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Reproduction as a life process

The life of each individual is limited in time. The species to which a particular individual belongs shall remain on Earth much longer than this individual. The species comprise all similar and related individuals living in the past and those that will live in future generations. ReproductionreproductionReproduction is a condition for the existence of the species, not for the existence of an individual, and generally leads to an increase in the number of individuals.

In the process of reproduction, organisms transfer genetic information to the offspring, i.e. specific instructions according to which the offspring will develop and function. There are two types of reproduction: asexual reproductionasexual reproductionasexual reproduction and sexual reproductionsexual reproductionsexual reproduction. Asexual reproduction leads to the formation of individuals very similar to the parental individuals, and sexual reproduction results in organisms with a much greater diversity of features than those of the parental individuals.

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Ilustracja przedstawia schemat sposobów rozmnażania. Schemat zaczyna się od hasła rozmnażanie, reproduction. Od niego dwie strzałki wskazują dwa żółte prostokąty jeden pod drugim, opisane: rozmnażanie płciowe, sexual reproduction i rozmnażanie bezpłciowe, asexual reproduction. Od rozmnażanie bezpłciowego prowadzą cztery strzałki do różowych prostokątów po prawej. Są to po kolei sposoby rozmnażania bezpłciowego: podział komórki, cell division pączkowanie, gemmation, zarodniki, spores, fragmentacja, fragmentation.

Asexual reproduction

Asexual reproduction leads to the production of offspring which is formed of one parental individual and is almost an identical copy of it. This type of reproduction does not require a search for a breeding partner. Asexual reproduction may be beneficial for the species in case of nutrition abundance and absence of risks. When living conditions are stable, all organisms adapted to them can develop and reproduce without major obstacles, but under unfavourable living conditions, each of organisms with identical features is equally threatened with negative impact of the environment. If the parental individuals and the offspring react to temperature changes in the same way, they can die simultaneously in case of severe frost.

Many organisms reproduce asexually in nature. The simplest and quickest form of such reproduction is cell division into two daughter cells. Another way of asexual reproduction is buddingbuddingbudding, which occurs in some single‑cell fungi and aquatic organisms belonging to sedentary species. Fungi, moss and ferns reproduce by producing sporessporespores. Quite a common manner of asexual reproduction is body division (fragmentation), which occurs in part of plants, fungi, lichens, algae and in some animals. Plants reproduce asexually through the growth and division of aboveground or underground organs.

Sexual reproduction

Sexual reproduction in animals usually takes place with the participation of two parental individuals. It is preceded by the formation of reproductive cells called gametesgametegametes. Females produce female gametes, i.e. egg cellegg cellegg cell, males – male gametes, i.e. spermspermsperm. Fusion of the sperm nucleus and the egg cell nucleus is called fertilisationfertilisationfertilisation. As a result of this process, a new individual with an individual and unique set of features inherited from the parents is created.

Sexual reproduction therefore leads to the production of offspring similar to each other and to the parents, but with different features at the same time. The diversity of these features makes offspring have different chances of survival in case of changing environmental conditions. Therefore, even under very difficult conditions, some of the animals created as a result of sexual reproduction may be endowed with a feature useful in a given environment and have a higher chance of survival and reproduction. Most animals are dioeciousdioeciousdioecious, which means that a single individual is either a female or a male. However, there are species in which one individual produces both female and male gametes. Such organism is called a hermaphrodite.hermaphroditehermaphrodite.

Plants and fungi also reproduce sexually.

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Ilustracja przedstawia plemnik człowieka. Ma on wydłużony kształt. Jajowata główka po lewej stronie przechodzi w cieńszą szyjkę i wstawkę z licznymi mitochondriami. W prawo ciągnie się długa witka.

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Grafika przedstawia ludzką komórkę jajową. Komórka jajowa otoczona dwiema warstwami. Od wewnątrz otacza ją osłonka. Od zewnątrz promienisty wieniec utworzony z innych komórek.

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Grafika przedstawia proces inseminacji. Widoczny jest plemnik, który przedostał się przez zewnętrzną i wewnętrzną osłonę do wnętrza komórki jajowej. Długa witka plemnika odpadła od główki i pozostała poza wnętrzem komórki jajowej. Do samego wnętrza komórki jajowej dostała się tylko główka plemnika.

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Grafika przedstawia procesy powstawania zygoty, po połączeniu się główki plemnika z jądrem komórki jajowej. Zygota przedstawiona jest jako owal złożony z dwóch różnych połówek. Jedna z nich oznaczona jest kolorem fioletowym, druga z nich kolorem różowym

Fertilisation

Animals can be fertilised in two ways. Internal fertilization occurs when the gametes are fused inside the body of the parental organism. In animals, it usually occurs during the act of copulation, in which the male introduces sperm into the female's genital tract by means of an intromittent organ. This type of fertilisation is characteristic mainly of terrestrial animals. External fertilisation takes place outside the body of the parental organisms, usually in aqueous environment. It consists in the secretion of gametes by males and females into the water. This usually happens at the same time and results from the mating activity of the animals. This type of fertilisation is found, for example, in most fish and amphibians.

Development after fertilisation

Fusion of sperm and the egg cell leads to the formation of a zygotezygotezygote, from which embryoembryoembryo develops in multicellular organisms. During embryo development, many intense changes take place. All cells and organs of a multicellular individual are formed from a single cell, i.e. a zygote.

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Ilustracja przedstawia różne etapy rozwoju zarodka. Od lewej w klamrze znajdują się: komórka jajowa, egg cell i plemnik, sperm. Proces ich połączenia to zapłodnienie, fertilization. Dalej strzałka w prawo wskazuje zygotę, zygote, której jądro ma dwa kolory. Dalej w prawo strzałka wskazuje skupisko podobnych komórek, przedstawiające zarodek, embryo. Na końcu ukazano duży zarodek z pierwszymi narządami, embryo with first organs między innymi głową i ogonem.

The process of embryo development requires a lot of energy. In some animals, the amount of nourishment necessary for embryonal development can be collected in egg yolks, in others it is supplied by blood vessels from the mother's organism. In case of animals producing a lot of eggs, these eggs generally contain small quantities of egg yolks. For this reason, the individuals that hatch from them are not fully developed. After hatching, they must prey on their own before they grow up. At this stage of development they are called larvae. The development during while the organism undergoes the larvae stage is called complex development. Animals which provide the embryos with a lot of nutrition during their development are characterized by a simple development, without the larvae stage.

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Ilustracja przedstawia porównanie rozwoju prostego i złożonego. Na jednej grafice ukazano prosty rozwój, simple development krowy od zygoty (zygote - fertilized egg), przez młodego osobnika (young specimen) do osobnika dorosłego (adult specimen). Na drugiej grafice przedstawiono złożony rozwój, complex development pszczoły od zygoty (zygote - fertilized egg), przez larwę (larva), przez poczwarkę (pupa) do osobnika dorosłego (adult specimen).

In spermatophytes, the embryo develops in the seed – an organ filled with a reserve of nutrients – which is covered by the seed husk. Under favourable conditions, the seed sprouts and a new plant develops from the embryo.

Conclusion

• Reproduction is a condition for the existence of species.

• Reproductive cells (gametes) produced by parental organisms are involved in sexual reproduction.

• Thanks to sexual reproduction, the individuals differ from each other, which increases the chances of the species survival.

• As a result of asexual reproduction, many offsprings produced from the same parental organism are substantially similar in genetic terms to the parental organism and to each other.

Keywords

sexual reproduction, asexual reproduction, gametes

Glossary