life cycles of plants
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LIFE CYCLES OF PLANTS. ALTERNATION OF GENERATIONS. DEFINITIONS OF THE FOLLOWING:. 1. Alternation of generations : haploid (n) stage that produces gametes followed by diploid (2n) stage producing spores. - PowerPoint PPT PresentationTRANSCRIPT
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LIFE CYCLES OF PLANTS
ALTERNATION OF GENERATIONS
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1. Alternation of generations: haploid (n) stage that produces gametes followed by diploid (2n) stage producing spores.
2. Haploid: gametes; half the number of chromosomes in somatic cell, indicated by n, result of meiosis.
3. Diploid: somatic cells; double number of chromosomes in gametes, indicated by 2n; result of fertilisation.
DEFINITIONS OF THE FOLLOWING:
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1. Gametophyte: generation producing gametes in gametangium/ gametangia sexual stage.
2. Sporophyte: generation producing spores in sporangium/ sporangia – asexual stage.
NB: In higher plants the generations alternate in the life cycle of a plant; in lower plants reproduction is either through gametes (sexual) or spores etc. (asexual).
More terminology ...
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1. A haploid gametophyte generation that produces gametes (n) by mitosis ...
the gametes (n) fuse toform zygote (2n), the start of ...
2. A diploid sporophyte generation that
produces spores (n) by meiosis.
All terrestrial plants have two phases in their life cycles ...
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Mosses (Bryophyta) Ferns (Pterophyta/ Pteridophyta) Seed plants (Spermatophyta)1. Gymnospermae (cone-bearing plants producing
seeds)2. Angiospermae (flowering plants producing seeds)
Exam guidelines require study of mosses and flowering plants to highlight alternation of generations.
Three groups of plants with life cycles showing alternation of generations:
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MOSSES Leaf-like structures of moss gametophyte
Sporangium of moss sporophyte
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FERNS
Fern leaf (sporophyte) with sori (groups of sporangia
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SEED PLANTS
GYMNOSPERMS
ANGIOSPERMS
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Female cones at different ages Male cones
Gymnosperms have two types of sporangia – female and male
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Flowering plants
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Which generation is dominant in which plant group?Mosses
Gametophyte dominant
Ferns, gymnosperms and flowering plants
Sporophyte dominant
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LYFE CYCLES IN ANIMALS
INSECT METAMORPHOSIS
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Physical changes from one stage in life cycle to another.
TWO TYPES OF METAMORPHOSIS:
1. Complete metamorphosis
2. Incomplete metamorphosis
METAMORPHOSIS
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Comparison of metamorphosis
88 % of all insects 12 % of insects
COMPLETE Four stages: 1. egg 2. larva 3. pupa 4. adult
E.g. Butterflies, bees, housefly
INCOMPLETE Three stages: 1. egg 2. nymph 3. adult
E.g. Locust, cricket, cockroach
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Complete metamorphosis
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Incomplete metamorphosis
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Advantages and disadvantages of complete metamorphosis Advantages:1. Larva and adult
occupy different habitats and niches – not in competition.
2. Carrying capacity of ecosystem increased.
3. Egg and pupa stages allow insect to survive unfavourable conditions.
Disadvantages:1. Larval forms
vulnerable to predators.
2. Suitable habitat for laying of eggs needed.
3. Change from pupa to adult requires high energy-input.
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Advantages and disadvantages of incomplete metamorphosis Advantages:1. Vulnerable pupa
stage avoided.2. Less energy is
needed – no drastic changes from pupa to adult.
3. All stages can eat same food.
Disadvantages:1. Nymph and adult often
compete for same food.2. Insect vulnerable to
dehydration and predators when moulting.
3. Entire insect population could be threatened by environmental changes.
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Amphibian metamorphosis
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