Download 20 Photosynthesis

20
Photosynthesis: The Power of the Sun A/P Biology
Pages 185-207
Name: _____________________________ Date: ____________ Period: _________
What is photosynthesis? Well we can break the word down to photo (Greek) meaning
“light”, and synthesis meaning “put together”. “Light putting together”, refers to the
sugar construction by the “Calvin cycle”. The light reaction in the plant occurs in the
thylakoids membranes of the chloroplasts’ grana. See picture below:
As you can see, the chloroplast is a disc-shaped organelle (that probably was once a free
living organism) that contains a matrix like interior set of structures. The stacks of
thylakoids are called grana and collect light in a way that is a mirror image of the step
down process of cellular respiration. In this case, electrons (donated by water) are
excited by the sunlight instead of being donated by NADH. This is the exact reverse of
cell respiration which has electron transport as the last step in producing ATP producing
in the last step: water (see equation below). Here, the first step is to break water.
Let us review briefly the over-all cellular respiration reaction “equation”:
Cellular Respiration
C6H12O6 + 6 O2
glucose + oxygen
6 CO2 +
6 H2O
carbon dioxide + water + energy
Page 2 (Cont. #20 A/P Biology)
What we learned about respiration was that carbohydrates contain high energy bonds, or
electrons which were passed down from molecule to molecule and we extracted that
energy by storing it into ATP by substrate level synthesis (by the use of enzyme substrate
complexes to produce ATP) and stored it into what I call “biological batteries” NADH
and FADH2. These batteries were primarily made within the Mitochondrial Matrix via
the Krebs’ Cycle (Citric Acid Cycle) and the NADH/FADH2 “batteries” reduced protein
complexes within the inner-membrane of the mitochondria. GO BACK and review the
basic process of NADH/FADH2 being oxidized and how the protons power ATP synthase
to make ATP within the mitochondria.
SO- the take home message for cellular respiration is:
1.) the final acceptor of electrons is the oxygen (why this is called “oxidative”
respiration), producing water!
2.) The entire molecule of glucose gets “taken apart”, carbon by carbon, yielding
carbon dioxide (CO2)!
In plants, one of the first things that are done is to “split water”. This is
accomplished through a series of reactions that involves light. (The explanation of how
pigments and light work together and about wavelengths and energy are handled in your
book pages 194-197. This is fairly well presented so I will not go over it specifically
here, you ARE required to read this and answer
questions at the end of this handout.)
Photosynthetic pigments absorb sunlight. The
most abundant pigments in plants are chlorophyll a
and chlorophyll b.
Pictures of these are in your book (pg. 192) but the
key to the molecule is the Magnesium “Porphyrin”
ring. This “ring” is a common structure found in
“Cytochrome c” and similar to the one found in
hemoglobin (except that Fe is in the center compared
to Magnesium). See diagram to right…
The absorption of light excites
electrons (originating from water that was
split) to a higher energy state, thus
converting the energy of sunlight to
potential chemical energy. The
photosynthetic pigments are arranged into
photocenters in the thylakoids membrane.
(See picture to the right.) Each of these
photocenters contains hundreds of pigment
molecules. The photocenter is located on the
surface of the thylakoid:
Page 3 (Cont. #20 A/P Biology)
All the pigment molecules work together to form a network or electron collecting
“antennae”. These many pigments absorb light and transfer their excited electrons to a
“central” chlorophyll molecule that serves as the reaction center. The reaction center
then transfers this “excited” electron to Pheophytin molecule that transfers the electron
through a series of membrane carriers, coupled to the synthesis of ATP and NADPH
(NADPH is similar to NADH in that it is a “biological battery”).
There are essentially two Photosynthesis Centers – PS1 and PS2 (Photosystem 1 and 2).
As you can see the basic creation of a “proton gradient” remains the same. The protons
are pumped from the area around the thylakoid, into the center of thylakoid (that is
essentially the reverse of how it is “done” in the mitochondria). When the protons are
“let back” in through the ATP synthase (see above), ATP is created. The chemiosmotic
production of ATP is called photophosphorylation because the initial energy input is
light energy. Compare this to oxidative phosphorylation in cellular respiration. Light
drives photophosphorylation, NADH, and FADH2 drives oxidative phosphorylation. The
mirror image of these two processes will become clear in a moment.
LIGHT is the energy that excites the electrons to create a proton gradient
that produces ATP and drives the transfer of electrons from water (leaving the
½ O2 that quickly combines with another ½ O2 to form elemental oxygen) to NADP+
to NADPH. The elemental oxygen O2 is released from the plant through openings
called stoma. Animals (such as humans) breathe in this oxygen to supply oxygen as the
final acceptor in the electron transport chain in the mitochondria-to form water!
The above description of photosynthesis only describes one part of the
reaction. This is the “Light” reaction (Light Dependent reaction). Simply stated, this is
because it requires light. The other half of the photosynthetic reaction requires no light
and was dubbed the “Dark” reaction, which is a misnomer. Actually, the other half is
more often called the “non-light” reaction (Light Independent Reaction)!
We have accounted for the oxidation of water to oxygen (O2 – which WE
breathe in!) but we have not used or fixed CO2 – which is the other by-product of
cellular respiration (see equation- page 1 of this handout). Where does this come in?
Page 4 (Cont. #20 A/P Biology)
Plant Photosynthesis
sunlight
6 CO2 + 6 H2O
C6H12O6 + 6 O2
carbon dioxide + water
glucose + oxygen
The over-all reaction of carbon dioxide and water in the presence of chlorophyll
and sunlight yields glucose and oxygen. Simple? Well-obviously not so simple.
We have previously stated that plants are producers because they create their own
food. That is they produce sugar (in fact plants or plant-like organisms) produce nearly
all the sugar in the world! In the reactions so far we have not seen anything that remotely
looks like the production of sugar. Well, the production of NADPH and ATP are used to
drive a cycle found only in the stroma of the chloroplast outside the thylakoid (just like
the Krebs cycle is found only in the inner matrix of the mitochondria). This cycle is
called the Calvin cycle. It looks something like this (Light Independent Reaction:
This reaction is not as neat and clean as either the glycolytic cycle or the Krebs Cycle.
You will note that some of the reactions require the use of ATP and NADPH. These
molecules are provided by the light reaction. The “DARK” or “Non-Light” reaction
Page 5 (Cont. #20 A/P Biology)
(Calvin cycle) simply does not require the presence of light. This reaction can occur
ANYTIME there is a shortage of sugar that is used for cell walls, DNA/RNA, storage
(primarily in roots as starch), or most importantly- TO BREAK DOWN SO ATP CAN
BE MADE! We store energy as sugar in the complex form of glycogen (and fats). Plants
store sugar in the form of starch (in fruits and roots) and of course store energy as fats.
If we were to abbreviate this complex cycle so that you could see where the ATP
and NADPH were used, we could summarize the use of these high-energy compounds
and display the sugars that are made. It is important to note that this reaction works best
in the ABSENCE of oxygen (anaerobic). See diagram:
To “Step” you through this…
1.) Three CO2 is attached to THREE Five carbon sugars (i.e. CO2 is “fixed” onto a
five carbon molecule) to briefly make a six carbon molecules which immediately
gets broken into two 3 carbon molecules. This process is controlled by an enzyme
called Rubisco (Rubisco is a sloppy enzyme and will sometimes use oxygen
instead of CO2 to deviate the reaction away from producing 2 molecules of 3PGA. In reality THREE CO2 molecules are attached to 3 Five Carbon sugars to
make 3 six carbon sugars that are broken down to 6 three carbon sugars.
2.) 6 ATP and 6 NADPH ( from the light reaction!) molecules are used up converting
the six 3 carbon molecules to SIX glyceraldehyde-3-phosphate (G-3-P). ONE of
these is available to help make sugar in the plant cell - accounting for ALL three
CO2 molecules that were “FIXED” onto the five carbon sugar! HERE is where
the mirror image comes in…In the mitochondria 3 CO2 molecules are pulled
off from pyruvate (completing the breakdown of a six carbon sugar
molecule) and in the DARK reaction three CO2 molecules are added to a 5
carbon sugar to MAKE a 6 carbon sugar (if only briefly).
Page 6 (Cont. #20 A/P Biology)
3.) The other 5 three carbon molecules are converted to 3 Five carbon sugars to allow
the Calvin Cycle to start over again.
NOTE: If you add up all the ATP molecules (9) and NADPH molecules (6) and convert
these into ATP equivalents…
9 ATP molecules equals…..
9ATP molecules TIMES 2 = 18 ATP
6 NADPH molecules equals…..
18 ATP molec. TIMES 2= 36 ATP
Total
54 ATP
Why multiply times 2? Each cycle of the Calvin cycle produces only ONE 3 carbon
molecule (G3P or glyceraldehyde 3 phosphate), we need an equivalent of a six carbon
glucose molecule, so we multiply by 2!)
SOOOOO- it takes 54 ATP equivalencies to “make one glucose molecule--- yet we only
“get” 36 ATP (net) from breaking down glucose. Why the difference? Breaking down
glucose is inefficient. We cannot get 100% conversion and as always, with each
enzymatic conversion, we lose some heat!
SOOO- why doesn’t the EARTH eventually run out of energy? Energy is supplied by the
sun (plants can “trap” this energy with photosynthesis) and thus we constantly lose heat
energy through inefficiency, but we gain much more via the sun energy supplied free.
Last take-home message. Oxygen is the final acceptor of electrons/protons in oxidative
phosphorylation (electron transport) so oxygen must be present in the
mitochondria…AND-oxygen is PRODUCED in the chloroplast. So- both the
mitochondria and the chloroplast must contain oxygen at some point. One makes it, the
other reduces it to make water.
Page 7 (Cont. #20 Photosynthesis AP Bio.)
Answer the following questions on this sheet and on your scan-tron:
_____ 1.) Photosynthesis has two separate parts. What is the name of the part that
actually makes sugar?
a.) Photosystem 1
b.) Photosystem 2
c.) Light Dependent reaction
d.) Light Independent reaction
_____ 2.) What part(s) of the photosynthesis breaks water (mark all that apply?
a.) Photosystem 1
b.) Photosystem 2
c.) Light Dependent reaction
d.) Light Independent reaction
_____ 3.) Where in Cell Respiration is water produced?
a.) in the glycolytic cycle
b.) at the level of breaking down pyruvate to acetyl-CoA
c.) just before the electron transport d.) just after the electron transport (last step)
_____ 4.) Where is water broken down in Photosynthesis?
a.) Photosystem 2 (first step)
b.) Photosystem 1
c.) Calvin cycle
d.) during the production of ribulose-5-phosphate
_____ 5.) What are the two “high energy” compounds made in the light dependent
reaction that are used to “power” the Calvin cycle?
a.) water and ATP b.) water and NADH c.) NADH and ATP d.) NADPH and ATP
_____ 6.) Where is elemental gas oxygen produced in photosynthetic reaction? (Mark all
that apply.) (See diagrams, this handout.)
a.) in the cytoplasm of the plant cell
b.) in the mitochondria of the plant cell
c.) in the chloroplast of the plant cell
d.) in Photosystem 2
_____ 7.) The photosynthetic reaction shown on page 4 of this handout, indicates that a
six carbon sugar is made at the end. This is not exactly correct (just like the situation we
had when pyruvate entered the mitochondria). According to the diagram on page 6 of
this handout, what compound is produced through “one” cycle of the Calvin enzyme
cycle? (Internet?) (NOT pyruvate, but that would be convenient)
a.) Glucose b.) Sucrose c.) glyceraldehyde-3-phosphate d.) Cellulose
_____ 8.) We can extract approximately 36 ATP from completely breaking down glucose
with Cell respiration (glycolysis and mitochondrial Krebs and electron transport). It takes
approximately 54 ATP to “make” Glucose. This means that we can garner only about
66.6% of the energy from glucose when we break it down in comparison to the amount to
make it with photosynthesis. However, it really costs more energy than 54 ATP to make
glucose. What accounts for the missing energy not calculated?
a.) amount of energy released from water
b.) amount of energy released by oxygen
c.) amount of energy absorbed from the sun d.) amount of energy “left in glucose”
Page 8 (Cont. Handout #20
Photosynthesis AP Biology)
_____ 9.) Compare the “heme” structure in hemoglobin with the Chlorophyll structure in
plants. (Internet). You should note that there are many structural similarities. The central
metal molecule in the heme structure is iron, what is the central metal structure in the
chlorophyll molecule?
a.) copper b.) manganese c.) cobalt d.) magnesium e.) sodium
_____ 10.) Where is the oxygen produced in the plant cell?
a.) in the cytoplasm of the plant cell
b.) in the mitochondria of the plant cell
c.) in the chloroplast of the plant cell
d.) in the nucleus of the plant cell
______ 11.) In the above question, where is oxygen used in the plant cell?
a.) in the cytoplasm of the plant cell
b.) in the mitochondria of the plant cell
c.) in the chloroplast of the plant cell
d.) in the nucleus of the plant cell
_____ 12.) Oxygen, in high enough quantities is toxic to animal cells and plant cells.
Look in the Internet and determine what process would be hurt by the presence of
oxygen.
a.) the Calvin cycle b.) the glycolytic cycle c.) the Krebs cycle d.) electron transport
_____ 13.) What part of photosynthesis produces NADPH?
a.) PS1 b.) PS2 c.) ATP synthase d.) Calvin cycle e.) None of these
_____ 14.) What is another name for the electron transport system in the chloroplast?
a.) Calvin cycle b.) PS1/PS2 c.) water clock d.) cytoplasm cycle
_____ 15.) Why are most plants green?
a.) because chloroplast absorb most other colors and reflect green
b.) because green is the color of glucose that is made by photosynthesis
c.) because the sun is yellow (green is the complementary color)
d.) because water absorbs all other color wavelengths
Answer the following questions on this handout only.
_____ 16.) you place some water plants such as Elodea (Internet) in a test tube of water at
room temperature. You expose the test tube to white light and notice bubbles forming on
the leaves. These bubbles are most likely to be?
a.) carbon dioxide the plant will use for photosynthesis
b.) oxygen the plant loses during photosynthesis
c.) glucose the plant makes during photosynthesis
d.) water bubbles the plant uses in photosynthesis
Page 9 (Cont. Handout #20 AP Bio.)
_____ 17.) In question #16, You note that when white light is used at a distance of 15
cm, the number of bubble produced by the plant is 33 per minute. You move the light
source AWAY from the test tube to 30 cm. Speculate how many bubbles will be formed
in one minute. (Internet)
a.) 99 b.) 66 c.) 17 d.) none
_____ 18.) In the above question you bring the light source back to 15 cm and change the
color of the light to blue. Speculate how many bubbles will be formed.
a.) 66 b.) 28 c.) 12 d.) none
_____ 19.) In the above question you change the light to red?
a.) 38 b.) 27 d.) 5 d.) none
_____ 20.) In the above question you change the light to green?
a.) zero b.) 4 c.) 27 d.) 33
21.) Define photosynthesis and explain in some detail where it takes place.
22.) What did Jan Baptista van Helmont prove? Could we prove the same thing today?
If van Helmont and isolated the tree in an air proof container, preventing the exchange of
air, and there was no other living organism inside the air proof container, would the plant
survive (assuming you could add water to the plant periodically without any disturbance
of the air quality). Explain why or why not.
23.) What is the difference between the light and dark reaction? Why did Blackman (pg.
211) increase the temperature of the air? What did this increase in temperature
accomplish? Why would temperatures above 30oC cause the consumption of CO2 to fall
off rapidly?
24.) What is “carbon fixation” and how does it fit in with the Calvin cycle (hint: use pg.
212 and diagrams of Calvin cycle to determine the answer to this question)?
25.) What are the characteristic absorption spectrum of chlorophyll a and chlorophyll b?
How does this contribute to the over-all color of most plants? Which light has the most
energy, Red light or Violet light?
26.) What are the “waste products” from cellular respiration and why are they important
in photosynthesis?
27.) Where do all the electrons that were excited by the sun-light “go” in the
photosynthetic reaction? Are the electrons ever released? Why or why not?
Page 11 (Cont. Handout #20 AP Bio.)
28.) What molecules are produced by the light reaction? What is the difference between
the PS1 and the PS2 reaction?
29.) Keeping in mind that plant cells have mitochondria, determine why the mitochondria
is needed if the light reaction produces all that ATP and NADPH and the dark reaction
produces sugar?
30.) What are the different destinies for the sugar produced by the Calvin Cycle? For
each destiny, speculate as to the reason or strategy for each of these destinies.
Evolution of Photosynthesis
Date: ______________________________
Lesson Plan for Handout #20 A/P Biology
Objective: TLWD ability to explain how the general over-all equation of photosynthesis
is dependent on cellular respiration and the learner will be able to demonstrate how
carbon dioxide is fixed in the C3 vs. C4 reaction and compare it to the CAM reaction.
Moreover, the learner will be able to explain why certain wavelengths are better at
exciting electron than others and why there is a difference in the light reaction versus the
dark reaction when given handout #20.
Content: Photosynthesis, light reaction, dark reaction, carbon dioxide fixation, C3, C4,
and CAM
Method: Power point, white board, discussion.
Homework: Handout #20
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