biology4alevel600

1 A habitat is a place where an organism lives. The niche of an organism is the role that it plays in the community. 2 A population is a group of organisms of the same species, living in the same place at the same time, that can interbreed with one another. A community is all the organisms, of all the diff erent species, living in the same place at the same time. 3 An ecosystem is an interacting system of organisms and their environment, more or less self-contained.  4 Energy flows from one organism to another in the form of chemical energy in organic molecules in food. The pathways of energy flow can be shown in a food chain or food web, in which the arrows show the direction of energy flow.  5 The first organism in a food chain or food web is a producer. In most food chains, plants are the producers. They transfer energy from sunlight into chemical energy in organic molecules in the process of photosynthesis. All other organisms in a food chain are consumers.  6 Each s...

Living organisms need nitrogen because nitrogen atoms are an essential part of proteins, nucleic acids and ATP. The air contains about 78% nitrogen gas. However, this is in the form of nitrogen molecules, in which two nitrogen atoms are held together by a very strong triple covalent bond. This is very unreactive. Nitrogen molecules freely diffuse in and out of the bodies of living organisms, but take no part in the metabolic reactions inside their cells. Nitrogen fixation For nitrogen to become involved in metabolic reactions, it must first be converted to a different form by combining with oxygen or hydrogen. This process is called nitrogen fixation . It can be done by: • Lightning , which provides very high temperatures that can cause nitrogen and oxygen molecules in the air to combine to form nitrogen oxides; these can then be washed to the ground in rain. • Industrial processes in which nitrogen is combined with hydrogen to produce ammonia, NH3; this is then used to manufacture fer...

Ecology is the study of the ways in which organisms interact with their environment. Levels of ecological organisation A habitat is a type of environment in which an organism lives. For example, the habitat of a giraffe is grassland ( savannah ) with groups of trees such as Acacia . The habitat of a woodlouse ( Oniscus ) is a humid, dark place such as beneath the bark of a rotting log. The habitat of a mangrove tree is a muddy sea shore that is regularly flooded by the tide. A population is a group of organisms of the same species that lives in the same place at the same time. If the species is a sexually-reproducing one, the organisms in the population are able to interbreed with one another. For example, all the giraffes in a particular area of savannah make up the giraffe population. A community is all the organisms, of all the different species, that live in the same place at the same time. For example, all the giraffes and other animals, all the plants, all the fungi and all the...

• Levels of ecological organisation • Energy flow through ecosystems • Recycling of nitrogen Learning Outcomes Candidates should be able to: (a) define the terms habitat, niche, population, community and ecosystem and be able to recognise examples of each ; (b) explain the terms autotroph, heterotroph, producer, consumer and trophic level in the context of food chains and food webs ; (c) explain how energy losses occur along food chains and discuss the efficiency of energy transfer between trophic levels; (d) describe how nitrogen is cycled within an ecosystem, including the roles of nitrogen-fixing bacteria (e.g. Rhizobium) and nitrifying bacteria (Nitrosomonas and Nitrobacter); (e) use the knowledge gained in this section in new situations or to solve related problems. Note: An ecosystem should be studied in relation to an area familiar to the candidates.