FOR EXAM 2
A. Photosynthesis in Nature
1. Plants and other autotrophs are the producers of the biosphere.
a. Photosynthesis nourishes almost all of the living world directly or indirectly.
1. All organisms require organic compounds for energy and for carbon skeletons.
b. Autotrophs produce their organic molecules from CO2.
1. Autotrophs are the ultimate sources of organic compounds for all nonautotrophic organisms.
c. Photosynthesis occurs in plants, algae, some other protists, and some prokaryotes. (Fig. 10.1)
d. Heterotrophs live on organic compounds produced by other organisms.
1. The most obvious type of heterotrophs (like humans) feed on plants and other animals.
2. Other heterotrophs (like fungi and some bacteria) decompose and feed on dead organisms and on organic litter, like feces and fallen leaves.
3. Heterotrophs are dependent on autotrophs for food and for oxygen, a byproduct of photosynthesis.
2. Chloroplasts are the sites of photosynthesis in plants
a. Leaves are the major site of photosynthesis for most plants.
b. The color of a leaf comes from chlorophyll, the green pigment in the chloroplasts.
1. Chlorophyll plays an important role in the absorption of light energy during photosynthesis.
c. Chloroplasts are found mainly in mesophyll cells forming the tissues in the interior of the leaf. (Fig. 10.2)
d. O2 exits and CO2 enters the leaf through microscopic pores, stomata, in the leaf.
e. A typical mesophyll cell has 30-40 chloroplasts.
f. Each chloroplast has an inner and outer membrane and a central aqueous space, the stroma.
g. In the stroma are membranous sacs, the thylakoids, which contain chlorophyll molecules.
1. These have an internal aqueous space, the thylakoid space.
2. Thylakoids may be stacked into columns called grana.
B. The Pathways of Photosynthesis
1. Chloroplasts split water molecules.
a. Powered by light, the green parts of plants produce organic compounds and O2 from CO2 and H2O.
b. The reaction describing the process of photosynthesis is:
6CO2 + 6H2O + light energy --> C6H12O6 + 6O2
c. One of the first clues to the mechanism of photosynthesis came from the discovery that the O2 given off by plants comes from H2O, not CO2.
d. Water is split and electrons and H+ are transferred to CO2, forming sugar. (Fig. 10.3)
1. Polar covalent bonds (unequal sharing) are converted to nonpolar covalent bonds (equal sharing).
2. Light boosts the energy of electrons as they move from water to sugar.
e. The oxygen from water is released to the atmosphere (where it will be used in respiration).
2. The light reactions and the Calvin cycle cooperate in converting light energy to chemical energy of food: an overview
a. Photosynthesis is two processes, each with multiple stages.
1. The light reactions convert solar energy to chemical energy.
2. Using energy from the light reactions the Calvin cycle incorporates CO2 from the atmosphere into an organic molecule (sugar).
b. In the light reactions light energy absorbed by chlorophyll in the thylakoids drives the transfer of electrons and hydrogen from water to NADP+ (nicotinamide adenine dinucleotide phosphate), forming NADPH.
1. NADPH, an electron and H carrier, provides energized electrons to the Calvin cycle.
c. The light reactions also generate ATP by powering the addition of phosphate to ADP.
1. The ATP provides energy for the Calvin cycle.
d. The light reactions occur at the thylakoid membrane.
e. The Calvin cycle is named for Melvin Calvin who, with his colleagues, worked out many of its steps in the 1940s.
1. It begins with the incorporation of CO2 into an existing organic molecule via carbon fixation.
2. This new piece of carbon backbone is provided with electrons by NADPH to make sugar.
3. ATP from the light reactions also powers parts of the Calvin cycle.
4. The Calvin cycle occurs in the stroma.
FOR EXAM 3
3. The light reactions convert solar energy to the chemical energy of ATP and NADPH: a closer look
a. The nature of sunlight
1. Light, like other form of electromagnetic energy, travels in waves.
2. The distance between crests of electromagnetic waves is called the wavelength.
3. Wavelengths of electromagnetic radiation range from short (gamma rays) to long (radio waves).
4. The entire range of electromagnetic radiation is the electromagnetic spectrum. (Fig. 10.5)
5. The most important segment for life is a narrow band between 380 to 750 nm (visible light).
6. Photons are units of light with specific amounts of energy.
a. Photons with shorter wavelengths have more energy.
7. While the sun radiates a full electromagnetic spectrum, the atmosphere selectively screens out most wavelengths, permitting only visible light to pass in significant quantities.
b. Photosynthetic pigments
1. When light meets matter, it may be reflected, transmitted, or absorbed. (Fig. 10.6)
a. Different pigments absorb photons of different wavelengths.
b. A leaf looks green because chlorophyll, the dominant pigment, absorbs red and blue light, while transmitting and reflecting green light.
2. An absorption spectrum plots a pigment�s light absorption versus wavelength. (Fig. 10.8)
3. In the thylakoid membrane are several pigments that differ in their absorption spectrum.
a. Chlorophyll a, the dominant pigment, absorbs best in the red and blue wavelengths, and least in the green.
b. Only chlorophyll a participates directly in the light reactions. c. Accessory photosynthetic pigments absorb light and transfer energy to chlorophyll a.
1. Chlorophyll b, with a slightly different structure than chlorophyll a, has a slightly different absorption spectrum and funnels the energy from these wavelengths to chlorophyll a.
2. Carotenoids can funnel the energy from other wavelengths to chlorophyll a.
c. Excitation of chlorophyll
1. When a pigment molecule absorbs a photon, one of that molecule�s electrons is elevated to an orbital with more potential energy.
a. The only photons that a pigment molecule can absorb are those whose energy matches exactly the energy difference between the 2 energy levels.
b. A pigment molecule absorbs only photons corresponding to specific wavelengths.
2. Photons are absorbed by clusters of pigment molecules in the thylakoid membranes.
3. The energy of the absorbed photon is converted to the energy of an electron raised from a low energy level to a higher one.
d. Photosystems
1. In the thylakoid membrane, chlorophyll is organized along with proteins and smaller organic molecules into photosystems. (Fig. 10.11)
2. A photosystem acts like a light-gathering �antenna complex� consisting of a few hundred chlorophyll a, chlorophyll b, and carotenoid molecules.
3. When any antenna molecule absorbs a photon, it is transmitted from molecule to molecule until it reaches a particular chlorophyll a molecule, the reaction center.
4. How is the energy passed?
a. Excited electrons are unstable.
b. Generally, they drop to their ground state in a billionth of a second, releasing heat energy.
c. Some pigments, including chlorophyll, release a photon of light, in a process called fluorescence, as well as heat.
d. Another pigment molecule absorbs the photon.
5. At the reaction center is a primary electron acceptor which removes an excited electron from the reaction center chlorophyll a.
6. There are two types of photosystems.
a. Photosystem I (discovered first) has a reaction center chlorophyll, the P700, that has an absorption peak at 700nm.
b. Photosystem II (discovered second) has a reaction center with a peak at 680nm.
c. The differences between these reaction centers (and their absorption spectra) lie not in the chlorophyll molecules, but in the proteins associated with each reaction center.
7. These two photosystems work together to use light energy to generate ATP and NADPH.
e. The light reactions (Fig. 10.16)
1. Photosystem II captures the energy that makes ATP.
a. Light strikes antenna complex.
b. Energy passes from pigment to pigment until it reaches the reaction center.
c. An electron in chlorophyll a receives the energy from the antenna complex and is boosted to a high energy level.
d. Chlorophyll a transfers the electron to an electron acceptor (quinone).
e. The chlorophyll a has lost an electron that must be replaced.
1. A water splitting enzyme removes electrons and H+ from water.
2. An electron fills the void left in chlorophyll a.
3. H+ stays in thylakoid space and O2 is released.
f. Quinone passes the excited electron to a carrier (plastoquinone).
g. Plastoquinone carries the electron to the cytochrome complex.
h. The electron gives some of its energy to the cytochrome complex which uses it to pump H+ from the stroma into the thylakoid space.
i. H+ rushes down its concentration gradient through ATP-synthase channels.
j. Energy stored in the H+ gradient is used to make ATP on the stroma side of the membrane.
2. Photosystem I captures the energy that makes NADPH.
a. The electron held in the cytochrome complex still contains about half of the energy it originally absorbed from the antenna complex.
b. Plastocyanin (PC) carries the electron to photosystem I.
c. An electron in chlorophyll a in Photosystem I receives the energy from the antenna complex and is boosted to a high energy level.
d. Energy from the electron from photosystem II is added to the new electron-old electron replaces the one lost by photosystem I.
e. New high energy electron is passed to ferredoxin (electron carrier) on outside (stromal side) of thylakoid membrane.
f. Ferredoxin carries electrons to NADP reductase.
g. NADP reductase uses 2 electrons, H+ from the stroma and NADP+ to make NADPH (electron carrier).
4. The Calvin cycle uses ATP and NADPH to convert CO2 to sugar: a closer look
a. The Calvin cycle regenerates its starting material after molecules enter and leave the cycle.
b. CO2 enters the cycle and leaves as sugar.
c. The cycle spends the energy of ATP and the electrons and H carried by NADPH to make the sugar.
d. The actual sugar product of the Calvin cycle is not glucose, but a three-carbon sugar, glyceraldehyde-3-phosphate (G3P).
e. The Calvin cycle has three phases. (Fig. 10.17)
1. In the carbon fixation phase, each CO2 molecule is attached to a five-carbon sugar, ribulose bisphosphate (RuBP).
a. The six-carbon intermediate splits in half to form two molecules of 3-phosphoglycerate per CO2.
2. In the reduction phase each 3-phosphoglycerate receives another phosphate group from ATP.
a. A pair of electrons and one H from NADPH are added to form G3P.
b. G3P exits the cycle and can be made into other sugars.
3. In the regeneration phase, G3P molecules are rearranged to form RuBP molecules.
a. To do this, the cycle must spend more ATP (one per RuBP) to complete the cycle and prepare for the next.
f. For the net synthesis of one G3P molecule, the Calvin recycle consumes nine ATP and six NADPH.
g. The G3P from the Calvin cycle is the starting material for metabolic pathways that synthesize other organic compounds, including glucose and other carbohydrates.
5. Photosynthesis is the biosphere�s metabolic foundation: a review
a. In photosynthesis, the energy that enters the chloroplasts as sunlight becomes stored as chemical energy in organic compounds.
b. Sugar made in the chloroplasts supplies the entire plant with chemical energy and carbon skeletons to synthesize all the major organic molecules of cells.
1. About 50% of the organic material is consumed as fuel for cellular respiration in plant mitochondria.
2. Carbohydrate in the form of the disaccharide sucrose travels via the veins to nonphotosynthetic cells.
3. There, it provides fuel for respiration and the raw materials for anabolic pathways including synthesis of proteins and lipids and building the extracellular polysaccharide cellulose.
4. Plants also store excess sugar by synthesizing starch.
c. Heterotrophs, including humans, consume plants for fuel and raw materials.
d. On a global scale, photosynthesis is the most important process to the welfare of life on Earth.
1. Each year photosynthesis produces 160 billion metric tons of carbohydrate.
e. The energy cycle
1. The products of photosynthesis (sugar and O2) are used in cellular respiration.
2. The products of cellular respiration (CO2 and water) are used in photosynthesis.