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Activity # 5
Metabolism of carbohydrates , fats ,proteins with oxygen
Carbohydrates are organic molecules composed of carbon, hydrogen, and oxygen
atoms. The family of carbohydrates includes both simple and complex sugars.
Glucose and fructose are examples of simple sugars, and starch, glycogen, and
cellulose are all examples of complex sugars. The complex sugars are also
called polysaccharides and are made of multiple monosaccharide molecules.
Polysaccharides serve as energy storage (e.g., starch and glycogen) and as structural
components (e.g., chitin in insects and cellulose in plants).
During digestion, carbohydrates are broken down into simple, soluble sugars that
can be transported across the intestinal wall into the circulatory system to be
transported throughout the body. Carbohydrate digestion begins in the mouth with
the action of salivary amylase on starches and ends with monosaccharides being
absorbed across the epithelium of the small intestine. Once the absorbed
monosaccharides are transported to the tissues, the process of cellular
respiration begins (Figure 1). This section will focus first on glycolysis, a process
where the monosaccharide glucose is oxidized, releasing the energy stored in its
bonds to produce ATP.
Glycolysis is the process of breaking down a glucose molecule into
two pyruvate molecules, while storing energy released during this process
as ATP and NADH. Nearly all organisms that break down glucose utilize
glycolysis. Glucose regulation and product use are the primary categories in which
these pathways differ between organisms. In some tissues and organisms, glycolysis
is the sole method of energy production. This pathway is common to both anaerobic
and aerobic respiration. Glycolysis consists of ten steps, split into two
phases. During the first phase, it requires the breakdown of two ATP
molecules. During the second phase, chemical energy from the intermediates is
transferred into ATP and NADH. The breakdown of one molecule of glucose results
in two molecules of pyruvate, which can be further oxidized to access more energy
in later processes. Glycolysis can be regulated at different steps of the process
through feedback regulation. The step that is regulated the most is the third step.
This regulation is to ensure that the body is not over-producing pyruvate molecules.
The regulation also allows for the storage of glucose molecules into fatty
acids. There are various enzymes that are used throughout glycolysis. The
enzymes upregulate, downregulate, and feedback regulate the process.
Krebs Cycle/Citric Acid Cycle/Tricarboxylic Acid Cycle
The pyruvate molecules generated during glycolysis are transported across the
mitochondrial membrane into the inner mitochondrial matrix, where they are
metabolized by enzymes in a pathway called the Krebs cycle (Figure 4). The Krebs
cycle is also commonly called the citric acid cycle or the tricarboxylic acid (TCA)
cycle. During the Krebs cycle, high-energy molecules, including ATP, NADH, and
FADH2, are created. NADH and FADH2 then pass electrons through the electron
transport chain in the mitochondria to generate more ATP molecules.
To start the Krebs cycle, citrate synthase combines acetyl CoA and oxaloacetate to
form a six-carbon citrate molecule; CoA is subsequently released and can combine
with another pyruvate molecule to begin the cycle again. The aconitase enzyme
converts citrate into isocitrate. In two successive steps of oxidative decarboxylation,
two molecules of CO2 and two NADH molecules are produced when isocitrate
dehydrogenase converts isocitrate into the five-carbon α-ketoglutarate, which is then
catalyzed and converted into the four-carbon succinyl CoA by α-ketoglutarate
dehydrogenase. The enzyme succinyl CoA dehydrogenase then converts succinyl
CoA into succinate and forms the high-energy molecule GTP, which transfers its
energy to ADP to produce ATP. Succinate dehydrogenase then converts succinate
into fumarate, forming a molecule of FADH2. Fumarase then converts fumarate into
malate, which malate dehydrogenase then converts back into oxaloacetate while
reducing NAD+ to NADH. Oxaloacetate is then ready to combine with the next
acetyl CoA to start the Krebs cycle again (see Figure 4). For each turn of the cycle,
three NADH, one ATP (through GTP), and one FADH2 are created. Each carbon of
pyruvate is converted into CO2, which is released as a byproduct of oxidative
(aerobic) respiration.
Oxidative Phosphorylation and the Electron Transport Chain
The electron transport chain (ETC) uses the NADH and FADH2 produced by the
Krebs cycle to generate ATP. Electrons from NADH and FADH2 are transferred
through protein complexes embedded in the inner mitochondrial membrane by a
series of enzymatic reactions. The electron transport chain consists of a series of four
enzyme complexes (Complex I – Complex IV) and two coenzymes (ubiquinone and
Cytochrome c), which act as electron carriers and proton pumps used to transfer
H+ ions into the space between the inner and outer mitochondrial membranes (Figure
5). The ETC couples the transfer of electrons between a donor (like NADH) and an
electron acceptor (like O2) with the transfer of protons (H+ ions) across the inner
mitochondrial membrane, enabling the process of oxidative phosphorylation. In
the presence of oxygen, energy is passed, stepwise, through the electron carriers to
collect gradually the energy needed to attach a phosphate to ADP and produce ATP.
The role of molecular oxygen, O2, is as the terminal electron acceptor for the ETC.
This means that once the electrons have passed through the entire ETC, they must
be passed to another, separate molecule. These electrons, O2, and H+ ions from the
matrix combine to form new water molecules. This is the basis for your need to
breathe in oxygen. Without oxygen, electron flow through the ETC ceases.
The electrons released from NADH and FADH2 are passed along the chain by each
of the carriers, which are reduced when they receive the electron and oxidized when
passing it on to the next carrier. Each of these reactions releases a small amount
of energy, which is used to pump H+ ions across the inner membrane. The
accumulation of these protons in the space between the membranes creates a proton
gradient with respect to the mitochondrial matrix.
Also embedded in the inner mitochondrial membrane is an amazing protein pore
complex called ATP synthase. Effectively, it is a turbine that is powered by the flow
of H+ ions across the inner membrane down a gradient and into the
mitochondrial matrix. As the H+ ions traverse the complex, the shaft of the complex
rotates. This rotation enables other portions of ATP synthase to encourage ADP and
Pi to create ATP. In accounting for the total number of ATP produced per glucose
molecule through aerobic respiration, it is important to remember the following
A net of two ATP are produced through glycolysis (four produced and two
consumed during the energy-consuming stage). However, these two ATP are
used for transporting the NADH produced during glycolysis from the
cytoplasm into the mitochondria. Therefore, the net production of ATP during
glycolysis is zero.
In all phases after glycolysis, the number of ATP, NADH, and
FADH2 produced must be multiplied by two to reflect how each glucose
molecule produces two pyruvate molecules.
In the ETC, about three ATP are produced for every oxidized NADH.
However, only about two ATP are produced for every oxidized FADH2. The
electrons from FADH2 produce less ATP, because they start at a lower point in
the ETC (Complex II) compared to the electrons from NADH (Complex I)
The Fatty Acid Oxidation Pathway Intersects the TCA Cycle
In 1904, Knoop, in a classic experiment, decisively showed that fatty acid oxidation
was a process by which two-carbon units were progressively removed from the
carboxyl end fatty acid molecule. The process consists of four reactions and
generates acetyl-CoA and the acyl-CoA molecule shortened by two carbons, with
the concomitant reduction of FAD by enzyme acyl-CoA dehydrogenase and of
NAD+ by β-hydroxyacyl-CoA dehydrogenase. This pathway is known as βoxidation because the β-carbon atom is oxidized prior to when the bond between
carbons β and α is cleaved The four steps of β-oxidation are continuously repeated
until the acyl-CoA is entirely oxidized to acetyl-CoA, which then enters the TCA
cycle. In the 1950s, a series of experiments verified that the carbon atoms of fatty
acids were the same ones that appeared in the acids of TCA cycle.
Amino Acid Transamination/Deamination Contributes to the TCA
Two points must be considered regarding the use of amino acids as fuels in energy
metabolism. The first is the presence of nitrogen in amino acid composition, which
must be removed before amino acids become metabolically useful. The other is that
there are at least twenty different amino acids, each of which requires a different
degradation pathway. For our purpose here, it is important to mention two kinds of
reactions involving amino acid: transamination and deamination. In the first kind of
reaction, the enzymes aminotransferases convert amino acids to their respective αketoacids by transferring the amino group of one amino acid to an α-ketoacid. This
reaction allows the amino acids to be interconverted. The second kind of reaction,
deamination, removes the amino group of the amino acid in the form of ammonia.
In the liver, the oxidative deamination of glutamate results in α-keto-glutarate (a
TCA cycle intermediate) and ammonia, which is converted into urea and excreted.
Deamination reactions in other organs form ammonia that is generally incorporated
into glutamate to generate glutamine, which is the main transporter of amino groups
in blood. Hence, all amino acids through transamination/deamination reactions can
be converted into intermediates of TCA cycle, directly or via conversion to pyruvate
or acetyl-CoA
(b) Exersices that burn fat:
(1) Shadow Boxing (To Burn Fat)
it’s one of the cardio exercises you can do at home or maybe anywhere when
you want.
It doesn’t require any equipment and it can help you burn a ton of calories!
you can burn up to 400 calories within 30-40 minutes of shadowboxing.
Obviously, if you have a budget to buy a good punching bag and/or an opponent
against whom you can box, then even better. This way you can increase the
number of calories burned by about 30-50%.
But the best part about it is Shadow-boxing helps you work on your abs!
Boxing, in general, is a very compound exercise because it essentially activates
almost every muscle in your body. Certain movements such as the uppercut and hook
really work on the upper-body rotation have an extreme impact on your obliques and
upper abs.
You can increase your calorie expenditure by adding movements that are of higher
intensity, such as knee kick, elbow hit (which also works your abs), spinning-backfist, and more.
(2) Rowing (The Fat Burning Exercise)
Rowing is one of the fat killer cardio exercises that speed up your heart rate,
entangling almost every muscle in the body.
The main downside is that you need a Rowing Machine or an actual boat and river
where you can, you know, row.
Reasons to love rowing machine exercise
You can burn up to 400 calories in an interval of 30 minutes.
It’s a multi-joint compound exercise that activates almost every muscle in the
It gives a very good pump for the feet, back and biceps.
Improves core strength.
Boosts muscle growth.
 It improves muscle coordination.
 It is perfect for muscle hydration (pump).
What is also really comfortable is that the machine allows you to adjust the force
and thus alter the intensity of the rowing.
This is perfect because it allows you to do very explosive HIIT in the gym.
As I have already mentioned, exercise kills three large muscle groups – the back,
legs, and abdomen.
How is that of any importance?
Essentially, large foundation muscles such as the legs, abs, chest, and back require
far more calories to move and recover than other smaller muscle groups like the
biceps and triceps.
(3) Burpee (Best Cardio To Burn Fat)
One of the simplest exercises that can be done anywhere at any time. That leaves
your heart thumping against your chest, your lungs on fire, and every muscle
completely exhausted.
Burpees are estimated to burn approximately 355 calories within a 30-minute timeframe.
The Burpee is definitely one of those cardio exercises that don’t just help you burn
fat but it will also help you build some muscle definition.
From a standing position, you go down into a squat. You put your hands to the
ground and keep them extended and you kick your feet behind you – you are now in
a plank position.
Instead of stopping there drop yourself to the ground and do a push-up. From there
as you push yourself back and you return back to your starting position as you jump
in the air you grab on a pull-up bar and you do a pull-up.
This is going to leave your body completely exhausted after a couple of repetitions.
There are also a couple of other really good variations of the classic Burpee that
can help you increase the intensity and burn some extra calories.