Published on February 26, 2014
PHOTOSYNTHESIS 12a Bilegdemberel.M
An over view of Photosynthesis • Photosynthesis is a series of reactions in which energy transferred as light is transformed to chemical energy. Energy from light is trapped by chlorophyll, and this energy is then used to i. Split apart the strong bonds in water molecules to release hydrogen ii. Produce ATP iii. Reduce a substance called NADP NADP stands for nicotinamide adenine dinucleotide phosphate and is a coenzyme like NAD. Overall equation of Photosynthesis is 6CO2+6H2O=C6H12O6+ O2 There are many different steps in photosynthesis, which can be divided into two main stages – Light-dependent reactions and Light independent reactions.
Chloroplasts • Photosynthesis takes place inside chloroplasts. These organelles surrounded by two membranes, called an envelope.
Chloroplast Structures Functions Ribosomes in stroma Synthesising enzymes – Ribulose bisphosphate carboxylase, for example. Starch Grain The soluble sugars made in photosynthesis cannot be stored. They can only be stored if they are converted into an insoluble form. Grana Creates large surface area for chlorophyll so lots of light can be absorbed. Thylakoid Membranes Driven reactions to occur within the chloroplasts at the same tiime. DNA Codes for some of the chloroplast proteins.
Light dependent reaction • The first reactions of photosynthesis require light energy, and are called light dependent reactions. • When light is absorbed by PS I and PS II, electrons in the chlorophyll molecule are boosted to higher energy levels. They are emitted and passed on to electron carriers. • The loss of electrons from PS II causes the splitting of water. The water gives up its electrons to PS II to fill the gap. • This leaves oxygen, which is given off as waste, and hydrogen ions. • The electrons are passed through a series of electron carriers at successively lower energy levels. This means that energy is released and used to form ATP out of ADP and P. •
Light dependant reactions • The electrons from PSI are passed along a different chain of carriers to NADP. The NADP also picks up the hydrogen ions from the split water molecules. The NADP becomes reduced NADP. • We can show all of this in a diagram called the Z-scheme. The higher up the energy level.
Light Independent reactions • This reaction takes place in the stroma of the chloroplast, where the enzyme ribulose bisphosphate carboxylase, usually known as rubisco, is found. • Carbon dioxide diffuses into stroma from the air spaces within the leaf. It enters the active site of rubisco, which combines it with a 5carbon compound called ribulose biphosphate,RuBP. The products of this reaction are two 3-carbon molecules, glycerate 3-phosphate, GP. The combination of CO2 with RuBP is called carbon fixation. • The energy from ATP and hydrogen from reduced NADP are then used to convert the GP into triose phosphate, TP. Triose phosphate is the first carbohydrate produced in photosynthesis. • Most of the TP is used to produce RuBP, so that more CO2 can be fixed.
Light independent reactions • This series of reactions is known as the Calvin cycle.
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