CONCEPTUAL LIFE SCIENCE
Energy Capture by Plants
INTRODUCTION
Green plants produce food for humans and other animals by means of the process
of photosynthesis. Without green plants, life as we know it would soon cease to exist.
Green plants absorb light energy from the Sun and convert it to stored chemical energy.
Because of this, green plants are the basis of the food chain.
The Sun is a star. It is the nearest star to the Earth and it is the principal
component of the Solar System. The Sun gives off electromagnetic energy.
Electromagnetic energy consists of radiation in the form of waves. These waves have
different wavelengths. The shorter the wavelength, the more energy the radiation has.
The various types of electromagnetic radiation form the Electromagnetic Spectrum that is
illustrated in Figure 6-1.
Figure 6-1. The electromagnetic spectrum. Light is an example of electromagnetic
energy. Heat and ultraviolet radiation are also examples of electromagnetic radiation.
Physicists explain that the waves of electromagnetic radiation, such as light, are
made up of little particles called photons. Photons are little bundles of energy. The green
plant contains chlorophyll, which has the ability to absorb photons of light. The plant
uses red and blue light. It absorbs these wavelengths. It does not use green light. That is
why it looks green. The plant reflects green light because it does not use it.
6-2
When light is absorbed by chlorophyll, the chlorophyll gets excited and expels an
electron. For each pair of these electrons, which contain electrical energy, the plant can
make molecules that store chemical energy. These molecules are known as ATP and
NADPH. The abbreviations are identified as follows:
ATP = Adenosine Triphosphate,
NADPH = Nicotinamide Adenine Dinucleotide Phosphate.
The processes that produce these molecules are called Cyclic and Non-cyclic
Photophosphorylation.
Basically photophosphorylation means making of ATP (phosphorylation) using
energy from light (photo). ATP is a widely encountered energy-storage molecule in
living systems.
Definition of photosynthesis
Photosynthesis is the capture and use of light energy to produce organic materials
from inorganic raw materials. In photosynthesis, light energy from the Sun is trapped
and converted to chemical energy. The chemical energy is used to change CO2 into
glucose, a simple sugar. Details of these two components follow below.
Figure 6-2. General reaction of photosynthesis. The CO2 entering at the left is converted
to glucose, which has the formula C6H12O6. Oxygen is the waste product of
photosynthesis. The oxygen results from photolysis of water, a process that replaces the
electrons of non-cyclic photophosphorylation.
Light energy from the Sun is trapped and converted to chemical energy
Chlorophyll is located in the grana of the chloroplast. The primary locale of
photosynthesis in green plants is in the leaves. Leaf tissue contains layers of cells that
have large numbers of chloroplasts to absorb solar energy.
In the light reaction of photosynthesis, light energy from the Sun is trapped by
chlorophyll and converted to chemical energy in the two forms of ATP and NADPH.
Cyclic photophosphorylation produces only ATP. Non-cyclic photophosphorylation
produces both ATP and NADPH. In order to complete the process, a source of electrons
is necessary. Green plants use H2O and extract the electrons from it using a reaction
called photolysis of water.
6-3
Cyclic photophosphorylation
In this process, light causes a pair of electrons to be released from a pair of
chlorophyll a molecules. These electrons travel a cycle where some of their energy is
used to make ATP. The electrons will return to the chlorophyll a molecules from which
they came. This is why the pathway is cyclic.
Figure 6-3. Cyclic photophosphorylation.
The pair of electrons in Photosystem I receives energy from red light. The
electrons lose some of their energy as they pass to postulated molecule X. As they
continue through the cycle, energy is lost each time they move from one molecule to
another. Sometimes, some of the energy is retained in the form of ATP. At the end of
the cycle, the electrons have lost all of their energy and they return to chlorophyll a.
X. The current explanation predicts that there is an unknown molecule that
receives the two electrons from chlorophyll a before they are sent to ferredoxin.
Ferredoxin (Fd). Ferredoxin is a special electron-transferring molecule that
accepts electrons from molecule X. Ferredoxin easily adds electrons to other molecules.
Cytochromes. Cytochromes are enzymes that transfer electrons to other
substances. In certain cases the energy is not completely lost but some of it is used to
make ATP.
Phosphorylation. Phosphorylation is the process of making adenosine
triphosphate (ATP). The cell begins with the diphosphate (ADP) and adds an inorganic
phosphoryl group (Pi) to it. These phosphate compounds are derived from phosphoric
acid. Every ATP stores 7,400 calories of energy when it is made. Cyclic
photophosphorylation does not make NADPH, so non-cyclic photophosphorylation is
necessary.
6-4
Non-cyclic photophosphorylation
The process of non-cyclic photophosphorylation involves three systems. These
are as follows:
 Photosystem I
 Photosystem II
 Photolysis of Water
We have already been introduced to Photosystem I as it is also part of the cyclic
photophosphorylation process. All three systems are required to produce NADPH.
Photosystem I
Photosystem I sends two electrons to NADP+ in the non-cyclic
photophosphorylation process. The electrons begin travelling the first part of the cyclic
pathway. They make one molecule of ATP during this passage. In non-cyclic
photophosphorylation, however, the electrons are sent from ferredoxin to NADP+ to
produce NADPH. The electrons do not return to the chlorophyll a molecules of
Photosystem I. Photosystem II is used to replace the electrons that did not return to
Photosystem I.
Photosystem II
Photosystem II replaces the electrons lost by Photosystem I. Photosystem II has
both chlorophyll a and b and it responds to both red and blue light. After leaving
Photosystem II, the electrons travel to Coenzyme Q (an electron-transferring enzyme)
and then the other part of the cyclic pathway back to Photosystem I. The electrons do not
return and must be replaced. During this pathway, another ATP molecule is made. Both
photosystems make ATP when they are operational.
Photolysis of water
Photosystem II gets new electrons by photolysis of water. “Lysis” in biology
implies something being broken down, getting destroyed or exploding. In this case, the
chloroplast has a way of destroying the water molecule and making it into three
components. The first component consists of protons (H+). These protons are used as
part of the reaction that makes NADPH. The second component consists of electrons.
The electrons replace the electrons lost by Photosystem II. The third component is
oxygen gas (O2). Oxygen gas is released by the plant into the atmosphere through the
stomates. Oxygen is the waste product of photosynthesis.
6-5
Figure 6-4. Non-cyclic photophosphorylation.
Photosystem I sends electrons to reduce NADP+. Then, Photosystem II sends
replacement electrons to Photosystem I. Finally, photolysis of water replaces the
electrons lost by Photosystem II. Therefore, water is the ultimate source of electrons for
photosynthesis. The oxygen released by the plant comes from photolysis of water.
Oxygen is the waste product of photosynthesis.
Summary of light reaction
The light reaction of photosynthesis comprises all of the light-dependent or
photochemical reactions. For each pair of electrons, the plant makes one NADPH and
two ATP molecules. These molecules are used to produce glucose sugar during the
carbon-fixation reactions of photosynthesis. Photolysis of water provides the electrons
used to produce the NADPH and to carry the electrical energy necessary to make ATP.
Oxygen is released from photolysis of water as a waste product.
The chemical energy is used to change CO2 into glucose, a simple sugar
The production of glucose from carbon dioxide (CO2) takes place all the time,
whether the Sun is shining or not. As it is the only part of photosynthesis that works
without light, it is sometimes called the dark reaction of photosynthesis. The dark
reaction of photosynthesis contains the carbon-fixation reactions. These reactions are
localized in a pathway known as the Calvin-Benson Cycle.
The Calvin-Benson Cycle
Overall reaction
6 RuDP + 6 CO2 + 18 ATP + 12 NADPH  6 RuDP + 1 Glucose + 18 ADP + 18 Pi + 12 NADP +
6-6
Figure 6-6. The Calvin-Benson cycle. The cycle uses ATP and NADPH produced by the
photochemical reactions. This energy is used to incorporate CO2 into RibuloseDiphosphate (RuDP). The resulting molecules PGA and PGAL lead into nine more
reactions. For every six turns of the cycle, one molecule of glucose results.
The Calvin-Benson cycle constitutes the essence of the dark reactions of
photosynthesis. This pathway is of a type known as anabolic. Anbolic pathways build
things. In this case, the molecule being built is glucose. Glucose is built by reduction of
CO2. This means that the Calvin-Benson pathway is also a reductive pathway.
Reductive, anabolic pathways are commonly known as biosynthesis.
Study guide for photosynthesis
A.
General considerations
1.
Photosynthesis requires light, chlorophyll and CO2.
2.
The overall equation of photosynthesis is:
energy + 6 CO2 + 12 H2O  C6H12O6 + 6 O2 + 6 H2O
3.
4.
B.
The raw materials for photosynthesis are H2O and CO2.
The waste product of photosynthesis is O2, which comes from the
breakdown of water.
The photochemical (light) reactions
1.
Light is absorbed by chlorophyll
a.
Red and blue light are absorbed.
b.
Green light is not absorbed, it is reflected.
6-7
2.
3.
4.
5.
6.
C.
The process of photosynthesis occurs in the chloroplasts of plant cells.
Water is broken down by photolysis to replace the electrons in the noncyclic pathway.
a.
The H is incorporated into the glucose sugar.
b.
The O is released as a waste product.
The light energy is converted to electrical energy.
The electrical energy is converted to chemical energy and is stored in the
form of ATP and NADPH.
The photochemical reactions occur only in the light.
The carbon-fixation (dark) reactions
1.
The carbon-fixation reactions of photosynthesis occur all of the time,
whether there is light or not.
2.
These reactions do not occur without CO2.
3.
These reactions use the energy stored in ATP and NADPH to convert CO2
to glucose.