Bio 211 Intro Molecular and Cell Biology
Lecture 16 “Photosynthesis I”
Reading: Campbell Chap. 10 pp. 168-179
The last two segments of cell chemistry will continue with some metabolic
processes that are unique to plants and blue-green algae, the ability to harvest energy from
the sun as chemical energy in the process of photosynthesis. The processes we’ve talked
about so far, respiration and fermentation, are used by all living things, including plants to
derive energy from sugars. Plants and blue-green algae have in addition the special
ability to make the sugars themselves.
Outline:
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2.
3.
4.
Producers and consumers
Photosynthesis: overview
Photosynthesis: key experiments
Light reactions
1. Producers and consumers
Plants and other photosynthetic organisms produce most of the organic material
that feed the planet. Plants are autotrophs, meaning they make their food from inorganic
raw materials available in the environment. Autotrophs are the producers of the
biosphere, that is they make food available. Figure 10.1 gives some examples of
organisms that harvest light to produce food (photoautotrophs). These include land
plants--mosses, ferns, flowering plants: aquatic plants such as kelp, a multicellular algae;
single-celled protists like Euglena; and photosynthetic bacteria, such as cyanobacteria and
purple sulfur bacteria.
Animals and parasitic organisms are heterotrophs. Heterotrophs are unable to
make their own food, but most rely on compounds produced by other organisms.
Heterotrophs are the consumers of the biosphere.
We can also look at the interactions of producers and consumers at the level of the
processes that go on in plant and animal cells. Fig. 9.1 summarizes some of these key
processes.
Light energy from the sun is captured by plants and other photoautotrophs.
Particular organelles in plant cells called chloroplasts are the site of photosynthesis,
where light energy is captured as chemical energy in organic molecules such as sugars.
Another byproduct of photosynthesis is oxygen, which is crucial to the process of
respiration.
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Organisms (both plants and animals) transfer the chemical energy from sugar and
other molecules into the chemical energy of ATP. To do this most efficiently, they must
carry out respiration which requires oxygen and the specialized organelle, the
mitochondrion. Extra energy is released as heat and other byproducts of respiration
include carbon dioxide and water. Conveniently, these “waste products” for respiration
also serve as the raw materials for photosynthesis. Therefore there is both a flow of
energy and a recycling of chemical matter that occurs in a balanced ecosystem.
Today we will focus on the process of photosynthesis as it occurs in plant cells,
however we do want you to keep in mind how important this process is to all life on
earth.
2. Photosynthesis: An overview
Photosynthesis is the harvesting of light energy from the sun and its conversion to
chemical energy in the form of sugars. Overall, we can describe the reaction that takes
place as:
6 CO2 + 6 H2O + light energy ----> C6H12O6 + 6 O2
Chemically, it appears to be the reverse of respiration, where glucose is oxidized
to give carbon dioxide and water. Plants do not simply reverse the process of respiration
to make sugars, they produce sugars by a separate process.
If we look closely at the process of photosynthesis, we learn that it has two major
stages. See Figure 10.4. We often refer to these stages as the “light reactions” and the
“Calvin cycle” or dark reactions. They each consist of many individual steps.
a. Light reactions: Energy from the sun is used to split water to produce oxygen,
to reduce the cofactor NADP+ to make NADPH, and to make ATP.
b. Calvin cycle: Can take place in the absence of light for a short while. Carbon
dioxide is “fixed” by attaching it to a sugar; repeated cycles make the sugar longer. This
process uses CO2, NADPH and energy in the form of ATP.
Like the different stages of respiration, there are different parts of the cell that are
important for different aspects of photosynthesis. In plants, photosynthesis takes place in
specialized organelles called the chloroplasts. Within the chloroplasts the light reactions
occur on internal membranes in the chloroplast called the thylakoids. The Calvin cycle
occurs in the internal fluid portion of the chloroplast called the stroma.
3. Photosynthesis: Key experiments
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Before we study the light and dark reactions of photosynthesis in a little more
depth, let’s briefly take a look at how scientists learned about the process of photosythesis
by experimentation.
As I presented earlier, the process of photosynthesis can be summarized as
follows:
6 CO2 + 12 H2O + light energy ----> C6H12O6 + 6 O2 + 6H2O
In the reaction, oxygen is released. Scientists trying to figure out the process of
photosynthesis wondered, is the oxygen coming from the carbon dioxide or from the
water? We now know the oxygen comes from the water, but how could you show that
was the case.
Thus we can propose two alternative hypotheses:
OR
(1) The oxygen comes from the water.
(2) The oxygen comes from the carbon dioxide.
Once we have proposed a hypothesis, we need to devise an experiment to test it.
The first indirect test of this hypothesis came from a study of photosynthesis in
bacteria rather than in plants by C. B. van Niel in the 1930s. One group of bacteria
required hydrogen sulfide (H2S) rather than water (H2O) for photosynthesis. He found
that these bacteria carried out the following reaction during photosynthesis:
CO2 + 2 H2S ---> CH2O + H2O + 2 S
Instead of using water to produce oxygen, they seemed to split hydrogen sulfide
to produce sulfur. He predicted that in plant photosynthesis, the oxygen should be
coming from the splitting of water, as in the following reaction:
CO2 + 2 H2O ---> CH2O + H2O + O2
To test this directly in plants required new technology. The method that was used
involved using a heavy isotope of oxygen, 18O. Using heavy oxygen, it would be possible
to follow the fate of either the oxygen in the carbon dioxide or the oxygen in the water.
Thus you could devise two different experiments, that would directly test each of these
possible hypotheses:
Expt. 1: Test whether the oxygen is released from water. Tag the water oxygen
and determine whether it ends up in the oxygen after photosynthesis.
CO2 + 2 H2 18O ---> CH2O + H2O +
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18
O2
Expt. 2: Test whether the oxygen is released from the carbon dioxide. Tag the
oxygens in CO2 and determine whether it ends up in the oxygen. In this case, the tag
showed up not in oxygen, but in some of the released water.
C18O2 + 2 H2O ---> CH2O + H218O + O2
Thus by using isotopic tags for the atoms of oxygen, it is possible to demonstrate that the
oxygen of photosynthesis comes from the splitting of water.
4. Light reactions
To talk further about the light reactions of photosynthesis requires some
background. Therefore we will divide this discussion up into several parts.
a. Light absorbance
b. Chlorophyll and photosystems
c. Non-cyclic pathway
Light and other forms of electromagnetic energy travel in waves. Visible light
consists of different wavelengths of light in a narrow range of sizes, between about 380 to
750 nanometers. Fig. 10.5 shows the different types of electromagnetic energy; visible
light is but a narrow range of wavelengths that the human eye can detect.
Photosynthesis depends on the ability of certain pigment molecules called
chlorophylls to absorb visible light. We can measure the wavelengths of light absorbed
by photosynthetic pigments using an instrument called a spectrophotometer. Fig. 10.7a
shows the data obtained by measuring the light absorbance of photosynthetic pigments.
The chlorophylls and pigments most important for photosynthesis absorb light in
the blue and red range and reflect light in the green range. Therefore leaves appear green.
The blue and red light contribute the energy needed to split water in photosynthesis. This
is further confirmed by an action spectrum, which measures the effectiveness of different
wavelengths of light in stimulating photosynthesis. An action spectrum is shown in Fig.
10.7b.
The key pigments involved in photosynthesis are:
chlorophylls a and b (Fig. 10.8)
carotenoids
In the light reactions of photosynthesis, these pigment molecules absorb light and
ultimately transfer their energy to a special chlorophyll a molecule called “the reaction
center”. Figure 10.10 shows how a series of pigment molecules, called a “photosystem”
harvests light energy. Most of the carotenoid and chlorophyll molecules make up the
antenna complex. The each absorb light, then transfer their energy between them, and
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ultimately to a reaction center chlorophyll. This chlorophyll can then energize an electron
in photosynthesis.
Brief synopsis of the “light reactions”. Let’s return to our summary diagram to
describe what happens in the light reactions, so we can determine what the significance of
these pigments and reaction centers is. Fig. 10.11, top panel. The mission of the light
reactions is to absorb light and split H2O to form oxygen. The role of the reaction center
is to absorb light energy and to power up some electrons derived from the splitting of
water. This is shown in the lower part of figure 10.11. The electrons then pass through
an electron transport chain, similar to that found in mitochondrial respiration. As they
pass along the chain, they give up their energy and need a recharge from the second
photosystem. Ultimately the electrons go to reduce NADP+ to make NADPH.
What are the major products of the light reactions?
(1) water is split to form oxygen
(2) the electrons from oxygen pass to the electron transport chain
(3) the flow of electrons is coupled to the pumping of protons
(4) a proton gradient is used to make ATP
(5) electrons ultimately go to reduce NADP+ to make NADPH, needed for the
Calvin cycle.
For homework, you will need to answer, “How do the light reactions lead to the
formation of ATP? To answer this, you will need to read closely pages 177-179 of the
textbook and study Figures 10.14 and 10.15.
Summary: Today, we started to examine the critical role played by photosynthetic
organisms to life on earth. We mostly focused on the elements critical to the light
reactions of photosynthesis, how the photosystems harvest light from the sun and use it to
split water, forming oxygen. In the last cell chemistry lecture, we will explain how the
light reactions make ATP and then trace what happens to the products of the light
reactions in the Calvin cycle or “dark reactions of photosynthesis”.
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