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AP Bio- Metabolism 3.3: Photosynthesis
Transcript of AP Bio- Metabolism 3.3: Photosynthesis
The Light Reactions
Versatility and Regulation
A Quick Recap
Light Makes Us Food!
There is a reciprocal relationship between chemoheterotrophic nutrition and photoautotrophic nutrition.
The inputs of one are the outputs of the other.
This accounts for this curious fact:
Aerobic Cellular Respiration:
C H O + 6O
6CO + 6H O
C H O + 6O
6CO + 6H O
Plants and Such
Plants are NOT the only photoautotrophs
Most of the oxygen in the atmosphere is generated by cyanobacteria and aquatic protists
Purple Sulfur Bacteria
In Plants, photosynthesis happens at the leaves, organs which are specialized for the process.
At the leaf, mesophyll cells are full of chloroplasts, the site of photosynthesis.
Light = Energy
Light is a form of electromagnetic radiation.
It is produced by the movment of electrons between orbitals.
Visible light is just one tiny slice of the larger electromagnetic spectrum.
Why are plants green?
Chlorophyll is a pigment.
Pigment: any molecule that interacts with light energy to produce a color.
Chlorophyll comes in two main varieties: Chlorophyll A, and Chlorophyll B.
While chlorophyll is the main photosynthetic pigment, it is NOT the only pigment found in chloroplasts.
Accessory Pigments: other pigments that allow the chloroplast to absorb a wider range of light, and protect the chloroplast from light-related damage (example: carotenoids, xanthophylls)
Sunlight contains almost all wavelengths of visible light.
Chloroplasts do not absorb all wavelengths of light equally.
When plants are exposed to light, chloroplasts preferentially absorb light in the blue and red parts of the spectrum.
Chlorophylls have an absorption spectrum that is highest in the blue and red portions of the visible light spectrum
The accessory pigments expand the useful range of light (the "action spectrum"), but green is still the least useful.
The unequal utility of different wavelenghts of light was first noticed by Theodore Engelmann, who obeserved higher rates of growth of aerobic bacteria on algae grown in blue and red wavelengths of light.
Chloroplasts consist of a series of membranous disks (thylakoids), arranged in stacks (grana).
The grana are inside of the inner membrane of the chloroplast.
The fluid/space surrounding the grana is called the stroma.
Photosynthetic prokaryotes use specialized cell membrane regions to accomplish photosynthesis.
Photosynthesis is a 2-part process:
1. The light reactions: Occur in the thylakoid membranes. Light is used to drive the production of ATP and NADPH (an electron shuttle). Water provides the electrons needed and is converted to oxygen gas (a waste product).
2. The Calvin Cycle: Occurs in the stroma. The ATP and NADPH produced in the light reactions are used to incorporate carbon dioxide into a 3-carbon sugar.
Light + Chlorophyll = Electrons!
When photons of light interact with chlorophyll, electrons in the Magnesium atom become excited.
This happens with ~1 % of all sunlight that strikes the surface of the earth.
Isolated chlorophyll will flouresce when exposed to light, as the excited electrons return to the ground state.
Complexes of protein and pigment molecules that are embedded in the thylakoid membrane.
Direct incoming photons into the "reaction center" where chlorophyll a molecules produce excited electrons which are transferred to an electron transport chain (remember them?).
Photosystem II: central chlorophyll works best at a light wavelength of 680 nm (P680). Found at the "start" of the ETC.
Photosystem I: central chlorophyll works best at a light wavelength of 700 nm (P700). Found at the "end" of the ETC.
Since chlorophyll is not going to have the electrons return to it, new electrons are needed.
Water provides the replacement electrons ("photolysis"). This creates 4 protons and 1 molecule of oxygen gas for every 2 water molecules consumed.
The oxygen gas is released as a waste product, becoming a major input for aerobic cellular respiration (take a breath!).
As the electrons move through the ETC, they provide the energy for chemiosmosis, in a fashion almost identical to in cellular respiration.
A few notable differences:
In respiration, the energy comes from oxidation of glucose ("oxidative phosphorylation"). In photosynthesis, the energy comes from photons ("photophosphorylation")
In respiration, protons were pumped from the matrix into the intermembrane space by the ETC. In photosynthesis, electrons are pumped from the stroma into the thlakoid space.
Non-Cyclic Electron Flow
Electrons move from photosystem II to photosystem I via the ETC. From photsystem II, they are transfered to the enzyme NADP-Reductase which uses them to reduce NADP+ into NADPH.
Produces both ATP and NADPH
Cyclic Electron Flow
Electrons move from photosystem I to the ETC before returning to photosystem I.
Only produces ATP.
Does not require water.
The Calvin cycle will require 9 ATP and 6 NADPH for every sugar produced.
Even if you are growing plants indoors, you are still using sunlight to do it, just a version that was stored in the chemical bonds of the fossil fuels that are being used to power the electric lights.
The Calvin cycle is named for Melvin Calvin, who discovered it by using radioactive C-14 to trace the path of carbon through the cycle.
He recieved a Nobel Prize for his efforts in 1961.
It is also commonly referred to as simply "Carbon Fixation"
It is never, ever, called "The Dark Reactions"
9 ADP + Pi
Enjoy this Analogy!
Every step of the Calvin Cycle is controlled by an enzyme (not shown)
1. Carbon Fixation: Ribulose Bisphosphate Carboxylase (aka "RuBisCo") mediates the transfer of a molecule of Carbon Dioxide onto a molecule of Ribulose Bisphosphate (RuBP- 5 C)
2. Reduction: ATP and NADPH are used to rearrange RUBP into Glyceraldehyde 3-phosphate (G3P, aka PGAL) a three-carbon sugar.
3. Regeneration: ATP is used to reconstitute RuBP from G3P
Where's the Sugar?
In order to get 1 G3P as a product of the Calvin Cycle, 3 molecules of carbon dioxide have to be joined to three molecules of RuBP.
This makes 6 molecules of G3P, 1 of which is a net product.
The other 5 G3P are used to regenerate three molecules of RuBP.
G3P is a sugar building block. 2 G3P can make 1 6 carbon sugar. Many G3P can make a polysaccharide.
*-if they are present
Consider everyone you know, ever pet you have ever had, every ancestor in your lineage...they have all been able to exist for the simple fact that photoautotrophs make more food than they need and produce oxygen gas as a waste product.
Modern industry is more and more interested in using plants to do all sorts of things (like make biofuels, for instance).
An evolutionary quirk
Rubisco evolved in conditions of low oxygen gas concentration.
As a result, its active site has a high affinity for oxygen gas.
Which is a problem.
The metabolic pathway that occurs when rubisco incorporates Oxygen instead of Carbon Dioxide into RuBP.
A metabolic dead end. Uses ATP but produces no sugar.
Best if avoided.
Lots of times, not a problem.
As long as a plant can keep its stomates open and exchanging gas with the environment, photorespiration is kept to a minimum.
2 major adaptations
Carbon fixation occurs in mesophyll cells.
Carbon dioxide is incorporated into a 4C organic acid (malate) by the enzyme PEP carboxylase (which has a very low affinity for oxygen).
The 4C acid is then transported to bundle sheath cells, where the carbon dioxide is cleaved from the 4C acid.
Since the bundle sheath cells are surrounded by mesophyll, their oxygen gas concentration remains low, even as the light reactions occur in the mesophyll cells.
But there are environments where keeping stomates open will lead to dessication.
Closed stomates = increasing [oxygen gas]/decreasing [carbon dioxide]
= increased photorespiration.
= NOT GOOD!
No adaptations for minimizing photorespiration
Both stages of photosynthesis occur in the same cell simultaneously.
Oxygen and Carbon Dioxide are exchanged with the environment through the stomates.
Sugars are transported to vascular tissue for transport throughout the plant.
Carbon fixation occurs during the evening, when open stomates will not lead to dessication.
The carbon dioxide is stored in an organic acid.
During the day, the organic acid store is used to supply the calvin cycle with carbon dioxide.
Make Sure You Can:
C H O + 6O
6CO + 6H O
Water will be oxidized (it is the "reducing agent").
Carbon will be reduced (it is the "oxidizing agent").
An anabolic, endergonic process
How do living systems process energy?
Explain how photoautotrophic energy processing allows for the production of useful energy for organisms.
Explain why and how photoautotrophic energy processing is controlled.
Identify the reduction and oxidation reactions that occur in photosynthesis.
Explain the processes and identify all inputs and outputs of all steps of photosynthesis.
Relate the different steps of photosynthesis to their locations in the cell.
Compare the adaptations that have been made to reduce photorespiration.
Compare photosynthesis with cellular respiration.
Explain how photosynthesis provides the energetic foundation for the vast majority of life on Earth.
Without light, you wouldn't be able to eat anything. And you want to eat, don't you? DON'T YOU?!?!
It would apologize, if it could...
Organic Molecules (ex. glucose)
Coach says it's called the "Z-Scheme"
ADP + Pi, NADP+
Sing a Silly Song