a. Where? - on the inner membrane of prokaryotes or chloroplasts, involving suites of proteins organized into "photosystems" or "electron transport chains"
b. How?
1. PRIMITIVE SYSTEM: "sulphur bacteria"
- these bacteria have a double membrane (two bilayers)... proteins are nested in the inner membrane
- light hits these groups of proteins arrayed around a "chlorophyll" molecule - with an atom of Magnesium at the center. This group of proteins = Photosystem I
- the Magnesium is large - the electrons in the outer shell are far away and easy to excite... the photon transfers energy to the electron - it is raised to a higher energy state, correlating with moving further from the nucleus...in fact, it is lost completely by the atom and transferred to an 'electron acceptor molecule'.
- This electron acceptor molecule then passes the electron to a series of proteins nested within the inner membrane of this bacterium... the ELECTRON TRANSPORT CHAIN (ETC)
- ESSENTIALLY, each time the electron is transferred, protons follow ('electrostatically') and are 'pumped across the inner membrane into the intramembrane space. This builds up an electrostatic charge differential across the membrane. There are protein channels ('initially closed') that, when opened, allow the H+ to flood through in response to the charge gradient. This electric discharge energy is transferred to ADP, linking it to P to create ATP. This is called 'chemiosmotic synthesis' .
- In this case, the electron that has now lost its energy can be returned to the photosystem.... so this process that has produced ATP is 'cyclic phosphorylation' (the ADP was 'phosphorylated by adding a P).
- Something else can happen, too. Instead of the Electron Acceptor giving the electron to the ETC, it can give the electron to NADP... another 'energy transport molecule' like ADP. When this happens, the NADP gains energy and a negative charge and is NADP-. It reacts with free H+ ions that are always present in aqueous solutions (you should know why...), to make the high energy transport molecule, NADPH. In this case, the electron isn't returned to the Magnesium.... photosynthesis would stop, unless the photosystem can strip electrons from other molecules in solution.
Sulphur bacteria have photosystems that can strip electrons from Hydrogen Sulphide (H2S). This releases 2H+ ions and S as a waste product..... NOT OXYGEN
So, sulphur bacteria, that are still present today, photosynthesize in sulphur springs and do not produce oxygen as a waste product.
This explains an interesting geological pattern: The oldest fossil life on record are photosynthetic bacteria that date to 3.8 billion years old. However, the first evidence of oxygen in the Earth's atmosphere occurs at about 2 billion years ago. So, how can you now explain how there were photosynthetic bacteria present for 1.8 billion years, without any oxygen being produced?
- The problem was, and still is, that these bacteria are limited to living in spots where H2S is abundant - sulphur springs. These are rare. If something evolved a system that could strip electrons from a more abundant source, like water, then these new organisms could exploit a wider range of environments....
1. ADVANCED SYSTEM: most other photosynthetic bacteria, and photosynthetic eukaryotes.
- In photosynthetic Eukaryotes(photosynthetic protists and plants), these reactions occur on the inner membrane of the Chloroplast - a specific membrane-bound organelle very much like a bacterium within the larger eukaryotic cell.
- In these organisms, there is a second photosystem (PSII) that is 'stronger' and can strip electrons from WATER (which holds the electrons more strongly than H2S does...)
- light strikes the thylakoid;
electron in PSII released from the Mg in the chlorophyll molecule.
- Light also strikes PSI
- it loses an excited electron, too. This electron is accepted by an electron
acceptor, and then passed to NADP, which accepts a H+ to balance the charge,
making the high energy molecule, NADPH.
- the electron from PSII
is passed to an electron acceptor, and then to molecules in the electron
transport chain. As the electron is passed down the chain, ATP is produced
by chemiosmosis (SEE Fig 10.16 - you are responsible for understanding this process in respiration and photosynthesis ). When this electron has lost it's energy,
it fills the "hole" in PSI created when that photosystem lost its electron.
- How does PSII replace
its lost electron? Water is split into oxygen, 2 H+, and 2 electrons. The
electrons are passed to the cholorophyll, excited by light, and energized.
- The oxygen reacts with
another oxygen atom to produce oxygen gas, which is released as a waste
product.
- So, the presence of oxygen in the atmosphere 2 billion years ago, indicated by the presence of sedimentary deposits with oxidized iron (red beds), correlates with the evolution of this more advanced type of photosynthesis that occurred about 2 billion years ago.
c. Summary:
- so, radiant energy excites
electrons, which are passed down an electron transport chain to make ATP.
- Water is the ultimate
source of these electrons; the splitting of water produces O2
as a waste product.
- Ultimately, the electrons
are passed to NADP, making NADPH.
- Water + ADP + NAD -->
in the presence of light --> ATP + NADPH + O2 (waste)
b. How?
Calvin-Benson Cycle:
- 6 CO2 molecules
bind to 6 C5 molecules of Ribulose Biphosphate (RuBP), making
6 C6 molecules. (ATP is broken and the energy that is
released is used to link CO2 to RUBP - I don't think I mentioned
this in class)
- These energized C6
molecules are unstable; the split into 12 C3 molecules.
- 2 C3 molecules
are used to form 1 glucose (C6) molecule.... more ATP is used,
and NADPH is used, too, and H is transferred to make the glucose.
- the 10 remaining C3
molecules (30 C total) are rearranged, using ATP and NADPH, and 6 C5
molecules are generated (30 C total).
c. Summary:
- 6 CO2 molecules
are used to make one molecule of glucose.
- 6 RuBP molecules are involved,
and are recycled
- the ATP and NADPH formed
in the light reaction are used to power this reaction; the energy in these
molecules is used top make bonds between the CO2, and the H
from NADPH is used to reduce the CO2 to form glucose (C6H12O6).
- ATP is a short-lived molecule. Even though the light
reaction produces ATP, cells can't store energy in this form. So, a more
stable energy storage molecule must be built (glucose).
- After the light independent reaction, photosynthetic organisms
have glucose, that they can break down as they need it. If they are eukaryotic
orgs (plants, photosynthetic protists), then the breakdown of glucose to
harvest the energy would occur in mitochondria. Remember - plants have
BOTH mito's and chloro's.
- All of the energy in living systems (besides geothermal
vent communities) is ultimately derived from the sun. We use this energy
to stick amino acids together to make our proteins, etc. Even the gas and
oil that powers our industrial societies was initally stored as glucose
produced by photosynthesis. Coal, gas, and oil are just fossilized plants
- and we "burn" that energy millions of years after it was converted from
sunlight. We are powering our societies with sunlight that hit the Earth
millions of years ago.... how cool is that?
5. Photorespiration
a. Problem:
- RuBP will bind to BOTH CO2 and O2.
And when RuBP binds to O2, it is split and transformed to the
amino acid serine, with the production of CO2 as waste.
Essentially, it is digested. These reactions use the ATP and NADPH produced
in the light reaction, too.
- So, when RuBP binds O2, it does the
exact opposite of photosynthesis - it uses the energy of the light reaction
to BREAK DOWN a carbohydrate (RuBP) and RELEASE CO2.
- When will this happen? This happens when
the relative concentrations of O2 and CO2 cross a
critical threshold, If O2 is super-abundant and CO2
is scarce, then photorespiration will ocur.
- This happens to many plants on HOT, DRY days,
when there is a lot of sun for photosynthesis to proceed (so O2
concentrations rise in the leaf and CO2 concentrations fall),
and it is DRY so the stomates are closed and gases can't be exchanged with
the environment (so O2 builds up in the leaf).
- Why does this happen? Maybe because photosynthesis
evolved before oxygen was present, so this shortcoming of RuBP was not
important initially.
b. Solutions
- "C3 plants" (as described above - called C3 because the first STABLE product of carbon fixation is a C3 molecule) have chloroplasts in their mesophyll
and not their bundle shealth..and they suffer from photorespiration on
hot dry days.
- "C4 plants" have chloroplasts in both cells; the
bundle shealth has RuBP, but the mesophyll has a different binding molecule
(PEP) with a higher affinity for CO2. So, PEP (a C3
molecule) can bind CO2 at low concentrations, and then the product
(a C4 product) is passed to the bundle shealth.
In the bundle shealth the CO2 is dissociated from the C4, leaving the C3 PEP. The PEP is returned to the mesophyll. This keeps the concentration
of CO2 in the bundle shealth high enough for RuBP to keep fixing
CO2, even though the leaves may be closed. This allows
C4 plants to maintain glucose production even on hot dry days when their stomates are closed. Grasses
are classic C4 plants, and they have adapted physiologically (and morphologically)
to their environment. So, PEP "pumps" CO2 into the bundle shealth, keeping the concentration of CO2 high enough that photosynthesis (and not photorespiration) will occur.
- CAM plants fix CO2 at night as
malate (C4). Then, in the day when stomates are closed
but light is available, they harvest energy and release to CO2
to RUBP, allowing both reactions to proceed.
A. Photosynthesis
B. Respiration
All living cells - bacteria, protists, fungi, plants, and animals, take complex organic molecules and break them down into their monomers. These catabolic reactions release energy that drives coupled anabolic reactions that make ATP from ADP and P. As such, some of the energy in the covalent bonds of the initial organic molecules is transformed into chemical energy in bonds of ATP. In subsequent catabolic reactions, the energy can be released when ATP is broken to drive other anabolic reactions that link monomers into polymers needed by the cell. All four classes of biological molecules (carbo's, fats, proteins, and nucleic acids) are broken down for energy harvest. The process of carbohydrate metabolism, however, is the central process. Fats, proteins, and nucleic acids are broken into their monomers, these are modified, and then these products can be shunted into the carbohydrate digestion process.
1. Glycolysis
a. Overview:
- Metabolic process used by ALL cells (primitive); occurs in the cytoplasm.
- Does not require oxygen (primitive)
- reaction: glucose (C6) + 2 ATP + 2NAD ----> 2 pyruvate (C3) + 4 ATP + 2NADH There is a net release of energy that is stored in NET profit of 2 ATP and 2NADH. The electrons are accepted by NAD-->NAD- (H + proton added to balance charge).
b. Requirements:
In order for glycolysis to occur (and all subsequent respiration and energy harvest!!!) the cell must have all three reactants - Glucose, ATP, and NAD. Obviously, if glucose is absent then the cell starves. That's moot. And, As glycolysis proceeds, there is always a net surplus of ATP produced by previous glycolysis reactions. But, NAD is used up and converted to NADH. If NAD is not present, glycolysis stops (very BAD).
c. solutions:
So, NADH must give up its electrons (and H+) to something else, so that the NAD can be recycled and used in glycolysis. This happens two ways, depending on whether oxygen is present (aerobic respiration) or absent (anearobic respiration)..
Study Questions:
1. Draw what happens in the primtive light reaction of sulphur bacteria, and explain the events that occur.
2. What is the electron donor for sulphur bacteria? What type of limitation does this impose on where these organisms can live?
3. Draw and explain what happens in the more advanced light dependent reaction. Why can we call this an 'adaptation'? (Why is this an improvement over the the more primitive system, considering the habitats available on Earth?)
4. Describe the correlations between these observations:
- the oldest fossils are 3.8 billion years old and look like photosynthetic organisms
- eukaryotic photosynthetic organisms about 2 billion years ago
- 'red beds', the oldest sedimentary deposits that include oxidized minerals, date to about 2 billion years
- previous to these red beds, minerals in sedimentary deposits are in their reduced state, suggesting that they were not exposed to an oxidizing atmosphere during their erosion and deposition, suggesting that the atmosphere contained no oxygen gas.
5. Draw the Light Indepedent reaction and describe the events that occur.
6. Explain photorespiration and describe two different adaptations of plants that live in hot, dry, environments.
7. When, where, and why is oxygen produced by photosynthesis? What is the primary function of photosynthesis?
8. All organisms can harvest energy by digesting biomolecules. This process is called _____.
9. Describe glycolysis.