Shiek Sultan Zayed H. Mohametano Veronica G. Nacaytuna

Photosynthesis, Biosynthesis & Fermentation

History

In 1648, Johan Baptist van Helmont considered water to be the source of life and the basic nutrient

for plants. He devised an experiment by which he showed that small potted willows can thrive on soil and

water alone while they gain their substance (weight) solely from the “water” as the weight of the soil in the

pots did not decrease significantly. In 1779, Jan Ingen-Housz demonstrated the green parts of the plant

could manufacture oxygen by the sun’s light and not its heat. He also proposed the idea that plants used CO2

while producing O2; and in 1881 a German botanist name Theodor Engelmann discovered that

photosynthesis occurs in the chloroplast.

Photosynthesis

Green plants, algae, and certain bacteria transform light energy (photons) to chemical energy stored

in the bonds of the sugar they make from carbon dioxide and water. Through photosynthesis, it produced

the oxygen man and all other aerobic microorganisms for cellular respiration. With an equation of,

Photosynthesis occurs on plant’s leaves. Their green color is from chlorophyll, a light-absorbing

pigment in the chloroplasts that plays a central role in converting solar energy to chemical energy.

Chloroplast is concentrated in the cells of the mesophyll, the green tissue in the interior of the leaf. Carbon

dioxide enters the leaf, and oxygen exits, by the way of tiny pores called stomata (singular, stoma, meaning

“mouth”). In the chloroplast, an envelop of two membranes encloses an inner compartment in the

chloroplasts, which is filled with a thick fluid called stroma. Suspended in the stroma is a system of

interconnected membranous sacs, called thylakoids, which encloses another compartment, called the

thylakoid space. Chlorophyll pigments harvest energy (photons) by absorbing certain wavelengths (blue-420

nm and red-660 nm are most important). Plants are green because the green wavelength is reflected, not

absorbed. However, some leaves are not green; it is because chloroplasts contain several pigments:

Chlorophyll a, Chlorophyll b and Carotenoids. Calvin cycle (see figure 1) also known as Carbon Fixation, is a

light independent reaction. The C3 plants comprise 80% of plants on earth. It occurs in the stroma of the leaf

and uses ATP and NADPH from light reactions, and uses CO2 to produce glucose which takes 6 turns and

uses 18 ATP and 12 NADPH. Figure 2 shows a diagram that depicts the summary of the chemical processes

of photosynthesis.

Figure 1. The diagram of Calvin cycle Figure 2. Summary of chemical processes of photosynthesis

Biosynthesis

It is also called as anabolic pathways or biogenesis and is defined as: the production of a complex

chemical compound from simpler precursors in a living organism, usually involving enzymes (to catalyze the

reaction) and energy source such as ATP. It includes: Photosynthesis, Chemosynthesis, Amino Acid synthesis,

Nucleic Acid synthesis, ATP synthesis, and Krebs cycle.

Krebs cycle (see figure 3) is also known as tricarboxylic acid cycle (TCA) or citric acid cycle. It is a

series of chemical reactions used by all aerobic organisms to generate energy through the oxidation of

acetate derived from the carbohydrates, fats and proteins into carbon dioxide and chemical energy in the

form of Adenosine Triphosphate(ATP) as well as NADH.

Figure 3. Schematic Diagram of Krebs cycle

Junction Step for glycolysis and Kreb’s cycle: upon entering the mitochondrion via active transport, pyruvate is first converted to a compound called acetyl CoA. There are 8 Reactions of TCA cycle: Citrate synthase, Aconitase, Iso-citrate dehydrogenase, a ketoglutarate dehydrogenase, Succinyl-Coenzyme A synthetase, Succinate dehydrogenase, Fumerase and Malate dehydrogenase. Step 1(Citrate Synthase Reaction), Acetyl CoA adds its two-carbon acetyl group to oxaloacetate, producing citrate.

Step 2 (Aconitase Reaction), Citrate is converted to its isomer, isocitrate, by removal of one water molecule and addition of another.

Step 3 (Isocitrate Dehydrogenase), Isocitrate is oxidized, reducing NAD+ to NADH. Then the resulting

compound loses a CO2 molecule.

Step 4 (α-ketoglutarate dehydrogenase), Another CO2 is lost, and the resulting compound is

oxidized, reducing NAD+ to NADH. The remaining molecule is then attached to coenzyme A by an unstable

bond.

CH3

O

O

O

pyruvate

CO2HSCoA

CH3

SCoA

O

acetyl CoA

NADHNAD+

pyruvate dehydrogenase complex

acetyl CoAoxaloacetate

CoASH

citrate synthase

citrate

OH2

CH3

C

O

SCoA

C O

CH2

C

O

COO

O

COO

CH2

C

CH2

C

OH CO

O

O O

+

citrate

aconitase

isocitrate

COO

CH2

C

CH2

C

OH CO

O

O O

COO

CHCH

CH2

C

CO

O

OO

OH

isocitrate

NAD NADH CO2

isocitrate dehydrogenase

alpha ketoglutarate

COO

CHCH

CH2

C

CO

O

OO

OH

COO

C

CH2

CH2

C

OO

O

alpha ketoglutarate

NAD NADHCoASH

CO2

succinyl CoA

alpha ketoglutaratedehydrogenase

COO

C

CH2

CH2

C

OO

OC

CH2

CH2

COO

OSCoA

Step 5 (Succinyl CoA synthetase), CoA is displaced by a phosphate group, which is transferred to

GDP, forming GTP, a molecule with functions similar to ATP.

Step 6 (Succinate dehydrogenase), Two hydrogens are transferred to FAD, forming FADH2 and

oxidizing succinate.

Step 7 (Fumarase), Addition of water molecule rearranges bonds in the substrate.

Step 8 (Malate Dehydrogenase), the substrate is oxidized, reducing NAD+ to NADH and regenerating

oxaloacetate. Products of the first turn of the cycle are: one GTP (or ATP), three NADH, one QH2 and two

CO2. Because two acetyl-CoA molecules are produced from each glucose molecule, two cycles are required

per glucose molecule. Therefore, at the end of two cycles, the products are: two GTP, six NADH,two QH2 and

four CO2. The total number of ATP obtained after complete oxidation of one glucose in glycolysis, citric acid

cycle, and oxidative phosphorylation is estimated to be between 30 and 38.

Fermentation

Fermentation is a metabolic process that converts sugar to acids, gases and/or alcohol. It occurs

in yeast and bacteria. And also occur in muscle cells (Lactic acid fermentation). Fermentation takes place in

the absence of oxygen, when the electron transport chain is unusable. It is the complete pathway, including

Glycolysis and the re-oxidation of NADH. There are two types of fermentation Lactic Acid Fermentation and

Alcoholic Fermentation (see figure 5). Despite the many methods bacteria employ to ferment organic

compounds, there are some unifying concepts that are true of all fermentations. These are:

NAD+ is almost always reduced to NADH. Electrons are removed from the carbon denoted in red

(see figure 4) and donated to NAD+. An inorganic phosphate is attached to the carbon

succinyl CoA

GDP GTP CoASH

succinate

succinyl CoAsynthetase

C

CH2

CH2

COO

OSCoA

C

CH2

CH2

COO

OO

succinate

FAD FADH2

succinyl CoAdehydrogenase

fumarate

C

CH2

CH2

COO

OO

CC

C

COO

O O

H

H

fumarate

OH2

malate

fumarase

CC

C

COO

O O

H

H

C

CH

CH2

COO

O

OH

O

Figure 4. Reduction of NAD+ to NADH

Fermentation results in a excess of NADH. Accumulation of NADH causes a problem for anaerobes.

They have too much of it and it prevents further oxidation of substrate due to a lack of an NAD+ pool to

accept electrons. In many fermentation pathways, the steps after energy generation are performed in part

to get rid of the NADH.

Pyruvate is often an important intermediate. It is a valuable intermediate because it can be used

for cell synthesis and many different enzymes can act on it. It gives the microbe flexibility.

Energy is derived from Substrate-Level Phosphorylation (SLP). The substrate is converted to a

phosphorylated compound and in subsequent reactions the high energy phosphate is transferred to ATP.

Energy yields are low. SLP is an inefficient process and much of the energy of the electrons is lost.

Typically energy yields are 1-4 ATP per substrate molecule fermented.

Oxygen is not involved. Fermentation can involve any molecule that can undergo oxidation. Typical

substrates include sugars (such as glucose) and amino acids. Products typically depend upon the substrate

but can include organic acids (lactic acid, acetic acid), alcohols (ethanol, methanol, butanol), ketones

(acetone) and gases (H2 and CO2)

Figure 5. The schematic diagram of Alcohol Fermentation and Lactic Acid Fermentation