allow flowering to occur;
m. Pfr acts as inhibitor of flowering in short-day plants;
Outline how knowledge of photoperiodism can be used to induce short-day plants to flower out of season.
Application: Methods used to induce short-day plants to flower out of season
- Gardeners and other horticulturalists can manipulate the flowering of certain plants by controlling their exposure to light
- To do this, a specific amount of UNINTERRUPTED night length is provided to the plant (depending on which type of plant it is)
Long-day plants (spinach, carnations, lettuce): to promote flowering, plants are purposefully exposed to a light source or a burst of light during the night (lengthening light exposure and minimizing time of darkness)
Short-day plants (chrysanthemums, poinsettias): to promote flowering, plants are purposefully covered with a black cloth for 12-15 hours a day until flower buds begin to grow/ show color. Note that if the period of darkness is interrupted with a burst of light, short-day plants will not flower.
