Cellular Functions

By GERARD CHRETIEN BIOLOGY

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 electrons to another substance is called an oxidation-reduction reaction, or redox-reaction.

The gain of one or more electrons by an atom, ion, or molecule is called reduction. The loss of one or more electron is called oxidation.

Oxidation and reduction always occur together.

In a redox reaction, energy is transferred.

The coenzyme NAD is a key electron carrier in redox reactions

The main pair of oxidizing and reducing agents in cells is based on the compound NAD (NICOTINAMIDE ADENINE DINUCLEOTIDE).

An Overview: Releasing Energy from Glucose

The three energy-extracting processes of cells may be divided into distinct pathways:

A. When O2 is available as the final electron acceptor, four pathways operate. Glycosis takes place first, and is followed by the three pathways of cellular respiration: pyruvate oxidation, the citric acid cycle, and the respiration.

B.When O2 is unavailable , pyruvate oxidation, the citric acid cycle , and the respiratory chain do not function, and fermentation is added to the glycolytic pathway.

In prokaryotes, the enzymes the used in glycolysis, fermentation, and the citric acid cycle are soluble in the cytosol.

In eukaryotes, glycosis and fermentation take place in the cytoplasm outside of the mitochondria.

Glycosis begins the breakdown of glucose.

Cellular Respiration operates when O2 is available, yielding Co2 and H2o as products.

In pyruvate oxidation, the end product of glycosis(pyruvate) is oxidized to acetate, which is activated by the addition of a coenzyme and further metabolized by the citric acid cycle.

The Citric Acid cycle is a cycle series of reactions in which the acetate becomes completely oxidized, forming Co2 and transferring electrons (along with their hydrogen nuclei) to carrier molecules.

The fourth energy-extracting pathway for aerobic cells is the Respiratory Chain., which releases energy from the reduced NADH+H+ in such a way that it can be used to form ATP.

Glycosis:From Glucose to Pyruvate

Glycosis can be divided into two groups of reactions: energy-investing reactions that use ATP, and energy-harvesting reactions that produce ATP.

The energy-investing reactions of glycolysis require ATP

The first five reactions are endergonic; that is, the cell is investing free energy rather than gaining it during the early reactions of glycosis.

A kinase is an enzyme that catalyzes the transfer of a phosphate group from ATP to another substrate.

The energy-investing reactions of glycolysis yield ATP and NADH + H+

Substrate-Level Phosphorylation is called substrate-level phosphorylation

Glycolysis May Be Allowed By Fermentation

A review of the glycolytic shows that the beginning of glycolysis, two molecules of ATP are used per molecule of glucose, but that ultimately four molecules

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