Chapter 7
Photosynthesis: Using Light to Make Food
Overview
In this chapter we will examine the process of photosynthesis and the importance of this process to life on this
planet. Photosynthesis and cellular respiration (Chapter 8) are linked by the equation:
The reactants on the left are the inputs to photosynthesis and the products are the indicated on the right.
Photosynthesis consists of two phases: a light dependent phase in which light energy is converted to chemical
energy (ATP) and a light independent phase (Calvin-Benson cycle) which forms the sugar (glucose) molecule.
There are two types of light dependent reactions. The first, and most ancient is the cyclic method while the
more efficient method is called non-cyclic.
The final section of this unit will examine the modifications to photosynthesis that have enabled some plants to
adapt to harsh environmental conditions which otherwise would have made the photosynthetic process
unfeasible.
Assigned Reading
Text, Pages 107-118
PowerPoint Presentation
Chapter Review, Page 121-122
Testing Your Knowledge, Page 122
Key Terms
 Chlorophyl
 Chloroplast
 Thylakoid
 Stroma
 Stomata
 Grana
 Autotroph
 Heterotroph
 Cyclic, Noncyclic Pathways
 Light-Dependent, Light-Independent Pathways
 NADPH
 RuBP
 Chemiosmosis (Electron Transport System)
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

Photosystems (P680/P700)
Phosphorylation
Calvin Cycle
Introduction
In the previous lesson we examined the role of enzymes in metabolic reactions. In this chapter we will begin to
examine the processes by which living organisms acquire energy. Each of the pathways in this, and the next
chapters require multiple enzymes to function. Each of these enzymes must be regulated in some manner.
Metabolic pathways are thus very complex, and while we will simplify them for this course, what we are
examining here today is the result of decades of scientific investigation.
Read the introduction carefully, focusing on the definitions of autotrophs and heterotrophs. In this chapter we
will examine the autotrophs, while Chapter 6 examined the hetrotrophic organisms. We will see however, that
both types of organisms are interconnected.
Overview of Reactions
One of the first things that you should focus on is Figure 7.5 which shows the cellular locations of the reactions
of photosynthesis. Note that each cell may contain 20-30 chloroplasts, and that each chloroplast contains
hundreds of the plate like structures called thylakoids. In these thylakoids, the reactions of photosynthesis
occur. So a single leaf may have millions of cells, each with hundreds of thylakoids. In other words, trees
represent massive photosynthetic pathways.
Sunlight and Photosynthetic Pigments
We all know that plants need sunlight for energy.
But the sunlight that we see as humans is much
more complex that what it appears to be. The first
part of this section does a very nice job talking
about the spectrums of energy and the absorption
spectrum of pigments.
The process of photosynthesis will convert solar
energy (kinetic energy) into ATP (potential energy)
for use by the plant to make sugars. To do that we
must capture the photons. This is done by the
photosynthetic pigments located with the
chloroplast. Note that the photosynthetic pigments
have different absorption spectrums (see image).
They actually function like TV antennae - focusing the solar energy into a central molecule called a reaction
center. There - the photon strikes an electron and transfers its kinetic energy.
Electrons and ATP
Remember that the end product of photosynthesis is sugar (glucose), and that this sugar will be assembled from
carbon dioxide and water. The making of a larger molecule requires an input of energy (Second Law of
Thermodynamics). Sunlight is that energy. So a cell must generate energy using sunlight.
There are two types of reactions in photosynthesis - light dependent reactions and light independent reactions.
They used to be called "light" and "dark" reactions, but that gave people the idea that the latter only occurred at
night, which is not the case. We will focus on the light dependent reactions first.
Cyclic Light-Dependent Photosynthesis
Remember that the first stage of photosynthesis converts light energy (photons) into chemical energy (ATP).
The easiest way to do this is a cyclic process. In the cyclic process an electron on the reaction center of the
photosystem is struck by a photon. This excited electron leaves the photosystem where it is captured by an
electron transport system (ETS). The ETS harvests the energy from the electron, and when the energy level has
returned to normal, returns the electron to the photosystem where it began.
This process could be demonstrated if I formed a bunch of students into a circle and handed the first student a
hot rock. As they passed the rock around, some of the heat would be retained in the palm of each person until
the rock reached room temperature. In the ETS, the enzymes (proteins) harvest the energy of the electron in
much the same manner. Note that this process uses a photosystem called P700, and that it recycles electrons.
Non-cyclic Light-Dependent Photosynthesis
The process above, which just uses P700, is the more ancient of the pathways. The modern pathway is shown in
your text. You need to understand this pathway thoroughly. In the non-cyclic pathways, the electrons are not
returned to their original photosystem and we will use two photosystems (P680 and P700) in the process.
Note that the electron from P680 replaces the electron lost by P700 during the reactions. This still forms ATP (1
ATP per 3 photons). But the electron lost by P700 does not return to P680. Instead, it is captured by a molecule
called NADP+. When we add an electron (-) to this positively charged molecule, we form NADPH ( since
hydrogen has a single proton and electron). Thus, if we follow the hydrogen atoms in these reactions, we can
follow the path of the electrons. NADPH will become important in the next section.
But what happens to P680? We have lost an electron and not replaced it. To regenerate this electron, P680
splits water (a process called photolysis) into hydrogen, electrons and oxygen. The electrons fill the "holes" in
P680, while the oxygen is released into the atmosphere. Therefore, oxygen is a waste product of non-cyclic light
dependent photosynthesis.
Chemiosmosis or the Electron Transport System (ETS)
So how exactly is that ATP formed during these reactions? Remember that ATP is a high energy molecule and
that ADP represents a lower energy molecule. The process of phosphorylating ADP to form ATP requires energy.
Electrons represent energy. So what happens in the thylakoid is that the electrons are pumped to one side of
the membrane. As this side of the membrane becomes more negative, and electric difference of potential is
generated across the membrane.
A great analogy to this is a hydroelectric dam. In a dam, water (electrons) are pumped to one side of the dam.
When needed a channel is opened (an integral protein). As the water enters, its energy is tapped to do work, in
our case, the formation of ATP from ADP.
Remember that this process is being conducted by autotrophs, and that autotrophic organisms make their own
food. For plants, that food is glucose. Glucose has the chemical formula C6H12O6, thus to form glucose we will
need a few things: carbon, oxygen, hydrogen and energy to hold it all together.
The metabolic pathway that assembles glucose is the Calvin cycle (also called the Calvin-Benson cycle). Notice
that it is not dependent on light, and thus has no photosystems or oxygen involved. The Calvin cycle is a cyclic
type of pathway ( see Figure 7.16). We will not spend a lot of time at the moment in this pathway since the next
chapter examines the reverse process.
From our initial equation on photosynthesis we observed that plants require water, carbon dioxide and energy
to form sugar. We have talked about the water and energy already. The Calvin-Benson cycle requires and input
of carbon dioxide. As a gas, CO2 is difficult to work with. So the first thing that is done is that the CO2 is attached
to a molecule called ribulose bisphosphate (RuBP). Now we have all of the materials needed. But first a quick
review of where they came from:
 Carbon from CO2
 Hydrogen from NADPH in non-cyclic light-dependent reactions
 Oxygen from CO2
 ATP from non-cyclic light-dependent reactions.
The Calvin cycle uses these to form sugar. Note that in the process it forms NADP + and ADP, which are returned
to the non-cyclic light dependent reactions.
Environmental Variations
Just when you have it all figured out, the environment changes. Not all plants have the exact same
photosynthetic processes. Due to temperature or carbon dioxide availability, some plants have altered the
above pathways to adapt to their environment. Be able to recognize that the C4 and CAM plants represent
environmental adaptations to photosynthesis.
Links of Interest
Photosynthesis Center at ASU—check out the page on "what is photosynthesis" for more information on this
process
Concepts
 Understand the relationship between heterotrophs and autotrophs.
 Understand the principles of photosynthesis.
 For both the light-dependent and –independent reactions, know where the reaction occurs, the major
molecules involved, the inputs to the cycles, and the outputs.
 Recognize the difference between the cyclic and non-cyclic photosynthesis and the significance of the
latter.
 Understand the environmental modifications of the photosynthetic process in some plants (e.g. C4,
CAM).
Review Material
MyBiology.com—Study guides and resource for this text. Specifically look at MP3 Tutor and all Web Activities