Topic 9: Plant Science HL

9.1.1 Draw and label plan diagrams to show the distribution of tissues in the stem
and leaf of a dicotyledonous plant. Either sunflower, bean or another dicotyledonous plant with similar tissue distribution should be used. Note that plan diagrams show distribution of tissues (for example, xylem, phloem) and do not show individual cells. 

DIAGRAM PRESENT IN MARKSCHEME:
PLAN DIAGRAM OF SUNFLOWER STEM

Award [1] for each of these structures clearly drawn and labelled.epidermis shown on the outside with thickness less than 10% of overall diameter;
cortex labelled between the outer layer of the stem and the vascular bundles;
xylem shown on the inner side of the vascular bundles;
phloem shown on outer side of the vascular bundles;
vascular bundle with some way of indicating the entire structure;
pith shown in centre;
cambium shown between xylem and phloem;

9.1.2 Describe the differences in the structures of dicotyledonous plants and monocotyledonous plants.

Monocotyledonous plants	Dicotyledonous plants
Leaves with parallel veins	Leaves with branched net of veins
One seed leaf	Two seed leaves
Scattered vascular bundles	Vascular bundles around edge
Adventitious roots	Branched tap roots
Petals usually in groups of 3	Petals usually in groups of four or fives

monocotyledon seeds contain one cotyledon/seed leaf;
dicotyledon seeds contain two cotyledons/seed leaves;
monocotyledons have parallel veins;
dicotyledons have net-like veins;
monocotyledon stems have scattered vascular bundles;
dicotyledon stems have vascular bundles around edge;
monocotyledon roots are adventitious/fibrous;
dicotyledon roots are from radicle/tap root/branched;
monocotyledon flower parts/petals are (usually) in threes;
dicotyledon flower parts/petals are (usually) in fours or fives;

Structure	Monocotyledonous	Dicotyledonus
leaf	parallel veins	branched (net of) veins;
seed	one cotyledon	two cotyledons;
flower	floral parts in multiple of 3	floral parts in multiple of 4 or 5;
stem	scattered vascular bundles	ring of vascular bundles around central pith;
root	adventitious roots	branched tap roots;

9.1.3 Explain the relationship between the distribution of tissues in the leaf
and the functions (absorption of light, gas exchange, support, water conservation, and the transport of water and products of photosynthesis)  of these tissues in dicotyledonous plants






9.1.4 Identify modifications of roots, stems and leaves for different functions: bulbs, stem tubers, storage roots and tendrils.




9.1.5 State that dicotyledonous plants have apical and lateral meristems.
9.1.6 Compare growth due to apical and lateral meristems in dicotyledonous
plants.




9.1.7 Explain the role of auxin in phototropism as an example of the control
of plant growth.

Auxin is a plant hormone produced by the tip of the stem, causing transport of hydrogen ions from cytoplasm to cell wall. The H+ ions break bonds between cell wall fibres, making the wall more flexible. Hormone auxin causes cell growth due to alteration of gene expression.
Auxin moved to side of stem with least light, causing those cells to grow faster.
Positive phototropism is growth towards light, shoot tip sense direction of brightest light.


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;
auxin makes cells enlarge/grow; gene expression also altered by auxin to promote cell growth;
auxin moved to side of stem with least light/darker side causes cells on dark side to elongate/cells on dark side grow faster;
(positive) phototropism is growth towards light; shoot tip senses direction of (brightest) light;
auxin produced at apical meristem / tip;
transported to growing area / zone of cell growth;
lateral transport to cells on shade side;
results in cell expansion;
shoot “grows” towards light source;
experimental detail;

Transport in angiospermophytes
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.

• Water, which passes through cytoplasm of endodermis, is absorbed by osmosis. Thus, solute concentration inside root is higher than in outside soil.
• Ions are actively, transport

water is absorbed by osmosis; water has to pass through cytoplasm of endodermis;
solute concentration inside the root is higher than in the soil / outside;
roots have a large surface area (in relation to their volume);
branching / lateral roots (increases the surface area); root hairs increase the surface area;
due to active transport of ions into the root;
apoplastic and symplastic transport across the root;
cortex cell walls (increase the surface area);
apoplastic route is through the cell walls (and intercellular spaces);
symplastic route is through the cytoplasm (and plasmodesmata);
water carried up stem through xylem tubes.
water movement in xylem due to pulling force / transpiration pull;
cohesion between water molecules;

9.2.2 List ways in which mineral ions in the soil move to the root.
There are three processes: diffusion of mineral ions, fungal hyphae (mutualism), and mass flow of water in the soil carrying ions.

• Diffusion of mineral ions
• Fungal hyphae
• Mass flow of water in the soil carrying ions

9.2.3 Explain the process of mineral ion absorption from the soil into roots by active transport.

• Concentration of ions in roots exceeds that of ions in soil
• Roots actively pump H+ out of their own tissue into the soil, creating an electrochemical gradient.
• Inside of root cells more negative

9.2.4 State that terrestrial plants support themselves by means of thickened cellulose, cell turgor and lignified xylem.

9.2.5 Define transpiration.
Loss of water vapour from leaves and stems of plants.

Aim 7: Data logging with pressure sensors, humidity, light or temperature probes to measure rates of transpiration can be performed.

9.2.6 Explain how water is carried by the transpiration stream, including the
structure of xylem vessels, transpiration pull, cohesion, adhesion and
evaporation.
Limit the structure of xylem vessels to one type of primary xylem.

transported in xylem (vessels);
passive / no energy used by plants;
evaporation / transpiration causes low pressure / suction / pull;
transpiration stream / continuous column of water from roots to leaves;
water molecules are cohesive (so transmit the pull) / hydrogen bonding;
root pressure can move water up the plant;

apoplastic pathway is through cell walls;

9.2.7 State that Guard cells can regulate transpiration by opening and
closing stomata. 

9.2.8 State that the plant hormone abscisic acid causes the closing of stomata.

9.2.9 Explain how the abiotic factors light, temperature, wind and humidity,
affect the rate of transpiration in a typical terrestrial plant.

light: [2 max]
causes stomatal opening in morning, increasing transpiration;
increasing light increases transpiration;
because stomatal opening increases;
no light causes stomatal closure, reducing transpiration;
wind: [3 max]
removes water vapour from around leaf;
increases water vapour / humidity gradient so increases transpiration;
increases transpiration / lack of wind can reduce transpiration;
no increase in transpiration if humidity is 100%;
humidity : [3 max]
high humidity reduces water vapour gradient so lowers transpiration;
high humidity lowers transpiration rate;
lowering humidity can increase transpiration rate (to a point);

To receive full marks responses must address all three parts.

less transpiration as (atmospheric) humidity rises; smaller concentration gradient (of water vapour); more transpiration as temperature rises; faster diffusion / more kinetic energy (of water molecules); faster evaporation (due to more latent heat available); more transpiration as wind (speed) increases; humid air / water vapour blown away from the leaf; increasing the concentration gradient (of water vapour); more transpiration in the light; due to light causing stomata to open; wider opening with brighter light hence more transpiration; CAM plants opposite; narrower stomata with high carbon dioxide concentration hence less transpiration;

at very low humidity stomata may shut down;

9.2.10 Outline four adaptations of xerophytes that help to reduce transpiration.
These could include: reduced leaves, rolled leaves, spines, deep
roots, thickened waxy cuticle, reduced number of stomata, stomata
in pits surrounded by hairs, water storage tissue, low growth form,
CAM (crassulacean acid metabolism) and C4 physiology.

• Reduced leaves, rolled leaves and spines (actual leaf is a spine) to reduce evaporation surface
• Deep roots to reach for underground water
• Thickened waxy cuticle to reduce evaporation from contact with air
• 
  

xerophytes are plants that live in dry conditions;
reduced leaves/spines to prevent water loss (by transpiration);
rolled leaves to prevent water loss / stomata on the inside / sunken stomata;
thick waxy cuticle/hairs on leaves to prevent water loss (by transpiration);
reduced stomata to prevent water loss (by transpiration) / stomata on
one side of leaf;
deep/widespread roots to obtain more water;
special tissue for storing water;
take in carbon dioxide at night / CAM plant to prevent water loss;

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).

Reproduction in angiospermophytes

9.3.1 Draw and label a diagram showing the structure of a dicotyledonous animal-pollinated flower. Limit the diagram to sepal, petal, anther, filament, stigma, style

and ovary.

LOOK AT STUDY GUIDE!

9.3.2 Distinguish between pollination, fertilization and seed dispersal.

9.3.3 Draw and label a diagram showing the external and internal structure of a named dicotyledonous seed. The named seed should be non-endospermic. The structure in the diagram should be limited to esta, micropyle, embryo root, embryo shoot and cotyledons.

9.3.4 Explain the conditions needed for the germination of a typical seed.

water needed to rehydrate the seed; gibberellin released / active after water absorbed; gibberellin needed to produce amylase; water needed to allow substances inside the seedling to be transported; oxygen needed for (aerobic) cell respiration; warmth needed to speed up metabolism/enzyme activity; warmth indicates that it is a favourable season for germination/spring; some seeds need a cold period to stimulate germination; some seeds need fire to stimulate germination; some seeds need to pass through an animal (gut) to stimulate germination;

9.3.5 Outline the metabolic processes during germination of a starchy seed. Absorption of water precedes the formation of gibberellin in the embryo’s cotyledon. This stimulates the production of amylase, which catalyses the breakdown of starch to maltose. Maltose diffuses to the embryo for energy release and growth. 

• Absorption of water
• Embryo increases respiration and secretes gibberellin 
• This stimulates amylase production
• Amlyase catalyses breakdown of starch to maltose
• Maltose diffuses to embryo for energy release and growth

absorption of water; (embryo) increases respiration; (embryo) secretes gibberellin to (aleurone layer); (stimulates) production of amylase; digestion of starch to smaller sugars / maltose; mobilize to tissues / transport of foods / nutrients to embryo;
9.3.6 Explain how flowering is controlled in long-day and short-day plants, including the role of phytochrome. Limit this to the conversion of Pr (red absorbing) to Pfr (far-red absorbing) in red or white light, the gradual reversion of Pfr to Pr in darkness, and the action of Pfr as a promoter of flowering in long day plants and an inhibitor of flowering in short-day plants.


Flowering is controlled in long-day and short-day plants. Flowering is affected by light, i.e., phytochrome which exists in two forms: Pfr is active form and Pr is inactive form.
Red-absorbing Pr converted to far-red absorbing Pfr in red/day light
Sunlight contains more red than far-red light so Pfr predominates during the day.
Pfr is reversed to Pr during darkness.

flowering affected by light;
phytochrome; exists in two (interconvertible) forms/Pfr and Pr; Pfr is active form / Pr is inactive form;
Pr (red absorbing/660 nm) converted to Pfr (far-red/730 nm absorbing) in red/day light;
sunlight contains more red than far red-light so Pfr predominates during the day;
gradual reversion of Pfr to Pr occurs in darkness;
in long-day plants, flowering induced by dark periods shorter than
a critical length / occurs when day is longer than a critical length;
enough Pfr remains in long-day plants at end of short nights to stimulate flowering;
Pfr acts as promoter of flowering in long-day plants;
short-day plants induced to flower by dark periods longer than a
critical length/days shorter than a critical value;
at end of long nights enough Pfr has been converted to Pr to allow flowering to occur;
Pfr acts as inhibitor of flowering in short-day plants;