Biology Practical

Biology Practical
Aim: The aim of this practical experiment is to compare the rate of utilisation of food reserves during the germination of seeds of the two following species/types: Monocotyledons, and Dicotyledons. Background Knowledge: Using the knowledge I have gained from my A-level and course, and from the various sources of information listed in my references page, I have found the following information on seed growth and germination: There are two main types of seeds: Monocotyledon and Dicotyledon. They differ from each other as shown in the diagram below: [image][image] Germination is the onset of growth of the embryo in a seed, usually after a period of dormancy. Dormancy is the state where germination will not occur, even if the conditions are right. Seeds undergo a number of stages during germination and various conditions have to be met for a seed to start its growth cycle: The first stage of germination in water uptake: The seeds must have 40-60% moisture content for germination to occur. The next stage is the Digestion and Translocation stage. The diagram below shows how the seed begins its growth cycle
The barley seed shown is a typical monocot. It has a seed coat , a large endosperm area filled with starch, and an embryo. Barley lacks any special dormancy?germination is initiated by water and reasonably warm temperature. The seed takes up water from the environment in the process known as imbibition. The water passes through the embryo, picking up the germination signal: the hormone Gibberellic Acid. The water moves the hormone from the embryo to the aleurone layer of the endosperm. This layer of cells stores much protein. The water activates hydrolysis enzymes that degrade the storage protein into amino acids. The gibberellic acid activates the dna gene coding for the enzyme amylase in the aleurone cells. Transcription in the nucleus and translation by ribosomes in the cytosol results in the production of amylase inside the aleurone cells. The amino acids from hydrolysis of storage protein are used in the translation of amylase. The amylase is shipped by ER into the Golgi, sorted and packaged into vesicles, and exported through the cell membrane by exocytosis. The amylase is then dumped into the endosperm interior. There the amylase catalyzes the hydrolysis of starch into sugar. In this case the sugar is maltose, which is transported to the embryo. The sugar fuels respiration in the embryo so it can grow. The radicle protrudes from the seed coat, and germination is complete. A typical dicot follows near enough the same process. One difference I found was the fact that monocots store starch in endosperms, and only break it down, or use it, as they need it. In comparison to this dicots store starch in cotyledons, and the endosperm is reduced to a papery coating around the embryo. How does amylase break down starch? Amylase, like other enzymes, works as a catalyst, i.e. it is unchanged by the reaction, but makes the reaction easier by reducing the energy required for it to happen. Catalysts speed up the reaction. The theory behind the working is called the ?lock and key? theory: the enzyme is shaped so that the products fit into them, react and are released. Amylase digests starch by catalysing hydrolysis, which is splitting by the addition of a water molecule. Therefore starch plus water becomes maltose (which is equivalent to two joined glucose molecules). Factors affecting Germination: There are a number of environmental factors, which affect the germination process: Factor Effect Temperature Temperature regulates germination. It affects both the % and rate of germination. Enzymes catalyse the reactions in germination. Therefore these enzymes have an optimum temperature. Too higher or lower temperatures means less productive germination. Light Sensitivity Light sensitivity occurs mostly in non-cultivated seeds ? weed seeds. Ecologically it keeps seed from germinating where it can?t grow such as under a canopy where there is not enough light. Water There has to be the right balance of water. Seeds are just as sensitive to having too much water, as too little. Water is needed for inhibition and transport. Oxygen atp supplies the energy from respiration and breakdown of starch. atp works best when oxygen is readily available. Preliminary Tests: Test Observation 1 I am going to weigh and use the dry mass of the seeds. This is weight of the seed alone, after all water and moisture has been removed (normally by using an oven). This is a much more accurate way of measuring the amount of food in the seed, as some seeds may absorb more water than others. 2 I decided that 25 seeds per test would be adequate. As there are plenty, just in case some of the seeds die off. And also 25 seeds will always show up on the electronic scales. If using under 10 seeds, sometimes there would be no reading on the scales. 3 I decided that pea seeds should be dried for 2 ½ hours and wheat seeds for 1 hour. Any longer than this and the seeds start to burn. Apparatus: Apparatus Used for Seeds 750 pea seeds (dicotyledon) and 750 wheat seeds (monocotyledon) Seed Trays Used for planting the seeds in for germination to take place. Cotton Wool To lay at the bottom of the seed trays to allow a base for growth and to hold water moisture. Scales For weighing the seeds Beakers To soak the seeds in over night. Oven Heating oven to dry out the seeds. Permanent Pen To label the beakers and seeds Method:
Start with 750 pea seeds (dicotyledon) and 750 wheat seeds
Use the following procedure for both types of seeds:
1. Split the seeds up into groups of 25. (Fewer than 25 with not show up well on the scales, as they would be too light). This means you would have 30 groups of seeds. 2. Then separately take each group of seeds, and using electronic scales, weigh the group of seeds, and record in the table given below (Table 1) 3. Then place the group of seeds in a beaker, which has been half filled with ordinary tap water. And label it so that it matches the table, i.e. as below. (Repeat this with each group of seeds) Day 1 Test 1 Test 2 Test 3 Day 2 Test 1 Test 2 Test 3 Day 3 Test 1 Test 2 Test 3 Day 4 Test 1 Test 2 Test 3 Day 5 Test 1 Test 2 Test 3 Day 6 Test 1 Test 2 Test 3 Day 7 Test 1 Test 2 Test 3 Day 8 Test 1 Test 2 Test 3 Day 9 Test 1 Test 2 Test 3 Day 10 Test 1 Test 2 Test 3 4. These must then be left over night (for 24 hours), so that the seeds can absorb the water and the first stage of germination can commence. 5. Then place the group of seeds in a seed tray, which have been filled with cotton wool that has been soaked in water. 6. Label the dish so that it matches the table, and therefore the beaker they have just come from. Repeat this procedure with each group of seeds so that you have 30 seed trays filled with 25 seeds. 7. The seed trays should be stored in a light place, for example on a windowsill, but they must all be stored together so that they are under the influence of the same conditions. 8. The next day (known as Day 1 in the table), take the 3 seed trays labelled Day 1. 9. Then place the groups of seeds on separate pieces of paper towels (making sure that you label which set of seeds is from which test) which can then be placed in a drying cupboard. Leave the peas in the cupboard for around 2 ½ hours and the wheat for around 1 hour. Any longer than this and the seeds begin to burn. 10. Then take each group of seeds out, and weigh them, using the same set of electronic scales that were used previously. (You can either take the seeds off the towel and weight them separately, or weigh them with the paper towel. Then weigh the paper towel separately and subtract the difference) 11. Record the weights in the table (table 1), and work out the calculations. 12. On the second day repeat this procedure using the peas labelled day 2 tests. Record the results in the table. 13. Follow this procedure everyday for the various groups of seeds and record everything in the table. 14. When all results have been obtained, fill in table 2, and then using this info plot a graph of comparison (see graph 1). Variables: The variable being tested/changed is the number of days the seeds are being germinated for. There are a number of things that must be kept constant and not changed for the test to be fair. These include:
Drying Time
Factors/conditions for germination including temperature, light,
water, oxygen.
Make sure both seed types are left germinating for the same amount
of time.
Use the same set of scales for weighing the seeds.
Prediction: From my previous knowledge and research I can make 3 predictions as to what will happen: Prediction Reasoning 1 I think that the mass of the seeds will decrease during germination as the starch in the seeds is used up during respiration. The waste products of respiration (water and carbon dioxide) will not be weighed as they are not part of the dry mass, and will be lost when the seed is heated. 2 The second prediction is that the pea seeds (dicotyledons) will loose mass quicker than the wheat seeds (monocotyledons). This is because monocots contain an endosperm, which contains the starch. This hydrolyses and uses the reserves as needed. But dicots store starch in the cotyledons and they use up the reserve very quickly so that the endosperm is reduced to a papery coating around the embryo. I found this on the following website: 3 A second reason why the dicotyledons will loose mass quicker is the fact that the starch is stored in the cotyledons, which are directly linked to the embryo. The monocotyledons starch is stored in the endosperm and therefore it has to diffuse across to the embryo rather than having a direct link

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