Essay title :
Discuss whether life is possible without water
6B Lam Wing Yan
Remark: Teacher’s comment is printed in red.
Life is not possible without water. Water is essential for plants and animals to maintain their lives.
For plant, it has needed water already when it is a seed. Without water, the seed can’t germinate and grow up to a plant. The seed absorbs water through the micropyle. The water is used to soften the seed coat to allow the radicle to emerge. And it makes the seed wall swells to break the seed coat. Besides, it activates enzymes in the seed so that the stored food can be converted into a form which can be used as nutrients by the embryo. Also, water acts as a solvent to transport soluble food to the growing regions of the embryo. The life of plant can’t get started without water. (a very good starting paragraph)
When the seed has germinated and become a plant, it still needs water. It is needed in the photosynthesis. During the light reaction of the photosynthesis, water is needed. The light break the water molecules into hydrogen and oxygen so that there is hydrogen atoms transfer into the dark reactions to make carbohydrates. Without water, the plants can’t make food from photosynthesis. Besides, water is needed for the transport of chemical substances. Since water is a good solvent, the sucrose can be transported through the phloem and minerals can be transported through xylem when water is present. (As plants form the basis of most food chain, this means that H2O is a necessity for all life forms.)
Furthermore, water is needed for providing support in some plants. Without water, the cells in the stems become flaccid and can’t support the stem. During transpiration, water is lost through the stomata. If the plants can’t uptake enough water to compensate the water loss, the cells would become flaccid and the stomata will be closed. As a result, the plants can’t get in carbon dioxide through the stomata and so photosynthesis can’t be carried out. So, during transpiration, if water is not present, some plants will die because they lose the support by turgidity of cells and some may die because they have no food.
According to the above reasons, the life of plant is impossible without water. (Without the green plants, most organisms die too because they depend directly or indirectly on green plants for food since they can’t make their own food.) (Put this part in your 3rd paragraph) Despite this reason, animals can’t survive without water too because of the following reasons.
Firstly, water is needed in the process hydrolysis, e.g. digestion of food. Without water, the animals can’t digest proteins, fats, polysaccharides and so that can’t use it as their nutrients to provide energy. Other than digestion of food, conversion of stored glycogen in liver into glucose also need water. Without water, this can’t be done and so, during fasting or starvation, the animals can’t get energy from the oxidation of food.
Secondly, (water is the major composition of cytoplasm.) water provide a site for chemical reaction. Water provides an ideal place for chemical reactions to take place because water has a high specific heat capacity so that the chemical reactions will not be greatly affected when the temperature outside the organisms changes. Similarly, due to this special features of water, the heat provided in the chemical reactions doesn’t affect the body temperature of the organisms very much. (No other liquid of similar molecular weight has so high specific heat capacity.)
Fourthly, water is essential to the diffusion of material across the surfaces such as the lungs or alimentary canal. During gaseous exchange, gases must be first dissolved in the film of water on the alveoli. If water is not present, oxygen can’t be uptaken by the cells and the wasted gases can’t be (diffuse across the respiratory surfaces). (Breathing is applied to limited classes of animals)
Fifthly, water acts as a solvent in animals too. Since it can dissolve other substances easily, this makes it very useful. It is used in transport, removal of wastes, and secretions. For the transport, water contribute a great content in blood plasma, tissue fluid and lymph. Without water, nutrients can not be transported to the cells for them to use, (the white blood cells can’t be transported, so the body defence strength decreases too.)(This is not a key point, because many animals do not have white blood cells.) For the removal of wastes, the metabolic wastes like ammonia and urea are removed from the body in solution of water. Without water, such things can’t be removed and these toxic substances will accumulate in the body and it is harmful, and eventually the organisms may die. For the secretions, most of them comprise substances in aqueous (medium) too. For example, most digestive juices have salts and enzymes in solution.
Sixthly, water is needed for the regulation of body temperature (in warm-blooded animals). Evaporation of water during sweating and panting is used to cool the body in warm-blooded animals. Without water, the body temperature will continue to increases during exercise or in a hot environment. And the enzymes’ activities are affected. This in turn affects the metabolism.
Due to the above reasons, animals can’t be live without water either. Water must be present when life is possible.
Teacher’s overall comment:
A well-organized essay. Development of ideas is very good. Well relevant to the question asked. Sufficient content for an essay type question. Later when you learn more about reproduction in lower plants, you’ll see water is important in the reproduction of all plant forms.