Anabolism and catabolism are opposite processes, and yet one is not simply the reverse ofthe other. Usually several key reactions of the anabolic pathway need different enzymesand/or produce different intermediate compounds than those used in catabolism.Anabolism and catabolism can take place in the same cell, but they often take place in different cell compartments (such as lipid anabolism in the cytosol and lipid catabolism inthe mitochondrion) and probably at different times. Anabolism and catabolism are notsimply opposites but have a cyclic interaction, whereby the one follows the other andprepares for the other. There is no catabolism without there first being something built upthat can be broken down. There is no anabolism without there being energy fromcatabolism to be built into larger structures.
A biochemical cycle in the organism as a whole
different cell compartments (such as lipid anabolism in the cytosol and lipid catabolism inthe mitochondrion) and probably at different times. Anabolism and catabolism are notsimply opposites but have a cyclic interaction, whereby the one follows the other andprepares for the other. There is no catabolism without there first being something built upthat can be broken down. There is no anabolism without there being energy fromcatabolism to be built into larger structures.
A biochemical cycle in nature
A carbohydrate metabolic cycle occursbetween organisms in nature.Green plants reduce carbon dioxide andwater to carbohydrates and oxygen in theprocess called photosynthesis. They buildup their organism with the carbohydratesformed in photosynthesis and releaseoxygen. This is a reductive, anabolic processin the plant using external sunlight as theenergy source.The oxygen and the plant carbohydrates produced in photosynthesis are used up byhigher organisms in oxidative processes that break down the carbohydrates in thedigestive process. The metabolic cycle that exists between plants on the one hand, andhumans and animals on the other, involves different organisms in nature.The overall role of animal and human metabolism in nature is catabolic. Plant metabolismplays a prototypically anabolic role in nature and provides the energy required by theoverall catabolic metabolism of higher organisms
The citric acid cycle
A carbohydrate metabolic cycle occursbetween organisms in nature.Green plants reduce carbon dioxide andwater to carbohydrates and oxygen in theprocess called photosynthesis. They buildup their organism with the carbohydratesformed in photosynthesis and releaseoxygen. This is a reductive, anabolic processin the plant using external sunlight as theenergy source.The oxygen and the plant carbohydrates produced in photosynthesis are used up byhigher organisms in oxidative processes that break down the carbohydrates in thedigestive process. The metabolic cycle that exists between plants on the one hand, andhumans and animals on the other, involves different organisms in nature.The overall role of animal and human metabolism in nature is catabolic. Plant metabolismplays a prototypically anabolic role in nature and provides the energy required by theoverall catabolic metabolism of higher organisms mitochondrial membrane to convert theenergy to ATP with the help of the reductionof O2 to H2O. These are for instance NADH(the reduced form of nicotinamide adeninedinucleotide) and FADH2 (the reduced formof flavin adenine dinucleotide). The citricacid cycle is also the starting point forgluconeogenesis. And it provides intermediatesfor the synthesis of proteins andlipids and for the heme group ofhemoglobin. In the citric acid cycle anabolicand catabolic pathways connect. Cyclesinterconnect with other cycles with the citricacid cycle at the center.
Acetyl-CoA is possibly the key molecule in metabolism (see also sections 2.1.3 and chapter
5). In addition to being the starting molecule for the citric acid cycle it is also the starting
point of fatty acid and cholesterol synthesis.
In bacteria and plants, acetyl-CoA can be converted to oxaloacetate and otherintermediates of the citric acid cycle via the glyoxylate pathway and as such can becomethe starting point for the synthesis of both amino acids and carbohydrates. The glyoxylatepathway is not available in mammals, which precludes them from converting fats tocarbohydrates. This is the reason that mammals can not exist on a diet that contains onlyfats. Many bacteria use just acetic acid via a conversion to acetyl-CoA for the synthesis oftheir organism’s compounds.
Oxaloacetate must be kept at specifically sufficient levels in themitochondrion to allow acetyl-CoA to enter the citric acid cycle.Oxaloacetate is also the starting point of gluconeogenesis.
Biochemical cycles in time
The emphasis on either anabolic or catabolic processes in biochemical cycles is different
at different times of the day (circadian rhythm) and at different times in the life cycle of
an organism.
Photosynthesis in plants
Photosynthesis mainly occurs in the leaves of green plants. It actually represents two
processes, the light reactions and the dark reaction.
1. The light reactions:
The light reactions occur under the influence of light which is absorbed by chlorophyll
(chlorophyll is a biochemical compound in plants similar to hemoglobin in the blood of
higher organisms). The light reactions involve the oxidation of water to produce oxygen.
H2O ➝ 2H+ +1/2 O2
The energy freed in this catabolic process is captured in the plant, via the conversion of
NADP+ to NADPH, by the photophosphorylation of adenosine diphosphate (ADP) to ATP,
a conversion that is coupled to the oxidation of water. The second light reaction is:
ADP + Pinorganic ➝ ATP
ATP is an energy-carrying compound and it represents a biochemical form of directly
available energy (see also section 3.3.). Under normal circumstances the light reactions of
photosynthesis are daytime processes. The light reactions use solar energy to free the energy that holds together the water molecule, which in turn is converted via NADPH to
ATP.
The light reactions convert sunlight to biochemical energy in the plant.
Light drives the transfer of protons from one substance to another in a thermodynamically
uphill direction.
2. The dark reaction:
The second process of photosynthesis is the dark reaction. It involves the fixation of CO2
for the production of sugars:
6 CO2 + 12 H+ ➝ [C6 H12 O6] + 3 O2
The energy for this anabolic process comes from the ATP formed in the light reactions. The
dark reaction results in the formation of disaccharides and polysaccharides (starch and
cellulose). As the name indicates, this reaction is not directly dependent on sunlight, only
indirectly.
The overall equation for photosynthesis is:
6 CO2 + 6 H2O + sunlight ➝ carbohydrate (C6 H12O6) + 6 O2
Chlorophyll in the plant absorbs the sunlight during the day and makes it directly
available for the light reactions and indirectly, converted to chemical energy, for the dark
reaction of photosynthesis. Plants are our example for using solar energy!
The plant builds up its organism in the dark reaction of photosynthesis, it grows visibly as
a result of it. The light reactions do not result in visible growth but in energy production,
which is invisible to the naked eye. The visible growth of the plant takes place where the
light does not shine. The growth of plants towards the sun and the intricate process of the
turning of the sunflower’s head towards the light are based on more intensive growth on
the side of the plant turned away from the sun, i.e. where the dark reaction can take place.
On the illumined side of the plant, solar energy is converted to biochemical energy as
water is oxidized.
The light reactions of photosynthesis cannot take place at night unless an artificial light
source is provided. Plant metabolism is linked to the cycle of day and night. The inner time
clock of plants is normally set by the rhythm of the sun’s light. Plant rhythms can be easily
influenced by changing their exposure to light as is done in artificially lit greenhouses.
QUESTION: When does the plant grow more, during the day
or at night?
A biochemical cycle in the organism as a whole
different cell compartments (such as lipid anabolism in the cytosol and lipid catabolism inthe mitochondrion) and probably at different times. Anabolism and catabolism are notsimply opposites but have a cyclic interaction, whereby the one follows the other andprepares for the other. There is no catabolism without there first being something built upthat can be broken down. There is no anabolism without there being energy fromcatabolism to be built into larger structures.
A biochemical cycle in nature
A carbohydrate metabolic cycle occursbetween organisms in nature.Green plants reduce carbon dioxide andwater to carbohydrates and oxygen in theprocess called photosynthesis. They buildup their organism with the carbohydratesformed in photosynthesis and releaseoxygen. This is a reductive, anabolic processin the plant using external sunlight as theenergy source.The oxygen and the plant carbohydrates produced in photosynthesis are used up byhigher organisms in oxidative processes that break down the carbohydrates in thedigestive process. The metabolic cycle that exists between plants on the one hand, andhumans and animals on the other, involves different organisms in nature.The overall role of animal and human metabolism in nature is catabolic. Plant metabolismplays a prototypically anabolic role in nature and provides the energy required by theoverall catabolic metabolism of higher organisms
The citric acid cycle
A carbohydrate metabolic cycle occursbetween organisms in nature.Green plants reduce carbon dioxide andwater to carbohydrates and oxygen in theprocess called photosynthesis. They buildup their organism with the carbohydratesformed in photosynthesis and releaseoxygen. This is a reductive, anabolic processin the plant using external sunlight as theenergy source.The oxygen and the plant carbohydrates produced in photosynthesis are used up byhigher organisms in oxidative processes that break down the carbohydrates in thedigestive process. The metabolic cycle that exists between plants on the one hand, andhumans and animals on the other, involves different organisms in nature.The overall role of animal and human metabolism in nature is catabolic. Plant metabolismplays a prototypically anabolic role in nature and provides the energy required by theoverall catabolic metabolism of higher organisms mitochondrial membrane to convert theenergy to ATP with the help of the reductionof O2 to H2O. These are for instance NADH(the reduced form of nicotinamide adeninedinucleotide) and FADH2 (the reduced formof flavin adenine dinucleotide). The citricacid cycle is also the starting point forgluconeogenesis. And it provides intermediatesfor the synthesis of proteins andlipids and for the heme group ofhemoglobin. In the citric acid cycle anabolicand catabolic pathways connect. Cyclesinterconnect with other cycles with the citricacid cycle at the center.
Acetyl-CoA and the citric acid cycle
Acetyl-CoA is the molecule that starts the citric acid cycle by binding to its ”end” productoxaloacetate. All metabolites of carbohydrates and lipids enter this cycle as acetyl-CoA,and it is the link to ultimate oxidative breakdown for many amino acids.Acetyl-CoA is possibly the key molecule in metabolism (see also sections 2.1.3 and chapter
5). In addition to being the starting molecule for the citric acid cycle it is also the starting
point of fatty acid and cholesterol synthesis.
In bacteria and plants, acetyl-CoA can be converted to oxaloacetate and otherintermediates of the citric acid cycle via the glyoxylate pathway and as such can becomethe starting point for the synthesis of both amino acids and carbohydrates. The glyoxylatepathway is not available in mammals, which precludes them from converting fats tocarbohydrates. This is the reason that mammals can not exist on a diet that contains onlyfats. Many bacteria use just acetic acid via a conversion to acetyl-CoA for the synthesis oftheir organism’s compounds.
Oxaloacetate must be kept at specifically sufficient levels in themitochondrion to allow acetyl-CoA to enter the citric acid cycle.Oxaloacetate is also the starting point of gluconeogenesis.
Biochemical cycles in time
The emphasis on either anabolic or catabolic processes in biochemical cycles is different
at different times of the day (circadian rhythm) and at different times in the life cycle of
an organism.
Photosynthesis in plants
Photosynthesis mainly occurs in the leaves of green plants. It actually represents two
processes, the light reactions and the dark reaction.
1. The light reactions:
The light reactions occur under the influence of light which is absorbed by chlorophyll
(chlorophyll is a biochemical compound in plants similar to hemoglobin in the blood of
higher organisms). The light reactions involve the oxidation of water to produce oxygen.
H2O ➝ 2H+ +1/2 O2
The energy freed in this catabolic process is captured in the plant, via the conversion of
NADP+ to NADPH, by the photophosphorylation of adenosine diphosphate (ADP) to ATP,
a conversion that is coupled to the oxidation of water. The second light reaction is:
ADP + Pinorganic ➝ ATP
ATP is an energy-carrying compound and it represents a biochemical form of directly
available energy (see also section 3.3.). Under normal circumstances the light reactions of
photosynthesis are daytime processes. The light reactions use solar energy to free the energy that holds together the water molecule, which in turn is converted via NADPH to
ATP.
The light reactions convert sunlight to biochemical energy in the plant.
Light drives the transfer of protons from one substance to another in a thermodynamically
uphill direction.
2. The dark reaction:
The second process of photosynthesis is the dark reaction. It involves the fixation of CO2
for the production of sugars:
6 CO2 + 12 H+ ➝ [C6 H12 O6] + 3 O2
The energy for this anabolic process comes from the ATP formed in the light reactions. The
dark reaction results in the formation of disaccharides and polysaccharides (starch and
cellulose). As the name indicates, this reaction is not directly dependent on sunlight, only
indirectly.
The overall equation for photosynthesis is:
6 CO2 + 6 H2O + sunlight ➝ carbohydrate (C6 H12O6) + 6 O2
Chlorophyll in the plant absorbs the sunlight during the day and makes it directly
available for the light reactions and indirectly, converted to chemical energy, for the dark
reaction of photosynthesis. Plants are our example for using solar energy!
The plant builds up its organism in the dark reaction of photosynthesis, it grows visibly as
a result of it. The light reactions do not result in visible growth but in energy production,
which is invisible to the naked eye. The visible growth of the plant takes place where the
light does not shine. The growth of plants towards the sun and the intricate process of the
turning of the sunflower’s head towards the light are based on more intensive growth on
the side of the plant turned away from the sun, i.e. where the dark reaction can take place.
On the illumined side of the plant, solar energy is converted to biochemical energy as
water is oxidized.
The light reactions of photosynthesis cannot take place at night unless an artificial light
source is provided. Plant metabolism is linked to the cycle of day and night. The inner time
clock of plants is normally set by the rhythm of the sun’s light. Plant rhythms can be easily
influenced by changing their exposure to light as is done in artificially lit greenhouses.
QUESTION: When does the plant grow more, during the day
or at night?
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