WJEC Biology for A2: Student Bk

Knowledge check Identify the missing word or words. The three main requirements for germination are water, a suitable temperature and ................................... Water is needed for the mobilisation of ............................. The young root or .......................... grows first, providing anchorage and water. The food store in the ..................... is hydrolysed by enzymes into soluble molecules, resulting in the growth of the embryo. The young shoot or ............................ grows upwards through the soil and when above soil level makes food by the process of ............................ by which time the food store is almost depleted. 31 Going further Gibberellins are used in the brewing industry to speed up germination so that barley seeds need to be kept warm for less time and more malted barley can be produced. Examtip Be prepared to explain the relative changes in the dry mass of the embryo, seedling and endosperm. The graph shows that as a seed germinates, its dry mass has increased after about 10 days. The dry mass of the embryo as it develops into a seedling increases while the dry mass of the cotyledons, providing its food, decreases. Changes in dry mass as a seed germinates The effect of gibberellin The brewing industry uses germinating barley seeds to make beer, so a lot of research has been done on barley germination. The terms ‘malt’ and ‘malting’ used in brewing refer to the maltose generated when the starch in barley is digested. It is likely that similar mechanisms occur in other species. GA and germination 1. The barley embryo secretes a plant growth regulator, gibberellic acid, which diffuses through the endosperm to the aleurone layer. This is a layer of cells towards the outside of the seed, which has a high protein content. 2. The gibberellic acid switches on genes in the cells of the aleurone layer, resulting in transcription and translation, producing enzymes including protease and amylase. 3. The proteases hydrolyse protein in the aleurone layer to amino acids, which are used to make amylase. 4. The amylase diffuses out of the aleurone layer and hydrolyses the starch stored in the endosperm cells. 5. The maltose and glucose produced diffuse back through the endosperm to the plumule and radicle of the embryo. 6. They are respired for energy, which fuels biosynthesis and cell division and which brings the seed out of dormancy. 0 2 4 6 8 10 12 Time from start of germination / days dry mass / mg 0 100 embryo 200 300 cotyledons seed Starch Protein Protease Amylase Amino acids GA 1 2 3 4 5 6 Sugars endosperm embryo aleurone layer Sexual reproduction in plants 189