Biology gcse short notes
Transcript of Biology gcse short notes
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BIOLOGY
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Asexual reproductionAsexual reproduction needs only one parent, unlike sexual reproduction,
which needs two parents. Since there is only one parent, there is no
fusion of gametes and no mixing of genetic information. As a result, the
offspring are genetically identical to the parent and to each other. Theyare clones.
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PlantsAsexual reproduction in plants can take a number of forms. Many plants develop
underground food storage organs that later develop into the following year's
plants. Potato plants and daffodil plants do this, for example.
A daffodil bulb at the beginning and end of the growing season, with a lateral bud where the new
plant will grow
Types Of Asexual Reproduction In Plants
1. Rhizomes
Plants such as the grasses, cattails and sedges produceunderground stems or rhizomes. As these stems grow throughthe soil, they will periodically produce adventitious roots and a newabove ground shoot. If the rhizome subsequently dies, a newseparate plant will have been formed
Plant Reproduction by rhizomes.
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2. Tubers
Tubers are actually modified rhizomes. They are formed in suchplants as Irish potatoes. They develop when specialized stembranches grow down into the ground and swell up with starchcontaining cells. Buds on the tubers will grow into new plants.
Plant Reproduction by the use ofTubers.
3. Runners
These are horizontally growing stems that produce few, if any,leaves. At the spot where a leaf would normally develop a node, theseplants will produce adventitious roots down into the soil, and newabove ground shoots.
Strawberry plant with runners - stems
growing sideways
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4. Plantlets
A few seed plants such as the duckweed produce miniature plants
on the margin of their leaves. These drop off and develop intomature plants. The duckweed, which is an aquatic plant,reproduces almost entirely by this method.
Chlorophytum with plantlets - sidebranches growing
5. Bulbs
Onions, chives and lilies over-winter in the form of a bulb. Each bulbhas a very short stem which is surrounded by fleshy leaves. In thespring, the shoot apex begins to grow using the nutrients storedin the leaves.
Plant Reproduction by the Use
Bulbs.
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6. Corms
This structure is similar to bulbs except that there are no storageleaves. The nutrients are, instead, stored in the swollen stem.
Plant Reproduction by the Use of Corms.
Another type is the growing of plants from cuttings. A piece of a plant
stem, with a few leaves attached is cut from a healthy plant. This is
planted in damp compost, where it will grow roots and develop into a
new plant
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Animals
In asexual reproduction, one individual produces offspring that are genetically identical
to itself. These offspring are produced by mitosis. There are many invertebrates,including sea stars and sea anemones for example, that produce by asexual reproduction
Types Of Asexual Reproduction In Animals
Budding
In this form of asexual reproduction, an offspring grows out of the body of the
parent.
Hydras exhibit this type of reproduction.
Gemmules (Internal Buds)
In this form of asexual reproduction, a parent releases a specialized mass
of cells that can develop into offspring.
Sponges exhibit this type of reproduction.
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Fragmentation
In this type of reproduction, the body of the parent breaks into distinct pieces,
each of which can produce an offspring.
Planarians exhibit this type of reproduction.
Regeneration
In regeneration, if a piece of a parent is detached, it can grow and developinto a completely new individual.
Echinoderms and Hydra exhibit this type of reproduction.
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Asexual reproduction in animals does occur - it happens in sea
anemones and starfish, for example - but it is less common than sexual
reproduction.
Advantages of Asexual Reproduction in Animals
Animals that remain in one particular place and are unable to look for
mates would need to reproduce asexually.
Another advantage of asexual reproduction is that numerous offspring can
be produced without "costing" the parent a great amount of energy or
time.
Disadvantages of Asexual Reproduction in Animals
A disadvantage of this type of reproduction is the lack of genetic variation.
All of the organisms are genetically identical and therefore share the same
weaknesses.
If the stable environment changes, the consequences could be deadly to
all of the individuals.
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Sexual reproductionThe genetic information passed from parent to offspring is contained in
genes carried by chromosomes in the nucleus.
Sexual reproduction produces offspring that resemble their parents, but
are not identical to them.
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Sexual Reproduction In Plants Plants produce specialized, haploid sex cells in their flowers. The male sex
cells are the pollen grain and the female sex cells are ova.
The male sex cells must be transferred to the female sex cells. This is
called pollination.
Pollination is normally carried out either by wind or insects. Following
pollination, fertilisation takes place and the zygote formed develops into
a seed, which, in turn, becomes enclosed in a fruit.
The sex cells are produced by meisosis in structures in the flowere. Pollen
grains are produced in anthers of the stamen. The ova are produced in the
ovules in the ovaries.
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PollinationTransfer of pollen grains to the stigma is called pollination.
If the pollen grains are transferred to the stigma of the same flower, the pollination is
called self-pollination
If the pollen grains are transferred to the stigma of another flower of the same
species, the pollination is called cross-pollination.
Cross pollination is brought about by various agencies like wind, water, bees, birds, bats
and other animals including man.
Methods of Pollination
Insect Pollination: It is the form of pollination where pollen or spores are distributed by
insects such as bees, flies and moths.
Wind Pollination: Its the transfer of pollen from one individual plant to another. Wind
pollination can take place in both, flowering and non-flowering plants.
Feature ofFlower
Type Of FlowerInsect Pollinated Wind Pollinated
Position Of
Stamens
Enclosed within flower so that
insect must make contact
Exposed so that the wind
can easily blow the pollen
away
Position Of Stigma Enclosed within flower so that
insect must make contact
Exposed to catch the pollen
blowing in the wind
Type Of Stigma Sticky so pollen grains attachfrom insects Feathery, to catch pollengrains blowing in the wind
Size Of Petals Large to attract insects Small
Colour Of Petals Brightly coloured to attract
insects
Usually green
Nectaries Present Absent
Pollen Grains Larger, Sticky grains to stick
to insects bodies
Smaller. Smooth inflated
grains to carry in the wind
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Fertilisation
Pollination transfers the pollen grain to the stigma.
However, for fertilization to take place, the nucleus of the pollen grain must fuse
with the nucleus of the ovum, which is inside and ovule in the ovary.
To transfer the nucleus to the ovum, the pollen grain grows a tube this digests its
way through the tissue of the style and into the ovary
Then, the tube grows around to the opening in an ovule.
The tip of the tube dissolves and allows the pollen grain nucleus to move out of
tube and into the ovule
Here it fertilizes the ovum nucleus.
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Seed And Fruit FormationOvary will lead to the fertilized ovule becoming a seed and the ovary in which
its found becoming a fruit.
Four changes take place:
1. The zygote develops into an embryonic plant with small root(radical) and
shoot(plumule).
2. The other contents of the ovule develop into cotyledons which will be a food store
for the young plant when the seeds germinate
3. The ovule becomes the seed coat ortesta
4. The ovary wall becomes the fruit coat; this can take many forms depending on the
type of fruit.
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Seed Dispersal And
Germination In Plants Seed dispersal is the scattering of offspring away from each other
and from the parent plant.
Seed germination is the restart of growth by the plant embryo
using the food stored in the seed.
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Seed Dispersal
Need For Dispersal
Improved chance of success by reducing competition and overcrowding.
Enables colonisation of new suitable habitats
Increased chance of species survival.
Methods Of Dispersal
1. Wind
Light weight seeds and high air resistance help the dispersal of seed using wind
Sycamore and ash produce fruit with wings
Dandelions and thistles produce fruit with parachute-like devices
2. Water
Water dispersal happens when light, air filled fruits float away on water Buoyant Fruit: sedge
Buoyant seed: water lily
3. Animal
Animals too help with dispersing in various ways
Animals seek the fruit as a food source, eventually the seed passes
through the digested system unharmed and excreted. The seed is nowdispersed. Eg: Strawberries, blackberries
Fruits with hooks cling to the hair of an animal and its carried away.
Eg:Goose grass
4. Self Scattering
Some fruits explode when they dry out and flicks the seed away
Peas and beans; the drying pod flicks out the seeds
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Germiation
A seed contains a plant embryo, consisting of a root (radical),shoot (plumule) and one or two seed leaves called cotyledons.
It also contains a food store. During germination, the food store is
used up, providing the nutrients to allow the radical and plumule
to grow.
The radical grows down into the soil where it will absorb water mineral
ions.
The plumule grows upwards towards the light, where it can start
photosynthesis Once the seedling is able to photosynthesize, germination is over
The Conditions Needed For Germination
When seeds are dispersed from the parent plant, theyre usually
very dry, containg only 10% water.
This low water content restricts a seeds metabolism, so that it can
remain alive but dormant for a long time.
When a seed germinates, dormancy is broken.
The seeds food store is broken down by enzymes and respired
aerobically.
This means germination needs the following conditions:
Warm temperature- for activation of enzymes and for it to act efficiently
Water- for chemical reactions to take place in solution
Oxygen- for respiration