Citric acid is a versatile organic acid found in many fruits, especially citrus fruits like lemons, oranges, limes, and grapefruits. Its chemical formula is C6H8O7, and it's classified as a weak acid. Citric acid has a wide range of applications, from food and beverage production to household cleaning and skincare. In this comprehensive description, I'll delve into its properties, uses, production methods, health effects, and environmental impact. *1. Properties of Citric Acid:* Citric acid appears as a white crystalline powder or granules. It's odorless and has a tart, sour taste. It's highly soluble in water, making it easy to incorporate into various products. Citric acid is stable at room temperature but decomposes at higher temperatures, losing its acidic properties. It's a chelating agent, meaning it can bind to metal ions, making it useful in certain industrial processes and household cleaners. *2. Sources of Citric Acid:* While citric acid occurs naturally in citrus fruits, it's also produced commercially through microbial fermentation, primarily using strains of the fungus Aspergillus niger. This method allows for large-scale production of citric acid to meet the demand in various industries. Additionally, it can be synthesized chemically, although this method is less common due to higher production costs and environmental concerns. *3. Uses of Citric Acid:* *- Food and Beverage Industry:* Citric acid is widely used as a flavoring agent, acidity regulator, and preservative in the food and beverage industry. It enhances the flavor of many products and provides a tart taste in sodas, candies, jams, and preserves. It also acts as a preservative, extending the shelf life of packaged foods and preventing discoloration in fruits and vegetables. *- Pharmaceutical Industry:* Citric acid is used in pharmaceuticals as a pH regulator, excipient in tablets and capsules, and as a flavoring agent in syrups and liquid medications. *- Cleaning Products:* Due to its chelating properties, citric acid is used in household cleaning products such as descalers, bathroom cleaners, and dishwashing detergents. It effectively removes mineral deposits and stains without the need for harsh chemicals. *- Cosmetics and Personal Care:* Citric acid is found in skincare products like exfoliating scrubs, facial peels, and anti-aging creams. It helps to promote skin renewal by gently removing dead skin cells and promoting collagen production. *- Industrial Applications:* Citric acid is used in various industrial processes, including water softening, metal cleaning, and the production of detergents and surfactants. *4. Production Methods:* *- Microbial Fermentation:* This is the most common method for commercial production of citric acid. It involves fermenting glucose or sucrose-containing substrates with strains of Aspergillus niger in large-scale bioreactors. The fungus produces citric acid as a byproduct of its metabolism, which is then extracted and purified. *- C

1. Fatty Acids Biosynthesis

3. • Arachidonic acid is referred to as an ω-6 fatty
acid because the closest double bond to the
end begins six carbons from ω end.
• Linoleic acid, is the precursor of arachidonic
acid, the substrate for prostaglandin synthesis
• linolenic acid, the precursor of other ω -3
fatty acids important for growth and
development

5. SYNTHESIS OF FATTY ACIDS
• FA are mainly supplied by diet
• Excessive carbohydrates and proteins are
converted to FA which are stored as TAG
• In humans, fatty acid synthesis occurs
– primarily in the liver and lactating mammary glands
– To a lesser extent in adipose tissues
• The process incorporates carbons from acetyl
CoA into the growing fatty acid chain, using ATP
and NADPH

7. Production of cytosolic acetyl CoA
• 1st step Transfer of acetate units from
mitochondrial acetyl-CoA to the cytosol
• The coenzyme A portion of acetyl-CoA cannot
cross the mitochondrial membrane; only the
acetyl portion is transported to the cytosol.
• Citrate is produced by the condensation of
oxaloacetate (OAA) and acetyl CoA.

10. Carboxylation of acetyl CoA to form
malonyl CoA
• The energy for the carbon-to- carbon
condensations in fatty acid synthesis is
supplied by the process of carboxylation and
then decarboxylation of acetyl groups in the
cytosol.

12. Formation of malonyl-CoA
• Irreversible process
• Catalyzed by acetyl-CoA
carboxylase.
• The bacterial enzyme has three
separate polypeptide subunits
• In animal cells, all three activities
are part of a single multifunctional
polypeptide.
• In all cases, the enzyme contains a
biotin prosthetic group

13. • In all organisms, the long carbon chains of fatty acids are assembled
in a repeating four-step sequence, catalyzed by a system collectively
referred to as fatty acid synthase.
• There are two major variants of fatty acid synthase:
– fatty acid synthase I (FAS I), found in vertebrates and fungi,
– fatty acid synthase II (FAS II), found in plants and bacteria.
• The FAS I is a single multifunctional polypeptide chain (Mr 240,000).
• The mammalian FAS I is the prototype.
• Seven active sites for different reactions lie in separate domains.
• The mammalian polypeptide functions as a homodimer (Mr
480,000).
• The subunits appear to function independently.

16. Fatty acids elongation
• Long-Chain Saturated Fatty Acids Are Synthesized from
Palmitate
• fatty acid elongation systems present in the smooth
endoplasmic reticulum and in mitochondria
• Different enzyme systems are involved, and coenzyme A
rather than ACP is the acyl carrier in the reaction, the
mechanism of elongation in the ER is otherwise identical
to that in palmitate synthesis:
• The double bond is introduced into the fatty acid chain
by an oxidative reaction catalyzed by fatty acyl–CoA
desaturase, a mixed-function oxidase

18. Fatty Acid Biosynthesis Is Tightly Regulated
• The reaction catalyzed by acetyl-CoA carboxylase is the rate-
limiting step in the biosynthesis of fatty acids, and this
enzyme is an important site of regulation.
• In vertebrates, palmitoyl-CoA, the principal product of fatty
acid synthesis, is a feedback inhibitor of the enzyme;
• citrate is an allosteric activator

19. • When the concentrations of mitochondrial
acetyl-CoA and ATP increase, citrate is
transported out of mitochondria.
• it then becomes both the precursor of
cytosolic acetyl-CoA and an allosteric signal for
the activation of acetyl-CoA carboxylase
• citrate inhibits the activity of
phosphofructokinase-1, reducing the flow of
carbon through glycolysis.

20. • Acetyl-CoA carboxylase is also regulated by
covalent modification.
• Phosphorylation, triggered by the hormones
glucagon and epinephrine, inactivates the
enzyme and reduces its sensitivity to activation
by citrate, thereby slowing fatty acid synthesis.
• In its active form, acetyl-CoA carboxylase
polymerizes into long filaments phosphorylation
is accompanied by dissociation into monomeric
subunits and loss of activity.

21. • when animals ingest an excess of certain
polyunsaturated fatty acids, the expression of
genes encoding a wide range of lipogenic
enzymes in the liver is suppressed.
• Β-oxidation is blocked by malonyl-CoA, which
inhibits carnitine acyltransferase-I