Cellular Metabolism: Photosynthesis, Anabolism, and Biosynthesis Pathways

Review of Photosynthesis Fundamentals

Which coenzyme is necessary for non-cyclic photophosphorylation?

NADP⁺

What happens to the light energy absorbed by antenna pigments?

It transfers to other pigments in the form of excitation energy.

Which organisms possess chloroplasts?

Algae and plants.

What is the electron donor that replaces the electron lost by Chlorophyll P680?

Water.

What provides the electron that replaces the electron lost by Chlorophyll P700?

The plastocyanin.

What molecule fixes CO₂ in the Calvin cycle?

Ribulose-1,5-bisphosphate carboxylase (RuBisCO).

What is the first stable compound that appears in the Calvin cycle after CO₂ fixation?

3-Phosphoglyceric acid.

Which statement describes a characteristic of chemosynthesis?

It is performed only by certain types of bacteria.

Heterotrophic Anabolism: Biosynthesis Pathways

1. Defining Heterotrophic Anabolism

Heterotrophic anabolism refers to the metabolic processes involving the formation of complex organic molecules from simpler organic molecules, known as precursor molecules. This process occurs in both autotrophic and heterotrophic cells.

Heterotrophic anabolism typically involves two phases:

1. First phase: Biosynthesis of monomer precursors.
2. Second phase: Biosynthesis of polymers using monomers.

Simple organic precursor molecules can originate from several sources:

• From the catabolism of reserve substances in both autotrophic and heterotrophic cells.
• From the digestion of food in heterotrophic cells.
• From photosynthesis or chemosynthesis, exclusively in autotrophs.

It is important to note that not all enzymes can catalyze a reaction in both directions (anabolism and catabolism). Heterotrophic anabolism is fundamentally a reduction process. The energy required for all anabolic processes is obtained from the dephosphorylation of ATP molecules.

Most anabolic pathways occur in the cytosol, with notable exceptions:

• Nucleic acid synthesis, which occurs in the nucleus, chloroplasts, and mitochondria.
• Protein synthesis, which occurs in ribosomes.
• The synthesis of phospholipids and cholesterol, which takes place in the smooth endoplasmic reticulum.
• The glycosylation of lipids and proteins, which begins in the endoplasmic reticulum and continues in the Golgi apparatus.

2. Heterotrophic Carbohydrate Anabolism

Carbohydrate anabolism has two main phases:

Synthesis of Glucose (Gluconeogenesis)

Glucose can be obtained from pyruvic acid via gluconeogenesis. In autotrophic cells, glucose can also be synthesized from processes originating in the Calvin cycle.

Synthesis of Glucose Polymers or Other Hexoses

The most important polymers are glucose polymers linked by alpha bonds. In plant cells, starch is synthesized (a process called amylogenesis). In animal cells, glycogen is synthesized (a process called glycogenogenesis).

The Gluconeogenesis Pathway

Gluconeogenesis is the reverse process of glycolysis. It is the synthesis of glucose from non-carbohydrate precursors. In animals, it is essential during periods of fasting.

Precursors for gluconeogenesis can come from:

• Deamination of amino acids.
• Transformation of lactic acid.
• Degradation of fatty acids (in plants, algae, and bacteria, but generally not in animals, as animal cells lack the necessary enzymes to convert acetyl-CoA into glucose precursors).

The pathway involves three key regulatory steps that bypass irreversible steps of glycolysis:

1. The conversion of pyruvic acid to phosphoenolpyruvate: Pyruvic acid enters the mitochondria where the enzyme pyruvate carboxylase transforms it into oxaloacetic acid. Since oxaloacetic acid cannot cross the inner mitochondrial membrane, it is transformed into malic acid, which can pass into the cytosol. In the cytosol, malic acid is converted back to oxaloacetic acid, which finally becomes phosphoenolpyruvate.
2. The conversion of fructose-1,6-bisphosphate into fructose-6-phosphate.
3. The conversion of glucose-6-phosphate into glucose.

Glycogenogenesis and Amylogenesis

Glycogenogenesis is the synthesis of glycogen from glucose-6-phosphate. This occurs especially in the liver and muscles, where muscle glycogen serves as a specific glucose reserve for muscle cells.

Amylogenesis is the synthesis of starch. This process is similar to glycogenogenesis, but the activating molecule is ATP (or ADP-glucose) rather than UTP (or UDP-glucose, typically used in glycogen synthesis).

3. Anabolism of Lipids, Nucleotides, and Amino Acids

3.1. Heterotrophic Anabolism of Lipids

The most important function of lipids is energy storage, primarily as fats or triglycerides.

1) Obtaining Fatty Acids

The main source of fatty acids in animals is dietary fat. The secondary source is the biosynthesis of fatty acids, which occurs in the cytosol starting from acetyl-CoA.

Biosynthesis begins with the release of acetyl-CoA from the mitochondria into the cytosol. The required acetyl-CoA is activated to become a three-carbon molecule called Malonyl-CoA. The new carbon addition comes from dissolved bicarbonate ion (CO₂).

The union of Malonyl-CoA (3C) to Acetyl-CoA (2C) results in a four-carbon molecule and the release of CO₂. The fatty acid being formed is typically palmitic acid, which can then be used to synthesize stearic acid or oleic acid.

Differences between Fatty Acid Biosynthesis (Anabolism) and Degradation (Catabolism):

• It occurs in the cytosol, instead of the mitochondria.
• The fatty acid is joined to the Fatty Acid Synthase Complex (FAS) enzyme group, not to CoA.
• The two-carbon units added are provided by Malonyl-CoA, not acetyl-CoA.
• The hydrogen carrier is NADPH, not NADH or FADH₂.

2) Obtaining Glycerol

Glycerol must be in the form of glycerol-3-phosphate before it can join the fatty acids.

3) Formation of Triglycerides

Fatty acid molecules join together via ester bonds. First, a monoacylglyceride is formed, followed by a diacylglyceride, and finally, a triglyceride.

3.2. Heterotrophic Amino Acid Anabolism

Not all organisms are capable of synthesizing all 20 amino acids (AA). Humans and many animals are unable to synthesize 10 specific amino acids; these are known as essential amino acids (AA).