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Q12:
Define external fertilisation. Mention its disadvantages.
Solution :
Definition of External Fertilisation
In biological reproduction, external fertilisation is defined as the process wherein the fusion of male and female gametes (syngamy) occurs completely outside the body of the reproducing organisms. This mode of fertilisation is highly dependent on an external medium, almost exclusively an aquatic environment (such as marine or freshwater ecosystems). It is characteristic of most aquatic organisms, including the majority of algae, fishes (e.g., Osteichthyes or bony fishes), and amphibians (e.g., frogs).
For external fertilisation to be successful, a biological phenomenon known as synchronicity or simultaneous spawning is required. The male and female organisms must release their respective gametes (sperm and ova) at the exact same time and in close spatial proximity. [From an evolutionary biology perspective, this process relies on environmental cues such as photoperiod, temperature, and lunar cycles to trigger simultaneous gamete release.]
Mathematical Probability of Syngamy in External Environments
Because the gametes are expelled into a vast three-dimensional fluid space, the probability of successful fertilisation ($P_f$) is relatively low. It can be modeled conceptually as directly proportional to the number of sperm ($N_s$) and the number of eggs ($N_e$), and inversely proportional to the volume of the dispersion medium ($V$) and the diffusion rate of the aquatic currents ($D$):
$$P_f \propto \frac{N_s \times N_e}{V \times D}$$
To overcome the denominator (volume and diffusion), organisms practicing external fertilisation must drastically increase the numerator, necessitating the production of millions of gametes.
Visual Representation of External Fertilisation
The following schematic demonstrates the simultaneous release of gametes and subsequent external syngamy in an aquatic medium.
Disadvantages of External Fertilisation
While external fertilisation is effective in aquatic environments, it carries several significant biological and ecological drawbacks:
- Vulnerability to Predators (High Mortality Rate): Because the fertilised eggs (zygotes) and subsequently the developing embryos are left exposed in the open environment, they serve as a massive food source for aquatic predators. Consequently, a staggeringly low percentage of offspring survive to reach adulthood. [This is typical of an r-selected reproductive strategy, where high quantity is prioritized over individual survivability.]
- Extreme Environmental Dependence: The success of syngamy is entirely at the mercy of environmental variables. Fluctuations in water temperature, changes in pH, desiccation (if stranded on land), or strong water currents can either destroy the gametes or wash them away before fertilisation can occur.
- High Biological Wastage and Energy Expenditure: To ensure that at least a few gametes successfully meet and fertilise, the organism must allocate an enormous amount of metabolic energy to synthesize millions of ova and spermatozoa. The vast majority of these gametes fail to fertilise, representing a monumental waste of biological resources.
- Total Absence of Parental Care: Once spawning occurs, the parents generally abandon the gametes. Unlike internal fertilisation (where embryos are protected inside the maternal body or inside a hard-shelled amniotic egg), embryos resulting from external fertilisation possess no physical defense mechanisms or maternal nourishment beyond the initial yolk.
Final Solution: External fertilisation is the biological process where the fusion of male and female gametes (syngamy) occurs strictly outside the body of the organism, usually in an external aquatic medium like water. Its major disadvantages include a massive biological wastage of gametes, an extreme vulnerability of the developing embryos to predators and environmental fluctuations, and a near-total absence of parental care, resulting in severe mortality rates before the offspring reach adulthood.
More Questions from Class 12 Biology Reproduction in Organisms EXERCISES
- Q1: Why is reproduction essential for organisms?
- Q1: Why is reproduction essential for organisms?
- Q2: Which is a better mode of reproduction: sexual or asexual? Why?
- Q2: Which is a better mode of reproduction: sexual or asexual? Why?
- Q3: Why is the offspring formed by asexual reproduction referred to as clone?
- Q3: Why is the offspring formed by asexual reproduction referred to as clone?
- Q4: Offspring formed due to sexual reproduction have better chances of survival. Why? Is this statement always true?
- Q4: Offspring formed due to sexual reproduction have better chances of survival. Why? Is this statement always true?
- Q5: How does the progeny formed from asexual reproduction differ from those formed by sexual reproduction?
- Q5: How does the progeny formed from asexual reproduction differ from those formed by sexual reproduction?
- Q6: Distinguish between asexual and sexual reproduction. Why is vegetative reproduction also considered as a type of asexual reproduction?
- Q6: Distinguish between asexual and sexual reproduction. Why is vegetative reproduction also considered as a type of asexual reproduction?
- Q7: What is vegetative propagation? Give two suitable examples.
- Q7: What is vegetative propagation? Give two suitable examples.
- Q8(a): Define (a) Juvenile phase,
- Q8(a): Define (a) Juvenile phase,
- Q8(b): Define (b) Reproductive phase,
- Q8(b): Define (b) Reproductive phase,
- Q8(c): Define (c) Senescent phase.
- Q8(c): Define (c) Senescent phase.
- Q9: Higher organisms have resorted to sexual reproduction in spite of its complexity. Why?
- Q9: Higher organisms have resorted to sexual reproduction in spite of its complexity. Why?
- Q10: Explain why meiosis and gametogenesis are always interlinked?
- Q10: Explain why meiosis and gametogenesis are always interlinked?
- Q11(a): Identify each part in a flowering plant and write whether it is haploid (\(n\)) or diploid (\(2n\)). (a) Ovary ———————————
- Q11(a): Identify each part in a flowering plant and write whether it is haploid ((n)) or diploid ((2n)). (a) Ovary ———————————
- Q11(b): Identify each part in a flowering plant and write whether it is haploid (\(n\)) or diploid (\(2n\)). (b) Anther ———————————
- Q11(b): Identify each part in a flowering plant and write whether it is haploid ((n)) or diploid ((2n)). (b) Anther ———————————
- Q11(c): Identify each part in a flowering plant and write whether it is haploid (\(n\)) or diploid (\(2n\)). (c) Egg ———————————
- Q11(c): Identify each part in a flowering plant and write whether it is haploid ((n)) or diploid ((2n)). (c) Egg ———————————
- Q11(d): Identify each part in a flowering plant and write whether it is haploid (\(n\)) or diploid (\(2n\)). (d) Pollen ———————————
- Q11(d): Identify each part in a flowering plant and write whether it is haploid ((n)) or diploid ((2n)). (d) Pollen ———————————
- Q11(e): Identify each part in a flowering plant and write whether it is haploid (\(n\)) or diploid (\(2n\)). (e) Male gamete ———————————
- Q11(e): Identify each part in a flowering plant and write whether it is haploid ((n)) or diploid ((2n)). (e) Male gamete ———————————
- Q11(f): Identify each part in a flowering plant and write whether it is haploid (\(n\)) or diploid (\(2n\)). (f) Zygote ———————————
- Q11(f): Identify each part in a flowering plant and write whether it is haploid ((n)) or diploid ((2n)). (f ) Zygote ———————————
- Q12: Define external fertilisation. Mention its disadvantages.
- Q13: Differentiate between a zoospore and a zygote.
- Q13: Differentiate between a zoospore and a zygote.
- Q14: Differentiate between gametogenesis from embryogenesis.
- Q14: Differentiate between gametogenesis from embryogenesis.
- Q15: Describe the post-fertilisation changes in a flower.
- Q15: Describe the post-fertilisation changes in a flower.
- Q16: What is a bisexual flower? Collect five bisexual flowers from your neighbourhood and with the help of your teacher find out their common and scientific names.
- Q16: What is a bisexual flower? Collect five bisexual flowers from your neighbourhood and with the help of your teacher find out their common and scientific names.
- Q17: Examine a few flowers of any cucurbit plant and try to identify the staminate and pistillate flowers. Do you know any other plant that bears unisexual flowers?
- Q17: Examine a few flowers of any cucurbit plant and try to identify the staminate and pistillate flowers. Do you know any other plant that bears unisexual flowers?
- Q18: Why are offspring of oviparous animals at a greater risk as compared to offspring of viviparous animals?
- Q18: Why are offspring of oviparous animals at a greater risk as compared to offspring of viviparous animals?
CBSE Solutions for Class 12 Biology Reproduction in Organisms
Chapters in CBSE - Class 12 Biology
- Human Reproduction
- Reproductive Health
- Principles of Inheritance and Variation
- Molecular Basis of Inheritance
- Evolution
- Human Health and Disease
- Strategies for Enhancement in Food Production
- Microbes in Human Welfare
- Biotechnology: Principles and Processes
- Biotechnology and Its Applications
- Organisms and Populations
- Ecosystem
- Biodiversity and Conservation
- Environmental Issues
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