Home Notes Papers

Sexual reproduction in plants

Paper 1Paper 2Paper 3Paper 4Paper 5Paper 6

This topic is examined in Paper 1, Paper 2, Paper 3, Paper 4, Paper 5, and Paper 6.

Anatomy of an Insect-Pollinated Flower
To understand sexual reproduction, we must first identify the reproductive structures. In insect-pollinated flowers (e.g., roses, sunflowers), parts are adapted to attract and guide insects.
Function
Protect the flower bud before it opens.
Attract insect pollinators with colour and scent.
Produces and releases pollen grains containing male gametes.
The stigma traps pollen; the ovary contains ovules (which contain female gametes).

Key Visual Cues for Diagrams:

  • Anthers: Look for sacs hanging from filaments, often positioned centrally or slightly above the stigma.
  • Stigma: Look for a sticky, lobed, or textured surface at the top of the central structure.
  • Ovary: Look for the swollen base below the style, often containing small dots (ovules) inside.
Pollination
Pollination is defined as the transfer of pollen grains from an anther to a stigma.

It is crucial to distinguish this from fertilisation. Pollination is merely the transport step. It does not involve the fusion of nuclei.

Wind-Pollinated Flowers: Identification and Adaptations
Wind-pollinated flowers (e.g., grasses, maize) do not need to attract insects. Therefore, their structures are adapted for releasing and catching pollen carried by the wind.
Wind-Pollinated Flower
Long filaments that hang outside the flower to sway in the wind and release pollen easily.
Large, feathery, and protruding outside the flower to catch airborne pollen grains effectively.
Small, colourless, or absent (no need to attract insects).
Light, smooth, and produced in large quantities to increase the chance of landing on a stigma.

Identifying Wind-Pollinated Structures (LO 3):
When looking at a diagram of a wind-pollinated flower, identify:

  1. Anthers: They are typically exposed on long filaments extending well beyond the other floral parts.
  2. Stigma: It is distinctly large, feathery (branched), and extends outwards to maximize surface area for catching pollen.
⚠︎ Confusing Pollination with Fertilisation
The Error: Students often describe fertilisation when asked about pollination, or vice versa.

The Correction:

  • Pollination is the transfer of pollen from anther to stigma. It happens outside the ovary.
  • Fertilisation is the fusion of the male nucleus (from the pollen) with the female nucleus (in the ovule). This happens inside the ovule.

Why this matters: Examiners look for specific keywords. If you write 'fusion of nuclei' for pollination, you get zero marks because that is the definition of fertilisation.

Describing Structural Adaptations
When to use: When asked to 'describe' or 'explain' how a flower part is adapted for its function.

Examiner Expectation: You must link the structure directly to the function. Do not just list features; explain why that feature helps.

Correct Phrasing Example:

  • Incorrect: "The stigma is feathery."
  • Correct: "The stigma is large and feathery to increase surface area, which allows it to catch more pollen grains carried by the wind."

Reasoning: Examiners accept answers that explicitly state the mechanical advantage (e.g., 'increases surface area', 'allows easy release') rather than just naming the feature.

Self-Pollination vs. Cross-Pollination
Pollination can occur in two ways, which have significant implications for the genetic diversity of the plant population.
Genetic Consequence
Produces offspring that are genetically identical (clones) to the parent. Low variation.
Produces offspring with genetic variation. Increases diversity.

Impact on Population (LO 11):

  • Cross-pollination is generally advantageous for survival because genetic variation allows the population to respond better to changes in the environment (e.g., new diseases or climate change). Some individuals may have traits that allow them to survive.
  • Self-pollination relies on no external agents, which is useful if pollinators are scarce, but it leads to a lack of variation, making the population vulnerable to extinction if conditions change.
Fertilisation and Pollen Tube Growth (LO 12)
After pollination, the process of fertilisation begins. This is a sequence of events inside the carpel.
Step-by-Step Process:

  1. Germination: The pollen grain lands on the sticky stigma and absorbs moisture/nutrients.
  2. Pollen Tube Growth: A pollen tube grows down through the style. It is not passive; the tube actively extends towards the ovary.
  3. Entry into Ovule: The pollen tube enters the ovule through a small opening called the micropyle.
  4. Fusion: The nucleus of the pollen grain (male gamete) travels down the tube and fuses with the nucleus in the ovule (female gamete).

Note on Gametes: The female gamete is located inside the ovule (specifically within the embryo sac). Do not refer to the entire ovule as the gamete; the ovule becomes the seed after fertilisation.

Fertilisation
Fertilisation occurs when a pollen nucleus fuses with a nucleus in an ovule.

This fusion creates a zygote, which develops into an embryo. The ovary surrounding the ovule develops into the fruit, and the ovule itself develops into the seed.

⚠︎ Misidentifying Female Gametes

The Error: Students often call the female gamete an 'ovum' or 'egg cell' (animal terminology) or confuse the ovule with the gamete.

The Correction:

  • In plants, the structure containing the female gamete is the ovule.
  • The female gamete itself is inside the ovule. After fertilisation, the ovule becomes the seed.
  • Do not say 'the ovule fuses with the pollen'. Say 'the nucleus in the ovule fuses with the pollen nucleus'.
Seed Germination Conditions (LO 8)
Germination is the process where a seed begins to grow into a seedling. It requires specific environmental conditions.
Why is it required? (Function)
  1. Softens the seed coat to allow the embryo to emerge.
  2. Activates enzymes that break down stored food reserves (e.g., starch) into soluble sugars for respiration.
Required for aerobic respiration. This releases energy needed for cell division and growth of the embryo.
Enzymes involved in respiration and growth work best at an optimal temperature. Too cold = enzymes inactive; too hot = enzymes denatured.
Common Misconception: Light is NOT a general requirement for seed germination. Most seeds germinate in dark soil. (Note: Some specific seeds like lettuce require light, but this is an exception, not the rule).
Explaining Germination Requirements
When to use: When asked 'Why is oxygen/water needed for germination?'

Examiner Expectation: You must link the condition to a biological process.

Correct Phrasing Example:

  • Incorrect: "Seeds need oxygen to breathe."
  • Correct: "Oxygen is required for aerobic respiration, which releases energy for growth and cell division."
  • Correct: "Water activates enzymes that break down stored food reserves into soluble substances that can be transported to the growing embryo."

Reasoning: Examiners accept answers that mention 'respiration', 'energy', 'enzymes', or 'breakdown of food'. Vague terms like 'breathe' or 'drink' are not accepted.

Past Paper Style Questions
Q:
Identify the structure in the diagram that produces pollen grains. [1]
A:
Anther
Q:
Describe how the stigma of a wind-pollinated flower is adapted for its function. [2]
A:
  1. It is large/feathery (1).
    2. This increases surface area to catch pollen grains from the wind (1).
Q:
Distinguish between self-pollination and cross-pollination. [2]
A:
Self-pollination is transfer within the same flower or same plant. Cross-pollination is transfer to a different plant of the same species.
Q:
State two environmental conditions required for seed germination. [2]
A:
  1. Water
    2. Oxygen (or suitable temperature)
Q:
Describe the process of fertilisation in a flower. [2]
A:
The pollen nucleus fuses with the nucleus in the ovule.
Q:
Explain why cross-pollination is beneficial for a plant population. [2]
A:
  1. It produces genetic variation (1).
    2. This allows the population to respond to changes in the environment / survive new diseases (1).
Beta v0.7.8 Free while we're in beta — it transitions to paid post launch. Thank you for supporting us at this stage!