Download Seed germination information Sheet

A2 Biology
Assessed Coursework
Seed Germination
Parts of a seed:
Testa – outer seed coat
Micropyle – small hole in testa where water enters seed
Radicle – seed root
Plumule – seed shoot
parts of the embryonic plant
Cotyledon – seed leaf
Endosperm – food store (in seeds that don’t store food in the cotyledons)
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Non-endospermic seed in section (LS)
Endospermic seed in section (LS)
The seed contains some stored food – often starch (You should know why it’s stored as starch &
not glucose!)
What is “germination”?
Germination is the process in which the young plant emerges from the
seed & becomes independent. This requires the embryonic plant inside the seed to use the
stored food to grow enough to push the radicle out of the testa. As soon as the radicle
appears, the seed can be said to have germinated. Even in tiny seeds, this is quite easy to see as
the radicle is almost white compared with the dark testa.
What happens during germination:
Germination begins with water uptake through the testa &
micropyle. At first, water enters by imbibition – this means that water is absorbed by the dry
substances, like cellulose in the cell walls, inside the seed. This causes the seed to swell &
rupture the testa. Water then enters the embryonic plant cells by osmosis.
Water is needed to activate the enzymes that catalyse the
biochemical reactions of germination.
Bio SWW\A2 Biology\Assessed Coursework
A2 Biology
Assessed Coursework
3 These enzymes then hydrolyse the stored food substances so 4 the products e.g. sugars can be
translocated (transported) to the growth regions of the embryo. (Water is needed for this
hydrolysis & transport & also fills the vacuoles of growing cells.) 1 The production of more
hydrolytic enzymes is triggered by the release of a hormone, gibberellic acid (GA) from the
embryo. 2 Gibberellic acid diffuses to the food store & acts on the nuclei of the storage cells,
activating genes that code for hydrolytic enzymes. (Numbers refer to points in the diagram below)
3
2
4
1
A suitable temperature will be the optimum for the
enzymes involved in the hydrolysis of the food store. This varies from species to species but is
generally between 1-35oC.
Growth – making more cells & making those cells bigger –
requires energy from aerobic respiration (another reaction catalysed by enzymes) &
germinating seeds respire very rapidly.
The food store provides the energy source for
germination & growth of the seedling’s first green leaves. Once these have developed, the
seedling is then able to photosynthesise to produce its own glucose & starch.
All diagrams from:
Clegg, C. J. & Mackean, D. G. Advanced Biology – Principles & Applications, John Murray:
London, 1994
Bio SWW\A2 Biology\Assessed Coursework