Photosynthesis

Photosynthesis is how plants harness the radiant energy from sunlight. This process is important since it's originally how all of the energy in fossil fuels (except for the abiogenic fossil fuels) was captured. This process is also indirectly how animals and humans get energy from the sun in the form of food.

The levels of atmospheric oxygen also come from photosynthesis, since that's how oxygen is chemically liberated from carbon dioxide to allow the atmosphere of Earth to support life. Oxygen is a by-product of the photosynthesis process.

The Process

Figure 1. A diagram of the chloroplast.

Chloroplasts are the sites of photosynthesis for a plant. Listed are the important parts of this cell for photosynthesis to occur:

• They consist of two membranes to hold the cell together, an outer membrane and an inner membrane.
• The inside space of a chloroplast is called the stroma, it is a dense fluid with high pH (low concentration of hydrogen ions).
• In the stroma fluid, there are little pancake-shaped organelles called thylakoids, and a stack of these thylakoid pancakes is called a granum.
• Lastly, the interior space of the thylakoid is called the lumen, and it features a low pH (high concentration of hydrogen ions) which will be further explored in light-dependent reactions below.

Reference Figure 1 for a labelled diagram of what the chloroplast and its organelles looks like. Reference Figure 2 for where each step of photosynthesis occurs in the chloroplast cell.

Figure 2. An overview of the photosynthesis reaction in a chloroplast cell.

Overall reaction

The overall reaction of photosynthesis is endergonic, meaning that energy is a reactant and the energy is being chemically stored in the product. In this case, glucose (C₆H₁₂O₆) is the product storing energy in its bonds. Below is the overall chemical reaction of photosynthesis. Reference Figure 2 to see how this occurs in a cell.

Light Energy + 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

In this reaction, we begin with energy, carbon dioxide (CO₂), and water (H₂O). Carbon dioxide is a low energy reactant that is reduced into glucose (C₆H₁₂O₆), which is a high energy product. Water is oxidized into atmospheric oxygen.

There are many intermediate reagents and elementary steps to yield this result. The two main stages are traditionally described as the light-dependent and light-independent reactions. But even though these "dark" (light-independent) reactions don't directly use light energy as a reactant, the Calvin-Benson Cycle still requires products from the light-dependent reactions and is optimized with sunlight.

Light-dependent reactions

The thylakoid is important because the thylakoid membrane is where the light-dependent reactions occur. Along its membrane are protein complexes that conduct the chemical reactions. See Figure 3 for a visual representation.

It begins at Photosystem II: Photosystems consist of proteins and pigments, namely chlorophyll. A photon will hit the chlorophyll, and that light energy will split a water molecule into atmospheric oxygen, hydrogen ions (also known as protons), and an electron (e^-). This is an oxidation reaction, meaning that there is a loss of electrons.

• The oxygen is a by-product that leaves the cell through stomata.
• The protons further increase the concentration of H⁺ within the thylakoid lumen compared to the stroma, contributing to the concentration gradient. This proton concentration gradient is important because it's a way of storing energy which will later drive ATP synthesis (this is known as chemiosmosis).
• The electron travels along the Electron Transport Chain.

The Proton Pump: is a protein complex that uses the energy from the travelling electron to pump hydrogen ions into the thylakoid lumen, against their concentration gradient. This is a form of active transport since it requires energy.

The electron arrives at Photosystem I: where a photon hits a chlorophyll pigment once again and re-excited the electron. This electron finally contributes to a reduction reaction, where NADP⁺ (Nicotinamide Adenine Dinucleotide Phosphate) yields NADPH (the reduced form of NADP⁺).

• The NADPH is sent to the Calvin-Benson Cycle (below).

ATP Synthase: utilizes the H⁺ concentration gradient to yield Adenosine Triphosphate (ATP). This is a reduction reaction where the movement of the proton along its concentration gradient (passive transport) powers the conversion of inorganic phosphate (P_i) and Adenosine Diphosphate (ADP) into ATP.

• The ATP is sent to the Calvin-Benson Cycle (below).

Figure 3. Visual representation of the Light Dependent Reactions of photosynthesis.

Light-independent reactions: The Calvin-Benson Cycle

The Calvin-Benson Cycle occurs in the stroma of the chloroplast. There are 3 main stages: carbon fixation, reduction, and regeneration. See Figure 4 to visualize these steps.

Carbon dioxide enters for Carbon Fixation: the carbon dioxide enters this cycle through membrane proteins called stomata, which open and close. Rubisco enzyme (RuBP) combines with the carbon dioxide to create a 6-carbon molecule. But, this 6-carbon molecule is unstable and settles as 3-carbon molecules.

The next stage is Reduction: where these 3-carbon molecules are gaining electrons (effectively being reduced). This is done with the help of NADPH and ATP, which are reducing agents (give their electrons). These reactions yield G3P (Glyceraldehyde 3-phosphate, a 3-carbon sugar). One G3P leaves the cycle, and the rest are recycled (they move on to the regeneration stage). The G3P that leaves needs to find another in order to combine and make glucose.

During Regeneration: the G3P are further reduced by ATP and are turned into RuBP enzymes. As such, the cycle begins again with carbon fixation.

Figure 4. Visual representation of the Calvin-Benson Cycle of photosynthesis.

Glucose is the final end-product of photosynthesis for the plant. It's the plants way of storing energy in chemical bonds. The glucose can later be used in cellular respiration where an organism makes ATP from glucose to use in other functions.

Photosynthesis is incredibly important to life, and worth learning more about: two good places to start are hyperphysics (particularly nice is the discussion of how photosynthesis allows plants to appear to overcome the second law of thermodynamics by converting disorder into order) and UC Davis's chem wiki, which has an extensive overview of photosynthesis.

For Further Reading

• Energy in plants
• Radiant energy
• Sunlight
• Redox reaction
• Or explore a random page