1.Explain how photosynthesis works from sunlight to sugar storage. Include all steps and processes. Use labled drawings, pictures, and explanations with the drawings from the chapter in the book.

Photosynthesis is the biological process by which the energy of sunlight is absorbed and used to power the formation of organic compounds from carbon dioxide and water. Although primarily associated with green plants, photosynthesis also occurs in algae and a variety of bacteria. This process ultimately supplies the energy required by all living organisms for their continued survival.

The photosynthetic process can be divided into two stages: absorption of light energy, known as the light reactions, and carbon fixation, known as the dark reactions.

In this stage colored pigments called chlorophylls absorb, or trap, light energy from the Sun. This causes the flow of a tiny electrical current, which converts the energy into two high-energy chemicals, adenosine triphosphate (ATP) and nicotinamide adenine dinucleotide phosphate (NADPH) .

Carbon fixation uses the energy contained in ATP and NADPH to drive reactions that form organic compounds, such as starch and sugars, from the simple molecules of carbon dioxide and water. The organic compounds formed can subsequently be broken down during respiration to release energy and sustain life.

An important outcome of the photosynthetic reaction is that the light-induced electrical current causes the water to break down. Water is the source of electrons for the electrical current and one of its component atoms, oxygen, is released as molecular oxygen due to this decomposition. This replenishes the atmospheric oxygen supply, which would otherwise be depleted by respiratory processes of organisms or by burning substances, and also depletes the carbon dioxide that it produces. The reactions are summarized as follows:

Respiration: organic compounds + oxygen ---> carbon dioxide + water

Photosynthesis: carbon dioxide + water ---> organic compounds + oxygen

Thus, the process of photosynthesis is the opposite of respiration.

The entire process of photosynthesis occurs in chloroplasts, the tiny green organelles inside the cells of green plant tissue. They were first observed by Hugo von Mohl in 1837, but their role in photosynthesis was not fully understood until the experiments of Julius von Sachs nearly 30 years later, which showed that starch is produced only in those chloroplasts that are exposed to light. Chloroplasts are generally 4 to 6 microns in length, 1 to 2 microns in width, and somewhat disc- or ellipse-shaped. Each chloroplast is surrounded by an envelope composed of two lipoprotein membranes, each 60 angstroms thick. Inside is an extensive array of green membranes called thylakoids, in a granular fluid known as the stroma. The thylakoid membranes occur in pairs and enclose a lumen space, forming disks that tend to be stacked into ordered structures called grana. The grana are connected sporadically by unstacked thylakoids. This structural complexity plays a role in controlling the interactions between different components. Those parts of the apparatus involved with the light reactions are bound into, or closely associated with, the thylakoid membranes. The enzymes involved in carbon fixation by contrast are located in the stroma. Chloroplasts also contain their own DNA and ribosome complement, providing the necessary genetic information and apparatus for producing many of the required protein molecules. The remainder of the required protein molecules are produced in the cytoplasm from genes of the nucleus and are selectively imported into the chloroplast by processes that are not yet well understood.

The major light receptors in higher plants are chlorophylls. They are similar in structure to the heme molecules that give blood its red color. Both are composed of a similar organic molecule, termed a porphyrin ring, which surrounds a metal atom: iron in heme and magnesium in chlorophyll. Chlorophylls strongly absorb red and blue light, the region of the light spectrum where solar energy has its maximum output. They do not absorb green bands well, and this light is reflected so that the leaves appear green. The two major types, chlorophyll a and chlorophyll b, have slightly different absorption bands, allowing a greater proportion of the visible light to be absorbed. Other red, yellow, and orange pigments, termed carotenoids and xanthophylls, are also present and these absorb some further light that the chlorophyll itself cannot capture.

In order to capture a large part of the light, many pigment molecules are necessary. These molecules are bound to specific proteins and arranged into large well-ordered structures, each containing as many as 600 chlorophyll molecules, that act as antennae for light. These antennae touch the proteins that activate the chemical reactions of photosynthesis, called the photosystems. The photosystems contain a variety of components, including a specialized chlorophyll, termed the reaction center chlorophyll. Light absorbed by pigments of the antennae is transferred extremely rapidly, with the loss of very little energy, to the reaction center chlorophyll by a process called resonance transfer. The transferred energy causes an electron from the reaction center chlorophyll to leave the chlorophyll molecule and to be transferred to an electron-accepting species. These reactions occur very quickly--the fastest in picoseconds and the slowest in microseconds--and are called the primary photochemistry of photosynthesis. There are two quite independent types of photosystem in higher plants, each with its own antennae system, and each carrying out quite different reactions. The first is photosystem II, and its reaction center chlorophyll is called P 680 (for 680 nanometer light absorbing pigment). The second is photosystem I, and its reaction center chlorophyll is P 700 (700 nanometer light-absorbing pigment).

When photosystem II is light-activated, the primary photochemistry results in an electron transfer: the P 680 loses an electron and an acceptor, termed the primary acceptor or Q, gains it. Photosystem II is attached to a set of proteins that are called the oxygen evolving complex. The oxygen evolving complex contains manganese atoms (probably four) in a structure that is not yet known. The P 680 can regain its electron from this manganese-containing site when the manganese takes electrons from water. Four sequential transfers of an electron to the P 680 are required to produce molecular oxygen from water. The overall reaction is:

2H(2)O ---> O(2) + 4H+ + 4e-

The activation of photosystem II also results in electron transfer to its primary acceptor, Q. These electrons flow along a series of carriers, finally to be transferred to the reaction center chlorophyll of photosystem I. There are three major components in this electron-transfer chain. The first is a small organic molecule, plastoquinone, which is dissolved in the thylakoid membrane, and capable of diffusing quite rapidly with its electrons to the next component in the series. This next component is a fairly large protein complex embedded in the thylakoid membrane, called the cytochrome bf complex. It has four different iron-containing species--three are heme groups and the fourth is a structure containing iron and sulfur atoms. The complex transfers electrons from the plastoquinone to plastocyanin, a small, blue, copper-containing protein that is dissolved in the lumen space inside the thylakoids.

Light activation of photosystem I also results in electron transfer from its reaction center chlorophyll to an electron acceptor. The reaction center chlorophyll regains its electron from the plastocyanin. The electrons gained by the primary acceptor pass along a chain of components, still incompletely understood, to a water-soluble protein, ferredoxin, containing iron and sulfur atoms and located in the stroma. An enzyme catalyzes the transfer of its electrons to NADP, converting it to the NADPH form required for carbon fixation.

The above processes result in electron flow from water to NADP, with the formation of oxygen and NADPH. Concurrently as electrons flow, some energy is released and used to drive the formation of ATP from its precursors,ADP (adenosine diphosphate) and inorganic phosphate. The process is termed photophosphorylation. The way in which electron transfer is coupled to ATP synthesis was first correctly described by the chemiosmotic theory of Peter MITCHELL, a theory which subsequently earned him a Nobel Prize in 1978. The essence of the mechanism is that the electron transfer reactions result in protons being moved from one side of the thylakoid membranes to the other. Since the membranes are impermeable to protons, the protons instead flow back across the membrane through an enzyme, the ATP synthase. This is another thylakoid-embedded enzyme, and ATP is synthesized as the protons flow through it. The mechanisms of electron transfer and ATP synthesis are collectively called noncyclic photophosphorylation, and it is likely that they do not produce the correct ratio of ATP:NADPH required for carbon fixation. An alternative electron-transfer pathway of photosystem I is able to produce ATP without NADPH reduction. This is called cyclic photophosphorylation because the electron cycles around a loop of carriers without forming NADPH. By varying the noncyclic and cyclic photophosphorylation, the correct ratio of ATP to NADPH for carbon fixation is achieved.

Melvin CALVIN and his coworkers elucidated the pathway of carbon fixation in the late 1940s. This pathway, known as the Calvin cycle, utilizes the ATP and NADPH produced by light reactions. A key reaction is carbon fixation, which involves the addition of a carbon dioxide molecule to a 5-carbon-atom sugar, ribulose-1,5-diphosphate (RUDP), producing two 3-carbon-atom molecules of 3-phosphoglycerate. The remaining reactions of the cycle involve the regeneration of ribulose-1,5-diphosphate. For each molecule of carbon dioxide that is fixed, three molecules of ATP and two molecules of NADPH are required:

3CO(2) + 9ATP + 6NADPH + 6H+ + 6H(2)O ---> 1/2(glucose) + 9ADP + 9HPO4 + 6NADP+

The final stable end products of the Calvin cycle are starch and sucrose, both produced from the glucose. Starch is a polysaccharide with a chemical structure similar to that of glycogen; it acts as the main energy reserve in plant cells and is also synthesized in the chloroplasts. Sucrose is the main form of carbohydrate exported into the rest of the plant and is the energy source equivalent to the sugar glucose in animals. Sucrose is synthesized outside the chloroplast in the cytoplasm. To do this, carbon is first transported out of the chloroplast as a carbon-containing chemical, dihydroxyacetone phosphate, in exchange for a phosphate molecule from the cytoplasm. The enzyme which catalyzes the carbon fixation reaction is ribulose-1,5-diphosphate carboxylase (RuBisCo). Besides its normal reaction with carbon dioxide, it can also catalyze a reaction of ribulose-1,5-diphosphate with oxygen:

RUDP carboxylase

Ribulose diphosphate + O2 ---> 3-phosphoglycerate + phosphoglycolate

The chloroplast is able to remove the phosphate group of the phosphoglycolate to produce glycolate but is unable to metabolize the glycolate further. The glycolate is further metabolized by enzymes that occur in the cytoplasm, the peroxisomes, and the mitochondria. This process is known as photorespiration and leads to carbon dioxide production and oxygen uptake. Photorespiration decreases the efficiency of photosynthesis and is particularly apparent when the carbon dioxide level in cells is much less than in the surrounding air; for instance, when photosynthesis is occurring in high light intensities or at high temperatures, or when the plant has to restrict the opening of its stomata--the tiny pores through which carbon dioxide enters and oxygen leaves the plant--in order to restrict water loss by transpiration.

Many species of plants have adapted in particular ways to their environment. For instance, many tropical and subtropical plants expend additional energy to increase the carbon dioxide concentration in the chloroplasts, showing virtually no photorespiration. They are called C(4) plants because the initial product of carbon dioxide fixation is a 4-carbon acid, as opposed to the 3-carbon acid produced in other plants. Another adaptation, associated with slow growth but very efficient conservation of water, is crassulacean acid metabolism. Carbon dioxide is fixed into C(4) acids during the night, and during the day the stomata are closed and light energy is used to release carbon dioxide from the C(4) acids. Cacti and succulents, which grow where daytime humidity is very low, utilize this type of metabolism.

The pictures of photosynthesis are unavailable because I was not able to put them on the computer.

2.Explain how respiration works from glucose to ATP. Include all steps and processes. Use labled drawings, pictures, and explanations with the drawings from the chapters in the book.

All organisms capable of independent life (in other words, all except the viruses) are made principally of fats, proteins, carbohydrates , and nucleic acids. Proteins are polymers of smaller molecules--the amino acids--and nucleic acids are made up of mononucleotides. These polymers are suspended in a watery solution containing salts and small quantities of other materials. Carbohydrates also can form polymers--the STARCH of plants and the GLYCOGEN of animals--and fat can be broken down into smaller molecules of fatty acids and glycerol. All these compounds form the structure of the cell, including the membranes and the organelles, or smaller distinct structures, within the cell such as the nucleus, nucleolus, mitochondria, robosomes, and lysosomes. Fats and carbohydrates can be broken down to simpler compounds--fatty acids, glycerol, and simple sugars--which may be used for immediate energy production. Fatty acids in excess of immediate needs can be rebuilt into fat and stored in special cells making up adipose tissue. Sugars in excess of current needs are either converted to fatty acids and stored as fat, or formed into a giant polymer--in plants, a starch granule; in animals, a glycogen molecule. Protein is degraded to amino acids, which, if not used to make new protein, can be converted to derivatives of fatty acids or sugars (depending on which amino acids are involved) and utilized accordingly. Because carbohydrates form many weak bonds to water, carbohydrates are always surrounded by water molecules; sugars and starches, as a result, weigh a great deal per calorie of energy, being the weight of the carbohydrate plus the weight of a considerable amount of water surrounding it. Fats, on the other hand, repel water and, per calorie of energy, weigh much less than carbohydrates. For this reason, organisms that move, primarily animals, have the bulk of their reserve energy stored as fat, with a small amount of stored sugar for rapid energy production; nonmoving organisms, such as the higher plants (trees, shrubs, and grasses), principally store sugars and starches. Seeds, the only portion of such plants that must move, typically store fat.

The major source of energy in the cell is obtained from the oxidation of hydrogen obtained from food by respired oxygen to form water. The greatest single source of hydrogen molecules for oxidation is the Krebs cycle, named for its discoverer, Sir Hans Adolf KREBS. It is also called the tricarboxylic acid (TCA) cycle, or CITRIC ACID cycle. In animal cells, the entire cycle is found within the mitochondrion, an organelle of the cytoplasm; it is found in a more distributed fashion in bacterial cells. Plants can make high-energy compounds and carbohydrates by capturing the energy of light, a very different process (photosynthesis).

The cycle consists of nine compounds, each convertible into the next; the last of the nine, oxaloacetic acid, is converted to the first, citric acid, by the addition of an activated two-carbon compound, acetyl coenzyme A, completing one turn of the cycle and simultaneously beginning the next. Each of the steps in the cycle is catalyzed by a specific enzyme. On each turn of the cycle an acetyl molecule is oxidized, converting the carbons to carbon dioxide and removing the hydrogens, which are bound to a vitamin-derived carrier compound, nicotinamide adenine dinucleotide (NAD), involved in the electron transport system discussed below. The Krebs cycle intermediates--the individual members of the cycle--can be made from glucose as well as from a number of amino acids. The acetyl groups, which must be supplied in a steady stream, can be obtained from fatty acids or from glucose and are activated by the addition of coenzyme A, a derivative of the vitamin called pantothenic acid. Thus all three major foodstuffs can be used to produce high-energy compounds in reactions beginning in the Krebs cycle. The two-carbon acetyl group of acetyl coenzyme A is attached to the last member of the cycle, four-carbon oxaloacetate, to form six-carbon citric acid. This is converted to five-carbon ketoglutaric acid, which in turn becomes four-carbon succinic acid, eventually being converted to oxaloacetic acid. Each time, the lost carbon becomes carbon dioxide, eventually appearing in expired air. A steady state is preserved, with acetyl groups entering, and carbon dioxide and hydrogens attached to NAD leaving. Because the intermediates are readily interconverted to glucose or amino acids, the cell has little difficulty in maintaining their concentration. The Krebs cycle can be looked on as a machine for removing hydrogens from foodstuffs; the hydrogens are sent to the electron transport system, where they are combusted to water, and the free energy obtained is used to form the crucial high-energy compound, ATP.

Within and on the inner membrane of the mitochondrion are large molecules capable of rapidly alternating oxidation and reduction reactions. These make up the electron transport system (ETS). Each molecule of the hydrogen carrier mentioned above, NAD, delivers two electrons and one proton of a hydrogen molecule to the ETS. The system passes the electrons along the entire sequence of reactions; the protons go into the solution after the first few compounds, including a derivative of the vitamin riboflavin, several iron-sulfur complexes, and a quinone called coenzyme Q. The electrons are then passed through a series of cytochromes (close relatives of hemoglobin), the last of which catalyzes the formation of water from the electrons, protons, and oxygen derived from respired air. Each member of this series has an increasingly greater affinity for electrons, so that the entire series runs "downhill" and energy is produced. If the energy is not used for chemical work, it is dissipated as heat.

The energy produced by the ETS is used to form a chemical bond between adenosine diphosphate (ADP) and inorganic phosphate to form ATP. In fact, as a pair of electrons passes down the ETS from beginning to end, where it is captured by oxygen, enough energy is trapped to synthesize three ATP molecules. In fully functional cells, electron transport is tightly coupled to oxidative phosphorylation. That is, if ATP synthesis is prevented (which would happen if there were a lack of inorganic phosphate, ADP, or oxygen), electron transport will not take place. The ATP generated is used throughout the cell to drive most of the otherwise energetically unfavorable reactions. In certain vertebrate tissues, notably skeletal muscle and the brain, an extra store of energy is maintained by using excess ATP to convert creatine to creatine phosphate, which is also a high-energy compound. Creatine phosphate can quickly transfer the phosphate group to ADP to reform ATP when the latter is needed. Invertebrates use phosphoarginine in a similar fashion.

All cells can synthesize some ATP in the absence of oxygen by means of anaerobic reactions. Usually glucose, the most important sugar in the cell, is broken down to pyruvic acid, which is then converted to lactic acid and excreted from the cell. As an example, bacteria present in milk absorb milk sugar and convert it to lactic acid in a metabolic process that produces enough ATP to meet the bacteria's needs. The lactic acid is excreted and sours the milk. Similarly, in animals, muscles can continue to function for short periods of time without oxygen; the lactic acid is excreted into the bloodstream. If the muscle cell is deprived of oxygen for a longer period, the acidity prevents further metabolism and the cell begins to die.

When oxygen is present the pyruvic acid obtained from glucose does not become lactic acid but is instead converted either to oxaloacetic acid or to acetyl coenzyme A, depending on the cell's needs at the time. In either case it enters the TCA cycle. The steps from glucose to pyruvic acid may be called glycolysis, the anaerobic pathway (anaerobic meaning not requiring oxygen), or, in honor of two of its discoverers, the Embden-Myerhof pathway. It must be stressed that the TCA cycle is a much more efficient producer of ATP than glycolysis. In fact, the complete combustion of a molecule of glucose to carbon dioxide and water results in the production of 36 molecules of ATP, of which 34 are produced from the TCA cycle and only 2 from glycolysis in the absence of oxygen. The TCA cycle is often called the final common pathway of energy production because acetyl coenzyme A originates from sugars, fats, and proteins.

In order to build new cell constituents, one or more of the following processes must take place. One process, polymerization, can involve the production of proteins by joining together large numbers of amino acids in specific arrangements; it can also involve the production of RNA or DNA from mononucleotides. Polymerization requires the presence of either ATP or certain other high-energy compounds, many of which are derived from ATP. Another process, hydrogenation, also called reduction, is the addition of hydrogen to a molecule. This process usually requires a carrier molecule, such as a pyridine nucleotide, and a hydrogen source, usually from the degradation of glucose. Most large molecules are built by chemical linkage of small molecules, a process whose immediate source of energy usually is ATP.

The pictures of cellular respiration are unavailable because I was not able to put them on the computer.