# 12.5: Part 2 - Photosynthesis


Photosynthesis is a process used to harness the energy from sunlight to ultimately bond carbon atoms from carbon dioxide into molecules of glucose. This occurs in two major phases--the light dependent phase and the light independent phase--and requires chlorophyll a. Both phases take place inside of the chloroplasts of eukaryotic photosynthesizers or across folded membranes within prokaryotes. For simplicity’s sake, we will focus on the eukaryotes and describe these processes in relation to chloroplast anatomy (sorry, bacteria, back seat again).

## Anatomy of a Chloroplast

Chloroplasts are surrounded by at least two membranes. The two membranes were derived from the original cyanobacterium that was engulfed but not digested (Keeling, 2004). Inside of the inner membrane, the chloroplast has a jelly-like matrix, much like the cytosol of the cell, called the stroma. The stroma surrounds a series of folded membrane-bound structures, thylakoids, that resemble pancakes and are stacked (again, much like pancakes) into columns called grana (granum, singular). The thylakoids appear dark green, as the thylakoid membrane is full of chlorophyll molecules. Enclosed inside each thylakoid is a region called the thylakoid space.

Label the bolded structures in the diagram of the chloroplast. Indicate the origin of each membrane of the chloroplast.

## Light-dependent Phase: The electron Transport Chain

This phase is called the light-dependent phase because to begin this process, a particle of light, called a photon, of the correct wavelength must be absorbed by a molecule of chlorophyll a, embedded in a complex called photosystem II (PSII).

The energy from the photon causes an electron to be knocked off of the chlorophyll molecule and this electron is shuttled to an adjacent protein complex. More on this in a second.

The chlorophyll molecule needs to replace the electron it lost so that it is ready to respond to the next photon. A water molecule is split at the base of PSII, releasing two electrons and producing oxygen and 2 protons ($$\ce{H+}$$) into the thylakoid space. Make note of each time H+ is added to the thylakoid space, as this creates a buildup of positive charges, which repel each other.

Back to that first electron. The protein complex it is shuttled to is a proton pump. When the electron enters the protein complex, it effectively switches on the proton pump, causing $$\ce{H+}$$ to be pumped from the stroma and into the thylakoid space.

The electron then jumps to another proton pump, causing another H+ to be pumped from the stroma into the thylakoid space.

This electron has depleted much of its energy, so it must enter another photosystem (PSI) to be reenergized by another photon.

The re-energized electron is then transported to an enzyme called NADP+ reductase, which reduces (adds electrons to) a molecule of NADP+ to create high-energy NADPH. This requires two electrons, but this process involves a continuous flow of electrons, so another one arrives shortly after the first.

This is the end of the electron transport chain. However, there is another component to this phase. Inside the thylakoid membrane, $$\ce{H+}$$ are building up, creating a storage of energy across the thylakoid membrane as they repel against each other. Even though they are tiny (a single proton), they cannot pass freely through the membrane due to the positive charge.

Instead, an enzyme called ATP synthase allows the $$\ce{H+}$$ passage to the other side. The flow of $$\ce{H+}$$ through the enzyme causes it to spin, much like a turbine, converting the stored energy into kinetic energy (movement).

ATP synthase converts this kinetic energy into chemical energy by using it to add phosphate groups onto molecules of ADP. This creates unstable, high energy ATP molecules that can be used to power other cellular processes.

Apply it: Work with your labmates to design a model that illustrates the light-dependent phase of photosynthesis. Consider the materials available in lab (including, potentially, other students), as well as whether your model will have moving parts. What is the best way to communicate this process?

Below is an example of a model of the electron transport chain in photosynthesis. Can you talk your way through what is happening? Try explaining this process to a partner.

## Light-independent Phase: The Calvin Cycle

The light-independent phase takes place in the stroma of the chloroplast. During this phase, called the Calvin Cycle, the chemical energy stored in the NADPH and ATP produced during the light-dependent phase is used to build molecules of glucose. This happens in three major stages.

1. Carbon Fixation: Carbon dioxide ($$\ce{CO2}$$) enters the plant through the stoma, diffuses into the cells, then into the chloroplast. An enzyme called RuBisCO (a much easier way to say Ribulose-1, 5-bisphosphate carboxylase/oxygenase) bonds $$\ce{CO2}$$ to a 5-carbon molecule called RuBP. This breaks into two 3-carbon molecules of 3-PGA.
2. Reduction: NADPH donate electrons and ATP donates a phosphate group to convert each 3-PGA into G3P, two of which are required to make glucose. For every six G3P made, only one goes on to make glucose, the rest must be recycled.
3. Regeneration: To keep the cycle going, RuBP must be regenerated. Using energy and phosphate donated by more ATP, five G3P are recycled to make three more RuBP.

This process can happen during day or night, but it requires a steady input of ATP, NADPH and $$\ce{CO2}$$.

The reactants above are required to complete this process. What are the products of the light independent phase?

For each of the products and reactants in the chemical reaction for photosynthesis written below, identify where it was either produced or consumed.

Table $$\PageIndex{1}$$: Summary Table for Photosynthesis

Phase

Where does it take place?

Reactants

Products

Light dependent

Light independent