Sexual Reproduction in PlantsCambridge IGCSE Biology: Revision notes
Section 1
What is sexual reproduction in plants?
Sexual reproduction in plants involves the fusion of two gametes (sex cells) to produce offspring. In flowering plants, male gametes are produced in the anther (part of the stamen) and female gametes are produced in the ovule (inside the ovary). The process requires pollination (transfer of pollen) followed by fertilisation.
Unlike asexual reproduction, sexual reproduction produces genetically different offspring because each gamete contains a different combination of genetic material. This genetic variation is a key characteristic that distinguishes sexual from asexual reproduction.
Examiners test whether you understand that sexual reproduction produces genetically different offspring. Be explicit about this—say 'offspring are not clones' or 'offspring have different alleles' rather than just 'offspring are different'.
Section 2
What is fertilisation and why does it matter?
Fertilisation is defined as the fusion of the nuclei of two gametes to form a single nucleus. This is a critical concept because it explains how genetic material from two parents combines in offspring.
During fertilisation:
- A male gamete (sperm nucleus) enters the ovule
- The nucleus of the male gamete fuses with the nucleus of the female gamete (egg nucleus)
- A new nucleus forms, creating a zygote
The zygote then divides by mitosis to develop into an embryo, which eventually grows into a new plant. Fertilisation is therefore the moment when two separate genetic contributions merge into one organism.
Students often confuse pollination with fertilisation. Remember: pollination is the transfer of pollen (a physical process), while fertilisation is the fusion of nuclei (a biological process). Pollination must happen before fertilisation can occur.
Think of pollination as a delivery service bringing a parcel to your door, and fertilisation as opening the parcel and mixing its contents with yours.
Section 3
Why are haploid gametes and diploid zygotes important?
Understanding chromosome numbers is essential to explaining how sexual reproduction works:
| Stage | Chromosome Number | Cell Type | Significance |
|---|---|---|---|
| Gamete | Haploid (n) | Sperm or egg | Contains only half the chromosomes; formed by meiosis |
| Zygote | Diploid (2n) | Fertilised egg | Contains full chromosome set; formed when gamete nuclei fuse |
Why this matters:
- Gametes are haploid (n): They contain half the number of chromosomes as the parent plant. In humans, gametes have 23 chromosomes.
- Zygotes are diploid (2n): They contain the full complement of chromosomes. The zygote has chromosomes from both parents.
- If gametes were diploid, the chromosome number would double with each generation, which would be lethal.
This is why meiosis produces haploid gametes—it reduces the chromosome number by half, so fertilisation restores the normal diploid number in the zygote.
Examiners specifically ask about haploid and diploid. You must state: 'Gamete nuclei are haploid' and 'The zygote nucleus is diploid' using these exact terms. Explain that fusion of two haploid nuclei (n + n) produces a diploid nucleus (2n).
In a plant with 14 chromosomes in its body cells (diploid = 2n = 14), a gamete will have 7 chromosomes (haploid = n = 7). When a male gamete (7 chromosomes) fuses with a female gamete (7 chromosomes), the zygote has 14 chromosomes (diploid = 2n = 14).
Section 4
What are the advantages of sexual reproduction in wild populations?
Sexual reproduction provides significant benefits to wild plant populations:
Genetic Variation
- Offspring are genetically different from parents and siblings because they receive different combinations of alleles from each parent
- This variation means the population is not all identical, reducing the risk of a disease or environmental change wiping out the entire population
Adaptation to Changing Environments
- If the environment changes (e.g. temperature, drought, new predator), some individuals may already possess advantageous alleles
- These individuals survive and reproduce, passing beneficial alleles to offspring
- Over time, the population adapts without needing to rely on random mutations
Resilience
- Genetic diversity makes populations more stable and resilient to environmental stress
- A genetically uniform population (like those produced by asexual reproduction) is vulnerable to disease and environmental change
Disadvantage in Wild Populations
- Sexual reproduction is energetically costly: plants must produce flowers, attract pollinators, and grow seeds
- Slower reproduction rate compared to asexual reproduction—takes longer to establish a population
- Requires pollination (not guaranteed) rather than self-reproduction
When writing about advantages, use phrases like 'genetic variation means the population can adapt if the environment changes' or 'some individuals may have alleles that allow survival in new conditions'. Link variation directly to population survival.
Section 5
What are the advantages and disadvantages of sexual reproduction in crop production?
Farmers face different priorities than wild populations, so sexual reproduction has distinct benefits and drawbacks in agriculture:
Advantages for Crop Production
- Genetic diversity allows selective breeding—farmers can choose plants with desirable traits (high yield, disease resistance, larger fruit) and cross them to produce improved varieties
- New varieties can be developed to meet changing needs (e.g. crops resistant to new pests or adapted to new climates)
- Hybrid vigour (heterosis): offspring from crosses between different varieties often show superior traits and higher yields
Disadvantages for Crop Production
- Unpredictable offspring: Genetic variation means crops are not uniform in appearance, size, or ripening time. Farmers cannot guarantee consistent product quality for markets
- Lower consistency: Some seeds may produce weak or poor-quality plants, reducing reliability compared to asexual reproduction
- Time-consuming: Sexual reproduction and selective breeding take many generations (years) to develop new varieties, whereas asexual reproduction produces identical copies instantly
- Pollination requirements: Crops may require bees or hand pollination, adding cost and complexity
- Higher seed costs: Cross-bred seeds are expensive to produce and may not breed true in subsequent generations
Why asexual reproduction is often preferred in crops
- Produces genetically identical offspring with guaranteed uniform quality, size, and ripening
- Faster establishment of large uniform crops
- Can reproduce desirable varieties without genetic segregation
Examiners test whether you can compare sexual and asexual reproduction for crops. Structure answers as: 'Sexual reproduction allows [advantage], but produces [disadvantage]. Asexual reproduction ensures [advantage], but cannot [disadvantage].' This shows contrast and understanding.
A farmer growing strawberries through sexual reproduction gets genetically varied plants—some large, some small, ripening at different times (bad for market consistency). Using asexual reproduction (runners), all plants are clones, ripening simultaneously with identical fruit size (perfect for supermarket sale).
Must Know
- Sexual reproduction fuses two haploid gamete nuclei to form a diploid zygote, producing genetically different offspring
- Fertilisation is defined as the fusion of the nuclei of two gametes; it is distinct from pollination
- Gamete nuclei are haploid (n) with half the chromosomes; zygote nuclei are diploid (2n) with the full chromosome number restored
- In wild populations, sexual reproduction provides genetic variation, allowing adaptation to environmental change and increasing population resilience (main advantage), but requires energy and time (disadvantage)
- In crop production, sexual reproduction allows selective breeding and development of new varieties (advantage), but produces genetically variable, unpredictable offspring lacking uniformity (disadvantage)
- Asexual reproduction is preferred in many crops because it produces genetically identical, uniform offspring reliably, but cannot adapt to new environmental conditions
That's the notes covered.
Carry on to the next subtopic.