Chapter 7: How do Organisms Reproduce?
Reproduction and variation
During reproduction the DNA of the cell is copied. Small errors cause variation, which helps a species survive in a changing environment.
Asexual reproduction
Binary fission (Amoeba), multiple fission (Plasmodium), fragmentation (Spirogyra), regeneration (Hydra, Planaria), budding (Hydra, yeast), vegetative propagation (potato, Bryophyllum, rose cutting) and spore formation (Rhizopus). Only one parent is involved.
Sexual reproduction in flowering plants
A flower has the stamen (anther, filament) as its male part and the pistil (stigma, style, ovary) as its female part. After pollination the pollen tube reaches the ovary and fertilisation takes place. The zygote becomes an embryo, the ovule becomes a seed and the ovary becomes a fruit.
Reproduction in humans
Male: the testes make sperms and testosterone. Female: the ovaries make eggs. Fertilisation takes place in the oviduct, the embryo is implanted in the uterus and gets nutrition through the placenta. If the egg is not fertilised, menstruation occurs. Knowing about contraception and about preventing sexually transmitted diseases is also important.
Important questions
Q1. Why is variation important for a species?
In a changing environment some individuals survive, so the species continues to exist.
Q2. What is the importance of DNA copying in reproduction?
It gives new cells the same body design. Small errors in copying also create variations.
Q3. What are binary fission and multiple fission?
In binary fission a cell divides into two, as in Amoeba. In multiple fission a cell divides into many cells, as in Plasmodium.
Q4. What is fragmentation?
When an organism breaks into pieces and each piece grows into a new individual, as in Spirogyra.
Q5. What is regeneration? Give examples.
When a complete organism grows back from a cut part, as in Hydra and Planaria.
Q6. What is budding?
A bud grows as an outgrowth on the parent body and later detaches, as in Hydra and yeast.
Q7. What is vegetative propagation? Give examples.
A new plant grows from the root, stem or leaf of a plant, such as the eye of a potato, the leaf of Bryophyllum or a rose cutting. The new plants are identical to the parent.
Q8. What is spore formation?
Spores formed in sporangia of the parent germinate under favourable conditions to form a new plant, as in Rhizopus.
Q9. What is the main difference between asexual and sexual reproduction?
Asexual reproduction needs one parent and the offspring are almost identical. Sexual reproduction needs two parents and the offspring show variation.
Q10. Name the male and female reproductive parts of a flower.
Male: stamen (anther and filament). Female: pistil (stigma, style and ovary).
Q11. What is pollination?
The transfer of pollen from the anther to the stigma. It can be self-pollination or cross-pollination, carried by wind, water or insects.
Q12. How does fertilisation happen in a flower after pollination?
The pollen tube grows through the style to the ovary, and the male gamete fuses with the egg to form a zygote. The ovule becomes the seed and the ovary becomes the fruit.
Q13. What is the function of the testes?
To make sperms (male gametes) and the hormone testosterone.
Q14. Why are the testes outside the body cavity, in the scrotum?
Sperm formation needs a temperature lower than the normal body temperature.
Q15. State the functions of the ovary.
To make eggs (female gametes) and female hormones. One egg matures and is released every month.
Q16. Where does fertilisation take place in humans?
In the oviduct (fallopian tube). The zygote then gets implanted in the uterus.
Q17. What is the function of the placenta?
It provides nutrition and oxygen to the embryo from the mother’s blood and removes waste products.
Q18. What happens if the egg is not fertilised?
The thick lining of the uterus breaks down and leaves the body with blood and mucus. This is menstruation, which happens about once every month.
Q19. Name two methods of contraception.
Barrier methods (such as a condom), oral pills, the copper-T (IUD), and surgical methods (sterilisation).
Q20. Name two sexually transmitted diseases and one way to prevent them.
Gonorrhoea, syphilis and HIV-AIDS. They can be prevented by using condoms.
Quiz
1. By which method does Amoeba reproduce?
Amoeba divides into two to reproduce.
2. From which part does a new Bryophyllum plant grow?
Buds grow on the edges of a Bryophyllum leaf.
3. Which is the male reproductive part of a flower?
The stamen is the male reproductive part.
4. Where does fertilisation take place in humans?
Fertilisation takes place in the oviduct.
5. Which method is seen in Plasmodium?
Plasmodium reproduces by multiple fission.