Bio 211 Intro Molecular and Cell Biology
Lecture 17 “Photosynthesis II”
Reading: Campbell Chap. 10 pp. 179-185
Homework: Chemiosmotic hypothesis for ATP synthesis in chloroplasts
In the last lecture, we started our discussion of the process of photosynthesis.
Who can tell me what some of the key elements of the process are?
(1)
(2)
(3)
(4)
Absorbance of light energy (Light reactions)
Water is split to form oxygen (Light reactions)
Takes place in the chloroplasts of photosynthetic organisms
Synthesis of sugars (Calvin cycle)
In today’s lecture, we will spend a little more time examining the light reactions in
more detail, particularly looking at how ATP is made. We will also take a closer look at
the dark reactions. At the end of the period we will take additional time to go over the
format of next week’s exam and to answer general questions on the content of the exam.
If you have made notes to answer the homework question, you may get those ready now.
Outline:
1. Light reactions: chemiosmotic hypothesis
2. Light reactions: non-cyclic vs. cyclic photophosphorylation
3. Calvin cycle
1. Light reactions: chemiosmotic hypothesis
If you remember from last time, the light reactions of photosynthesis involve the
absorbance of light by the photosynthesis, which leads to the splitting of water. Water
provide electrons which are then shuttled down an electron transport chain and used to
make NADPH from NADP+. Another important product of the light reactions is ATP.
Your homework from the last period was to find out how the chloroplast makes
ATP during the light reactions. To answer this, you were to read pages 177-179 closely
and look at Figures 10.14 and 10.15.
How do the light reactions make ATP?
Answer: They make ATP by pumping protons. When electrons are transported in
the electron transport chain, protons are moved toward the inside of the thylakoid
membranes. The proton gradient drives ATP formation as protons move back through
the ATP synthase complex.
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Figure 10.15 shows how the different complexes might be arranged in the light
reactions. The electrons split off from water are activated in the photosystems, then
passed among the carriers in the electron transport chain. These pump protons toward the
inside of the thylakoid membranes, creating a gradient. Then the protons move back
through the ATP synthase complex to make ATP.
Fig. 10.14 shows ways in which ATP made from the light reactions
(photophosphorylation) and ATP made during respiration in mitochondria are similar.
Similarities (photophosphorylation and oxidative phosphorylation)
both involve electron transport chain
both require membranes (organelles) to be intact
as electrons move, protons are carried across the membrane
movement of protons through ATP synthase drives ATP synthesis
Differences
photophosphorylation: chloroplasts;
ox phos: mitochondria
proton gradient: pH is lower (more acidic) outside of mitochondria
pH is lower (more acidic) inside thylakoid membrane
Thus the light reactions of photosynthesis involve the transfer of electrons in a
chain resulting in the synthesis of ATP.
2. Light reactions: cyclic vs non-cyclic photophosphorylation
The pathway of electron transfer we’ve looked at so far is also called the noncyclic pathway. Using this pathway makes all the products we attribute to the light
reactions of photosynthesis: ATP, oxygen from water and NADPH.
However, sometimes the plant cell only needs to make more ATP. For example,
in the synthesis of sugars in the Calvin cycle, slightly more ATP is needed than NADPH.
In this case the electrons are “recycled” via a cyclic electron transport pathway. Fig.
10.13 shows this alternate route. Electrons from photosystem I are stimulated by the
absorbance of light, then shuttled through an alternate system of carriers. At the same
time, a proton gradient is formed which can drive the synthesis of additional ATP.
The cyclic pathway allows more ATP to be made, without producing more oxygen
or NADPH.
3. Calvin cycle
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In the last part of the lecture today, we will take a brief look at the Calvin cycle.
These reactions are sometimes called the “dark reactions” because they do not require
light directly and can take place in the dark as long as the plant has not run out of
substrates for the reactions, namely ATP and NADPH, as carbon dioxide is usually
available.
You may remember from last time that the overall process of photosynthesis can
be summarized as:
6 CO2 + 12 H2O + light energy ----> C6H12O6 + 6 O2 + 6 H2O
With the light reactions we’ve shown that some of the waters on this side of the
equation are split in the presence of light to give oxygen. The carbon dioxide is used in
the Calvin cycle in the synthesis of sugars.
Figure 10.16 summarizes the major stages of the Calvin cycle.
I. In phase I, carbon dioxide is attached to a 5 carbon sugar (ribulose
bisphosphate) by the enzyme nicknamed Rubisco and then split into two 3 carbon sugars.
“Carbon fixation”
II. In phase II, the 3 carbon sugar (PGA=3-phosphoglycerate) is reduced to give
another 3 carbon sugar, (3GP=glyceraldehyde 3-phosphate). These reactions require ATP
and NADPH. “Reduction”
III. In phase III, the 5 carbon sugar is regenerated so the Calvin cycle can happen
again. This requires more ATP. “Regeneration of CO2 acceptor”
For the net synthesis of one 3 carbon sugar (G3P), the Calvin cycle uses 9
molecules of ATP and 6 molecules of NADPH and has to go around the cycle three
times. The ATP and NADPH used in the Calvin cycle must come from the light
reactions of photosynthesis, thus the plant can’t make sugars in the absence of light. Note
that glucose, a 6 carbon sugar can be made from two of the 3 carbon sugars from phase II
using other biosynthetic pathways. Refer to Fig. 10.19 for summary of photosynthesis.
Unit Summary: In this unit on cell chemistry, we have looked at the molecules
and metabolic pathways critical to life on earth. The chemistry of cells we learned is
mostly based on the chemistry of carbon and the key macromolecules of cells such as
sugars and proteins have carbon as their main structural component. Key to the survival
of cells and organisms has been their ability to derive energy from their environment.
Among the processes we’ve looked at are photosynthesis, the ability of of plants and
blue-green algae to harvest light energy and transform it into chemical energy and
glycolysis and respiration which enable most organisms to make ATP from the chemical
energy stored in sugars.
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What I haven’t been able to show you are many of the ways organisms can differ
in their metabolism. These differences have evolved over millions of years of evolution.
If you were to read to the end of Chap. 10, you would find that some plants adapted to
hot, dry conditions have different ways of fixing carbon dioxide that help them minimize
the loss of water. Many of these alternate pathways of other organisms may turn out to be
beneficial to us. For example, some bacteria have the ability to use oil slicks or toxic
waste as a source of nutrition. We may be able to use these organisms to help us clean up
some of our mistakes in the environment.
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