Glucose
Glucose (C6H12O6), also known as D-glucose, dextrose, grape sugar, and the more
common corn sugar or blood sugar, is found in everyday life. Andreas Sigismund Marggraf, a
German chemist from Berlin and a pioneer of analytical chemistry, was the first to extract
glucose out of beets using a method with alcohol. It was later purified by his student, Franz
Achard. This discovery leads the way to much advancement in the field of organic chemistry
because of this molecules importance in many organisms. Glucose may seem like a normal
molecule, but it has much significance, not only in humans, but also in animals, plants, and
everyday life. I would put the significance of glucose on the first paragraph to get the reader’s
attention.
This significant molecule has a molar mass of 180.15g/mole, due to the more massive
oxygen, a density of 1.54 g/cm3, and it can be found in the form of a white powder. Glucose is
always colorless. It is very soluble in water as well as acetic acid and several other solvents and
it can be crystallized from these solvents into three major forms: alpha-glucopyranose, betaglucopyranose, and beta-glucopyranose hydrate.
Glucose is essential in many life processes, in the human body, and in other living
creatures. These include plants, diabetes, glycogen, gluconeogenesis, glycolysis, alpha-beta
adrenergic receptors, metabolism, glucose uptake in the liver and muscle, and in certain diseases.
In plants, glucose is stored as starch, which is repeating units of sugar. Plants produce sugar by
converting carbon dioxide and water in the presence of light. This process is important in all
photosynthetic organisms so that they may produce this essential molecule to be consumed in our
diet. Glucose also serves some major purposes in plants. Starch is the major component in cell
walls of photosynthetic cells, providing structure. If you can break down plant fibers, they are
very nutritious due to the hundreds of thousands of sugar molecules bounded together. Not only
humans, but plants also use glucose as an energy source. However, plants use it during periods
of darkness. Photosynthetic organisms still require energy when it cannot perform
photosynthesis. They acquire it through aerobic respiration using oxygen and glucose and
breaking it down to water and carbon dioxide.
A key biological process that requires glucose is glycolysis. Glycolysis is the breakdown
of glucose into pyruvate with a net production of 2 ATP and the reduction of 2 NAD+ to NADH.
The entire process is energetically downhill and mediated by 10 enzymes. Variable
concentrations of AMP and ADP control glycolysis by phosphorylation and dephosphorylation
of phosphofructokinase. Glycolysis is the breakdown of glucose into pyruvate with a net
production of 2 ATP and the reduction of 2 NAD+ to NADH. The entire process is energetically
downhill and mediated by 10 enzymes. Variable concentrations of AMP and ADP control
glycolysis by phosphorylation and dephosphorylation of phosphofructokinase.
Another key process that requires glucose is gluconeogenesis. The main site of
gluconeogenesis is the liver in animals. Under certain conditions, it occurs in the small intestine
and the kidney to a more limited extent. This process forms glucose from fats and proteins,
substances other than carbohydrates. Gluconeogenesis is described by the following reaction: 2
pyruvate + 2 NADH + 4 ATP + 2 GTP à Glucose + 2 NAD+ + 4 ADP + 2 GDP + 6 Pi. Pyruvate
Carboxylase expends ATP and bicarbonate to convert pyruvate into oxaloacetate. GTP’s high
energy phosphate bond is broken by PEP Carboxykinase to produce phosphoenolpyruvate. It
then turns into 2-phosphoglycerate with the release of water from Enolase. 2-phosphoglycerate
turns into 3-phosphoglycerate by Phosphoglycerate Mutase. Phosphoglycerate Kinase uses ATP
to convert 3-phosphoglycerate into 1,3-Bisphosphoglycerate. It then becomes glyceraldehyde 3phosphate from Glyceraldehyde-3-phosphate Dehydrogenase with the help of
NADH. Glyceraldehyde-3-phosphate can be interchanged with dihydroxyacetone-phosphate by
Triosephosphate Isomerase. Aldolase converts glyceraldehyde-3-phosphate and
dihydroxyacetone-phosphate into fructose-1,6-bisphosphate. This molecule is then converted
into fructose-6-phosphate with the addition of water by fructose-1,6bisphosphatase. Phosphoglucose Isomerase transforms fructose-6-phosphate into glucose-6phosphate and then it finally becomes glucose with the addition of water from Glucose-6phosphatase. This pathway is spontaneous, but overall, it consumes six phosphate bonds of GTP
and ATP.
During fasting or starvation of carbohydrate, glucose can still be obtained. The source of
oxaloacetate or pyruvate for gluconeogenesis is obtained from amino acid catabolism. Protein
obtained from muscle may have to be broken down to supply the body with amino acids. They
are transferred to the liver and deaminated to convert to gluconeogenesis inputs. Fat cells also
contribute to gluconeogenesis inputs. They provide triacylglycerol, which is hydrolyzed to
glycerol. When the blood glucose level is low, the hormone glucagon triggers liver cells to
activates the effects of cyclic AMP cascade. Protein Kinase A or cAMP-Dependent Protein
Kinase stimulates gluconeogenesis by phosphorylating regulatory proteins and enzymes in the
liver.
To prevent gluconeogenesis and the production of glucose when it is unnecessary (there
is already a high concentration of glucose), fructose-1,6-bisphosphatase is inhibited by
AMP. When there is a high concentration of AMP and a low concentration of ATP, glucose will
not be synthesized because the cell would have to expend energy.
During exercise, when ATP is needed, the Cori Cycle occurs. A short explanation about
what the Cori Cycle is would be useful. In a short span of time, a lot of ATP is required to
produce energy for exercising. The high energy phosphate bond is taken from phosphocreatine
and used by muscle cells. When the supply of phosphocreatine is out, ATP is taken from
glycolysis, obtaining it from the glucose uptake from the body’s blood or from glycogen
breakdown.
The Cori Cycle is also operated during cancer. This cycle is activated when the body’s
metabolism is changed to anaerobic metabolism due to the underdevelopment of blood vessels
compared to the growth of a tumor. The tumor causes a decrease in the oxygen
concentration. Because of the tumor, this cycle spends six phosphate bonds from the liver in
order to produce two phosphate bonds from glycolysis, resulting in more energy spent than it is
gained. Even with a regular intake of food/glucose, the body is still losing weight in late-stage
cancer.
Another disease that glucose largely impacts is diabetes. Diabetes affects nearly 25.8
million children and adults in the United States Citation?. If left untreated, it can result in
blindness, multiple cardiovascular diseases, high blood pressure, and in serious cases,
amputation of limbs. People with this deadly disease must perform regular checks on their
glucose level in their blood. The most inexpensive and widely used way of monitoring glucose
level is to poke their fingers to draw the blood needed for the test. A 2011 report projected that
the global blood glucose test strips and meters market will reach US$21.5 billion by 2017.
I would add the normal blood glucose levels here.
Officially known as diabetes mellitus, this is a disease that is caused by a decreased
response to insulin in its target tissues or a deficiency of insulin. Diabetes mellitus is one of the
best-known endocrine disorders, which is marked by elevated levels of blood glucose. There
are two types of diabetes, type 1 diabetes mellitus and type 2 diabetes mellitus. Type 1 diabetes
mellitus is insulin-dependent. This is an autoimmune disorder that occurs when the immune
system destroys the beta cells in the pancreas. Type 2 diabetes mellitus is more common. This
type is non-insulin-dependent. This occurs when there is a deficiency in insulin or there is a
reduced response to the target cells because the insulin receptors have been changed or modified,
which will not recognize insulin at its receptors.
With glucose being so closely linked to diabetes, researchers around the world have been
tirelessly looking for ways to cure this disease. While no known cure was discovered, a great
deal of progress has been made on the tracking of glucose levels. Recently Google has teamed
up with researchers at the University of Washington to develop a contact lens that can measure
the blood glucose levels in a person’s tears and display the reading on their mobile phone. If this
forward-thinking contact lens project is successful, people with or without the disease may be
able to stop drawing blood to measure their sugar levels.
According to Google, the glucose-sensing contact lens will function similarly to an
electronic ID card used to gain access to a building. Like the card, the lens doesn’t contain its
own power supply. Instead, the antenna integrated in the contact lens will pick up the radio
waves from a mobile to provide it with a sustainable power to measure the glucose level. This is
one of the first steps in a new and innovative way towards measuring a person’s glucose level, a
step closer against fighting diabetes.
The technology in Google’s glucose lenses goes well beyond electronics – it contains
enzymes and electrodes built into the materials used to make regular contact lenses. This
combines advances made in biochemistry, electronics and material sciences during the past
decades.
So far the problem of this technological advancement is whether or not the sensor can
pick up an accurate reading of the glucose level. While the success of this new glucometer isn’t
guaranteed, this vast improvement in the control of blood glucose would provide health benefits
to diabetes patients. Also the potential expansion on other biological information that the sensor
may be able to obtain makes this glucose contact lens a promising and worth waiting product of
the future.
As an energy storage, glucose is stored in the body in the form of glycogen and broken
down when the body needs it. When glucose levels are low in the blood, the pancreas responds
by producing the hormone, glucagon from alpha cells. Glucagon acts as a ligand and binds to the
glucagon receptors on liver cells. Cyclic adenosine monophosphate acts as a messenger to
promote glycogen degradation into glucose, which brings the blood glucose level back to
homeostasis. When glucose levels are high in the blood, the pancreas responds by producing the
hormone, insulin produced by beta cells of the pancreas. Insulin acts as a ligand and binds to the
insulin receptors located on both muscle and liver cells. As glucose molecules enter both liver
and muscle cells via glucose transporter proteins, glucose is taken up as glycogen. Glycogen
degradation is also promoted when the hormone, epinephrine, binds to alpha-adrenergic
receptors on liver cells, which leads to increased cytosolic calcium ions. These events also bring
the body back to homeostasis.
Insulin from the liver directly stimulates glycogen synthesis by inhibiting the glycogen
synthase kinase 3-beta, which increases the activity of glycogen synthase by decreasing
phosphorylation. Glucose itself inhibits phosphorylase a through binding of the enzyme in its
inactive T state and producing a conformational changes, shifting from the R to T state. As a
result, this shift exposes the serine amino acid’s phosphate group, which will now be open to
dephosphorylation. The new enzyme is then called phosphorylase b. Therefore, a high glucose
concentration stimulates glycogen synthesis by converting phosphorylase a to phosphorylase b.
glycogen synthase is activated by the release of phosphoprotein phosphatase-1. The liver will
store excess glucose in the form of glycogen. Too much glycogen uptake by the liver will result
in a “fatty liver”.
Works Cited
Michael McDarby. Online Introduction to the Biology of Animals and Plants. The Basic Needs
for Photosynthesis. 2014. http://faculty.fmcc.suny.edu/mcdarby/Animals&PlantsBook/Plants/01Photosynthesis.htm
Joyce J. Diwan. Gluconeogenesis; Regulation of Glycolysis and Gluconeogenesis. Molecular
Biochemistry I. 2007.
https://www.rpi.edu/dept/bcbp/molbiochem/MBWeb/mb1/part2/gluconeo.htm#intro
TechCrunch. Google Unveils Smart Contact Lens That Lets Diabetics Measure Their Glucose
Levels. 2014. http://techcrunch.com/2014/01/16/google-shows-off-smart-contact-lens-that-letsdiabetics-measure-their-glucose-levels/
Google Blog. Introducing Our Smart Contact Lens Project. 2014.
http://googleblog.blogspot.com/2014/01/introducing-our-smart-contact-lens.html
Voet, Donald, Judith G. Voet, and Charlotte W. Pratt. Fundamentals of Biochemistry: Life at the
Molecular Level. Hoboken, NJ: Wiley, 2013. Print.