If you set a potato on fire, the energy of the potato released as heat can make a glass
of water boil.
Now, instead of igniting your potato, eat it.
Just like the energy from the burning potato boiled the water, the potato you ate will
go on to fuel you.
Except, there are no flames and heat.
Instead, the body, very slowly, breaks the potato down into basic molecules called nutrients
to supply your body with energy.
With this energy you can do maths, run, lift weights, and even laze on the couch.
So, how does the body use that potato to create energy?
Cellular respiration.
Body What is cellular respiration?
Cellular respiration is how your cells convert the food you eat, through a multi-step process
into energy that your cells can use.
In a snapshot, this is what respiration looks like:
C6H12O6 + 6O2 → ATP + 6CO2 + 6H20 Many organisms, like animals, plants, and
some bacteria eat food and breathe in oxygen to generate energy - ATP, and two by-products:
carbon dioxide and water, which the cell eventually throws out.
Some organisms like some bacteria can produce ATP without oxygen.
This is called anaerobic respiration.
Before we go ahead with the process of cellular respiration, let’s look at ATP.
Adenosine Triphosphate is the cell's energy currency.
Just like how you might exchange money for a shiny new “thing”, the cell spends ATP
in exchange for energy which it then uses to do some work.
Chemically, ATP stores energy through the phosphates.
The three phosphates hanging like a tail at the end of the adenosine molecule are negatively
charged and since they all have the same charge, they repel each other.
So, when an enzyme breaks the bond holding the two phosphates together, it relieves some
of the electrostatic repulsion, which results in a release of energy that the cell uses
to do work.
This two phosphate molecule is ADP.
ADP will be recycled to make more ATP by adding another phosphate to it.
How does cellular respiration happen?
Your cells love sugars — carbohydrates — particularly glucose, and will preferably use that to generate
ATP.
In fact, the brain almost exclusively runs on sugar.
In case glucose runs out, cells move on to breaking down fats, and as a last resort,
proteins.
One glucose molecule can give up to 32 molecules of ATP through cellular respiration.
This is only an estimate and scientists are still fine tuning this number, but it is the
number we’ll use.
pause Converting glucose into ATP occurs in three
steps Glycolysis
The Krebs cycle And finally a process called Oxidative phosphorylation.
These processes are nothing but chemical reactions.
But instead of happening in a test tube, they happen in our cells.
Most of these reactions are very slow and so cells have molecules called enzymes that
speed everything up.
Enzymes are nothing but catalysts, chemical matchmakers that bring the two molecules required
for reaction and help them react.
There are more than a 100 enzymes involved in cellular respiration and taking all their
names would take a very long time.
So, we’ll skip most of the enzymes, except a few crucial ones.
Step 1: Glycolysis Imagine eating your morning toast.
The digestive system breaks down the bread into smaller nutrients.
Through the bloodstream, the glucose makes its way into the cells where glycolysis takes
place.
The goal of glycolysis is to break down the six-carbon glucose into two three-carbon molecules
called pyruvic acid or pyruvates...and...produce some ATP in the process.
Just like how a business needs some starting investment money before it can turn a profit,
the cell cell first invests some energy -- 2 ATP molecules — to begin glycolysis.
Later reactions yield energy — 4 ATP molecules, giving a total net energy profit of 2 ATP
molecules.
Glycolysis also produces two molecules of NADH.
We’ll come to NADH in the next section.
Step 2: Krebs Cycle Some bacteria and eukaryotes like yeast don't
perform the Krebs cycle in certain conditions.
They instead perform fermentation, converting the pyruvate into various other chemicals
like lactic acid and alcohols.
In fact, human muscles also produce lactic acid through a similar route when they don't
get enough oxygen.
This is why your muscles feel sore after you exercise.
But, in the organisms that do perform the krebs cycle, the pyruvates will make their
way to the POWERHOUSE OF THE CELL, THE MITOCHONDRIA!
A rapid overview of the mitochondria: two membranes, outer and inner, between them is
a fluid filled space called intermembrane space.
The inner membrane folds into squiggles called cristae.
The inside of the mitochondria is the matrix.
In the matrix, the pyruvate is converted into a two-carbon molecule called Acetyl coenzyme
A or simply acetyl CoA.
This molecule will now enter the Krebs cycle.
Acetyl CoA will react with the starting molecule -- oxaloacetate to create citric acid, which
is why this process is also called the citric acid cycle.
After this, a series of 8 reactions occur which produce by-products like carbon dioxide
and water, and the process regenerates oxaloacetate once more, which is why it is called a cycle.
Add a new acetyl CoA molecule and the cycle will begin anew.
But, the most important product that the Krebs cycle yields is NADH and FADH2.
So, what are these and what do they do?
Chemically, they are nothing but the vitamins B3 Niacin and the vitamin 2 riboflavin.
Chemically, they hold with them "energised electrons" which can be used to generate ATP.
Think of it like this.
You're exchanging gold for its weight in money.
NADH gives 2.5 ATP molecules and FADH2 gives 1.5 ATP molecules.
One cycle of the Krebs cycle yields 3 NADH and 1 FADH2.
Step 3: The Electron Transport Chain and Oxidative phosphorylation
To cash in these molecules and get ATP, the NADH and FADH2 must donate their electrons
to the electron transport chain.
The electron transport chain is made of 4 large protein complexes in the inner mitochondrial
membrane.
They play pass the parcel with the electrons.
As they pass the electron, the protein complexes transfer protons from the matrix to the intermembrane
space.
This creates an electrochemical gradient: there are more charged protons in the intermembrane
space than in the matrix.
The gradient has chemical potential energy and balancing this gradient is what creates
ATP through the enzyme ATP synthase.
Perhaps the coolest enzyme in the entire cell, ATP synthase converts ADP to ATP by harnessing
the gradient's stored energy.
This contraption spins as it allows protons back into the matrix and in doing so harnesses
the chemical potential energy to generate ATP.
The final electron acceptor is oxygen.
Without oxygen to accept the electron, the whole system gets clogged and ATP synthesis
will halt.
Cyanide acts as a poison by blocking electron transfer to oxygen.
Eventually, cells can’t produce enough ATP and die.
Results of cellular respiration Energy in the form of ATP (for cell to do
work) So, how much total energy do we get from one
cycle of cellular respiration.
If we do the math from the number here, it should be about 32.
But...this is just an estimate.
There is other research providing other numbers and research is still going on.
Intermediates chemicals.
The Krebs cycle creates many molecules that are intermediates.
They can be used as starting molecules to create other molecules which can go on to
become a part of proteins and nucleic acids.
CO2, H2O and heat (former is exhaled) Like in combustion, respiration produces CO2,
water and some heat as by products.
We mostly discard this waste.
The heat produced from respiration is sometimes used by animals in winter to heat their bodies
up.