jimtrue.com : school : BSC2010 : CH 10: Photosynthesis
Posted by Jim True on March 4, 2004 5:29 AM. Last Updated October 22, 2006 9:23 PM
Disclaimer for all material noted here is at the bottom of this web page.
CH 10: Photosynthesis
Obtaining Energy
- Virtually all forms of life rely either directly or indirectly on energy from the sun for life.
- There are two modes for obtaining energy:
- Autotroph ("auto" -- self; "troph" -- nourish, food) -- An organism that is able to produce its own food via metabolic processes. It does not need to eat or ingest other organisms or substances from other organisms for energy. Do not have to eat; do have to take in raw materials which they can get from diffusion or active transport.
- There are two subcategories of autotrophy depending on what initial energy source is converted to food:
- chemosynthetic autotroph (or chemoautotroph) -- Production of organic compounds (food) through the oxidation of inorganic chemical compounds. Vent communities in the deep ocean that are collecting energy from the sea floor vents.
- Photosynthetic autotroph (or photoautotroph) -- Production of food through a two stage process involving the initial conversion of light energy.
- Photoautotrophy is far more common, driving about 99% of all life on Earth.
- Regardless of the subcategory, because they generate food for themselves and also serve as the initial food sources for other organisms, autotrophs are also referred to as producers.
- Heterotroph ("hetero" -- other). Organisms that cannot manufacture their own food; they must consume other organisms or compounds produced by other organisms.
Photosynthesis (PSYN)
Photosynthesis ("photo" -- light, "synth" to make) uses special molecules called pigments to capture light energy.
- In prokaryotic cells that photosynthesize, these pigments are embedded in the cell membrane.
- In eukaryotes, the pigments are enclosed within the organelle of PSYN, the chloroplast.
Chloroplast
- The chloroplast has many structural similarities to the mitochondrion.
- The chloroplast structure includes an outer membrane surrounding a highly folded inner membrane. The inner membrane folds are arranged as series of stacked disks.
- Each individual disk is called a thylakoid. One stack of thylakoids is called a granum, and one chlorplast may hold many grana, just as each plant cell may hold many chloroplasts.
- Surrounding the grana is a liquid region called the stroma
- The number of chloroplasts in plants cells is highly variable, with many plant cells (e.g. roots) lacking them entirely.
- Embedded within the thylakoid membranes are pigment molecules, as well as electron transport chain molecules.
- PSYN will take place in and across the thylakoids and in the stroma.
(figure 10.2, p.178)
Light and Pigments
- Light is part of the electromagnetic spectrum, a range of energy that radiates as a wave.
- These waves range in length from 10's of meters to nanometers (one one-billionth of a meter).
- The longer the wavelength, the less energy and vice versa. Long waves include radio and micro-waves, short include x-rays and UV radiation. Very narrow band of wavelength called visual light, 380-750 nm. Should be called the human visual light spectrum.
- One portion of the EM spectrum between 400 and 700 nm is called the visible light spectrum
- The different wavelengths of visible light are divided into the seven primary colors ROY G BIV -- longest to shortest wavelength. (Red is the top of a rainbow!)
- Light travels as particles. The basic particle of light is the photon.
- Photons travel in waves. Depending on the wavelength, photons possess different amounts of energy.
- They travel at enormous speeds and when they strike molecules, they transfer a small portion of their energy to the molecule. X-Rays and Gamma rays are traveling at such velocity that their energy can break apart the receiving molecules.
- This energy is transferred to electrons (remember, these are the POTENTIAL ENERGY source for the molecule!).
- When an electron receives additional energy, it is boosted out of its energy level (shell) to a higher energy level, ie, the electron is said to be excited.
- One of the two things happens to an excited electron:
- It drops back to its original energy level by releasing a brief burst of energy in the form of heat or light. The light release is called flourescence.
- The electron is "captured" by another molecule or substance and its excess energy is used to do work.
- Pigment -- A substance that absorbs certain wavelengths of light and reflects others.
- "The color reflected is the color detected"©
- If a pigment is absorbing certain wavelengths of light, it is absorbing a certain amount of ENERGY!
- So, any substance pigmented with different colors absorb some wavelengths of visible light and reflect others.
- BLACK -- the absence of all color. It ABSORBS all wavelengths of visible light.
- WHITE -- The presence of all color. It REFLECTS all wavelengths of visible light.
- Any energy form converts to heat, thus, a black surface becomes hotter than white when exposed to sunlight.
- There are wide variety of pigments present in various photoautotrophs.
- Green plants have four main pigments:
- Chlorophyll -- two different types, a and b, both of which are green pigments. Chlorophyll is the main pigment in green plants.
- At the center of each chlorophyll molecules is an atom of Magnesium (Mg) which is the source of excited electrons.
- There are also several accessory pigments, so named because there are fewer of them and they assist in PSYN.
- Xanthophyll -- a yellow pigment.
- Carotenoids -- these are orange-red pigments.
- Different plants and other PSYN organisms will have variety of pigments, but all contain chlorophyll in some amount.
(Figures 10.6 & 10.7, p.182; 10.8, p.183
Photosystems
- Embedded in numerous locations in the membranes of each thylakoid is a cluster of pigment molecules called a photosystem.
- Each photosystem is is made up of numerous pigment molecules (often called the "antenna complex") surrounding TWO chlorophyll a molecules at the reaction center. Molecular arrangement resembles a satellite dish; auxiliary outlying pigments focus the light energy into one channelling location.
- There are two different photosystems (PS) known in eukaryotic cells (different in one respect):
- PS I, also known as P700, and
- PS II, also known as P680.
- The numbers refer to the peak absorption wavelength in the red range for the chlorophylls in these photosystems.
- When a photon strikes an antenna complex, its energy is transferred from molecule to molecule until it reaches the reaction center. Center of the chlorophyll has a magnesium atom at it's center; this is where the primary energy is channelled, an electron knocked off the magnesium atom.
- At the reaction center, the magnesium atoms in each of the two chlorophyll a molecules received the transferred energy.
- Each has an electron that is excited and trasferred to a primary electron acceptor, which removes it from the chlorophyll.
- Thus, chlorophyll is OXIDIZED by light energy (Photooxidation)! The loss of one or more electrons is oxidation.
PSYN Overview
- PSYN is actually a two stage process, with numerous reactions in each:
- Light-dependent (AKA Light) Reaction -- this requires light to photooxidize chlorophylls in the photosystems.
- This will drive the formation of temporary electron acceptors called NADPH and ATP by phosphorylation. Two temporary products, ATP and NADPH and waste O2.
- Since the phosphorylation reaction is driven by light energy, it is termed photophosphorylation.
- Calvin Cycle (AKA Calvin-Benson Cycle or Light Independent Reaction) - It does not require light directly, however, it needs the ATP and electrons from the NADPH to run. No such thing as a 'dark reaction'.
- In this stage, the atmospheric CO2 is incorporated into an organic molecule which can ultimately be converted to glucose (food).
- The process is generally termed carbon fixation.
- This is the PSYN reaction formula:
- 6 CO2 + 6 H20 --> C6H12O6 + 6O2 (reverse of cellular respiration)
Light Dependent Reaction
- In most modern plants, the light dependent reaction occurs via a process called non-cyclic photophorylation (NCPP) or non-cyclic electron flow, which involves both PSI and PSII.
- The term non-cyclic refers to the fact that the excited electrons that are released from the chlorophylls in the photosystems do NOT return to their original photosystem.
- To initiate NCPP, light MUST be present at some range of intensity, hence the name, light DEPENDENT reaction.
- Light photons strikes both photosystems. Energy is transferred to the reaction center, which excites an electron in each Mg atom in the chlorophylls.
- The excited electrons are transferred to the first of several electron transport molecules in the thylakoid membrane (photooxidation).
- When a photosystem loses an excited electron (both electrons, one from each chlorophyll a), it is temporarily deactivated until another electron replaces the missing one.
- We'll look at each PS separately and then connect the two in the reaction sequence.
- PSII releases its electron to an electron acceptor transport molecule.
- In order to reactivate PSII, a molecule of water is split by an enzyme.
- This releases Oxygen gas as a waste product.
- The excited electrons from PSII will then pass through an electron transport chain that is very similar to that in the mitochondrion.
- The energy released from this electron is used to drive a proton pump (H+ ions).
- The transported H+ ions are pumped into the space within the thylakoids.
- The H+ ions can only diffuse back into the matrix (stroma?) AT SPECIFIC CARRIER PROTEINS!
- At these proteins, in the presence of a special enzyme called ATP synthase, as H+ ions diffuse across, ADP + P --> ATP.
- Because this phosphorylation process is driven by light energy, it is called PHOTOphosphoryation.
- While this is going on, PSI has also been struck by a photon, and electrons from its reaction are excited.
- PSI also releases its electrons to electron acceptor transport molecules, and is deactivated.
- However, the electrons are then passed to the temporary electron acceptor NADP+ to form NADPH.
- To reactivate PSI, it receives the electrons from PSII once they have been drained of all their excess energy in the electron transport chain.
- Cyclic Electron Flow (AKA Cyclic Photophosphorylation) -- involves only PSI and produces only ATP.
- This occurs when ATP for Calvin cycle runs low and more needs to be produced quickly.
- PSI electrons are excited and release electrons to primary electron acceptor as usual. These electrons are then shuttled directly to the electron transport chain to drive the proton pump, chemiosmosis and photophosphorylation.
(figure 10.14, p.187)
- NCPP is a LINEAR biochemical reaction sequence.
- The products of NCPP are ATP and NADPH, both needed for the Calvin Cycle, plus O2 gas, released as a waste product.
Calvin Cycle
- It is also sometimes incorrectly referred to as the "dark reaction".
- It does NOT take place in the dark normally.
- However, the reaction does not need light to proceed if there is a source of NADPH and ATP, but these are used up when light intensity drops, so the Calvin cycle stops as well.
- The Calvin cycle is a series of 5 to 6 enzyme mediated reaction steps that begin and end with a 5 carbon molecule Ribulose biphosphate (RuBp).
- RuBp is combined with inorganic CO2 gas from the atmosphere to form an unstable 6 carbon compound (carbon fixation).
- This reaction is mediated by Ribulose Biphosphate Carboxylase (Rubisco), the most abundant plant protein on Earth.
- The 6 carbon intermediate breaks into two 3 carbon compounds.
- Using ATP and electrons from NADPH these molecules are converted into a 3 carbon molecule that is the EXACT SAME molecule that is formed when glucose first breaks in two in glycolysis!
- The Calvin cycle "spins" 6 times using 1 CO2, 2 ATP and 2 NADPH each time.
- It ultimately produces 2 excess 3 carbon molecules than can combine to form glucose or other carbohydrates.
- Thus, organic "food" is formed during the Calvin cycle by "fixing" inorganic CO2 into an organic compound. Carbon fixation.
Photorespiration
- Green plants absorb CO2 gas needed for PSYN through special pores in the leaves (and some stems) called stomata ("stoma" -- mouth).
- When these pores are open, plants also lose water through evaporation.
- Because this loss can be very significant and even harm the plant when it is very hot and dry, the stomata are closed during such weather.
- However, this shuts off CO2 supplies.
- Thus, when the CO2 level drops to a low level, PSYN production ceases and may even reverse in a process called photorespiration.
- In this process, the enzyme Rubisco accepts oxygen instead of carbon dioxide and forms a 2 carbon organic molecule that is converted to CO2, which is released from the plant.
- However, unlike CR, photorespiration produces no ATP, so the plant produces neither food nor energy. Very dangerous for the plant; no food and no energy. This is what often occurs during a drought that will kill even large plants.
- As a result, the plant suffers if this continues for too long (it is why new seedlings are shielded from hot, bright sun at nurseries).
- Some plants, however, are adapted to perform PSYN in hot, dry climates via slightly different food production pathways.
- Most plants are referred to as C3 plants because they convert RuBP to a 3 carbon compound in the presence of Rubisco and CO2. 3 Carbon products. Hot weather plants are usually referred to as C4 plants.
C4Pathway
- However, a number of species are C4 plants because they fix carbon from CO2 to a 4 carbon compound.
- They also have two different groups of photosynthetic cells, unlike C3 plants.
- One group of cells (mesophyll cells) use a different enzyme which has a very high affinity for CO2 and can fix it at very low concentrations (such as in hot, dry climates when the stomata are closed). If it's a hot, dry climate, and the stomata are closed, when do the stomata ever open? At night, especially in hot dry climates, the air gets very, very cold and the stomata open at night.
- The four carbon compound in the mesophyll cells is then broken to a 3 carbon compound with the release of CO2.
- The CO2 passes into the second type of cell (bundle-sheath cells) in which the Calvin cycle proceeds normally.
- C4 plants include important crop plants which do well in hot climates, e.g., corn and sugarcane. Their very own Carbon-dioxide factories within the plant to create CO2 for the Calvin cycle.
CAM Pathway
- Found in many plants that live in very hot, dry climates.
- Includes plants such as cacti and pineapples and other succulents, plants that store water in their tissues, their leaves or stems. These plants have very, very, thick, fleshy, watery skins or leaves.
- In these plants, stomata are closed during the day, preventing any CO2 uptake.
- So, they take up CO2 at night it is cooler and the stomata are open.
- The CO2 is fixed into a variety of organic acids.
- During the day, the organic acids release CO2 within the plant tissues and the Calvin cycle can proceed using that CO2.
- This path is referred to as Crassulacean Acid Metabolism, CAM for short.
- Thus, in the C4 pathway, there is a physical or spatial separation of the steps allowing the Calvin cycle to occur.
- In the CAM pathway, there is a temporal separation.
- Regardless, all C3, C4 and CAM plants ultimately use the Calvin Cycle to produce food.
Disclaimer: These are MY notes taken from classroom lectures while I'm in the classroom. While I'm perfectly happy to share my notes with my classmates and I know I take very good notes, you should still make every effort to attend the class and TAKE YOUR OWN NOTES. I will not transcribe everything the instructor says in the classroom, and I will NEVER post pre-exam reviews. My notes will not replace the value of actually attending class and taking your own class notes.I also cannot attest to their accuracy, other than they are what was provided in the lecture; you should not reference my notes as "expert opionion" by any means, and if you notice an error or omission, please do me the favor of e-mailing me with the correction and I will re-post my notes. End of Disclaimer.