Where It Starts - Photosynthesis Chapter 7 Where It Starts - Photosynthesis

Photosynthesis - Intro Photosynthesis – makes food (sugar and other compounds) by using sunlight as an energy source carbon dioxide as the carbon source Releasing water and oxygen

7.0 Sunlight and Survival Plants are autotrophs, or self-nourishing organisms; they get Carbon and energy from the environment and make their own food. The first autotrophs filled Earth’s atmosphere with oxygen, creating an ozone (O3) layer The ozone layer became a shield against deadly UV rays from the sun, allowing life to move out of the ocean and diversify.

Other types of organisms Heterotrophs – Cannot obtain energy and C from the environment. They feed on autotrophs, one another, and organic wastes. Plants are photoautotrophs. They make sugars and other compounds using sunlight as the energy source and CO2 and release great amounts of oxygen. Plants around the world produce 220 billions tons of sugar each year

Prokaryotes The first prokaryotes were chemoautotrophs, they did not have the enzymes needed. (So they extracted energy and carbon from methane and hydrogen sulfide, there was little free oxygen.) Some prokaryotes evolved to neutralize toxic oxygen radicals and flourished. Those that did not perished.

7.1 Sunlight as an Energy Source Plants (Photoautotrophs) utilize the energy from the sun, in the form of photons , particles of wavelengths of visible light. Organisms use only a small range of wavelengths for photosynthesis (380 – 750nm) Light energy is packed as photons.

Electromagnetic Spectrum Shortest Gamma rays* wavelength X-rays* UV radiation* Visible light Infrared radiation Microwaves Longest Radio waves wavelength * alters or breaks bonds in DNA and Proteins. Threat to all organisms.

Photons Packets of light energy Each type of photon has fixed amount of energy Photons having most energy travel as shortest wavelength (blue-violet light) Photons having least energy travel in longer wavelengths. (red Light, pg. 108)

Visible Light Wavelengths humans perceive as different colors Violet (380 nm) to red (750 nm) Longer wavelengths, lower energy Figure 7-2 Page 108

Pigments Pigments are a class of molecules that absorb photons in certain wavelengths only. The color you see is the wavelength not absorbed Light-catching part of molecule often has alternating single and double bonds These bonds contain electrons that are capable of being moved to higher energy levels by absorbing light

Pigment Molecules Pigment molecules on the thylakoid membranes absorb photons. Chlorophyll a (major) pigments absorb violet and red, but reflect green & yellow (leaves) Chlorophyll b (accessory) pigments reflects green & blue. Carotenoid pigments absorb blue-violet and blue-green but reflect yellow, orange, and red The light-catching portion is the flattened ring structure (see page 109)

Pigment Molecules Xanthophylls reflect yellow, brown, purple, or blue light Anthocyanins – reflect red and purple light in fruit and flowers Phycobilins – reflect red or blue-green light and are accessory pigments found in red algae and cyanobacteria

Pigments in Photosynthesis Bacteria Pigments in plasma membranes Plants Pigments and proteins organized into photosystems that are embedded in thylakoid membrane system

7.2 Harvesting the Rainbow Wilhelm Theodor Engelmann, a botanist, knew that plants use sunlight, water and something in the air. What he wanted to know what which parts of sunlight do plants favor? He designed an experiment using photosynthetic alga, Cladophora and aerobic bacterial cells.

Englemann’s Experiment He knew that certain bacterial cells will move toward places where oxygen concentration is high He also understood that Photosynthesis produces oxygen He identified violet and red light are best at driving photosynthesis, as most of the bacterial cells gathered at this point.

7.3 Overview of Photosynthesis Reactions Photosynthesis proceeds in two reaction stages. See page 111. Light-dependent reactions -thylakoid membrane Sunlight energy is converted to chemical bond energy of ATP Water molecules are split, and typically the coenzyme NADP+ accepts the released hydrogen and electrons, thus becoming NADPH, oxygen is released

Photosynthesis – Second Stage Light Independent reactions - stroma Runs on energy delivered by ATP. This energy drives the synthesis of glucose and other carbohydrates. The building blocks are the hydrogen atoms and electron from NADPH, as well as carbon and oxygen atoms stripped from carbon dioxide and water.

light-dependant reactions light-independant reactions Two stages of Photosynthesis SUNLIGHT H2O O2 CO2 NADPH, ATP light-dependant reactions light-independant reactions NADP+, ADP sugars CHLOROPLAST Fig. 7-6c, p.111

7.4 Light-Dependent Reactions Occurs in the thylakoid membrane of the chloroplast Hundreds of Pigments absorb light energy, give up e-, which is transferred to a photosystem. Two molecules of chlorophyll a are at the center of a photosystem. Chloroplasts have two kinds of photosystems, type I and type II. (Focus on type II)

Light Dependent continued The freed e- enter an electron transfer chain, an orderly array of enzymes and co- enzymes. This is the first step in the conversion of light to chemical energy. Water molecules split, ATP and NADH form, and oxygen is released.

Light Dependent continued Pigments (PS II)that gave up electrons get replacement electrons (from water molecules –through the process of photolysis) H+ moves from the stroma into the thylakoid compartment. The e- now enter photosystem 1 This is also known as the noncyclic pathway of ATP formation.

cross-section through a disk-shaped fold in the thylakoid membrane LIGHT- HARVESTING COMPLEX PHOTOSYSTEM II P 680 sunlight PHOTOSYSTEM I P 700 H+ NADPH e- e- e- e- e- e- NADP + + H+ e- H2O H+ H+ H+ H+ thylakoid compartment H+ H+ H+ H+ H+ H+ O2 H+ thylakoid membrane stroma ADP + Pi ATP cross-section through a disk-shaped fold in the thylakoid membrane H+ Fig. 7-8, p.113

Noncyclic pathway of ATP This pathway of photosynthesis photon energy forces electrons out of photosystem II to an electron transfer chain, which sets up H+ gradient that drive ATP formation, and ultimately end up in NADPH. The electrons are not cycled back into photosystem II.

Cyclic Pathway of ATP Photosystem I may run independently so that cells can continue to make ATP. It is a cyclic because the electrons that leave photosystem I get cycled back to it. These electrons pass through an ETC that moves H+ into the thylakoid compartments. This drives ATP formation, but no NADPH forms.

7.5 Energy Flow in Photosynthesis Most pigments in photosystem are harvester pigments When excited by light energy, these pigments transfer energy to adjacent pigment molecules Each transfer involves energy loss

Photosystem Function: Reaction Center Energy is reduced to a level that can be captured by molecule of chlorophyll a This molecule P700 (I) or P680 (II) is the reaction center of a photosystem Reaction center accepts energy and donates electron to acceptor molecule

Cyclic Electron Flow Electrons are donated by P700 in photosystem I to acceptor molecule flow through electron transfer chain and back to P700 to be reused Electron flow drives ATP formation This is the process of the first anaerobic photoautotrophs. See page 114

Noncyclic Electron Flow Two-step pathway for light absorption and electron excitation Uses two photosystems: type I and type II e_ that leave PS II are not returned to it. Produces ATP and NADPH Involves photolysis - splitting of water See page 114

Chemiosmotic Model of ATP Formation Electrical and H+ concentration gradients are created between thylakoid compartment and stroma H+ flows down gradients into stroma through ATP synthesis ( ATP synthase) Flow of ions drives formation of ATP

7.6 Light-Independent Reactions Synthesis part of photosynthesis – make sugar Takes place in the stroma and can proceed in the dark Steps – carbon fixation, Rubisco mediated Calvin-Benson cycle – a series of enzyme mediated reactions

Carbon Fixation A Carbon atom from CO2 becomes attached to an organic compound. Plants get the carbon dioxide from the air, and algae get it from carbon dioxide dissolved in water. Rubisco (ribulose bisphosphate carboylase/oxygenase meditates this step in most plants.

Calvin-Benson Cycle This is known as the sugar factory It is a series of enzyme mediated reactions that take place in the stroma. Reactants – carbon dioxide attaches to rubisco, and splits into phosphoglycerate. ATP, and NADPH Products – Glucose, ADP, and NADP+ This is cyclic and RuBP is regenerated

phosphorylated glucose Calvin- Benson Cycle Six turns to make one glucose molecule 6CO2 ATP 6 RuBP 12 PGA 12 6 ADP 12 ADP + Calvin-Benson cycle 12 Pi ATP 12 NADPH 4 Pi 12 NADP+ 10 PGAL 12 PGAL 1 Pi 1 phosphorylated glucose Fig. 7-10b, p.115

7.7 The C3 Pathway Gases diffuse in/out of a plant through stomata. In Calvin-Benson cycle, the first stable intermediate is a three-carbon PGA Because the first intermediate has three carbons, the pathway is called the C3 pathway

Photorespiration in C3 Plants On hot, dry days stomata close Inside leaf Oxygen levels rise Carbon dioxide levels drop Rubisco attaches RuBP to oxygen instead of carbon dioxide (photorespiration) Only one PGAL forms instead of two See page 117

C4 Plants Carbon dioxide is fixed twice In mesophyll cells, carbon dioxide is fixed to form four-carbon oxaloacetate Oxaloacetate is transferred to bundle-sheath cells Carbon dioxide is released and fixed again in Calvin-Benson cycle. See page 117.

CAM Plants Crassulacean Acid Metabolism Carbon is fixed twice (in same cells) Never uses oxygen, no matter the concentration Night – stomata open Carbon dioxide is fixed to form organic acids Day Carbon dioxide is released and fixed in Calvin-Benson cycle. See page 117.

7.8 Autotrophs and the Biosphere Photoautotrophs Carbon source is carbon dioxide Energy source is sunlight Heterotrophs Get carbon and energy by eating autotrophs or one another

Linked Processes Photosynthesis Aerobic Respiration Energy-storing pathway Releases oxygen Requires carbon dioxide Aerobic Respiration Energy-releasing pathway Requires oxygen Releases carbon dioxide

Photoautotrophs Capture sunlight energy and use it to carry out photosynthesis Plants Some bacteria Many protistans See Figure 7-14, on page 120 for an overview