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Week 6
Plants are very important in biology. In fact, one could argue that they are the single
most important organism on the planet (also, one could argue that bacteria are just as
important but lets just stick with plants for now!).
Although there is a great deal of diversity in plants, the majority of plants are composed
of two systems: the shoot system and the root system. In order for most plants to
survive, both systems are needed.
The root system of plants generally consists of the parts below the ground. Roots
function in anchoring a plant into the ground, absorbing minerals and water, and often as
a storage site for organic molecules (such as starch).
The shoot system of plants can generally be thought of as the parts above the ground.
The shoot system consists primarily of the stems and leaves. Leaves are the main
photosynthetic organ of most plants.
Many plants also have flowers as part of the shoot system. Many flowers have BOTH
the male and female parts on same flower. The male parts of the flower (called the
stamen) make pollen. The female parts of the flower (called the carpel) are where the
egg is made.
Remember, also, from last week, two main types of tissues in plants (but not the only
two) are xylem (used to transport water) and phloem (used to transport food).
The leaves of plant (like the other parts of a plant) are made up of cells. These cells have
organelles in them called chloroplasts. Chloroplasts contain little sacks called
thylakoids. The thylakoids have a type of membrane around them very much like that of
the mitochondria and the cell or plasma membrane of the entire cell. The thylakoids are
the place where all the action takes place. Here is a little picture to help you visualize all
of that: In your book, you will see that not all plants look like this. There are many kinds
of plants so just realize this is a generalization.
Photosynthesis, just like cellular respiration, can be divided up into parts. There are two
main parts to photosynthesis: 1) The Light Reactions and 2) The Dark Reactions. The
Dark Reactions are also often referred to as the “Light Independent Reactions” or the
“Calvin cycle”. Once again, refer to the SUMMARY CHART to help keep track of
what you need to know.
Step 1-Light Reactions: In the Light reactions, Light Energy is converted into Chemical
Energy. This is done in a somewhat complex way. First, let’s look at the basic idea of a
photosystem. When light hits chlorophyll (a chemical that is sensitive to light), it alters
the chemical structure of the molecule. Basically, it alters the structure by changing the
way the electrons move around the molecule. When light hits chlorophyll, the energy is
transferred to the center of the molecule.
Next, another molecule, a protein called a primary electron acceptor, grabs hold of one
of those electrons which is basically the source of that increased energy. Therefore, the
primary electron acceptor is grabbing hold of a high energy electron.
So now that you have the basic idea of how a photosystem works, let’s look at how these
photosystems operate in the light reactions. In the light reactions, there are two
photosystems. We call them photosystem I and photosystem II (this is the order in
which they were discovered). We will start our story with photosystem II (it makes more
sense in this order. Use the picture below as a reference:
Sunlight hits the chlorophyll in photosystem II. This causes a change in the structure of
of the chlorophyll that is in photosystem II and creates a high energy electron. Basically,
the light energy in sunlight is transformed into a type of chemical energy in terms of how
the electron moves about in a molecule. The primary electron acceptor (a type of protein)
grabs that high energy electron. That high energy electron is then passed from the
primary electron acceptor from protein to protein as it moves down an electron transport
chain. The energy that is generated in this electron transport chain is used to make some
ATP. This electron transport chain is very similar to the electron transport chain in
cellular respiration. In fact, it works the same exact way. If you remember the electron
transport chain in cellular respiration, the final electron acceptor was oxygen. This time,
instead of oxygen, photosystem I will be the electron acceptor. Photosystem I is doing
the same basic thing as photosystem II. Light hits it and the energy is transferred in the
form of moving an electron to a primary electron acceptor. The energy in the electron
transport chain of photosystem I, however, is not used to make ATP; instead it is used to
make NADPH. NADPH is very similar to NADH (in cellular respiration). You can
think of NADPH has a high energy electron “holder”. The main goal of the light
reactions is to make NADPH.
Once a photosystem has had a beam of light hit it and the high energy electron removed,
it cannot work any more until it replaces that electron. In other words, only one electron
in the photosystem is able to obtain that high energy state. Once the primary electron
acceptor takes it away, it can no longer get an electron in that high energy state. So it
needs to replace it in order to get it to work again. Photosystem I replaces its electron by
getting one from photosystem II. But how does photosystem II replace its electron?
Here, a very special reaction called water splitting occurs. Water is basically split into
pieces (by an enzyme) and the electrons are removed from the hydrogens. This reaction
releases oxygen. Everyone has heard (or at least they should have) that plants give off
oxygen. Well, this is where they do it - in the light reactions of photosystem II!
Here is another picture showing the light reactions. This one actually shows how it takes
place across they thylakoid membrane. This should look very similar to something we
learned last week (the electron transport chain of cellular respiration). It basically works
the same way. High energy electrons are passed down a series of proteins and energy is
released to conduct active transport of molecules to build a gradient. This is then used to
generate ATP and NADPH in the case of photosynthesis.
Step 2-Dark Reactions: In the dark reactions, the NADPH that we produced in the light
reactions is going to be used. What happens in the dark reactions is this: The plant is
going to take in several molecules of carbon dioxide (CO2). The carbon dioxide gets into
the plant through the leaves through tiny little openings called stomata. An enzyme
called “rubisco” is going to basically glue those molecules of CO2 together. The high
energy electrons from the NADPH are going to be transferred to this new molecule. The
ATP is also used here (remember, we made some ATP during the light reactions) and the
energy is transferred to this new molecule we are building. We end up with a molecule
called G3P. G3P is actually a molecule that you get when you first split a glucose
molecule in cellular respiration. Therefore, what we have really made when we make
G3P is the equivalent of a glucose molecule.
Now, you may not realize the importance of all of this yet. Right now, it might just seem
like a big mess of stuff. But you should realize this, the main goal of the whole process
of photosynthesis is to make glucose (or more accurately, G3P). This is done by plants
taking in water, carbon dioxide, and sunlight. So, basically, plants are transforming light
energy into chemical energy. Here is a picture summarizing the dark reactions:
Remember that all living things need energy in order to survive. Even you. So let’s say
you are eating a hamburger in order to meet those daily energy requirements. Where
does the bread come from? Bread has energy in it stored up from plants. Where does the
meat come from? It comes from cows. But the cows got all of their energy from eating
grass and hay and whatever a cow eats! These plants went through the process of
photosynthesis also. Basically, you absolutely need to realize, that all living things on
earth are dependent on photosynthesis. Without photosynthesis, you don’t have any light
energy being converted into chemical energy.
Let’s change gears for just a minute. What is the number one agriculture state in the
U.S.? You might have images of field after field of hay and barley and corn from some
Midwest state like Nebraska or Kansas. But the number one agriculture producing state
in the U.S. is actually California! In other words, more than any other state, California
produces the most food overall. Now what else do you have going on in California as
well as many other places in the U.S.? Development. When you drive around southern
California, you will of course notice that nearly every inch of ground is being developed.
It wasn’t always that way. There was a time, not even that long ago, that southern
California was filled with agriculture fields. I grew up in Oxnard (we call it “the nard”)
and my housing tract had 4 or 5 good sized agriculture fields within ½ mile of my house
This was in the late 1980’s. Now, they are all gone. One was turned into a Wal-Mart
(because you really need to make sure you have enough Wal-marts!), one is a high
school, and the others are housing tracts. One is actually a park now, but those aren’t
very good odds overall. So, what are we going to do? More and more people, less and
less land, less and less plants, less photosynthesis, put it all together. This is not a good
thing. We as humans make a very, very bad assumption that as long as the sun comes up
tomorrow and I can still text message and drive my car, everything is fine! But it is
it….More on this come at the end of the semester.
Here is a summary of inputs and outputs of Photosynthesis. This is what you should
really focus on remembering:
SUMMARY CHART:
Step:
Inputs:
Outputs:
Main Goal:
Light Reactions
Water, ADP, NADP+
Oxygen, ATP, NADPH
Make NADPH and ATP
Dark Reactions
CO2, NADPH, ATP,
Rubisco (an enzyme)
Glucose (G3P)
Make Glucose (G3P)
Now go back and compare the inputs and outputs of cellular respiration to the inputs and
outputs of photosynthesis. You should realize that the inputs of cellular respiration are
basically the outputs of photosynthesis and vice versa. This will be important for the
final exam.
Coming Up….(but you need to know at least this part
for this coming quiz!)…
There are many kinds of organisms in the world. Any particular kind of organism is
called a species. There are many different definitions of what a species is, but one that is
commonly used is any group of organisms that are capable of reproducing fertile
offspring are considered to be the same species (although this definition does not always
work).
Species can be classified based on their various characteristics. The science of naming
and classifying organisms is called taxonomy. Taxonomy was most influenced by
Carolus Linnaeus who wrote a book called Systema Naturae in 1735. In this book,
Linneaus gave everything a two part name. The first part of the name was called the
genus and second part of the name was called the species. This system is still used today
to name and classify organisms.
In addition to the genus and species, organisms can further be classified based on
characteristics that they share in common with other organisms. Dogs and cats, for
example are separate species but they both have hair so they are also said to share certain
characteristics that make them both mammals. The other groupings we used (going from
more general to more specific are) Kingdom, Phylum, Class, Order, and then Family.
Organisms can be identified based on physical characteristics, DNA, behavior, or all of
these. We often use physical characteristics because they are the easiest to observe. In
identifying organisms, a dichotomous key is often used. A dichotomous key is a chart
that basically allows you to examine a species for various characteristics and as you make
decisions, the key guides you to the name of the specific organism (check out lab 7 for an
example).
ASSIGNMENT #6 – Print this sheet off and turn it in with your lab next week. This
sheet of paper goes on top (then the lab).
1) Electron that are lost during photosystem I reactions are directly replaced from:
A) The splitting of water
B) photosystem II
C) oxygen
D) NADPH
E) ATP
2) Which of the following is an enzyme that is involved in the dark reactions of
photosynthesis?
A) Sucrase
B) Glucase
C) Rubisco
D) Phosphorylase
E) Nitrogenase
3) The female parts of a flower are collectively called the:
A) Filaments
B) Stamens
C) Anthers
D) Pistil
E) Carpel
4) The male parts of a flower are collectively called the:
A) Filaments
B) Stamens
C) Anthers
D) Pistil
E) Carpel
5) Plants mainly get water from their:
A) Stomata
B) Roots
C) Shoots
D) Chlorophyll
6) What is the most abundant protein on earth?
7) What are three different types of photosynthesizers?
8) How much (in weight) can photosynthesis produce every day?
Words that you may be asked to define or use in fill-in-the blank types of questions:
Shoot System, Root System, Light Reactions, Dark Reactions, Calvin Cycle, Rubisco,
NADPH, Water, Water Splitting, Chlorophyll, Chloroplasts, Thylakoid, Photosystem I,
Photosystem II, Stomata, G3P, Glucose, Kindgom, Phylum, Class, Order, Family, Genus,
Species, Dichotomous Key, Carolus Linnaeus, Systema Naturae, Taxonomy, Xylem,
Phloem