Still, the vast majority of mitochondrial proteins are synthesized from nuclear genes and transported into the mitochondria. These include the enzymes required for the citric acid cycle, the proteins involved in DNA replication and transcription, and ribosomal proteins. The protein complexes of the respiratory chain are a mixture of proteins encoded by mitochondrial genes and proteins encoded by nuclear genes.
Proteins in both the outer and inner mitochondrial membranes help transport newly synthesized, unfolded proteins from the cytoplasm into the matrix, where folding ensues Figure 3. This causes diffusion of the tethered protein and its receptor through the membrane to a contact site, where translocator proteins line up green. When at this contact site, the receptor protein hands off the tethered protein to the translocator protein, which then channels the unfolded protein past both the inner and outer mitochondrial membranes.
Mitochondria cannot be made "from scratch" because they need both mitochondrial and nuclear gene products. These organelles replicate by dividing in two, using a process similar to the simple, asexual form of cell division employed by bacteria. Video microscopy shows that mitochondria are incredibly dynamic. They are constantly dividing, fusing, and changing shape. Indeed, a single mitochondrion may contain multiple copies of its genome at any given time. Logically, mitochondria multiply when a the energy needs of a cell increase.
Therefore, power-hungry cells have more mitochondria than cells with lower energy needs. For example, repeatedly stimulating a muscle cell will spur the production of more mitochondria in that cell, to keep up with energy demand. Conclusion Mitochondria, the so-called "powerhouses" of cells, are unusual organelles in that they are surrounded by a double membrane and retain their own small genome. Figure 2. Glycolysis Overview. During the energy-consuming phase of glycolysis, two ATPs are consumed, transferring two phosphates to the glucose molecule.
The glucose molecule then splits into two three-carbon compounds, each containing a phosphate. During the second phase, an additional phosphate is added to each of the three-carbon compounds. The energy for this endergonic reaction is provided by the removal oxidation of two electrons from each three-carbon compound. During the energy-releasing phase, the phosphates are removed from both three-carbon compounds and used to produce four ATP molecules. Watch this video to learn about glycolysis.
Glycolysis can be divided into two phases: energy consuming also called chemical priming and energy yielding. The first phase is the energy-consuming phase, so it requires two ATP molecules to start the reaction for each molecule of glucose. Importantly, by the end of this process, one glucose molecule generates two pyruvate molecules, two high-energy ATP molecules, and two electron-carrying NADH molecules.
The following discussions of glycolysis include the enzymes responsible for the reactions. When glucose enters a cell, the enzyme hexokinase or glucokinase, in the liver rapidly adds a phosphate to convert it into glucosephosphate.
A kinase is a type of enzyme that adds a phosphate molecule to a substrate in this case, glucose, but it can be true of other molecules also.
This conversion step requires one ATP and essentially traps the glucose in the cell, preventing it from passing back through the plasma membrane, thus allowing glycolysis to proceed. It also functions to maintain a concentration gradient with higher glucose levels in the blood than in the tissues. By establishing this concentration gradient, the glucose in the blood will be able to flow from an area of high concentration the blood into an area of low concentration the tissues to be either used or stored.
Hexokinase is found in nearly every tissue in the body. Glucokinase, on the other hand, is expressed in tissues that are active when blood glucose levels are high, such as the liver. Hexokinase has a higher affinity for glucose than glucokinase and therefore is able to convert glucose at a faster rate than glucokinase. This is important when levels of glucose are very low in the body, as it allows glucose to travel preferentially to those tissues that require it more.
In the next step of the first phase of glycolysis, the enzyme glucosephosphate isomerase converts glucosephosphate into fructosephosphate. Like glucose, fructose is also a six carbon-containing sugar. The enzyme phosphofructokinase-1 then adds one more phosphate to convert fructosephosphate into fructosebisphosphate, another six-carbon sugar, using another ATP molecule. Aldolase then breaks down this fructosebisphosphate into two three-carbon molecules, glyceraldehydephosphate and dihydroxyacetone phosphate.
The triosephosphate isomerase enzyme then converts dihydroxyacetone phosphate into a second glyceraldehydephosphate molecule. Therefore, by the end of this chemical-priming or energy-consuming phase, one glucose molecule is broken down into two glyceraldehydephosphate molecules.
The second phase of glycolysis, the energy-yielding phase, creates the energy that is the product of glycolysis. Glyceraldehydephosphate dehydrogenase converts each three-carbon glyceraldehydephosphate produced during the energy-consuming phase into 1,3-bisphosphoglycerate. Because there are two glyceraldehydephosphate molecules, two NADH molecules are synthesized during this step.
Each 1,3-bisphosphoglycerate is subsequently dephosphorylated i. The enzyme phosphoglycerate mutase then converts the 3-phosphoglycerate molecules into 2-phosphoglycerate. The enolase enzyme then acts upon the 2-phosphoglycerate molecules to convert them into phosphoenolpyruvate molecules. Plants use much of this glucose, a carbohydrate, as an energy source to build leaves, flowers, fruits, and seeds.
They also convert glucose to cellulose, the structural material used in their cell walls. Most plants produce more glucose than they use, however, and they store it in the form of starch and other carbohydrates in roots, stems, and leaves. The plants can then draw on these reserves for extra energy or building materials. Each year, photosynthesizing organisms produce about billion metric tons of extra carbohydrates, about 30 metric tons for every person on earth.
Photosynthesis has far-reaching implications. Like plants, humans and other animals depend on glucose as an energy source, but they are unable to produce it on their own and must rely ultimately on the glucose produced by plants. Moreover, the oxygen humans and other animals breathe is the oxygen released during photosynthesis. Humans are also dependent on ancient products of photosynthesis, known as fossil fuels, for supplying most of our modern industrial energy.
These fossil fuels, including natural gas, coal, and petroleum, are composed of a complex mix of hydrocarbons, the remains of organisms that relied on photosynthesis millions of years ago. Thus, virtually all life on earth, directly or indirectly, depends on photosynthesis as a source of food, energy, and oxygen, making it one of the most important biochemical processes known.
One plant leaf is composed of tens of thousands of cells, and each cell contains 40 to 50 chloroplasts. The chloroplast, an oval-shaped structure, is divided by membranes into numerous disk-shaped compartments. These disklike compartments, called thylakoids, are arranged vertically in the chloroplast like a stack of plates or pancakes. A stack of thylakoids is called a granum plural, grana ; the grana lie suspended in a fluid known as stroma. Embedded in the membranes of the thylakoids are hundreds of molecules of chlorophyll, a light-trapping pigment required for photosynthesis.
Additional light-trapping pigments, enzymes organic substances that speed up chemical reactions , and other molecules needed for photosynthesis are also located within the thylakoid membranes. Because a chloroplast may have dozens of thylakoids, and each thylakoid may contain thousands of photosystems, each chloroplast will contain millions of pigment molecules.
In the first stage, the light-dependent reaction, the chloroplast traps light energy and converts it into chemical energy contained in nicotinamide adenine dinucleotide phosphate NADPH and adenosine triphosphate ATP , two molecules used in the second stage of photosynthesis. In the second stage, called the light-independent reaction formerly called the dark reaction , NADPH provides the hydrogen atoms that help form glucose, and ATP provides the energy for this and other reactions used to synthesize glucose.
These two stages reflect the literal meaning of the term photosynthesis, to build with light. AThe Light-Dependent Reaction Photosynthesis relies on flows of energy and electrons initiated by light energy. Electrons are minute particles that travel in a specific orbit around the nuclei of atoms and carry a small electrical charge. Light energy causes the electrons in chlorophyll and other light-trapping pigments to boost up and out of their orbit; the electrons instantly fall back into place, releasing resonance energy, or vibrating energy, as they go, all in millionths of a second.
Chlorophyll and the other pigments are clustered next to one another in the photosystems, and the vibrating energy passes rapidly from one chlorophyll or pigment molecule to the next, like the transfer of energy in billiard balls. Light contains many colors, each with a defined range of wavelengths measured in nanometers, or billionths of a meter.
Certain red and blue wavelengths of light are the most effective in photosynthesis because they have exactly the right amount of energy to energize, or excite, chlorophyll electrons and boost them out of their orbits to a higher energy level. Other pigments, called accessory pigments, enhance the light-absorption capacity of the leaf by capturing a broader spectrum of blue and red wavelengths, along with yellow and orange wavelengths.
The mitochondrial genome retains similarity to its prokaryotic ancestor, as does some of the machinery mitochondria use to synthesize proteins. Each year, photosynthesizing organisms produce about billion metric tons of extra carbohydrates, about 30 metric tons for every person on earth.
Mitochondrial genomes are very small and show a great deal of variation as a result of divergent evolution.
Glycolysis Fermentation is glycolysis followed by a process that makes it possible to continue to produce ATP without oxygen. Chapter Indeed, a single mitochondrion may contain multiple copies of its genome at any given time. Because a chloroplast may have dozens of thylakoids, and each thylakoid may contain thousands of photosystems, each chloroplast will contain millions of pigment molecules. The chloroplast, an oval-shaped structure, is divided by membranes into numerous disk-shaped compartments. Certain red and blue wavelengths of light are the most effective in photosynthesis because they have exactly the right amount of energy to energize, or excite, chlorophyll electrons and boost them out of their orbits to a higher energy level.