ESSENTIAL KNOWLEDGE 2A2PHOTOSYNTHESIS

How the photosystems collect light energy and convert it to chemical energy.

How photosystem II produces ATP by chemiosmosis in the light reactions.

How the double membrane structure of the chloroplasts enable their function in chemiosmosis.

How photosystem I produces NADPH in the light reactions.

How the Calvin cycle uses the products of the light reactions to synthesize sugar (G3P).

The causes of and metabolic costs of photorespiration.

The evolutionary adaptations of C4 and CAM plants that enable them to efficiently perform the Calvin cycle in hot, dry habitats.

The commonalities and distinctions between photosynthesis in chloroplasts and aerobic respiration in mitochondria.

light-dependent reactions

Calvin cycle light-independent

reactions photosystem photosystem II photosystem I carbon fixation absorption spectrum pigments

chlorophyll chlorophyll a chlorophyll b action spectrum carotenoid NADPH cyclic

photophosphorylation noncyclic

photophosphorylation

Calvin cycle Rubisco RuBP glyceraldehyde-3-

phosphate C3 photosynthesis photorespiration C4 photosynthesis crassulacean acid

metabolism (CAM)

PHOTOSYNTHESIS

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PHOTOSYNTHESIS OVERVIEW Energy for all life on Earth ultimately

comes from photosynthesis

6CO2 + 12H2O C6H12O6 + 6H2O + 6O2

Oxygenic photosynthesis is carried out by Cyanobacteria 7 groups of algae All land plants – chloroplasts

CHLOROPLAST Thylakoid membrane – internal membrane

Contains chlorophyll and other photosynthetic pigments

Pigments clustered into photosystems Grana – stacks of flattened sacs of

thylakoid membrane Stroma lamella – connect grana Stroma – semiliquid surrounding thylakoid

membranes

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STAGES Light-dependent reactions

Require light1. Capture energy from sunlight2. Make ATP and reduce NADP+ to NADPH

Carbon fixation reactions or light-independent reactions Does not require light3. Use ATP and NADPH to synthesize organic

molecules from CO2

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DISCOVERY OF PHOTOSYNTHESIS Jan Baptista van Helmont (1580–1644)

Demonstrated that the substance of the plant was not produced only from the soil

Joseph Priestly (1733–1804) Living vegetation adds something to the air

Jan Ingen-Housz (1730–1799) Proposed plants carry out a process that

uses sunlight to split carbon dioxide into carbon and oxygen (O2 gas)

F.F. Blackman (1866–1947) Came to the

startling conclusion that photosynthesis is in fact a multistage process, only one portion of which uses light directly

Light versus dark reactions

Enzymes involved

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C. B. van Niel (1897–1985) Found purple sulfur bacteria do not

release O2 but accumulate sulfur Proposed general formula for

photosynthesis CO2 + 2 H2A + light energy → (CH2O) + H2O +

2 A Later researchers found O2 produced

comes from water Robin Hill (1899–1991)

Demonstrated Niel was right that light energy could be harvested and used in a reduction reaction

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PIGMENTS Molecules that absorb light energy in the

visible range Light is a form of energy Photon – particle of light

Acts as a discrete bundle of energy Energy content of a photon is inversely

proportional to the wavelength of the light Photoelectric effect – removal of an

electron from a molecule by light

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ABSORPTION SPECTRUM When a photon strikes a molecule, its

energy is either Lost as heat Absorbed by the electrons of the molecule

Boosts electrons into higher energy level Absorption spectrum – range and

efficiency of photons molecule is capable of absorbing

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Organisms have evolved a variety of different pigments

Only two general types are used in green plant photosynthesis Chlorophylls Carotenoids

In some organisms, other molecules also absorb light energy

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CHLOROPHYLLS Chlorophyll a

Main pigment in plants and cyanobacteria Only pigment that can act directly to

convert light energy to chemical energy Absorbs violet-blue and red light

Chlorophyll b Accessory pigment or secondary pigment

absorbing light wavelengths that chlorophyll a does not absorb

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PIGMENTS

Structure of chlorophyll

porphyrin ring Complex ring

structure with alternating double and single bonds

Magnesium ion at the center of the ring

Photons excite electrons in the ring

Electrons are shuttled away from the ring

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Action spectrum Relative effectiveness of different

wavelengths of light in promoting photosynthesis

Corresponds to the absorption spectrum for chlorophylls

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Carotenoids Carbon rings linked

to chains with alternating single and double bonds

Can absorb photons with a wide range of energies

Also scavenge free radicals – antioxidant

Protective role Phycobiloproteins

Important in low-light ocean areas

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PHOTOSYSTEM ORGANIZATION Antenna complex

Hundreds of accessory pigment molecules Gather photons and feed the captured light

energy to the reaction center Reaction center

1 or more chlorophyll a molecules Passes excited electrons out of the

photosystem

ANTENNA COMPLEX Also called light-harvesting complex Captures photons from sunlight and

channels them to the reaction center chlorophylls

In chloroplasts, light-harvesting complexes consist of a web of chlorophyll molecules linked together and held tightly in the thylakoid membrane by a matrix of proteins

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REACTION CENTER Transmembrane protein–pigment

complex When a chlorophyll in the reaction

center absorbs a photon of light, an electron is excited to a higher energy level

Light-energized electron can be transferred to the primary electron acceptor, reducing it

Oxidized chlorophyll then fills its electron “hole” by oxidizing a donor molecule

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LIGHT-DEPENDENT REACTIONS1. Primary photoevent

Photon of light is captured by a pigment molecule2. Charge separation

Energy is transferred to the reaction center; an excited electron is transferred to an acceptor molecule

3. Electron transport Electrons move through carriers to reduce NADP+

4. Chemiosmosis Produces ATP

Capt

ure

of li

ght e

nerg

y

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In sulfur bacteria, only one photosystem is used

Generates ATP via electron transport Anoxygenic photosynthesis Excited electron passed to electron

transport chain Generates a proton gradient for ATP

synthesis

CYCLIC PHOTOPHOSPHORYLATION

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CHLOROPLASTS HAVE TWO CONNECTED PHOTOSYSTEMS

Oxygenic photosynthesis Photosystem I (P700)

Functions like sulfur bacteria Photosystem II (P680)

Can generate an oxidation potential high enough to oxidize water

Working together, the two photosystems carry out a noncyclic transfer of electrons that is used to generate both ATP and NADPH

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Photosystem I transfers electrons ultimately to NADP+, producing NADPH

Electrons lost from photosystem I are replaced by electrons from photosystem II

Photosystem II oxidizes water to replace the electrons transferred to photosystem I

2 photosystems connected by cytochrome/ b6-f complex

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NONCYCLIC PHOTOPHOSPHORYLATION Plants use photosystems II and I in

series to produce both ATP and NADPH Path of electrons not a circle Photosystems replenished with

electrons obtained by splitting water Z diagram

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PHOTOSYSTEM II Resembles the reaction center of purple

bacteria Core of 10 transmembrane protein subunits

with electron transfer components and two P680 chlorophyll molecules

Reaction center differs from purple bacteria in that it also contains four manganese atoms Essential for the oxidation of water

b6-f complex Proton pump embedded in thylakoid membrane

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PHOTOSYSTEM I Reaction center consists of a core

transmembrane complex consisting of 12 to 14 protein subunits with two bound P700 chlorophyll molecules

Photosystem I accepts an electron from plastocyanin into the “hole” created by the exit of a light-energized electron

Passes electrons to NADP+ to form NADPH

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CHEMIOSMOSIS Electrochemical gradient can be used

to synthesize ATP Chloroplast has ATP synthase enzymes

in the thylakoid membrane Allows protons back into stroma

Stroma also contains enzymes that catalyze the reactions of carbon fixation – the Calvin cycle reactions

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PRODUCTION OF ADDITIONAL ATP Noncyclic photophosphorylation generates

NADPH ATP

Building organic molecules takes more energy than that alone

Cyclic photophosphorylation used to produce additional ATP Short-circuit photosystem I to make a larger

proton gradient to make more ATP

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CARBON FIXATION – CALVIN CYCLE To build carbohydrates cells use Energy

ATP from light-dependent reactions Cyclic and noncyclic photophosphorylation Drives endergonic reaction

Reduction potential NADPH from photosystem I Source of protons and energetic electrons

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CALVIN CYCLE Named after Melvin Calvin (1911–1997) Also called C3 photosynthesis Key step is attachment of CO2 to RuBP

to form PGA Uses enzyme ribulose bisphosphate

carboxylase/oxygenase or rubisco

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3 PHASES1. Carbon fixation

RuBP + CO2 → PGA2. Reduction

PGA is reduced to G3P3. Regeneration of RuBP

PGA is used to regenerate RuBP 3 turns incorporate enough carbon to

produce a new G3P 6 turns incorporate enough carbon for 1

glucose

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OUTPUT OF CALVIN CYCLE Glucose is not a direct product of the

Calvin cycle G3P is a 3 carbon sugar

Used to form sucrose Major transport sugar in plants Disaccharide made of fructose and glucose

Used to make starch Insoluble glucose polymer Stored for later use

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ENERGY CYCLE Photosynthesis uses the products of

respiration as starting substrates Respiration uses the products of

photosynthesis as starting substrates Production of glucose from G3P even uses

part of the ancient glycolytic pathway, run in reverse

Principal proteins involved in electron transport and ATP production in plants are evolutionarily related to those in mitochondria

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PHOTORESPIRATION Rubisco has 2 enzymatic activities

Carboxylation Addition of CO2 to RuBP Favored under normal conditions

Photorespiration Oxidation of RuBP by the addition of O2 Favored when stoma are closed in hot

conditions Creates low-CO2 and high-O2

CO2 and O2 compete for the active site on RuBP

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TYPES OF PHOTOSYNTHESIS C3

Plants that fix carbon using only C3 photosynthesis (the Calvin cycle)

C4 and CAM Add CO2 to PEP to form 4 carbon molecule Use PEP carboxylase Greater affinity for CO2, no oxidase activity C4 – spatial solution CAM – temporal solution

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C4 PLANTS Corn, sugarcane, sorghum, and a number of

other grasses Initially fix carbon using PEP carboxylase in

mesophyll cells Produces oxaloacetate, converted to malate,

transported to bundle-sheath cells Within the bundle-sheath cells, malate is

decarboxylated to produce pyruvate and CO2 Carbon fixation then by rubisco and the

Calvin cycle

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C4 pathway, although it overcomes the problems of photorespiration, does have a cost

To produce a single glucose requires 12 additional ATP compared with the Calvin cycle alone

C4 photosynthesis is advantageous in hot dry climates where photorespiration would remove more than half of the carbon fixed by the usual C3 pathway alone

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CAM PLANTS Many succulent (water-storing) plants,

such as cacti, pineapples, and some members of about two dozen other plant groups

Stomata open during the night and close during the day Reverse of that in most plants

Fix CO2 using PEP carboxylase during the night and store in vacuole

When stomata closed during the day, organic acids are decarboxylated to yield high levels of CO2

High levels of CO2 drive the Calvin cycle and minimize photorespiration

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COMPARE C4 AND CAM Both use both C3 and C4 pathways C4 – two pathways occur in different

cells CAM – C4 pathway at night and the C3

pathway during the day

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