Transcript Plant Science - HS Biology IB

Topic 9: Plant Science
9.1: Plant Structure and Growth
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9.1.1:Draw and label plan diagrams to show the distribution of tissues in the
stem and leaf of a dicotyledonous plant.
9.1.1
IB Question: The main parts of growing plants are roots, stems and leaves. Draw a plan
diagram to show the arrangement of tissues in the stem of a dicotyledonous plant. [5]
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9.1.2:Outline three differences between
the structures of dicotyledonous and
monocotyledonous plants.
Describe the differences in the structures
of dicotyledonous plants and
monocotyledonous plants. [5]
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9.1.3: Explain the relationship between the
distribution of tissues in the leaf and
the functions of these tissues.
Draw a labeled diagram showing the
tissues present in a dicotyledonous leaf. [4]
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9.1.3: Explain the relationship between the distribution & function of leaf tissues
Leaf
tissues
Function
Epidermis
The epidermis is an outer support tissue that holds the leaf together. The
upper epidermis is exposed to direct sunlight so it has a waxy coating (called
the waxy cuticle) to prevent water loss. The lower epidermis has specialized
guard cells. Guard cells form adjustable pores that control the rate of
transpiration (water loss) as well as gas exchange.
Pallisade
mesophyll
The center of a leaf is composed of mesophyll, which has 2 layers: palisade
mesophyll and spongy mesophyll. Palisade mesophyll is an upper layer of
elongated cells that contain many chloroplasts to absorb light and carry out
photosynthesis.
Spongy
mesophyll
The spongy mesophyll is a lower layer containing loosely packed cells. The
loose arrangement of cells allows water, O2 and CO2 to diffuse easily. Rapid
diffusion is necessary for transporting CO2 for the light independent reactions
of photosynthesis.
Vascular
tissue
Leaves contain vascular bundles in the spongy mesophyll layer. Vascular
bundles contain xylem tissue and phloem tissue. Water moves through xylem
tubes and dissolved glucose moves through phloem tubes. The phloem
transports the products of photosynthesis to other parts of the plant
9.1.4:Identify modifications of roots, stems
and leaves for different functions:
bulbs, stem tubers, storage roots and
tendrils.
9.1.5: STATE: Dicotyledonous plants have apical and lateral meristems
9.1.6: Compare growth due to apical and
lateral meristems in dicotyledonous
plants.
Lateral meristems
Apical meristems
Occur in the tips of stems and
roots
Occur between xylem and phloem in stems
Produces soft tissues
Produces hard xylem tissue: wood
Lengthens roots and stems
Widens stems to support the weight of tall
plants
Allows plants to develop special Allows trees to grow tall, helping them to
structures like leaves and flowers compete effectively for light.
Found in all phyla of plants
Absent in mosses and horsetails.
9.1.7: Explain the role of auxin in
phototropism as an example of the
control of plant growth.
Explain the role of auxin in phototropism.
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9.1.7: Explain the role of auxin in phototropism as an example of plant growth
Plants use hormones to control the growth of roots and stems. When a plant releases a
growth hormone in response to an external stimulus we call the resulting directional
growth a tropism.
One type of tropism is phototropism: growth in response to light. Phototropism may be
either positive (towards the light) or negative (away from the light).
Phototropism requires the absorption of light by proteins known as phototropins.
Phototropins change to a new conformation (a new shape) when they absorb certain
wavelengths of light. The new shape causes phototropins to act as ‘on switches’ for a
gene that regulates the activity of auxins.
Auxins cause cells to become longer. Therefore, by releasing auxins on one side of a
stem but not on the other side, a stem will bend because one side becomes longer than
the other.
When a stem detects directional light it moves auxins from its sunny side to its shady
side, which causes the shady side to bend toward the light. Bending toward light allows
plants to absorb more sunlight and be able to photosynthesize at a faster rate.
Auxins cause cells to become larger in the following way: 1) they cause cells to actively
transport hydrogen ions out of the cell, making the outside acidic; 2) the acid outside the
cell makes the cell wall softer; 3) softer cell walls make the cells more stretchable; and
4) stretchy cells are bigger because the internal pressure inside the cell causes the cell
wall to bulge out.
auxin is a plant hormone;
produced by the tip of the stem/shoot tip;
causes transport of hydrogen ions from cytoplasm to cell wall;
decrease in pH / H+ pumping breaks bonds between cell wall fibres;
makes cell walls flexible/extensible/plastic/softens cell walls;
auxin makes cells enlarge/grow;
gene expression also altered by auxin to promote cell growth;
(positive) phototropism is growth towards light;
shoot tip senses direction of (brightest) light;
auxin moved to side of stem with least light/darker side
causes cells on dark side to elongate/cells on dark side grow faster; [8 max]
Accept clearly annotated diagrams for phototropism marking points.
9.2: Transport in
angiospermophytes
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9.2.1: Outline how the root system provides a large surface area for mineral ion and
water uptake by means of branching and root hairs.
9.2.1
Outline the adaptations of plant roots for
absorption of mineral ions from the soil. [5]
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9.2.2: List ways in which mineral ions in the
soil move to the root.
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Mineral ions move through soil to the
roots of plants by one of three means:
1) Diffusion
2) Mass flow of water in the soil carrying
ions
3) Mutualistic association with hyphae of
fungi
9.2.3: Explain mineral ion absorption from the soil into roots by active
transport
Plants take up mineral ions by active transport. In active transport, mineral
ions are moved against a concentration gradient, which requires: 1) numerous
mitochondria in root hair cells for ATP production; 2) protein channels in the
cell membrane for active transport; and 3) oxygen in the soil that is absorbed
by root hairs for cell respiration.
In order for mineral ions to be pumped into the roots the mineral ions must
make physical contact with protein pumps on cell membranes of root hair
cells.The mineral ions move into contact with root hair proteins in one of two
ways: 1) diffusion and 2) mass flow. Mass flow is when draining water carries
minerals.
Diffusion and mass flow are slow processes because mineral ions bind to the
surface of soil particles. Therefore many plants evolved mutualistic
relationships with fungi to improve the rate of mineral absorption. The long
thread-like hyphae of the fungus intertwine with the root hairs of the plant, and
extend into the soil. Hyphae are highly efficient at absorbing mineral ions from
the soil, which they share with the plant roots in exchange for sugars. The
photo below shows the thread-like hyphae of a fungus growing amongst a
plant's root hairs
9.2.4: STATE: Terrestrial plants support themselves by
means of thickened cellulose, cell turgor and lignified xylem.
9.2.5 : Transpiration is the loss of water vapour from he leaves and stems of plants
transpiration pull
adhesion
Root pressure
9.2.6: Explain how water is carried by
the transpiration stream, including
the structure of xylem vessels,
transpiration pull, cohesion, adhesion
and evaporation.
Describe how water is carried by the
transpiration stream. [7]
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Explain the effect of light intensity and
temperature on the rate of photosynthesis.
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Photosynthesis and transpiration occur in
leaves. Explain how temperature affects
these
processes. [8]
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Define the term transpiration and explain
the factors that can affect transpiration in a
typical terrestrial plant. [9]
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9.2.6: Explain how water is carried by the transpiration stream
The xylem is a system of long hollow tubes responsible for replacing water lost during
transpiration and photosynthesis. The xylem is made of two kinds of cells: tracheids and
vessels. Xylem cells die before they are functional: after they die they become long,
narrow tubes with pores at each end that allow water to pass through them.
The xylem sap moves from roots, through the stem, to the leaves without any energy
being spent by the plant.
Three processes cause water to rise up the xylem tube:
Root pressure: Water moves into roots by osmosis because the roots have high
concentrations of solute. This causes a positive pressure that forces sap up the xylem
towards the leaves. Root pressure is highest in the morning before the stomata open
and allow transpiration to begin.
Capillary action: The xylem is a long tube that is microscopically thin. When water
molecules contact the surface of the xylem there is adhesion. Adhesion tends to pull
water molecules upward by a process called capillary action.
Transpiration pull: When water molecules evaporate from leaves the water potential
drops at the stomata. The low pressure then pulls new water molecules towards the
stomata from the xylem vessels. As these water molecules move they pull on water
molecules behind them due to cohesion (caused by hydrogen bonding). The pull is
transmitted from one water molecule to the next, all the way to the roots.
9.2.7: STATE: Guard cells can regulate transpiration by opening and closing stomata.
9.2.8: STATE: The plant hormone abscisic acid causes closing of the stomata.
9.2.9
9.2.9: Explain how abiotic factors affect transpiration rate in a terrestrial plant
Transpiration is the loss of water (by evaporation) from the leaves and stems of plants.
In a typical terrestrial mesophytic plant, the rate of transpiration:
Decreases: when humidity increases because at high humidity, the air is saturated with
water so evaporation stops.
Increases: when light intensity increases because more stomata open in strong light to
maximize the rate of photosynthesis.
Increases: when temperature increases because water molecules evaporate faster
creating negative pressure at the stomata.
Increases: as wind speed increases because air movement carries water vapor away
from stomata creating negative pressure at the stomata.
9.2.10: Outline four adaptations of
xerophytes that help to reduce
transpiration.
Outline adaptations of xerophytes. [4]
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9.2.11: Outline the role of phloem in active
translocation of sugars (sucrose)
and amino acids from source
(photosynthetic tissue and storage
organs) to sink (fruits, seeds, roots).
9.3: Reproduction in angiospermophytes
9.3.1: Draw and label a diagram showing the structure of a
dicotyledonous animal-pollinated flower.
9.3.2Distinguish between pollination,
fertilization and seed dispersal.
Pollination is the attachment of a pollen grain on the stigma of a flower, by wind or by
an animal. After pollination, a pollen grain grows a long pollen tube that stretches down
the style to the ovary. The pollen tube enters the ovary through a small opening, the
micropyle, and releases sperm to fertilize the eggs.
Fertilization is the fusion of an egg and sperm to form a zygote. A zygote develops
into an embryo, and in flowering plants the embryo is packaged in a seed coat with food
reserves.
Mature seeds typically spread out from their parent plant, a process called
seed
dispersal. Plants have evolved diverse methods of seed dispersal: some seed pods
are explosive, some seeds are attached to sails that blow in the wind, some seeds spin
like helicopter blades, and others rely on birds to eat them and spread them in their
droppings.
9.3.3: Draw and label a diagram showing
the external and internal structure of
a named dicotyledonous seed.
9.3.4: Explain the conditions needed for the germination of a typical seed
Mature seeds are dormant with very few metabolic processes going on.
The resumption of growth in a seed is called germination.
Seeds need oxygen, water and warmth to germinate.
Without water, enzymes are not activated.
Without oxygen, cellular respiration isn't possible.
Warmth is also important for germination because the
enzymes involved in growth are more active at warmer
temperatures.
Germination is usually triggered by a change in the environment; e.g., warmer
temperature, wetter soil, erosion of the seed coat by fire, etc.
Explain the conditions that are needed to
allow a seed to germinate. [5]
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9.3.5: Outline the metabolic processes during germination of a starchy seed
1. Absorption of Water
2. Plant hormone called gibberellin is produced in the cotyledons of
the seeds.
3. Gibberellin stimulates the production of amylase.
4. Amylase catalyzes the digestion of starch into maltose.
Maltose is transported to grow plant in different regions of
seedling including embryo root and the embryo shoot.
5. Maltose is hydrolysed into glucose
used in aerobic cell respiration as a source of energy
used to synthesize cellulose (which is necessary to produce the
cell wall of new cells) or substances needed for growth.
When the leaves of seedling reach the light, it can start processing
photosynthesis which can then supply the seedling with foods.
Draw the external and internal structure of
a named dicotyledonous seed. [4]
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9.3.5: Explain how flowering is controlled
in long-day and short-day plants,
including the role of phytochrome.
9.3.6: Explain how flowering is controlled in long-day and short-day plants
Phytochrome is a protein pigment found in most plants. It acts as a photoreceptor, which
means it detects light.
When PR absorbs red light (660 nm) it gets converted into PFR; and when PFR absorbs
far-red light (730 nm) it switches back to PR.
Switching to PFR from PR is a fast process; but changing from PFR to PR is a slower
process. Thus, in sunlight, PR is quickly converted into PFR; but at night PFR is slowly
converted into PR.
This difference in conversion rates means that PFR levels are highest in plants at the
peak of summer, when day-length is greatest. Thus plants can use PFR levels like a
calendar, to determine the date of mid-summer.
Long-day plants
Plants that start to flower in mid-summer are called long-day plants. Long-day plants use
PFR to trigger the flowering process.
In mid-summer, nights are too short to convert all of the PFR into PR. This results in
many PFR proteins becoming bound to receptor proteins, which in turn ‘turn on’ genes
that produce flowers.
Short-day plants
Plants that start to flower in autumn are called short-day plants. Short-day plants use
PFR to inhibit the flowering process.
Thus, near mid-summer, the protein receptors of short-day plants act to ‘turn off’ the
flower-producing genes. By autumn, day-length is short so PFR levels drop too low to
inhibit flowering, thus flowering begins.
9.3.6
730 nm
660 nm
FAST (in light)
SLOW (in darkness)
High Pfr concentration is an inhibitor in
short day plants but will allow flowering
when levels Pfr drop to a “critical” level
High Pfr concentration is the trigger for
flowering in long day plants by turning on genes
for flowering
In Long Day Plants Pfr accumulates and acts as a transcription factor, turning
on the genes for flowering. E.g. Clover
When Pfr levels fall low enough (depending on the species) short day
plants will flower. E.g. Strawberry
Explain how flowering is controlled in longday and short-day plants. [7]
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