High School Biology: 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
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,
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
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
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
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 - The Basic Reaction
||Carbohydrate & Other
| + ---
|+ CO2 + Water
Overview of Respiration(See Lewis fig. 7.5)
(see Lewis, figures 7.7 & 7.14)
- Glucose* Activation (use of ATP*)
- Enzyme Reactions
- Energy Extraction (Production of ATP* & NADH*)
- Production of Pyruvic Acid*
- Outer membrane
- Inner membrane systems
- Electron Carriers of repiratory chain
- ATP Synthase
- Matrix (liquid area within the inner membrane)
- Enzymes of Krebs Cycle)
- Intermembrane space (liquid area between the inner and outer membranes)
- Concentration of Protons)
- Pyruvic Acid* converted to Acetyl CoA* + CO2
- Enzyme Reactions
- Acetyl CoA* combined with 4C compound to make Citric Acid*
- Citric Acid* broken down step-by-step
- CO2 released
- NAD+ reduced (e- added) to NADH*
- 4C compound regenerated
Proteins and Fat as energy Sources
- NADH* provides e- to respiratory chain producing NAD+
- O2 accepts the e-, combines with H+, and produces H2O
- e- flow through respiratory chain pumps protons to intermembrane
space producing a Proton Gradient*
- Proton movement* through ATP synthase
- ADP + Phosphate ions ----ATP
(see Lewis, figure 7.17 & 7.19)
- Occurs when O2 is not available in plants & yeasts
- Alcoholic Fermentation
- Pyruvic Acid* converted to Ethyl Alcohol* and CO2
- A small amount of ATP* is produced)
- Lactic Acid Fermentation
- Occurs when O2 is not available in animals & some bacteria
- Pyruvic Acid* converted to Lactic Acid*
- A small amount of ATP* is produced)