PHOTOSYNTHESIS Chapter 6 Photosynthesis is the process by which plants, algae & cyanobacteria capture the energy in sunlight and convert it into chemical energy. Many consider photosynthesis to be the most important chemical process on earth, because it was not until photosynthesis began about 2 billion years ago that oxygen began to build up in the earth’s atmosphere. All oxygen in the air we breathe has cycled through photosynthetic organisms. Chapter 6

A. Light Visible light makes up only a small portion of the electromagnetic spectrum. Sunlight consists of: 4% Ultraviolet (UV) radiation 44% Visible light 52% Infrared (IR) radiation Of these 3 types of radiation, we are primarily concerned with visible light because it provides the right amount of energy to power photosynthesis. [UV radiation is too powerful, while infrared radiation is not powerful enough]

Characteristics of Visible Light: is a spectrum of colors ranging from violet to red consists of packets of energy called photons photons travel in waves, having a measurable wavelength ()  = distance a photon travels during a complete vibration [measured in nanometers (nm)] 1 nanometer = a billionth of a meter The wavelengths of visible light range between 390 (violet end) and 760 nanometers (red end).

A photon’s energy is inversely related to its wavelength... ...the shorter the , the greater the energy it possesses. Which of the following photons possess the greatest amount of energy? Green photons  = 530nm Red photons  = 660nm Blue photons  = 450nm Blue photons are the most energetic because they have the shortest wavelengths.

What happens to light when it strikes an object? reflected (bounces off) transmitted (passes through) absorbed Reflected or transmitted wavelengths determine the color of the object. Leaves appear to be green because the pigments they possess reflect green wavelengths of light. Objects that reflect all wavelengths of light (absorb none) are white, while objects that reflect none (absorb all) are black. Only absorbed wavelengths of light function in photosynthesis.

B. Photosynthetic Pigments Molecules that capture photon energy by absorbing certain wavelengths of light. 1. Primary pigments Bacteriochlorophyll - green pigment found in certain bacteria. Chlorophylls a & b - bluish green pigments found in plants, green algae & cyanobacteria.

Chlorophyll a is the dominant pigment in plant cells.

Each pigment absorbs a particular range of wavelengths. 2. Accessory Pigments Carotenoids - red, orange, yellow pigments found in plants, algae, bacteria & archaea. Phycoerythrin - red pigment found in red algae. Phycocyanin - blue pigment found in red algae & cyanobacteria. Each pigment absorbs a particular range of wavelengths. Only 3 accessory pigments are listed here. Table 6.1 in text lists several others.

Chlorophylls absorb red (600-700nm) & blue (400-500nm) wavelengths of light best. Since these are the primary photosynthetic pigments, photosynthesis occurs maximally under red & blue lights (reason why grow-lamps have a purple hue). Accessory pigments function to capture wavelengths of light that chlorophylls cannot. They then pass that energy to the chlorphylls. Carotineoids absorb blue wavelengths (460-550nm) of light best. Phycoerythrin absorbs green & yellow wavelengths of light best. Since phycoerythrins are not found in plants, green wavelengths of light contribute little to photosynthesis in plants. Phycocyanin absorbs yellow wavelengths of light best.

C. Chloroplasts Sites of photosynthesis in plants & algae. Concentrated in mesophyll cells of most plants. Are usually 40-200 chloroplasts / cell. Note stoma (opening) in cross section of leaf. they function in gas exchange, allowing CO2 to enter & O2 to exit leaf. However, stoma will close during hot dry conditions to conserve water. As we shall see later, this will impair photosynthesis.

Chloroplast structure: Double membrane surrounds stroma. Granum = stack of thylakoids. Stroma - gelatinous matrix; contains ribosomes, DNA & various enzymes. Thylakoid - flattened membranous sac; embedded with photosynthetic pigments.

D. Photosynthesis Occurs in two stages: 6CO2 + 12H2O  C6H12O6 + 6O2 + 6H2O Occurs in two stages: Light reactions - harvest photon energy to synthesize ATP & NADPH. Carbon reactions (Calvin cycle) - use energy from light reactions to reduce CO2 to carbohydrate. Photosynthesis will be described as it occurs in most eukaryotic cells.

Overview of Photosynthesis

1. Light Reactions require light occur in thylakoids of chloroplasts involve photosystems I & II (light harvesting systems). Photosystems contain antenna complex that captures photon energy & passes it to a reaction center. Antenna complex contains about 300 chlorophyll molecules & 50 accessory pigments. Reaction center contains a pair of reactive chlorophyll a molecules. Reaction center of photosystem I contains a pair of P700 chlorophyll a molecules (P stands for pigment; they absorb light energy mostly at 700nm). Reaction center of photosystem II contains a pair of P680 chlorophyll a molecules (they absorb light energy mostly at 680nm).

Light Reactions of Photosynthesis Both photosystems and electron transport chains are embedded in thylokoid membranes. Light reactions begin with photosystem II. 1. Light strikes PSII, exciting 2 electrons, which are passed to an electron acceptor. [Electrons lost from PSII must be replaced - replacement electrons are obtained by splitting water (O2 is released as a byproduct of the light reactions)]. 2. Electrons (from PSII) flow down ETC, providing energy for production of ATP by chemiosmotic phosphorylation. 3. Light strikes PSI, exciting 2 electrons, which are passed to an acceptor molecule. Electrons reaching bottom of ETC are passed to PSI as replacement electrons. 4. Excited electrons flow down a 2nd ETC, providing energy for production of NADPH. Note: electrons released when water was split eventually end up in NADPH!!!!

ATP Production by Chemiosmotic Phosphorylation ATP is produced as electrons from PSII flow down the electron transport chain toward PSI. 1. As electrons flow down the chain, energy is released. Energy is used to pump H+ (protons) from the stroma into the thylakoid space, creating a proton gradient. 2. Protons within the thylakoid space flow back into the stroma through channels called ATP synthases. 3. As protons flow through the ATP synthase, ADP is phosphorylated, forming ATP. The coupling of ATP formation to energy release from a proton gradient is called chemiosmotic phosphorylation.

2. Carbon Reactions (Calvin cycle; C3 cycle) do NOT require light (occur in both darkness & light as long as ATP & NADPH are available) occur in stroma of chloroplasts require ATP & NADPH (from light reactions), and CO2 Called Calvin cycle in honor of the American biochemist Melvin Calvin. Called C3 cycle because CO2 is fixed as a 3C compound (PGA). NADPH is often the limiting factor of carbon reactions, because cells have only 1 mechanism for its production (light reactions of photosynthesis). NADPH cannot be made at night, so when it runs out, carbon reactions cease. Not likely that ATP will be a limiting factor of carbon reactions because cells have other mechanisms for making ATP.

Calvin Cycle 1. Carbon fixation: The enzyme rubisco “fixes” CO2 [attaches CO2 to the 5-carbon sugar, ribulose biphosphate (RuBP)]. The resulting 6C compound is unstable & immediately splits to form two 3C molecules (PGA). Rubisco is one of the most important & abundant enzymes in the world. 2. PGAL synthesis: The energy in ATP & NADPH is used to convert PGA PGAL (phosphoglyceraldehyde). PGAL is the direct carbohydrate product of the carbon reactions. 3. PGAL molecules are siphoned off & combined to form glucose, sucrose, starch & other organic molecules. 4. Regeneration of RuBP: Some of the PGAL is rearranged to regenerate RuBP. [essential step in perpetuating the cycle]

Plants that use only the Calvin cycle to fix carbon are called C3 plants. Ex. cereals, peanuts, tobacco, spinach, sugar beets, soybeans, most trees & lawn grasses. 85% of all plant species are C3 plants.

E. Photorespiration Process that counters photosynthesis. Occurs when stomata close under hot, dry conditions: O2 levels in plant increase CO2 levels in plant decrease Under these conditions, rubisco fixes O2 (rather than CO2). Thus, PGAL is NOT produced. Stoma close on hot dry days (to conserve water). Thus, CO2 is steadily being depleted, while O2 is steadily increasing inside the plant. Photorespiration severely hampers photosynthesis in C3 plants.

F. C4 and CAM Photosynthesis Adaptations that allow certain plants to conserve water and reduce photorespiration at higher temperatures. 1. C4 Photosynthesis C4 plants reduce photorespiration by physically separating the light reactions and Calvin cycle. Called C4 photosynthesis because carbon dioxide is fixed as a 4C compound [malic acid] before it enters the Calvin cycle. C4 plants include sugarcane, corn, millet & sorghum. About 0.4% of plant species are C4 plants.

Leaf anatomy of a C4 plant C4 Photosynthesis: Light reactions occur in chloroplasts of mesophyll cells. Calvin cycle occurs in chloroplasts of bundle sheath cells. C4 plants have an additional biochemical pathway that allows them to fix CO2 even when levels within the plant fall very low. CO2 is fixed initially in mesophyll cells using the enzyme PEP carboxylase. PEP carboxylase has a high affinity for CO2. PEP carboxylase converts CO2 into a 4C compound, malic acid. Mesophyll cells actively pump malic acid into bundle sheath cells. CO2 is released & enters Calvin cycle (fixed by rubisco). This adaptation keeps CO2 levels high in bundle sheath cells, so rubisco functions optimally (photorespiration does not occur). Note: C4 plants dominate in hot, dry environments because they have a distinct advantage over C3 plants (able to inhibit water loss & reduce photorespiration). However, C4 plants are not as abundant in other habitats because they are at an energetic disadvantage. They must use use energy to pump malic acid into bundle sheath cells.

2. CAM Photosynthesis CAM plants reduce photorespiration by acquiring CO2 at night. Night: mesophyll cells fix CO2 as malic acid malic acid is stored in vacuoles. Day: malic acid releases CO2 which enters Calvin cycle. Malic acid CAM plants include cacti, pineapple, Spanish moss, orchids, some ferns & the wax plant. About 10% of plant species are CAM plants.