Photosynthesis

What is photosynthesis? The capturing of sunlight in order to make energy is called photosynthesis.

So where does photosynthesis occur??? Photosynthesis occurs in little sacs called thylakoids. When thylakoids are stacked together in groups, they are referred to as a granum. These grana (plural form of granum) are suspended in a semiliquid substance known as the stroma. All of the grana and stroma are contained in chloroplasts, the organelle where photosynthesis occurs.

Pigment Molecules that are good at absorbing certain light in the visible spectrum are known as pigments. When groups of photosynthetic pigments are clustered together, they form a photosystem. This is the starting point of photosynthesis.

Chlorophyll and Carotenoids The main photosynthetic pigment the chloroplast is the pigment chlorophyll a. There is a secondary pigment that backs up chlorophyll a called chlorophyll b. These two chlorophylls absorb mostly violet-blue and red light. Carotenoids- help support chlorophyll by giving an even wider range of light for light absorption. Gives leaves their orange/yellow color in the fall.

The formula This formula states what the overall goal is in photosynthesis. 6 CO2 + 6 H2O + Light  C6H12O6 + 6 O2

What are all these numbers and letters? These letters and numbers represent the molecules and amounts of each that are needed. Reactants 6 CO2 = six molecules of carbon dioxide are required 6 H2O = 6 molecules of water are required Light = Sunlight is required Products C6H12O6= One molecule of glucose is produced 6 O2= six molecules of oxygen are produced

Light In photosynthesis, light energy is required to produce our glucose product, but is all of light in general what we need to get our final product? The answer is no. The colors we see are colors that are reflected off of the surface of the object we are viewing. In the case of chlorophyll, the green color we see is the result of the green color being reflected back into our eyes because the chlorophyll does not absorb green light.

Does photosynthesis happen all at once? No. Photosynthesis takes place in two distinct stages. They are the light reactions and the dark reactions.

Stage One: The Light Dependent Reactions This means that light is required for this process to occur.

The Light Reactions A photon (light) strikes the pigment in photosystem II, causing an electron to get excited. This electron couples with a proton that was stripped from a water molecule (one of our reactants) by and enzyme found in the stroma of the chloroplast.

The Light Reactions: Continued When the water is split, the oxygen is released from the cell. (This is where the oxygen on the product side of our general equation comes from). Those excess hydrogen ions are then stored in the thylakoid.

The Light Reactions: Continued The excited electron is then brought to the proton pumps by a carrier molecule. These proton pumps are embedded in the thylakoid membrane. These membranes that the electron passes through are called the electron transport chain.

What happens to all of the excess hydrogen ions from the light reactions?: chemiosmosis All of these extra hydrogen ions have began to build up, and need to have some purpose. These excess hydrogen ions are used in conjunction with an enzyme called ATP synthase to bind an additional phosphate group onto an ADP molecule, thus creating ATP. This ATP will prove to be crucial later on.

Photosystem I After traveling down the electron transport chain, the electron arrives at photosystem I. When a photon hits photosystem I, this electron is launched out of photosystem I, and is carried by a special electron carrier called ferrodoxin. This electron is donated to NAD+ to form NADPH.

So what Am I going to do with all of this NADPH and ATP? NADPH and ATP will be used in a second series of reactions. These reactions are called the Calvin cycle. The Following words all have the same meaning: Light Independent Reactions Calvin Cycle Dark Reactions

Light Independent / Dark Reactions So far energy has been stored in the form of ATP and NADPH. This cycle uses that energy to produce sugar molecules.

What’s first? 3 CO2 molecules react with a 5-carbon sugar molecule called RuBP. This process is called carbon fixation. RuBP

What’s next? When the RuBP and 3CO2 are rearranged, we wind up with 3 six carbon sugars, each with its own phosphate group. Then, 6 ATP will add another phosphate group to the mix, creating 6 three carbon sugars, each with 2 phosphate groups.

After That These 6 three carbon sugars are reduced (meaning they gain an electron) by the NADPH. This provides us with 6 three carbon sugars that are of high energy. 5 of these three carbon sugars are then rearranged to regenerate the RuBP that was used in the beginning of the reaction.

So what does this all mean??? The 3 CO2 molecules that we started with have now been locked into a 3 carbon sugar molecule. Every time we go through one cycle of the Calvin cycle we use 9 ATP and 6 NADPH. These molecules are regenerated in the light reactions.

Oh, so how do I get glucose? This 3 carbon sugar is our actual product of the Calvin cycle. Once the cycle repeats, and we have two three carbon sugars, the cell can rearrange these 3 carbon sugars into one 6 carbon sugar, known as glucose! These sugars made by the cell are stored as starch.