Friday, October 22, 2021

Light Dependent Reactions

Biology Index

Where are we going with this? The information on this page should increase understanding related to this standard:  Understand how photosynthesis turns light energy into chemical energy.

Article includes ideas, images, and content from Troy Smigielski (2021-10)

Light Dependent Reactions
(So, explaining the magic?)

The photosynthesis process includes two distinct steps. The light dependent reactions (LDR) and the light independent reactions. The net result of photosynthesis is the production of oxygen that is released and sugar that is stored to be used by the plant.

The LDR produce oxygen that gets released by the plant as a byproduct. This is the oxygen we breathe, but it is not our only source of oxygen. There are phytoplankton in the ocean that produce about 50% of our oxygen.

While it is very nice of plants to produce oxygen that we can breath, oxygen is actually a byproduct… it is something that just happens to be formed; something the plant actually does not need.

The main function of the light dependent reactions is to make energy for the light independent reactions to use. Remember, photosynthesis converts light energy into chemical energy. The light dependent reactions is the first step in creating the chemical energy.

The energy of cellular functioning is called adenosine triphosphate
(ATP). ATP is a fairly complex molecule in which is a chain of three phosphates. The "links" in the chain are chemical bonds, and it is in the bonds that energy is stored. 

ATP is an energy-rich molecule that stores energy in the 3rd phosphate bond.

Breaking off one of the phosphates releases energy and results in one adenosine diphosphate and one phosphate. 

When the energy from ATP is used, it becomes ADP because the 3rd bond has been broken off.

ATP = adenosine triphosphate = adenosine + 3 phosphates
ADP = adenosine diphosphate = adenosine + 2 phosphates

So, back to photosynthesis… 

The plant's goal is to make sugar it can use. It uses the energy in the ATP to create sugar. So, the first step in photosynthesis is to make ATP.

The LDR convert ADP into ATP.

When an organism uses the energy in ATP, the 3rd bond is broken, which releases energy. When this happens, the molecule becomes ADP again. (ADP only has 2 phosphates.)

The LDR also convert NADP+ into NADPH. This conversion attaches an electron (energy) to the electron carrier. Remember, NADPH is the electron carrier molecule in photosynthesis.

It is important to note that when NADP+ picks up a Hydrogen (which carries an electron), it becomes NADPH. When NADPH drops off its electron, it will become NADP+.

So… Quick review…

When ADP picks up energy, it becomes ATP. Does ADP have a lot of energy? Does ATP?

ATP has a lot of energy which it releases to become ADP.

When NADP+ picks up energy, it becomes NADPH.

Light dependent reactions take place in the thylakoid membrane. Thylakoids are the individual flattened, membranous disks in the chloroplast. The membrane of the thylakoid is also made up of a phospholipid bilayer.

Light Reaction Steps
So, then, it's kinda like magic, right?

While it is complicated and detailed, the light dependent reactions are not magic. 

Step 1: Light is absorbed into the chloroplast by pigments in photosystem II (PSII). The main pigment absorbing the light is chlorophyll.

Step 2:
The energy from the light splits up a water molecule (H2O) which provides two H+ ions, two high-energy electrons, and the oxygen we breath.

As more light comes in and this continues to happen, the H+ ions start to build up inside the thylakoid membrane, so naturally they want to leave through facilitated diffusion. 
To do this, they pass through a protein channel called ATP Synthase. As H+ ions flow out, ATP Synthase spins and makes ATP. 

Both electrons move down the electron transport chain (ETC) into photosystem I (PSI). As they travel down the ETC, they release energy. 
As electrons travel down the ETC, they release energy. This energy is used to push more H+ ions into the cell.

Step 3: Once in Photosystem I, the electrons from water receive more light energy and are attached to NADP+. This makes NADPH.

What? What? Just… what?

Light comes into PSII and chops up water into oxygen, H+ ions, and electrons.

Oxygen leaves, the H+ ions create ATP through ATP Synthase, and the electrons travel down the ETC to PSI.

In PSI, the electron is attached to NADP+ which forms NADPH.

Light Dependent Reactions Bottom Line
Hmm… I should probably memorize this, right?

1. LDRs happen in the thylakoid membrane and need light.

2. LDRs purpose is to make energy molecules (ATP and NADPH) for the Calvin Cycle. They also make oxygen during this process.

3. LDRs start in photosystem II and move to PS I. 

LDR Application

Think about this…

  • In the fall, plants lose chlorophyll and/or their leaves. What would happen if the plant loses chlorophyll? 

  • Would they be able to make oxygen in the fall?

  • If there are less leaves in the winter, what happens to the carbon dioxide levels in the atmosphere?


This is the Keeling Curve, which measures CO2 in the atmosphere. Notice the annual fluctuations.

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