Obtaining Cellular Energy
Photosynthesis
Photosynthesis is the process of converting carbon dioxide, water and light energy
into oxygen and high-energy sugar molecules. The chemical equation representing
this process is 6CO2 + 6 H2O + light C6H12O6 + 6 O2. Plants, algae and some
bacteria can use the sugar molecules produced during photosynthesis to make
complex carbohydrates such as starch of cellulose for food. The process of
photosynthesis consists of two basic stages: Light-dependent reactions and lightindependent reactions (also called the Calvin cycle).
Photosynthesis takes place inside an organelle called a chloroplast. A chloroplast is
one type of organelle called a plastid. Plastids engage in photosynthesis and store
the resulting food. The chloroplast is a specific organelle with a double membrane
that contains stacks of sac-like membranes called thylakoids. The thylakoid
membrane contains a green pigment called chlorophyll. Pigments are substances
that absorb light. Light-dependent reactions take place inside the thylakoid
membrane. Light-independent reactions take place in the stroma. In the lightdependent phase, sunlight hits the leaf of the plant where it is absorbed by the
pigments in the leaf. There are several plant pigments, but the most familiar is
chlorophyll, the green pigment. Chlorophyll is stored in the chloroplasts of the plant
cell.
When light hits the chlorophyll, electrons absorb the energy, become excited and
leave that chlorophyll molecule. Carrier molecules transport the electrons which
follow an electron transport chain. Electron acceptor molecules pick up the
electrons in a series and pass them from one molecule to another. As this occurs,
energy is released, and ATP is formed. The final electron acceptor is NADP+.
Splitting a molecule of water replaces the electrons released from the chlorophyll.
These electrons, now available, combine with the NADP+ to form NADPH. The next
stage of photosynthesis uses the NADPH, while oxygen leaves as an end product of
the reaction.
The end products of the light-dependent reactions are ATP, oxygen and NADPH.
The ATP and NADPH are used in the light-independent reactions, and the oxygen is
released into the atmosphere.
The next phase, the light-independent or carbon fixation reactions, uses the ATP
formed during the light-dependent reaction as an energy source. In this phase,
carbon, from carbon dioxide, and NADPH are used to form glucose. To accomplish
this, a five-carbon sugar (called a pentose) uses a carbon atom from carbon dioxide
to create a six-carbon sugar (a hexose). Glucose is the end result, after several
conversions have taken place. The glucose can then be used as food to enter cellular
respiration, or it can be converted to other carbohydrate products such as sucrose
or starch.
Cellular Respiration
Cellular respiration is the process of breaking down food molecules to release
energy. Plants, algae, animals and some bacteria use cellular respiration to break
down food molecules. There are two basic types of cellular respiration: aerobic and
anaerobic. Aerobic respiration occurs in the presence of oxygen and is represented
by the chemical equation below. The energy released through cellular respiration is
used to create ATP. Cellular respiration occurs in three phases: glycolysis, Krebs
cycle, and electron transport. The process starts with a molecule of glucose and the
reactions of cellular respiration occur with the use of enzymes. Respiration is the
primary means by which cells obtain usable energy.
Glycolysis is the first phase in cellular respiration. This step occurs in the cytoplasm
of the cell, and it can occur whether or not oxygen is present. In this phase, the
glucose molecules (a 6-carbon sugar) is broken in half through a series of reactions.
The energy released by breaking down the glucose is used to produce ATP.
Additionally, some high-energy electrons are removed from the sugar during
glycolysis. These electrons pass on to an electron carrier called NAD+, converting it
to NADPH. These electrons will alter b used to create more energy.
In aerobic respiration, the 3-carbon sugars produced from glycolysis enter the
mitochondria along with oxygen. As the sugars enter the mitochondria they convert
to citric acid in phase two of cellular respiration. The citric acid cycle, or Krebs
cycle, is the cyclical process that breaks down the citric acid through a series of
reactions The citric acid cycle produces more ATP, as well s some GTP (a highenergy molecule similar to ATP). More high-energy electrons are released, forming
NADH from NAD+.
The last phase of cellular respiration is the electron transport chain, which occurs
on the inner mitochondrial membrane. In this phase the NADH releases the
hydrogen ions and high-energy electrons it picked up during glycolysis and the citric
acid cycle. The energy from these electrons is used to convert large quantities of
ADP into ATP. The electrons transfer through a series of carrier proteins. At the
end of the electron transport chain, the free electrons and H+ ions bond with
oxygen. The oxygen and H+ ions form water, which is released from the cell as
waste product. Each electron transfer releases energy. The multi-step processes
carried out during aerobic cellular respiration are very efficient. From the input of
one glucose molecule, 36 ATP molecules can be produced.
Anaerobic Respiration
Anaerobic respiration, or fermentation, is the process by which sugars break down
in the absence of oxygen. Our muscle cells, fungi and some bacteria are capable of
carrying out anaerobic respiration. These cells convert the products of glycolysis
into either alcohol or lactic acid. Glycolysis releases energy, while the production of
alcohol or lactic acid provides NAD+, the electron carrier needed for glycolysis.
Yeast and some bacteria can carry out alcoholic fermentation. Yeast produces
ethanol through alcoholic fermentation. The chemical equation representing this
process is shown below. During the fermentation process, carbon dioxide gas is
released. This carbon dioxide gas is responsible for the holes in bread. The
fermentation of yeast cells produces carbon dioxide, which becomes trapped in the
dough, forming small bubbles and causing the bread to rise. Other uses of alcoholic
fermentation are the making of beer, wine, and liquor.
Animal cells use a form of anaerobic cellular respiration called lactic acid
fermentation. This generally occurs when the organism is engaging in strenuous
exercise. During this period, the organism is not able to take in enough oxygen to
meet the demand of all the cells in the body. So in order to keep working, the cells
begin to break down glucose in the absence of oxygen. The products of the reaction
are energy and lactic acid.
Lactic acid fermentation proceeds at a very fast rate, but the amount of energy
produced is far less than with aerobic respiration. Remember the 36 ATP molecules
theoretically produced during aerobic respiration? By comparison, lactic acid
fermentation produces only 2 ATP molecules.
So, can you tell if your cells are respiring aerobically or anaerobically? Well, you can
assume that if you are doing anything that causes muscle fatigue-like weight lifting,
those muscles have needed extra energy very quickly, and they obtained it through
lactic acid fermentation. Your suspicions will be confirmed the next day if you are
sore. Muscle soreness occurs because of lactic acid build-up in the muscle.
Comparing Photosynthesis and Cellular Respiration
All organisms must be able to obtain and convert energy to carry out life functions,
such as growth and reproduction. Photosynthesis is one way that organisms can
rap energy from the environment and convert it into a biologically useful energy
source. Cellular respiration is a way that organisms can break down energy sources
to carry out life’s processes. Photosynthesis takes place in plants, protists, algae and
some bacteria. Cellular respiration takes place in all eukaryotic cells and some
prokaryotic cells.
Function
Location
Reactants
Products
Chemical equation


Photosynthesis
Energy storage
Chloroplast
Carbon dioxide and water
Glucose and oxygen
Cellular Respiration
Energy release
Mitochondria
Glucose and oxygen
Carbon dioxide and water
6CO2 +6 H2O C6H12O6 + 6O2
C6H12O6 + 6O2 6CO2 + 6H2O

Section Review Questions
1. Define the following terms:
a. Photosynthesis
b. Calvin cycle
c. Chloroplast
d. Thylakoid
e. Chlorophyll
f. Pigment
g. Plastid
h. Light-dependent phase
i. Light-independent phase
j. Carbon fixation
k. Cellular respiration
l. Aerobic respiration
m. Anaerobic respiration
n. Glycolysis
o. Krebs cycle
p. Electron transport chain
q. Alcoholic fermentation
r. Lactic acid fermentation
s. chemosynthesis
2. What form of energy is used by cells?
a. Enzymes
b. Cofactors
c. ATP
d. DNA
3. The process of releasing energy from the chemical breakdown of
compounds in a cell is
a. Hesitation
b. Expiration
c. Elimination
d. respiration
4. What is released when ATP is broken down into ADP and one
phosphate?
a. Oxygen
b. Water
c. Energy
d. hydrogen
5. The process by which the energy from the Sun is used to create
glucose molecules is known as
a. Cellular respiration
b. Photosynthesis
c. Chemosynthesis
d. fermentation
6. What is a plastid’s function within a cell?
a. Digest food and break down wastes
b. Produce proteins
c. Carry on cellular respiration
d. Carry out photosynthesis and provide color
7. Compare and contrast aerobic and anaerobic respiration.
8. Write the chemical equation for photosynthesis and cellular
respiration using words instead of chemical formulas.