Introduction: Nutritional Ingredients
The primary nutrients are used by the organism in a variety of ways. Their breakdown in
the digestion results in metabolites that can be utilized for further metabolic processes in
the organism. They can be used for the formation of large compounds that contribute to
the structure of the organism (like the carbohydrates in the cell wall of plants, proteins in
collagen, or the lipids in membranes). The metabolites can also be converted to
substances that regulate the organism and its functions (as for instance in immune
recognition, neurotransmission, or as enzymes). And they can be oxidized further to
provide energy for the above processes and bio-mechanical and active transport needs
through their ultimate breakdown, as happens for instance with glucose.
Carbohydrates, proteins, and lipids can each be used for a variety of different functions;
within limits they can also be converted one into the other with the help of intermediates
in the citric acid cycle, which interconnects their anabolic/catabolic cycles. And they each
have an area of characteristic activity, which will be discussed in the following chapters for
the respective compounds. This chapter deals with carbohydrate metabolism and function,
chapter 4 discusses protein metabolism and activity and chapter 5 the metabolism and
activity of the lipids.
Carbohydrates and water
In section 1.2.4 we found how the breakdown of water stands at the beginning of the
synthesis of carbohydrates in photosynthesis. This is one important process that shows the
relationship between carbohydrates and water. Water is oxidized to convert sunlight into
biochemical energy for the synthesis of carbohydrates from CO2:
6 CO2 + 6 H2O + light ( C6 H12 O6 (carbohydrates) + 6 O2
Animals and humans breathe in the O2 and use it to break down carbohydrates to CO2
and water in the citric acid cycle and oxidative phosphorylation (section 1.2.2.). This
oxidative catabolic process, together with the reductive anabolic process of
photosynthesis, forms a cycle in nature between plants and the organisms of animals and
human beings (see fig. 1.3) in which water is alternately oxidized and formed.
The water/carbohydrate connection is also found when sugar is burnt without the
addition of oxygen, i.e. made into carbon. This happens when sugar is heated up in a
closed container so that no oxygen can enter. The result will be carbon and water, the
latter escaping as steam. This phenomenon would imply that the general formula for
carbohydrate monomers could be written as Cn (H2O)n. This is the reason they were
originally given the name carbo-hydrate. The general formula for glucose for instance is C6
H12 O6.
Carbohydrates have a relation to water both in their structure and in their metabolic
processes. In the formation of polysaccharides, as in forming the peptide bonds in
proteins, one water molecule is released from glucose in forming the glycosidic bond with
every addition of a glucose molecule.
Functions of carbohydrate polymers
Carbohydrate polymers, like all large compounds, hold within their structure potential
energy that becomes available at their breakdown as metabolic and other energy for the
organism. Many common carbohydrates are polysaccharides.
Polysaccharides are polymers of sugars (see section 2.1.1.). The most commonly occurring
monomer is glucose (section 3.3.). Carbohydrates also have structural functions.
Storage function
Glycogen and starch are storage forms for glucose. Glycogen is usually more branchedthan starch, which allows for the glucose to be cleaved off at more points at the sametime. Therefore glycogen, which is the mainglucose polymer in higher organisms, cangive a greater rate of supply of glucose indemand situations than starch, whichexists in plants only, would be able to.Animals and humans have more glucosequicker at their demand than plants. Asmentioned before in section 2.1.1. glycogendoes not occur in heart and brain cells. Itoccurs mainly in liver and muscle cells, butonly the liver can convert lactate toglucose. Muscle cells are dependent on theliver for gluconeogenesis (see the Cori cycle, section 1.2.1.) and the serum glucose level isprimarily buffered by the liver. The liver’s carbohydrate function is prototypical in higheranimals and humans.Glycogen has no structural support function in higher organisms and therefore can bebroken down without affecting their structure. Glycogen stores in the liver form an idealenergy supply for the organism. However, human glycogen stores are estimated to bedepleted after 10-15 hours of fasting and therefore need constant replenishing.
Amylopectin Glycogen
(Campbell, 1999)
Structural function
Cellulose is a structural glucose polymer in plants (see fig. 2.2.). Structural carbohydrates
in non-plants have amino acids or contain amino acid sequences as monomers to form
chitin or peptidoglycans (section 2.1.1.). The breakdown of cellulose does affect the
structure of the plant, as does the breakdown of peptidoglycans by antibiotics affect the
structure of bacteria, and the breakdown of chitin affects the structure of invertebrates
that carry it in their exoskeleton.
Glucose cannot be cleaved off of cellulose without the presence of special enzymes called
cellulases, which are found in bacteria and in the digestive tract of, for instance, cattle.
Humans do not have cellulases in their digestive tract. Cellulose is the main insoluble part
of dietary fiber (bulk or roughage) in our diet, which stimulates peristalsis and binds some
of the potentially toxic substances in food.
Glucose and Energy
Glucose is the most abundant of the carbohydrate monomers in nature. Glucose
metabolism is central to the energy supply of living organisms. It can be transported in
the body to the place where energy is required.
Anaerobic breakdown of glucose in glycolysis yields pyruvate or, under lasting anaerobic
conditions, lactate and 2 molecules of ATP per glucose molecule. Its further aerobic
breakdown occurs in the citric acid cycle (section 1.2.3.), which can yield 30 more ATP
molecules. The catabolism of glucose releases energy, which results in the
phosphorylation of ADP to ATP. ATP is the carrier of immediate energy for organisms. It
contains two phosphoanhydride bonds linking the phosphates. These bonds hydrolyze
easily, with a consequent immediate release of the energy (less than 1 minute). As such, it
is directly available biochemical energy, which must, however, be used “right away”. It can
be used to convert the biochemical energy to bio-mechanical energy (as in the case of the
exercising muscle), bio-electrical energy (as in the nervous system), light (as in
phosphorescent bacteria), active transport, etc.
The freed up energy from glycolysis and oxidation in the citric acid cycle can also be used
in the form of ATP for reductive anabolic processes that result in compounds such as
proteins and lipids. Glucose is a reducing sugar. When glucose is oxidized, another
compound can be reduced in an anabolic reaction with the energy that becomes available
(transferred via ATP).
Fig. 3.1 The conversion of energy-carrying compounds to energy-requiring processes
Since it is not needed in the structures of the organism, glucose and glycogen can be
broken down without consequences for the structure of the body. Thus glucose becomes
the main supplier of energy for living organisms. The prototypical function of glucose in
organisms is to provide energy.
Summary and conclusion
Carbohydrate functions
Carbohydrates can be used for a variety of functions in organisms. They serve different
functions in plants than in higher organisms. Plants are the only organisms that use
homopolysaccharides to build up their structural components, as well as using starch and
glucose for an energy source. In lower animals carbohydrates can also have structural
functions. Then their conformation includes amino acids or amino acid derivatives. In
higher organisms carbohydrates only provide energy for the metabolic needs of the
organism. They do not provide structural support. Carbohydrates serve different functions
in plants than in higher organisms.
The prototypes of carbohydrate polymer appear in plants, in the form of starch and
cellulose. Glycogen in the liver is the prototypical appearance of carbohydrate polymer in
higher animals and humans.
Starch in plants and glycogen in animals and humans, can freely release their store of
potential energy through their breakdown without disturbing the structural integrity of
the organism they are part of. This makes these carbohydrates the ideal energy source.
Glycogen is broken down to glucose. Glucose is an intermediate energy-carrying
compound. Its breakdown yields the energy carrying compound ATP, which needs to be
used right away because it hydrolyzes readily. The energy can be used for organism
functions, varying from anabolic biochemical to transport to bio-mechanical processes.
Glucose is a reducing sugar in that its oxidation also provides the organism with the
energy needed for reductiveprocesses that build up the organism. It makes the synthesis of
compounds, tissues, organs, and organisms possible.
Characterization: water, carbohydrates and plants
Water plays an important role in carbohydrate structure and processes. The role of water
can be compared to the role of carbohydrates in organisms and the role of plants in
nature.
The relation between water and carbohydrates becomes visible in the process of
photosynthesis. The oxidation of water by sunlight in photosynthesis stands at the
beginning of the synthesis of carbohydrates in plants. The oxygen that is released in this
process is essential for the function of higher organisms.
H2O ➝ H+ + O2 ➝ carbohydrate synthesis + higher organism function
The role of carbohydrates in the metabolism is to make energy available through
carbohydrate breakdown for anabolic processes and the functions of the organism.
carbohydrates ➝ ATP and CO2 ➝ organism synthesis and organism function
The place of the plant in the whole of nature became visible in section 1.2.2. Plants are an
important part of the nutrient cycle in nature. They are the most important food supply of
many species. The typical function of plants in nature is to help make energy available in
higher organisms through their breakdown in the digestive system of higher animals and
humans.
plants ➝ metabolites and CO2 ➝ higher animal and human organism structure + function
Conclusion: The breakdown of water,which is accompanied by the conversionof sunlight to biochemical energy,provides indirectly for the energyrequired in living organisms. Thefunction of water in plants is similar tothe function of carbohydrates in higherorganisms is similar to the function ofplants in the whole of nature. Wateris basic to all life. Carbohydratemetabolism in organisms is “plant-like.”
The primary nutrients are used by the organism in a variety of ways. Their breakdown in
the digestion results in metabolites that can be utilized for further metabolic processes in
the organism. They can be used for the formation of large compounds that contribute to
the structure of the organism (like the carbohydrates in the cell wall of plants, proteins in
collagen, or the lipids in membranes). The metabolites can also be converted to
substances that regulate the organism and its functions (as for instance in immune
recognition, neurotransmission, or as enzymes). And they can be oxidized further to
provide energy for the above processes and bio-mechanical and active transport needs
through their ultimate breakdown, as happens for instance with glucose.
Carbohydrates, proteins, and lipids can each be used for a variety of different functions;
within limits they can also be converted one into the other with the help of intermediates
in the citric acid cycle, which interconnects their anabolic/catabolic cycles. And they each
have an area of characteristic activity, which will be discussed in the following chapters for
the respective compounds. This chapter deals with carbohydrate metabolism and function,
chapter 4 discusses protein metabolism and activity and chapter 5 the metabolism and
activity of the lipids.
Carbohydrates and water
In section 1.2.4 we found how the breakdown of water stands at the beginning of the
synthesis of carbohydrates in photosynthesis. This is one important process that shows the
relationship between carbohydrates and water. Water is oxidized to convert sunlight into
biochemical energy for the synthesis of carbohydrates from CO2:
6 CO2 + 6 H2O + light ( C6 H12 O6 (carbohydrates) + 6 O2
Animals and humans breathe in the O2 and use it to break down carbohydrates to CO2
and water in the citric acid cycle and oxidative phosphorylation (section 1.2.2.). This
oxidative catabolic process, together with the reductive anabolic process of
photosynthesis, forms a cycle in nature between plants and the organisms of animals and
human beings (see fig. 1.3) in which water is alternately oxidized and formed.
The water/carbohydrate connection is also found when sugar is burnt without the
addition of oxygen, i.e. made into carbon. This happens when sugar is heated up in a
closed container so that no oxygen can enter. The result will be carbon and water, the
latter escaping as steam. This phenomenon would imply that the general formula for
carbohydrate monomers could be written as Cn (H2O)n. This is the reason they were
originally given the name carbo-hydrate. The general formula for glucose for instance is C6
H12 O6.
Carbohydrates have a relation to water both in their structure and in their metabolic
processes. In the formation of polysaccharides, as in forming the peptide bonds in
proteins, one water molecule is released from glucose in forming the glycosidic bond with
every addition of a glucose molecule.
Functions of carbohydrate polymers
Carbohydrate polymers, like all large compounds, hold within their structure potential
energy that becomes available at their breakdown as metabolic and other energy for the
organism. Many common carbohydrates are polysaccharides.
Polysaccharides are polymers of sugars (see section 2.1.1.). The most commonly occurring
monomer is glucose (section 3.3.). Carbohydrates also have structural functions.
Storage function
Glycogen and starch are storage forms for glucose. Glycogen is usually more branchedthan starch, which allows for the glucose to be cleaved off at more points at the sametime. Therefore glycogen, which is the mainglucose polymer in higher organisms, cangive a greater rate of supply of glucose indemand situations than starch, whichexists in plants only, would be able to.Animals and humans have more glucosequicker at their demand than plants. Asmentioned before in section 2.1.1. glycogendoes not occur in heart and brain cells. Itoccurs mainly in liver and muscle cells, butonly the liver can convert lactate toglucose. Muscle cells are dependent on theliver for gluconeogenesis (see the Cori cycle, section 1.2.1.) and the serum glucose level isprimarily buffered by the liver. The liver’s carbohydrate function is prototypical in higheranimals and humans.Glycogen has no structural support function in higher organisms and therefore can bebroken down without affecting their structure. Glycogen stores in the liver form an idealenergy supply for the organism. However, human glycogen stores are estimated to bedepleted after 10-15 hours of fasting and therefore need constant replenishing.
Amylopectin Glycogen
(Campbell, 1999)
Structural function
Cellulose is a structural glucose polymer in plants (see fig. 2.2.). Structural carbohydrates
in non-plants have amino acids or contain amino acid sequences as monomers to form
chitin or peptidoglycans (section 2.1.1.). The breakdown of cellulose does affect the
structure of the plant, as does the breakdown of peptidoglycans by antibiotics affect the
structure of bacteria, and the breakdown of chitin affects the structure of invertebrates
that carry it in their exoskeleton.
Glucose cannot be cleaved off of cellulose without the presence of special enzymes called
cellulases, which are found in bacteria and in the digestive tract of, for instance, cattle.
Humans do not have cellulases in their digestive tract. Cellulose is the main insoluble part
of dietary fiber (bulk or roughage) in our diet, which stimulates peristalsis and binds some
of the potentially toxic substances in food.
Glucose and Energy
Glucose is the most abundant of the carbohydrate monomers in nature. Glucose
metabolism is central to the energy supply of living organisms. It can be transported in
the body to the place where energy is required.
Anaerobic breakdown of glucose in glycolysis yields pyruvate or, under lasting anaerobic
conditions, lactate and 2 molecules of ATP per glucose molecule. Its further aerobic
breakdown occurs in the citric acid cycle (section 1.2.3.), which can yield 30 more ATP
molecules. The catabolism of glucose releases energy, which results in the
phosphorylation of ADP to ATP. ATP is the carrier of immediate energy for organisms. It
contains two phosphoanhydride bonds linking the phosphates. These bonds hydrolyze
easily, with a consequent immediate release of the energy (less than 1 minute). As such, it
is directly available biochemical energy, which must, however, be used “right away”. It can
be used to convert the biochemical energy to bio-mechanical energy (as in the case of the
exercising muscle), bio-electrical energy (as in the nervous system), light (as in
phosphorescent bacteria), active transport, etc.
The freed up energy from glycolysis and oxidation in the citric acid cycle can also be used
in the form of ATP for reductive anabolic processes that result in compounds such as
proteins and lipids. Glucose is a reducing sugar. When glucose is oxidized, another
compound can be reduced in an anabolic reaction with the energy that becomes available
(transferred via ATP).
Fig. 3.1 The conversion of energy-carrying compounds to energy-requiring processes
Since it is not needed in the structures of the organism, glucose and glycogen can be
broken down without consequences for the structure of the body. Thus glucose becomes
the main supplier of energy for living organisms. The prototypical function of glucose in
organisms is to provide energy.
Summary and conclusion
Carbohydrate functions
Carbohydrates can be used for a variety of functions in organisms. They serve different
functions in plants than in higher organisms. Plants are the only organisms that use
homopolysaccharides to build up their structural components, as well as using starch and
glucose for an energy source. In lower animals carbohydrates can also have structural
functions. Then their conformation includes amino acids or amino acid derivatives. In
higher organisms carbohydrates only provide energy for the metabolic needs of the
organism. They do not provide structural support. Carbohydrates serve different functions
in plants than in higher organisms.
The prototypes of carbohydrate polymer appear in plants, in the form of starch and
cellulose. Glycogen in the liver is the prototypical appearance of carbohydrate polymer in
higher animals and humans.
Starch in plants and glycogen in animals and humans, can freely release their store of
potential energy through their breakdown without disturbing the structural integrity of
the organism they are part of. This makes these carbohydrates the ideal energy source.
Glycogen is broken down to glucose. Glucose is an intermediate energy-carrying
compound. Its breakdown yields the energy carrying compound ATP, which needs to be
used right away because it hydrolyzes readily. The energy can be used for organism
functions, varying from anabolic biochemical to transport to bio-mechanical processes.
Glucose is a reducing sugar in that its oxidation also provides the organism with the
energy needed for reductiveprocesses that build up the organism. It makes the synthesis of
compounds, tissues, organs, and organisms possible.
Characterization: water, carbohydrates and plants
Water plays an important role in carbohydrate structure and processes. The role of water
can be compared to the role of carbohydrates in organisms and the role of plants in
nature.
The relation between water and carbohydrates becomes visible in the process of
photosynthesis. The oxidation of water by sunlight in photosynthesis stands at the
beginning of the synthesis of carbohydrates in plants. The oxygen that is released in this
process is essential for the function of higher organisms.
H2O ➝ H+ + O2 ➝ carbohydrate synthesis + higher organism function
The role of carbohydrates in the metabolism is to make energy available through
carbohydrate breakdown for anabolic processes and the functions of the organism.
carbohydrates ➝ ATP and CO2 ➝ organism synthesis and organism function
The place of the plant in the whole of nature became visible in section 1.2.2. Plants are an
important part of the nutrient cycle in nature. They are the most important food supply of
many species. The typical function of plants in nature is to help make energy available in
higher organisms through their breakdown in the digestive system of higher animals and
humans.
plants ➝ metabolites and CO2 ➝ higher animal and human organism structure + function
Conclusion: The breakdown of water,which is accompanied by the conversionof sunlight to biochemical energy,provides indirectly for the energyrequired in living organisms. Thefunction of water in plants is similar tothe function of carbohydrates in higherorganisms is similar to the function ofplants in the whole of nature. Wateris basic to all life. Carbohydratemetabolism in organisms is “plant-like.”
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