C0 5: Cellular respiration (processes by which cells consume O2 and produce CO2)

CITRIC ACID CYCLEElectron transportPHOSPHORYLATIONPHOTOSYNTHESIS

Obligate anaerobes, organisms that grow only in the absence of oxygen, avoid the gas by living in highly reduced environments such as soil. They use fermentative processes to satisfy their energy requirements. Aerotolerant anaerobes, also depend on fermentation for their energy needs, possess detoxifying enzymes and antioxidant molecules that protect against oxygen's toxic products. Facul- tative anaerobes not only possess the mechanisms needed for detoxifying oxygen metabolites, they can also generate energy by using oxygen as an electron acceptor when the gas is present. Obligate aerobes highly dependent on oxygen for energy production. They protect themselves from the potentially dangerous consequences of exposure to oxygen with elaborate mechanisms composed of enzymes and antioxidant molecules.

Types of Cell Respirations:

In the course of electron transfer, the large amount of energy released isconserved in the form of ATP, by a process called oxidative phosphorylation

1. Oxidation of organic fuels (fatty acids, glucose, and some amino acids) yields acetyl-CoA.

2. Oxidation of acetyl groups in the citric acid cycle includes four steps in which electrons are abstracted.

3. Electrons carried by NADH and FADH2 are funneled into a respiratory chain, ultimately reducing O2 to H2O. This electron flow drives the production of ATP.

Respiration Main steps:

The citric acid cycle is a metabolic pathway in which two carbon fragments derived from organic fuel molecules are oxidized to form CO2 and the coenzymes NAD+ and FAD are reduced to form NADH and FADH2, which act as electron carriers.

The electron transport pathway, also referred to as the electron transport chain (ETC), is a mechanism by which electrons are transferred from reduced coenzymes to an acceptor (usually O2).

Oxidative phosphorylation, synthesis of ATP from ADP ( the energy released by electron transport is captured in the form of a proton gradient that drives the synthesis of ATP, the energy currency of living organisms.

Explanation:

In each turn of the citric acid cycle, 2 carbon atoms enter as the acetyl group of acetyl-CoA and 2 molecules of CO2 are released.

Main Reactions of the Citric Acid Cycle. Oxaloacetate, (4 C ) condenses with acetyl-CoA to form citrate (6 C). Also formed : 2 molecules of CO, 3 molecules of NADH, 1 molecule of FADH2,

The net reaction for the citric acid cycle is as follows:

After its transport into the mitochondrial matrix, pyruvate is converted to acetyl- CoA in a series of reactions catalyzed by the enzymes in the pyruvate dehydrogenase complex. The net reaction, an oxidative decarboxylation, is as follows:

glycolysis

OVERALL REACTION

TCA is Amphibolic : ( function in both anabolic and catabolic processes)

Amphibolic

anabolic pathway a series of biochemical reactions in which large complex molecules are synthesized from smaller precursors anabolism energy-requiring biosynthetic pathways

catabolic pathway a series of biochemical reactions in which large complex molecules are degraded into smaller, simpler products

The citric acid cycle is obviously catabolic, because acetyl groups are oxidized to form CO2 and energy is conserved in reduced coenzyme molecules. The citric acid cycle is also anabolic, because several citric acid cycle intermediates are precursors in biosynthetic pathways

ETC is a series of electron carriers in the inner membrane of the mitochon- dria of eukaryotes and the plasma membrane of aerobic prokaryotes.

Electron Transport Chain. Complexes I and II transfer electrons from NADH and succinate, respectively, to UQ. Complex III transfers electrons from UQH2 to cytochrome c. Complex IV transfers electrons from cytochrome c to O2.

Complex I, also referred to as the NADH dehydrogenase complex, catalyzes the transfer of electrons from NADH to UQ. The major sources of NADH include several reactions of the citric acid

complex II ( The succinate dehydrogenase complex ) consists primarily of the citric acid cycle enzyme succinate dehydrogenase and two iron-sulfur pro- teins. Complex II mediates the transfer of electrons from succinate to UQ.

Complex III (cytochrome bcj complex). transfers electrons from reduced coenzyme Q (UQH2) to cytochrome c.

Complex IV (Cytochrome oxidase) is a protein complex that catalyzes the 4-electron reduction of O2 to form H2O.

ETC INHIBITOR

Several molecules specifically inhibit the electron transport process The inhibition can be measured with an oxygen electrode. (Oxygen consumption is a sensitive measure of electron transport.) When electron transport is inhibited, oxygen consumption is reduced or eliminated.

Example of inhibitors: •antimycin A inhibits cyt b.. •rotenone and amytal, which inhibit NADH dehydrogenase (complex I). •Carbon monoxide (CO), azide (N3), and cyanide inhibit cytochrome oxidase.

Oxidative phosphorylation,

Oxidative phosphorylation the process whereby the energy generated by the ETC is conserved by the phosphorylation of ADP to yield ATP,

The Chemi osmotic Theory. The flow of e through the ETC complexes is coupled to the flow of protons across the inner membrane from the matrix to the intermembrane space. This process raises the matrix pH. In addition, the matrix becomes negatively charged with respect to the intermembrane space.

Oxygen can accept single electrons to form unstable derivatives, referred to as reactive oxygen species (ROS). This ROS can seriously damage living cells

Examples of ROS : superoxide radical, hydrogen peroxide, the hydroxyl radical, and singlet oxygen.

ROS formation is usually kept to a minimum by antioxidant defense mechanisms.

Oxidative stress

Examples of circumstances that may cause serious oxidative damage :•overconsumption of certain drugs•exposure to intense radiation, •repeated contact with certain contaminants (e.g., tobacco smoke).

To protect themselves from oxidative stress, living organisms have developed several antioxidant defense mechanisms. These mechanisms employ several metalloenzymes and antioxidant molecules.

The major enzymatic defenses against oxidative stress are provided by : •superoxide dismutase, •glutathione peroxidase, •catalase.

Antioxidants are substances that inhibit the reaction of molecules with oxygen radicals.

Antioxidants are effective because they are more easily oxidized than the atom or molecule being protected.

Photosynthesis : the trapping of light energy and its conversion to chemical energy, which then reduces carbon dioxide and incorporates it into organic molecules

Photorespiration: a light-dependent process occurring in plant cells actively engaged in photosynthesis that consumes oxygen and liberates carbon dioxide

The essential feature of photosynthesis is the absorption of light energy by spe- cialized pigment molecules.

The chlorophylls are green pigment molecules that play the principal role in eukaryotic photosynthesis, because its absorption of light energy directly drives photochemical events. Chlorophyll b acts as a light-harvesting pigment by absorbing light energy and passing it on to chlorophyll a.

The orange-colored carotenoids : molecules that either function as light-harvesting pigments (e.g., lutein, a xanthophyll, ) or protect against reactive oxygen species (ROS)

PHOTOSYNTHETIC PIGMENTS

Photosynthetic pigments use primarily the visible light portion of the electromagnetic spectrum

1. Pigment is a substance that absorbs visible light

2. Two major photosynthetic pigments are chlorophyll a and chlorophyll b

3. Chlorophylls absorb violet, blue, and red wavelengths; they reflect green, this is why leaves appear green.

Photosynthesis takes place in chloroplasts.

Photosynthesis consists of two major phases: the light reactions and the light-independent reactions.

During the light reactions, water is oxidized, O2 is evolved, and the ATP and NADPH required to drive carbon fixation are produced.

During the light-independent reactions, CO2 is incorporated into organic molecules. The first stable product of carbon fixation is glycerate-3-phosphate.

PHOTOSYNTHESISVirtually all energy on earth comes from

sunlight. Plants use energy from the sun to make te bonds which hold organic molecules together. When these bonds are broken the

energy is ultimately transferred to ATP, which is then moved about cells and organisms to

power their needs. Photosynthesis overview

12H20 + 6CO2 ----- light -----> 6O2+ C6H12O6 + 6H20

Photosynthesis - The Basic Reaction

CO2 +H2O +---Plants (Chloroplasts)Light Energy---Simple

Sugars+ O2

PHOTOSYNTHETIC PIGMENTS

Photosynthetic pigments use primarily the visible light portion of the electromagnetic spectrum

1. Pigment is a substance that absorbs visible light that behave as packets of energy called photons.

2. Two major photosynthetic pigments are chlorophyll a and chlorophyll b

3. Chlorophylls absorb violet, blue, and red wavelengths; they reflect green, this is why leaves appear green. Different colors/wavelengths of light (ROYGBIV). 

CAROTENOIDS

1. Carotenoids are yellow-orange pigments which absorb light in violet, blue, and green regions

2. When pigments absorb light, electrons are boosted to a higher energy level and the energy is captured in a chemical bond

Photosynthesis occurs in chloroplasts

Chloroplasts have two parts:1. A double membrane encloses a fluid-filled space called

the stroma  or ground substance  2. Thylakoids = flattened sacs organized into stacks called

grana3. Chlorophylls and other pigments involved in absorption

of solar energy are embedded within thylakoid membranes; these pigments absorb solar energy

REACTIONS IN PHOTOSYNTHESIS

Photosynthesis has two sets of reactions

1. Light-dependent reactions = light energy is converted to chemical energy (ATP&NADPH). Occur in the thylakoid

2. Light-independent reactions =

The energy in the ATP and NADPH are used to power the light-independent reactions (to make carbohydrates)

Light-independent reactions

• Takes place in the stroma of the chloroplast • Energy from the light reactions (ATP, NADPH) is used to

form several molecules of 3-phosphoglyceraldehyde (PGAL). This PGAL is then used to produce glucose

• Takes place in the stroma of the chloroplast • Energy from the light reactions (ATP, NADPH) is used to

form several molecules of 3-phosphoglyceraldehyde (PGAL). This PGAL is then used to produce glucose

THE CALVIN CYCLE

• The incorporation of CO2 into carbohydtrate by eukaryotes in the stroma, in the absence of light and presence of ATP and NADPH produced by light reactions, is often referred as Calvin cycle.

• It is light-independent reaction and also referred as reductive pentose phosphate cycle (RPP cycle) and photosynthetic carbon reduction cycle (PCR cycle).

• The 3 phases of Calvin cycle are: carbon fixation; reduction by NADPH; and regeneration of glyceraldehyde-3-phosphate.

Carbon fixation

• It is a mechanism in which inorganic CO2 is incorporated into organic molecules.

• Ribulose-1,5-biphosphate carboxylase (often described as the world’s most abundant enzyme) catalyzes the carboxylation of ribulose-1,5-biphosphate to form 2 molecules of glycerate-3-phosphate.

• Plants that produce glycerate-3-phosphate as the first stable product of photosynthesis are referred as C3 plants.

Photorespiration

• It is a light-dependent process consuming oxygen (both ATP and NADPH) and releasing fixed CO2. Hence it is a wasteful process.

• It is a serious problem for C3 plants (e.g. Soy-beans and oats) in hot, dry environments, where these plants close stomata to conserve water and reduce the CO2 concentration within the leaf tissue

C4 plants (e.g. Sugar-cane and maize)• These plants develop C4 metabolism to surpress

photorespiration and in hot weather open their stomata only in the cooler night temperature.

• In the C4 biochemical pathway in the mesophyll cells, that are in direct contact with the air space in the leaf, take up CO2, and use it to synthesize oxaloacetate, which is then reduced to malate

• Malate diffuses to bundle sheath cells where it is reconverted to pyruvate and the CO2 released in the reaction is used in the Calvin cycle to yield triose phosphate molecules

• Triose phosphate is converted to starch or sucrose.• Pyruvate returns to the mesophyll.

Crassulacean Acid Metabolism

• CAM plants (e.g. Succulent Cacti) that grow in high light intensity and limited water supply, employ C4 metabolism, by opening their stomata in the night, when CO2 is incorporated into oxaloacetate by PEP (phosphoenolpyruvate) carboxylase to form malate.

• Malate is stored in the vacuole until photosynthesis begins the next morning, when CO2 is regenerated

• Malic acid accumulated in the leaves of the plant may give an unpleasant taste.