Photosynthesis: The light dependent reactions

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  • Created on: 18-03-13 21:44
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PHOTOSYNTHESIS
The reactions in photosynthesis require an input of energy from light. The energy needed to break the bonds within carbon dioxide is greater than the
energy released when the products (glucose and oxygen) are formed. Therefore the products of the reaction are at a higher energy level than the
reactants and act as a store of energy.
GLUCOSE IS A FUEL WHICH CAN LATER BE OXIDISED DURING RESPIRATION TO RELEASE ENERGY
OXYGEN IS A WASTE PRODUCT AND IS RELEASED INTO THE ATMOSPHERE
1. PHOTOLYSIS occurs: the splitting of water into hydrogen and oxygen which requires energy from sunlight.
2. The hydrogen reacts with carbon dioxide in order to `store' the hydrogen: carbon dioxide is reduced to form the carbohydrate fuel glucose (this
can be stored or converted to other organic molecules)
3. This fuel has the potential to release large amounts of energy when the hydrogen stored in the carbohydrate reacts with oxygen during
respiration. In aerobic respiration the hydrogen combines with oxygen to make water, and energy and carbon dioxide are released.
THE IMPORTANCE OF PHOTOSYNTHESIS
The organic molecules made are passed on through food webs to other organisms.
Animals can't make all the organic compounds they need therefore they are entirely dependent on plants for their existence.
Plants release oxygen into the atmosphere, without which no organism would be able to carry out aerobic respiration.
The Light Dependent Reaction: occurs when light is absorbed by chlorophyll in the thylakoid membranes of
the chloroplast.
1. Energy from the light raises two electrons in each chlorophyll molecule to a higher energy level ­ electrons in the
chlorophyll molecule are now in an `excited' state.
2. The electrons pass along a series of electron carrier molecules (which are embedded in the thylakoid membrane). The
molecules constitute the electron transport chain.
3. The electrons pass from one carrier to the next in a series of oxidisation and reduction reactions, losing energy in the
process. This energy is used to convert ADP and inorganic phosphate into ATP (PHOTOPHOSPHORYLATION)
4. The electrons lost must be replaced if the flow of electrons along the electron transport chain is to continue.

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Within the thylakoid space, an enzyme catalyses the photolysis to give oxygen, hydrogen ions and electrons. These
electrons replace those that were emitted from the chlorophyll molecule, so it is no longer positively charged.
6. The electrons that have passed along the chain combine with the co-enzyme NADP and hydrogen ions from the water to
form reduced NADP.…read more

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