Edexcel: Topic 5, Ecosystems rely on energy transfer


The arctic food chain begins with photosynthesising plants like all ecosystems.  In the arctic oceans nutrient rich cool water rises to support phytoplankton (microscopic plants) which provide food for animal plankton which in turn feed fish populations and whales.

Producers and productivity

The rate of energy being incorporated into organic molecules in an ecosystem is the primary productivity of the ecosystem. examples of Producers/autotrophs (organisms that can make their own organic compounds from inorganic compounds) are green plants, algae and some bacteria.  When light falls on producers, some energy is transfered to a chemical energy store by producing organic fuels e.g. glucose however not all producers are photosynthetic , some are chemosynthetic autptrophs.  They make organic molecules using energy released from chemical reactions.


An overview

The equation for photosynthesis is:(http://www.pleasanton.k12.ca.us/fhsweb/kelso/laptop/chap_8/notes/Pictures/equation.gif)

Carbon dioxide is reduced when hydrogen and electrons from water are added to it to form a carbohydrate.  Reactions in photosynthesis require light energy.  The energy needed to break the bonds in carbon dioxide and water is greater than the energy released when the products are formed.  The products of the reaction (glucose and oxygen) are at a higher energy level than the reactants (carbon dioxide and water) making them a good store of energy.  Oxygen is a waste product and is released into the atmosphere.  Glucose can be oxidised during respiration to release energy.  The hydrogen is separated from the water and stored in a carbohydrate.  When energy is needed within the cell the hydrogen reacts with oxygen in respiration releasing a large amount of energy.

Releasing Hydrogen from water

This requires energy, photosynthesis uses energy from sunlight.  Photolysis is the process of splitting the water using light.

Storing hydrogen in carbohydrates

The hydrogen reacts with the carbon dioxide to store the hydrogen.  The carbon dioxide is reduced to form the carbohydrate fuel glucose which can be stored or used to store other organic molecules.

Using the glucose

This fuel has the potential to release large amounts of energy when the hydrogen reacts with the oxygen during respiration.  In aerobic respiration the glucose is pulled apart, the hydrogen and oxygen combine to create water and energy as well as carbon dioxide are released.  Some of the glucose is converted into chemicals needed to create new cells using enzymes e.g. starch, cellulose, fats, amino acids, nucleic acids.

The importance of photosynthesis

The organic molecules are passed onto other organisms as animals cannot make these themselves and depend on plants for their existence.  Plants also release oxygen which enables organisms to carry out aerobic respiration.

How photosynthesis works

Photosynthesis is a series of reactions controlled by enzymes which occur in 2 main stages

- Light-dependent reactions use energy from light and hydrogen from photolysis of water to produce reduced NADP, ATP and the waste product oxygen which is either used in respiration or released to the atmosphere

-Light-independent reactions use the reduced NADP and ATP to reduce carbon dioxide to carbohydrates.

What is meant by reduction?

During the




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