3.5: Photosynthesis

Now that you know how enzymes work, it's time to learn about one of their most important uses. Topic 3.5 goes over the complex (and quite technical) process of photosynthesis. Feel free to watch the video and read the lesson multiple times to make sure you get the hang of this process.

Vocab List

Written Explanation

A Macro Analysis:

Before looking at the specifics of energy production in cells, we should zoom out to learn about the organisms that produce and use this energy. Those organisms can be organized into two general groups: autotrophs and heterotrophs. Autotrophs make their own energy, which they store in energy rich molecules (like glucose). Heterotrophs cannot make their own energy, and instead consume the energy stored by autotrophs.

Autotrophs Heterotrophs
Produce their own energy from light (photoautotroph) or chemicals (chemoautotroph) Get energy by processing complex organic molecules (which come from other organisms)
These can be plants, algae, and certain bacteria These are animals (herbivores, omnivores, and carnivores), and other unicellular organisms

A Micro Analysis:

Photosynthesis is the process by which all plants, algae, and certain unicellular organisms get their own energy. Using energy from sunlight, it converts carbon dioxide and water into glucose and oxygen. If it weren't for photosynthesis, not only would animals not have energy (glucose), but they also wouldn't have the oxygen they need to breathe.

The photosynthesis equation

The equation above is the general idea behind photosynthesis, but College Board expects you to know a few more details, starting with:

Chloroplasts:

Photosynthesis occurs in chloroplasts, which are small organelles present in plant cells. These organelles help separate the enzymes that perform photosynthesis from the rest of the cell, making the process more efficient (compartmentalization). Additionally, this allows the buildup of an electrochemical gradient (more on that later). Chloroplasts contain small disk-shaped sacs called thylakoids, which is where photosynthesis begins. A green pigment, known as chlorophyll, is housed in thylakoids, which is what absorbs sunlight and gives plants their signature green look. Photosynthesis ends in the stroma, which is the gel-like liquid that fills chloroplasts (like the cytosol of a cell).

Diagram of a chloroplast

Stomata:

Another component of plant cells which facilitates photosynthesis is the stomata, which are holes in the leaf that allow CO2, O2, and H2O to enter and exit the plant (these are all components of photosynthesis).

Diagram of a stomata

How plants get energy:

The first half of photosynthesis is directly fueled by sunlight. These reactions are called the light-dependent reactions of photosynthesis. They occur in the membrane and lumen of thylakoids.

Diagram of the light-dependent reactions

Starting in Photosystem II (because it was discovered second), light hits a type of chlorophyll pigment called P680. The photons (small particles) in light energize two electrons of the compound, moving them into a higher energy state. These energy rich electrons are then passed through the reaction center of the system, before finally being captured by the primary electron acceptor and being carried out of the protein complex. To summarize: light energy is used to energize electrons, which is then exported to the next stage.

Diagram of PSII

Since photosynthesis (and specifically PSII) needs to keep running, the lost electrons must be replenished from somewhere. Plants cells do this by splitting a water molecule into its oxygen part, which forms O2 and some H+ (protons), and electrons. The oxygen is released back out of the cell from the stomata, the electrons are used to refill PSII, and the hydrogen ions are used in another part of photosynthesis. This type of reaction (such as splitting water) is called an oxidation reaction because it causes the water molecule to lose electrons.

Diagram of the oxidation of water molecules

Next, electrons are passed from Photosystem II to Photosystem I via the electron transport chain (ETC). The ETC is composed of several cytochrome integral proteins (literally defined as electron carrying proteins).

Diagram of the ETC

At the end of the ETC, electrons have lost some of their energy, and enter Photosystem I (because it was discovered first). In PSI, a slightly different chlorophyll pigment called P700 is energized by light (similarly to P680 in PSII). Once again, the energized electrons are passed to a primary electron acceptor, and then moved down a different electron transport chain. The electrons from PSII are the ones that replace the exported electrons. This time, the destination of the electrons is to be added to a NADP+ electron carrier molecule, turning it to an NADPH molecule. This type of reaction is called a reduction reaction, meaning that it adds electrons to a molecule.

Diagram of PSI

ATP Synthesis:

The last stage of light-dependent reactions uses all the previously mentioned parts to generate usable energy. Throughout the light-dependent reactions, an electrochemical gradient has built up. Just like a strong concentration gradient involves a difference in substrate concentration, a strong electrochemical gradient involves a difference in charge between the inside and outside of the relevant membrane (in this case, the thylakoid membrane). Since opposite charges want to attract each other, an electrochemical gradient forces positively and negatively charged particles towards each other.

In PSII, hydrogen ions (positively charged protons) are accumulated in the lumen of thylakoids as a result of the oxidation of water molecules. Additionally, negatively charged electrons are built up on the other side of the membrane, as the electrons travel from PSII, to the ETC, and then to PSI. Most importantly, some of the ETC's integral proteins are active transport proteins that use the energized electrons' energy to move an H+ into the thylakoid. The work of these ETC proteins moves the largest amount of protons across the thylakoid membrane.

Buildup of the electrochemical gradient

If you remember back to the rules of passive diffusion, molecules must be small, nonpolar, and uncharged to pass freely across the plasma membrane. Since hydrogen ions are charged, they can only cross through dedicated channels. Chloroplasts utilize both the selective permeability of thylakoid membranes and the strong forces that arise from the electrochemical gradient. Through chemiosmosis, H+ are passed through an integral channel protein called ATP synthase, powering the phosphorylation (adding a phosphate group) of an ADP molecule.

How plants store energy:

So plants get their energy from the sun in light-dependent reactions. But how do they store that energy for later use? Well, some of it is generated and used immediately in the form of ATP. The rest, however, is stored as glucose through the light-independent reactions. These reactions, unlike the light-dependent reactions, occur in the stroma of the chloroplast.

Summary of the light-independent reactions

The light-independent reactions constitute one major element: the Calvin Cycle. The cycle is divided into three distinct phases. First, CO2 is added to a 5 carbon sugar (which is done by an enzyme called Rubisco). This is known as fixing the carbon (turning it into an organic compound). Then, some ATP from the ATP Synthesis stage, and NADPH from PSI are used in a series of reactions to turn those 6 carbon sugars into 2 G3P. For every three input CO2, one G3P leaves the cycle to help create glucose. Lastly, another series of reactions combines 2 G3P into one glucose molecule. Ultimately, 6 turns of the Calvin Cycle are needed to create one molecule of glucose, as there are 6 CO2 molecules which need to be fixed.

Diagram of the Calvin Cycle