Photosynthesis 2

Light-dependent photosynthesis produces ATP, Reduced NADP and oxygen

• Light energy is absorbed by PS2.
• The light energy excites electrons in chlorophyll.
• The electrons move to a higher energy level (i.e they have more energy).
• These high-energy electrons move along the electron transport chain to PS1.

• As the excited electrons from chlorophyll leave PS2 to move along the electron transport chain, they must be replaced.
• Light energy splits water into protons (H+ ions), electrons and oxygen.

• The excited electrons lose energy as they move along the electron transport chain.
• This energy is used to transport protons into the thylakoid so that the thylakoid has a higher concentration of protons than the stroma. This forms a proton gradient across the membrane.
• Protons move down their concentration gradient, into the stroma, via an enzyme called ATP synthase. The energy from this movement combines ADP and Pi to form ATP.

• Light energy is absorbed by PS1, which excites the electrons again to an even higher energy level.
• Finally, the electrons are transferred to NADP, along with a proton (H+ ion) from the stroma, to form reduced NADP.

• The Calvin Cycle takes place in the stroma of the chloroplasts.
• It makes a molecule called triose phosphate from carbon dioxide and RuBP (a 5-carbon compound). Triose phosphate can be used to make glucose and other useful oranic substances.
• There are a few steps in the cycle, and it needs ATP and H+ ions to keep it going.
• The reactions are linked in a cycle,which means the starting compound, RuBP is regenerated.

• Carbon dioxide enters the leaf through the stomata and diffuses into the stroma of the chloroplast.
• Here, it is combined with RuBP, a 5-carbon compound. This gives an unstable 6-carbon compound, which quickly breaks down into two molecules of a 3-carbon compound called glycerate 3-phosphate (GP).
• Ribisco (an enzyme) catalyses the reaction between carbon dioxide and RuBP.

•  Now ATP, from the light-dependent reaction, provides energy to turn the 3-carbon compound, GP, into a different compound called triose phosphate (TP).
• This reaction also requires H+ ions, which come from reduced NADP (also from the light-dependent reaction.) Reduced NADP is recycled to NADP.
• Triose phosphate is then convertred into many useful organic compounds. e.g glucose.

•  Five out of every six molecules of TP produced in the cycle aren't used to make hexose sugars, but to regenerate RuBP.
• Regenerating RuBP uses the rest of ATP produced by the light-dependent reaction.

The Calvin Cycle is the starting point for making all the organic substances that a plant needs. TP and GP are molecules needed to make carbohydrates,lipids and proteins.

• Carbohydrates - hexose sugars, like glucose, are made by joining two triose phosphate molecules together and larger carbohydrates (e.g sucrose, starch and cellulose) are made by joining hexose sugars together in different ways.
• Lipids- these are made using glycerol, which is synthesised from triose phosphate, and fatty acids, which are synthesised from glycerate 3-phosphate.
• Proteins - some amino acids are made from glycerate 3-phosphate, which are joined together to make proteins.

Here's the reason why:

• Three turns of the cycle produces six molecules of triose phosphate (TP), because two molecules of TP are made for every CO2 molecule used.
• Five out of six of these TP molecules are used to regenerate RuBP.
• This means that for three turns of the cycle only one TP molecule that can be used to make a hexose sugar is produced.
• A hexose sugar has six carbons though, so two TP molecules are needed to form one hexose sugar.
• This means the cycle must turn six times to produce two molecules of TP that can be used to make one hexose sugar.
• Six turns of the cycle need 18 ATP and 12 reduced NADP from the light-dependent reaction.
• This might seem a bit inefficient, but it keeps the cycle going and makes sure that there is always enough RuBP ready to combine with CO2 taken in the atmosphere.