Making and Using Food

The flow of energy through living organisms begins with photosynthesis. This process stores energy from sunlight in the chemical bonds of glucose. By breaking the chemical bonds in glucose, cells release the stored energy and make the ATP they need. The process in which glucose is broken down and ATP is made is called cellular respiration. Photosynthesis and cellular respiration are like two sides of the same coin. This is apparent from Figure below. The products of one process are the reactants of the other. Together, the two processes store and release energy in living organisms. The two processes also work together to recycle oxygen in Earth’s atmosphere.

Photosynthesis

Photosynthesis is often considered to be the single most important life process on Earth. It changes light energy into chemical energy and also releases oxygen. Without photosynthesis, there would be no oxygen in the atmosphere. Photosynthesis involves many chemical reactions, but they can be summed up in a single chemical equation:

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

Photosynthetic autotrophs capture light energy from the sun and absorb carbon dioxide and water from their environment. Using the light energy, they combine the reactants to produce glucose and oxygen, which is a waste product. They store the glucose, usually as starch, and they release the oxygen into the atmosphere.

Cellular Respiration

Cellular respiration actually “burns” glucose for energy. However, it doesn’t produce light or intense heat as some other types of burning do. This is because it releases the energy in glucose slowly, in many small steps. It uses the energy that is released to form molecules of ATP. Cellular respiration involves many chemical reactions, which can be summed up with this chemical equation:

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Chemical Energy (in ATP)

Cellular respiration occurs in the cells of all living things. It takes place in the cells of both autotrophs and heterotrophs. All of them burn glucose to form ATP.

Photosynthesis: Sugar as Food

Plants and other autotrophs make food out of “thin air”—at least, they use carbon dioxide from the air to make food. Most food is made in the process of photosynthesis. This process provides more than 99% of the energy used by living things on Earth. Photosynthesis also supplies Earth’s atmosphere with oxygen.

An overview of photosynthesis is available at http://www.youtube.com/user/khanacademy#p/c/7A9646BC5110CF64/26/-rsYk4eCKnA (13:37).

Stages of Photosynthesis

Photosynthesis occurs in two stages, which are shown in Figure below.

1. Stage I is called the light reactions. This stage uses water and changes light energy from the sun into chemical energy stored in ATP and NADPH (another energy-carrying molecule). This stage also releases oxygen as a waste product.
2. Stage II is called the Calvin cycle. This stage combines carbon from carbon dioxide in the air and uses the chemical energy in ATP and NADPH to make glucose.

Before you read about these two stages of photosynthesis in greater detail, you need to know more about the chloroplast, where the two stages take place.

The Chloroplast: Theater for Photosynthesis

The “theater” where both stages of photosynthesis take place is the chloroplast. Chloroplasts are organelles that are found in the cells of plants and algae. (Photosynthetic bacteria do not have chloroplasts, but they contain structures similar to chloroplasts and produce food in the same way.) Look at the Figure below. The figure is a high power microscopic photo of the upper part of a Winter Jasmine leaf. If you could look at a single leaf of this plant under a microscope, you would see small green ovals, like those shown. These small green ovals are chloroplasts.

Figure below shows the components of a chloroplast. Each chloroplast contains neat stacks called grana (singular, granum). The grana consist of sac-like membranes, known as thylakoid membranes. These membranes contain photosystems, which are groups of molecules that include chlorophyll, a green pigment. The light reactions of photosynthesis occur in the thylakoid membranes. The stroma is the space outside the thylakoid membranes. This is where the reactions of the Calvin cycle take place. You can take a video tour of a chloroplast at the link below. http://www.cells.de/cellseng/1medienarchiv/Zellstruktur/Plastiden/Chloroplasten/Feinaufbau/Flug_Chloroplast/index.jsp

Photosynthesis Stage I: The Light Reactions

The first stage of photosynthesis is called the light reactions. During this stage, light is absorbed and transformed to chemical energy in the bonds of NADPH and ATP. You can follow the process in the figure as you read about it below.

Steps of the Light Reactions

The light reactions occur in several steps, all of which take place in the thylakoid membrane, as shown in Figure above.

• Step 1: Units of sunlight, called photons, strike a molecule of chlorophyll in photosystem II of the thylakoid membrane. The light energy is absorbed by two electrons (2 e^-) in the chlorophyll molecule, giving them enough energy to leave the molecule.
• Step 2: At the same time, enzymes in the thylakoid membrane use light energy to split apart a water molecule. This produces:

1. two electrons (2 e^-). These electrons replace the two electrons that were lost from the chlorophyll molecule in Step 1.
2. an atom of oxygen (O). This atom combines with another oxygen atom to produce a molecule of oxygen gas (O₂), which is released as a waste product.
3. two hydrogen ions (2H⁺). The hydrogen ions, which are positively charged, are released inside the membrane in the thylakoid interior space.

• Step 3: The two excited electrons from Step 1 contain a great deal of energy, so, like hot potatoes, they need something to carry them. They are carried by a series of electron-transport molecules, which make up an electron transport chain. The two electrons are passed from molecule to molecule down the chain. As this happens, their energy is captured and used to pump more hydrogen ions into the thylakoid interior space.
• Step 4: When the two electrons reach photosystem I, they are no longer excited. Their energy has been captured and used, and they need more energy. They get energy from light, which is absorbed by chlorophyll in photosystem I. Then, the two re-energized electrons pass down another electron transport chain.
• Step 5: Enzymes in the thylakoid membrane transfer the newly re-energized electrons to a compound called NADP⁺. Along with a hydrogen ion, this produces the energy-carrying molecule NADPH. This molecule is needed to make glucose in the Calvin cycle.
• Step 6: By now, there is a greater concentration of hydrogen ions—and positive charge—in the thylakoid interior space. This difference in concentration and charge creates what is called a chemiosmotic gradient. It causes hydrogen ions to flow back across the thylakoid membrane to the stroma, where their concentration is lower. Like water flowing through a hole in a dam, the hydrogen ions have energy as they flow down the chemiosmotic gradient. The enzyme ATP synthase acts as a channel protein and helps the ions cross the membrane. ATP synthase also uses their energy to add a phosphate group (Pi) to a molecule of ADP, producing a molecule of ATP. The energy in ATP is needed for the Calvin cycle.

Summary of Stage I

By the time Step 6 is finished, energy from sunlight has been stored in chemical bonds of NADPH and ATP. Thus, light energy has been changed to chemical energy, and the first stage of photosynthesis is now complete.

For a more detailed discussion see http://www.youtube.com/user/khanacademy#p/c/7A9646BC5110CF64/27/GR2GA7chA_c (20:16) and http://www.youtube.com/user/khanacademy#p/c/7A9646BC5110CF64/28/yfR36PMWegg (18:51).

Photosynthesis Stage II: The Calvin Cycle

The second stage of photosynthesis takes place in the stroma surrounding the thylakoid membranes of the chloroplast. The reactions of this stage can occur without light, so they are sometimes called light-independent or dark reactions. This stage of photosynthesis is also known as the Calvin cycle because its reactions were discovered by a scientist named Melvin Calvin. He won a Nobel Prize in 1961 for this important discovery. In the Calvin cycle, chemical energy in NADPH and ATP from the light reactions is used to make glucose. You can follow the Calvin cycle in Figure below as you read about it in this section. You can also watch an animation of the Calvin cycle at this link: http://www.science.smith.edu/departments/Biology/Bio231/calvin.html.

Steps of the Calvin Cycle

The Calvin cycle has three major steps: carbon fixation, reduction, and regeneration. All three steps take place in the stroma of a chloroplast.

• Step 1: Carbon Fixation. Carbon dioxide from the atmosphere combines with a simple, five-carbon compound called RuBP. This reaction occurs with the help of an enzyme named RuBisCo and produces molecules known as 3PG (a three-carbon compound, 3-Phosphoglyceric acid).
• Step 2: Reduction. Molecules of 3PG (from Step 1) gain energy from ATP and NADPH (from the light reactions) and re-arrange themselves to form G3P (glycerate 3-phosphate). This molecule also has three carbon atoms, but it has more energy than 3PG. One of the G3P molecules goes on to form glucose, while the rest of the G3P molecules go on to Step 3.
• Step 3: Regeneration. The remaining G3P molecules use energy from ATP to form RuBP, the five-carbon molecule that started the Calvin cycle. This allows the cycle to repeat.

Summary of Stage II

The Calvin cycle takes over where the light reactions end. It uses chemical energy stored in ATP and NADPH (from the light reactions) and carbon dioxide from the air to produce glucose, the molecule that virtually all organisms use for food.

The Calvin Cycle is discussed at http://www.youtube.com/user/khanacademy#p/c/7A9646BC5110CF64/29/slm6D2VEXYs (13:28).

Stages of Cellular Respiration

You have just read how photosynthesis stores energy in glucose. How do living things make use of this stored energy? The answer is cellular respiration. This process releases the energy in glucose to make ATP, the molecule that powers all the work of cells.

An introduction to cellular respiration can be viewed at http://www.youtube.com/user/khanacademy#p/c/7A9646BC5110CF64/19/2f7YwCtHcgk (14:19)

Cellular respiration involves many chemical reactions. As you saw earlier, the reactions can be summed up in this equation:

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Chemical Energy (in ATP)

The reactions of cellular respiration can be grouped into three stages: glycolysis, the Krebs cycle (also called the citric acid cycle), and electron transport. Figure below gives an overview of these three stages, which are also described below.

Cellular Respiration Stage I: Glycolysis

The first stage of cellular respiration is glycolysis. It takes place in the cytosol of the cytoplasm.

Splitting Glucose

The word glycolysis means “glucose splitting,” which is exactly what happens in this stage. Enzymes split a molecule of glucose into two molecules of pyruvate (also known as pyruvic acid). This occurs in several steps, as shown in Figure below. You can watch an animation of the steps of glycolysis at the following link: http://www.youtube.com/watch?v=6JGXayUyNVw.

Results of Glycolysis

Energy is needed at the start of glycolysis to split the glucose molecule into two pyruvate molecules. These two molecules go on to stage II of cellular respiration. The energy to split glucose is provided by two molecules of ATP. As glycolysis proceeds, energy is released, and the energy is used to make four molecules of ATP. As a result, there is a net gain of two ATP molecules during glycolysis. During this stage, high-energy electrons are also transferred to molecules of NAD⁺ to produce two molecules of NADH, another energy-carrying molecule. NADH is used in stage III of cellular respiration to make more ATP.

A summary of glycolysis can be viewed at http://www.youtube.com/user/khanacademy#p/c/7A9646BC5110CF64/22/FE2jfTXAJHg.

Anaerobic and Aerobic Respiration

Scientists think that glycolysis evolved before the other stages of cellular respiration. This is because the other stages need oxygen, whereas glycolysis does not, and there was no oxygen in Earth’s atmosphere when life first evolved about 3.5 to 4 billion years ago. Cellular respiration that proceeds without oxygen is called anaerobic respiration. Then, about 2 or 3 billion years ago, oxygen was gradually added to the atmosphere by early photosynthetic bacteria. After that, living things could use oxygen to break down glucose and make ATP. Today, most organisms make ATP with oxygen. They follow glycolysis with the Krebs cycle and electron transport to make more ATP than by glycolysis alone. Cellular respiration that proceeds in the presence of oxygen is called aerobic respiration.

Structure of the Mitochondrion: Key to Aerobic Respiration

Before you read about the last two stages of aerobic respiration, you need to know more about the mitochondrion, where these two stages take place. A diagram of a mitochondrion is shown in Figure below.

As you can see from Figure above, a mitochondrion has an inner and outer membrane. The space between the inner and outer membrane is called the intermembrane space. The space enclosed by the inner membrane is called the matrix. The second stage of cellular respiration, the Krebs cycle, takes place in the matrix. The third stage, electron transport, takes place on the inner membrane.

Cellular Respiration Stage II: The Krebs Cycle

Recall that glycolysis produces two molecules of pyruvate (pyruvic acid). These molecules enter the matrix of a mitochondrion, where they start the Krebs cycle. The reactions that occur next are shown in Figure below. You can watch an animated version at this link: http://www.youtube.com/watch?v=p-k0biO1DT8&feature=related.

Before the Krebs cycle begins, pyruvic acid, which has three carbon atoms, is split apart and combined with an enzyme known as CoA, which stands for coenzyme A. The product of this reaction is a two-carbon molecule called acetyl-CoA. The third carbon from pyruvic acid combines with oxygen to form carbon dioxide, which is released as a waste product. High-energy electrons are also released and captured in NADH.

Steps of the Krebs Cycle

The Krebs cycle itself actually begins when acetyl-CoA combines with a four-carbon molecule called OAA (oxaloacetate) (see Figure above). This produces citric acid, which has six carbon atoms. This is why the Krebs cycle is also called the citric acid cycle. After citric acid forms, it goes through a series of reactions that release energy. The energy is captured in molecules of NADH, ATP, and FADH₂, another energy-carrying compound. Carbon dioxide is also released as a waste product of these reactions. The final step of the Krebs cycle regenerates OAA, the molecule that began the Krebs cycle. This molecule is needed for the next turn through the cycle. Two turns are needed because glycolysis produces two pyruvic acid molecules when it splits glucose. Watch the OSU band present the Krebs cycle: http://www.youtube.com/watch?v=FgXnH087JIk&feature=related.

Results of the Krebs Cycle

After the second turn through the Krebs cycle, the original glucose molecule has been broken down completely. All six of its carbon atoms have combined with oxygen to form carbon dioxide. The energy from its chemical bonds has been stored in a total of 16 energy-carrier molecules. These molecules are:

• 4 ATP (including 2 from glycolysis)
• 10 NADH (including 2 from glycolysis)
• 2 FADH₂

The Krebs cycle is reviewed at http://www.youtube.com/user/khanacademy#p/c/7A9646BC5110CF64/23/juM2ROSLWfw.

Cellular Respiration Stage III: Electron Transport

Electron transport is the final stage of aerobic respiration. In this stage, energy from NADH and FADH₂, which result from the Krebs cycle, is transferred to ATP. Can you predict how this happens? (Hint: How does electron transport occur in photosynthesis?)

See http://www.youtube.com/watch?v=1engJR_XWVU&feature=related for an overview of the electron transport chain.

Transporting Electrons

High-energy electrons are released from NADH and FADH₂, and they move along electron transport chains, like those used in photosynthesis. The electron transport chains are on the inner membrane of the mitochondrion. As the high-energy electrons are transported along the chains, some of their energy is captured. This energy is used to pump hydrogen ions (from NADH and FADH₂) across the inner membrane, from the matrix into the intermembrane space. Electron transport in a mitochondrion is shown in Figure below. You can also see an animation of the process at this link: http://www.youtube.com/watch?v=Idy2XAlZIVA&feature=related.

Making ATP

The pumping of hydrogen ions across the inner membrane creates a greater concentration of the ions in the intermembrane space than in the matrix. This chemiosmotic gradient causes the ions to flow back across the membrane into the matrix, where their concentration is lower. ATP synthase acts as a channel protein, helping the hydrogen ions cross the membrane. It also acts as an enzyme, forming ATP from ADP and inorganic phosphate. After passing through the electron-transport chain, the “spent” electrons combine with oxygen to form water. This is why oxygen is needed; in the absence of oxygen, this process cannot occur. You can see how all these events occur at the following link: http://www.sp.uconn.edu/~terry/images/anim/ATPmito.html.

A summary of this process can be seen at the following sites: http://www.youtube.com/user/khanacademy#p/c/7A9646BC5110CF64/24/mfgCcFXUZRk (17:16) and http://www.youtube.com/user/khanacademy#p/c/7A9646BC5110CF64/25/W_Q17tqw_7A (4:59).

How Much ATP?

You have seen how the three stages of aerobic respiration use the energy in glucose to make ATP. How much ATP is produced in all three stages? Glycolysis produces 2 ATP molecules, and the Krebs cycle produces 2 more. Electron transport begins with several molecules of NADH and FADH₂ from the Krebs cycle and transfers their energy into as many as 34 more ATP molecules. All told, then, up to 38 molecules of ATP can be produced from just one molecule of glucose in the process of aerobic respiration.