Cellular Functions

Cellular Pathways

Several principles govern metabolic pathways in the cell:

A. Complex chemical transformations in the cell do not occur in a single reaction, but in a number of small steps that are connected in a pathway.

B. Each reaction is catalyzed by a specific enzyme.

C. Metabolic pathways is catalyzed by a specific enzyme.

D. Many metabolic pathways are compartmentalized, with certain steps occurring inside an organelle.

E. Metabolic pathways in organisms are regulated by the activities of a few enzyme.

Obtaining Energy and Electrons from Glucose

The most common fuel for living cells is the sugar Glucose.

Cells trap energy while metabolizing glucose

If glucose is burned in a flame, it readily forms carbon dioxide, water, and a lot of energy----but only if oxygen gas(O2) is present. The balance equation for this combustion reaction is:

C6 H12 O6 + 6 O2 ---- 6 CO2 + 6 H2O + ENERGY (HEAT AND LIGHT)

This same equation applies to the metabolism of glucose in cells, except that metabolism is a multi-step, controlled series of reactions, ending up with almost half of the energy captured in ATP.

Three metabolic processes play roles in the utilization of glucose for energy: GLYCOSIS, CELLULAR RESPIRATION, AND FERMENTATION.

A. Glycosis is a series of reactions that begins the metabolism of glucose in all cells and produces the three-carbon product pyruvate. A small amount of the energy stored in the glucose is released in usable form.

B. Cellular Respiration occurs when the environment is aerobic (contains oxygen gas , O2), and essentially converts pyruvate to carbon (CO2). In the process, a great deal of the energy stored in the covalent bonds of pyruvate is released and trapped in ATP.

C. Fermentation occurs when the environment is anaerobic (lacking in O2). Instead of energy-poor CO2, relatively energy-rich molecules such as lactic acid or ethanol are produced, so the energy extracted from glucose is far than under aerobic conditions.

Redox reactions transfer electrons and energy

a.Reaction in which one substance transfers one or more electron...

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... in glycolysis. This molecule then passes through the rest of glycolysis and the citric acid cycle,where its energy is extracted in NADH and ATP.

B. Lipids are converted to their substituents, glycerol and fatty acids. Glycerol is converted to dihydroxyacetone phosphate, an intermediatein glycolysis, and fatty acids to acetate and then acetyl CoA in the mitochondria. In both cases, further oxidation to CO2 and release of energy of energy then occur.

C. Proteins are hydrolyzed to their amino acid building blocks. The 20 amino acids feeds into glycosis or the citric acid cycle at different points.

Catabolism and Anabolism are Integrated

Glucose is an excellent source of energy. Polysaccharides and fats have no such catalytic roles. The level of acetylCoA rises as a fatty acids are broken down.

Allostery regulates metabolism

Glycolysis, the citric acid cycle, and the respiratory chain are regulated by allosteric control of the enzymes involved. The main control point in glycolysis is the enzyme PHOFRUCTOKINASE. The main control in the citric acid cycle is the enzyme ISOCITRATE DEHYROGENASE, which converts isocitrate to ketoglutarate..