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Biology 30
Module 1
Lesson 4
Food and Energy: A Closer Look
Copyright: Ministry of Education, Saskatchewan
May be reproduced for educational purposes
Biology 30
Lesson 4
Biology 30
Lesson 4
Lesson 4
Food and Energy:
A Closer Look
Directions for completing the lesson:
Text References for suggested reading:
Read BSCS Biology 8th edition
Photosynthesis Chapter 19 Pages 86; 489-495
Cellular respiration Chapter 4 Pages 86; 377-384
Nelson Biology
Photosynthesis Chapter 3 Pages 87-90
Cellular Respiration Pages 92-96
ATP Page 84
Study the instructional portion of the lesson.
Review the vocabulary list.
Do Assignment 4.
Biology 30
Lesson 4
aerobic respiration
anaerobic respiration
Calvin Cycle
Biology 30
dark reaction
light dependent reaction
light independent reaction
Thylakoid membranes
Lesson 4
Food and Energy: A Closer Look
Previous lessons presented information on matter and energy, including ways in
which these could be related to each other. Atomic and molecular structures,
different methods of bonding and the formation or breakup of organic compounds
have been described in order to show some of the characteristics of substances and
actions occurring within living cells. An important part was omitted at the time. That
part relates to the actual "capture" and utilization of energy and matter by living
Probably the two most important and complex biological processes, photosynthesis
and cellular respiration, are part of this. Even though the term "biological" is used to
show relationship to living things, the actions also include principles of Physics and
Chemistry. Although many of the general actions and steps are understood, there
are still some which scientists have yet to fully explain. In the descriptions of
photosynthesis and respiration in this lesson, explanations will be presented as an
attempt to bring about a general understanding rather than the full details of all the
individual steps.
To deprive any living organism of either matter or energy, or both, would eventually
result in death after a period of time. Matter, whether inorganic or organic, is
necessary for providing the building material of cells and tissues. It usually flows in
continuous cycles between the living and nonliving with the help of decomposers. A
continuous source of energy is needed to maintain cell and body actions. Unlike
matter, energy cannot be recycled and a large part of it is eventually lost from most
living systems as heat.
Photosynthesis is the single most important link on this
planet in making matter and energy available to all
organisms. Green plants are able to convert light energy and
inorganic substances into organic compounds which they
can utilize themselves or pass on to other organisms through
food chains.
Cellular respiration occurs in both functioning plant and
animal cells. Organic compounds are broken down into
smaller units, with the energy trapped in the chemical bonds
being released for various cell or body uses.
Biology 30
Lesson 4
After completing this lesson, you should be able to:
Biology 30
describe the general nature and role of ATP in cells.
explain the roles of chlorophyll and light in
indicate how electrons transform light energy into
chemical energy.
describe the light phase or light reactions of
describe the dark phase or dark reactions of
list some of the organic compounds which are possible
from the products of photosynthesis.
distinguish between photosynthesis and
distinguish between respiration and cellular respiration.
describe the first steps common to the different types of
describe the aerobic stage of cellular respiration.
describe two forms of fermentation: alcohol fermentation
and lactic acid fermentation.
summarize the main differences between photosynthesis
and cellular respiration.
apply different testing results to help explain some
features relating to photosynthesis.
explain possible differences in gas movements in green
plants over a 24 hour period.
Lesson 4
Energy Storage and Use – ATP
The organic compound which is most available
as an energy source for cellular use is sugar specifically glucose. Although starch is
regarded as a fuel, it is a long chain and is an
insoluble (doesn't dissolve) compound. It must
be broken down into shorter sugar chains to be
usable. Evan a glucose molecule would release
too much energy for a specific cell action. The
work of enzymes and life sustaining cell actions
require energy in smaller "packets" - more
suited to the action that can be released as
needed. A real life example would be the
inconvenience of using a ten-dollar bill to buy a few jelly beans instead of a loonie.
Similarly, the large amount of energy in a sugar molecule is inconvenient for a cell to
use. A large amount of the energy would be given off - "wasted" as heat.
Organisms and cells solve the energy problem by having special compounds in which
chemical (bond) energy is stored in smaller, easily available amounts that can be
released as needed. Of these compounds the most common and probably best known
is adenosine triphosphate or ATP.
ATP may be likened to a number of different things in the manner in which it
functions. In the case of money, it could be like a small change purse ready with its
smaller handouts of coins; in terms of electrical energy, it could be like a battery for a
light source or for an engine. In such situations, change purses and batteries cannot
function indefinitely without being “re-charged” from time to time. ATP and similar
energy carriers operate on somewhat the same principle. ATP can store energy when
there is an excess of it and then release it as needed. Then it needs to be “recharged”.
ATP is made up of:
a nitrogen base (adenine),
a five-carbon sugar (ribose),
three phosphate groups.
Identify each in the diagram.
Biology 30
Lesson 4
The three phosphate groups are the main features to remember of this molecule.
The last two phosphate groups are the important ones. The wavy lines joining each
phosphate group to the one before it are high energy bonds. The breaking apart of
the high energy bonds releases energy that is needed for cell activities such as
transporting molecules through membranes, or movement. Breaking off of two
phosphate groups leaves adenosine monophosphate or AMP. Usually just the last
phosphate group is broken off, releasing energy, leaving the two-phosphate molecule
adenosine diphosphate, or ADP. When there is a surplus of energy, it is used to
bond a phosphate group to the ADP to form ATP once more. Within cells, there is a
constant cycle involving these compounds and energy. This cycle can be represented
by the following diagram.
This process can be likened to batteries in a flashlight. When the energy is all used,
the batteries can be taken out and recharged. They are then ready for the energy to
be used again.
The energy used in forming ATP comes from either
A high energy electron of chlorophyll which has been acted on by light, or
The chemical bond(s) of sugar being broken down during cellular respiration.
Formation of ATP occurs in:
In the mitochondria, ATP molecules are also broken down to release energy if it is
Biology 30
Lesson 4
The energy compound, ATP is found in all living microorganisms, plants and animals.
It provides the source of energy which is the most directly usable for the greatest
number of cell activities.
All cellular activities involving the synthesis or breakdown of substances and energy
movements could be included under the general term of metabolism. Those
metabolic activities where energy is used to synthesize or build organic compounds
are part of anabolism. When organic compounds are broken down to release stored
energy for active uses, the actions are part of catabolism.
Although anabolic and catabolic actions are common to both plants and animals,
green plants can take anabolism to a degree not possible in other organisms. Green
plants can use radiant (light) energy to change simple, inorganic matter, such as
carbon-dioxide and water, into the beginnings of organic forms. These initial organic
compounds then become the sources of energy, the "building blocks" for all other
living tissue - not only in plants, but in all organisms found in food chains and webs.
Since green plants can actually make their own organic compounds, they are
independent or autotrophs. Organisms which rely on other organisms for their
organic compounds are said to be dependent. They are called heterotrophs.
Ultimately, even heterotrophs which are high-order consumers (such as carnivores)
depend on green plants for their energy and other nutrient requirements. In this
respect, the action of photosynthesis assumes great importance as a major
life-sustaining process for all organisms.
NOTE: Photosynthesis is a very complex process. It can be difficult to
understand. Thinking of electrons in minute structures like
chloroplasts is not something that is commonly done. So to give an
overview, this lesson will begin by explaining the role of some of the
participants of photosynthesis and then move into explaining what
occurs during photosynthesis.
Biology 30
Lesson 4
Chloroplasts and Their Role
Photosynthesis occurs in plant organelles called chloroplasts. Chloroplasts capture
light energy and produce food. They contain a green pigment, chlorophyll. There are
five kinds of chlorophyll designated as chlorophyll a to e. The two kinds that are the
most important to photosynthesis are chlorophyll a and b. They absorb most
wavelengths of the light spectrum but not green light. Plants can only use energy
from the absorbed light. Green light is reflected, thus the reason for plants being
green in colour. In the fall, chlorophyll a is broken down, then the other pigments
such as carotenes, beta-carotenes, and xanothophylls (yellows, oranges, and browns)
show up.
Chlorophyll acts in the same way as catalysts do in chemical reactions. They initiate
and control rates of reactions but do not enter into the final products. The exposure
of the chlorophylls and the pigments to the energy of light gives them extra energy.
This begins the actions eventually leading to the formation of organic compounds.
This will be seen as the steps of photosynthesis are laid out.
A chloroplast is a complex structure. Electron microscope studies of chloroplasts
have shown an outer membrane and internal specialized membranes called
thylakoid membranes. The thylakoid membranes are arranged in stacks of disks
that are altogether called grana. Chlorophyll and other pigments and enzymes are
embedded in the thylakoid membranes. The gel-like fluid that surrounds the grana
is called the stroma. It too, contains enzymes as well as DNA, RNA, and ribosomes.
An illustration
that will help to
remember this
arrangement of
the chloroplast is
to think of stacks
of four quarters
altogether each
stack is called a
dollar (grana).
These stacks of
quarters are in a
bowl (outer
membrane) filled
with colourless
gelatin that is not quite set yet. This gelatin represents the stroma.
Biology 30
Adapted from
Lesson 4
The Role of Light in Photosynthesis
The amount of light.
The importance of light is best shown by testing for the presence of starch in leaves of
plants that have been placed in varying light conditions. It is easier to test for
starch than testing for glucose (the product of photosynthesis) because after glucose
is formed, it very quickly combines into a larger molecule of starch. A test for starch
is done by treating the leaf to remove all pigment, then applying iodine to the leaf. If
starch is present, a blue-black colour will appear. This is a positive test. A plant left
in dim to no light produces no starch. A plant left in good light conditions shows the
most starch production.
Another indication of the importance of light is often shown by plants, such as
grasses, which may have been covered by boxes or other objects for a time.
Uncovering these plants later has shown that many or all had died.
The last type of situation just mentioned has also shown that light appears to be
important in another way. Without knowing the exact manner in which it occurs,
scientists have indicated that certain plants or plant parts must be exposed to light
for a certain amount of time in order for chlorophyll to develop. Stems, leaves and
some seeds kept in the dark will lose their color as the chlorophyll that they had
breaks down and is not replaced; or, will not develop in the first place. The
greenness of the exposed parts of carrot roots and potato tubers shows the opposite
situation, with light inducing (green) chlorophyll formation.
Light can be broken up into a visible spectrum which includes a range of colors that
are red, orange, yellow, green, blue, indigo, violet. The different colors have different
wavelengths and different energy levels as well. Infrareds and reds have the longest
wavelengths, but the lowest energy levels. Violets and ultraviolets have short
wavelengths, but large amounts of energy. It appears that plants absorb very little of
the ultraviolet or infrared light rays. Some scientists also feel that part of the middle
portion of the spectrum, consisting of green and adjacent bands, is either reflected or
transmitted right through – accounting for the green colors. However, other
scientists feel that greens are absorbed almost equally as well and that there are only
very slight differences in absorption and reflection. If this is accurate, the ability to
absorb much of the visible light spectrum means that most plants can really be
grown under artificial light. The only requirement is a light intensity much higher
than most buildings normally have.
Despite the large amounts of natural light to which plants can be exposed, estimates
indicate that approximately 50% of the light striking plants is either reflected or
transmitted right through plant surfaces. Of the amount available to the plants, only
1 to 3% is converted to chemical energy.
Biology 30
Lesson 4
Through studies carried out by a Belgian physician and scientist, Van Helmont, the
conclusion was made that “plant tissues came from water”. Although not entirely
right, Helmont’s conclusion led to further investigations as to the origin of matter
making up plant bodies. These investigations and results, generally accepted as
fairly accurate at the present time, do indicate that approximately 94% of plant
matter is developed through photosynthesis while only 6% comes from the soil. It is
also known that atmospheric CO2 is a major contributor to plant matter, along with
water. These are considered to be the “raw materials” for photosynthesis.
What happens in Photosynthesis?
An overall chemical equation that sums up what happens in photosynthesis is:
6 CO2
carbon dioxide
12 H2O
C 6 H12O6
6 O2
6 H2O
In words, this equation says: carbon dioxide and water combine in the presence of
chlorophyll and light to produce glucose, oxygen and water. This, of course, is very
simplified representation of what actually goes on in the process of photosynthesis
but it is a good reminder for when you are studying.
Note: You should know this equation.
The overall process of photosynthesis:
absorbs light energy (Occurs in the thylakoid membranes of the grana.)
converts light energy (Also occurs in the thylakoid membranes.)
stores chemical energy in sugars. (Occurs in the stroma.)
The absorbing and converting of light energy and storing of chemical energy happens
in two major processes in Photosynthesis. They are:
The Light Dependent Reaction or Photolysis; and
The Light Independent Reaction, also called The Calvin Cycle or the Dark
Note: Yes, you should be aware and know all the previous names.
Biology 30
Lesson 4
The Light Dependent Reactions (also called
The ATP’s formed also carry
energy to the dark reaction.
There are two different types of molecule clusters scattered in the Thylakoid
membranes of the chloroplasts. They are given the names Photosystem I and
Photosystem II.
Each photosystem has chlorophylls, pigments, and enzymes slightly different than the
other. This results in each cluster absorbing light of different wavelengths.
Note: The organic-making process in green plants begins in
Photosystem II. Please read each step through carefully. Be
certain to compare each step number back to the diagram so
it is easier to follow and understand. You may have to go
back and do this several times.
Refer to the end of the lesson for overall Photosynthesis diagram.
Biology 30
Lesson 4
Photosystem II
Non-cyclic electron flow (#1 in the diagram)
Light hits chlorphylls and pigments in photosystem II (see #1 in diagram).
This causes the electrons to become "excited" and they move to higher
energy levels. Absorbing light sets up a flow of electrons.
The now high-energy electrons are taken through several electron carriers
until they are accepted by NADPH+ (#6) which is a coenzyme (electron
carrier). (NADP is nicontinamide adenine dinucleotide phosphate.) The
high-energy electrons, with a negative charge reduce the positive charge so
the molecule becomes NADPH. This energy rich molecule carries energy to
the dark reaction. Light energy has now been changed, by means of
energized electrons, into chemical bond energy (in NADPH).
Cyclic flow - (#2 in diagram)
Some “excited” electrons are picked up and moved along by several electron
carriers. These electrons give up some of their energy to bond a phosphate
group to ADP to form ATP. The electrons then return to the chlorophyll and
pigments and are ready to begin the cycle again. This cycle flow occurs very
quickly. ATP is an energy carrier that moves into the dark reaction.
Photosystem I (#3 in the diagram)
Electrons from other pigments and chlorophylls are energized when light hits
These "excited" electrons e  move through a series of electron carriers. As they
do this they release energy to add a phosphate to ADP to form ATP.
These electrons now move into photosystem II (#1 & 2) to keep the supply of
electrons flowing.
 
The ATP's that have been formed in both the photosystems carry energy into the dark
reaction, also called the Light Independent Reaction.
The Water Molecule is split (also called Photolysis) (#5 in diagram)
The chlorophylls and pigments of photosystem I need a source of
The meaning of
photolyis is light
electrons. Water is the source of electrons for the light reaction.
splitting. Light
A special group of enzymes remove electrons e from water.
energy splits the
H2O molecule.
This splits the water molecule (#5). Oxygen is given off as a
"waste" product of photosynthesis.
The Hydrogen ions H  from the molecule of water combine with NADP (#4) and
give it a positive charge NADPH . NADPH now has greater attraction for the
high energy electrons from photosystem II.
The oxygen from the split water molecule is given off as a gas.
Biology 30
 
 
Lesson 4
Review of the Light Dependent Reaction (Photolysis)
The Light Reaction must occur in the light.
The raw materials that are used are water, light, chlorphyll
The water molecule is split by the light energy. Enzymes remove electrons from
the water H2 O molecule (#5) to supply the photosystems I & II with a
continuous supply of electrons. The Hydrogens (H+) join with NADP to form
NADPH+ and oxygen O2  is given off as a gas.
Light shines on the chlorophylls/pigments in the thylakoid membranes of the
The excited electrons are accepted by the coenzyme NADPH+ to become NADPH
(#6 in diagram)
The energy rich compound NADPH moves through for use in the Dark reaction.
Another set of "excited" electrons (#2 & #3) release their energy to add a
phosphate to ADP to form ATP and then return to the chlorophyll to start over
again. ATP carries energy for use in the Dark Reaction.
Two very important energy carriers are produced: ATP and NADPH. They
move into the Dark Reaction (Light Independent Reaction) for use in forming
organic compounds.
The Calvin Cycle - The Light-Independent Reactions
(also known as The Dark Reaction)
The Light-Independent Reaction, also known as the Calvin Cycle, does not require
light and occurs in the stroma of the chloroplasts. Even though this part of
Photosynthesis does not require light, it does need the ATP and NADPH that was
formed in the Light Dependent Reaction.
The term carbon fixation can also be applied to this phase as this is actually what is
happening. Inorganic carbon from carbon dioxide is fixed into organic compounds.
The enzyme, RuBP carboxylase, is very important as it promotes carbon
fixation. Almost half of the protein in a chloroplast is part of this enzyme.
It is the most abundant protein in all organic life.
Enzymes are important in driving the whole process of photosynthesis (Psis).
Biology 30
Lesson 4
Note: Look back at the overall Photosynthesis equation. So far we have talked about the H2 O , the
chlorophyll and the light. Now the raw material, CO2 enters the reaction. This is the first time it has
been mentioned.
Steps of the Calvin Cycle
Carbon Fixation
With enzymes aiding the reaction, CO2 joins with ribulose biphosphate (RuBP), a
5-carbon sugar at #1.
The 6-carbon sugar that is formed is unstable and splits into two molecules of 3carbon PGA (phosphoglycerate) (see #2 in diagram)
Energy from Light Reaction Added (#4 and #5)
The ATP (#4) and NADPH (#5) from the Light Reaction enter a series of reactions
that convert PGA (#3) into PGAL (#6) (phosphoglyceraldehyde), a 3-carbon sugar
Chemical energy has now successfully been built in and stored in an organic
Most of the PGAL, with the actions of other enzymes and additional ATP, is
converted to RuBP to keep the cycle going. (#7)
some PGAL leaves the cycle (to form glucose)
Biology 30
Lesson 4
There are a number of actions that can occur with the remaining useable PGAL.
 PGAL is the first major nutrient/organic compound of photosynthesis
 Some could be used immediately for cell activities needing energy. PGAL can go
straight to cellular respiration as a fuel.
 PGAL can be stored in stems and roots because more PGAL is produced than is
needed by the cells right away.
 PGAL is quite reactive. Before the completion of photosynthesis much of the PGAL
is changed into a less reactive but moveable form called glucose, a 6-carbon
sugar, the second nutrient of photosynthesis.
 PGAL can be used to synthesize more complex sugars, starches (for storage), oils,
amino acids, proteins, lipids and other compounds that are needed.
After reviewing this description of photosynthesis (which has not included all the
details) one can see that the original formula given earlier is very simplified.
Changing kinetic light energy into potential chemical bond energy involves
transferring energy by means of high energy electron movements.
In the description of Cellular Respiration in the next section we will see the role of electrons once again.
Conditions Affecting Photosynthesis
The rate of photosynthesis can be affected by a number of conditions. Some of these
 kinds (wavelengths) and intensity of light
 the amounts of the "raw" materials of carbon-dioxide and water.
 External temperatures could increase or decrease rates; extremes of warmth or
cold could stop activity completely.
 Kinds and rates of plant activities often seem to be "synchronized" with
photoperiods. These are the actual lengths of light and darkness in certain time
 The types of plants (or perhaps the kinds of chlorophylls they have) injuries or
general plant health could have some effects as well.
These factors can affect each other so the combined effect must be looked at. Any
one factor not at its optimum can effect the overall rate of photosynthesis. One type
of plant may not grow at the maximum rate in a cold climate with full sun because it
needs higher temperatures.
Biology 30
Lesson 4
Importance of Photosynthesis
Although the relative importance of photosynthesis as an organic and energy link for
all living organisms has been mentioned more than once, the process has many other
effects. Wherever plant products or plant uses are involved, such as animal feeds,
textiles, lumber, pulp, resins, medicines and many others, photosynthesis has to be
considered as the starting point. Scientists speculate that the atmospheric
conditions during the early development of this planet lacked oxygen or had very
little. The current twenty percent of the atmosphere that is oxygen has resulted
largely as a byproduct of photosynthesis. Another very significant contribution from
the process and plant products has been the development of our fossil fuel reserves
in the form of coals, oil and gases.
There is a concern with the destruction
of the world's forests. How is this
concern related to Photosynthesis?
Which part of the process would it affect
the most?
“Plants that photosynthesize are 'oxygen
factories.” Photosynthesizing plants 'put
out' O 2 and at the same time take in CO2 .
Destroying rain forests would reduce the
amount of O 2 put into the air and slows the
rate at which CO2 is removed from the
atmosphere. The increase of CO2 in the
atmosphere seems to affect 'global warming.'
Biology 30
Lesson 4
Earlier, organisms were mentioned as being divided on the basis of those which relied
on other organisms for organic nutrients (heterotrophs) and those which could make
their own organic compounds (autotrophs). Most autotrophs are green plants or
photosynthetic organisms. There is another group of autotrophs generally classed as
chemotrophs, which are capable of making food through chemotropism. Mainly
bacteria, found near soil surfaces, lake bottoms and ocean floors, these organisms
use special enzymes to break down inorganic chemical compounds such as sulfides
or oxides. The energy released by these chemicals can then be used to synthesize
organic compounds. In comparison to photosynthesis, however, chemosynthesis
produces only extremely small amounts of organic matter.
In referring to "respiration", one may form the idea of breathing gases or taking
certain gases into the body (commonly oxygen) and expelling others (carbon-dioxide).
On a cellular level, there is a slightly different meaning, even though movements of
gases are quite important.
Cellular respiration is the breakdown of organic compounds to
release energy for cell activities. The energy that is produced is
usually transferred to smaller units, such as ATP.
Cellular Respiration occurs in living cells all the time. Some respiration occurs in the
cytoplasm, but most occurs in the mitochondria, the “powerhouse of the cell.” In
humans, cellular respiration is usually aerobic. This means it requires oxygen which
is carried by the blood along with small food molecules. The oxygen and food
particles pass through the cell membrane by the process of diffusion to be used by
the cell. The most common food particle used as fuel for cellular respiration is
glucose, C 6H12 O 6 , although starches, lipids and amino acids can also be used as
energy sources.
The overall reaction, shown below, simplifies a "complex series of chemical reactions."
(carbon dioxide)
6H2O 
Note: You should know this equation.
Biology 30
Lesson 4
It is this catabolic part of metabolism, where energy is released from
organic compounds that will be considered as respiration throughout the
rest of this lesson.
Respiration can be a very complex topic. We will begin by giving a brief summary.
Read the summary and the charted diagram below to gain an overview of the
catabolic process.
Respiration can be divided into three general steps:
1. Glycolysis
takes place in the cytosol of the cytoplasm of the cell.
Glucose molecules are split with a limited amount of energy released in
the form of ATP.
Intermediate products are formed and move into the mitochondria.
2. Krebs Citric Acid Cycle
Occurs in the mitochondria.
The remaining structure of the glucose molecule is dismantled.
All carbons of the original glucose are released as carbon dioxide.
Only a limited amount of ATP is released.
3. Electron Transport
Hydrogen atoms and electrons removed from various compounds are
moved along a series of carrier molecules. As this occurs significant
amount of energy is released and stored in ATP.
At the end of this stage the availability of oxygen plays a significant effect
on the entire process.
Now view the chart diagram which follows.
Biology 30
Lesson 4
Respiration Overview:
Again, this is a basic overview. We will now look at the entire process in a little more
detail. The main object here is to know what goes in, what happens to it, and what
comes out at the end of the process.
The Stages of Respiration
Occurs in the cytosol of the cytoplasm.
Oxygen is not necessary for this stage. It is anaerobic.
ATP (energy) must be used to start the reaction.
Glucose is split into two, 3-Carbon molecules called PGAL which are rearranged to
form two, 3-Carbon molecules of pyruvic acid.
Biology 30
Lesson 4
 The Hydrogen atoms that are given off are picked up by a carrier molecule NAD
(nicotinamide adenine dinucleotide) to become NADH.
 The electrons and protons from hydrogen separate. The electrons are moved along
a series of coenzymes (carriers). The energy that the electrons give off is used to
add a phosphate to ADP to form ATP.
 There is a net gain of 2 ATP's in glycolysis, the first stage of cellular respiration.
This is only about 5-7% of the total energy available in the glucose molecule, so
the pyruvic acid molecules still contain most of the original energy.
The end products of glycolysis are:
Two molecules of pyruvic acid (3-C)
Net gain of 2 ATP
Two NADH molecules
The move into the mitochondria:
 The availability of some oxygen will allow for the removal of one carbon from a
pyruvic acid (3-C) molecule. The carbon and oxygen combine to release CO2 . The
remaining 2-carbon portion becomes acetic acid.
oxygen present
 Each acetic acid is picked up by a coenzyme (a carrier molecule) called CoA
(coenzyme A) to form acetyl CoA.
The Coenzyme A delivers the acetic acid molecules to the Krebs Citric Acid Cycle in
the mitochondria.
Biology 30
Lesson 4
Krebs Citric Acid Cycle
 Occurs in the mitochondria.
 The 2-carbon acetic acid joins with a 4-carbon acid to form a 6-carbon acid called
citric acid.
 From here citric acid goes through several reactions.
 Chemical bonds are broken and atoms are given off or are re-arranged.
 Electrons are also given off in the form of carbon atoms (#1). These carbon atoms
are from the original glucose molecule. The carbon atoms combine with oxygen to
produce carbon dioxide (CO2). (A person exhales this). The carbon structure of
the original glucose is completely dismantled.
 Hydrogen atoms e  join with carriers NAD to form NADH (#2) to go to the next
stage, which is the Electron Transport System.
 Whenever bonds are broken, energy is given off so heat is released and ATP is
formed (#3).
 The series of acids produced in the cycle eventually lead back to the formation of
the original 4-carbon acid. Reproducing the original 4-carbon acid enables the
cycle to continue.
 
(a portion of Krebs cycle
from above)
Biology 30
Electron Transport
Lesson 4
 Moving into the final stage of cellular respiration the hydrogen atoms (electrons
and protons) are transported along a series of electron carriers where they lose
their energy to adding a phosphate to ADP to form ATP.
 Oxygen is most important at the end of the electron transport stage where it picks
up the hydrogen ions and the electrons to form water which is released. This
action enables the electrons to keep moving through the entire system.
 The availability of oxygen to keep removing the carbons and hydrogens (electrons)
enables this process to be known as aerobic respiration.
 From the beginning of glycolysis to the end of aerobic respiration, each glucose
molecule will produce 38 ATP (34 of which are formed in the electron transport
system). During the process, the sugar is broken down gradually as the bonds
between its atoms are broken. The 38 ATP produced represent only 40 to 60% of
the original energy that went into glucose. The rest of the energy is lost mostly as
heat. This is still a fairly good recovery rate compared to machines, which often
average 30 to 40% conversion to useful energy.
What happens if these processes are stopped? (NAD) Cyanide, a fast acting
poison, combines with the final carrier NAD in the electron transport system
and blocks any ATP production. Without any ATP, cell processes are not
able to continue. Unconsciousness soon follows.
Carbohydrates, fats and proteins are all sources of "fuel" for
the process of cellular respiration. Through digestion they
are broken down to the point that they can enter the Krebs
Citric Acid Cycle.
Carbohydrates are broken down to glucose so they are
ready to enter glycolysis.
Fats, as you learnt in lesson 3, are made up of fatty acids
and glycerol. Glycerol is broken down into a 3-carbon
compound and enters glycolysis, whereas fatty acids are
further broken into acetic acid and join with CoA to form
acetyl CoA to enter the mitochondria and enter the Krebs
Proteins, made up of amino acids, has to have the amino
group removed then the resulting carbon molecules can
enter at glycolysis or the Krebs cycle.
Biology 30
Lesson 4
Anaerobic Respiration
Not all cells or organisms are in conditions where sufficient amounts of oxygen are
available all the time, or even some of the time. Such cells or organisms still need
energy to maintain or carry out necessary life-sustaining activities. Respiration is
still carried out, but under conditions of oxygen deficiencies it is called anaerobic
A limited number of organisms, mainly some kinds of bacteria, carry out respiration
where there may be no free atmospheric oxygen. Anaerobes are able to break down
certain inorganic compounds such as nitrates (N03), carbon-dioxide and sulfates
(S04) and use the oxygen from these sources. In soils, especially those that are wet
and have little oxygen, denitrifying bacteria may have an effect on crop production as
they reduce the amounts of nitrates available to plants. In water-logged soils, such
as in some sloughs or marshes, some bacteria remove oxygen from sulfates and in
the process, form hydrogen sulfide. This produces the swamp gas or "rotten egg"
smell. Removal of oxygen from C02 leaves behind methane in other anaerobic
After glycolysis, if oxygen is in very short supply or is totally absent, a process called
fermentation occurs. If the hydrogen cannot be removed by oxygen, the process of
breaking down pyruvic acid (formed in glycolysis) stops. Fermentation does not
produce or release any more energy beyond the glycolysis stage. The 2 ATP's
produced in glycolysis are all that is produced so fermentation is a very inefficient
process. The products formed still have a lot of energy tied up in them.
There are two types of fermentation: lactic acid fermentation and alcoholic
Biology 30
Lesson 4
Alcohol Fermentation
Lactic Acid Fermentation
Yeasts and many bacteria carry out
alcohol fermentation. In the absence of
oxygen, carbon dioxide is released from
pyruvic acid and hydrogen is transferred
to the pyruvic acid to form ethyl alcohol.
Lactic acid fermentation is more
common to animals (especially in the
muscles) and to some bacteria. In this
type of fermentation, the single
reaction of hydrogen transferring to
pyruvic acid (see *) is enough to form
lactic acid. Muscle exertion and the
inability of the body to supply oxygen
quickly enough results in lactic acid
This type of fermentation has commercial
value in baking and in the brewing and
winemaking industry. (In the production
of "sweet wines", fermentation is halted
earlier so that the wine still contains a
large amount of sugar or glucose. "Dry"
wines are allowed to ferment longer to
convert more sugar to alcohol).
In baking, the yeast releases the CO2 into
the dough. The oven heat kills the yeast
and the bubbles break, leaving "holes" in
the bread or cake. Thus, the cake or
bread is fluffy and light to eat.
Biology 30
buildup which leads to muscle fatigue
and soreness. This process is
sometimes known as building up an
"oxygen debt". Rest allows oxygen and
aerobic respiration to break down lactic
acid. Again, an over-accumulation
could be toxic to the organisms in
which it is produced. Depriving the
human brain of oxygen for longer than
five minutes leads to permanent
damage and possible death. Certain
bacteria can also produce lactic acid as
they break down animal or plant
products. Sour milk, sauerkraut and
silage are some examples where lactic
acid is formed. The characteristic of
lactic acid being toxic at certain levels
and killing organisms has sometimes
been applied to preserving certain
substances or foods – such as silage or
Lesson 4
As mentioned shortly before, fermentation is not an efficient process in terms of
yielding energy for the organisms carrying it out. Glycolysis is the only part which
releases a limited amount of energy. Fermentation itself, where hydrogen combines
with pyruvic acid to form various substances, merely allows glycolysis to go on a little
longer. In the end, however, this could be fatal to organisms as substances such as
alcohol and lactic acid can be toxic in higher concentrations.
Uses of Energy
If cells and organisms are to maintain living conditions, they must have a continuing
supply of energy. Maintaining a living condition involves many diverse actions. The
release of energy also releases heat in many organisms. In most plants and
cold-blooded organisms, this is quickly lost to the external environments and has
little value for the organisms. For warm-blooded organisms, maintaining higher body
temperatures is necessary for proper body functioning and too great a drop could be
fatal. Much of the energy released initially during respiration goes towards the
formation of ATP. The energy can be temporarily stored this way and then when it is
required, it is readily available in smaller, more convenient amounts. Released from
ATP, a considerable portion is used to synthesize many kinds of cell parts and cell
substances. These include the three general groups of organic compounds, as well
as vitamins, pigments, organic acids and alkaloids. Energy is commonly associated
with motion as well. While obvious by the muscular contractions of many animals,
movements occur in plants as well. In both plants and animals, protoplasmic
streaming, chromosome movements and active absorption are some of the less
obvious motions.
The process of photosynthesis by green plants, in which inorganic matter is changed
to organic (food), is an action upon which most life depends. The conversion of light
energy into the chemical energy of PGAL, ATP and eventually other organic
compounds, provides sources of energy which are released by the process of
respiration in all living cells.
The following table illustrates some of the major differences between photosynthesis
and respiration.
Biology 30
Lesson 4
occurs in the green cells of plants
occurs in the living cells of all
must begin in the presence of
takes place all the time, in both
light and dark.
requires water and CO2.
requires organic matter and O2.
releases O2.
releases water and CO2.
radiant energy is converted to
chemical energy.
chemical energy is changed into
heat or transformed into smaller
units of ATP.
organic matter is produced.
organic matter is broken down.
Once light energy has been converted to chemical bond energy, different kinds of
actions can take place with the chemical energy and chemical compounds formed.
The following illustration shows some of the different pathways that could be
associated with the anabolic and catabolic stages of metabolism.
Pathways of Metabolism
Biology 30
Lesson 4
Processes of Photosynthesis
The Light Dependent Reactions (also called Photolysis)
The energy
ATP move
into the Dark
Reaction (#4,
#5 in the next
Steps of the Calvin Cycle (The Light-Independent/Dark Reaction)
Biology 30
Lesson 4
Overall Diagram of Cellular Respiration
Biology 30
Lesson 4