BIO103 TOPIC
BIO 103: 08 Fall-2014
Biology-I,
Energy of Life
Contents: Cellular respiration, Enzymes and Photosynthesis
1. Cellular respiration:
Cellular Respiration/Respiration: The process by which living cells break down
glucose molecules and release stored chemical potential energy. It occurs in the
mitochondria.
a. You must not confuse respiration with breathing. Breathing is simply the
exchange of gases between an organism and its environment. Cellular
respiration is the “burning” of glucose in cells to release the energy required to
support life process. The word “burning” is placed in quotation mark because
the really does not burn. A better term to use is oxidation.
b. Oxidation is the loss or removal of electron(s) from a substance. In cellular
respiration glucose is oxidized.
Figure 1: The difference between breathing and cellular respiration
Cellular Respiration vs. Oxidation
In some respects, oxidation in cell, or cellular respiration, is similar to oxidation, or
burning, of fuels such as coal, wood, and oil. When a fuel is burned, chemical
potential energy in the molecules of the fuel is released as heat and light energy.
In a similar manner, when glucose is oxidized in cells, chemical potential energy in
the glucose molecules is released partly as heat energy.
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BIO103
BIO 103:TOPIC 08Fall-2014
Biology-I,
However, there is one important difference between the burning of fuel and the
oxidation of glucose in living cells. The burning of fuel produces very high
temperatures. Clearly, such high temperatures cannot occur in cells. The cells would
be destroyed. Thus, during cellular respiration, the rate of oxidation must be
controlled. As a result, it occurs in several small steps. Each step is assisted by an
enzyme. The enzyme permits to take place that steps at the normal temperature of
the organisms.
Enzymes are proteins that act as catalysts to regulate the speed of the many chemical
reactions involved in the metabolism of living organisms.
The Summation Equation for Cellular Respiration
Cellular respiration begins with two substances, glucose and oxygen. The process
produces energy. Oxidation of most organic compounds produces carbon dioxide
and water. Therefore, we will assume that they are formed in cellular respiration.
We breathe out carbon dioxide and water vapour produced in our body by cellular
respiration. The following are the summation equations for cellular respiration:
Enzymes
Glucose + Oxygen Carbon dioxide + Water+ energy
Enzymes
C6 H12O6 + 6O2 6 CO2 + 6 H2O+energy
This summation equation only “sums-up” what happens during cellular respiration.
It tells us what cellular respiration starts with and what it ends with. However, it
does not tell us how this process occurs.
Anabolism and Catabolism (already discussed in first lecture, so provided in brief)
Anabolism is all of the metabolic processes that build bio-molecules. E.g. synthesis of
an amino acid from simpler molecules and, the synthesis of a protein from amino
acids;
Catabolism involves all of the metabolic processes that tear down bio-molecules.
E.g. cellular respiration, in which the sugar glucose is broken down in the presence
of oxygen to carbon dioxide and water
The sum of anabolism and catabolism is called the Metabolism.
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BIO103 TOPIC
BIO 103: 08 Fall-2014
Biology-I,
Figure 2: anabolism and catabolism at a glance
ATP-The major energy currency
ATP is the molecule found in all living organisms that plays the key role in the
storage, transfer, and release of energy in a cell. That is, it is the main immediate
source of usable energy for the activities of the cells. ATP is the major energy
currency molecule of the cell.
ATP has a complex structural formula. We will write it simply as A~~. The
A stands for adenosine and each stands for one phosphate group. The wavy lines
(~) represent high-energy phosphate bonds.
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BIO103
BIO 103:TOPIC 08Fall-2014
Biology-I,
ATP is built up by the metabolism of food stuffs in the cell in special compartments
called mitochondria. It is then distributed to all parts of the cell. The two bonds
between the three phosphate groups are high-energy bonds, that is, they are
relatively weak and yield their energy readily when split by enzymes. These bonds
play the key role in the storage, transfer, and release of energy in a cell. With the
release of the end phosphate group, 7 kilocalories of energy become available for
work, and the ATP molecule becomes ADP (adenosine diphosphate). Most of the
energy-consuming reactions in cells are powered by the conversion of ATP to ADP;
they include the transmission of nerve signals, the movement of muscles, the
synthesis of protein, and cell division. Usually, ADP quickly regains the third
phosphate unit by using food energy through the cellular respiration. The process
can be summarized in this equation:
A~~ A~ + + Energy
ATP ADP + + Energy
In words the equation says: ATP produces ADP, a phosphate group, and energy.
Also, ADP combines with a phosphate group and energy to produce ATP.
The change from ATP to ADP and from ADP to ATP occurs over and over again
throughout the cell. The energy is stored in ATP and transported to where it is
needed. There the ATP converts to ADP, releasing the needed energy.
The Role of ATP in Cellular Respiration
The energy stored in ATP molecules is the only energy that is directly usable by
cells. As a result, the energy released by glucose during cellular respiration must be
stored in the bonds of ATP.
Enzymes permit the energy of glucose to be released in a slow, controlled manner.
As it is released, it combines with a phosphate group, and ADP to form high-
energy bonds in ATP. The ATP travels to places in the cell where energy is needed. It
then loses a phosphate group and becomes ADP. The energy stored in the high-
energy bond is released at this time. That energy powers all life processes.
Biologists have shown that, for every molecules of glucose that is oxidized, 38
molecules of ATP are formed. Therefore, the summation equation for cellular
respiration showing the involvement of the ATP-ADP cycle could be written as:
Enzyme
s
C6 H12O6 + 6 O2 + 38 ADP + 38 P 6 CO2 + 6 H2O + 38 ATP
Store Energy
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BIO103
BIO 103:TOPIC 08Fall-2014
Biology-I,
Anaerobic Respiration
The process of cellular respiration requires free oxygen, oxygen gas. As a result, it is
called aerobic respiration. However many microorganisms, such as, yeast and some
bacteria can respire without free oxygen. This type of respiration is called anaerobic
respiration. Anaerobic respiration is the process by which certain microorganisms
break down glucose molecules in the absence of molecular oxygen and release
stored chemical potential energy.
Anaerobic respiration may be two types: alcohol fermentation, and lactic acid
fermentation.
a. Alcohol Fermentation
Bacteria and yeast that live in area where there is no oxygen must respire
anaerobically. Since one of the end products is alcohol, this process is often called
alcohol fermentation.
The summation equation for this type of anaerobic respiration is:
Enzymes
Glucose Ethyl alcohol + Carbon dioxide + Energy
Enzymes
C6 H12O6 2 C2H5OH + 2 CO2 + Energy
b. Lactic Acid Fermentation
Anaerobic respiration does occur in some of our body cells from time to time.
During periods of heavy physical activity, your muscle cells may not be able to get
oxygen fast enough to carry out the usual aerobic cellular respiration. When this
occurs, muscle cells begin to respire anaerobically. Glycogen, or animal starch, is
stored in muscles. During anaerobic respiration, this glycogen changed to glucose.
The glucose is then broken down anaerobically. No free oxygen is involved. Since the
end product is lactic acid, this process is often called lactic acid fermentation.
The summation equation for this type of anaerobic respiration is:
Enzyme
Glucose s Lactic acid + Energy
Enzyme
C6 H12O6 s 2 C3H6O3 + Energy
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BIO103
BIO 103:TOPIC 08Fall-2014
Biology-I,
2. Enzymes:
Definition
Enzymes are complex chemicals that control reactions in living cells. They are
biochemical catalysts speeding up reactions that would otherwise happen too
slowly. The chemical which an enzyme works on is called its substrate. An enzyme
combines with its substrate to form a short-lived enzyme/substrate complex. Once a
reaction has occurred, the complex breaks up into products and enzyme.
E+S ES EP E+P
Figure 3: An enzyme catalyzed reaction
The chemical nature of enzymes
1. Enzymes are specific: each enzyme usually catalyses only one reaction.
2. Enzymes combine with their substrates to form temporary enzyme-substrate
complex.
3. Enzymes are not altered or used up by the reactions they catalyze, so can be
used again and again.
4. Enzymes are sensitive to temperature and pH.
5. Many enzymes need cofactors in order to function.
6. Enzyme function may be slowed down or stopped by inhibitors.
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BIO103
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Classification of Enzyme:
Enzymes are protein. There are 6 types of reaction they can catalyze-
1. Oxidoreductases: These catalyze oxidation and reduction reactions.
2. Transferases: These catalyze the transfer of a chemical group from one
compound to another.
3. Hydrolases: These catalyze hydrolysis (splitting by use of water) reactions.
Most digestive enzymes are hydrolases.
4. Lyases: these catalyze the breakdown of molecules by reactions that do not
involved hydrolysis.
5. Isomerases: These catalyze the transformation of one isomer into another,
6. Ligases: These catalyze the formation of bonds between compounds, often using
the free energy made available from ATP hydrolysis.
To remember from the acronym: OTHLIL
3. Photosynthesis
The Need for Energy
All living things need a continuous supply of energy. This energy is needed to
support life profess such as movement, growth, and reproduction. Almost all energy
used by living things originally comes from the sun. Green plants and many other
organisms contain chlorophyll. These living things, through photosynthesis, store
some of the sun’s energy in the bonds of glucose molecules. They do this by
converting light energy into chemical potential energy. The glucose that is made
during photosynthesis is the basic energy source for almost all organisms.
Organisms break down glucose during the process of respiration. This releases some
of the energy that was stored in the bonds of the glucose molecules.
Organisms that produce their own food are called autotrophs. Therefore, organisms
that contain chlorophyll are autotrophs. They use the glucose that they produce
during photosynthesis as an energy source during reparation.
Organisms that depend on other organisms for food are called heterotrophs. All
heterotrophs depend directly or indirectly on autotrophs food. The dependence is
direct in the case of plant eaters, or herbivores, such as rabbits, deer, and cow. They
eat plants and convert stored carbohydrates in the plants into glucose. The glucose
is then “burned” during respiration to release needed energy. The dependence is
indirect in the case of flesh eaters, or carnivores, such as tiger, lion, and wolves.
These animals feed on herbivores such as rabbits. The rabbits eat autotrophic
organisms such as grass. Therefore, indirectly, the carnivores depend on autotrophs
for glucose that is needed for energy.
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BIO103
BIO 103:TOPIC 08Fall-2014
Biology-I,
Summary
1. Photosynthesis is the process by which light energy is changed to chemical
potential energy and stored in the bonds of glucose molecules.
2. Respiration is the process by which living cells break down glucose molecules and
release the stored chemical potential energy.
Photosynthesis takes place only in cells that contain chlorophyll. Photosynthesis also
requires light energy. In contrast, respiration takes place in all cells, all the time.
The Composition of Glucose
Glucose belongs to a family of organic compounds called carbohydrates. Its
molecular formula is C6H12O6. (We have already discussed in macromolecules
section).
If we write its formula as C6(H2O), we can see why it is called “carbo...hydrate”. It
contains carbon. It also contains hydrogen and oxygen in the same proportion as
water.
The Summation Equation for Photosynthesis
As the name implies, light energy (photo) is used in building complex substance
from simple substances (synthesis). Photosynthesis process occurs only in
organisms that contain chlorophyll. The simple substances used in photosynthesis
are water and carbon dioxide. During photosynthesis light energy is changed to and
stored as chemical potential energy in the bonds of glucose molecules.
Photosynthesis is the process by which green plants and certain other organisms
use the energy of light to convert carbon dioxide and water into the simple sugar
glucose. In so doing, photosynthesis provides the basic energy source for virtually
all organisms. An extremely important by-product of photosynthesis is oxygen, on
which most organisms depend.
Chlorophy
ll
Carbon dioxide + Water + Light Energy Glucose + Oxygen+ Water
Chlorophyll
6 CO2 + 12 H2O + Light Energy C6 H12O6 + 6 O2+ 6H2O
Theses equations are just summation equations. They simply “sum up” what
happens during photosynthesis. They tell us what photosynthesis starts with and
what it ends with. They indicate that chlorophyll must be present. However, these
equations do not tell us how the process occurs.
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BIO103
BIO 103:TOPIC 08Fall-2014
Biology-I,
Light energy
Oxygen
Products
Chlorophyll
Glucose
Carbon dioxide
Starting materials
Water
Figure 4: The diagram sums up what happens during photosynthesis. Water and
carbon dioxide enter the leaf. Glucose and oxygen are produced.
The Role of Chlorophyll
A green leaf appears green in white light. Therefore, it must contain pigments that
absorb the red and blue wave lengths of the spectrum but reflect green wave length.
Of course these pigments are chlorophyll. Nearly all the blue light is absorbed by the
chlorophyll. Also, much of the orange and red light is absorbed. However, little
green and yellow light is absorbed. Therefore, plants that contain chlorophyll as
their main pigment appear green because chlorophyll reflects mainly green and
yellow light.
Chlorophyll
Chlorophyll, the pigment found in plants, some algae, and some bacteria that gives
them their green colour and that absorbs the light necessary for photosynthesis.
Chlorophyll absorbs mainly violet-blue and orange-red light. There are at least five
chlorophyll molecules. All are somewhat alike in structure and properties. The two
most common types are chlorophyll a and chlorophyll b. Chlorophyll is found in cell
organelles called chloroplasts. The chlorophyll functions as a catalyst in the process
of photosynthesis. A catalyst is a substance that affects the rate of a chemical
reaction without being permanently changed itself. Thus chlorophyll speeds up the
process of photosynthesis, but is not used up in the process.
Apparently other pigments, the carotenes and the xanthophylls, also assist in
photosynthesis. It is believed that that they aid chlorophyll in absorbing the light
energy needed for photosynthesis.
_________________Good Luck Students__________________
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