Our Home: Earth, a Unique Life Sustaining Planet · Lesson 5 of 7
Sexual Reproduction in Plants and Animals
“Follow gametes through fertilisation and development, comparing flowers, seeds, eggs, and young animals.”
• Explain gamete fusion, formation of a zygote, and inherited variation. • Identify the reproductive roles of anthers, pollen, ovules, and ovaries. • Distinguish pollination from fertilisation. • Trace development from a fertilised egg to an embryo, seed, and growing plant. • Compare external and internal fertilisation and offspring development. • Investigate pollinator attraction, offspring survival, and parental care.
Two Gametes Contribute to a New Organism
Sexual reproduction involves the fusion of reproductive cells called gametes. In the familiar plant and animal examples, a male gamete and a female gamete combine their inherited information. This process produces a zygote, the first cell of the developing organism.
A reproductive cell that can fuse with another gamete during sexual reproduction. Each contributes one half of the usual genetic complement in these examples.
Fusion of a male gamete and a female gamete to form a zygote.
The cell formed by gamete fusion, containing the combined genetic contribution from the two gametes.
Each gamete contributes half the usual genetic complement, and the zygote receives a complete complement through their combination. This explains why the inherited complement does not simply double at every generation. The zygote divides and develops into an embryo, an early developing organism. We do not need to study the detailed cell divisions here to understand the main sequence: gametes, fusion, zygote, embryo, and further growth.
Combining inherited information can produce new combinations of features. Siblings can share many family features but differ because they do not necessarily receive the same combination. Sexual reproduction therefore links continuity with variation. It does not guarantee that every offspring looks exactly halfway between the visible appearances of its parents.
Problem
Two kittens from the same parents have different coat patterns. Must one have failed to inherit information?
- 1.Both kittens receive inherited information through gametes.
- 2.The inherited combinations can differ between offspring.
- 3.Different coat patterns are compatible with sexual reproduction and family resemblance; complete visible identity is not required.
Locate the Reproductive Parts of a Flower
Flowers contain structures that help produce gametes and bring them together. The anther produces pollen, which carries the cells involved in delivering male gametes. The female part includes a stigma that receives pollen and an ovary containing ovules.
Transfer of pollen from an anther to a stigma.
An ovule contains the egg cell, the female gamete. Keep the levels distinct: an ovule is a structure containing the egg, not the egg itself, and a pollen grain is not simply another name for a male gamete. The pollen helps deliver male gametes so that fertilisation can occur inside the ovule.
Pollination Prepares the Way for Fertilisation
Wind, insects, and other animals can transport pollen. An insect visiting a flower may pick up pollen and later bring some to a stigma. Bright colours or scents can help attract particular visitors, connecting a plant’s reproduction with other living organisms.
Pollen may reach a stigma of the same flower or another flower on the same plant, or it may travel between plants of the same species. Sexual reproduction still involves gamete fusion when the gametes come from one plant; it does not always require two separate individual plants. Suitable pollen transfer prepares the way, but pollination and fertilisation are distinct events.
After suitable pollen reaches a stigma, a pollen tube can grow toward an ovule and deliver a male gamete. Fusion of the male gamete with the egg produces a zygote. An insect landing on a petal is not by itself evidence of fertilisation: the pollen must reach a suitable stigma and the later processes must succeed.
Problem
A bee carries pollen from an anther to a stigma. A student calls this fertilisation. Correct the description.
- 1.The observed event is pollen transfer to the stigma, which is pollination.
- 2.Fertilisation is the later fusion of a male gamete with the egg inside an ovule.
- 3.Pollination can enable fertilisation, but observing the first event does not establish that the second has already happened.
From a Fertilised Egg to a New Plant
After fertilisation, the zygote develops into the embryo. The ovule develops into a seed, with structures that protect the embryo and provide stored food. The ovary generally develops into the fruit containing the seeds.
The zygote becomes the embryo, the ovule becomes the seed, and the ovary generally becomes the fruit. Saying only “the zygote becomes a seed” misses the surrounding structures. Fruit and seed formation connect the early developing plant with protection and opportunities for dispersal.
| Before development | Later structure | Main relationship |
|---|---|---|
| Zygote | Embryo | The new organism begins development |
| Ovule | Seed | The developing embryo is enclosed within the seed |
| Ovary | Fruit | The fruit generally surrounds the seeds |
Dispersal moves seeds away from the parent plant. A bird eating a banyan fruit can carry seeds and deposit them elsewhere, including in a wall crack. When moisture and other suitable conditions become available, a seed may germinate. Rain during a monsoon can help explain why plants suddenly appear in cracks where dry conditions previously prevented germination.
During germination, water helps restart growth, and the embryo uses stored food before it can support itself through photosynthesis. A root grows into the substrate and a shoot grows toward the light. Leaves develop and begin making food under suitable conditions. Seeds need conditions such as suitable moisture, air, and temperature; the requirements vary among plants.
Problem
A small banyan appears in a damp wall crack although nobody planted one there. Construct a plausible explanation.
- 1.A seed may have reached the crack through a bird that ate fruit, or another dispersal route.
- 2.The crack could retain a small amount of substrate and moisture.
- 3.Suitable rainy conditions allow germination using food stored in the seed.
- 4.The growing plant develops roots and leaves; its appearance does not mean the wall created the organism spontaneously.
Fertilisation Can Occur Outside or Inside Animals
In animals, the male gamete is a sperm and the female gamete is an egg. In many fish and frogs, eggs and sperm are released into water, where fusion can occur outside the body. This is external fertilisation, and water provides the setting where the gametes can meet.
Gamete fusion occurring outside the parent’s body, commonly in water in the examples studied here.
Gamete fusion occurring inside the female’s body in the animal examples studied here.
Birds and mammals have internal fertilisation. The location of fertilisation and the later location of embryo development are different questions: internal fertilisation does not always mean development continues inside the mother until birth. Birds undergo internal fertilisation and lay eggs, while most mammals keep the developing young inside the mother and give birth to live young.
Protecting and Nourishing Developing Young
A bird embryo starts developing before the egg is laid and continues development in the egg during incubation. Stored nutrients, including those in the yolk, support development. The egg protects the embryo, but suitable temperature and other conditions remain important until hatching.
In most mammals, the embryo develops inside the mother and receives nourishment and oxygen through specialised connections with her body. The mother later gives birth to a live young animal. “Most” matters: egg-laying mammals are exceptions, so the visible distinction between hens and dogs should not become a claim that every mammal gives live birth.
There are advantages and costs in different arrangements. Developing inside the mother can provide protection and steady support, but requires resources from her. Eggs can develop outside the parent, but may be exposed to predators or unsuitable conditions unless protected. A reproductive strategy succeeds through a combination of offspring number, development, surroundings, and care.
Investigate Survival, Care, and Flower Visitors
Many fish and frogs produce large numbers of eggs, while only a fraction may survive to adulthood. Producing many offspring can increase the chance that some survive when early losses are high. The costs include resources used to produce the eggs and the loss of many developing young; surviving young may also compete for limited resources. Large numbers do not mean every egg is fertilised or that all fish and frogs provide the same amount of care.
Compare parental care in birds and selected reptiles using reliable observations. Some birds incubate eggs, feed young, and guard nests; reptiles vary, with some guarding eggs or young. Avoid concluding that all snakes or all reptiles lack care. Ask how protection, food, predators, and habitat influence survival, and compare named examples rather than entire groups without evidence.
For a flower investigation, observe flowers with different colours or scents from a safe distance. Record the flower type, visitor, visit duration, and whether contact with anthers or stigma is visible. Keep observation times comparable. Visitor counts alone do not prove successful fertilisation; the investigation can show patterns of attraction and possible pollen transfer.
Some yeasts have sexual cycles in which compatible mating types fuse and combine genetic information. Bacteria can exchange genetic material, but that exchange is not the sperm–egg or gamete-fusion process described here; bacterial multiplication commonly occurs through division. Do not treat all microbial genetic exchange as the same reproductive mechanism.
Quiz
What forms when male and female gametes fuse?
Which event is pollination?
Which development relationship is correct?
What is external fertilisation?
Which statement about birds is correct?
What can different coat patterns in siblings illustrate?
Practice Problems
- Trace the sequence from male and female gametes to zygote, embryo, and further development.
- Explain why siblings can resemble their parents and still differ from one another.
- Distinguish anther, pollen, stigma, ovule, egg, and ovary by their roles.
- Describe pollination and fertilisation separately, including a case where the same plant supplies both gametes.
- Explain the relationships zygote–embryo, ovule–seed, and ovary–fruit, then trace seed germination.
- Explain how a banyan could appear in a wall crack during rainy conditions.
- Compare fertilisation and development in a frog, a hen, and a dog. Keep location of fusion separate from location of later development.
- Explain why many eggs may be useful where offspring survival is low, and how parental care can affect survival.
- Plan an observation of flower visitors and state what it could show and what it could not prove.
Key Takeaways
• Sexual reproduction combines gametes to form a zygote and passes inherited information to a new generation. • Combined inherited contributions can produce variation among offspring. • Pollination is pollen transfer; fertilisation is gamete fusion. • Pollen delivers male gametes, while an ovule contains the egg. • The zygote develops into the embryo; the ovule becomes the seed and the ovary generally becomes the fruit. • Dispersal and suitable germination conditions help new plants grow away from the parent. • Fertilisation location and embryo-development location are separate features to compare. • Egg resources, suitable surroundings, offspring number, and parental care affect survival.