9.3b Plant Reproduction
15/04/2013 § 1 Comment
Okay, so this last bit is a continuation of how plants reproduce. How do they reproduce – it’s called flowering, pollination, and fertilization. Flowering gets really interesting later, not to mention confusing, but I’ll try to explain it as best as I can.
After a seed germinates, it grows a stem, roots and leaves and enters the vegetative phase. The plant can remain in the vegetative phase for as short as a few weeks up to as long as a many years. When meristems in the shoot start to produce the structure for flowers instead of leaves, then the plant has entered the reproductive phase. The flowers of plants produce pollen, which hold male gametes, and hold it out so that insects can find them easily to brush pollen they’re already carrying (they may or may not already be carrying pollen) to the stigma. This is called pollination, which is the transfer of pollen from an anther to a stigma. From there, the plant is fertilized when the pollen (holding the male gametes) travels down a tube on the stigma to the ovule, in order to fertilize the ovary. The ovary would then grow to be a seed of a fruit.
Quite important to learn is the job of the pigment phytochrome. This pigment has two forms – the dormant, inactive form Pr, and the active and energized form Pfr. When there is sunlight (so, during the day), Pr can absorb red light, which immediately converts it into the Pfr version. This means that during the day, all phytochromes are in their Pfr form. If Pfr meets strong, far-red light, it also immediately converts back to Pr. Rarely does this happen though, so Pfr normally converts back to Pr gradually and over night, when there is no sunlight to keep it in its Pfr form.
What phytochrome does is that it either inhibits or stimulates a plant’s ability to flower. Phytochrome binds to the receptor of the plant that controls its ability to flower, and whether it is in its Pr or Pfr mode, a particular plant will flower.
And now there are plants that are short day or long day plants. If a plant is a short day plant, it flowers during a short day. Short days and long nights are days during the Autumn or Winter, which means that this particular plant would flower when there is more Pr (because the long nights allow all the Pfr to convert back to Pr). Conversely, if a plant is a long day plant, which means days during the Spring or the summer when the night is the shortest, it flowers when there is more sunlight. This means that long day plants flower when there is a lot of Pfr (because the night is too short to allow all the Pfr to convert back to Pr).
I hope that made sense – that’s how I understand it.
- The main part of growing plants are roots, stems and leaves. Draw a plant diagram to show the arrangement of tissues in the stem of a dicotyledonous plant.
- Draw a labelled diagram showing the tissues present in a dicotyledonous leaf.
- Explain the functions of the different tissues of a leaf.
- Explain the role of auxin in phototropism.
- Outline the adaptations of plant roots for absorption of mineral ions from the soil.
- Describe the process of mineral ion uptake into roots.
- Describe how water is carried by the transpiration stream.
- Explain how abiotic factors affect the rate of transpiration in a terrestrial plant.
- List three abiotic factors which affect the rate of transpiration in a typical mesophytic plant.
- Explain how wind affects the rate of transpiration from a leaf.
- Outline adaptations of xerophytes.
- Outline the role of the phloem in the active translocation of biochemicals.
- Draw the structure of a dicotyledonous animal-pollinated flower
- Describe the metabolic events of germination in a starchy seed.
- Explain the conditions needed for seed germination.
- Explain how flowering is controlled in long-day and short-day plants.
Wow, I need to work on these.
DATA BASED QUESTIONS
Page 132, sowing times for soybeans
1. Compare the growth of the soybean plants sown on the different dates. —> The soybean plant that began on May 2, earlier than the other four plants, produced a greater total number of nodes than the other four plants. The May 17 plant comes second, producing less than May 2, but more than May 30 and June 17. The May 30 plant is third as it only produces more than the June 17 plant. The June 17 plant produces nodes last out of all the plants and also produces the least number of nodes. All four plants stop growing more nodes at the same time and their rate of growth is quite similar.
- a) Deduce when the soybeans started to flower. —> August 18 – 20.
- b) Deduce with reasons, the factor that triggers flowering in soybeans. —> Flowering is caused when the meristem starts to produce flower structures and not vegetative structures, and perhaps this is caused by a hormone.
- a) Explain the advantage, in terms of soybean yields, of sowing the crop as early as possible. —> Based on the data, sowing the crop as early as possible would be quite beneficial because the output is much greater. Sowing soybeans in at the beginning of May instead of in mid June produces about 6 more nodes per plant. This is because soybeans seem to be long day plants and as summer progresses from May to June, the days gradually get shorter even if it’s still summer.
- b) Suggest two possible disadvantages of sowing soybeans earlier than the dates used in the trial. —> Anything earlier than May might not give the soybeans a sufficient amount of Pfr to produce nodes because the nights are longer and too much of the Pfr gets converted back to Pr. Also, the colder weather might alter the temperature (which is important!) the plant needs for its enzymes to work at an efficient rate.
Chapter 10 Questions (Answers following the “//” are accurate and checked.)
- i. Explain how plants increase the sugar concentration of phloem sap to such high levels —> The plants create a concentration gradient by actively pumping ions into the roots, creating a high concentration in the phloem sap compared to the surrounding environment. // Yes, plants do this by actively transporting sugar.
- ii. Explain how high sugar concentrations cause a high pressure to develop in the phloem. —> // A high solute concentration creates a chemiosmotic gradient that draws water in by osmosis.
- i. Describe the relationship between the sucrose concentration of phloem sap ingested by aphids and the percentage of oligosaccharides in the honeydew. —> // Up until 0.25 mols, the percentage of oligosaccharides is 0%. After 0.25 molars of sucrose concentration, the percentage of oligosaccharides increases at a very steep and rapid rate. At around 0.50 mols of sucrose concentration, and at around 85% of oligosaccharides, the percentage stops increasing and plateaus.
- ii. Suggest reasons for aphids secreting enzymes to reduce the solute concentration of the fluid in the gut. —> Too much solute concentration makes it difficult for them to function. // It reduces water loss in the aphids, through osmosis.
- i. Evaluate phloem sap as a source of amino acids for aphids. —> // It isn’t a good source of amino acids, because proteins, which is an actual source of amino acids, are at a lower percentage, therefore there aren’t that many amino acids…? The sap is mostly sugar anyway – and I’m really sorry but the answer in the book makes little sense.
- ii. Suggest reasons for the differences in amino acid content between phloem sap and aphid protein. —> // Amino acids are synthesized to make plant proteins and plants and aphid proteins are different as the organisms need to do different things to live.
- i. Explain how antibiotics could be used to obtain evidence for the role of Buchnera in aphids. —> // aphids feed on phloem sap with antibiotics, which will test the aphid’s growth rates, protein synthesis rates, as well as amino acid contents.
- ii. Using the data in this question, discuss the reasons for few animals using phloem sap as the main part of their diet. —> // phloem sap can help overcome certain physiological problems, such as dehydration due to osmosis or lacking essential amino acids
2. State one role of each of the following:
- a) apical meristems —> produces leaves and flowers // stem/root growth
- b) bulbs —> modified leaves // food storage
- c) cotyledons —> holds the … embryo? // food storage in seeds/photosynthesis
- d) guard cells —> regulates transpiration by opening and closing the stomata // controls the aperture
- e) palisade mesophyll —> I DON’T KNOW // photosynthesis (oh)
- f) Pfr —> transcripts flowering for long day plants and inhibits flowering for short day plants (long night plants). // stimulates/inhibits flowering
- g) spongy mesophyll —> How about no. // gas exchange
- h) tendrils —> modified leaves // support
- i) waxy cuticle —> avoids transpiration // yes, reduces water loss
- j) xylem —> distributes water and minerals to cells throughout leaf // transports water
3. Distinguish between pollination, fertilization and seed dispersal in the reproduction of flowering plants. —> Pollination is the transfer of pollen from one flower’s anther to another flower’s pistil. Fertilization is when the pollen meets the ovary in the carpel. Seed dispersal is how seeds travel long distances from the parent plant. // Pollination is done by wind, insects or a different agent. Fertilization is specifically the fusion of male and female gametes in the ovule wherein the pollen tube carries the male gamete with its nucleus to the ovule. Seed dispersal is specifically the transfer of seeds from the parent plant to a new site via the wind, animals or another agent.
- a) Outline the structure of roots and how it allows water to be absorbed. —> Roots branch out downwards into the soil to increase its overall surface area. The more surface area of the roots in the soil, the more the root is exposed to the extracellular fluid. The root hairs also greatly increase the surface area of the root to absorb water by using a concentration gradient to bring water inside the root. // Roots are narrow cylindrical structures that can form lateral branches.
- b) Explain how water is transported from the roots to the top of the tallest trees. —> Water is carried by the transpiration stream thanks to the structure of xylem vessels, as well as the transpiration pull. The xylem vessels are elongated tubes that the water can pass through, and the transpirational pull involves evaporation, cohesion, adhesion and transpiration, which all pull the water up. // So, through the transpiration stream, water evaporates from the surface of mesophyll cells. Leaf cells replace the evaporated water by drawing in new water. Water adhesion of cellulose in cell walls, cohesion in xylem vessels AND cell walls, low pressure created in xylem creates a transpiration pull that is enough to DEFY GRAVITY and draw the water up in the xylem from root to leaf.
- c) Suggest reasons why monocotyledonous plants do not grow into large trees whereas some dicotyledonous plants grow into very large trees. —> // No cambium leads to no thickening of the stem, so the stem cannot grow into a thick trunk. It won’t become a xylem. Without that support, the plant can’t really branch out.