Photosynthesis

Bio 391 – Ch4

How Exactly is Sunlight captured and converted into Food?

What are autotrophs?

Obtains energy from nonliving sources

Two typesPhotoautotrophsPhotoautotrophs

Photosynthesis

Sun energy converts CO2 into sugars

Enzymes convert sugars into amino acids and other needed compounds

ChemoautotrophsChemoautotrophsSpecialized bacteria

No sunlight – use energy of inorganic substances (Fe, S, etc.)

Electromagnetic SpectrumWide 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 LightVery small section of the electromagnetic spectrum

ROYGBIV

ChloroplastsStructureStructure

ThylakoidsHighly folded inner membrane

surface area

Holds pigments

GranumStack of thylakoid membranes

StromaLiquid between thylakoid and outer membrane of chloroplast

Have their own DNA & RNA

Chlorophyll & Accessory Pigments

Pigments = light absorbing molecules

Found on the thylakoid membrane

ChlorophyllChlorophyllTwo types – “a” and “b”

Absorbs violet-blue and orange-red colors

~ 350-500 nm & 650-700 nm

Reflects green plants have green color

AcceAccessoryssory Pigm Pigmentsents Absorb other colors of light and transfer Σ to chlorophyll-a

Most noticeable in the fall monthsEX: carotenoids

Absorption Spectrums of Pigments

Photosynthesis Simplified

Can be broken down into two steps:Light ReactionsLight Reactions

Pigments in thylakoids absorb lightLight converted into chemical energy

Calvin CycleCalvin Cycle (a.k.a. “Dark Reactions”)Chemical energy from light reactions used to make 3 carbon sugars from CO2

Used to make more complex sugars or other biochemical molecules

Overall ReactionOverall Reaction6CO2 + 6H2O C6H12O6 + 6O2

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 IILight 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 ILight 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 splitO2 – 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

http://highered.mcgraw-hill.com/sites/9834092339/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 CO2

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 atomsIt’s all about rearrangement of carbon atoms

CO2 enters cycle by attaching to RuBP RuBP is a 5-carbon molecule

Similar to Acetyl CoA entering Krebs cycle

Creates 2 PGAPGA 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 CO2 to a RuBP

RubiscoRubiscoEnzyme that catalyzes CO2 fixation

Activated by light thus Calvin cycle requires some level of light to occur

Can bind O2 if present

3 turns = 1 PGAL

“C3 plants” – those that fix 3 CO2 into 1 PGAL

Calvin Cycle Summary

PGAL

Light Reactionhttp://vcell.ndsu.edu/animations/photosynthesis/movie.ht

m

Calvin-Benson Cyclehttp://www.youtube.com/watch?v=mHU27qYJNU0

Photosynthesis

includes

of

take place intakes place in uses

to produce to produce

use

Light-dependentreactions

Calvin cycle

Thylakoidmembranes Stroma NADPHATPEnergy from

sunlight

ATP NADPH O2 Chloroplasts High-energysugars

Section 8-3

Concept Map

Factors Effecting the Rate of

Photosynthesis

Light IntensityMore light = higher rate

Reaches saturation pointEnzymes 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/H2O

OH- or H2O2 damages chloroplasts

PHOTOINHIBITION

CO2 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

O2 Concentration / Photorespiration

• REMEMBER Rubisco binds CO2 and O2 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 C4 and CAM plants

– Still makes some CO2 and thus some sugars

C3 vs C4 vs CAMhttp://wc.pima.edu/~bfiero/tucsonecology/

plants/plants_photosynthesis.htm

Leaf Anatomy – C3 vs. C4

C3 plants•CO2 pulled through stomata and immediately goes to mesophyll cells to complete photosynthesis•Called C3 because it makes PGA (3-Carbon molecule)•Stomata open during day•Efficient in cool and moist envir.

C4 plants•CO2 pulled through stomata and immediately goes to mesophyll cells then to the bundle sheath cells to complete photosynthesis•Called C4 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•CO2 pulled through stomata and stored as an acid. During the day, stomata close, CO2 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 SongPhotosynthesis Song 2: The Calvin Cycle