Akinyi Lester
Grade 10
Cellular respiration
Releasing energy
Respiration releases energy – it is an exothermic process.
Photosynthesis is the process by which plants, and some bacteria, synthesise food molecules
– which they then use, in addition to other things, for respiration. The process of
photosynthesis requires energy – it is endothermic.
Don't confuse respiration with breathing, which is ventilation. Respiration happens in cells.
Why organisms need energy
All organisms need energy to live. This energy is used:

to drive the chemical reactions needed to keep organisms alive – the reactions to build
complex carbohydrates, proteins and lipids from the products of photosynthesis in plants,
and the products of digestion in animals, require energy

movement – in animals, energy is needed to make muscles contract, while in plants, it is
needed for transport of substances in the phloem
Respiration is only around 40 per cent efficient. As animals respire, heat is also released. In
birds and mammals, this heat is distributed around the body by the blood. It keeps these
animals warm and helps to keep a constant internal temperature.
Energy is also used:

for cell division

to maintain constant conditions in cells and the body – homeostasis

to move molecules against concentration gradients in active transport

for the transmission of nerve impulses
Aerobic and anaerobic respiration
Aerobic respiration
Respiration using oxygen to break down food molecules is called aerobic respiration. Glucose
is the molecule normally used for respiration – it is the main respiratory substrate. Glucose
is oxidised to release its energy.
The word equation for aerobic respiration is:
glucose + oxygen → carbon dioxide + water + energy released
You need to be able to recognise the chemical symbols:
C6H12O6 + 6O2 → 6CO2 + 6H2O + energy released
Respiration is a series of reactions, but this summarises the overall process.
The first stages of respiration occur in the cytoplasm of cells, but most of the energy released
is in the mitochondria.
Anaerobic respiration
Most organisms cannot respire without oxygen but some organisms and tissues can continue
to respire if the oxygen runs out. These organisms and tissues use the process of anaerobic
respiration.
Human muscle can respire anaerobically for short periods of time – even though the process
is relatively inefficient, it's better to continue respiring and be able to run away from danger –
or run a race.
The glucose in muscle is converted to lactic acid:
glucose → lactic acid + energy released
Some plants, and some fungi such as yeast can respire anaerobically – it's preferable to
release less energy but remain alive.
Glucose in yeast cells is converted to carbon dioxide and ethanol, which we refer to simply as
'alcohol':
glucose → ethanol + carbon dioxide + energy released
Anaerobic respiration occurs only in the cytoplasm of cells.
Aerobic and anaerobic respiration compared:
Aerobic
Presence of
oxygen
Anaerobic
Present.
Absent or in short supply.
Complete
Incomplete. The products of respiration still contain
energy.
Products of
respiration
Carbon dioxide and water.
The products do not contain
stored chemical energy.
Mammalian muscle: lactic acid. Yeast: ethonol and
carbon dioxide. Some plants: ethonol and carbon
dioxide. The products still contain stored chemical
energy.
Amount of
energy
released
Relatively large amount.
Small amount, but quickly.
zOxidation of
glucose
Anaerobic respiration
Anaerobic respiration in yeast
Anaerobic respiration is economically important – many of our foods are produced by
microorganisms respiring anaerobically.
Yeast is used to make alcoholic drinks. When yeast cells are reproducing rapidly during beer
or wine production, the oxygen runs out. The yeast switches to anaerobic respiration.
Ethanol and carbon dioxide are produced.
Yeast can also be used to produce bread. Yeast respires using sugar added to the dough.
Bubbles of carbon dioxide make the bread rise. The alcohol that’s produced evaporates as
the bread is baked.
Anaerobic respiration in plants
Certain plants, and plant cells also respire anaerobically. These include plants that grow in
marshes, where oxygen concentrations will be low.
The Process of Cellular Respiration
Cellular respiration is the process of extracting energy in the form of ATP from the glucose in
the food you eat. How does cellular respiration happen inside of the cell? Cellular respiration
is a three step process. Briefly:
1. In stage one, glucose is broken down in the cytoplasm of the cell in a process
called glycolysis.
2. In stage two, the pyruvate molecules are transported into the mitochondria.
The mitochondria are the organelles known as the energy "powerhouses" of
the cells (Figure below). In the mitochondria, the pyruvate, which have been
converted into a 2-carbon molecule, enter the Krebs cycle. Notice
that mitochondria have an inner membrane with many folds, called cristae. These
cristae greatly increase the membrane surface area where many of the cellular
respiration reactions take place.
3. In stage three, the energy in the energy carriers enters an electron transport chain.
During this step, this energy is used to produce ATP.
Oxygen is needed to help the process of turning glucose into ATP. The initial step releases
just two molecules of ATP for each glucose. The later steps release much more ATP.
Glycolysis
Stage one of cellular respiration is glycolysis. Glycolysis is the splitting, or lysis of
glucose. Glycolysis converts the 6-carbon glucose into two 3-carbon pyruvatemolecules. This
process occurs in the cytoplasm of the cell, and it occurs in the presence or absence of
oxygen. During glycolysis a small amount of NADH is made as are four ATP. Two ATP are used
during this process, leaving a net gain of two ATP from glycolysis. The NADH temporarily
holds energy, which will be used in stage three.
What is glycolysis?
Glycolysis is a series of reactions that extract energy from glucose by splitting it into two
three-carbon molecules called pyruvates. Glycolysis is an ancient metabolic pathway,
meaning that it evolved long ago, and it is found in the great majority of organisms alive
today2,32,3start superscript, 2, comma, 3, end superscript.
In organisms that perform cellular respiration, glycolysis is the first stage of this process.
However, glycolysis doesn’t require oxygen, and many anaerobic organisms—organisms that
do not use oxygen—also have this pathway.
Highlights of glycolysis
Glycolysis has ten steps, and depending on your interests—and the classes you’re taking—
you may want to know the details of all of them. However, you may also be looking for a
greatest hits version of glycolysis, something that highlights the key steps and principles
without tracing the fate of every single atom. Let’s start with a simplified version of the
pathway that does just that.
Glycolysis takes place in the cytosol of a cell, and it can be broken down into two main
phases: the energy-requiring phase, above the dotted line in the image below, and the
energy-releasing phase, below the dotted line.

Energy-requiring phase. In this phase, the starting molecule of glucose gets rearranged, and
two phosphate groups are attached to it. The phosphate groups make the modified sugar—
now called fructose-1,6-bisphosphate—unstable, allowing it to split in half and form two
phosphate-bearing three-carbon sugars. Because the phosphates used in these steps come
from ATP, two ATP molecules get used up.
The Krebs Cycle
In the presence of oxygen, under aerobic conditions, pyruvate enters the mitochondria to
proceed into the Krebs cycle. The second stage of cellular respiration is the transfer of the
energy in pyruvate, which is the energy initially in glucose, into two energy carriers, NADH
and FADH2. A small amount of ATP is also made during this process. This process occurs in a
continuous cycle, named after its discover, Hans Krebs. The Krebs cycle uses a 2-carbon
molecule (acetyl-CoA) derived from pyruvate and produces carbon dioxide.
Overview of the KREBS CYCLE
In eukaryotes, the citric acid cycle takes place in the matrix of the mitochondria, just like the
conversion of pyruvate to acetyl CoA. In prokaryotes, these steps both take place in the
cytoplasm. The citric acid cycle is a closed loop; the last part of the pathway reforms the
molecule used in the first step. The cycle includes eight major steps.
In the first step of the cycle, acetyl CoA combines with a four-carbon acceptor molecule,
oxaloacetate, to form a six-carbon molecule called citrate. After a quick rearrangement, this
six-carbon molecule releases two of its carbons as carbon dioxide molecules in a pair of
similar reactions, producing a molecule of NADH each time. The enzymes that catalyze these
reactions are key regulators of the citric acid cycle, speeding it up or slowing it down based
on the cell’s energy 222 .
The remaining four-carbon molecule undergoes a series of additional reactions, first making
an ATP molecule—or, in some cells, a similar molecule called GTP—then reducing the
electron carrier FAD to FADH2 and finally generating another NADH. This set of reactions
regenerates the starting molecule, oxaloacetate, so the cycle can repeat.
Overall, one turn of the citric acid cycle releases two carbon dioxide molecules and produces
three NAD, one FADH2, and one ATP or GTP. The citric acid cycle goes around twice for each
molecule of glucose that enters cellular respiration because there are two pyruvates—and
thus, two acetyl CoA —made per glucose.
The Electron Transport Chain
Stage three of cellular respiration is the use of NADH and FADH2 to generate ATP. This occurs
in two parts. First, the NADH and FADH2 enter an electron transportchain, where their energy
is used to pump, by active transport, protons (H+) into the intermembrane space of
mitochondria. This establishes a proton gradient across the inner membrane. These protons
then flow down their concentration gradient, moving back into the matrix by
facilitated diffusion. During this process, ATP is made by adding inorganic phosphate to
ADP. Most of the ATP produced during cellular respiration is made during this stage.
For each glucose that starts cellular respiration, in the presence of oxygen (aerobic
conditions), 36-38 ATP are generated. Without oxygen, under anaerobic conditions, much
less (only two!) ATP are produced.
The electron transport chain is a series of proteins and organic molecules found in the inner
membrane of the mitochondria. Electrons are passed from one member of the transport
chain to another in a series of redox reactions. Energy released in these reactions is captured
as a proton gradient, which is then used to make ATP in a process called chemiosmosis.
Together, the electron transport chain and chemiosmosis make up oxidative phosphorylation.
The key steps of this process, shown in simplified form in the diagram above, include:

Delivery of electrons by NADH and FADH22start subscript, 2, end subscript. Reduced electron
carriers (NADH and FADH22start subscript, 2, end subscript) from other steps of cellular
respiration transfer their electrons to molecules near the beginning of the transport chain. In
the process, they turn back into NAD++start superscript, plus, end superscript and FAD,
which can be reused in other steps of cellular respiration.

Electron transfer and proton pumping. As electrons are passed down the chain, they move
from a higher to a lower energy level, releasing energy. Some of the energy is used to pump
H++start superscript, plus, end superscript ions, moving them out of the matrix and into the
intermembrane space. This pumping establishes an electrochemical gradient.

Splitting of oxygen to form water. At the end of the electron transport chain, electrons are
transferred to molecular oxygen, which splits in half and takes up H++start superscript, plus,
end superscript to form water.

Gradient-driven synthesis of ATP. As H++start superscript, plus, end superscript ions flow
down their gradient and back into the matrix, they pass through an enzyme called ATP
synthase, which harnesses the flow of protons to synthesize ATP.
We'll look more closely at both the electron transport chain and chemiosmosis in the
sections below.
The electron transport chain is a collection of membrane-embedded proteins and organic
molecules, most of them organized into four large complexes labeled I to IV. In eukaryotes,
many copies of these molecules are found in the inner mitochondrial membrane. In
prokaryotes, the electron transport chain components are found in the plasma membrane.
As the electrons travel through the chain, they go from a higher to a lower energy level,
moving from less electron-hungry to more electron-hungry molecules. Energy is released in
these “downhill” electron transfers, and several of the protein complexes use the released
energy to pump protons from the mitochondrial matrix to the intermembrane space, forming
a proton gradient.
[Click to see a free energy diagram]

NADH is very good at donating electrons in redox reactions (that is, its electrons are at a high
energy level), so it can transfer its electrons directly to complex I, turning back into
NAD++start superscript, plus, end superscript. As electrons move through complex I in a
series of redox reactions, energy is released, and the complex uses this energy to pump
protons from the matrix into the intermembrane space.

FADH22start subscript, 2, end subscript is not as good at donating electrons as NADH (that is,
its electrons are at a lower energy level), so it cannot transfer its electrons to complex I.
Instead, it feeds them into the transport chain through complex II, which does not pump
protons across the membrane.
Summary



Most of the steps of cellular respiration take place in the mitochondria.
Oxygen and glucose are both reactants in the process of cellular respiration.
The main product of cellular respiration is ATP; waste products include carbon dioxide
and water.
Stage
Direct products (net)
Ultimate ATP yield (net)
Glycolysis
2 ATP
2 ATP
2 NADH
3-5 ATP
Pyruvate oxidation
2 NADH
5 ATP
Citric acid cycle
2 ATP/GTP
2 ATP
6 NADH
15 ATP
2 FADH22start subscript, 2, end subscript
3 ATP
Total
30-32 ATP