Photosynthetic bacteria; also known as blue-green bacteria, or blue-green algae.
electron transport chain
A series of electron carrier molecules that transfers electrons.
light reactions
The reactions of photosynthesis that only occur during daylight hours in which the energy of sunlight is captured; also known as the light-dependent reactions.
NADPH
A high energy electron carrier produced during the light reactions; carries the energy in the electrons to the Calvin Cycle.
photosynthesis
The process by which plants use the sun's energy to make their own “food” from carbon dioxide and water; process that converts the energy of the sun, or solar energy, into carbohydrates, a type of chemical energy.
stomata
Special pores in leaves; carbon dioxide enters the leaf and oxygen exits the leaf through these pores.
stroma
Fluid in the chloroplast interior space; surrounds the thylakoids.
thylakoid
Flattened sacs within the chloroplast; formed by the inner membranes.
Points to Consider
How is glucose turned into an usable form of energy called ATP?
How do you gain energy from the food you eat?
What would provide more energy- a bowl of pasta or a small piece of candy?
What “waste” gas do you exhale?
Lesson 4.3: Cellular Respiration
Lesson Objectives
Write and explain the chemical formula for cellular respiration.
Explain the two states of cellular respiration.
Compare photosynthesis with cellular respiration.
Describe the results of fermentation and understand when fermentation is needed.
Check Your Understanding
Where does the energy captured at the beginning of photosynthesis originate from?
What is the form of chemical energy produced by photosynthesis?
Introduction
How does the food you eat provide energy? When you need a quick boost of energy, you might reach for an apple or a candy bar. Although foods with sugars can give you a quick boost of energy, they cannot be used for energy directly by your cells. Energy is simply stored in these foods. Through the process of cellular respiration, the energy in food is converted into energy that can be used by the body's cells. In other words, glucose (and oxygen) is converted into ATP (and carbon dioxide and water). ATP is the molecule that provides energy for your cells to perform work, such as contracting your muscles as you walk down the street or performing active transport. Cellular respiration is simply a process that converts one type of chemical energy, the energy stored in sugar, into another type, ATP.
Overview of Cellular Respiration
Most often, cellular respiration proceeds by breaking down glucose into carbon dioxide and water. As this breakdown of glucose occurs, energy is released. The process of cellular respiration includes the conversion of this energy into ATP. The overall reaction for cellular respiration is as follows:
C6H12O6 + 6O2 6CO2 + 6H2O
Notice that the equation for cellular respiration is the direct opposite of photosynthesis. While water was broken down to form oxygen during photosynthesis, in cellular respiration oxygen is combined with hydrogen to form water. While photosynthesis requires carbon dioxide and releases oxygen, cellular respiration requires oxygen and releases carbon dioxide. This exchange of carbon dioxide and oxygen in all the organisms that use photosynthesis and/or cellular respiration worldwide, helps to keep atmospheric oxygen and carbon dioxide at somewhat stable levels.
Cellular respiration doesn’t happen all at once, however. Glucose is broken down slowly so that cells convert as much sugar as possible into the usable form of energy, ATP. Still, some energy is lost in the process in the form of heat. When one molecule of glucose is broken down, it can be converted to a net total of 36 or 38 molecules of ATP. Although the process is not 100% efficient, it is much more efficient than, for instance, a car engine obtaining energy from gasoline.
Cellular respiration can be divided into three phases.
Glycolysis: the breakdown of glucose.
The citric acid cycle: the formation of electron carriers.
The electron transport chain: the formation of ATP.
In eukaryotic cells, the first phase takes place in the cytoplasm of the cell, while the other phases are carried out in the mitochondria. This organelle is known as the “powerhouse” of the cell because this is the organelle where the ATP that powers the cell is produced.
Glycolysis
The first step of cellular respiration is glycolysis. During glycolysis, the 6-carbon glucose is practically "cut in half," broken down into two 3-carbon pyruvate molecules. Glycolysis requires an initial energy-investment step, although in the end, glycolysis produces more energy than was initially invested. Two ATP molecules are used to convert glucose into the two 3-carbon pyruvate molecules. These 3-carbon molecules are then oxidized, which means that they lose electrons, as electrons are transferred to the high energy electron acceptor NAD+, producing the electron carrier NADH. This oxidation step helps produce 4 ATP molecules from ADP. That means, taking into account the initial investment of 2 ATP molecules, glycolysis has a net production of 2 ATP.
An Overview of Glycolysis Inputs Outputs
Glucose (6-carbon molecule) 2 pyruvate (3-carbon molecule)
2 NAD + 2 NADH (electron carrier)
2 ATP (energy) 2 ADP
4 ADP 4 ATP (energy)
After glycolysis, the pyruvate can go down several different paths. If there is oxygen available, the pyruvate moves inside the mitochondrion to produce more ATP during further break-down stages. In the absence of oxygen, the fermentation process begins.
Inside the Mitochondria
If oxygen is available, the next step of cellular respiration is moving the pyruvate into the mitochondria. The mitochondria have a double membrane. The inner membrane is known as the cristae, and is folded to form many internal layers. Some steps of cellular respiration occur in the cristae, while others take place in the matrix, the inner compartment of the mitochondrion that is filled with enzymes in a gel-like fluid.
Figure 4.12
Most of the reactions of cellular respiration are carried out in the mitochondria.
Within the mitochondria the Kreb’s Cycle or citric acid cycle occurs. The citric acid cycle is a series of oxidation steps that produce NADH and FADH2, another type of electron carrier. These electron carriers will be used in the final step of cellular respiration. To begin the Kreb's Cycle, the 3-carbon pyruvate from glycolysis must be converted into a 2-carbon molecule, which then can enter the cycle. During the cycle carbon dioxide is produced. Two molecules of ATP are also produced per each initial glucose molecule. A graphic of the mitochondria is shown in Figure above.
In the final steps of cellular respiration, the electron transport chain accepts the electrons from glucose that are being carried by NADH and FADH2. These electrons are passed along the chain until they are finally combined with oxygen, which with the addition of hydrogen ions, becomes water. That is the key reason why this process only occurs in the presence of oxygen. Illustrated in Figure below.
As the electrons move down the electron transport chain, energy is released and later used to synthesize ATP. The process of ATP synthesis is exactly the same as photosynthesis; hydrogen ions are pumped across the cristae of the mitochondria, forming a chemiosmotic gradient, and ATP synthase uses the energy of the movement of hydrogen ions back across the membrane, from high to low concentration, to make ATP.
Because oxygen is the final electron acceptor in this process, the electron transport chain can only occur in the presence of oxygen. This is known as aerobic respiration. However, there is not always enough oxygen present for aerobic respiration to occur. In this case, the next step after glycolysis will be fermentation instead of the citric acid cycle.
&
nbsp; An Overview of the Citric Acid Cycle Inputs Outputs
2 two-carbon molecules 4 CO2
6 NAD+ 6 NADH (electron carrier)
2 FAD+ 2 FADH2 (electron carrier)
2 ADP 2 ATP (energy)
Figure 4.13
During electron transport, electrons from glucose (carried by NADH and FADH) are passed along until they are finally combined with oxygen, which with the addition of hydrogen ions, becomes water. Meanwhile, hydrogen ions are pumped across the cristae of the mitochondria, forming a gradient, and ATP synthase uses the energy of the movement of hydrogen ions back across the membrane, from high to low concentration, to make ATP.
Fermentation
Sometimes cellular respiration is anaerobic, occurring in the absence of oxygen. In the process of fermentation, the NAD+ is recycled so that is can be reused in the glycolysis process. No additional ATP is produced during fermentation, so the organism only obtains the two net ATP molecules per glucose from glycolysis.
Yeasts (single-celled eukaryotic organisms) carry on alcoholic fermentation in the absence of oxygen, making ethyl alcohol (drinking alcohol) and carbon dioxide. Alcoholic fermentation is central to bread baking. The carbon dioxide bubbles allow the bread to rise, and the alcohol evaporates. In wine making, the sugars of grapes are fermented to produce the wine.
Animals and some bacteria and fungi carry out lactic acid fermentation. Lactate (lactic acid) is a waste product of this process. Our muscles undergo lactic acid fermentation during strenuous exercise, when oxygen cannot be delivered to the muscles quickly enough. The buildup of lactate is what makes your muscles sore after vigorous exercise. Bacteria that produce lactate are used to make cheese and yogurt (Figure below). Tooth decay is also accelerated by lactate from the bacteria that use the sugars in your mouth. In all these types of fermentation, the goal is the same: to recycle NAD+ for glycolysis.
Figure 4.14
Products of fermentation include cheese (lactic acid fermentation) and wine (alcoholic fermentation).
Lesson Summary
Cellular respiration is the breakdown of glucose to release energy in the form of ATP.
Glycolysis, the conversion of glucose into two 3-carbon pyruvate molecules, is the first step of cellular respiration.
If oxygen is available, the pyruvate enters the mitochondria and goes through a series of reactions, including the citric acid cycle, to produce more ATP.
If oxygen is not available, the pyruvate is reduced during the process of fermentation to free up more NAD+ for glycolysis, and there is no net gain of ATP.
Review Questions
What are the products of alcoholic fermentation?
What is the metabolic process where glucose is ultimately converted to two molecules of pyruvate?
Why do your muscles get sore after vigorous exercise?
What is the purpose of fermentation?
Where does the citric acid cycle take place?
Write the chemical reaction for the overall process of cellular respiration.
Which is more efficient, aerobic or anaerobic cellular respiration?
What are the important electron-accepting enzymes in cellular respiration?
What is chemiosmosis?
Further Reading / Supplemental Links
http://en.wikipedia.org/wiki/Cellular_respiration
http://biology.clc.uc.edu/Courses/bio104/cellresp.htm
http://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookGlyc.html
http://biology.clc.uc.edu/Courses/bio104/cellresp.htm
http://www.science.smith.edu/departments/Biology/Bio231/glycolysis.html
Vocabulary
aerobic respiration
Cellular respiration in the presence of oxygen.
alcoholic fermentation
Fermentation in the absence of oxygen; produces ethyl alcohol (drinking alcohol) and carbon dioxide; occurs in yeasts.
anaerobic respiration
Cellular respiration in the absence of oxygen; fermentation.
ATP
A usable form of energy inside the cell; adenosine triphosphate.
cellular respiration
The process in which the energy in food is converted into energy that can be used by the body's cells; in other words, glucose (and oxygen) is converted into ATP (and carbon dioxide and water).
citric acid cycle
Middle phase of cellular respiration; formation of electron carriers occurs during this phase; also known as the Kreb’s cycle.
cristae
The inner membrane of the mitochondria.
electron transport chain
Last phase of cellular respiration; used to power the formation of ATP occurs during this phase.
FADH2
Electron carrier produced during the Kreb’s cycle.
fermentation
Anaerobic respiration in which NAD+ is recycled so that is can be reused in the glycolysis process.
glycolysis
First phase of cellular respiration; breakdown of glucose occurs during glycolysis; produces two 3-carbon pyruvate molecules.
lactic acid fermentation
Anaerobic respiration that recycles NAD+ for glycolysis; occurs in animals and some bacteria and fungi.
matrix
The inner compartment of the mitochondrion that is filled with enzymes in a gel-like fluid.
mitochondria
Organelle where cellular respiration occurs; known as the "powerhouse" of the cell because this is the organelle where the ATP that powers the cell is produced.
NADH
Electron carrier produced during glycolysis and the citric acid cycle.
Points to Consider
Now that we know how the cell gets its energy, we are going to turn our attention to cell division. Cell division is a highly regulated process.
What do you think could happen if your cells divide uncontrollably?
When new life is formed, do you think it receives all the DNA of the mother and the father?
Why do you think you might need new cells throughout your life?
Chapter 5: Cell Division, Reproduction, and DNA
Lesson 5.1: Cell Division
Lesson Objectives
Explain why cells need to divide.
List the stages of the cell cycle and explain what happens at each stage.
List the stages of mitosis and explain what happens at each stage.
Check Your Understanding
What is the cell theory?
In what part of your cells is the genetic information located?
Introduction
Imagine the first stages of a life. In humans, a sperm fertilizes an egg, forming the first cell. From that one cell, an entire baby with trillions of cells will develop. How does a new life go from one cell to so many? The cell divides in half, creating two cells. Then those two cells divide. The new cells continue to divide and divide. One cell becomes two, then four, then eight, and so on (Figure below ). Rapid cell division allows the development of new life, but cell division must be tightly regulated. If the body’s close regulation of cell division is disrupted later in life, diseases such as cancer can develop. Cancer involves cells that divide in an uncontrolled manner. Therefore, much research into cell division is underway across the globe in effort to further understand this process and find a cure for cancer.
Figure 5.1
Cells divide repeatedly to produce an embryo. Previously the one-celled zygote divided to make two cells (a). Each of the two cells divides to yield four cells (b), then the four cells divide to make eight cells(c), and so on. Through cell division, an entire embryo forms from one initial cell.
Why Cells Divide
Besides the development of a fetus, there are many other reasons that cell division is necessary to life. To grow and develop, you must form new cells. Imagine how often your cells must divide during a growth spurt. Growing just an inch requires countless cell divisions.
Another reason for cell divisio
n is to repair damaged cells. Imagine you cut your finger. After the scab forms, it will eventually disappear and new skin cells will grow to repair the wound. Where do these cells come from? Remember that according to the cell theory, all cells must come from preexisting cells. In order to make new skin cells, some of your existing skin cells had to undergo cell division.
Besides suffering physical damage, your cells can simply wear out. Over time you must replace old and worn-out cells. Again, cell division is essential to this process. You can only make new cells by dividing similar preexisting cells.
The Cell Cycle
The process of cell division in eukaryotic cells is carefully regulated. The cell cycle which in essence is the lifecycle of a cell, is composed of a series of steps that lead to cell division (Figure below ). These steps can be divided into two main components: interphase and mitosis. Interphase is when the cell mainly performs its “everyday” functions; for example, it is when a kidney cell does what a kidney cell is supposed to do. On the other hand, mitosis is when the cell prepares to become two cells. Some cells, like nerve cells, do not complete the cell cycle and divide, while others divide repeatedly.
Most of the cell cycle consists of interphase, the time between cell divisions. During this time the cell carries out its normal functions and prepares for the next stage. Interphase can be divided into three stages: the first growth phase (G1), the synthesis phase (S), and the second growth phase (G2). During the G1 stage, the cell doubles in size and doubles the number of organelles. Next, during the S stage, the DNA is replicated. In other words, an identical copy of all the cell’s DNA is made. This ensures that each new cell that results after cell division has a set of genetic material identical to that of the parental cell. DNA replication will be further discussed in lesson 3. Finally, in the G2 stage proteins are synthesized that will aid in cell division. In the end of interphase, the cell is ready to enter the mitotic phase.
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