Biology gcse short notes

7/28/2019 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

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