Where It Starts: Photosynthesis Chapter 6 Photosynthesis

Photosynthesis Equation 6H 2 O + 6CO 2 6O 2 + C 6 H 12 O 6 watercarbon dioxide oxygenglucose LIGHT ENERGY

Photosynthesis

Sunlight and Survival  Autotrophs use nonliving sources to build their own food: producers  Chemoautotrophs  Photoautotrophs  Heterotrophs feed on living organisms or their remains: consumers  Most bacteria, fungi, animals  Carnivores, omnivores, detritivores

Sunlight and Survival  Plants are photoautotrophs; they use sunlight, CO 2, and water to produce sugar in the process of photosynthesis

Energy from the Sun  Visible light: wavelengths humans see as different colors Violet (380 nm wavelength) to red (750 nm) Violet (380 nm wavelength) to red (750 nm) ROY G BIV ROY G BIV

Plant Pigments  Pigments are molecules that capture light energy from absorbed wavelengths  pigments are specific for certain wavelengths  “Chlorophyll a” absorbs red and blue-violet  Color that you see is from wavelengths not absorbed - you see what is reflected! (“Chlorophyll a” looks green)  Light energy causes bonds to become unstable and boosts electrons to higher energy levels

Variety of Pigments Primary pigment : Chlorophyll a Accessory pigments : PigmentReflect Chlorophyll b yellow/green Carotenoids orange,red,yellow Xanthophylls yellow, brown, purple, blue Anthocyanins red, purple Phycobilins red, blue-green -- Each absorbs different wavelengths of visible light -- This makes photosynthesis very efficient

Chlorophyll A

Breakdown of chlorophyll a reveals accessory pigments. Fig. 5-2, p.74 Fall Colors

Light Receptors  Pigments capture light energy (photons) - like a solar panel which boosts electron levels. photon Light-Harvesting Complex

Photosynthesis Equation 6H 2 O + 6CO 2 6O 2 + C 6 H 12 O 6 watercarbon dioxide oxygenglucose LIGHT ENERGY

a A look inside the leaf b One of the photosynthetic cells inside leaf: contains chloroplasts leaf’s upper epidermis photosynthetic cell in leaf leaf vein leaf’s lower epidermis Leaf Structure

stroma thylakoid compartment thylakoid membrane system two outer membranes Chloroplast Organelle of photosynthesis in plants and algae

Two Steps in Photosynthesis 1. Light-dependent reactions -- in thylakoid disc (inside stroma) -- require light. 2. Light-independent reactions -- in stroma (a semi-fluid interior surrounded by two membranes) -- do not use light; sugars made here.

Photosystems I and II

Fig. 5-4d, p.75 sunlight Where the two stages of photosynthesis occur inside the chloroplast light- dependent reactions light- independent reactions CO 2 Sugars NADPH, ATP NADP +, ADP O2O2 H2OH2O Two Steps in Photosynthesis Thylakoid disc Stroma

Light dependent reactions  Photosystems (PS) I and II  These are reaction centers  Made of 100’s of pigments and other molecules  PS I and II are surrounded by lots of light harvesting systems.

Photosystem I & II

Role of Electron Transfer Chains  Adjacent to photosystems  Pigments absorb photons, excite electrons to higher energy level  Acceptor molecule accepts electrons from photosystem  As electrons pass along chain, energy released drives synthesis of ATP

Light-Dependent Reactions  Noncyclic pathway Make ATP and NADPH Make ATP and NADPH Water is split (photolysis) and oxygen released in noncyclic pathway from splitting of water into electrons, H +, and O 2 Water is split (photolysis) and oxygen released in noncyclic pathway from splitting of water into electrons, H +, and O 2 Requires PS I and II Requires PS I and II PS I can run by itself if NADPH builds up and ATP is still needed PS I can run by itself if NADPH builds up and ATP is still needed Oxygen bubbles from leaves of Elodea.

ATP Formation in the Noncyclic Pathway  Photolysis and electron transfer chains create electrical and H + concentration gradients across thylakoid membrane (H + builds up in thylakoid compartment)  H + ions flow down gradients into stroma through ATP synthases  Flow of H + ions through ATP synthases drives formation of ATP from ADP and phosphate

Photosystem I e–e– H2OH2O NADP + + e - + H + = NADPH 1/2 O 2 + 2H + Fig. 5-6a, p.76 energy light Photosystem II e–e– a The Noncyclic Pathway ATP formed

Light-Dependent Reactions  Cyclic pathway ATP forms from electron flow (no NADPH) ATP forms from electron flow (no NADPH) Requires photosystem I Requires photosystem I Electrons Electrons are donated by chlorophyll a in photosystem I to an acceptor moleculeare donated by chlorophyll a in photosystem I to an acceptor molecule flow through electron transfer chain and back to photosystem Iflow through electron transfer chain and back to photosystem I are recycledare recycled

Photosystem I and II

NADPH NADP + + H + thylakoid compartment thylakoid membrane stroma ATP ADP + P i H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ Photosystem I sunlight Photosystem II Light- Harvesting Complex Fig. 5-7, p.77 H+H+ e–e– e–e– e–e– e–e– e–e– e–e– H+H+ e–e– O2O2 H2OH2O cross-section through a disk- shaped fold in the thylakoid membrane Summary of Light-Dependent Reactions sunlight ATP Synthase Electron Transfer Chains

Light-Independent Reactions  “Synthesis” part of photosynthesis Glucose is made from CO 2 Glucose is made from CO 2  Proceed in the light or the dark using energy (ATP and NADPH) formed in the light-dependent reactions  Take place in the stroma  Called the Calvin-Benson cycle

Calvin-Benson Cycle  Reactants - In  Carbon dioxide  ATP  NADPH  Products- Out  Glucose (converted to sucrose and starch)  ADP  NADP + Reaction pathway is cyclic.

Fig. 5-8, p.78 REACTIONS PROCEED IN CHLOROPLAST’S STROMA Calvin-Benson cycle 12 PGAL glucose 1 6 ATP 12 ATP 12 NADPH 6 RuBP 12 PGA 6 CO 2 Calvin-Benson Cycle 10 PGAL 2 PGAL carbon fixation by rubisco enzyme

Calvin - Benson

The C3 Pathway  In Calvin-Benson cycle, the first stable intermediate is a three-carbon PGA (phosphoglycerate)  Because the first intermediate has three carbons, the pathway is called the C3 pathway

upper epidermis palisade mesophyll spongy mesophyll lower epidermis stoma veinair space Leaves of a C3 Plant Basswood, a typical C3 plant

Photorespiration in C3 Plants  On hot, dry days, stomata close  Inside leaf Oxygen levels rise Oxygen levels rise Carbon dioxide levels drop Carbon dioxide levels drop  Only get one PGAL instead of two  Not very efficient

C4 and CAM Plants  Plants suited for hot, dry conditions  Carbon dioxide is fixed twice

Summary of Photosynthesis 6H 2 O sunlight Calvin- Benson cycle 6O 2 Light Dependent Reactions Light Independent Reactions NADP + ADP + P i 6 RuBP 12 PGAL P end products (e.g., sucrose, starch, cellulose) phosphorylated glucose 6CO 2 ATP NADPH x