High School Biology: Cell Metabolism

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CELL METABOLISM

A. Metabolism- It is the total energy that is released and consumed by a cell. Metabolism is the sum of the total energy of catabolism and anabolism.

1. Catabolism- The energy releasing process in which a chemical (food) is broken down, via decomposition or degradation, into its smaller constituents. An example is the breakdown of glucose into CO2 and water and 38 molecules of ATP.

2. Anabolism- Anabolism is the opposite of catabolism. In this class of metabolism, the cell consumes energy to produce larger molecules via smaller ones. An example is the synthesis of glycogen from glucose and ATP.

Note: The produced energy in catabolism may be used to provide heat energy for other cellular catabolism or anabolism. Yet, some of the energy that is used by anabolism may be used later for catabolism. Example. glycogen is an storage site for both glucose and ATP since, when glucose is released from glycogen, the sugar molecule can be used by the cell to produce 38 ATP via catabolism.

B. Respiration- Is the energy releasing process whereby sugar molecules are broken down via 3 consecutive steps (1. glycolysis, 2 Kreb's cycle and 3. Electron transport system) to CO2 and water and 38 ATP molecules.

C. Redox Reaction- This is a simultaneous oxidation-reduction process whereby cellular metabolism occurs. It is:

ENERGY RICH MOLECULES- In a cellular respiration there are three kinds of energy rich molecules. These are ATP, NADH2 and FADH2. However, for NADH2 and FADH2 molecules to be useful by the cell they are both converted, via other mechanisms, into ATP.

1. Adenosine Triphosphate-This is the most common energy carrier molecule in respiration. It is used in cellular respiration, metabolism, catabolism, neuron action potentials and muscle contractions as examples. Its structure is :

The ATPase is consumed and can be broken down into ADP and AMP via hydrolysis;

Phosphorylation of adenosine monophosphate is a process whereby the low energy AMP formed creates a higher energy bond ADP and ATP.

2. Flavin Adenine Dinucleotide (FAD)-Another molecule that is used by cellular respiration to carry energy from one place to another. FAD is used in the Kreb's Cycle.

Every molecule of FADH2 has the energy equivalent of 2 molecules of ATP.

3. Nicotinamide Adenine Dinucleotide (NADH)-After ATP, NADH is the second most commonly used energy carrier.

Every molecule of NADH2 has enough energy to promote 3 molecules of ATP.

Video: Glycolysis and Cellular Respiration

Introduction to Glycolysis

RESPIRATION- Respiration is the process whereby a sugar molecule is broken down into CO2, H2O and 38 ATP via 3 consecutive steps. These are 1. Glycolysis, 2. Kreb's cycle, and 3. Electron Transport System.

I. Glycolysis- Or anaerobic respiration, is the process that occurs in either the absence or presence of oxygen molecules in the cytoplasm of a cell. The process starts when a glucose molecule is phosphorylated and is broken down into two 3 carbon molecules of pyruvic acid. In this process 2 molecules of ATP start the hydrolysis and via subsequent steps 2 molecules of ATP and 2 molecules of NADH2 are produced, resulting in total production of 2 ATP molecules to be used by the cells energy. Note: The energy stored in NADH2 (equivalent to 6 ATP molecules) is used again by the Kreb's cycle itself. Thus this energy is not counted toward glycolysis energy release. Glycolysis is as follows:

II. Fermentation- This step occurs in eucaryotes only when there is a shortage of oxygen. It occurs in anaerobic and facultative aerobic bacteria, for instance.

1. In Muscle Cells- During extraneous activities, the oxygen in the muscle tissue is decreased to an extent that aerobic respiration does not occur at a sufficient rate. Hence;

2. In Yeast- The fermentation end product is ethyl alcohol;

Note: In fermentation, pyruvic acid is the final electron acceptor. Also, when oxygen is present in the lactic acid accumulated muscle cells, then, via the enzyme lactate dehydriogenase the lactic acid is converted back into pyruvic acid to be used again in oxidation respiration.

III. Aerobic Metabolism- Or aerobic respiration, is the process that occurs only in the mitochondria. This is the most energy yielding chemical reaction within the cell. Namely 36 of 38 ATP are produced in the mitochondria, hence its nick name " The cell's Powerhouse". There are three stages where oxidation respiration occurs. 1. Production of Acetyl-COA from pyruvic acid. 2. Tricarboxylic acid cycle. 3. Electron Transport system

1. Acetyl-Coa Pathway- This is a cycle where a single molecule of pyruvic acid is bonded to a molecule of coenzyme-A, and the released energy forms a molecule of NADH2+. Yet, the pyruvic acid in the process has lost its carboxylic acid end in the form of CO2 gas. The end product is called Acetyl-COA.

Note: The prefix 2 is written because a 6 carbon sugar molecule yields 2 molecules of pyruvic acid. Thus, from here on the prefix 2 represents the total molecules in each step produced from theoriginal single molecule of glucose.

2. Tricarboxylic Acid Cycle (Tca)- Or the Kreb's or Citric Acid Cycle. This is the cycle where Acetyl-COA binds to an Oxaloacetic acid (from the cycle) to form a citric acid, the initiating molecule of the TCA cycle. The TCA cycle produces 24 ATP molecules. The cycle is as follows:

SUMMARY: The three stages of energy liberating pathway of glucose metabolism can be written in the form: ...

IV. Electron Transport System (ETS)- This is a system that is the most complicated of all, whereby the energy stored in the NADPH2+ and FADH2+ molecule is released via different steps so that molecules of ADP and Pi are combined to form ATP molecules. All of the processes in this system occur within the inner mitochondrial membrane, via various enzymes and protein changes. It should be noted that the whole process is to use the energy stored in either NADH2+ and FADH2+ and consume it to create a concentration gradient of H+ between the outer mitochondrial membrane compartment (higher H+ concentration) and inner mitochondrial membrane compartment. The process is as follows:

1. When NADH2+ binds to the flavoprotein (FP) it reduces an e- and a H+. Two H+ are released into the outer mitochondrial matrix, while the electron moves along the proteins of the inner mitochondrial membrane.

2. Two electrons are bounded to the flavoprotein then these two electrons move from FP to the protein containing iron and sulfur (FeS + FeSB), then to the cytochrome b. Cytochrome b gives the e- to the coenzyme Q (Q).

3. Coenzyme Q is an enzyme that moves across the inner matrix membrane when it carries an e- within . This movement also transports 2H+ to the outer matrix membrane. Again e- moves from Q to cytochrome b2----> Cyt e-------> Cyt a and Cyt a3.

With the exception that 2H+ molecules are imported into the outer membrane matrix from inner membrane matrix. B. The electron when it reaches Cyt a3 is absorbed by highly electronegative oxygen and 2 H+ to form a water molecule.

4. Due to the consumption of energy by ETS the e- has the highest energy when it is at FP, and it loses energy as it goes down the ETS until it reaches the Cyt a3.

5. ATP permease- This is a pump that is used by the inner matrix membrane to bring H+ and ADP+ Pi into the inner matrix from outer matrix, and yet the ATP permease functions in exporting the newly synthesized ATP + OH- from the inner matrix to the outer matrix.

6. The last but the most important enzyme on the pathway is the F complex or the ATP permease. This enzyme uses the electrochemical gradient between the outer membrane matrix and inner membrane matrix to synthesize ATP from ADP + Pi. For every inflow of a pair of H+ there will be a new ATP synthesized.

7. If you add the highlighted number of H+ in the paragraphs 1 and 3 you will see that a single molecule of NADH2+ initiates the release of 6H+ from the inner membrane matrix to the outer membrane matrix. Thus when 3 pairs of H+ are imported back from the outer membrane matrix there will be 3 ATP molecule synthesized.

8. In the case of FADH2, the lower energy FADH2 molecule will bind the electron transport system at coenzyme Q. This results in the export of 2 pairs of H+ from the inner matrix to the outer matrix (4H+ in paragraph 3 only). As a result there will be only 2 ATP produced. Note: The inner matrix membrane without its ATP permease and ATP synthase is not permeable to the passage of H+. Thus the outer membrane matrix is positively charged with an electrochemical gradient and osmotic pressure with respect to the negatively charged inner matrix....

Respiration Links
Respiration - The Basic Reaction
O2 + Carbohydrate & Other
Organic Compounds*
+ --- Living Cells --- ATP
& Heat*
+ CO2 + Water

Overview of Respiration(See Lewis fig. 7.5)

Organic Compounds*
From Food
--- Glycolysis
(Enzyme
Reactions
in Cytoplasm)
--- Pyruvic
Acid*

some ATP*
---------
Krebs Cycle
(Enzyme Reactions
in Mitochondria)
--- NADH*
& CO2
--- Proton
Gradient*
--- Lots
of
ATP*
Mitochondrion Structure (see Lewis, figures 7.7 & 7.14) Proteins and Fat as energy Sources
Anaerobic Fermentation (see Lewis, figure 7.17 & 7.19)