Biology Revision for "O" level examinations

-          Blood transports materials such as oxygen, hormones and nutrients. In order for the nutrients to reach the individual cells, tissue fluids are used. They surround the tissues, and act as a ‘middle man’ between the blood and the tissues. -           The lymphatic system drains this tissue fluid away from the cells using lymph vessels. -           Lymph glands pass to the lymph glands which act as filters. The glands contain many white blood cells. The glands may become swollen during an infection, due to increase production of white blood cells. -           Lymph fluid pass on only one direction, because of the presence of valves (similar to veins) Functions of Lymphatic system -           Transport tissue fluids back into the blood -           Transport fats: from villi to blood -           Removes excess fluid, protein, and foreign materials from tissue spaces -           Defends body against infections 

Points to note  -           - Usually, carbon does not exist alone. It forms molecules or compounds. E.g. it can exist as CO 2 and it is also a component of protein, lipids  carbohydrates that found in living organisms -           - Carbon enters the Carbon cycle through photosynthesis when plants, algae or some bacteria use carbon dioxide to manufacture food. -          -  Glucose made, contains carbon (C 6 H 12 O 6 ). The carbon is also used to make plant proteins, and lipids. Therefore, when animals feed on plants, the carbon is transferred to the animals -          -  Plants and animals undergo respiration, and the glucose (C 6 H 12 O 6 ) breaks down. Carbon Dioxide (CO 2 ) is then released into the atmosphere -           - When animals and plants die, or release waste, then microorganisms will feed on the waste, and release the carbon -           Animals and plants remains, after millions of years, become fossilized. When the fossil fuel is burned, it releases ca

-          -  Nitrogen not used directly by living organisms -          -   Nitrogen gas constitute 78% of the air Nitrogen fixation This describes how nitrogen gas from the air is converted into nitrates. Plants use the nitrates as fertilizer. This occurs by two methods 1.       Lightening : Lightening causes N2 gas to react with H2O molecules in the atmosphere and form ammonia and nitrates 2.       Nitrogen fixing bacteria: some plants, especially leguminous plants, have bacteria living in their root nodules. These bacteria convert the atmospheric nitrogen to nitrates. E.g. of bacteria: Rhizobia Nitrification Animals and plants produce waste. Also, when they die, their body starts to decompose. Ammonia is released, which is then converted to nitrites. The Nitrifying bacteria, such as Nitrosomanas , convert the ammonia to nitrites, and the nitrites to nitrates, which can then be used by plants for growth Denitrification In this process, the nitrates and nitr