Ch. 10 Photosynthesis

The Process That Feeds the Biosphere

Photoautotrophs

•Use light energy to drive the synthesis of organic molecules from CO2 and H20. (They can make their own food and ours as well)

•Heterotrophs in comparison must consume organic molecules for energy and carbon.

Chloroplasts: The site of Photosynthesis in plants.

•All green parts of a plant have chloroplast, but the leaves are the major organs of photosynthesis.

•The Players;

•Chloroplasts are plant cells organelles that are mostly located in the cells that make up the mesophyll tissue.

•The exterior of the lower epidermis of a leaf cell contains tiny pores called stomata, through which CO² enters and O² and H²O exit the enters and O² and H²O exit the leaf.leaf.

The players cont.

•Chloroplasts have an outer Chloroplasts have an outer membrane and an inner membrane and an inner membrane. Inside the inner membrane. Inside the inner membrane is the membrane is the stroma, stroma, which is a dense fluid filled which is a dense fluid filled area. Within the stroma is a area. Within the stroma is a vast network of interconnected vast network of interconnected sacs called the thylakoids. sacs called the thylakoids. Within theWithin the t thylakoidshylakoids is a is a compartment known as the compartment known as the thylakiod space.thylakiod space.

•Chlorophyll Chlorophyll is located in the is located in the thylakoid membrane and is the thylakoid membrane and is the light absorbing pigment that light absorbing pigment that drives photosynthesisdrives photosynthesis

The Splitting of water

The oxygen given off by plants through their stomata is derived from water and not carbon dioxide.

The chloroplast split H2O into water H and O.

Old belief was that O2 came from CO2 that photosynthesis was just the reverse of respiration.

The overall chemical change is the same but the metabolic processes are more oomplicated

Photosynthesis The Basic Idea

•Basically, plant cells produce organic compounds using light energy, carbon dioxide and water. Oxygen is release in the process.

•The two main parts of photosynthesis are the light reactions and the Calvin cycle.

Light Reactions of photosynthesis

•Solar energy is converted to chemical energy.

•Light is absorbed by chlorophyll and drives the transfer of electrons from water to NADP+ to NADPH ( the p is for photosynthesis) …you have mastered Ch 9 don’t go confusing the players.

•Water is split during these reactions and O² is released.

•Photophsophorylation during the light reactions leads to the production of ATP fromADP.

•The net products of the light reactions are NADPH (which stores electrons), ATP and Oxygen.

The Calvin Cycle

•In the Calvin Cycle, CO² from the air is incorporated into organic molecules in carbon fixation.

•The fixed Carbon is then used to make carbohydates.

•NADPH is used to power carbon fixation.

•The Calvin cycle also uses ATP in the course of its reactions.

Solar Energy

•Light is electromagnetic energy, and it behaves as though it is made up of discrete particles, called photons- each of which has a fixed quantity of energy.

•White light is a mixture of all the wavelengths of visible light. A prism can sort white light into its component colors by bending light of different wavelengths at different angles.•(Think Raindrops and Rainbows)

•Visible light drives photosynthesis

Why leaves are green… Interaction of light with chloroplasts.

•The chlorophyll molecules of chloroplasts absorb violet –blue and red light (the colors most effective in driving photosynthesis) and reflect.

Lab # 4

1. White light is separated into colors by a prism

2. One by one the different colors are passed through the sample(chlorophyll extracted from spinach leaves) Ex. Green light and blue light are used here.

3. The transmitted light strikes a photoelectric tube, which converts the light energy to electricity.

4. The electrical current is measured by a galvanometer. The meter indicates the fraction of light transmitted through the sample, from which we can determine the amount of light absorbed

Absorption Spectra

•The three curves show the wavelengths of the light best absorbed by three types of pigments of chloroplast pigments.

•An Action spectrum (rate of photosynthesis vs. wavelength can be derived from this information. The Action spectrum lumps all of the photosynthetic activity of all three pigments into one.

•See example on the board.

Chlorophyll a and b:slight structural changes dramatically alter absorption spectra.

•Chlorophyll a- vs. b a difference in 1 functional makes chlorophyll b less efficient in absorbing light energy

Excitation of isolated chlorophyll by light

•Absorption of a photon cause a transition of the chlorophyll molecule from its ground state

The photosystems of the light reactions

•Photons of light are absorbed by certain groups of pigment molecules in the thylkoid membrane of the chloroplast.•These groups are called photosystems.•Have a light harvesting complex made up of chlorophyll and carotenoid molecules.

•How it works:When a photon strikes a pigment molecule in a light harvesting complex, the energy is passed from molecule to molecule until it reaches the reaction center.At the Rxn. Center an excited e- from one of the 2 chlorophyll a molecules is captured by the primary electron acceptor

Noncyclic Electron Flow(The gold arrows trace the current of light-driven electrons fromH20 to NADPH)1. Photosystem II aborbs light

energy in the 680nm wavelength range. An electron in the rxn. Center cholorphyll (called 680) becomes excited and is captured by a primary electron acceptor. The rxn. Center chlorophyll is oxidized and needs an electron.

2. An enzyme splits a H2O molecule into 2 H+ ions, 2 e-, and an oxygen atom.

3. The original excited electron passes from the primary electron acceptor of PS II to PSI through an ETC.

4. The energy from the transfer of electrons down the ETC is used to phosphorylate ADP to ATP in the thylakoid membrane ( this is noncylic e flow and is similar to Chemiosmosis)

cont.

5. The electrons that reach the end of the ETC are donated to the chlorophyll of P700 in PS I ( This need for an e- by PS I is created when light energy excites an electron in P700 and that e- is taken up by the primary acceptor of PS I).

The primary e- acceptor of PS I passes the excited electrons along to another ETC, which transmits them to Ferredoxin and then finally to NADP+, which is reduced to NADPH, the second of the two important light –reaction products

Mechanical Analogy for the light reactions

PS II : a photon of light is absorbed and raises an e- to an higher energy level.

The e- is passed down the ETC consistanly losing E.

In PS I it is reboosted by the light E absorb in PSI.

At the top of PSI it reuces NADP= to NADPH which will be shuttled to the Calvin Cycle.

Cyclic Flow

•Uses PSI but not PS II

•Only makes ATP ( this is because the Calvin Cycle makes uses more ATP than NADPH.

•Probably a mechanism to keep up with the need of ATP.

•Noncyclic makes equal parts ATP to NADPH and since the Calvin Cycle needs more ATP but the body doesn’t like to waste. Cyclic is a way to get ATP Without generating NADPH that is not needed.

•Reduce ,reuse , recycle.

Mitochondria vs. Chloroplasts

Chemiosmosis Compared

The pumping is reversed spatially

Mitochondria: H+ are pumped from the matrix to the intermembrane space.

Chloroplast: H+ are pumped from the stroma to the thylakoid space

3 Phases of the Calvin Cycle

1. Carbon Fixation2. Reduction3. Regeneration of the CO2 Acceptor (RUBP)

Phase 1CO2 Fixation

•3 CO2 molecules entering one at a time are attached to 3 molecules of RuBP; these reactions are catalyzed by Rubisco. They produce a unstable intermediate that immediately splits into 2 3-carbon compounds called 3-phosphoglycerate.

•The 3-phosphoglycerate molecules are phosphorylated to become 1,3 biphos.

3 Phases of the Calvin Cycle

1. Carbon Fixation2. Reduction3. Regeneration of the CO2 Acceptor (RUBP)

Phase 2Reduction

•6 NADPH reduce the six 1,3 biphos. To six G3P.

•One G3P leaves the cell to be used by the plant.

3 Phases of the Calvin Cycle

1.Carbon fixation2.Reduction3.Regeneration of RUBP the (CO2 acceptor)

Phase 3 Regeneration of RUBP ( the

CO2 receptor)

•RuBP is regenerated as the 5 G3P’s are rearranged into 3 of the starting molecules with the expenditure of 3 ATP molecules.

The C4 plant uses PEP instead of Rubisco

•CO2 Fixation in the mesophyll•Calvin Cycle in the Bundle sheath cells.

Alternative Mechanisms of Carbon fixation evolved in hot, arid climatesC4 plants•The 1st carbon compound formed has 4C’s not 3.•have bundle sheath cells and mesophyll cells.•Spatial separation between the light reactions and the Calvin Cycle

CAM plants•Stomata kept closed during the day to prevent water loss ( also prevents gas exchange).•Night –stomata open and CO2 is taken in. Day CO2 releases for photosynthesis•Temporal Separation

•C3, C4 and CAM all use the Calvin Cyce they just have different methods for getting there.

A review of photosynthesis

Light reactions:•Are carried out by molecules in the thylakoid membranes.•Conver light E to the chemical energy of ATP and NADPH•Split H20 and release O2 to the atmosphere.

Calvin Cycle:•Take place in the stroma•Use ATP and NADPH to convert CO2 to the Sugar G3P.•Return ADP ,inorganic phosphate , and NADP+ to the light reactions