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Introduction
Metabolism is the total amount of the biochemical reactions
involved in maintaining the living condition of the cells in an
organism. All living organisms require energy for different
essential processes and for producing new organic
substances.
The metabolic processes help in growth and reproduction
and help in maintaining the structures of living organisms.
The organisms respond to the surrounding environment due
to metabolic activities. All the chemical reactions occurring
in the living organisms from digestion to transportation of
substances from cell to cell require energy.

WHAT IS METABOLISM?
Metabolism is the set of life-sustaining chemical reactions
in organisms. The three main functions of metabolism are:
the conversion of the energy in food to energy available to
run cellular processes; the conversion of food to building
blocks of proteins, lipids, nucleic acids, and
some carbohydrates; and the elimination of metabolic
wastes. These enzyme-catalyzed reactions allow organisms to
grow and reproduce, maintain their structures, and respond
to their environments. The word metabolism can also refer to
the sum of all chemical reactions that occur in living
organisms, including digestion and the transportation of
substances into and between different cells, in which case the
above described set of reactions within the cells is called
intermediary (or intermediate) metabolism.

TYPES OF METABOLISM
• There are two types of metabolic process:
• Catabolism
• Anabolism
Catabolism – This process is mainly involved in breaking
down larger organic molecules into smaller molecules. This
metabolic process releases energy.
Anabolism – This process is mainly involved in building up or
synthesizing compounds from simpler substances required by
the cells. This metabolic process requires and stores energy.
The processes of metabolism depend on the nutrients that
get digested to produce energy. This energy is necessary to
synthesize nucleic acids, proteins and other biomolecules in
our body.

Anabolism
Anabolism is a biochemical process in metabolism where the simple molecules combine to
generate complex molecules.
This process is endergonic, which means it is not spontaneous and requires energy to progress
the anabolic reaction. The complex molecules obtained are further used to store energy in the
form of ATP (Adenosine Tri Phosphate).
Cells use an anabolic process to make polymers, repair, and grow tissues. For example:
– Formation of disaccharides and water with the help of combining simple sugars
C6H12O6 + C6H12O6 → C12H22O11 + H2O
– Formation of dipeptides by combining Amino acids
NH2CHRCOOH + NH2CHRCOOH → NH2CHRCONHCHRCOOH + H2O
– Formation of lipids when glycerol reacts with fatty acids
CH2OHCH(OH)CH2OH + C17H35COOH → CH2OHCH(OH) CH2OOCC17H35
– Process of photosynthesis to form glucose and oxygen
6CO2 + 6H2O → C6H12O6 + 6O2
There are three stages in anabolism. They are
Production of precursors such as monosaccharides, nucleotides, amino acids, and isoprenoids.
Activation of the above-mentioned precursors into reactive forms with the help of energy
from ATP.
Assemble the precursors to form complex molecules such as polysaccharides, nucleic acids,
proteins, and lipids.

Energy Sources for Anabolic Processes
• Autotrophs – Formation of complex organic molecules such as proteins
and polysaccharides in plant cells from simple molecules like water and
carbon dioxide with the help of sunlight as an energy source.
• Heterotrophs – They require complex substances such as amino acids
and monosaccharides to produce these complex molecules.
• Photoheterotrophs and photoautotrophs – They obtain energy from light
• Chemoheterotrophs and chemoautotrophs get energy from inorganic
oxidation reactions.
Functions of Anabolism
• The nutrients from food are broken down into small blocks in the
catabolic pathway. Macromolecules are formed when small molecules
are combined. During the anabolic pathway, energy is utilized to
generate large molecules by forming chemical bonds between the
smaller molecules. These macromolecules are further used to build new
cells or structure the cells. Anabolism is essential for maintenance,
growth, and development of a cell

Catabolism
• Catabolism is the break down of complex molecules. Catabolism is the breakdown of
complex substances to their constituent parts (glucose, amino acids and fatty acids) which
form substrates for metabolic pathways.
Stages of Catabolism
• Catabolism can be broken down into 3 main stages. The three stages are as explained as
follows-
• Stage 1 – Stage of Digestion
• The large organic molecules of organic chemistry like proteins, lipids, and polysaccharides
are digested into their smaller components outside cells. This stage acts on starch, cellulose
or proteins that cannot be directly absorbed by the cells.
• Stage 2 – Release of energy
• Once the molecules are broken down, these molecules are taken up by cells and converted
to yet smaller molecules, usually acetyl coenzyme A, which releases some energy.
• Stage 3 – Energy Stored
• The released energy is stored by reducing the coenzyme nicotinamide adenine dinucleotide
into NADH.
• This process provides the chemical energy necessary for the maintenance and growth of
cells. Some examples of the catabolic processes include glycolysis, the citric acid cycle, the
breakdown of muscle protein in order to use the amino acids as substrates for
gluconeogenesis, the breakdown of fat in adipose tissue to fatty acids, and oxidative
deamination of neurotransmitters by monoamine oxidase.

Functions of Catabolism
Energy Production:
Catabolism provides the chemical energy (ATP) that fuels anabolic processes.
This energy is released during the breakdown of complex molecules like carbohydrates, proteins, and fats.
Examples of catabolic pathways include glycolysis, the citric acid cycle, and oxidative phosphorylation.
2. Nutrient Cycling and Recycling:
Catabolism breaks down molecules, releasing their building blocks (amino acids, fatty acids, etc.).
These building blocks can then be used for anabolic processes like protein synthesis or the creation of new
molecules.
This ensures a constant supply of essential nutrients for the body.
3. Cell Growth and Maintenance:
The energy and building blocks provided by catabolism are essential for cell growth, repair, and
maintenance.
These processes are crucial for maintaining the body's structure and function.
4. Regulation of Metabolism:
Catabolic and anabolic processes are tightly regulated to maintain a balance of energy and nutrient levels
in the body.
Hormones and other signaling molecules play a role in coordinating these processes.
5. Removal of Waste Products:
Catabolic reactions often produce waste products like carbon dioxide and water, which are then removed
from the body.
This ensures that the body's internal environment remains stable.

CHARACTERISTIC FEATURES OF ANY ONE ANABOLIC PROCESS
– Photosynthesis
• Photosynthesis is a process by which phototrophs convert light energy into
chemical energy, which is later used to fuel cellular activities. The chemical
energy is stored in the form of sugars, which are created from water and carbon
dioxide. Photosynthesis is also used by algae to convert solar energy into
chemical energy. Oxygen is liberated as a by-product and light is considered as a
major factor to complete the process of photosynthesis.
• Photosynthesis occurs when plants use light energy to convert carbon dioxide
and water into glucose and oxygen. Leaves contain microscopic cellular
organelles known as chloroplasts.
• Each chloroplast contains a green-coloured pigment called chlorophyll. Light
energy is absorbed by chlorophyll molecules whereas carbon dioxide and oxygen
enter through the tiny pores of stomata located in the epidermis of leaves.
• Another by-product of photosynthesis is sugars such as glucose and fructose.
• These sugars are then sent to the roots, stems, leaves, fruits, flowers and seeds.
In other words, these sugars are used by the plants as an energy source, which
helps them to grow. These sugar molecules then combine with each other to
form more complex carbohydrates like cellulose and starch. The cellulose is
considered as the structural material that is used in plant cell walls.

CHARACTERISTIC FEATURES OF ANY ONE CATABOLIC PROCESS -
Glycolysis
Glycolysis is a set of reactions that converts glucose to pyruvate or lactate.
This is the first metabolic pathway to be elucidated and hence is considered
as a paradigm of metabolic pathways. Glycolysis is also called Embden-
Meyerhoff pathway. The complete set of reactions occurs in the cytoplasm
of virtually every animal cell. The entire process occurs without molecular
oxygen. Glycolysis consumes 2 ATP and generates 4ATP. Thus, the process
results in the generation of 2 net ATP. The process also generates 2 NADH.
What happens to pyruvate depends upon the presence or absence of
mitochondria in the cell or upon the availability of oxygen in mitochondria
– containing cell. For the glycolytic pathway to continue, NAD+ has to be
regenerated. In erythrocytes (no mitochondria) and in mitochondria –
possessing cells under anaerobic conditions, NAD+ is regenerated from
NADH during the conversion of pyruvate to lactate. In mitochondria –
possessing cells under aerobic conditions, NAD+ is regenerated by either
malate – aspartate shuttle or α-glycerophosphate shuttle, which transfer
the reducing equivalents from NADH into mitochondria for electron
transport chain, thus regenerating NAD+ in the cytoplasm.

CONTRIBUTION OF ANY ONE EMINENT SCIENTIST - Sir Hans Adolf Krebs
• Hans Krebs significantly advanced our understanding of metabolism
by elucidating the citric acid cycle (also known as the Krebs cycle or
tricarboxylic acid cycle). This cycle is a central metabolic pathway in many
organisms, including animals, for oxidizing carbohydrates, fats, and amino
acids to generate energy. He also made important contributions to
understanding urea synthesis and amino acid metabolism.
Impact:
• Nobel Prize:
Krebs received the Nobel Prize in Physiology or Medicine in 1953 for his discovery
of the citric acid cycle, sharing the award with Fritz Lipmann.
• Foundation of Biochemistry:
His work revolutionized our understanding of metabolic processes and laid the
foundation for much of modern biochemistry.
• Clinical Significance:
His insights into metabolic pathways have implications for understanding and
treating metabolic disorders and in developing clinical applications such as
parenteral nutrition.

Key Contributions
Discovery of the Citric Acid Cycle:
In 1937, Krebs presented a comprehensive description of the citric acid cycle,
a series of biochemical reactions that oxidize acetyl-CoA (a molecule derived
from carbohydrates, fats, and amino acids) to produce energy in the form of
ATP, NADH, and FADH2.
Urea Cycle:
He and Kurt Henseleit identified the urea cycle, which is responsible for
removing excess nitrogen from the body in the form of urea.
Amino Acid Metabolism:
Krebs's research on amino acids included studies on proteolysis (the
breakdown of proteins), urea synthesis, and the metabolism of glutamine.
Intermediary Metabolism:
He focused on intermediary metabolism, exploring various pathways
involved in the breakdown and synthesis of different nutrients.

Conclusion
In conclusion, metabolism is a vital biochemical process
that underpins all life forms, encompassing both catabolic
and anabolic pathways. These processes are crucial for
energy production, growth, and cellular maintenance.
Understanding metabolism helps illuminate how
organisms adapt to their environments and manage
energy efficiently. Insights from scientists like Hans Krebs
have expanded our knowledge of metabolic pathways,
particularly the Krebs cycle. This understanding is
essential not only for advancing biological science but
also for addressing health issues such as metabolic
disorders. Ultimately, studying metabolism is key to
promoting health and improving quality of life.

Bibliography
• https://en.wikipedia.org
• https://www.ncbi.nlm.nih.gov
• https://byjus.com
• https://my.clevelandclinic.org
• https://www.britannica.com

Acknowledgement
I would like to express my heartfelt gratitude to my principal
for their support and encouragement throughout this
project. I am also thankful to my teachers for their invaluable
guidance and insights, which have greatly enhanced my
understanding of metabolism. Special thanks to my parents
for their unwavering support and for providing the necessary
resources for my research. Their belief in my abilities
motivated me to excel. This project has deepened my
appreciation for biology, and I am truly grateful to everyone
who contributed to its completion.

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