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4.7.7Additional lessons

Seed Dispersal and Germination — IB Biology HL Revision Notes & Topical Questions

5 min read 5 parts Lesson 61 of 101

📚 IB Biology HL Hub / Topical Revision
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Section 1 of 5Seed Dispersal

4.7.7 Seed Dispersal and Germination
Why disperse
  • Moving seeds away reduces competition with the parent for light, water and minerals, and helps colonise new habitats.
Wind
  • Seeds or fruits that are light, winged or plumed, such as sycamore and dandelion, are carried over long distances.
Animals
  • Hooked fruits attach to fur, and fleshy fruits are eaten so that seeds pass out in faeces.
Water and explosion
  • Buoyant fruits float, as in coconut, while dried pods can burst open and throw seeds out.

Section 2 of 5Conditions for Germination

Seeds contain an embryo and food reserves that fuel early growth until photosynthesis begins.

Water
  • Needed to rehydrate the seed, activate enzymes and allow metabolism to restart.
Oxygen
  • Needed for aerobic respiration to supply energy for growth.
Suitable temperature
  • Enzymes in the seed work best within a particular range of temperatures.
Dormancy
  • Seeds with low water content and slow metabolism can remain inactive until conditions become suitable.

Section 3 of 5Events in Germination

1
The seed absorbs water by imbibition, which rehydrates the tissues and activates enzymes.
2
The embryo releases gibberellin, a plant growth hormone.
3
Gibberellin stimulates cells in the aleurone layer to make amylase.
4
Amylase hydrolyses starch in the endosperm into maltose, which is transported to the embryo for respiration and growth.
5
The radicle emerges first, followed by the shoot, and photosynthesis begins once leaves open.

Section 4 of 5Seed Parts

Testa
  • The tough outer seed coat that protects the embryo.
Embryo
  • The young plant, consisting of the radicle (future root) and plumule (future shoot).
Food store
  • Starch in the endosperm or cotyledons supplies energy for early growth.
StageEventHormone or enzyme
ImbibitionWater uptakeNone
SignallingEmbryo releases signalGibberellin
DigestionStarch to maltoseAmylase

Section 5 of 5Exam Checklist & Tips

4.7.7 – Key Points for the Exam

  • Germination The start of growth of an embryo into a seedling.
  • Imbibition Uptake of water by a dry seed.
  • Gibberellin A plant hormone that triggers amylase production in germinating seeds.
  • Dormancy A state of inactivity in which a seed does not germinate.

Common Mistakes

  • Saying seeds need light to germinate in all species; most need water, oxygen and warmth.
  • Stating that amylase breaks down proteins.
  • Mixing up dispersal (movement of seeds) with pollination.
  • Thinking the seed makes food by photosynthesis before it has leaves.

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