1. Explain the process of fertilization in flowering plants.
Answer:
Fertilization in flowering plants involves several key steps. First, pollen grains are transferred from the male reproductive part (anther) to the female reproductive part (stigma) of a flower, a process called pollination. After pollination, the pollen grain germinates on the stigma, forming a pollen tube that grows down through the style toward the ovary. The sperm cells travel down the pollen tube. One sperm cell fuses with the egg cell in the ovule, leading to the formation of a zygote. This process is called syngamy. After fertilization, the ovule develops into a seed, and the ovary transforms into a fruit.
2. What is internal fertilization and how does it differ from external fertilization?
Answer:
Internal fertilization occurs when fertilization takes place inside the female’s body. This process is common in mammals, reptiles, and birds. It provides a more protected environment for the embryo. Sperm are deposited inside the female reproductive tract, where they meet the egg for fertilization.
In contrast, external fertilization occurs outside the female’s body, typically in aquatic animals such as fish and amphibians. The female releases eggs into the water, and the male releases sperm over them, allowing fertilization to occur externally.
3. Describe the role of the acrosome in fertilization in animals.
Answer:
The acrosome is a cap-like structure present in the head of sperm cells. It contains enzymes such as hyaluronidase and acrosin, which are critical for the sperm to penetrate the zona pellucida surrounding the egg. When a sperm cell comes into contact with the egg, the acrosome releases its enzymes, breaking down the protective layers of the egg and allowing the sperm to fuse with the egg’s membrane. This fusion leads to fertilization, forming a zygote.
4. What are the key stages of fertilization in mammals?
Answer:
In mammals, fertilization follows these stages:
- Gamete Recognition: The sperm recognizes and binds to the egg’s zona pellucida.
- Acrosomal Reaction: The sperm releases enzymes from its acrosome to digest the egg’s protective layer.
- Sperm-Egg Membrane Fusion: The sperm and egg membranes fuse, allowing the sperm nucleus to enter the egg.
- Cortical Reaction: Once the sperm enters the egg, the egg releases cortical granules to prevent other sperm from entering (preventing polyspermy).
- Zygote Formation: The sperm and egg nuclei fuse to form a zygote, which then undergoes the first cell division.
5. What is the significance of the zona pellucida in mammalian fertilization?
Answer:
The zona pellucida is a glycoprotein layer surrounding the egg cell in mammals. It plays a crucial role in fertilization by serving as a barrier that the sperm must penetrate before it can fuse with the egg. The acrosomal reaction of the sperm releases enzymes that help break down this layer. After fertilization, the zona pellucida undergoes a change to prevent polyspermy, which is the fertilization of the egg by multiple sperm.
6. Discuss the fertilization process in external fertilization and provide examples.
Answer:
In external fertilization, both the eggs and sperm are released into the surrounding environment, where fertilization occurs outside the female’s body. This process is common in aquatic animals like fish and amphibians, where environmental conditions, such as water, facilitate the movement and union of gametes. The female releases eggs into the water, and the male releases sperm in the same area. Fertilization happens when sperm encounters eggs in the water. The process relies heavily on environmental factors like temperature and water quality, and is typically synchronized to ensure a high chance of fertilization.
7. How does fertilization contribute to the development of a new organism in animals?
Answer:
Fertilization marks the beginning of the developmental process in animals. The union of male and female gametes (sperm and egg) forms a zygote, a single-cell organism with a complete set of chromosomes (half from each parent). The zygote undergoes repeated cell divisions, starting with cleavage, which leads to the formation of a blastula. The blastula then develops into a gastrula, establishing the primary germ layers (ectoderm, mesoderm, and endoderm), which will later differentiate into various tissues and organs. Through these stages, fertilization is the first step in the development of a new individual.
8. What are the advantages and disadvantages of external fertilization in aquatic animals?
Answer:
Advantages of external fertilization include:
- High genetic diversity, as sperm from multiple males may fertilize eggs.
- Low energy investment, as the male does not need to invest in internal fertilization or parental care.
- Large numbers of offspring are produced, increasing the chances of some surviving.
Disadvantages include:
- Environmental dependency, as successful fertilization depends on water temperature, currents, and other external factors.
- High risk of predation, as eggs and larvae are more vulnerable to predators in the environment.
9. Explain the role of the sperm’s tail in fertilization.
Answer:
The tail of a sperm, also known as the flagellum, is essential for motility. It propels the sperm through the female reproductive tract toward the egg. This movement is driven by the energy stored in the sperm’s mitochondria. The tail enables the sperm to swim and navigate towards the egg, facilitating its encounter with the egg for fertilization. The movement of the sperm’s tail is crucial for successful fertilization, particularly in internal fertilization, where sperm must travel through the cervix and uterus.
10. What is the function of the cortical reaction in preventing polyspermy?
Answer:
The cortical reaction occurs immediately after the sperm fuses with the egg’s plasma membrane. This reaction involves the release of cortical granules from the egg’s cytoplasm, which causes changes to the zona pellucida. The zona pellucida hardens and forms a barrier that prevents any additional sperm from entering the egg. This ensures that the egg is fertilized by only one sperm, preventing polyspermy, which could result in an abnormal number of chromosomes and a non-viable zygote.
11. Describe the process of double fertilization in flowering plants.
Answer:
Double fertilization is a unique process in flowering plants. After pollination, the pollen grain lands on the stigma and germinates, forming a pollen tube. The sperm cells travel down the tube to the ovule, where one sperm cell fuses with the egg cell to form the zygote (this is the first fertilization event). The second sperm cell fuses with two other nuclei in the ovule to form the triploid endosperm, which will develop into tissue that nourishes the developing embryo. This process ensures the survival of the seed after fertilization.
12. What are the steps involved in sperm-egg fusion during fertilization in humans?
Answer:
The process of sperm-egg fusion in humans includes several key steps:
- Sperm capacitation: Sperm undergoes biochemical changes that enhance their ability to fertilize the egg.
- Acrosomal reaction: The sperm releases enzymes from the acrosome to break down the zona pellucida surrounding the egg.
- Sperm binding: The sperm binds to specific receptors on the egg’s membrane.
- Fusion of membranes: The sperm’s membrane fuses with the egg’s membrane, allowing the sperm’s nucleus to enter the egg.
- Cortical reaction: The egg’s cortical granules are released, preventing polyspermy.
- Zygote formation: The sperm and egg nuclei fuse to form the diploid zygote.
13. How does the sperm reach the egg in internal fertilization?
Answer:
In internal fertilization, sperm is deposited in the female reproductive tract, usually through copulation. The sperm then swim through the cervix into the uterus and towards the fallopian tubes, where fertilization typically occurs. The tail (flagellum) of the sperm propels it forward by whip-like movements. The sperm must overcome various obstacles in the female reproductive tract, including the acidic environment in the vagina, to reach and fertilize the egg.
14. What are the key differences between internal and external fertilization?
Answer:
The key differences between internal and external fertilization are:
- Location of fertilization: Internal fertilization occurs inside the female body, while external fertilization takes place outside in the environment (usually in water).
- Protection of gametes: In internal fertilization, the egg is protected within the female reproductive tract, whereas in external fertilization, eggs and sperm are exposed to environmental factors.
- Number of offspring: External fertilization generally produces a large number of offspring, while internal fertilization often produces fewer, but more protected offspring.
- Energy investment: Internal fertilization requires more energy investment in the form of parental care, while external fertilization involves less parental involvement after gametes are released.
15. What is the role of the egg’s jelly coat in fertilization?
Answer:
The jelly coat surrounding the egg in many animals (especially aquatic species like amphibians and fish) plays an important role in fertilization. It serves as a protective barrier and helps attract sperm to the egg. In some species, the jelly coat contains chemicals that trigger the acrosomal reaction in sperm. This reaction allows sperm to digest through the jelly coat and the egg’s outer layers to reach the egg’s membrane for fusion.
16. How does the fertilization process in animals differ in oviparous and viviparous species?
Answer:
In oviparous species (like many reptiles and birds), fertilization is often internal, but the embryo develops outside the mother’s body within an egg. After fertilization, the egg is laid, and the embryo develops within the egg until it hatches.
In viviparous species (like mammals), fertilization also occurs internally, but the embryo develops inside the mother’s body, where it receives nourishment through a placenta. The embryo is protected within the mother’s body until it is born.
17. Explain the concept of “polyspermy” and how it is prevented in animals.
Answer:
Polyspermy refers to the fertilization of an egg by more than one sperm, which leads to an abnormal number of chromosomes and results in a non-viable zygote. In animals, polyspermy is prevented by two key mechanisms: the cortical reaction, where the egg releases enzymes that alter the zona pellucida to form a block to additional sperm entry, and the fast block to polyspermy, where the egg’s membrane potential changes immediately after sperm fusion, preventing other sperm from fusing.
18. How does fertilization lead to genetic variation in offspring?
Answer:
Fertilization brings together the genetic material from two parents, each contributing half of the genetic information. The shuffling of alleles during gamete formation (meiosis) results in a unique combination of genes in the zygote. This genetic recombination during fertilization ensures that offspring inherit a mix of traits from both parents, promoting genetic diversity, which is essential for evolution and adaptation to changing environments.
19. Discuss the process of fertilization in amphibians and how it differs from that in mammals.
Answer:
In amphibians, fertilization is usually external, where eggs are laid in water and sperm are released over them by the male. This process relies on water to facilitate the union of sperm and egg.
In contrast, mammals generally undergo internal fertilization. Male sperm is deposited inside the female reproductive tract, and fertilization occurs within the female’s body. Mammals also exhibit more extensive parental care and embryonic development inside the uterus.
20. Describe the role of the male reproductive system in fertilization.
Answer:
The male reproductive system plays a critical role in fertilization. It consists of the testes, which produce sperm, and the accessory glands (like the prostate and seminal vesicles) that provide the fluids in semen. During ejaculation, sperm travel through the vas deferens and are expelled from the body through the urethra. The sperm are then deposited in the female reproductive tract, where they travel to meet the egg for fertilization. The male reproductive system ensures the delivery of sperm in optimal condition for successful fertilization.
