Photosynthesis Bio 391 – Ch4 How Exactly is Sunlight captured and converted into Food?

What are autotrophs? Obtains energy from nonliving sources Two typesPhotoautotrophs Photosynthesis Sun energy converts CO 2 into sugars Enzymes convert sugars into amino acids and other needed compoundsChemoautotrophs Specialized bacteria No sunlight – use energy of inorganic substances (Fe, S, etc.)

Electromagnetic Spectrum Wide range of energy types – travels in waves – energy is defined by their wavelength λ = wavelength = distance between two adjacent wave crests or wave troughs Visible Light Very small section of the electromagnetic spectrum ROYGBIV

ChloroplastsStructure Thylakoids Highly folded inner membrane surface area Holds pigments Granum Stack of thylakoid membranes Stroma Liquid between thylakoid and outer membrane of chloroplast Have their own DNA & RNA

Chlorophyll & Accessory Pigments Pigments = light absorbing molecules Found on the thylakoid membraneChlorophyll Two types – “a” and “b” Absorbs violet-blue and orange-red colors ~ nm & nm Reflects green  plants have green color Accessory Pigments Absorb other colors of light and transfer Σ to chlorophyll-a Most noticeable in the fall months EX: carotenoids

Absorption Spectrums of Pigments

Photosynthesis Simplified Can be broken down into two steps: Light Reactions Pigments in thylakoids absorb light Light converted into chemical energy Calvin Cycle Calvin Cycle (a.k.a. “Dark Reactions”) Chemical energy from light reactions used to make 3 carbon sugars from CO 2 Used to make more complex sugars or other biochemical molecules Overall Reaction 6CO 2 + 6H 2 O  C 6 H 12 O 6 + 6O 2

Light Dependent Reactions Broken into Photosystem II and Photosystem I Reactants: light, water Use: ADP and Pi to make ATP NADP+ to make NADPH (similar to NAD+/NADH) Happens on the thylakoid membrane

Light Dependent Reactions Photosystem II Light hits the chlorophyll molecules and excites them – releasing two high energy electrons Electrons are used to create a H+ gradient across the thylakoid membrane This gradient drives the formation of ATP (similar process to the ETC in respiration) Photophosphorylation

Light Dependent Reactions Photosystem I Light hits the chlorophyll molecules and excites them – releasing two high energy electrons Electrons from Photosystem II replace the electrons that leave chlorophyll molecule Electrons are captured by NADP+ to make NADPH

Light Dependent Reactions ATP and NADPH are used in the light independent reactions How are electrons from Photosystem II replaced? Water is split O 2 – waste product – released by the plant Electrons – go into chlorophyll to replace lost e’s H + - used to make gradient to help make ATP

LIGHT DEPENDENT REACTIONS

Cyclic v. Noncyclic Photophosphorylation Cyclic – photosystem I only – electrons are recycled (use no NAPDH) Chemiosmosis – process of using proton movement to join ADP and Pi hill.com/sites/ /student_view0/chapter39/cyclic_and_noncyclic_photophosphorylation.html Simple vs. Complex Autotrophs Generates ATP but not NADPH. Why?

Light Independent Reactions Also called the Calvin Cycle Reactants: ATP, NADPH, and CO 2 Use: ATP to make ADP and Pi NADPH to make NADP+ Sugars are created Happens in the stroma

Calvin Cycle Keys to understanding…. It’s all about rearrangement of carbon atoms CO 2 enters cycle by attaching to RuBP RuBP is a 5-carbon molecule Similar to Acetyl CoA entering Krebs cycle Creates 2 PGA PGA is a 3-carbon molecule PGA turns into PGAL PGAL is a PGA molecule that has been energized by the ATP and NADPH

Calvin Cycle Summary Each turn fixes 1 CO 2 to a RuBPRubisco Enzyme that catalyzes CO 2 fixation Activated by light thus Calvin cycle requires some level of light to occur Can bind O 2 if present 3 turns = 1 PGAL “C 3 plants” – those that fix 3 CO 2 into 1 PGAL

Calvin Cycle Summary

PGAL

Light Reaction Calvin-Benson Cycle

Photosynthesis includes of take place in takes place in uses to produce use Light- dependent reactions Calvin cycle Thylakoid membranes StromaNADPH ATP Energy from sunlight ATPNADPHO2O2 Chloroplasts High-energy sugars Section 8-3 Concept Map

Factors Effecting the Rate of Photosynthesis

Light Intensity More light = higher rate Reaches saturation point Enzymes of light reaction going as fast as possible Higher than saturation point  PS declines Chlorophyll accumulates light faster than it can transfer it to ETS Extra energy goes to oxygen producing OH - when reaction w/H 2 O OH - or H 2 O 2 damages chloroplasts PHOTOINHIBITION

CO 2 Concentration Similar to light intensity Hits a saturation point Does not decline after saturation

Temperature Optimal temperature range If too high… Proteins denature If too low… Molecular movement is slower High Temps = cause stomata to close Prevents water loss Increases photorespiration C4 and CAM adaptations A metabolic pathway in plants that consumes oxygen, produces carbon dioxide, generates no ATP, and reduces photosynthesis

O 2 Concentration / Photorespiration REMEMBER  Rubisco binds CO 2 and O 2 equally as well Molecular shapes are similar Halves productivity of PGA Carbon fixation = 2 PGA Photorespiration = 1 PGA Glycolate = toxic to plant Benefits of photorespiration? Occurs when stomata close Dry and hot Evolutionary of C 4 and CAM plants Still makes some CO 2 and thus some sugars

C 3 vs C 4 vs CAM ts/plants_photosynthesis.htm

Leaf Anatomy – C 3 vs. C 4 C 3 plants CO 2 pulled through stomata and immediately goes to mesophyll cells to complete photosynthesis Called C 3 because it makes PGA (3-Carbon molecule) Stomata open during day Efficient in cool and moist envir. C 4 plants CO 2 pulled through stomata and immediately goes to mesophyll cells then to the bundle sheath cells to complete photosynthesis Called C 4 because it makes a 4- carbon molecule first (using PEP carboxylase Stomata open during day Efficient in higher temps and higher light intensity

Reducing Photorespriation: CAM plants CAM plants Crassulacean Acid Metabolism CO 2 pulled through stomata and stored as an acid. During the day, stomata close, CO 2 is released, then the cell goes through the Calvin cycle Stomata open during night Close during the day to prevent water loss Efficient in extremely hot and dry environments

Photosynthesis Song 1: The Light Reactions SongThe Light Reactions Song Photosynthesis Song 2: The Calvin CycleThe Calvin Cycle