Notes 

Chapter 8

Photosynthesis: The process whereby energy from light is captured by plant, algal, or bacterial cells and used to synthesize organic molecules from CO2 and H2O (or H2S).

*Plant life, algae, and cyanobacteria make O2

*20 % of the Earth’s O2 comes from the Amazon Rain Forest.

*The Rain Forests once covered 14 % of the Earth’s surface, but now it is less than 6 %.

Jan Baptisata Van Helmont grew a tree for 5 years in a pot.  The tree added 164 pounds, but the soil only lost 2 pounds.

*He concluded the plant got most of the nutrients from water.  Was he correct?

1770’s-Jan Ingenhous immersed green plants and discovered bubbles.

*He determined plants needed sunlight to produce these bubbles.

*Jean Senebier found CO2 is required for plant growth.

Biosphere: The regions on the surface of the Earth and in the atmosphere where living organisms exist.

*We now know that most of the mass comes from CO2.

1845-Julius Von Mayer proposed plants convert light energy into chemical energy.

Heterotrophs: Organisms that cannot produce their own organic molecules and thus must obtain organic food from other organisms.

Autotrophs: Organisms that have metabolic pathways that use energy from either inorganic sources, such as CO2 and H2O, or light to make organic molecules.

Conelis Van Niel proposed a general equation for photosynthesis(p.158)

*Photosythetic bacteria can use H2S instead of H2O and release sulfur NOT oxygen.

Photoautotroph: An autotroph that use light as a source of energy to make organic molecules.

Most species rely on photosynthesis for oxygen release and energy(glucose).

*Energy cycle Figure 8.1 on p.158.

Chloroplasts: Semiautonomous organelles found in plant and algal cells that carry out photosynthesis.

*like mitochondria, chloroplasts contain an inner and outer membrane.

Chlorophyll: A photosynthetic green pigment found in chloroplasts of plants, algae, and some bacteria.

*all green parts of the plant contain chlorophyll and can perform photosynthesis.

*the bulk of photosynthesis occurs in the leaves.

Mesophyll: The internal part of the leaf that contains cells with chloroplasts that carry out the bulk of photosynthesis in plants.

Stomata: Surface pores on plant surfaces that can be closed to retain water or open to allow the entry of carbon dioxide needed for photosynthesis and the exit of oxygen and water vapor.

*cells need H2O and CO2 for photosynthesis.

Thylakoid Membrane: A third membrane within the chloroplast that forms many flattened, fluid-filled tubules that enclose a single, convoluted compartment. It contains chlorophyll and is the site where the light-dependent reactions of photosynthesis occurs.

Thylakoids: Flattened, plate like membranous region found in cyanobacterial cells and the chloroplasts of photosynthetic protists and plants; the location of the light reactions of photosynthesis.

Thylakoid Lumen: The fluid-filled compartment within the thylakoids.(Think of a water balloon.)

Granum: A structure composed of stacked membrane-bound thylakoids within a chloroplast.(Stack of pancakes)

Stroma: The fluid-filled region of the chloroplast between the thylakoid membrane and the inner membrane.

*study Figure 8.2 (p. 159)

Light Reactions: The first of two stages in the process of photosynthesis. During the light reactions, photosystem II and photosystem I absorb light energy and produce ATP, NADPH, and O2.

*Takes place in the thylakoid membrane.

Calvin Cycle: The second stage of photosynthesis. ATP is used as a source of energy, and NADPH is used as a source of high-energy electrons so that CO2 can be incorporated into carbohydrates. The Calvin Cycle occurs in the stroma.

*Takes place in the stroma.

NADPH: Nicotinamide adenine dinucleotide phosphate is an electron carrier that can accept two electrons. It is an energy intermediate that provides energy and electrons to drive the Calvin Cycle.

NADH (Figure 6.11) – Nicotinamide ring – 8.2 reactions that Harness light energy

Wavelength: The distance between peaks in a wave pattern of light or sound.

Electromagnetic Spectrum: All possible wavelengths of electromagnetic radiation, from relatively short wavelengths (gamma rays) to much longer wavelengths (radio waves).

Visible light – Electromagnetic spectrum from 380 nm violet to 740 nm red

*visible light is what drives photosynthesis.

Photons: Discrete particles that make up light. They are massless particles that travel in a wavelike pattern and move at the speed of light.

*Energy absorbed from gamma rays, x-rays and uv rays can be devastating. (DNA mutation/ cancer)

*When light strikes an object it can 1) pass through 2) change its path 3) be absorbed

Pigment: A molecule that can absorb light energy.

Chlorophyll a: A type of chlorophyll pigment found in plants, algae, and cyanobacteria.

 Absorb other colors and reflect green.

Structures on p. 161 (Determined by Hans Fischer)

Chlorophyll b: A type of chlorophyll pigment found in plants, green algae, and some other photosynthetic organisms.

Delocalized electrons – Electron not restricted to a single atom.

Delocalized electrons can absorb light energy.

Carotenoid: A type of photosynthetic or protective pigment found in plastids that imparts a color that ranges from yellow to orange to red.

Absorption Figure 8.5 P. 161  The excited electron jumps to the next energy level.   When the electron drops back down it may:

 1) release heat (street gets hot)

2) release light (jelly fish glow)

3) captured by another molecule (photosynthesis)

Absorption Spectrum: A diagram that depicts the wavelengths of electromagnetic radiation that are absorbed by a pigment.

*Chlorophyll a and b absorb light most strongly in red and violet.

* carotenoids absorb light in blue and blue-green regions

Action Spectrum: The rate of photosynthesis plotted as a function of different wavelengths of light. (Fig. 8.7b)p. 162

Photosystem I (PSI): A distinct complex of proteins and pigment molecules in chloroplasts that absorbs light during the light reactions of photosynthesis.

*Discovered first, but occurs second.

*Key role of PSI is to make NADPH (Fig 8.8 p.163)

The formation of NADPH contributes to a H+ electrochemical gradient across the thylakoid membrane.

Photosystem II (PSII): A distinct complex of proteins and pigment molecules in chloroplasts that generates oxygen from water during the light reactions of photosynthesis.

*Discovered second, but occurs first.

Electron Transport Chain: A group of protein complexes and small organic molecules within the inner membranes of mitochondria and chloroplasts and the plasma membrane of prokaryotes. The components accept and donate electrons to each other in a linear manner and produce a H+ electrochemical gradient.

*E.T.C. is located in the thylakoid membrane

*Functions similar to the E.T.C. of the mitochondria.

*ATP synthesis in the chloroplasts is achieved by chemiosmotic mechanisms

*These are similar to those used to make ATP in mitochondria.

ATP synthesis is driven by the flow of H+ ions in the thylakoid.

The gradient is produced by:

1)                     Splitting water which puts H+ ions in the thylakoid.

2)                     Movement of electrons from PSII to PSI which pumps H+ ions into the thylakoid lumen.

3)                     Formation of NADPH which consume H+ ions in the stroma.

Noncyclic Electron Flow: The combined action of photosystem II and photosystem I in which electrons flow in a linear manner to produce NADPH.

*This is favored when the NADP+ is high and the NADPH is low.

Cyclic Photophosphorylation: During photosynthesis, a pattern of electron flow in the thylakoid membrane that is cyclic and generates ATP alone.

Cyclic Electron Flow: Another name for cyclic photophosphorylation due to the path of the electrons.

*This is favored when the NADP+ is low and the NADPH is high.

*Under these conditions there is sufficient NADPH to run the calvin cycle.

Homologous Genes: Two or more genes that are derived from the same ancestral gene that have accumulated random mutations that make their sequences slightly different.

*There are homologous genes in the E.T.C. of the mitochondria and the chloroplast

Light-Harvesting Complex: A component of photosystem II and photosystem I composed of several dozen pigment molecules that are anchored to proteins in the thylakoid membranes of a chloroplast. These complexes absorb photons of light.

*Energy is transferred, NOT the electron.

*1932 Robert Emerson and William Arnold discovered the light – Harvesting complex.

Resonance Energy Transfer: The process by which energy (not an electron itself) can be transferred to adjacent pigment molecules during photosynthesis. (p680*à p680+ + e-)

Antenna Complex: Another name for the light-harvesting complex because it acts like an antenna that absorbs energy from light and funnels that energy to P680 in the reaction center.

Primary Electron Acceptor: The molecule to which a high-energy electron from an excited pigment molecule such as P680* is transferred during photosynthesis.

*The energy transfer happens in a few pico seconds, so heat and light are not given off. P.165

Photosytem II

Photosystem II can produce Oxygen which is released into the atmosphere.

Manganese Cluster: A site where the oxidation of water occurs in photosystem II during photosynthesis.

*They are composed of four Mn2+, one Ca2+, and one Cl1-

X-Ray Crystallography: A technique in which researchers purify molecules and cause them to form a crystal. When a crystal is exposed to X-rays, the resulting pattern can be analyzed mathematically to determine the three-dimensional structure of the crystal’s components.

*Used in 2004 by SoIwata, James Barber to determine the 3D structure.

Enhancement Effect: The phenomenon in which maximal activation of the pigments in photosystems I and II is achieved when organisms are exposed to two wavelengths of light.

*In the 1950’s Robert Emerson discovered this while measuring CO2 uptake/O2 production

*The low rate of photosynthesis occurred at 680-nm and 700-nm, but when both wavelengths of light were shown at the same time, the rate tripled.

Z Scheme: A model depicting the series of energy changes of an electron during the light reactions of photosynthesis. The electron absorbs light energy twice, resulting in an energy curve with a zigzag shape.

8.4 Sythesizing carbohydrates via the Calvin Cycle

Calvin cycle (Calvin-Benson) determined by Melvin Calvin and Andrew Adam Benson 1940s-1950s.

P. 168 Summarized

Three Phases are 1) Carbon Fixation 2) reduction and carbohydrate production and 3) regeneration of RuBP

Carbon Fixation: A process in which carbon from inorganic CO2 is incorporated into an organic molecule such as a carbohydrate.

*carbohydrates are critical for two reasons: 1) these are the precursors of most macromolecules in all living things

2) this involves the storage of energy

* this stored energy can be used by plants in the dark or by animals that consume plants

Rubisco (RuBP carboxylase/oxygenase): The enzyme that catalyzes the first step in the Calvin cycle in which CO2 is incorporated into an organic molecule.

*most abundant protein in chloroplasts and maybe even on earth.

8.5 variations of photosynthesis

C3 Plant: A plant that incorporates CO2 into organic molecules via RuBP to make 3PG, a three carbon molecule.

Photorespiration: The metabolic process occurring in C3 plants that occurs when the enzyme Rubisco  combine with oxygen instead of carbon dioxide and produces only one molecule of 3PG instead of two, thereby reducing photosynthetic efficiency.                                                                                                                                                

C4 Plant: A plant that uses PEP carboxylase to initially fix CO2 into a four-carbon molecule and later uses Rubisco to CO2 into simple sugars; an adaptation to hot, dry environments.

PEP Carboxylase: An enzyme in in C4 plants that adds CO2 to phosphoenolypyruvate (PEP) to produce the four-carbon compound oxaloacetate.

CAM Plants (Crassalacean Acid Metabolism): C4 plants that open their stomata at night to take up carbon dioxide.

*stomata are closed during the day and open at night

At night CAM plants open stomata and take in CO2, CO2 joins PEP to form oxaloacetate, oxaloacetate is converted to malate and is stored in vacuoles of the cell.

Calvin Cycle(Calvin-Benson) determined by Melvin Calvin and Adrew Adam Benson from 1940-1950. (p.168 summary)