how do organisms reproduce?

definition of reproduction

definition of reproduction

the production of new organisms from the existing organisms of the same species is known as reproduction

why reproduction is necessary

it is essential for the survival of a species on earth

ensure continuity of life on earth

gives rise to more organisms with the same basic characteristics as their parents

asexual reproduction

the production of a new organism from a single parent without the involvement of gametes (sex cells) is called asexual reproduction

in this method, body cells of the parent organism undergo repeated mitotic cell divisions to form two or more organisms of the same kind. there are six different methods;

fission, budding, spore formation, regeneration, fragmentation, vegetative propagation

fission

fission is of two types;

binary fission: the parent organism splits to form two new organisms (daughter cells). this happens by the elongation and division of the nucleus. in this process, the parent organism ceases to exist. examples: amoeba, paramecium, leishmania and all other protozoas

multiple fission: the parent organism splits to form many/ more than two new organisms (daughter cells). this happens when a cyst is formed around the parent cell. inside the cyst, the cell splits several times to form many daughter nuclei. when the cyst breaks, the daughter cells are released and form new organisms. example: paramecium (malaria parasite)

budding

in budding, a small part of the body of the parent organism grows out as a bud which then detaches and becomes a new organism. this is caused due to repeated cell division at one specific area

examples: hydra (multicellular), yeast (unicellular)

spore formation

in spore formation, the parent organism produces hundreds of microscopic reproductive units called spores. when the spore case bursts, they all spread into the air. under favourable conditions (damp, warm), they germinate and produce new organisms

example: bread mould (rhizopus). rhizopus consists of two parts; thread-like structures called hyphae and knob-like structures attached to the hyphae called sporangia which contains hundreds of pores

regeneration

some organisms cut their body into multiple parts which regenerate to form whole new organisms. the process of getting back a full organism from its body parts is called regeneration. however, regeneration is not exactly the same as reproduction because most animals would not depend on being cut into pieces to be able to reproduce

eg: hydra, planaria

fragmentation

the breaking up of the body of a simple multicellular organism into two or more pieces on maturing, each of which grow to form new organisms, is called fragmentation

eg: spirogyra, sea anemone

difference between fission and fragmentation

in fission, a unicellular organism breaks to form new organisms while in fragmentation, a multicellular organism breaks to form new organisms

vegetative propagation [natural]

in this method, new plants are produced from the vegetative parts like stem, root and leaves. plants produced by vegetative propagation produce fruits and flowers earlier than those produced from seeds. this process is due to the presence of dormant buds on the surface of the plant

eg: roots - carrot, radish, turnip

stem - onion, potato, ginger

leaves - bryophylium

seedless plants - jasmine, rose

vegetative propagation [artificial]

the process of growing many plants from one plant by man-made methods is called artificial propagation. the three methods are;

cutting: a small part of the plant which is removed by making a cut with a sharp knife is called cutting. cutting can be stem cutting or root cutting. examples: rose, sugarcane

layering: one of the mature/ lower branches is bent and placed in the soul and a heavy stone is placed over it. this bent branch starts developing roots and grows into an individual plant. examples: jasmine, strawberry

grafting: the stems of two different plants are cut and joined together in such a way that the two stems join to grow a single plant. the cut stem with roots is called stock while the cut stem without roots is called scion. examples: apple, peach

advantages and disadvantages of vegetative propagation

advantages

new plants resemble parent in every way, involves only one parent, no need of any external agents like wind

disadvantages

less variations, may lead to overcrowding, new plants will not be able to adapt to changes in the environment

sexual reproduction

sexual reproduction takes place by the combination of reproductive cells called ‘sex cells’ or gametes. a male gamete fuses with a female gamete to form a zygote, which develops and grows into a new organism

angiosperms

the plants in which the sex organs are carried within the flowers and the seeds are enclosed in a fruit are called angiosperms. they are also known as flowering plants

do organisms create exact copies of themselves in asexual reproduction

asexual reproduction usually results in the production of genetically identical offsprings called clones. the only genetic variations are due to inaccuracies/ errors during DNA copying (no biochemical reaction can produce 100% results). hence we can say that asexually reproduced organisms are ALMOST genetically identical to their parent

DNA

DNA, or deoxyribo nucleic acid is present in the form of chromosomes in the nuclei of all cells

importance of variations in organisms

variations during reproduction helps the species of various organisms to survive and flourish even in adverse environment

parts of a flower

receptacle: the base of the flower to which all the petals are attached to

sepals: the green, leaf-like parts in the outermost circle of the flower. it protects the bud in its initial stages

petals: the colourful parts of the flower which lies inside the sepals. it attracts insects for pollination and protects the reproductive organs

stamen: male reproductive organ. produces pollen grains (male gametes). it is made of two parts; the stalk of the stamen ‘filament’ and the top of the stamen ‘anther’ (which is where pollen grains are produced)

carpel/pistil: female reproductive organ. made of three parts; top part which receives the pollen grains ‘stigma’, which is sticky so that pollen can stick to it, the middle tube called ‘style’, and the bottom part ‘ovary’ which makes ovules (female gametes) and stores them

unisexual and bisexual flowers

unisexual: flowers which contain only one sex organ (either stamen or carpel). eg: watermelon, papaya

bisexual: flowers which contain both sex organs. eg: hibiscus, mustard

pollination in flowers

the transfer of pollen grains from the anther of a stamen to the stigma of a carpel is called pollination. it is of two types;

self-pollination: when the pollen grains from the anther of a flower is transferred to the stigma of the same flower (bisexual)

cross-pollination: when the pollen grains from the anther of a flower is transferred to the stigma of another flower

how pollen grains move to stigma

when an insect sits on the flower for sucking nectar, the pollen grains stick to its body. when the insect sits on another flower, the pollen grains on its body get transferred to the stigma. this causes cross-pollination

the blowing wind also carries pollen grains and helps in pollination

fertilisation in flowers

fertilisation in flowers occur when the male gamete present in pollen grains fuses with the female gamete present in ovule

when a pollen grain falls on the stigma, it bursts open and forms a pollen tube which goes down the style all the way to the ovary. the gamete then moves down the pollen tube and meets with the female gamete to form zygote (fertilised egg)

formation of fruits and seeds

the zygotes formed after fusion of male and female gametes in flowers grow to become seeds. the flower then develops into a fruit and protects the seeds inside it

germination of seeds

the beginning of the growth of seeds is called germination of seeds. it occurs when the seed absorbs water, swells and bursts through the seed coat

definition of male and female

an animal having male sex cells ‘sperms’ in its body is called male

an animal having female sex cells ‘ova’ in its body is called female

gametes

the two cells involved in sexual reproduction are called gametes or sex cells. the male gamete in animals is called ‘sperm’ while the female gamete is called ‘ova’ or ‘egg’. the fusion of these two gametes results in the formation of a zygote, which is a fertilised egg

internal and external fertilisation

the fertilisation which occurs inside the female body is called internal fertilisation. the male gamete ‘sperm’ is transferred into the female body during copulation where it fertilises the egg. example: mammals, birds, reptiles

the fertilisation which occurs outside the female body is called external fertilisation. the male gamete and female gamete are fused outside the body. example: frogs and fishes release sperms and eggs into the water where they fuse

advantages of sexual reproduction

promotes diversity of characters in offsprings by providing genetic variation

this genetic variation leads to the evolution of species to form better organisms

it allows offsprings to better adapt to changes in the environment

why the amount of DNA does not get doubled during sexual reproduction

normal body cells have 46 chromosomes (or 23 pairs). however, the gametes are special cells which contain only half the amount of DNA compared to normal cells, that is, 23 chromosomes. hence, when the male and female gamete fuse to form zygote during fertilisation, the zygote will have 46 chromosomes, which is the normal amount of chromosomes

steps involved in the fusion of gametes in animals

the male parent produces male gametes called sperms while the female parent produces female gametes called ova

the sperm enters the ovum to form a new cell called zygote (fertilised egg)

the zygote then divides again and again to form a large number of cells and ultimately develops into a baby

puberty

the age at which the sex hormones/ gametes begin to be produced, making boys and girls sexually mature, is called puberty. generally, boys attain puberty at the age of 13-14 while girls attain puberty at the age of 10-12. it is at this age that secondary sexual characters start to develop

on attaining puberty, testes start producing testosterone and male gametes called sperms while ovaries start producing estrogen and female gametes called eggs

male reproductive system

the human male reproductive system consists of the following;

testes: a man has two testes which lie outside the abdominal cavity, inside a pouch called scrotum. they make male gametes and testosterone. as sperm formation requires a lower temperature than the normal body temperature, the temperature in the testes is 3C lower

vas deferens: the sperms are carried to the urether in a long tube called vas deferens

seminal vesicles and prostate gland: along the way through vas deferens, the secretions of these two glands provide nutrition and add their secretions to the sperm to make it a thick liquid (semen)

female reproductive system

the human female reproductive system consists of the following;

ovaries: oval shaped organs near the kidneys which produce female gametes and estrogen/ progesterone

oviducts/fallopian tubes: tubes above ovaries where fertilisation takes place

uterus: bag-like organ to which the oviducts are attached to. the growth of zygote into baby takes place here

cervix: narrow opening which connects uterus to vagina

follicles

follicles are unripe eggs/ova which mature during puberty

how fertilisation in humans occurs

in human beings, internal fertilisation takes place. the sperms move into the oviducts, which contains an ovum released by the ovaries during ovulation. only one sperm fuses with the ovum to form a zygote

implantation

the embedding of embryo in the thick lining of the uterus is called implantation

placenta

after implantation, a disc-like tissue develops between the uterus wall and the embryo, which is called placenta. all requirements of the embryo like nutrition, respiration, excretion etc. are provided for through the placenta

gestation

thee time period from the fertilisation up to the birth of the baby is called gestation.. the average gestation period in humans is about 9 months or 38 weeks

embryo

an embryo is formed after repeated cell division of a zygote

it is an unborn baby in the early stages of development <8 weeks

it is multicellular, and features are not much developed

foetus

a foetus is formed by the growth and development of an embryo

it is an unborn baby in the later stages of development >8 weeks

it is multicellular, and a lot of body features can be identified

menstruation

in females, the ovaries start releasing ovum or egg once every 28 days (ovulation) from the age of puberty. before every ovulation, the uterus prepares itself by making its lining thick and soft with lot of blood capillaries. however, if the egg is not fertilised at the time of ovulation, the egg dies within a day and the lining breaks down and comes out of the vagina in the form of bleeding. this process is called menstruation

menarche and menopause

the first occurrence of menstruation at puberty is called menarche

the permanent stoppage of menstruation in a woman is called menopause

birth control methods

barrier method: physical devices such as condoms and diaphragm. they prevent sperms from meeting the egg by acting like a barrier in between them. they also prevent the spread of STDs

chemical method: the use of oral pills or vaginal pills which contain hormones to stop the ovaries from releasing eggs into the oviduct. vaginal pills also have chemicals called spermicides which kill the sperms

intra-uterine contraceptive device IUCD: the use of copper-T which is placed inside the uterus. it prevents the implantation of fertilised egg in the uterus

surgical method: vasectomy, in which the vas deferens is cut and tied preventing sperms from coming out, and tubectomy, where the oviducts are cut and tied

sexually transmitted diseases (STDs)

diseases which are spread by sexual contact with an infected person are called sexually transmitted diseases. some common STDs are;

gonorrhoea, syphilis (caused by bacteria)

AIDS (caused by virus)