Topic 9 - Plant science

9.1 Plant Structure and Growth

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. They are sometimes called “low-power” diagrams.

*add endodermis

9.1.2 Outline three differences between the structures of dicotyledonous and monocotyledonous plants

Monocotyledonous	Dicotyledonous
A. seed has one cotyledon (seed leaf) B. leaves have parallel venation C. unbranched, fibrous root system D. flower parts occur in multiples of 3 E. vascular bundles in stem have a random/scattered arrangement	A. seed has two cotyledons (seed leafs) B. leaves have net-like venation C. branched, lateral root system from a tap root D. flower parts occur in multiples of 4/5 E. vascular bundles in stem occur in a ring arrangement

9.1.3 Explain the relationship between the distribution of tissues in the leaf and the functions of these tissues.

Tissue	Distribution/Structure/Function
Palisade mesophyll	Function: light absorption - Cells of the palisade mesophyll are tightly packed, filled with chloroplast, and located at the upper surface of the leaf to maximize light absorption. Since it is densely packed where the light intensity is the highest,it is the main photosynthetic tissue in plants.
Spongy mesophyll	Function: gas exchange within leaf - Cells of the spongy mesophyll are spaced out, creating many air spaces to enhance the diffusion of gases. It is located at the lower surface of the leaf so gases can diffuse to and from the stomata.It is centrally positioned near all tissues.
Stomata (and guard cells)	Function: diffusion of gases into leaf, transpiration - Gases (carbon dioxide and oxygen) enter and exit the leaf through the stomata. Water vapour is also lost through the stomata (transpiration). They are located at the bottom of the leaf to reduce water loss from the sun’s heat. - Guard cells control water loss/transpiration by maintaining control of stomatal opening and closing by turgor pressure. Guard cells will close stomata to reduce transpiration. Their thin lateral/radial walls of cellulose fibrous and thick dorsal and ventral walls allow opening and closing to occur.
Epidermis	Function: support, water conservation - Protects the inner mesophyll layers. Therefore they are structurally thick walled for strength, and transparent to allow the passage of light into the photosynthetic layers. - Secretes a lipid layer called the waxy cuticle, which prevents water loss due to evaporation from the sun’s heat. It is thicker on the upper surface where sunlight is more intense.
Vascular bundle (xylem and phloem)	Function: transport of water and products of photosynthesis - Water, organic molecules and ions are transported by the xylem and phloem. They are located in the middle of the leaf so that all cells may have the most efficient pathway of access to these cells at all times. Xylem: water, mineral salts, ions (to the leaves) Phloem: products of photosynthesis, sugars (to flowers/ new leaves/ roots/ stem/ fruit)

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

bulbs - modified leaves	A modified leaf used for food storage. Eg. a bulb of onion
stem tubers - modified stem	A modified stem used for food storage. The stems grow laterally underground from the tap root and appear as swollen structures filled with starch. Eg. potato
storage roots - modified root	A modified root that becomes swollen for food storage, where the main root becomes swollen with the food reserve. Eg. carrot
tendrils - modified stem/leaf (depends on plant)	Used for supporting the plant and allowing it to stay in a favourable upright position. Narrow outgrowths rotate through the air until they anchor to some solid support, allowing the plant to climb. Eg. tendrils in cucumbers

9.1.5 State that dicotyledonous plants have apical and lateral meristems.

Dicotyledonous plants have apical and lateral meristems (cells undergoing continuous cell divison).

9.1.6. Compare growth due to apical and lateral meristems in dicotyledonous plants.

Apical meristem

A. occurs at the tips of stems and roots (the “apex”) B. responsible for primary growth, which is growth in length and height of stems and roots C. produces epidermis, ground tissue, and primary xylem and phloem D. apical meristems are the product of embryonic cells E. timing of activity: produces initial tissue in growing plants from the outset of growth

Lateral meristem

A. occurs at the cambium, between xylem and phloem tissue within the vascular bundle B. responsible for secondary growth, which is growth in circumference/girth and strength of the stem C. mainly forms secondary xylem and phloem D. lateral meristems (cambium tissue) are the leftover cells from primary growth E. timing of activity: occurs more actively in older stems and roots in herbaceous plants, and from the outset of growth in woody plants

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

●     The stem tip of plants show a positive tropism to light (the plant grows in the direction of the light).

●     Auxin is a plant growth substance whose function is to promote cell elongation.

○     It does so by secreting hydrogen ions that diffuse through the cell wall of cells, which causes loosening of the cell wall and space for elongation.

●     Auxin is produced by meristems, so it occurs in highest concentrations at the tips of stems and roots.

●     Light inhibits the production of auxin.

●     The result of auxin inhibition due to light is that auxin collects on the dark side of the stem but it is not present on the illuminated side. Cells on the darker side of a plant are longer while cells on the lighter side of a plant are shorter, thus creating a bend in the stem that points towards the light source.

Extra Notes: Plant growth in light vs plant growth in dark

Plant growth in light	Plant growth in dark (plant growth in dark is called “etiolation”)
- shorter, sturdier plants with broader leaves and green colour - shorter because light inhibits production of auxin, and auxin is responsible for cell elongation - green because the plant photosynthesizes in the presence of light, so it has chloroplasts - sturdy because the plant receives the nutrients of photosynthesis during growth	- long, thin plants with small leaves and yellow colour - long because light is not present to inhibit auxin, so cell elongation occurs in all cells - yellow because the plant does not photosynthesize due to the lack of light, so it does not have chloroplasts - thin and weak because the plant does not receive the nutrients of photosynthesis during growth

9.2 Transport in angiosperms

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.

Plants increase the surface area of water and mineral ion absorption through the extensive branching of roots and by the growth of root hairs. Roots have a large increased surface area in relation to their volume.  Cortex cell walls also increase the surface area. These adaptations increase the surface area for contact between the soil and the roots.

9.2.2 List ways in which mineral ions in the soil move to the root.

Mass flow	- The mass flow of water along the apoplast pathway in plant cells continuously delivers new ion solution to the plasma membranes of root hairs.
Diffusion	- Ions diffuse into cells due to a difference in concentration gradient inside the cell and in the soil
Fungal hyphae  (mycorrhizal fungi)	- Plants living in mutalistic relationships with species of  mycorrhizal fungi can absorbs mineral ions from the fungi while providing it with carbohydrates.

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

●     The concentration of minerals inside plant cells tend to be higher than the concentration in the soil. This is because plants keep a reserve of useful ions within the cell.

●     Active transport(uses ATP/energy)  is employed to absorb mineral ions  against the concentration gradient between the soil and the cytosol of root hair cells.

●     Root hairs increase the surface area for absorption since minerals are absorbed through the epidermis of root hairs.

●     Active transport is highly selective and ions that are more useful to the needs of the plant as a whole will be taken up more frequently than others (eg. NO3- will be taken up over Na+). Those absorbed will be drawn through cell walls and enter cell wall space or the cytoplasm (apoplastic/ symplastic pathway).

●     The membrane pump proteins in the plasma membrane of root hair cells provides the selectivity and site of active transport to move the ions across the membrane. The mitochondria within root hair cells provide the energy for active transport.

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

The stems of terrestrial plants support themselves in three ways:

1.     Some cells develop thickened cellulose walls.

a.     These are called collenchyma cells and this mechanism is used in herbaceous plants.

2.     Cells store water within their vacuoles to maintain cell turgor, which exerts pressure on the surrounding cells.

a.     These are calls parrenchyma cells and this mechanism is used in herbaceous plants.

3.     Xylem cells in woody plants are lined with lignan, which is hardened cellulose thickening.

a.     Occurs in woody plants.

9.2.5 Define transpiration.

Transpiration is the evaporation of water through the stomata of green plant leaves (and stems).

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

Structure of a mature xylem vessel

●     Long, hollow cell. The living content from the initial cell has broken down and turned to sap, which works to thicken the walls of lumen of the xylem vessel.

●     The walls are thickened with lignan in a ringed manner along the vessel. This strengthens the xylem vessel and allows it to withstand negative pressure.

●     Pores along the outter cellulose of the cell wall allow water to be conducted between the xylem vessel and adjacent leaf cells.

●     No plasma membrane are present in mature xylem cells, so water can move in and out freely

How transpiration occurs

●     Transpiration is passive (no energy used by plants) and only occurs upwards, as water is pulled from the roots to the leaves. The flow of water in the plant from the roots is called a “transpiration stream”.

●     Heat from the environment evaporates water from the cells of the spongy mesophyll of the leaves, which is replaced by water from the xylem vessel in the leaf. This creates negative(low) pressure, and the water flows from high pressure to low pressure against gravity.

●     Capillary action between water molecules and the walls of the xylem tubes help to pull up the stream up.

●     Cohesive forces between water molecules due to hydrogen bonding prevents the transpiration stream from breaking.

●     Root pressure results from the active transport of ions into the root hair epidermal cells and water molecules follow by osmosis. The build-up of this pressure forces water upwards. There is a continuous column of water from the roots to the leaves.

●     Water can travel through the apoplastic pathway which is through the cell walls.

●     Water has to pass through the cytoplasm of the endodermis because the Casparian strip blocks the cell wall pathway.

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

Guard cells can regulate transpiration by opening and closing stomata. They do so using turgor pressure--when the guard cells are filled with water, they bulge which causes stomatal opening. When the guard cells lack water they become flaccid and the stomata remains closed. The thin lateral walls and thick dorsal and ventral walls allow for this to occur.

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

The plant hormone abscisic acid causes closing of the 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	- Guard cells tend to be open in the light (for intake of CO2 so photosynthesis can occur in the light) so the rate of transpiration is increased during daytime. - Infrared waves from sunlight warms the leaves, and higher temperatures increase the rate of transpiration.
Temperature	- Heat is necessary for the evaporation of water from the spongy mesophylls, so with higher temperatures this occurs faster and the rate of transpiration is increased. - With higher temperatures, the evaporation of water molecules from the surface of the leaf is increased. This increases the rate of diffusion of gases within the spongy mesophyll, because the concentration gradient between the inside and the outside of the leaf is greater. This results in an increased rate of transpiration.
Humidity	- If humidity is high, the concentration gradient between the interior and exterior of the leaf will not be steep. Therefore diffusion will occur more slowly, which results in a decreased rate of transpiration.(Low humidity= high transpiration rate)
Wind	- Wind blows away water vapour that collects near the stomata of leaves, which ensures that the concentration gradient of water vapour inside and outside the leaf is always steep. This increases the rate of transpiration.
CO2	If the carbon dioxide concentration is high the stomata will close, lowering the transpiration rate.

TAWS WHAT ABOUT THIS:

Low air pressure and low levels of carbon dioxide= high rates of transpiration

high number of leaves/ open stomata= high rate

an actively growing/ photosynthesizing plant= high rate

9.2.10 Outline four adaptations of xerophytes that help to reduce transpiration.

●     Example of xerophyte species: Euphorbia virosa (type of cactus)

●     CAM physiology, which allows stomatal opening during night instead of day. This avoids significant water loss due to the heat of the day.

●     Decreased number of stomata. This decreases the loss of water from transpiration.

●     Layers of hair on the epidermis traps moist air over the leaf, reduces diffusion and reflects sunlight. They also have a thick waxy cuticle to prevent water loss.

●     Superficial roots exploit overnight condensation at the soil surface.

●     Deep and extensive(wide-spreading) roots exploit a deep water table in the soil.

●     Thick stems with water storage tissue. Stems are vertical to avoid mid-day sun.

●     Stomata may be located in pits or groves where moist air becomes trapped, reducing diffusion.

●     Leaf may become rolled up or folded due to flaccid cells when water is low, which reduces the area over which transpiration can occur. Xerophytes have small leaves, thick leaves, spines or no leaves.

●     Water storage tissues are present in the leaves.

●     Xerophytes are usually annual plants with short-life cycles.

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).

Ten Points to mention (Transpiration and Translocation) SAYS CARTMAN FOR 10 MARKS

1.     Source produces organic molecules

2.     Glucose from photosynthesis produced

3.     Glucose converted to sucrose for transport

4.     Companion cell actively loads the sucrose

5.     Water follows from xylem by osmosis

6.     Sap volume and pressure increased to give mass flow

7.     Unloading of organic molecules by the companion cell

8.     Sucrose stored as the insoluble and unreactive starch

9.     Water that is released is picked up by the xylem

10.  Water is recycled as part of transpiration to resupply the sucrose loading

Translocation according to quicknotes

1.    Translocation moves the organic molecules (sugars, amino acids) from their source through the tube system of the phloem to the sink by active transport.  Sources include photosynthetic tissue and storage organs, like the spongy mesophyll. Translocation is bidirectional. Phloem vessels have cross walls called sieve plates that contain pores.

2.    Companion cells actively load sucrose(soluble, not metabolically active since it won’t be used until in the sink) into the phloem. Companion cells contain nucleii since they cause chemical reactions to move the sugar by active transportation.

3.    Water follows the high solute in the phloem by osmosis (passive transport). A positive pressure potential develops moving the mass of phloem sap forward in a movement known as mass flow (high concentration of sugar in phloem, low concentration of sugar in xylem, so water moves).

4.    The sap (water + sugar has a high viscosity) must cross the sieve plate. There are numerous hypotheses as to how...

5.    The phloem still contains a small amount of cytoplasm along the walls, to facilitate the movement of sugar (to prevent it from sticking to the side) but the organelle content is greatly reduced (since cytoplasm is reduced there are no organelles).

6.    Companion cells actively unload (ATP used) the organic molecules.

7.    Organic molecules are stored (sugar as insoluble starch) at the sink. Sinks include roots, growing fruits and the developing seeds within them. Overall the sugars have been transported from high concentration (leaves) to low concentration (roots). Water is released and recycled in the xylem.

Extra Notes: Water uptake by roots

Mass flow (apoplast pathway)	- Water travels through the free spaces between cellulose fibers in plant cell walls. - Most favourable pathway (called “apoplast”) for water uptake because the living part of the cell (organelles) is completely avoided, thus making it the path of least resistance. - Mass flow is the most frequently used route
Diffusion (sympoplast pathway)	- Water travels through the cytoplasm of cells by diffusion - This pathway (called “sympoplast”) poses a lot of resistance to water uptake
Osmosis (through vacuoles of cells)	- Osmosis is a means by which individual cells uptake water; it is not a significant means of water uptake across the whole plant - Osmosis is driven by gradients in osmotic pressure

flowering is controlled by the length of day (photoperiodism), and phytochrome is the pigment system responsbile for photomorphogenesis in flowering plants. phytochrome exists in two forms, P(red absorbing) and Pfr (far red absorbing), and Pfr controls the onset of flowering. Pr converts to Pfr in red or white light, and Pfr reverts to Pr in the darkness.

therefore Pfr acts as a promotor of flowering in long day plants

and an inhibitor of flowering in short day plants.

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.

9.3.2 Distinguish between pollination, fertilization and seed dispersal

Pollination is the transfer of pollen from a mature anther to a receptive stigma.

●      The pollen may come from the anthers of the same flower or flowers of the same plant. This is self-pollination. Disadvantages include: no variation for natural selection, no new combination of alleles, more susceptible to infectious diseases, more prone to genetic disease since inbreeding will increase the chances of becoming homozygous for a disease. In order to prevent self-fertilization from a hermaphrodite flower there might be mechanisms in place such as differing maturation times for stigmas and the stamens. This means that when the stigma is receptive, the stamens are not releasing pollen grains and vice versa.

●      Pollen coming from flowers on a different plant of the same species is cross-pollination.

●      Transfer of pollen may happen by insects, winds, running water and animals. Insect-pollinated flowers produce nectar (sugar solution) which attracts insects to the flower. Certain flowers possess colourful petals in order to attract bees and birds.

Fertilization is the fusion of male and female gametes to form a zygote.

●      This can only occur in flowering plants after an appropriate pollen grain has landed on the stigma, and has germinated there.

●      The pollen grain produces a pollen tube which grows down between the cells of the style and into the ovule through the micropyle. The pollen tube delivers two male nuclei. One will fuse with the egg nucleus in the embryo sac, forming a diploid zygote. The other fuses with another nucleus which triggers the formation of the food store for the developing embryo. This “double fertilization” is unique to flowering plants.

Seed dispersal is the carrying of the seed away from the vicinity of the parent plant.

●      Plant structures have evolved to aid dispersal by taking advantage of their environment conditions (winds, passing animals, flowing water).

●      Some plants have developed an explosive mechanism to fling seeds far away from the parent.

●      Since seeds are compact and nutritious, many animals will eat seeds. Since they are dropped far away from parents, they have also been successfully dispersed.

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 testa, micropyle, embryo root, embryo shoot and cotyledons.

named non-endospermic seed: e.g. (kidney) bean/Phaseolus sp.;

common/scientific name of plant;

Award [3 max] for three of the following clearly drawn and correctly labelled.

hypocotyl/radicle/embryo root;

plumule/epicotyl/embryo shoot– epicotyl shown closer to plumule than hypocotylstesta/seed coat; (clearly drawn as a layer or with some thickness if view is internal)

micropyle;

cotyledon;

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

●      Seed must overcome their dormant period and have suitable conditions in order to germinate.

●      Water: the seed must uptake enough water so that it is fully hydrated and so that the embryo can be fully physiologically active.

●      Oxygen: the seed must have enough oxygen so that the seed can support aerobic respiration. Growth demands a continuous supply of metabolic energy in the form of ATP that is best supplied by aerobic cellular respiration in all the cells.

●      Suitable temperature: a temperature that is optimum for the enzymes involved in the mobilization of food reserves, the translocation of organic solutes in the phloem, and the synthesis of intermediates for cell growth. For example wheat seeds germinate in the range of 1-35 degrees celsius vs. maize in the range of 5-45 degrees celsius.

9.3.5 Outline the metabolic processes during germination of a starchy seed.

With favourable temperature and adequate oxygen, the water absorption eventually causes the testa (seed coat) to split. Within the embryo metabolic processes occur:

1.     Embryo produces GA

2.     GA passes to food storage cells

3.     Hydrolytic enzymes are produced, converting stored starch into glucose and stored proteins into amino acids.

4.     Glucose and amino acids are translocated to growing points of stem and root.

5.     Growth and development occur in stem and root.

6.     Seedling emerges and eventually becomes a self-sufficient autotrophic plant.

9.3.6 Explain how flowering is controlled in long-day and short-day plants, including the role of phytochrome.

●      Flowering is a response to light duration. Phytochromes are stimulated by a certain wavelength of light.

●      Phytochrome is a photoreceptor pigment that exists in all plants in two forms:

○      Pr, which absorbs white/red light (660nm)

○      Pfr, which absorbs dark/far-red light(730nm)

●      In white or red light Pr is converted to Pfr

●      In far-red light or in darkness, Pfr gradually reverts to Pr

●      Pfr controls the onset of flowering because it is the active pigment.

○      Pfr acts as a promoter of flowering in long-day plants. Long periods of daylight are required to cause the accumulation of Pfr, since Pr converts to Pfr in the daylight.

○      Pfr acts as an inhibitor of flowering in short-day plants. Long periods of darkness are needed to decrease the concentration of Pfr, because Pfr reverts to Pr in the darkness.

●      Phytochrome(membrane-embedded) is located in the thylakoid membrane, however the phytol tail (tail of phytochrome made of fatty acids used to absorb the light) is located in the stroma.