Photosynthesis

Chapter 10

Objectives Compare the overall reaction of photosynthesis with

the overall reaction for respiration Describe where the processes of photosynthesis occur Describe the basic processes involved in

photosynthesis: water splitting to obtain electrons, redox reactions of the electron transport chains, electron and energy shuttling by means of ATP and NADPH, and the coupling of the light-dependent reactions and the Calvin cycle

Explain how pigments capture light and excite electrons

Describe the structural and functional differences between photosystem I and photosystem II, and cyclic and noncyclic photophosphorylation

Outline the steps in the cyclic fixation of carbon in the Calvin cycle and where these occur

Contrast the C4 and CAM photosynthetic systems with the simpler C3 system, and how they are adaptions to hot, dry climates

Introduction

Overall equation is reversal of cellular respiration 6CO2+12H2O+energy--->C6H12O6+6O2+6H2O

Increasingly probing studies provided knowledge about how photosynthesis works van Helmont-developed early ideas about where

plants obtain materials for growth showed that soil not sufficient concluded that water important

Priestly-showed that plants restore “bad” air Ingenhousz-plants only restore air when

exposed to light

Autotrophs are Producers

Autotroph-means self-feeding applies to any organism that makes own food

without eating, decomposing or absorbing other organisms or organic molecules

Photosynthetic autotrophs include plants, algae and photosynthetic bacteria

Site of Photosynthesis

Photosynthesis occurs in chloroplasts in all photosynthetic organisms except monerans

Leaves (specifically, mesophyll cells) are primary site of photosynthesis

Light-absorbing pigment is chlorophyll located in protein complexes in internal

membranes of chloroplasts Sugars assembled in stroma

Underlying Processes Oxygen produced by splitting water

demonstrated using 18O-labeled reactants plant given C18O2 does not release 18O2

plant given H218O does give off 18O2

Photosynthesis is redox process H2O oxidized--->1/2O2+2H++2e-

CO2 reduced to glucose by addition of e-’s and H+’s

compare with respiration where glucose oxidized and O2 reduced

In photosynthesis, electrons travel “uphill” from water to glucose, adding light energy captured by chlorophyll

In respiration, electrons travel “downhill” from glucose to water, releasing energy to ATP

Overview

Photosynthesis is a two-stage process light-dependant reactions

convert light energy to chemical energy, releases O2 as waste product

occurs in thylakoid membranes and produces energy shuttles ATP and NADPH

Calvin cycle cyclic series of steps that assemble organic

molecules from CO2

occur in stroma and use energy and electrons from ATP and NADPH in carbon fixation

light not required but usually run during day as require shuttles from light-dependant reactions

The Light Reactions

Driven by visible light light is electromagnetic radiation only small fraction of em radiation perceived

by organisms different wavelengths=different colors

leaf absorbs some wavelengths (red-orange and blue-violet) and reflects others (green)

in plants light absorbed by chlorophyll a, chlorophyll b and carotenoids

only chlorophyll a directly involved in light reactions; other pigments act as “antenna” molecules to broaden range of energy absorbed

The Photosystems

Light behaves like particles-photons When pigment absorbs photon, energy level

of one electron is raised to excited, unstable state if pigment is isolated from molecular

environment, excited electron loses energy as heat or light and returns to normal level chlorophyll fluoresces red

In chloroplasts, 200-300 chlorophyll molecules grouped with proteins to form antenna assembly around two chlorophyll a molecules-reaction center chlorophylls excited electrons passed from antenna chlorophylls

to reaction center chlorophylls then to primary electron acceptor series of redox reactions

• final is oxidation of reaction center chlorophyll and reduction of primary electron acceptor

Two photosystems (antenna assembly+primary electron acceptor) identified absorb at different wavelengths

photosystem I-absorbs maximally at 700nm (P700) photosystem II-absorbs maximally at 680nm (P680)

function together to carryout non-cyclic electron transport also known as non-cyclic photophosphorylation

photosystem I can also carryout cyclic electron transport (cyclic photophosphorylation) thought to be the earliest form of photosynthesis

• present in many primitive photosynthetic bacteria

synthesizes only ATP

Chemical Energy Generation

Electron transport chains generate ATP, NADPH and O2

kinetic energy of light absorbed and excites electrons

excited electrons passed along electron transport chain-series of redox reactions

released energy used to generate ATP, NADPH and O2

production of NADPH requires 2 electrons supplied to PS I by PS II replaced in PS II by splitting water H2O ---> 1/2O2 + 2H+ + 2e-

Chemiosmosis

Powers ATP synthesis H+ ions from splitting water and those pumped

across thylakoid membrane by electron transport chain form gradient across thylakoid membrane (inside to outside)

ATP synthase provides port for H+ to diffuse back into stroma releases energy and phosphorylates ADP to ATP similar process to ATP generation in mitochondria known as photophosphorylation

Carbon Fixation

ATP and NADPH from light-dependant reactions power Calvin cycle net result of Calvin cycle is 3C molecules from

CO2 using energy and electrons in ATP and NADPH from light-dependant reactions

CO2 added to 5C intermediate ribulose-1,5-bisphosphate (RuBP) catalyzed by RuBP carboxylase/oxygenase (rubisco)

Number of rearrangements occur in many steps, using energy in ATP and oxidation of NADPH last step in cycle regenerates RuBP all steps occur simultaneously but ultimately

regenerate starting reactants, hence cycle

Three RuBP enter cycle for each 3C molecule released from chloroplast

Calvin cycle occurs in chloroplast stroma 3C molecules exported to cytoplasm

used to synthesize glucose and other organic molecules

Plants that use only Calvin cycle to fix carbon called C3 plants first identifiable product of carbon fixation is

3C molecule

Carbon-fixing Variations

C3 plants conserve water by closing stomata allows buildup of O2 in leaves

Rubisco fixes O2 rather than CO2

called photorespiration uses ATP and NADPH but makes no sugars

C4 plants adapted to conserve water and prevent photorespiration CO2 incorporated into 4C molecule in mesophyll

cells diffuses into bundle sheath cells and released enters Calvin cycle in bundle sheath chloroplasts

CAM (crassulacean acid metabolism) plants incorporate carbon during night stomata open at night, closed during day CO2 incorporated in 4C molecule and stored in

vacuole at night during day, 4C molecules exported into

cytoplasm and CO2 released

CO2 enters Calvin cycle

C4 separate carbon incorporation and fixation spatially

CAM plants separate carbon incorporation and carbon fixation temporally