CLASS XI BIOLOGY - Excellup.com

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CLASS XI BIOLOGY Plant Kingdom 1. What is the basis of classification of algae? Answer: The main basis of classification of algae has been done on the basis of presence or absence of pigments, which impart an algae its colour. Chlorophyceae contains chlorophyll a and b, giving it the green colour and the name ‘blue-green algae’. Phaeophyceae contains chlorophyll a and c and fucoxanthin. The fucoxanthin gives it the brown colour and hence the name ‘brown algae’. Rhodophyceae contains chlorophyll a and d and phycoerythrin. The phycoerythrin gives the disitinct red colour and hence the name ‘red algae’. 2. When and where does reduction division take place in the life cycle of a liverwort, a moss, a fern, a gymnosperm and an angiosperm? Answer: Reduction division or meiosis takes place in gametic cells. The role of meiosis is to halve the number of chromosomes so that once fertilization takes place the embryo thus formed will be having similar number of chromosomes as in parent somatic cells. In plants sometimes the dominant phase will contain all of diploid stage or all of haploid stage. There can be a brief interspersal of other stage. This is known as the alteration of generation. 1. Sporophytic generation is represented only by the one-celled zygote. There are no free-living sporophytes. Meiosis in the zygote results in the formation of haploid spores. The haploid spores divide mitotically and form the gametophyte. The dominant, photosynthetic phase in such plants is the free-living gametophyte. This kind of life cycle is termed as haplontic. e.g., algae 2. On the other extreme, is the type wherein the diploid sporophyte is the dominant, photosynthetic, independent phase of the plant. The gametophytic phase is represented by the single to few-celled haploid gametophyte. This kind of lifecycle is termed as diplontic. All seed-bearing plants i.e. gymnosperms and angiosperms, follow this pattern. Meiosis results in formation of male and female gametes. 3. Bryophytes and pteridophytes, interestingly, exhibit an intermediate condition (Haplo-diplontic); both phases are multicellular and often free-living. However, they differ in their dominant phases. The meiosis takes place in the sporophytes to produce haploid spores.

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Finish Line & Beyond 3. Name three groups of plants that bear archegonia. Briefly describe the life cycle of any one of them. Answer: Bryophytes, Pteridophytes and Gymnosperms bear distinct archegonia. In angiosperms the archegonia is reduced to just a few cells so the term is not used in case of angiosperms. Life Cycle of Gymnosperms: Reproduction: The gymnosperms are heterosporous; they produce haploid microspores and megaspores. The two kinds of spores are produced within sporangia that are borne on sporophylls which are arranged spirally along an axis to form lax or compact strobili or cones.

Male Gamete: The strobili bearing microsporophylls and microsporangia are called microsporangiate or male strobili. The microspores develop into a male gametophytic generation which is highly reduced and is confined to only a limited number of cells. This reduced gametophyte is called a pollen grain. The development of pollen grains takes place within the microsporangia. Female Gamete: The cones bearing megasporophylls with ovules or megasporangia are called macrosporangiate or female strobili. The male or female cones or strobili may be borne on the same tree (Pinus) or on different trees (Cycas). The megaspore mother cell is differentiated from one of the cells of the nucellus. The nucellus is protected by envelopes and the composite structure is called an ovule. The ovules are borne on megasporophylls which may be clustered to form the female cones. The


Finish Line & Beyond megaspore mother cell divides meiotically to form four megaspores. One of the megaspores enclosed within the megasporangium (nucellus) develops into a multicellular female gametophyte that bears two or more archegonia or female sex organs. The multicellular female gametophyte is also retained within megasporangium. Fertilization: The pollen grain is released from the microsporangium. They are carried in air currents and come in contact with the opening of the ovules borne on megasporophylls. The pollen tube carrying the male gametes grows towards archegonia in the ovules and discharge their contents near the mouth of the archegonia. Following fertilisation, zygote develops into an embryo and the ovules into seeds. 4. Mention the ploidy of the following: protonemal cell of a moss; primary endosperm nucleus in dicot, leaf cell of a moss; prothallus cell of a ferm; gemma cell in Marchantia; meristem cell of monocot, ovum of a liverwort, and zygote of a fern. Answer: (a) Protonemal cell of a moss develop from spores and are haploid. (b) Primary endosperm nucleus is formed after fertilization and is diploid. (c) Leaf cell of a moss is haploid as it develops from protonemal cells. (d) Prothallus of a fern is haploid and bears male and female sex organs and is called gametophyte. This upon fertilization starts the diploid stage. (e) Gemma cells in Marchantia are meant for asexual reproduction and are diploid. (f) Meristem cell of a monocot is somatic cell and is diploid. (g) Ovum and zygote will always be diploid. 5. Write a note on economic importance of algae and gymnosperms. Answer: Economic Importance of Algae: Algae are useful to man in a variety of ways. At least a half of the total carbon dioxide fixation on earth is carried out by algae through photosynthesis. Being photosynthetic they increase the level of dissolved oxygen in their immediate environment. They are of paramount importance as primary producers of energy-rich compounds which form the basis of the food cycles of all aquatic animals. Many species of Porphyra, Laminaria and Sargassum are among the 70 species of marine algae used as food. Certain marine brown and red algae produce large amounts of hydrocolloids (water holding substances), e.g., algin (brown algae) and carrageen (red algae) are used commercially. Agar, one of the commercial products obtained from Gelidium and Gracilaria are used to grow microbes and in preparations of ice-creams and jellies. Chlorella and Spirullina are unicellular algae, rich in proteins and are used as food supplements even by space travellers.


Finish Line & Beyond Economic Importance of Gymnosperms: Gymnospermous plants are widely used as ornamentals. Conifers are often featured in formal gardens and are used for bonsai. Yews and junipers are often low-growing plants cultivated for ground cover. Conifers are effective windbreaks, especially those that are evergreen. Most plywood is gymnospermous. Fibres of conifers make up paper pulp and may occasionally be used for creating artificial silk or other textiles. Conifers are frequently planted in reforestation projects. Conifer bark is often the source of compounds involved in the leather tanning industry. Bark is also used extensively as garden mulch. From conifer resins are derived turpentine and rosin. A hardened form of resin from a kauri (Agathis australis), called copal, is used in the manufacture of paints and varnishes. Some resins, such as balsam (from hemlock) and dammar (from Agathis) are used in the preparation of mounting media for microscope slides. Resins may also have medicinal uses. Many types of amber are derived from fossilized resin of conifers. Commercially useful oils are derived from such conifers as junipers, pines, hemlock, fir, spruces, and aborvitae. These oils serve as air fresheners, disinfectants, and scents in soaps and cosmetics. Seeds are often food sources. Pine seeds are a delicacy eaten plain or used as a garnish on bakery products. Seeds of Ginkgo and cycads may be poisonous unless detoxified. "Berries" (in reality the fleshy cones) of juniper are used to flavour gin. 6. Both gymnosperms and angiosperms bear seeds, then why are they classified separately? Answer: The seeds of gymnosperms are naked, while that of angiosperms are covered by a membrane. That is why they are classified separately. 7. What is heterospory? Briefly comment on its significance. Give two examples. Answer: Certain pteridophytes produce two kinds of spores. This phenomenon is called heterospory. The smaller one are called microspore and the bigger ones are called megaspore. Microspore produce male gametophytes and megaspores produces female gametophyte. In fact heterospory is the crucial step in evolution. This ultimately led to seed development in gymnosperms and angiosperms.


Finish Line & Beyond 8. Explain briefly the following terms with suitable examples:(i) protonema (ii) antheridium (iii) archegonium (iv) diplontic (v) sporophyll (vi) isogamy Answer: (i) Protonema: A protonema is a thread-like chain of cells that forms the earliest stage (the haploid phase) of a bryophyte life cycle. When a moss or liverwort first grows from the spore, it grows as a protonema which develops into a leafy gametophore. Moss spores germinate to form an alga-like filamentous structure called the protonema. It represents the juvenile gametophyte. While the protonema is growing by apical cell division, at some stage, under the influence of the phytohormone cytokinin, buds are induced which grow by three-faced apical cells. These give raise to gametophores, stems and leaf like structures (bryophytes do not have true leaves (megaphyll)). These gametophores are the adult form of the gametophyte. Protonema are characteristic of all mosses and some liverworts but are absent from hornworts. The protonema is also the photosynthetic part of a germinating fern spore. (ii) Antheridium: An antheridium (plural: antheridia) is a haploid structure or organ producing and containing male gametes (called antherozoids or sperm). It is present in the gametophyte phase of lower plants like mosses and ferns, and also in the primitive vascular psilotophytes. Many algae and some fungi, for example ascomycetes and water moulds, also have antheridia during their reproductive stages. An antheridium typically consists of sterile cells and spermatogenous tissue. The sterile cells may form a central support structure or surround the spermatogenous tissue as a protective jacket. The spermatogenous cells give rise to spermatids via mitotic cell division. In bryophytes, the antheridium is borne on an antheridiophore, a stalk-like structure that carries the antheridium at its top. (iii) Archegonium: An archegonium is a multicellular structure or organ of the gametophyte phase of certain plants producing and containing the ovum or female gamete. The archegonium has a long neck and a swollen base. Archegonia are typically located on the surface of the plant thallus, although in the horned liverworts they are embedded. (iv) Diplontic: The major part of the life cycle is composed of gametophytic satge and gametophytes produce haploid male and female gametes. This happens in all the higher plants and animals. The life cycle is called diplontic life cycle. (v) Sporophylls: Sporophyll is a leaf that bears sporangia. Both microphylls and megaphylls can be sporophylls. In heterosporous plants, sporophylls (whether they are microphylls or megaphylls) bear either megasporangia (and thus are called megasporophylls), or microsporangia (microsporophylls). The overlap of the prefixes


Finish Line & Beyond and roots makes nomenclature.

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Sporophylls vary greatly in appearance and structure, and may or may not look similar to sterile leaves. (vi) Isogamy: Isogamy refers to a form of sexual reproduction involving gametes of similar morphology, differing only in allele expression in one or more mating-type regions. Since both gametes look alike, they cannot be classified as "male" or "female." Instead, organisms undergoing isogamy are said to have different mating types, most commonly noted as "+" and "-" strains. Fertilization occurs when "+" and "-" gametes fuse to form a zygote. In many cases, isogamous fertilization is used by organisms that can also reproduce asexually through binary fission, budding, or asexual spore formation. The switch to sexual reproduction mode is often triggered by a change from favorable to unfavorable growing conditions. Fertilization frequently leads to the formation of a thick-walled zygotic resting spore that can withstand harsh environments and will germinate once growing conditions turn favorable again.


CLASS XI BIOLOGY - Excellup.com

CLASS XI BIOLOGY - Excellup.com

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