Cracking the AP Biology Exam

5

Photosynthesis

A CLOSER LOOK AT PHOTOSYNTHESIS

There are two stages in photosynthesis: the light reaction (also called the light-dependent reaction) and the dark reaction (also called the light-independent reaction). The whole process begins when the photons (or “energy units”) of sunlight strike a leaf, activating chlorophyll and exciting electrons. The activated chlorophyll molecule then passes these excited electrons down to a series of electron carriers, ultimately producing ATP and NADPH. The whole point of the light reaction is to produce two things: (1) energy in the form of ATP and (2) electron carriers, specifically NADPH.

Both of these products, along with carbon dioxide, are then used in the dark reaction (light-independent) to make carbohydrates.

THE LIGHT REACTION

Many light-absorbing pigments participate in photosynthesis. Some of the more important ones are chlorophyll a, chlorophyll b, and carotenoids. These pigments are clustered in the thylakoid membrane into units called antenna complexes.

All of the pigments within a unit are able to “gather” light, but they’re not able to “excite” the electrons. Only one special molecule—located in the reaction center—is capable of transforming light energy to chemical energy. In other words, the other pigments, called antenna pigments, “gather” light and “bounce” the energy to the reaction center.

There are two types of reaction centers: photosystem I (PS I) and photosystem II (PS II). The principal difference between the two is that each reaction center has a type of chlorophyll—chlorophyll a—that absorbs a particular wavelength of light. For example, P680, the reaction center of photosystem II, has a maximum absorption at a wavelength of 680 nanometers. The reaction center for photosystem I—P700—best absorbs light at a wavelength of 700 nanometers.

When light energy is used to make ATP, it is called photophosphorylation. We’re using light (that’s photo) and ADP and phosphates (that’s phosphorylation) to produce ATP.

Noncyclic Photophosphorylation

The noncyclic method of photophosphorylation produces ATP using both photosystem I and photosystem II. When a leaf captures sunlight, the energy is sent to P680, the reaction center for photosystem II. The activated electrons are trapped by P680 and passed to a molecule called the primary acceptor. They are then passed down to carriers in the electron transport chain and eventually enter photosystem I. Some of the energy that dissipates as electrons move along the chain of acceptors will be used to “pump” protons across the membrane into the thylakoid lumen. When the reaction center P680 absorbs light, it also splits water into oxygen, hydrogen ions, and electrons. That process is called photolysis. The electrons from photolysis replace the missing electrons in photosystem II.

Remember the chemiosmotic theory mentioned in aerobic respiration? Well, the same mechanism applies to photosynthesis. Here’s how it works: Hydrogen ions accumulate inside the thylakoids when photolysis occurs. A proton gradient is established. As the hydrogen ions move through ATP synthase, ADP and Pi produce ATP.

When these electrons in photosystem I receive a second boost, they’re activated again. The electrons are passed through a second electron transport chain until they reach the final electron acceptor NADP+ to make NADPH.

Cyclic Photophosphorylation

The cyclic method uses a much simpler pathway to generate ATP. The electrons in photosystem I are excited and leave the reaction center, P700. They are passed from carrier to carrier in the electron transport system and eventually return to P700:

At the end of this cycle, only ATP is produced. This pathway is called cyclic photophosphorylation because the electrons from P700 return to the same reaction center. Unfortunately, this method isn’t as efficient as the noncyclic pathway since it doesn’t produce NADPH. Plants use this method only when there aren’t enough NADP molecules to accept electrons. Keep in mind:

  • The light reaction occurs in the thylakoids.

Let’s review the cyclic and noncyclic phosphorylation steps of the light reaction:

Noncyclic phosphorylation:

  • P680 in photosystem II captures light and passes excited electrons down an electron transport chain to produce ATP.
  • P700 in photosystem I captures light and passes excited electrons down an electron transport chain to produce NADPH.
  • A molecule of water is split by sunlight, releasing electrons, hydrogen, and free O2.

Cyclic phosphorylation:

  • P700 in photosystem I captures light and passes excited electrons down an electron transport chain to produce ATP.
  • NADPH is not produced.
  • Water is not split by sunlight.

Both reactions occur in the grana of chloroplasts, where the thylakoids are found. Remember: The light-absorbing pigments and enzymes for the light-dependent reactions are found within the thylakoids.

THE LIGHT-INDEPENDENT REACTION

Now let’s turn to the dark reaction. The dark reaction uses the products of the light reaction—ATP and NADPH—to make sugar. We now have energy to make glucose, but what do plants use as their carbon source? CO2, of course. You’ve probably heard of the term carbon fixation. All this means is that CO2 from the air is converted into carbohydrates. This step occurs in the stroma of the leaf.

The Calvin Cycle: The C3 Pathway

We’re finally ready to make glucose. CO2 enters the Calvin cycle and combines with a 5–carbon molecule called ribulose bisphosphate (RuBP) to make an unstable six-carbon compound. The enzyme RuBP carboxylase, or rubisco, catalyzes this reaction.

The easiest way to view the Calvin cycle is to consider 6 RuBP and 6 CO2 at the start. Next, 12 ATP and 12 NADPH are used to convert 12 PGA to 12 G3P, an energy-rich molecule. ADP and NADP+ are released and then recycled into the thylakoid where they will again be available for the light-dependent reactions. Two of the G3P are used to make glucose while the remaining 10 are rearranged into 6 RuBPs ready for the next round of the cycle. Since G3P, a three-carbon molecule, is the first stable product, this method of producing glucose is called the C3pathway.

Photorespiration

Sometimes intensely bright light tends to stunt the growth of C3 plants. Why? Lighted conditions can trigger a process called photorespiration. Photorespiration is the pathway that leads to the fixation of oxygen. During this process, RuBP carboxylase reduces the CO2 concentration to the point that it starts incorporating O2 instead. This process makes CO2-fixing less efficient.

C4: The Alternative Pathway

The C3 pathway is not the only way to “fix” CO2. Some plants, such as sugar cane and corn, have a more efficient way to fix carbon dioxide. In these plants, carbon dioxide first “combines” with phosphoenolpyruvate (PEP) in mesophyll cells to form oxaloacetate, a four-carbon molecule. The enzyme that fixes PEP, PEP carboxylase, has a high affinity for CO2 even under unusually low concentrations. Oxaloacetate is then converted to malate. Malate enters the bundle sheath cells, a tissue surrounding the leaf vein, and is converted to pyruvate and carbon dioxide. Carbon dioxide is then released for uptake into the regular Calvin cycle to make glucose.

The C4 pathway works particularly well for plants found in hot, dry climates. It enables them to fix CO2 even when the supply is greatly diminished. For our purposes, just remember that the subscripts in both C3 and C4 refer to the number of carbons initially involved in making sugar. However, both pathways ultimately use the Calvin cycle to produce glucose.

Let’s summarize the important facts about the dark reaction:

  • The Calvin cycle occurs in the stroma of chloroplasts.
  • ATP and NADPH from the light reaction are necessary for carbon fixation.
  • CO2 is fixed to form glucose.

CAM Photosynthesis

In some plants, the stomates are closed during the day to reduce excessive water loss from transpiration. You might think that this would prevent these plants from carrying out photosynthesis during the day. However, desert plants have evolved a way to perform photosynthesis when their stomates are closed; it’s called CAM (crassulacean acid metabolism) photosynthesis. In CAM photosynthesis, a process similar to C4 photosynthesis, PEP carboxylase is used to fix CO2 to oxaloacetate, but oxaloacetate is converted to malic acid instead of malate (an ionized form of malate) and sent to the cell’s vacuole. During the day, malic acid is converted back to oxaloacetate and carbon dioxide is released for photosynthesis. CO2 then enters the Calvin cycle.