CIE IGCSE Biology

Revision Notes

16.1.2 Sexual Reproduction in Plants

Flowers & Pollination

  • Flowers are the reproductive organ of the plant
  • They usually contain both male and female reproductive parts
  • Plants produce pollen which contains a nucleus inside that is the male gamete
  • Unlike the male gamete in humans (sperm), pollen is not capable of locomotion (moving from one place to another)
  • This means plants have to have mechanisms in place to transfer pollen from the anther to the stigma
  • This process is known as pollination and there are two main mechanisms by which it occurs: transferred by insects (or other animals like birds) or transferred by wind
  • The structure of insect and wind-pollinated flowers are slightly different as each is adapted for their specific function

 

Parts of the flower

Parts of a Flower table, IGCSE & GCSE Biology revision notes

 

General flower structure

Flower structure, IGCSE & GCSE Biology revision notesStructure of a flower

Adaptations for Pollination

Features of an insect-pollinated flower

Adaptations for pollination A table, IGCSE & GCSE Biology revision notes

 

Features of a wind-pollinated flower

Adaptations for pollination B table, IGCSE & GCSE Biology revision notes

 

  • The pollen produced by insect and wind-pollinated flowers is also different:
    • Insect pollinated flowers produce smaller amounts of larger, heavier pollen grains that often contain spikes or hooks on the outside so they are better able to stick to insects
    • Wind pollinated flowers produce large amounts of small, lightweight pollen grains that are usually smooth 

Here is an example of a multiple-choice question asking students to use their knowledge to identify types of pollen grain:

Pollen grain question, IGCSE & GCSE Biology revision notesA multiple-choice question asking students to use their knowledge to identify types of pollen grain

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Self & Cross-Pollination

  • Cross-pollination occurs when the pollen from one plant is transferred to the stigma of another plant of the same species
  • This is the way most plants carry out pollination as it improves genetic variation
  • Occasionally, the pollen from a flower can land on its own stigma or on the stigma of another flower on the same plant – this is known as self-pollination
  • Self-pollination reduces genetic variety of the offspring as all the gametes come from the same parent (and are therefore genetically identical)
  • Lack of variation in the offspring is a disadvantage if environmental conditions change, as it is less likely that any offspring will have adaptations that suit the new conditions well
  • On the other hand, cross-pollination relies completely on the presence of pollinators and this can be a problem if those pollinators are missing (eg the reduction in bee numbers is of great importance to humans as bees pollinate a large number of food crops) – this doesn’t apply to wind-pollinated plants

Fertilisation: Basics

  • Fertilisation occurs when a pollen nucleus fuses with an ovum nucleus in the ovule
Extended Only

Fertilisation

  • As the pollen has no ‘tail’ to swim to the ovary of a plant, in order to reach the ‘female’ nucleus in the ovary it has to grow a pollen tube
  • This only happens if the pollen grain has landed on the right kind of stigma (i.e. of the same species as the flower the pollen came from)
  • The nucleus inside the pollen grain slips down the tube as it grows down the style towards the ovary
  • The ovary contains one or more ovules which each contain an ovum with a female nucleus that a male pollen nucleus can fuse with
  • Once the nuclei (pl) have joined together, that ovule has been fertilised and a zygote has been formed
  • The zygote will start to divide and eventually form a seed within the ovule
  • As different plants have different numbers of ovules, this explains why different fruits (which develop from the ovary) have different numbers of seeds (which develop from the ovules)

Growth of a pollen tube, IGCSE & GCSE Biology revision notesGrowth of a pollen tube

 

Fertilisation in a flowering plant, IGCSE & GCSE Biology revision notesFertilisation in a flowering plant

Exam Tip

Students often get confused between pollination and fertilisation in plants, but they are not the same thing.

Think of pollination as the plant’s equivalent to human sexual intercourse – after sex, the male sex cells (sperm) have been deposited into the female. But, for fertilisation to occur, the nucleus from a male sperm cell has to fuse with the nucleus of a female sex cell (egg) and the sperm has to travel to find the egg before this happens. It’s exactly the same in plants!

Author: Jenna

Jenna studied at Cardiff University before training to become a science teacher at the University of Bath specialising in Biology (although she loves teaching all three sciences at GCSE level!). Teaching is her passion, and with 10 years experience teaching across a wide range of specifications – from GCSE and A Level Biology in the UK to IGCSE and IB Biology internationally – she knows what is required to pass those Biology exams.
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