Where does photosynthesis occour?

Photosynthesis takes place in the chloroplats. The light dependend reactions occour in the thylakoids of the chloroplats. The phospholipids membranes in these layered structres contain different pigments that absorb light energy. Chlorophyll and carotenoid molecules are arranged in groups, centred on chlorophyll a molecules. The light independent reactions of photosynthesis occur in the solution in the Stroma. Both sets of reactions in the cell by the outer chloroplast membranes. The products of photosynthesis are rapidly removed from the chloroplast. Carbon dioxide readily diffuses through the outer membranes of the chloroplasts into the stroma. Carbohydrate can be transported out through the membrane. Starch grains form temporary sites for storing excess carbohydrates produced during the day. During the night, carbohydrate may be released from the starch grains and translocated to other parrts of the plant.

Light dependent reactions.

The light dependent reactions of photosynthesis provide the energy and electrons needed for the later, light independent reactions. What we call chlorophyll is in fact several different compounds. These include chlorophyll a, chlorophyll b and carotenoids. Energy captured by all of these pigments is transferred to groups of chlorophyll a molecules. Chlorophyll a is one of a special group of compaounds that can absorb light energy and use it to boost the energy level of electrons from the hydrogen atoms in water. Electrons can be compared to satellites orbiting earth. To move a satellite into a higher orbit requires energy, for example from firing a booster rocket. Similarly, energy is required to boost an electron into a higher orbit around the nucleus of an atom, from where it can be passed on to anotehr atom more easily.
Chlorophyll a absorbs light energy and the exited electrons that it then contains are used to powere several very important redox reactions,

PHOTOLYSIS, PHOTOPHOSPHORYLATION and NADP production.

Photolysis

Energy from exited electrons in chlorophyll a molelcules is use to split water molecules into protons and electrons and oxygen.

2H2O - Protons (4H+) + electrons (4e-) + oxygen (O2)

this reaction is known as photolysis

The oxygen is released to the atmosphere. This reaction is the source of almost all the oxygen that makes up 20% of the atmosphere, which covers the Earth with a gas blanket ten miles deep. That's a lot of oxygen. Millions of tonnes of oxygen are present in the atmosphere only because of the work of photosyntehic organisms over millions of years. Without it there would be no animal life on earth.

Photophosphorylation

Energy from exited electrons can also be used to produce ATP. Because the energy comes from light, this method of producing ATP is called photophosphorylation

ADP + Pi + Energy from excited electrons = ATP

Production of NADP

The electrons released from water molecules by photolysis are eventually used to reduce carbon dioxide to form glucose. However, electrons cannot react directly with carbon dioxide. The exited electrons in chlorophyll a molecules are transferred to a very useful compound called nicotinamide adenine dinucleotide phosphate. NADP.

With each electron that NADP takes up, it also accepts a proton, and so it does not become negativley charged. The transfer of electrons and protons to NADP requires energy that is obtained from exited electrons in other groups of chlorophyll a molecules.

Reduced NADP can donate electrons to reduce carbon dioxide directly, and for this reason is known as a reducing agent. The reduction of carbon dioxide occurs during the light independent reactions of photosynthesis.