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Science in Focus: Shedding Light: Highlights

Workshop 6

pyramid of numbers | pyramid of biomass | pyramid of energy | decomposition | law of conservation of energy | energy flow in ecosystems | aerobic respiration | balance of nature

Pyramid of Numbers

We can represent the numbers of producers and consumers co-existing in an ecosystem by constructing a pyramid of numbers. By counting the numbers of organisms in an ecosystem, it is generally found that there is a progressive drop in numbers from producers, to primary and secondary consumers. This decrease in numbers occurs because of the energy losses when one organism feeds on another.

Pyramid of Numbers

Pyramid of Biomass

It is possible to estimate the mass of all the organisms living in a given area or ecosystem. This estimation is known as the biomass. If the mass of the producers and each type of consumer is recorded, a pyramid can be constructed showing that the biomass of the producers is generally far greater than that of the consumers.

Pyramid of Biomass

Pyramid of Energy

Energy flows through ecosystems from producers to the various levels of consumers. Each time an organism eats another, not all the energy is transferred. Only about 10% of the energy of a producer is transferred to the consumer that eats it. Therefore, there is a progressive loss of energy at each level of a food chain. We can represent the amount of energy at each level as a part of a pyramid.

Pyramid of Energy



This is the process whereby the remains of dead animals and plants or their excreta, are broken down to their smallest components by organisms known as decomposers. Decomposers include fungi and bacteria. When an organism excretes or it dies, its matter is acted on by larger organisms, such as worms, insect larvae and larger fungi. These organisms digest the matter into smaller pieces. The organisms which complete the process of decomposition are the microbes (microscopic bacteria and fungi). They are too small to 'eat' solid food, so they release chemicals onto the food and form a liquid nutrient soup. They absorb nutrients from this soup, but some nutrients seep into the soil to be used by plants. Decomposers therefore recycle matter in ecosystems. Compost heaps make use of the process of decomposition. The nutrients released by the decomposers in the compost heap are placed on and in the soil surrounding plants.


Law of Conservation of Energy

Energy exists in different forms and can be transformed from one form into another. When these transformations occur, energy is not destroyed and neither is energy made. Transformation of energy just exchanges one form of energy for another.

In physics, the law of conservation of Energy says that in an isolated, closed system, the amount of energy is constant. Energy can change forms (such as from potential energy to kinetic energy), but the total amount energy remains the constant.

This idea of energy transformation without the destruction or formation of energy within a system is stated as a law in the field of science.

Energy Flow in Ecosystems

The organisms in any ecosystem are inter-related by their food requirements. When a green plant uses light energy to make its food (sugar) it only uses a small percentage minute amount of the energy in the Ssunlight ’s available energy fallen on it. In this food-making process, light energy is transformed into chemical energy. The plant uses the sugar it has made to build new cells., Iit also uses the sugar as a source of energy to drive the chemical reactions occurring in its cells. The transformations that the energy in sugar undergoes always result in some energy transforming into heat energy. Some of this heat energy escapes to the surroundings. When a snail eats a plant, it cannot get the heat energy back that the plant lost to its surroundings, so the snail never gets all of the the plant’s original energy the plant absorbed from the sunlight.. At all levels of the food chain, the animal doing the eating never gets all of the energy that its food source had (because heat energy will always have been lost to the surroundings and the animal rarely eats every part of its food source anyway).

So energy has to be constantly supplied to an ecosystem. because you can’t recycle energy in a food chain. Animals and plants cannot don’t have a way of reclaiming the heat energy lost to the surroundings. atmosphere.


Aerobic Respiration

This is the process whereby living things release the chemical energy in sugar for their use. The process occurs inside the cells of living things and requires oxygen (hence it is called aerobic) that has been gained by the organism from its surroundings. All of the energy that was stored in the sugar when it was made during photosynthesis is released during aerobic respiration. Some of the released energy is radiated from the organism as heat. The remainder of the energy is used to make a new chemical called ADENOSINE TRIPHOSPHATE (ATP), which can be broken down easily inside of cells to release the energy. An analogy is to think of sugar as a bank account of stored energy that has to be turned into cash. The ATP is the cash and is the immediately available energy for the cell’s use.

Not only is energy released from sugar during aerobic respiration, but sugar is also broken down into its constituents, carbon dioxide and water.

sugar + oxygen = co2 + water + ATP + radiated energy

Aerobic respiration and photosynthesis can therefore be seen to be complementary processes. Photosynthesis using the energy of light, joins the carbon dioxide made during respiration to hydrogen from water in order to make sugar and oxygen. The energy in sugar is released during aerobic respiration using the oxygen made as a bi-product of photosynthesis.

Photosynthesis and Respiration


Balance of Nature

Ecosystems are in a state of continuous change. There may be increases in the number of carnivores in a particular season, or decreases in the number of producers, but over a period of time the ecosystem adjusts to reset the relationship between the numbers of the various organisms. The balance of nature is rather like a see-saw going up on one side and down on the other, eventually returning to a middle, balanced position. Humans can upset this balance (e.g. when they pollute an ecosystem so that a particular group of susceptible organisms die) with such drastic effects that the ecosystem cannot recover.


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