Science in Focus: Shedding Light: Highlights
Seeds and Germination
Germination is the process whereby seeds become active and begin the production of new cells. All seeds must have a supply of water and oxygen for germination. Additionally, they germinate within a specific temperature range. For most seeds the lower temperature limit is freezing point, but some seeds, such as those from cucumbers, will not germinate unless the temperature is at least 18 degrees Celsius. The upper limit of temperature for most seeds is about 40 degrees Celsius.
Most seeds will germinate with or without light, but there are some seeds that are "light-sensitive." Some seeds need the merest flash of light exposure to cause germination, while others need a few hours of light exposure. Examples of seeds which need light to initiate germination are species of rhododendrons, lettuce, tobacco, and birch. The type of light is also important to these light-sensitive plants. For example, lettuce seeds germinate in response to lower energy visible light photons (red light). But if lettuce seeds are exposed to photons of even lower energy (at the far end of the red light and into the infrared region); they will not germinate at all. This light-sensitivity only seems to occur in newly gathered lettuce seeds. Old lettuce seeds do not need light at all to germinate.
Some seeds will not germinate if light is present. An example is certain species of onions.
The sensitivity of seeds to light is due to the presence of a pigment called phytochrome that can be found in plant cells. The reactions of seeds and plants to light is an area of on-going research.
Food Storage in Seeds
The type of food stored depends on the type of seed. Most commonly, seeds store food as fat, or they store food as starch. A few store large amounts of protein. These food reserves are stored in the cotyledons of seeds like beans, or in the endosperm in seeds like sunflower. (See images below.) At the start of germination, seeds absorb water. Water allows chemical reactions to proceed in cells and to bring about the conversion of the stored food, generally to sugar. Sugar provides energy for the newly developing cells as well as combining with other materials to make new cells.
The sugar made by plants is glucose. Glucose is a simple molecule made up of six carbon, twelve hydrogen and six oxygen atoms. Simple sugar molecules can be joined together to form larger molecules. Starch is an example of a large molecule made by joining lots of glucose molecules together.
Simple sugars and more complex sugars (like starch) belong to a family of molecules called carbohydrates. Carbohydrates are important sources of energy for living things because they have been made by plants using the energy of light photons. Some of the photons' energy is "locked up" in the carbohydrate molecules.
Plants make glucose predominantly in their leaf cells. They either pass the glucose to other cells or join together the glucose molecules they have made from starch. Starch is a more compact way of storing sugar in a cell. Also, it is too large to escape from the cell. The starch molecules are stored in little storage sacs in the cell that are called starch grains.
When the cell needs glucose, it breaks up the starch it has stored in the starch grains into glucose molecules.
Testing Leaves for Starch (Activity)
Remember to always wear safey glasses
- The first step in this process is to remove a leaf from a plant (non-waxy leaves work best). Hold the leaf with forceps (tweezers) and dip it into a beaker of boiling water for about 30 seconds. This prevents any further reactions from occurring in the leaf.
- The second step is
to put the leaf into a test-tube containing about 25ml of alcohol (rubbing
alcohol is fine). Now place the test tube into a beaker of boiling
CARE! CARE! CARE! The alcohol must not be close to a flame, so you must boil the water first. Turn off your heating appliance, and then put the test tube into the beaker of boiling water.
- Step three involves watching and waiting for the leaf's green color (chlorophyll) to be removed from the leaf. The alcohol will turn green. The more chlorophyll you can remove from the leaf the better.
- Step four requires you to remove your leaf from the alcohol and to wash it in cold water to remove the alcohol.
- Now you are ready for step five. Spread your
washed leaf on a clean, white surface (a clear plastic dish with a
white piece of paper or a white piece of tile). Using a dropper, COVER
the leaf with Iodine solution. (Tincture of iodine that you can buy
at the drug store is fine.)
CARE! CARE! CARE! Iodine solution should not be inhaled. If it gets in the eyes, it should be washed out immediately and professional advice should be sought.
If your leaf contains starch, then the golden-brown iodine solution will become black in color.
What Do Plants DO with the Sugar They Make?
All living things need a source of energy, and for plants, the sugar made in photosynthesis (using the energy of photons) is that source. Energy is needed to drive the chemical reactions that occur in cells. Some energy is also used to maintain the internal temperature of the organism.
Sugar that is not used for energy is combined with minerals from the soil to make other essential molecules such as proteins. Sugar molecules can also be joined together to make two important large molecule carbohydrates: starch and cellulose. Starch is stored in cells as an energy reserve, but cellulose is a molecule needed by plants to make their cell walls. Cellulose is a raw material for products such as cellophane and rayon, and cotton is almost pure cellulose. In plants such as trees, the spaces between the cellulose molecules in some cell walls become filled with a substance called lignin. It is this lignification of the cellulose of cell walls that produces the wood of trees.
So sugar is the energy supply for plants, but also the basic building block for all the structures that the plant develops. As sugar is made from carbon dioxide and water during the process of photosynthesis, these two substances (a gas and a liquid) contribute the most mass to any plant.