LECTURE                                                      PHOTOSYNTHESIS

In this lecture you will learn about:

ALL ENERGY IS ABOUT THE ELECTRONS IN COVALENT BONDS
- electrons are the "glue" of the bonds
- the energy of electrons is the source of usable energy to drive chemical reactions
Without a constant input of energy ENTROPY would set in and everything would wind down. 
There are two sources of this energy: inorganic chemicals and light from the sun
INORGANIC

"A lithotroph is an organism that uses an inorganic substrate (usually of mineral origin) to obtain reducing equivalents for use in biosynthesis (e.g., carbon dioxide fixation) or energy conservation via aerobic or anaerobic respiration.[1] Known lithotrophs are exclusively microbes; No known macrofauna possesses the ability to utilize inorganic compounds as energy sources." 
"Here are a few examples of lithotrophic pathways, any of which may use oxygen or sulfur as electron acceptor:

    * Iron bacteria oxidize ferrous iron (Fe2+) into ferric iron (Fe3+)
    * Nitrifying bacteria oxidize ammonia (NH3) into nitrite (NO?2) or, alternatively, nitrite (NO?2) into nitrate (NO?3).
    * Purple sulfur bacteria and some chemolithotrophs oxidize sulfide (S2?) into sulfur (S0). Here oxygen is the electron acceptor.
    * Sulfur bacteria use oxidized sulfur compounds to produce sulfide. They also can grow on a number of oxidized or partly oxidized sulfur compounds (e.g. sulfate (SO2?4), thiosulfate (S2O2?3), thionates, polysulfides, sulfite). Here, sulfur is the electron acceptor.
    * Hydrogen bacteria oxidize hydrogen to water.
    * Carboxydotrophic bacteria oxidise carbon monoxide to carbon dioxide.

In the following examples, compounds other than oxygen are used as electron acceptors:
    * Methanogens are Archaea capable of oxidising hydrogen, reducing carbon dioxide to methane.
    * Thiobacillus denitrificans is one of many known sulfur bacteria, oxidizing reduced sulfur compounds with nitrate instead of oxygen.
    * The recently discovered Anammox bacteria oxidise ammonia with nitrite as electron acceptor to produce nitrogen gas.
    * Phosphite bacteria oxidize phosphite into phosphate. They use sulfate as electron acceptor, and reduce it into sulfide."
http://en.wikipedia.org/wiki/Lithotroph


 
 
 
 

"Deep Sea Vent Communities:  Did Life Originate in the Abyss?"
http://www.indiana.edu/~g105lab/images/gaia_chapter_13/vent_communities.htm

"Chemolithotrophs use the above inorganic compounds for aerobic or anaerobic 
respiration. The energy produced by the oxidation of these compounds is enough 
for ATP production."

PHOTOTROPHS 

"Phototrophs use light as energy source. These bacteria are photosynthetic; photolithotrophic bacteria are found in the purple bacteria (e. g., Chromatiaceae), green bacteria (Chlorobiaceae and Chloroflexi) and Cyanobacteria"

"Photoautotrophs are organisms (usually plants) that carry out photosynthesis to acquire energy. Energy from sunlight is used to convert carbon dioxide and water into organic materials to be used in cellular functions such as biosynthesis and respiration.
http://en.wikipedia.org/wiki/Lithotroph
 

 


 
PHOTOSYNTHESIS - Link to Basic Photosynthesis
LIGHT AND ATOMS

- From the first semester you learned that light and atoms interact. Atoms absorb and emit photons of light in discreet packets of energy. 
Light interacts with the outermost electrons and rarely with inner electrons.

- THIS IS BASIS OF PHOTOSYNTHESIS. Chloroplasts are the organelles that contains the light absorbing machinery. 

- On the right are the wavelengths absorbed by two different organic molecules (light centers), chlorophyll in the UV and carotenoid in the red. 
 

ELECTRONS ARE NOT ALL THE SAME
- an electron that has absorbed a photon of energy is a lot more energetic than one that has lost the energy and fallen back to a low level (hot potato)

- remember they are not FREE wandering around by themselves unescorted. They are "handed off" from one molecule to another ...

- this is the "redox" idea ... and there are organic molecules some that move freely and some set in a membrane that handle the electrons

- the energy of the electron can be "sucked out" to do WORK

Where photosynthesis occurs
-- chloroplast are most commonly green because they absorb all light except green, which they reflect (you knew that, right?).
-- the light absorbing compound is chlorophyll, which absorbs in the blue and red wavelength.
- The production of sugar from CO2 + H2O is called CARBON FIXATION.
- It occurs in two basic steps, one that occurs in the light and one in the dark.
 
PHOTOSYNTHESIS
.. the overall reaction is: 
-- CO2 + H2O + light ----> SUGAR

This is the basic reaction.
-- What is necessary to get this reaction is an a chloroplast.
- Chloroplasts are found within an organelle.

- ORGANELLES are separate entities within cells that perform specific functions. Like our cells, organelles also have a "double membrane" around them that enclose them wimsketch.giflike a sack.

-- The simplest plant to contain chloroplast is a one celled "plant" called algae. They are abundant in the ocean and all water (also in fish tanks). They produce most of the oxygen for the world. (think of that next time somebody says we should dump garbage or toxic chemicals in the ocean).

THE LIGHT- DEPENDENT REACTION  click for youtube
-- Basically, a photon of energy is used to rip 2 hydrogens off of oxygen.
- Energy (ATP) is made. The oxygen is released. You saw this in lab with Elodea.

THE LIGHT- DEPENDENT REACTION: DETAILS
1. a photon of energy is absorbed by an electron.
2. The electron makes a quantum leap to a higher energy level.
3. Instead of falling and emitting the photon, the high energy electron is trapped by a protein called CYTOCHROME.
4. The energy of the electrons is carefully removed a little at a time to
a. make ATPs (the energy units).
b. remove 2 hydrogens (and their electrons) from H2O and stick them onto a molecules called NADPH. (Carriers)

-- little packets of energy are removed from the electron and high energy (covalent) bonds are made in ATP. When the electron has been sucked dry, it is returned to chlorophyll to absorb another photon.

- Think of the electrons as water which has been lifted to the top of a dam. As it flows over a series of dams, it turns the water wheels. As the water wheel turns, it makes energy (the ATP high energy bond) and takes H's off of H2O.

THE DARK REACTION

-- CO2 is fixed (sugar is made) using both the hydrogens ripped off water, and the energy of ATP made in the light phase.

THE DARK REACTION: DETAILS

-- The energy in the ATP bond is then used to add hydrogens (reduce) CO2 to CH4. Remember that burning (oxidizing) removes hydrogens (and their electrons) and releases energy. Reduction is adding hydrogens (and electrons).

- Unfortunately, adding the hydrogens directly to CO2 isn't all that easy. All of the possible combinations of H + CO2 are toxic. Cells get around this problem by sticking the CO2 and hydrogens onto a 5 carbon sugar to make a 6 carbon molecule. The 6 carbon molecule is broken into two three carbon molecules which are either:

a. built up to a sugar or
b. the 5 carbon sugar is regenerated and is ready to take another CO2
 

THESE REACTIONS ARE CALLED CYCLES- THIS ONE IS CALLED THE CARBON-FIXATION CYCLE or CALVIN CYCLEs
-- cycles are repetitive. The molecules of each cycle are not used up, but are regenerated to continue the cycle.

THE CALVIN CYCLE SIMPLIFIED

1. 5 C + 1 CO2 -> 6C (carbon fixation)

2. 6 C -> 2 (3C)  (broken down) 

3a. (1) 3C is added to another 3C -> glucose

3b. (1) 3C is regenerated to 5C to start the cycle again

this requires ATP

REVIEW QUESTIONS:
What is photosynthesis and where does it take place in the plant?
Which part of the wavelength of light is absorbed by chlorophyll?
Where a photon of light is absorbed in chlorophyll?
What is the basic reaction of photosynthesis?
What mainly occurs in the light reaction and dark reaction of photosynthesis?
What kind of a light/matter interaction is photosynthesis?